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'I1 Moulding. " of Plastics I
SECOND EDITION
LC - :A 1
E d M by n .'
R. J. Crawford
Chapter 3
Chapter 5
Chapter 6
Chapter 7
Rotational Moulding Machines W.H.Covington, Jr
1. Introduction 2. The rotation process 178 3. Rock and roll machines 183 4. Box oven machines 191 5. Shuttle-style machlnes 194 6. Clamshell machines 195 7. Vertical style machines 8. Fixed-arm turret machines 9. The independent-arm machine
Sheet Metal Moulds TJ.Taylor
Introduction Tool construction and design Surface finish Special features
Maintenance
Cast Aluminium Moulds S.Scaccia
1. History 2. Product design &a1 on bubbles 3. The pattern 4. Mould,design 5. Function and operation of mould 6 Maintenance
Design of Rotationally Moulded Products G.L.Beall
1. Designing rotationally moulded plastic products 2. Rotationally moulded plastic part design
2.1 Wall thickness 2.2 Wall thickness uniformity 243 2.3 Closely spaced parallel walls a rotating mould 245 2.4 Warpage 2.5 Stiffening ribs 2.6 Kiss-off ribs 250 2.7 Draft angles 2.8 Surface finish 2.9 Undercuts 254 2.10 Holes 2.11 Comer radii 2.12 Tolerances
I
1 Il,APTER 1
-ktroduction to Rotational Moulding
method for producing hollow plastic articles. The process 940's but in the early years it attracted little attention as a dow process which was restricted to a small number
two decades, improvements in process control &rs have resulted in a very significant increase the advantages which it has to offer in terms of
lex, stress-fiee articles has made it a very '9h rncaddhg and injection moulding. '&Q known as rotocastina or rotornouldina, is unique
b i k d rotation continues w e d aad the product is
ntst be hollow Ace M8hing brations ~ ~ ~ e , for example, right and left handed
4. Pa-& b v e $4 wall thichess to those produced by ~IXE~ZBCM ~h blow-
5. Prodm~ cm be virtuall ns are taken. No weld
A f e a m of rotational m d &>ility. A very w i d e v ~ o f s h a r > e s a a d sizes canbe tiny ear syringes to 100,CMM litre storage W ( 6 ) . Inserts may be moddd-ia d y and surf tex- tures such*^ m d grain, leather grain, etc map, be ~ccurately repr%ced. Rotomoulding is i W y suited for e~anofnic short pmdncdatl runs and indeed it has an ever increasing role in prototype work for other prow-,
1. Mod&g iugh since most plrtsfia sue pivdable as pellets
and have to tw redwed to a fine powder. The grkdhg s e e adds significantly as h e quality of the grind is crucial to
during moulding. Tn some cases the moulder ty to improve th8 txxawmics in regard to raw
material ~ast for latge production funs of small parts. This is mould must be h W from room temperature to a and then back to room tempem. This results in
3. IWazMs suited to rotational mouldings are limited. At present, polyethylene mmts for over 90% of the materials used but current research is extending '
h,rmge of p k t i ~ s which ap be r&omoulded. , 4. &ad@ slnd unbxhg is very I h u r intensive especially h wnagIlcN P-
5. $oms or solid r ibs cannot be easily mo;uldcd. Hews, k s wpz4 I&
% Crystallinity
rhis m m t a sac-
h e w b w density grada of
6 7
are short an8 Wir occurrence can be w ~ l i mmtfolled. The density sf LLDPE can be d j u s t d L W tmge 910-960 kghd The/ ts8& mtm @ar a d reIBned stnrcture of LLDFE means that it is stronger srnmS th LDPE at the sane density whilst retaining tmglmess ahd stress rack raf-.
In general, polyethylenes are resistant bQ mwt m l ~ t s at room temperature although mna t i c and chlorinated hydmwbm w%l2 came swelling. ILLDPE and HDPB have no known solvents at room te rnperm. EDFE, is relatively unaffected by polar solvents, for example, alcohols, phenols, mtm and ketones. However, caution is nmbd in regard to envimnme:ntaJ stress cmking This &labs to the cracking which can occur in a material when it is under stress in the presence of a polar liqrzld or its vapom. E n v w ~ u stress cmking is dm associated with detergents, s h e fluids, cblorofmn, xylem and p&. In polyethylenes the resistance b MII c k r ~ e s as the density increases and the high molecular weight grades give the best performance.
Polyethylenes are generally mistzlnt to water, vegetable oils, d i d i s and most eonm~tmtd acids a room t e m p e m . However, their light (m ultra-violet) resis- tmw i8 pwr. The &eqmt way to improve this is by the heorporntion of &n blwk~bnt h e r motbods are avttiiable. Water absorption is very low but will i m m w i f ~ b l a c l e i s u s e d .
U)PE b more p ~ & b l e to gases and V-S than LLDBE and WDPE. The penm&&y far cxgdc ~apours is least for aleoh015 and them immues &em &ids ' to &ehyiPes and l&%nm, mters, ethers, hy-bns d h a b g e ~ hy&&?ar- bons. Fluorine, a W g e n , slowly attacks pol past to :hpmve i& permeability. Polyethylene particular, impact strength) will fuming sulphuric acid. It is alsd attacked slowly by c ~ 8 r n ~ ~ p h o m i c acid and phosgene. stress crack resistance (10).
lyethylene is typicztlIy in the region of 2-5s. The As well as the rheological and ch the higher density grades and iue nature of the powder can have an
as carbn blsck. a
premaburely and prevent other particles from entering parti mould, for example, ribs or- carnets(9). In contrast, if there are
a
9
rack resistance arises because although the mechanisms of attack are
ites for potential attack. and EBA are becoming more common when
new developments such as those described in Chapter
Partlde size
structnre of PVC will sm to tt~zdts in disco- a
effect rezt~ting with %
11
C W ~ & ti160 very e~ecctive (18). e difficult to rotationally mould and are relatively expensive.
ortant to note that a number of reactive materials can be rotationally can h according to b w . These include thermosetting materials such as polyester, silicone rubber, pia&&a is mt e0mpatiM.e fbefl it
ia adwisxl t~ ?gabs clmdy with the r ~ i n supplier to select the appropriate M e for a, qy&fk appUc&~n,
ate detail. It is likely therefore that their usage will increase the future. Due to the low viscosity of most reactive resins,
and which me not in ~~ use but which offer significant potmfiai. xciting possibility of using them to impregnate fibre preforms 1wI-a~ which are used mmnaci@Iy are plywter, polyet~&onab, ayioa 6, uld. The moulding of liquid plastics is considered further in nylon 21, nylon 12, poIvjnyli&ae fluoride and f l u w ~ p l n ; sa& as ECTFE (4,12,19,23). The growing impo-ce of nylan ~TS a m t c , M b g mt~rial m-
often used to reduce photocat-
13
which rocks back and forward. Hence, this machine does not provide a Eull
an oven is used to heat the mould. In most cases the oven is air
Plate rotation
( 1 1 , A . * I Y , C f :, -.# / , ' - !
. 3. ROTATIONAL MO The basic requirements heated and then cooled
1
14 tr The simplest concept is a shuttle type machine where the mould runs on a track In recent years this type of machine has been made much more flexible in that
between the three stations (Fig. 7). For fast production rates, howeverb it is more the arms are not rigidly fixed together. This means that programmable logic can- common to have a multiple arm carousel mshine. uollers w be used to index each arm individually at the optimum time. Hence the
movement of the arms is not controlled by the slowest event, as happens on the Coding Charging fixed ann whines . Further refinements of the independent arm machines include
the inMUction of additional ovens and cooling areas so that production rates can be push& to levels which are competitive with blow moukhg.
One a&er m e of mmhina which is worth mentioning because of its increasing popularity is the Clam Shell machine. This is different to the machines described above in that the charging, heating and cooling all take place in the same chamber. This chamber opsns like a clam shell so that the mould can be charged with pow- der. f i e '&Al' hen closes around the mould which rotates biaxially in a heated environment (23+31). At a pre-set time the cooling cycle is activated and eventually the clam shell spens again to permit the moulding to be removed.
RE 7 Shuttle type rotational moulding machine The moulds used in rotational moulding are shell-like. They define the outside
The simplest form of this is a 3 arm machine as illustrated in Fig. 8. ~n this c shape of the pmbct but do not have an internal care. There is a wide choice of one set of moulds is being charged with powder and one set of moulds is in materials available for moulds, and the construction method may involve fabrica-
tion, electroforming or wtiag (17,23,32-34). Cast aluminium is used most extensively for multi-cavity moulds and for com-
ing of the other moulds. plex shapes. Typid waB $hiclmesms vary from 5-10 mrn. The cost of one cavity is high due to the need for a pattern but subsequent cavities cost progressively less. Electroformed modds &e fmowed for vinyl plastisol products. The reproduction
, of detail is very predse @' W method, For very flexible products (eg. dolls' heads) 1
there is no need for a parting line. Fabricated sheet metal moulds are generally the most economical and iie almost atways used for very large products.
The sheet metals used for muld fabrication can be aluminium, mild steel or 1
stainless steel. Machined moulds am wed ocasionally but their wsts tend to be prohibitive. Chapters 5 and 6 deal with the design aspects of sheet metal and cast aluminium moulds,
The basic requirements of a moukl .are as follows: (a) It should have a good thermal d u & v i g so that the heat is transferred to
(and from) the plastic as quickly as po$sible. (b)The mould should be able to wi thsfa~4 without warping, the thermal
cycling of the heating and cooIing stages of the process. (c) There should be the provision of quick release clamps to keep the halves of
the mould tightly closed during the hearing @ cooling stages but which facilitate fast opening of the mould at the en$ of& cy&.
(d) The mounting of the mould on the a d p l a t e 9hould not p~evenr~athe free ,,
passage of air over the entire surface of the mould - otk~wise hot spo8 will
oblems which are attributed to mould venting are often more directly
17
related to ather fators. For example, if the muld has a vent but this bmmes blocked at some stage during the heating cycle, then hm am be problems with warpagddistortion of the moulded pro8u~k This is because, during cooliag, there is inadficient ah to fill the space inside the moW at the lower temperature. This causes tx p;tllteisll vacuum inside the moulding WM a m b suEtiGient to pull the product away from the mould. If there had k e n no e o d g throughout the heating and cooling cycle then the reduced &md praastare would not have occurred and hence there would be no distortion - munzing that the mating surfaces of the mould 'halves' are airtight.
It has to be reoognised, of course, that in the majority of cases the mating sur- faces of the mould are m t airtight. This can muse two problems, Firstly, air cm escape during the heating stage and result in a pamal vauum effwt during cooling as described above. Secondly, if there is no vent md the parting line is not airtight, it is possible that the mlten plastic can be squeezed out to cause 'flash' and possi- ble defects in the product wall at the parting line.
Hence, to overcome these problems it is normal to have an adequate vent (10-14 mm per m3 of mould volume) to ensure equalisation of htemal and external pres- sure in the mould.
release of the moulded part (leads to warpage) and they mould, albeit in small amounts.
OR PLASTICS
The Heat Transfer characteristics of a mould can be impomt if it is to be mounted with other products on a carousel type m~hine . It k generally desirable to balance the cycle times of all the produa@ in order to amid ov&r- rn mbr-heating. factors need b be eonsidered. Finally, the Weight Capacity of the modding machine ia an impom practical consideration and care must be taken thtrt the mould materid a tlor m e this to
Long-berm Behadour Plastics exhibit a time-dependent swain response to a constant applied stress. This bekviour is called creep. In a similar f&m if the stress on a plastic is removed it &bits a timedependent recovgr of strain back towards its original dimen&ons. This is illustrated in Fig. 11. Another important consequence of the viscoalastic nature of plastics is that if they are m b j d toe particular strain and rhis s W is held constant it is found that as time progresses the stress necessary to maintain this strain decreases. This is termed Stress Relaxation and is of vital importance ha the design of gaskets, seals, springs and snap-fit assemblies. The influence w w these timedependent phenomena have on deign procedures for plastics is extremely important and this will be considered in the next section.
$ 8
Elastic recovery
Elastic deformation
FIGURE 11 Typicd aeep and recovery behaviour of plastics
Design Methods for Plastics using Deformation Data The most common method of displaying the interdependence of stress, strain and time is by means of creep curves. However, there are also other meEhods which may be more useful in particular applications (see BS 4618). The first of these is
23
specifid it is neoessary to have
ill be illustrated first.
W. 48F4@ - ~ ~ x u ~ ~ M x ~ P ~ ~ w + o & ~ B ~ iIsN - Y
As tjme progrrssm8, the &fieCtion will incrdm tnn r d ch d@i@ o a - tion of 5 rrrm & M o n at 1 yea. Table 1 gives typical vdum fm the the*- dent moduli Ear a rage of plastics.
ert &. into one consisting totally of m. 13@), ~n this case the flange width is W b 1 - Tmsile Creep Modulus GN/mZ at2~%, i% astrain @%gmes in parenthesis are exmpoiW dm]
kts of 3 or more materials,
* 8 ~ ~ ~ b u a d & i s 6 7 % c d ~ ~ ~ * u e r , 2 5 ~ o f d r y ~ ~ ~ ~ r ~ ~ d i l y h l 6 7 4 6 & d t y d o & w ~ h S M b f & y k y l u s
ner as the sandwich
iBlB 1.3 Equivalent sections
FIGURE 14 Solid/foam equivalent sections
Design 01 Fo,amd Sandwich Sections As indicated above, it is now becoming common in the rotational moulding indus- try to have partially foamed sections, as shown in Fig. 14(a). In such cases, the design procedure to calculate flexural stifl%ess is to convert the composite structure to one consisting only of the solid material. This is shown in Fig. 14(b). The web width is once again given by
vhere the subscripts refer to 'f-foam' and 's-solid'.
f l foam I
is 120 mfn wide and is supported over a lengul of 500 mm,
& P mlid polyethyleae beam which would have the same flexural
ys.ing b foam sandwich beam d the beam when it is subjected to a load of 20 N at
The only problem with eqn (3) is that although it is not difficult to get the modu- 1
L lus, Es, for the solid material, it is not so easy to get a value for Ef. This is because (i) ~h~ equivalent solid polyethylene section is shown in Fig.l?(b). The width there are a very large number of foam densities which are possible and it would not
feasible to expect the material supplier to provide data for all densities. of the web is given by , '- V7f vever, it has been shown (65) that the modulus ratio is given by
u ~ g t & ~ = f f @ t b
where p is the density of the material. will be given by 131, The use of this design approach is illustrated in the following Example.
type of dilemma it is now common p~a%dce m me DeskabiIity Factors to crrmpdue materids on a cast per unit property b&. TbBk 2 lis@ rr @1&n of these factors for some common loading situations (61,a).
For rhe exmple involving the polyprapy1xemve bem and using the 1 year madu- lus values from Table 1 and cost and density v&es for tbe individual plastics, it is possibk to compare the desirability factors oF.a.m&e of p l a s h for this application.
On this basis, rigid PVC would seem to b bast. M e - , thia is only t l ~ fust step in the design process. It is now n e c e s w to consider other factors such as moulding!fabfidon method, possible envirmmeatlll atrack, special featarm such as transparency, electrical properties and so on. Note that the same produce eould be used to differentiate between different grades of plyerhylene. The detailed design of rotornodded products is considered in more depth in Chapters 6 md 7.
Selecting Plastic8 Bwed on their Permeability To an ever-increasing extent plastics are being used as containers and as such their barrier propeaies am very important. In most cases b @astic co-r &s replac- ing metal or glass containers each of which have almost infinite bmier properties. Plastics on the other hand are susceptible to penneation by gasea and liquids although tfxe exunt vde8 widely. It m y be seen from T&le 3 &at EVQH, PVdC, PAN, PA ttnd PET hwe good barrier properties whereas the olefms, polystyrene and polycarbonate are not so good, particularly against oxygen and Cop. Polypropylene and high density polyethylene are, howeva, gQod lwrrie~~ w water vapour.
TABLE 3 BERME!ABLLIW COEFFICIENT K EOR DEPERENT POLYbBRS
Oxygen CO water vapor Polymer em3. I o o p cma. I dm g.cm
rn2.24hr.bar mz.24 hr.bar uia.24 ham - EVOH 0.4 1.2 aVOC
0.8 0.4 1.2 0.0
PAN 4.3 12.2 24.0 PA 22.5 47.3 23.6 PET 39.4 78.8 15.8 OPET 19.7 47.3 7.9 ffb% 39.4 118.2 11.8 O W C 27.6 59.1 7,2 EDTPE 433.4 1,182.8 2.9 PP 906.2 1,393.0 Z.0 PS 1,379.0 3, f92,O ZJ4: PC 1.891.2 2.955.0 &7,3
The reason why plastics are so popular as contheis is t$eot, unbr&ab16 and flexible, However, in most cagepr the
-r
plastics for a hi@ performance container may as impact resistance, low cost, rnd-,
REFERENCES 111. Mooney, P.J., 'An W y & af the No* Americao Rotational Molding Business', PCRS Report, New - -
~ a n a a h CT(USA), 1995. [2]. Barrett, J., 'Rotomoulding moveslnto mainstream', B u d & NOV. 1988, p. 56-59. 131. Schwarts, S.S. and Goodman, S.H., " Pd&?h&ls md Processes', Chapter [I41 Van Nostrand
Reinhold, New York, 1982. [4]. Taylor, P., 'Rotomoulding', Bntish PlaSkX a R&&, Feb. 1986, p. 22-27. 151. Ramazzott~, D., 'Rotational hboulding', Ch. 4 in M e aoducl Dsiign Handbook, ed. by E. Miller, . .
Marcel Dekker, New York, 1983. [61. Anon, 'Rotational Moulding' Plastics and Rubher Weekly, Jan. n, 1987, p. 10-18. 171. Saffert. R., 'PVC Slush Moulding for Car DashbMds', Symposhm No. 5,3rd ~ n n u a l Meeting of . .
Polymer Processing Society, Stuttgart, April 198'1, B1. Crawford, R.J. "Rotational Moulding of Plastics", PFogaess in Rubber md Plastics Tmhmlogy, VQI. 6,
No. 1 (1990) pp 1-29. €93 KLiene, R., 'Polyethylene materials f ~ r sotational mouldit&, p e p presented at BPF Roromoutrier
Seminar, Telford, Sept. 1989. [LO]]. Simonsson, E., 'Polyethylene materials for routi@al WdIng', Neate Seminar on Rotational
Moulding, Birmingham, June 1987. /Ill. Tomo, D., 'Rotat~onal Moulding of Polyethylene Powders', in M I L C ~ ~ ~ of %&tid M a w ,
edited by P.F. Bruins, Gordon and Breach, New York, 1971. 1121. hdge, P.T., 'Materials for Rotat~onal Moulding', ARM ~ e k b r W b hlMd~i3fl to ~ 0 t a t i o d
~ o u l d i n ~ , Chicago, 1985. [I31 k w , G.E.. 'Cross linkable Rotational Moulding High Dens@ Pbf-kh: SpB a Msy 1972,
1 p. 762-765. 4191. Ree~, R.L., "Cross linkable Polyethylene for Rotational Moulding", SPE Antec, May 14@, p 621 -62% 9
[IS]. Grabensetter, T., 'Plastisols in Rotational Moulding', ARM Rotational Moulding Seminar, New I-, JSm. 1981.
In 1933 two resewchers, Dr. R.O.Gibson and Dr. E.W.Fawcett, in their laborato~ at he m a l i Division of Imperial Chemical Industries in Winningfan, Ches-9 discovered that the gas ethylene, when subjected to high pressures in the presence of a eadyst, produced a solid substance with high molecular weight.
previous attempts to produce long chain molecules from ethylene had only in son waxes, greases and oils. This momentous dismvery was patented
by ICI but for a mmber of years the polymer was developed mainly because of its unique elstrical properties. They called this new material 'polyfhene' a term known in Britain almost as well as 'paper', or 'plastic', although the correct gener- ic description 'polyefhylene' is now used throughout the world. In effect 1CI could have registered the name 'Polythene' for their particular product, but they missed
and sgi~tered the tradename of 'Alkathene'. to link the discovery to their Alkali Division.
6 The Development of Polyethylene t
Polyhew or polyethylene's first applications were for dielectrics, and the Second World development of radar would not have been so dramatically successful but for its use. It is said that the early warning radar system used by the British was
- instrumental in the decisive victory of the 'Battle of Britain'. Wmime production requirements gave impetus to polyethylene's commercial
I f
development with pr-tion at ICI's plant in Northwich increasing from a few tomes to over 1000 tonnes a year by the end of the war. BY then work had begun into developing a wider range of grades to produce materials of differing molecular weight and density, within the lhitations of the process. By now the production of polyethyierie was expanding rapidly in the USA where both Du Pont and the Union Carbide and Carbon Corporation had taken out patent licences from ICI to increase wartime production needs (Ref. 1).
Grades of lower molecular weight than those used in cable extrusion processes could be readily produced and were found to be ideally suited for injection mould- - kg. By far the largest use of polyethylene, however, is in the manufacture of thin films for packaging which are manufactured by an extrusion process developed in the USA soon after the war.
a, Du Pont found that their Canadian offshoot Du Pont Canada
I
. ' was needed. This resulted in the development of a unique new variation now known as 'linear polyethylene' because of its reduced branching in the low to medium den-sity range. Using a modificdon of &e Ziegler process and the intro-
, duction of another monomer - butene - in relatiqely small proportions, they pro- , ' duced what is, in effect, a polyethylenefbu~ne wpolymer.
The introduction of linear polyethylenes, with vastly superior properties over the .. original high pressure materials, significantly expanded the patential for the rota- ' tional moulding process.
0
The First Polyethylene Rotational Mouldings The first polyethylene rotational mouldings did not appear until the. early 1950's and were produced from granules. Heat was applied to a mauld rotated at high speed so that centrifugal forces could provide the pressure necessary to deform the granules once they had softened. The success of this method was li@ited, as only relatively low molecular weight polyethylenes would soften and flow enough; and these, it was found, could suffer from a curious phenomenon known as 'environ- mental stress cracking'. To reduce the casting forces required in working chis mate- rial, easier fusion was needed particularly as the use of higher makular weight grades was also necessary to improve the physical properties of the end product. A means of addressing these problems would be ta reduce the granule s b , and this led to the use of powdered materials which will be discussed later. The p d e r s that were eventually produced by grinding were more fluid in their
behaviourh.tJeir unme1ted state than granules. In addition no pressure or mould- ing form were-required to muse adequate fusion and it was therefore an obvious step to adapt PVC plastisol rotational moulding machines with low rotation speeds (5-20 rpm) ta produce the f ~ s t hollow polyethylene pms.
The static moulding of powders (Engel process) was also an adaptation of the 'slush moulding' of PVC plastisols, whilst dip coating methods, in common use with PVC, were alse developed once suitable polyethylene powders k a m e avail- able.
2. POLYETHYLENE PROPERTIES The polyethylene grades used in the rotational moulding process are ohiefly classi- fied by reference to their Density and Melt Index or Melt Flow Index (MI?'J).
2.1 Melt Flow Index
Definition Melt flow index is a number that indicates the viscosity of a molten poip&X hLt a particular temperature. In other words it is a measure of its melt & temperature.
Measurement (ASTM D 1238) The equipmdfit on which MT;Z is meiburrx] L, in
(Fig.1). The material to be testtxl is p f d b a
36
excessive flow in the mould and the risk of d i e d oxidation. The processor can , , reduce his moulding cycle time, thus i m p v b g output and reducing costs. A high
It could be argued therefore that a high NlPl is desirable. U&r~uniitely, a resin with a high MFI will have poorer mechanical and stress i n f l u e d p~@es tharb those of resins of lower MF%. A high Mm, it should be rdmemberedy.&tes direct- ly to a lower molecular weight, with all the risks that this @plies, m..product will be weaker in use. It might suffer fmm the phenomenon barn as environmental
& f l be u:$ et)7stdlbe.
is not one that should be
I Ill .
44
1. Particle Sue and Distribution Test (AS'SM D 1921) A particle size analysis is carried out using a set or stack of sieves of varicus mesh sizes ranging from 150 microns to a s h coarser tharn the target maximum size - usually 500 microns. A sample of powder, 1100 grams, is shake0 through the stack of sieves for up to ten minutes and the quantity held on each she of sieve is weighed. ' he distribution of sizes is usually as shawn in Fig. 6. 'l'hk distribution is fairly typical and is achieved almost independently of particle hapa The object o the test is to observe and control the more critical ends of the size distribution curve, ie. the dust or 'fines', and the number and size of the coarsest particles.
96
3 5
30
25
20
I5
10
5
0 0 100 200. 300 400 500 600 700
Particle Size (Microns) - Pine + Average -4~- W n e
FIGURE 6 Particle size 4
1. Dry Flow Test ( A S h D 18995) The shape of the particles will affect the 'dry flow' properties of the powder. This is the rate at which the powder witl flow through a Eunnel of specified shape and size (Fig. 7). A quality powder will flow through this funnel evenly and steadily much like sand pouring through an hour glass. Fibrillation OF a tendency to 'hairi- ness' of the particles will slow this process and can, in e x m e cases, stop the flow altogether. A 100 gram sample should flow through the funnel in a b u t 30 seconds but more importantly the flow should be steady and udntermpted.
3. Bulk Density Tat (ASTM D 1895) The bulk density of a powder is the weight of powder fhs( is hd& a givm \al- ume, without packing it in under pressure. The material Flow funnel is caught in a receptwk of && sb ed powder occupies more space &a a a l d y wt
1
All measurements in ml l l imctrer
lOOg sample
GOOD POWDER
HIGH BULK DENSITY
BAD POWDER
UIW BULK DENSITY carred by
fibrillation
FKWRE 7 Apparatus for measuring powder l@3uRE 8 Effeet of powder grind qdity on flow and powder bulk density h& hsity
k demonstrated that a low bulk density (low weight p r w h e ) can Ere the result d poor grinding practice. However, it is not always good 8 guide to q d t y as
- @ 'Dry Flow' test, as abnormal particle size and size distributim will &O have an d k ~ t . Nevertheless it is another method of quality assessment in m m w n use.
, h d e r Characteristics and their Effect on Moulding , h ~ d e r s can be ground to the needs of individual processors and depending on the
melting rate, can range from fine to c o a r ~ . The desirability of varying the average particle size em only be assessed when the m~uldiug parameters are known. Fine powders with a rn- particle size: of 400 microns are not always ntwkd. Most polyethylene g r a b in the normal rmge of melt index will moul+peifqfly well with a maximum particle size of 500 micrans, and materials with very high MFI and low d~mi ty will flow out smoothly with even higher particle sizes (ie. 600 microns or more). 121e finer powders tend to have slower dry flow propertieti but will produce good surface detail (Table 4). On the other hand, coarse powders will need more heating to achieve similar surfaces if not compensated by a higher melt flow index.
Table 4. Powder Analysis, Typical Properties
-- -
Microns Fine Average Coarse
600 3
500 3 15
425 1 15 20
300 15 2 5 2 2
212 27 22 20
150 32 2 0 15
FAN 25 15 5
Bulk Density (g/cc) 0.30 0.3 5 0.40
Dry Flow Jseconds 3 5 30 2 5
1
Particle Size It is reasonable to expect a fine powder to melt more rapidly than a coarse powder of the same grade of polyethylene, and it is also true that in any given sample of powder there will be powder particles of all sizes varying from dust to the largest size. The presence of dust or at least a reasonable proportion of fine particles can
I improve both the melting rate and the surface finish of the moulding. 1 In the rotational moulding process a proportion of the powder in the mould tum-
bles over itself as the mould is slowly rotated. This tumbling action, gether with & some inevitable vibration, results in a natural sieving of finer particl etween the coarser particles so that a degree of layering takes place before the parfiq1t:s reach their melt temperature (Fig.9). Finer.particles will f110r downwards to t&e mould surface and in due course melt and adhere to it, while maser paaides tend f~ rise
f
MOULD WALL
L; FIc@~@P Filtration -powder p d c l e size layering
and event,@dy melt at the @ner s d a c e of the moulding. As the powder is a mix- ture of s& p&Pe and air, some trapping of air pockets will occur as the powder melts. T& ma&t of air that is trapped between the particles, and the size and
I dewable, too la@ portion of nclghbou srze downw,lrds TW WbxM & Q Mfer melting fhmughout, and, if ad con- trolled, rapld t h e r n d ( T b mo~liI@r needs to be ~fssured that for a given grade of polyet b has established will be reasonably con\tant and rt I sizes of particles and the?? distribution
that can reasonably be achieved by the grlndrng proces\
Particle Shape A poorly ground powdel 15 u\ually on? where some shredding or teziring has occuried resulting in a degree of fibrlllatlon Thia can clearly be seen at microscopt? magnificat~on\ ,i\ low a\ x 10 or x20 A particle may W e a 'fibrous tail' which can be two o r three tlrnes a\ long a5 the pxt lcle ~t \elf Large amounts of particles of this type wrll prevent the powder frorn flowing freely. The prewIce of 'tails' or ' h a m ' le\ults in a type of moulding fault Lnoun as 'brldglng' Injtead of flowing Into a narrow rcce\\ In the rnould cavlty the part~cle\ are held back, unable tQ enter po\\lbly locked b y Intertwined h a m , where they melt and then adhere b
melting, it can make the filling of corners with tight radii impossible. In addition the irregular shapes of several combined particles will in themselves entrap more
,i , air pockets and cause an increased Iwel of voids and enlarged bubbles in the melt. It is not suri)rising that mouldings containing bubbles of increased dimensions and
, I' ftequency in the moulded wail will have si@cantly lower impact strength, tenci1~
I : I strength and elongation. In fact all physical properties will be impaired.
Powder Bridging Voids
FIGURE 10 Bridging of poor quality powder
Even in less extreme cases where the particle shape is not ideal, but adequate, there will be a tendency for a larger than average number of bubbles to form. An increased heating time t be enough to improve the visual effect but this in turn can result in a greater f i g f oxidation and resultant reduction of end properties.
Finally, another effect may make its appearance. Fibrous particles can collect into fluffy 'balls' rolling about on the top surface of the powder when tumbling and distribution within the mould is still in progress. When all the distributed powder has melted to form the wall of the moulding these 'balls' will eventually become anchored to the melted inner surface and form large irregular shaped lumps some- times referred to as 'scrambled eggs' because of their appearance (see Fig.11). When this has occurred the moulding cannot be improved by extending the heating cycle in the hope that the unevenness will flow out. The internal surface may become glossy or shiny but the underlying irregularity will remain.
4. POLYETHYLENE ADDITIVES b Rotomouldable polyethylenes can contain a variety of additives which may be incorporated either during their manufacture or subsequently added t6 meet the needs of specific applications. As supplied, most rotational mddtng gnuit% will
SECTION THROUGH MOULD
Agglomeration Of hairy powder
SECTION T H R O U G H M O U L D I N G
' S c r a m b l e d eggs' e f fec t
FIGURE I 1 Effect of ag,olonleration o f poor quality powder particles - 'scrambled esps eflec~'
antioxidants to protect the material during long moulding cycles and in sub- t use where high service temperatures may be encountered. In addition.
y mouldings arc used outdoors in direct exposure to UV light, UV are usually present. However, the resins are otherwise provided in their te, which is in the form of granules with a cloudy, translucent white
coloured mouldings, pigments can be added. It should be remembered nts are very finely divided solid particles of organic or inorganic materials ain discrete from their resin host and as such will have some effect on
properties of the moulded part. For this reason the level of addition, or centration, should be kept to a minimurn. There are two methods of pig-
Qon, these being the addition by 'melt compounding' or by 'dry blending'.
f injection moulded or blow moulded parts is usually achieved by way melt compounding, the process taking place during extrusion of the
. Polyethylenes used in rotational moulding can be coloured using if appropriate extrusion equipment is used prior to grinding. en rotational moulding grade in granule form is fed to a heated extruder, merit, usually in the form of a 'masterbatch' or colour concentrate.
igrnents, is blended with it so that the product leaving the shade and intensity of colour. ered that polyethylenes can suffer from environmental stress
p$enee of pigment may reduce resistance to this phenomenon. kt @ Wt &ealised that the carrier for the pigment in the master-
Ylrrkt I &m'@@II WmP L r n ~ Qxilfw Brown
Black - Sadia8n Wfmw , Ultra
Marine
57
layer to layer wi1Ir.h a a sufficient time P a n c e of bubbles
I
Table 8. Typical Rotomoulding Applications
MATERIALS HANDLING PRODUCTS
~~~b ( ~ ~ ~ i ~ u t t u r a l , Chemical, Fuel. Septic) Chemical 1 ~ ~ ' s (Intermediate Bulk Containers) Chemical drums, Shipping containers Double (insulated) bins, Tote boxes
wheeled bins, Hoppers, Coal bunkers
INDUSTRIAL PRODUCTS
pump housings
ENVIRONMENTAL PRODUCTS
~ i ~ t ~ ~ bins, Sanitation bins, Grit bins Bottle b a n b
LEISURE PRODUCTS canoes, Kayab, Wlndsurf boards, Boats* Toys, playground furniture. Domestic furniture Point of display items, Mannequins* Planters
MARINE PRODUCTS
Floats, BUOYS, Pontoons, Life belts
ROAD FURNITURE PRODUCTS
~ o a d barriers, Road Cones, Road signs
Poat h. - ' . , I ,
nyim-6
HOOC-R-COOH + H2N-R-NH2, r of carbon &oms in the linear chain of the recurring polymer
I
those made addition reaction of monomeric compounds that contain both acid nylon-1 1 are the prime examples of this type. and amine groups called lactams:
' ' and those po?vmeriz$ b~-self-copd~sation of amino acids: ,
An of the first class of polyamides is nylon-6,6 which is polymerized from ' adipic acid and hexamethylene diamine:
fC0(cHd4 co-m(CH2)~ NH --fn
temperature and extended cycle W s demanded for this high melting point re& (225°C) in the rotomolding process. Nylon4,6 with a high melting point of 285"C, has also been evaluated for rotational molding and has been found to lack the good moldability and falling weight impact strength usually demanded for hollow- shaped rotomolded parts. Nylon-6, 9, nylon-6, 10, and nylon-6, 12 have melting points of 20S0C, 215"C, and 219°C which should make them good candidates for the rotomolding process; however, they are not widely used in the rotomolding industry. At the present time, the most commercially successful nylons for roto- molding are nylon-6, nylon-1 1, and nylon-12 with melting points of 215"C, 186°C and 178"C, respectively. A comparison of properties of some commercially avail- able nylon resins is compiled in Table 1.
2. MANUFACTURE OF NYLON RESINS K.K~;; ; ,- ,!I . . ' . I " . :\I;~('. '8' .4'!::!!g;;?,*p.,;;'a;!y;;l:; Yf?'. ;*%! ,'., :: ., .,, . . c I * . : I : # , 3.. . : p.:.,. <, ,,:I ',,, 1, ; 0 :,, ..! , el ~nan;ci.
1 k.g#$. *<,I.; .:L . .l.i,i$::,',*., s% 'Yk ,*" #&*w*&&*$ ,,,? :+. , , , .,, , , , ;::, ,8.a;+.,!? , ,, t
(
The basic raw materials for nylon-6 are crude oil and natural gas. Fig. 1 is a schematic showing conversion of benzene to nylon-6 resin. Benzene is obtained from rmde oil processed in refineries, and propylene and ammonia are derived from either crude oil or natural gas. To produce caprolactam, benzene and propy- lene are first reacted to form cumene which is then converted to phenol: When phe- I
no1 is combined with ammonia and hydrogen, caurolactam is the final ~roduct.
I PHENOI, PLANT Acelone
I 1
Alt~moniurn Sulfate
Nylon-6 I .O lbs. u FIGURE 1 Beraesre to nylon-6 OWVC&OII sehearaPi0
C ~ P ~ O ~ W ~ I Storm (Molten) 8 P c various procedures. One procedure is illustrated in
MixTank polymerization to nylm-6. In plymer- PO- + tk&live~ + 4%&0
Antdave -6; it is preferred that mare d m h ends are
fie~rmre Cycle groups [lo]. This can be acca-hed by
2m°C, 55 PSIG tion d a dicarboxylic acid, such as sebacic acid, during the p o l y m m o n
Ring Owning, ~ddition . The prwenm of more acid chain ends in the nylun-6 polymer provides the ~ a c u ~ m Cyde istan* to oxidation during the heating cycle of the mamolding process.
2m°C, Strut at 10 PSIG PoIy~ndemtion
Melt Extmsion derivatives, but is manufactured strand and Orandate 85 PSIG Nitrogen alcoholysis of castor oil pro- - Leaching undecylenic acid 11 11. The 110°C Water, ~ x t 1 m i b 1 ~ <1-2% -11, is made by the reac-
-DrJing reaction with ammonia. ' Rotary vacuumI)Iyer
2 Nylon4 batch poiymehtiOn is pol^^ eom the monome& lauryllactam, a petrochemical deriva- catalyzed reaction to lon-12 was first pro-
CO1MPOUNDING base resins for rotational molding require the addition of anti-oxidants heat stabilizers before they can be sold as engineering resins. Nylons are to thermal degradation to varying degrees depending on composition, and
, must be stabilized with anti-oxidants or processed with an inert atmo- side the mold during the rotomolding process. The additives are normally ded into nylon resins in a compounding unit shown schematically in Fig. 4.
vacuum Base Resins
Weigh Feeders and Mixers
71
e which can readily be dried, usually within a few hours. Extended will result in moisture absorption deep within the particle, and as 24 hrs. for satisfactory drying of the nylon. Fig. 6 illustrates the ,Ion-6 homopalymcr resin which has been exposed to W O differ- isme, 1.0% a$Q.45%.
4 . t l Machine Type
variation in thickness. < \...I. \ , . . . i , - ' . . .
4.2.2 Inert Gas Injection To pllevent discoloration and loss of some m
P parting line. In the latter case the mold should be v d Wollgh '1
------ ,
lac where it is first cooled by air and then by water spray of mist while ; b&&y rotated. It is best if the rotation ratio remains the same as that w ovm cycle in order to avoid the possibility of melted resin movement
surface of the mold. This possibility is greatly increased in thick-
-VU r..."*.L..YI. -Y
j m p d 8 or sphedtes continues until the tehperature falls below the & m p e m e , Xg, of the particular nylon. Below the Tg, the molecu-
littie additional c r y ~ W e development occurs. The nylon from the melr temperature to below the Tg, the
a b m a i t : k w s m i @ @ thr: lower will be iQ toughness and
&vellop w&& wiq cause poor transfer of heat upWir&le slow coding of molded part. For &add mmist of a short prbd of air cooling
eLiq& ~ 9 e . A typical mold-
I h e resin WPE) , the better co- ct wo'uld be one which develops ' wane chemical bonding be twm micd b3f~1i"~ h~ PFF~P+-A 1 either by modification of the nylon rgsh lg IES of a modi£kd resin for the other layer,-or by using an adhesive ply chemically mactire with both the nylon and the other =in. For example, if the objtmim ww to pmdlyce a good multi-layered producrt consisting of nylon-6 and high &@&ty ps1pfhybnq this~cauld be accom- plighed by using a nylon-6 graft coplym-e?r ma& with a modified olefinic monomer artd rotolnolding if with a high density poly&yle.ne resin. The polyethy- lene has gaod bonding capability with the crlefinic mdified ayhn-6 mpnl---- This construction cauld also be achieved with a homopolymer nylon-6 resb. ,, ,,- lizing a random copolymer o l e f ~ c product (such ss ethylene paylie acid copoly- , mer resin) as an adhesive ply between the nylon4 and HDPE.
, - -- ~BUM k 0.055 in. H i tbkdk&
Resi Resi Molc Mac
three layer conitruction is desired a second inte required. This method is @e most convenient however, it has the limitation of requiring a fairly large which to locate the insulated hopper.
I ,$h4.' -
4.5.4 Multi-layer Material Combinations with Nylon There are several useful combinations of nylon with other resins w in the multi-layer rotomolding process:
1. Polyethylenes - HDPE, LLDPE, Ethylene acrylic acid copolymer 2. Fluoropolymers: ethylene chlorotrifluoroethylene I
,' M
Dry Cond.b Dry CQIld.& m- QQ&.b
DIN 53479 -- 1.10 -- 1.12 -- i . r u -'
OC 214 -- 214 -- 213 -- DIN53714 % -- 3.1 -- 2.5 - 2.4
54 44 47 42 I 90 320 z i o 400
1
I
1 I . . ' :,;,.+. ,, .
I
i . . '
. . . - ..- - --r- ------- ~.pas insidebe oven, to prevent kplmio~)
5. Implroved t.en4~nthu-e eontral. CW ai-m zrerd sheet steel molds can be US&, but it is immrtant thaf 'h mhld be iifiwh~ in n t r l ~ r ta avnid rnlr+-r;nl
-- - - - -- -- - - - - - r-- .-- is ~ . ~ ~ n u n (~.ah.). ~ b t s na W PWE or silicone rub&~ , -. used Fn the parting line to prevwt 1
- - - - --- ---- - . ---" .--- ,
dfect of sQm C ~ S ~ S of m&erials on nvlons. This summaw nmvid~s Sr. & e m i d 1
testing art the actual expoy&&~tions.
I
Aromatic Solvents Aliphatic Solvents Golsokt, Gasoh01,Oils PsPeak Acids Strong Acids Weak Alkalies Strong Alkalies Alcohols
?I.,, $2-%,*7- * -'.
-.
,pact strength # ' w e d Isod, J f art UUtrJh 0286 40
.pact Strength , M S 8 60
?rope
Spec I
!,1 . . .> ' I - I
ter&era~es. In the case of nylon-11 and nylon-12, monomeric sulfon & gk as N-(wbutyl) benzene sulfonamide, are used to increase flexibilip
tet stre@ The of plasticizers on molded properties of nylon-11 am
10 20 30 40 50 60 70 80 YO 100 Relative humidity. 9h
FIGURE 14 Moistme content WS. re.hive humidity. - - nylon-6, - nylon-1 1, - ** nylon-12 1 ', , , &,I; ; v, h i ' , . '., . .: & I H.%(tb
1 . i . + . 11-A 1 - 12 is illustrated in Table 8. In the case of nylon-6, caprolactam which is not extract- ed or only partially removed from the polymerized product functions as an effec- tive plasticizer. Unwashed nylon-6 polymer generally contains 8-10% by weight of unreacted caprolactam. Table 9 compares the rotomolded properties of unplasti- cized and plasticized nylon-6 resin. The flexural modulus of the plasticized nylon-6 resin is about one-half that of unplasticized resin at room temperature. An examina- tion of the falling weight impact results reveals that, although the caprolactam plas-
-- -
TABLE lo. EFFECT &&ER COEF~ZNT ON PHYSICAL PROPERTIES a
property I
Nylon-6 Nylon- 11 Nylon- 12 Dry 50%RH Dry 50%RH Dry 50%RH
Tensile Strength at Yield, MPa 83 37 57 46 42 37 Elongation at Break,% 70 390 330 >330 >250 >250 Flexural Modulus, MPa 2700 11 00 980 640 1420 980
Notched Izod Impact, 65 215 I 69 78 J/m
a. Data compiled from trade literature (L
s k s fhat the eras-liakd pol W F , but but9ubtm~i9y lower exposed for 1gO days at 80°C in diesel fuel, P 120°C in
conuol at 80°C in an air-citculathg oven, In time is plotted for ~h of the envh.On-b. there was no decreme in f l e d str- &r ferent environments.
5.4 permeability Nylons offer low pmmdnlity ta oxygen which makes thm dab ica nay dr- tight packaging. my dso have low permeability to gasoline, diesel ft~ls, qlcne. and many other sdprents 1 ambient or elevated temperatures. Fig.23 ~~8~ the s
, gasoline permeation obtained in identical tanks rotomolded in a cross-linked ! polyethylene (CLRBJ re& and in oylon-6 resin. The results show that afm 70 1 days' exposure at mom temperature (23"C), there was over 8.5% might lws I
through the CL-PLI taL and less than 0.1% thmugh the nylon-6 tank. At elevated temperatures the sqerior plSwation misistanee to gasoline of nylons compared to poljethylene woull bc expeetad to be even more pronounced.
, nylon resins will require pduc t be shipped
to the end-user. The osval Bnishbg o p ~ m is the removal of any d d parting- line flash which occurred during p d g , S d nylons have a geater hardness and are substantially .Ww t b n plphy lae , it Is usually more difficult to remove flash from the mold W ~ o v d from the nylon part is less M- cult if it is removed wh the mold. Removal of flash is accomplished u (scrappers) h'dd SO that they pass over the flash
In many rotomolding applicariom it k a-m to mw, rout, m, or W the molded part to prepare it for its end;ctt~ M m . h In WS of m ~ m g opera- tion on nylon parts, it is important tb SBM&~ bat generation. The he& CVAUC- tivity of nylon is much less than metab, md a a result b a t p m by frbti0n between the nylon and the metal cutting -1 d1 b m08dy &s& by tbe C U ~ ~
tool 1211. The minimal heat conducted hto ;the nyim p h d c will W-sfer through the part cross-section, and as a c o n s e q m t,b m w of the surface layer will rise significantly. If this surface heat iS nor CCTII~R@& it c~BBE &Urn- ming and discoloration, and produce an unsatisfactory part. The @qer $&$ion of sharp cutting tools, cutting speeds, and water cooling & b1wE ds,& m w e s chips from the cutting area will help prevent overheating.
6.1 Sawing Power band sawing of nylon is widely used for making irregular or curved contours .. as well as straight cuts. For a material thickness of less than 0.5 in. (12.7mm), the '
best results are obtained using high carbon steel blades with a precision-type tooth form having 10-14 teeth/in. and a band speed of 3000 fpm. Since the dissipation of
the aluminum version of the fuel cank would h v e cost more than the entire tank r made out d nylon. Cross-linked polyethylene was considered, but it would have no rigidity at temperatures above 200°F an8 it lacked the resistance to permeation by diesel fuel.
Nylon-6 has found use in storage c o n h e r s for solvents. Tanks 380 to 1890 ' liters (100 to 500 gallons) are being used in the dry cleaning industry for storage of phlomthylene. The excellent resistance of nylon4 to perchloroethy1ene made it a logical choice for this application, which developed as a result of corro8ion prob lems encountered by users of epoxy-lined steel tanks. Since polyethylene was unac- ceptable due to severe swelling, distortion, and lack of adequate permeation resis- tance to this chemical, and since stainless steel tanks were too costly, nylon-6 was the cost/perfonnance choice for this application.
*
Significant cost reductions have been achieved by equipment manufacturers with the converstion of hydraulic oil reservoirs from fabricated steel construction to rotomolded nylon tanks. Besides savings on fabrication, in many cases novel con- figurations were possible which maximized tank capacity. In one lift truck applica- tion, problems of corrosion and rust associated with steel tanks were also eliminated.
Nylon rotomolding resins, as engineering thermoplastics, have substantial advantages over polyethylene which make them the best choice for applications with one or more of the following requirements:
(1) better h ~ a t resistance (2) greater tensile strength and flexural modulus (3) creep resistance ik , , . Id+,
(4) resistance to permeability by gasoline and other fuels ... *L .4 ., . (5) chemical resistance to aromatic solvents ,&:* :- ,.I. -b . +5 . (6) wpexior environmental stress crack resistance ,! , t . '73i ~ S ~ , , : : ~ L - .
(7) abrasion and wear ismce
i' c'iW4.W.4~ .
(8) ease of paintab' ty (nylon can be painted like metal without'any &.u$ac~ treatment; polyt ylene must be flame-treated prior to painting)
(9) toughness, especially in molded-in or machined threads PA range of applications which often require some of the above speeial qualitis !
of the rotomolding gradts (1) tanks - for gasoline, diese (2) ducting - for automobile pollution contd,
systems ;.- (3)Yiydraulic oiI reservoir tanks (4) oildisplacement tanks (5) tanks for use in vacuum systems 05 tG 4 6
(6) automobile dash board (7) housings for medical and marine
(10) inner liner for filament w
[20]. Recognized Cornpone [21]. SPI Plastics Englnee [22]. Ref. 21, pp. 684,685 [23]. Machining Data Ha
1980, p 49 (book) [%I. Ref 21, pp. 673,675 (bookg ;,' , [251 h o n , 'Nylon-6 Fuel Tank
(journal article)
lo2
The early rotational molding process used a "rock and roll system", as illustrated in Figure 1. The mold was piaced on a h h n a axis with a power driven roll pro- viding thrr rolling axis similar to a barn51 .M, Qbultaneously, Ehe entire framework on which the rding action takes p b equipped with a mechanical or hydradie drive system that enabled the frame to Lw tilted 45" to the left and 45" to the right; hence rock and roll.
The most conventional method of r o W n & is %hid rotation" as illustrated in Figures 2 and 3. Figure 2 is a straight arm, a mMon member which protrudes hrhntally from the machine center, with mdds positioned on either side of the major axis. The major axis is rotated by ;direct w m d to 'ihe gear motor, while a
ROTATION RATIO M)R TYPICAL S W E S XIbPP Em.REs
B t o 1 oblongs (Horta@ntal ~ w t e d ) Bhraight cRariz&tal Mounted)
5 t o 1 Soae rhaisoet.61 ~ u c t s
4 . 5 t o 1
3.3 te 1
4 t o 5 Balls - Odd Shapes RectangUlar boxes - Horses with bent legs
2 t o 1 Rings, Tires, Balls hny rectangle which shows two or
m o r e t h i n s ides when run a t 4 t o 1
Picture Frames - Kanneguins ... Round F la t Shppes Horses with s t r a i g h t lege
r Auto Crash Pade (Vertical Mounted)
Part8 which should run a t 2 t o 1 but show th in s i d e walls
'F la t rectangles (gas tanks - s u i t cases - t o p bin covers)
1 t o 4 T i res - Curh3 Air lbucGs Pipes Angles - Fla t r w t a n g l a s Bal ls whose si&s a r e th in
a t 4 t o 1 r a t i o (Vsrt ical Nountod Cylindess)
1 4 5 Cylindara ( V e r t i c a l @bpte+) . ,-
space inside the swept volume of the oven is increased, the weight ca~acitv of the '4
" respectively. I Heating Methods Rotational molds mav be heated bv either an nncn-flnm~ m~thnd n hnt n;r POP;V_
, - . - --- - - Q--- -, -- r-----. - placement ci&ating fan distributes & through a system of ducts into the swept volume of the oven. The capacity of the fan (cubic meters of air per minute), will determine the number of air changes per minute. On contemporary machines, air-
1 should be changed in the oven approximately 25-30 times per minute in order to provide an effective heating for the molds. Direction of the air in the oven is gener- ally caused by t b m t i o n a l louvers so that no "dead spots" are created. The static I
--- - - - - - r -- - - ~ h e medium for heating hot-lair ovens' may either be natural gas or oil wirh-a J
.modulating burner. In some cases, electric heaters are used to generate the hot-air environment.
The oven residence time necessary to cure a part will depend upon the thickness of . the wall of the part, the type of plastic polvmer being used to mold the art. and the
- - - - - - - - - - - - - - faster rate than does steel. Thinner gauge aluminum helps to increase the rate of heat transfer.
On the earliest rock and roll machines there was no heated oven; an open-flame method was used (Figure 1) whereby a manifold of gas jets was placed to evenly heat the mold. As the mold rotated about the major (rolling) axis, the heat was imparted directly onto the mold surface, and transferred through to the plastic. This machine was inexpensive to manufacture, but the operating costs were significantly more than the closed oven type of heating system. All of the thermal energy not imparted to the mold went into the atmosphere, creating increased temperature in the work environment and the loss of energy. Open flame machins are still used
large oven. 1n some instances, modern safety laws have out-moded the use of open flame systems.
'4
- 8 Bor ----
if .: 8 % '
w i n g Ovendootopen . FIGURE 1 1 Shuttbtyb machine concept
109
2: liquid plastisol PVC parts, but there are circumstances where it can be
- -- b oven machine uses biaxial rotation in the oven; heat in the oven is supplied or oil fired burner and a recirculating fan system. The cooling system is quite
/F with the mold assembly placed on a roller mechanism that is plunged, by pan air cylinder, into a water bath and held there until the optimum tempera- Use part has been reached. The air cylinder retracts, raising the mold assembly g Ba platform height. The mold assembly is shifted to the loadinglunloading sta-
the mold and removing the parts and recharging the mold. of the manual labor involved in the transferring of the molds from the
p t i o n to the oven, back to the cooling station, and then to the unloading machine has been generally outmoded in all but some primitive mold-
$" s. The machines have been replaced with modem carousel types.
~ E S T Y L E MACHINES beginning of rotational molding, shuttle-style machines were uti- large tanks and containers. Even today, one would be pressed to ore cost-effective way of producing large parts than the venerable
lustrated in Figures 1 0 and 11. These machines are simple to reasonably low costs for manufacturing.
operation concept is as follows: Shuttle A is transported oven, the oven door closes, and rotation on both axes is commenced.
-letion of the preset oven time, Shuttle A is removed from the oven and b cooling process, in a controlled waterlair environment. After Shuttle A is R@ oven, Shuttle B enters into oven to begin its curing cycle. While shuttle
aven, the mold on shuttle A is cooled, the part is removed. The mold is @ h r the next cyclf. @dvantages of shuttle-style machine are: b &ili& to mold large and small parts economically.
k to toperate with reasonably low maintenance.
i~paation is not as time-paced as automated carousel machines, thereby b o p y t m the opportunity to manage large parts and molds. #aache$ may be initially provided with one mold shuttle and one oven. mmchhe c.an be outfltted *th a second shuttle to improve the productivi-
. This versatility allows the shuttle machine to be a low cost entry b
ROTATIONAL MOLDING MACHINES
113
-ARM TURRET MACHINES if?: ular concept of rotational molding machine was first introduced in the late
early 1960's. Machine sizes range from small machines with 40" swing to machines as large as 150" swing diameter. The fixed turret style
dominates the usage in the field today. These machines are cost-effective, oductive, and easily maintained. -arm turret machine (Figure 16) is used in applications where the oven and cles require relatively equal process time and where the load/l~nload cycle
easily managed. The 4-arm version is required where one of the process loading or heating or cooling, requires more time than any of the other cycles. basic fixed oven machine has three stations - an oven station, a cooling , and a loading/unloading station (Figure 17). The ovens are usually forced
irculating ovens with power operated doors on the entry side and on the The cooling chambers are equipped with doors on the entry and exit sides
pped to provide a cooling medium of forced air and water spray or a 0th. The loading station provides a position where the machine opera-
16 Fixed-arm turret machine =
FIGURE 22 Independent-indexing carriage machine
Unlike the fixed-arm turret machine, one arm does not depend upon the other for indexing, and in fact, many times you will find dissimilar cycles on the various arms of the machine. Each carriage can have its own dwell times in the oven, cool- ing, and loading stations as required to produce an optimum part. Figures 23 thru 27 illustrate a typical cycle of a 4-arm machine and how its independent-carriage movement can provide advantages to the user.
The versatile independent-indexing carriage machines offer the advantages of shuttle machines k tha t each carriage can cany a very high load, enabling it to pro- duce large parts.
5 " g 2 z 6 g 4 m x s - m u au m E c m 3;;
CHAPTER 5
Sheet Metal Moulds TJ. Taylor . L
I
P ~ W its design and its is most w e a t e for the must hie considered to con-
1. INTRODUCTION Sheet Mould m g , in practical engin&g terms, presents no specific
-
technical M d t i e s because cold fomhg, we1dib.g and &@sing are all part of stllndard gfmt mttd p f a d ~ e . Unfortunately, the intruian of other factors tends to c~mpfi- tb OPtcome rather tlm the meam by whicfi the end d t is achieved. Wlth most specialised e a ~ ~ applicatims; practical and technical expeaise correct m o ~ king has to be combined with a detailed knowledge of the end v s s , to enable the rrmoulds can be hi@~ ~ U P P ] ~ qMdifY as & Qdmaket: Of muse, this is not unique to RoMonal klQuld@. The same is 4af toohaking for Injection Mouldihg, Blow Moulding, Compression and Tram& Moulding.
For the Designer the pwpneters are much simpler: b u w how; h o w why; Know who. To establish the 'know how' it is necessary to look simply at the three most kdely used forms of manufacture for moulds in the Rotational Moulding p m s s . Fabrication, Cast Aluminium, Electroforms are the techniques, but what factominfluence the choice af one of them? In particular, in the context of this chapter what leads to the selection of fabricated sheet steel as a meam of producing a production tool?
The three tests to apply are A. Complexity of Form B. Quantity of Moulds required C. The Appearance of the Product surface.
L ' l
A. Complexity of Form The selection process begins with an o v e d impression of product, which alone may be sufficient to e W a t e some of manufacture. From size the progression is towards the g a d complex are the component shapes? How many chanp gf key factor is to idenhfy what is practically possible. Is it B .Em ly translated to a layflat image or developed by a m d k e &a1 to a hll size model?
1.1. cost So far one important consideraticm hw hm omitted from the discussion - Cost! M t h m the actual cost will be gemrat& in the form of a quotation by the individ- ual Toolmaker, it is as well to underrr.taad how tihis @wt is devised. This represents the 'know why' factor for the designert Suppaw W the decision has been made that the mould is to be p r o d u d &urm Mild &mi Sheet by Fabrication. First of all, what information will need to be pas& C b T-er and then how will he determine his costs?
It should be emphasised at this stage much infomation as possible regarding production requirements should to the too^ but basically he must know:
1. The moulding material. 2. The tool cmstnrction nquimmnt of the madder (m00uk.i material
thickness). 3. I"Be psition of thei Parting Line. 4 . n e number of tools (or impressions). 5. R q M surface firdaih.
fhe cornpent drawing and with a knowledge of the moulder's require- mb, the ts3oItne sin begin to fhe number sf madhours required for fdrieation. l lw prtxess begins by formiag.a plan of conrstruction that will take into mmmt the foilwing key areas.
Par- Urn What is its positiw and how mmplex is it? l---- Can it be pduced in one plane or am &ere areas, above or below the eneral plane, which for the sake of part withdrawd, need to be at a point of parting?
Is it necessary to have more than one parting face and are there any special f-s though which the parting line must pass?
Each additional edmplication or deviation from a flat, single, plane parting face will accumulate extra hours.
' , Method of Construction This evolves h m the basic component geometry, an
I The toolmaker must decide within the capabilities of component parts can be produced in-house or need to as .Stock spinnings, or perhaps pressings, derived from a pure$a%ed. This will be done to keep the amount of hand Much of this part of h e calculation will be used for the mould. If this is not specified by the rho ably have a standmi which is best sdted to his tim.
Inmrts In assessing the nature and position i3f t$e
122
LABOUR COST TYPICAL Suh4MARY
1 PartingLie/s 20 hours
2 Method of Construction 120
3 Inserts &Cores
4 SpecialFeahes
5 Clamping
6 Finish
40 hours
10 hours
/ ,,, . : , 100 hours
TOTAL HOURS 295 x OIHEAD L
x LABOUR L m
MATERIAL CONTENT * .
1 General Construction
3 Proprietary Items
TOTAL FACTORY COST
FIGURE 2 i*. .
2. TOOL CONSTRUCTION AND DESIGN In the evaluation of mould cost, the framework of the mould making process has been outlined. It is now necessary to put substance to the framework and consider the subject of mould construction in more detail. In the following sections, the pro- cess of mould construction is described to assist the tnoulderldesigner in identify- ing the constraints under which the toolmaker works and which inevitably influ- ence the quality and cost of the mould.
2.1 Parting Line Since the construotion, operation and possible success of the mo&d pmhst evolve from the parting line it is fitting that it is considered &st in @ mould design and construction. It is obviously preferable; to be able t ~ ~ m y ' 3 % ~ parting line can be here, because that is where it is in- ta W fm @
I
a ~ e hhoO This paining smes is a fm l"@.hSm t h m & ~ ~ t ~ s t s u c - tim. b w b g the exact relati~mbip of autskk k w s aUow rhe -1- m d w to dekmina any mhligmena or tbe7fi.ewimi&i.S&&dhim,
FIGURE
and fro1 use Fig
sec ti01 cla
deposition may be difficult to achieve due to a heal insert may be constructed from:
(i) a lighter gauge material or (ii) a more heat conductive material.
Other examples of fixed inserts are: (a) 'Kiss points': areas of the mould which are
t differential.
raised rn fn~ --"-- -v -.,-a
with adjacent or parallel surfaces (see special features). (b) 'Engraved Logos or Instructions': pre6rmed on a 9e.1
. In this case the
w a narrow gap
material, as a machine function, and &&I tr (c) Machined components: Areas of ~recise or
--- - --
9 the mould ; m i f i r Clntr
-2arat.e piece of at a later stage. ail m r h i - h m-- L- .-- - -- YY- 1*1u\111 ball W
produced accurately by using a standard s$eering facility (Fig. 12). I
FIGURE 12 Fixed inserts
D R A W
(B) Interchangeable Ieserts These are basically simple inserts usually in the line oi or' situation for the component part. A c o m n ex-
' draw w& le in where n - -.--- =-- -- .------ I .
uct is used for more than one customer. The individual custom's - - can be inserted into a moulding face and changed to a blank ~ l a x e or i d h l ~ ~ ~ . . identification as the component source changes,
Since the production requirement is usually not one; can have a more semi-permanent nahue as opposed to the, clamping (see Fig. 13).
b b,
\ LqSo IWit
I"
k m mwmted and diverse range of h x t s and before ma-
th& regard must be @vea ta ~ ~ I U t h e ~ d r m c % u e tad a y to maintab (as%
tem'ov& is to &at de-modding but w W it wtu- divmity, A few of the mmt c~mmon uses as
D R A W
CLAMP
"#-: ..w;#,Lt.. d,:,& FIGURE 14 RemovaMe inserts .if i . , .- ah..r&E cwv&
..A, ..L't 1, :., ~ l& +,> -h
I - I (iii) ~ i d b t i o n : This is similar in some ways to thread shear but here the
shrinkage force is not axial to the thread diameter. Where thread shear or distortion has to be taken into account, the construction can be such as to allow the insert to mdte during contraction. The insert, however, must be firmly held in place during the moulding cycle. Where springs are used for this purpose (Fig. 16) it is important that the moulder appreciates the importance of regular checks for uniform and effective spring tension.
,-*,a
Rwovable Handles..'. I A &liar requirement with products such as drums, hoppers and containers is for
s099.e form of lifting handle. Invariably in the design, they will be a restriction to p@t removal and therefore will require the use of a removable insert. A particularly ~sefu l solution is shown in Fig, 17. Here, use is made of the dissimilar expansion of two metals to obtain a secure parting face at one end where clamping has no effect. TIM handle used is a casting (phosphor bronze or aluminium) and the bedding sur- f- is an insert of coppe:: -,. . ,
,,\*,:,*, , , f
.',I,,. ,jZd .., - Moulded-in Holes The use of copper, brass or dumini c u k advantage when wishing to produce 'through holes' against the 'Where the component details are
' t m small to allow heat to be applied along the inside surface, a more conductive ' IkMtWial will allow a transfer of heat from the main mould surface to take place (b Fig. 18). The inserts can either be removed before demoulding or after the moulding has been extracted depending on the method of fixing.
a 1
threaded nature. The grouping of this type of insert should be treated with care. When: pitching or precise placement is called for on the moulded component, regard must be given to the effect of shrinkage. It may be necessary to contain the group of inserts as a whole so that their relative psition is unchanged after mould- ing.
Another method of producing 'through holes' against the line of draw is with a 'moulded-in'iinsert (see Fig. 20). Here the hole is formed by a metal tube which stays with the moulding.
There are a continuing number of specific applications where the use of a removable insert is called for. However ingenious or complicated the solution, the underlying principles laid down at the outset of section always apply.
LOG t \
\Nh \ '\ '\
Fraw 20 r
3. SURFACE FINISH Probably the most labour intensive part of tool fabrication is that required to obtain a quality finish. It is important therefore that all the previous work in'assembling the tool has been correctly carried out. Good practices and care in construction enable the toolmaker to achieve the desired finish more readily, whether it be a matt surface or -or polish.
By the time finishing takes place, all welding is complete and all distortion has been removed. The mould is complete except for the feature which determines the quality of the product. There is no easy way to obtain the end result. It is ironic that the better the finish produced, the.more apparent become fhe mistakes or bad prac- tices which were left unattended in the early stages. The preparatpry work whether for mirror polish or somewhere below is the same; the difference is only deter- mined by where the work ceases.
Mild steel is a relatively soft material and is therefore susceptible to scratching. The toolmaker will systematically work those areas which have received previous attention (welding or forming) with an abrasive, gradually reducing the grit size until the desired effect is obtained. 7'he real object is to provide aa even overall smooth surface. Plain areas (where no previous work has been wed ouf) can be left until the later stages. There is little point in producing scratches on iin unworked surface in order to remove them later. .
~ ~ h C ~ f nat #&a. The cabless use of
~ ; ~ f l t h u s i a s t i u use 1-4 heat, thus pr In m d s which are king surface
1 using a 'Grit blast' process, excessive blast pressure or dwelling in one area long with the blast head can also induce metal movement. polishing process gives a limited range of surface finishes to steel tools. It is e to obtain textured surfaces but within strict limitations. Standard steel
s are obtainable with a patterned surface and these may be folded or formed. bvious disadvantage in this case is that, where joining or welding takes place,
attern will be removed by dressing or beating. patterned surface can be applied by the process of electro-chemical etching
again, a welded seam can mar the end result. No matter how well the weld has been flattened or dressed in the preparatory stages, the line is highlighted and is Iridble as a change in pattern intensity. In any event, this process is expensive to
., g d o r m and may rule out the choice of FABRICATION in fav~ur pf ,o ather methods, if a surface pattern is called for. !t;k~;i'!~.t,>m?~
, ? ' , I,. #L :*k,r'-&!~,,;b *',' , ,,I: s 4 , . ,,; i , : " . 8
4. SPECHL FEATURES j"$. ' ) [ : i & t c ' p - :d l t '7iji ( K , ? ) ' f ! ~ ; l k ' # ~ , : , i L *$~!,i r 4.1. Heat Inducement L '
One advantage of fabricat8 'moulds is that areas of dissimilar metals chn be 'built in'. This is particularly useful where it is required to induce heat more readily to the moulding surface. These areas are not easy to identify but generally can be said
(a) areas which enc6unter heat starvation, due to the mould construction, or (b) areas where, due to component geometry, the moulding material will be
forced to pass over more quickly. By inducing heat quickly into these areas, the moulding can be allowed to attain the thickness expected to be produced over the remainder of the surface.
I Copper with its excellent thermal conductivity is the normal insert material but
should only be added to form small areas. The addition of a large area of copper is likely to cause distortion or buckling due to differential expansion. If the area in question is likely to be large, the copper can be inserted in the form of ships. some cases it may well be'that the desired effect can be obtained by using a lighter gauge of steel, but this is limited and would result only in a small increase in thick- ness over a large area and would have no effect if applied to a small area.
The insertion of a dissimilar metal is only one solution. Two other methods are
I widely used:
(i) To construct on the outside of fhe mould a feature which will produce a faster air flow over the surface. Deep Qockets or ribs in moulds can pro- vide 'dead spots' where little or no movement of air is experienced or where surrounding convection currents exclude the hot air (see Fig. 21).
t ' (ii) The alternative method, which is well practised in cast moulds, is to use
a Heat Pipe. This is a proprietary item. It is inserted into the mould, often in groups, and effects rapid transfer of heat by means of an enclosed liquid which is vaporised and then condensed producing latent heat which is given up at the mould surface. A typical application is shown in Fig. 22.
(iii) A rarely used but effective method of overcoming mould 'dead areas' is by way of a forced air supply. Compressed air is fed to a storage
solution but most e requirement continu
(iv) A more recent in problems associated with (iii) above is to introduce an 'Air Mover' to the problem area. Air Movers are compact units which require a compressed air feed but utilise the hot oven air and therefore do not require an air reservoir. They work by Venturi principle, drawing in the surrounding air and then expelling it at high velocity towards the 'heat- starved' target area. They can be used as a single amplifier or grouped into a multiple array for larger, difficult areas.
FIGURE 2 1 Heat compartment deep rib or pocket
4.2 Heat Exclusion A more frequent requirement in a mould is to provide some form of insulated set- tion to exclude heat. This is intended to reduce material wastage by masking areas where material is not required, eg open top containers. The degree to which materi- al is excluded will obviously depend on the effectiveness of the insulatio material. Tightly packed mineral wool to a thickness of between 25 and 30 mm i&enerdY .
considered sufficient (see Fig. 24). The practice of masking or insulating is often used by moulders as a i ~ answer ..,
to some of the problems set in the 'Heat Inducement' section. Where Wre is a
it is necessary to utilise a Charge Hopper. the volume of the mould is such that it is not able to accept the full
e mould melts and adheres to the mould. an article is required to be moulded using two different powders. In this se the first material is charged into the mould and the second powder is
rial also requires an insulated reservoir 1 uch time as a valve or gate is activated
uction of the new material. .
. . , .
FIGURE 25
It is by far the best policy to consider mould mounting as part of the design exer- cise. Allow attachment points to form part of the tool construction and provide the attachments with drilled holes. Then if the finaI mounting or nrouvinn is reauired I
0 -, ---- locations should form part of the initial specifiiKtion t i e moul~ maker. 'P I 7. MAINTENANCE Once a mould has been commissioned it should be art o b all pond moiildin~ nrslr-
Areas for Attention: 1. Check for distortion or damage which is impairing:
(a) Release (minor problems can escalate)
- .I -- -- - r----- '-', then brass or stainless steel n& on s G b o l t s is a solution (or use a propri- etary brand of anti-seize compound). I
3. check all clamps and re-adjus't to cUfTe11t loading if necessary. 4. Look for areas of material leakage and 'Build Up'. Clean and re-bed surfaces
if necessary. 5. Check all loose inserts for fit and damage. 6. Check vent. 7. Check flange fixing for any undue signs of stress (attachment). 8. Look carefully for metal thinning - erosion or corrosion. The mould forms an integral part of any production plan and therefore should be
maintained in good order if production standards are expected to be kept. This is an I u - "
less expensive, these are not reasbns for neglecLg the good p'ractices of mainte- nance and regular inspection. 'I
b~ b e WS menti& in pkev- o t h m ~ ~ w k ~ ~ d ~ p * ~ ~ .
* Am them any molded-h inserts for h d w m to tke plastic pat? .
What tat b i r l d w s dard am b y table to be mviwd d\aing tke mold- @ sp€3mtion? (-re a ins& Ieteer}.
* Where am U. pr&g h e af thr: m i d be pl&9 S o m t h w asthetics of pat arid mdding pMcalty are odds on this.
*If-reaslon-h&&eWaftheprutkhgarti tnt ,wil l ,
smn&the* rib or HBS-Q& bG c#e%&d into the pan.? (Kigs-offs will be c j k x w s l itl mOre&taiEb.)
As jtau sekk? thdh we sewed faom d the part &sign whih have an impwt on b ~ Q M wrgn.
. > - * .u. >, -
spiUad fbn 1-g P-= Mold b s k ~ 00 ~kopthom shifhg using m ~ h @ d aWd wlidcl &&. spd @WVB pnioS lines have 3 m e tongue on one half of the mold and a ~ lum oa b Mller side m and hold the mold halves in place. type of dmim is rrmch moir. mold shifting during the heating c y c k md is mucb mas effslin 6or -@ar shaped parting linea th are not in a -B w. B q of i& - across its width the tongue and g r c x ? ? par ti^^ line is m o ~ e m~nlt 0 clean between moldings. For this reason tbe -&.I is usually pound into Un msls side of the mold, to prevent spillage on the. gro red area of the pvting line.
Removable Cores When an undercut design cannot be r e _ g v e d . h m critical aesthetics or hct ion, remov- cores into the mold. When the undercut is a pmtmioo into m e side be made for the tool. This pen must be -bed
SrSr d e m o l m h g the plolstic part on e w q cydc af the &is i& m t of cbagnmg in undercuts to a plastic
removabls part ts @a EPd an a seating area of the m& a d simply &%I@@ isErp p b . Fk un&mW that ~ n t a i n thraads such as filler necks, the
UiWlm%& &ma tbe r$mMtQ wfic piwe prior to demolding. $ &d!go@-hale the cm I b a fixed part of the mold or removabie, as
d+sW by tb adder. ., . , t
dlow pbtic pat ribs or kiss-&$ are af the part and com6ct tu the ira8id.e d
in the part (see figure 2). Since the rib or kiss-off t ouch the opposite inna ara IU during the mcllhg and forming process. the kiss-off becoma an integral piat ol %e finished plartic pan Mold design plays a critical part in the abi* to effecti~ -- utilize a s dirdcsip criterion. The mord
Q
as in a deep col. of the mold. This will cause improper material build up and the two sides will not touch, thus eliminating the strengthening effect of the kiss-off. It should be noted that this procedure cannot be done without some noticeable wit- ness spot on the, plastic part. If aesthetics are important perhaps kiss-offs should not
variations of these two techniques; however, the principle is basically the sa3nO. .
146
ltemovable Cores
Malded Inserts
. ,
1
but it eliminates the necessity of using bolts and spring& t B q do mt utilize any mechanical attachment of the inserts to the mold, the magnets allow the pm to shri&freely away from them, again minimizing warpage. There have been sev~a l
FIGURE 2 variations of these two techniques; however, the principle is basically the same. \
I
' AnaattaFTPB-:' in using a
3. risk ofpnc& xwji 4. &#ier
than n
ed nwhine. In many
6. M,4INTENANCE SevervlI areas on the molds
opening and closing these molds may not always take tlR necessary precaution to ensure the mold halves are properly aligned prior to assembling them. result is ciamirg% to the parting lime at the point of contact. Although each incident $ minor, repeated activity will cause permanent damage. When severe damage has occurre$, the mold should be returned to tbe mold maker for repairs. The refurbishing bf parting Iiws requites skill and experience and is best left to the experts.
* . 4
l& , .
of Rotationally Moulded Products
Q
One Piece Simple Mold
on. Put 1
Side Action Mdd
@wmbM mmpD. TlrjS ym Ire pltxiud with one mdty and DM miding qmf#iaa. lls cutants &ex trhc armmts mph a mom mmplkmd m l d with l o o f ~ o ~ .
~ ~ . u r i g a ~ ' f r ~ ~ a ~ t r & ~ ~ - ~ s p ~ @ a n b e p t r o d u ~ i n a s i Q l P l ~ ~ ~ ~ w a - p ~ t a D B d t n o n e m l ~ o p e ~ o d . . Al l~f i lhe l lmtmlErmdI~ p ~ ~ w i t h t h e ~ ~ p t s ~ v e ~ ~ . . .Tbfgtw~~epr$asno mtd paB copld be ww by the c h l o h in the s- PI water,. Tim(?: is no laa* my CQ- for the smmgh af the legs. MI things ~ s i d t m d , this h the best concc;pt for oc floating &ah- U d & w 8 the &mea dm&a c h s na gmseat a ~1~~ Eqqmmnw
2 Industrial h i g n considerations
. , ., };%'I? +, ,..I, , < , 1 . . 1 , - I , , .
b,makled parts can tibiglml6~ fht fatmad -%@& I
I J , ~ of the new I
iBw. drawing. It is in i < I
defmed in part drawings that are less lting products suffer accordingly.
piece part design is not being the at ti if^ d thh chapter will concentrate on the design guide&, ., 1 1
2. ROTATIONALLY MOLDED PLASTIC PART DESIGN A good quality piece part design k ths f&&t of a designer's undermnding of the capabilities, limitations and unique quimments of the material, the tooling and
. ' prowsing technique that have been chasm, coupled with attention to design details.
Rotational mokiing is a relatively new plastics designer who is contemplathg the use of should proceed with caution. One of the reoccurring problem with rotational molding is that many parts are kimed by enghers with no prior experience with the process. First time users of rotax'ond molding will enhance their chances of ! success by consulting with an experienced design enghmr, molder or material sup- plier d m the preliminary &sign phase of a new project. I
A little additional care aod aaen%ion in the part design phase of the new pkduct , development cycle will pay big dividends when that product goes into production. -
l&gw heating cycles. The possibility of thermal degradation Limits the mimum weU thickness that can be produced with a given plastic material. mere Yn Ways exceptions, but the following maximum and minimum recom-
mnded waTl mlhesscs can be used as a guide to what wall thicknesses are practical. polyethylae provides the designer with the widest range of wall thicknesses.
polyethylene pPLI will be at their best with a wall thickness in the range of 1.5 to B.Omm. Wall thicknesses of as thin as O.5mm have been successfully produced; m.Omm thick w d s have been.molded on large storage W s . A wall thickness of 1s.OOnuq is not uncommon. However, these thick walls result in long molding
2.1 vrll mckness The single most important fbing to remember about designing any plastic pad i g q &kdn a uniform wall thickness. I4 this regard, rotational molding is unique, as - the p m matee unikrm wdl,thicIcn~aes no matter what the designer specifies. In fact, it is difficult to pro8u@e a molded part with siflcmt changes in wall thickwm.
R.Otati&y molded paFOB am h i @ and dimensioned on only the outside sur- faces that are in contact with the inside surface of the cavity. The inside of the molded part will be f m f o m d and its size and shape will be depeadent u p the outside she and shape, minus Ehe wall thickness.
The of force and the absence of
cycles. "
' polyvinyl' cblodde (plastisols) have been reliably molded into small medical &vices with $dl thicknesses of less than 0.25mm. Most polyvinyl chloride p m b, designed with wall thicknesses in the range of 1.5 to 10.0mm.
Nylon pat@ en generally designed with wall thicknesses in the range of 2.5 to M . b . In specid cam, parts as thin as 1.5mm and as thick as 38mm have b a n
with satlfactory results. I
Polvcarbonata is typically used for molded parts with wall thicknesses in the I i
ange bf 1.5 to lo.& Rotation* nalded parts are hollow. They are produced in molds without inter-
nal cores. In many cases, this provides the designer with the impomt option of fhanging the thickness of a molded part after the production mold has been sarn- pled and actual molded parts have been evaluated. In most other processes, chang- ing the wall thichew would require extensive mold modifications.
Wall thickne~s has a direct effect on €he cost of the molded part. In addition to the added cost d .the m&srial used in a thicker wU, the cycle time and the energy required to heat snd )by1 the plastic will be directly related to the wall thickness. For exampl& aa0,76mm maease in the wall thickness of a Nylon 6 part will result
an incre&e-d M e time of approximately two minutes. One half minute of additional heating w4.U be r e g W for each Q.6m.m increase in the wall thickness of a polyethylene
32 Wall Thickness ~ni'fo~mWy'' Rotationally molded parts am gm.&d in mdds wirb no internal cores. As a result, fbe wall y. Depending on the shape of the part and the agreed that the wall thickness can be eontrolled to S O % . If wall thickness uniformity w m more important than cost, the yaritition can be reduced to *lo% in some case$. h the case of closed molding processes, such as inj&tiorra! c , ~ ~ ~ i o n mold-
@, the standard way of indicating the wall thickness of a Iti'olc&d p@ is to specify a specific thickness and an allowable tolerance; for example, "3.25&.2". Unfortunately, this same drafting practice is also applied to rotatiawy molded parts. A better method of specifying the wall thickness on a rotatiwraI1y mlded pttzt is to indicate the nominal wall thickness and the minimum allowable thic-
that will be accepted anywhere on the molded part. For example, "nominal wall thickness 3.25mm; minimum allowable wall thickness 2.60mm".
In many cases, the wall thickness is monitored and controlled on a production basis by weighing the molded part. Obviously, this only be used after the production mold has been built and the weight molded parts has been established.
The inability to spec+ a specific wall sions on a rotationally molded part, is unsettling to a designer who is accustomed to working with only closed molding processes such as injection molded structural foam or reaction injection molding. However, extrusion blow molding and twin sheet thennoforming share the same limitations. In fact, rotational molding maintains more uniform wall thicknesses than either one nf there other mm-t;t;ll- n+n--ra--
I . - - - - - - -- --- ---- , -- -..".a ' i produce double walled hollow such as one-piece insulated ice chests or the boat shown in the cross section illustratinn. Fio 5 Mnnv nf the.- hnllnx*r no.+- ..,:+h 1
161
---- -- ---.,- .,...". wV... y-uurv yrvrbuab~.
2.3 Closely Spaced Parallel Walls One of rotational molding's advantages over other Drocesses is the ahilitv tfi parillr
- -- - - - - - - - - r - -p -. ----.J V' u.vuv r x v u v w y'uK, ,="I(.',
paallel walls are filkd with self-rising foam to provide additional stiffness or insulation.
--, 0- - --- ------I ""'Y'b y V I. -I." A" ..ry.V-..&.,.J three times greater than the solidified molded part. Consideting this fa &e designer must provide a minimum average distance between p81:del of times the nominal wall thickness. A smaller own mace between the w d s mf tiw
together require extra care in butting the powde; in the ah. -In mm mold must be closed and the plastic material is then carefully pmd through a filling port. In extreme cases, the mold must be v i b m d all of the material into the cavity. ~ 1 1 of these extra o ~ o n s ~ a d d molding the part.
There is also a tendency for the powder to bridge across dm~&
FIGURE 5 Design rules for closely spaced walls
The bulk factor of finelv mound rntatinnal rnnlrlino nnurrlcrr i a annrnvimatelv I -- .- - - -
cavity would not provide enourrh volume for thk w\;dered ~btk. Walls I
This creates a solid wall, or dam, thk prevea I other areas in the cavity. This condition can
C
~srpage-zJ
PM;URB 7 Mold part flatness h i t s in i Wcrn
--0 -- --- 0- ---- --.---- appearance of warpage can be minimized by adding engraving or texture to dis- guise the condition. A highly polished or painted surface will accentuate warpage.
The d m of flatness that can be e x p t e d on a given part is dependent upon m y faetors: the size and shape of the p a the plastic material b e i molded; the quality of the mold; and the way the part is molded. Each plastic part must be wn- sidered individually. Fig. 7 lists the ideal, wmmmial and recision flamess toler- .
stiffness provided by the rib is determined by the distance cmf the wall of the art. A rib height of four times the nominal wa
reduces both material costs and cvcle time.
. - ances that can be used as guidelines in designing rotation&ly molded parts i t h e commonIy molded materials.
The ideal tolerance requires nothing more than normal good quality molding. The precision W e s s tolerances can only be achieved at a significant inamwe in costs. t
Thin walls are advantageous," but many mo than can be provided by the possible to make a desitable
:, the tw ribbed . .
ro six- shape
,1u- "-"-- --
provide the same s&ng& as the unribbed part with less plastic matena ana a shorter molding cycle.
bbing adds s b g t h while reducing material and molding cost Reprodu~ed courtesy of the Association of Rotational Molders
Depending upon how the part will be loaded, some consideration should be ren to the possib'ity of hollow ribs of this type failing, due to deforming like an xrdian. Hollow stiffening ribs, of the e p e preferred for the rotational molding process, : actually closely s p a 4 p a d e l walls. The minimum distance across a stiffening should be at least five times the thickness of the part, as shown in . Fig. .. 9. . . _,._ me np
dl thick projecrs
ness will . "
de a significant increase in stiffness. If this dimension becomes more than four the part's wall thickness, the width of the rib should be increased proportion-
A failure to provide an adequate width in the rib cavity will prevent uniform of the plastic material into that part of the cavity, resulting in bridging, internal I and an increase in mold shrinkage.
cts aove ovide the
FIGURE 9 Recommended minimum hollow h b proportions
required stiffness. In those cases, it is better to provide a multiplicity of short ribs instead of one tall rib.
In those cases where hollow ribs cannot be used, short solid ribs with the propor- tions shown in Fig. 10 can be used. These solid ribs are less desirable than hollow ribs, but they can be used as a last resort.
The problem with solid ribs is that the material bridges over and does not prop- erly fill out the rib cavity. If a solid rib cavity fills out completely, it will create an increase in wall thickness in that location. This thicker wall will take longer to cool
,
and it will shrink more than the thinner walls around the rib. The thicker rib is stronger than the tninner walls. When the thicker, stronger rib shrinks more, it can overpower the surrounding walls and contribute to warpage.
2.6 Kiss-off Ribs Another useful method of providing additional stiffness to a rotationally molded part with closely spaced walls is kiss-off or tack off ribbing, of the type shown in Fig. 11. Connecting the two walls together at the kiss-off creates a very strong box beam type structure.
The exact thickness of the material in the kiss-off area is, in the final analysis, established by trial and error. A thickness of 1.75 times the part's wall thickness has been found to be a good starting point.
While designing parts with kiss-off ribbing, care must be exercis to provide adequate space between two kiss-off ribs to allow the plastic mat% to freely come in contact with all surfaces of the cavity.
Kiss-off ribbing can be used to produce long stiffening ribs or s m d isolated areas of added strength. Inner columns, of the type shown in Fig. 12, are often
C
167
molded plastic parts have to be removed from the cavity that formed them. , plastic parts molded by any process will be easier to remove from their eavities if
the part is designed with draft angles on those surfaces which are peq~%&cular to the parting line of the mold. Anything that makes producing the part easier reduces the molding cost. Draft angles are one of these design features that reduce the cost
' of rotationally molded parts. The limiting factor on a rotationally molded part's cooling cycle time is that the
part must be cmkd enough to have regained sufficient strength to retain its s h w 7 , :,. after being remaved from the cavity. The part must also have regained mcient
smngth to resist the forces required to remove the part from the cavity. The liberal use of draft angle, wherever possible, will reduce the forces applied to the part dur-
k. h g h c demolding process. In reducing these forces, the cooling time, cost, induced
&u& 12 Kiss-off rib shapes stress and part warpage will be minimized. One of rotational molding's advantages over other processes is that many types
bs -o f f ribbing is sometimes used in the bottom of double walled tanks and., of part can be molded straight up and down, with no draft angle at all. his is made wkhXS provide the swngth required t@ s u p w the downward pressure of liq- possible by the fact that h e hollow parts are molded without internal cores. AS
ldd or m d a r products. A useful variation of the standard k i ~ s - ~ f f rib is the , O: the part is cool* it shrinks emd dnms away from the cavity, which makes it easy
‘‘ChOSt'' bs-off rib shown in Fig. 13B. , to remove from the moM. -Each of the various plastic materials has its own mold shrinkage chargcteristics.
Those materials which have a high mold shrinkage factor, such as polyethylene or nylon, will pull away from the ou'tside cavity wall more than the m a d & with a
, lower mold shrinkage factw, such as polycarbonate. . These same high shrinkage factors have the reverse effect on inside surf- of
the cavity, such as the truncated cone and rib detail shown in Fig. 14. -g causes the material to grip these surfaces very tightly. '
The softer, self-lubricating materials, such as polyethylene, are always ~ i ~ t to remove from cavities that have a minimal && angle.
The strong, rigid materials, such as nylon md polycarbonate, are more diffic~lt to remove frbm cavities with minimal draft angles.
FIGURE 13 (A and B) Kiss-off rib variation
The kiss-off rib, Fig. 13A, provides a lot of vertical support. the horizontal walls on both sides of the kiss-off bead in response to the
l*. This a stress concentration at the junction of the aupported and unsup- porS9i8 hfXh~ntal walls. The "almost" kiss-off rib provides the same vertical sup- % eliminating the stress concentration by allowing the horizontal wall to IBDw mlaciwe to the vertical support.
A @hilar kiss-off and "almost" kiss-off are & o m being used b~ Fig. 5. The ~nventional kiss-off rib at the keel provides additional s the heavi~y loaded arm. The "airnost" kiss-off ribs at the sides of the deck alloy the innet hull to gaia support from the outer hull, while allowing the two hulls to move relative to each other in response to inside or outside forces. FIGURE 14 Inside and outside draft angles are different .
, ,
168 169
&e cavity wit,.steel shot n abrasive materials of vwying Size works atll types of mold. There are some limitations on the blasting process's &de a uniform finish at the hottorn of deep narrow r m w . I and blasted finishes rqfxwnt a multiplicity of tiny ~ B F O U ~ S ia the gcttyity. When these finishes are used, the draft angles rw-nded in
be increased by one degree per side for each 0.025mm of U:Sure 1
+j & machined steel cavities will take the highest polish. Cast and el&- can be polisbed to a mirror finish; however, the occasional pres-
where th& p~esence will is a limitation. it& fhd md use environment, *%bhg can be labor intense. Depending on the size and shape of the
r: process by which it was originatly made, a highly polihed finish Rbs the most costly to pmvide and ~~.
g Whes can add significantly to the initial cost of a moldi as welt Eta ~ s q u h x l to maintain that finish. Designers should specify only &t in the hands of the end user. Care must be exembed the surfam finish.
required texture off of the etched, but the welded ma9
well with , ability to p
Texture( wall of the Table I shl depth.
Fabrical troformed ence of p0
Cavity llllI. cavity a d the ail1 norms
High qualit the ongoino r: finish reql to not ovei
2.9 Under - An underc
jecting wa the part m
An und Draft angles i part from the features on a I
It is difficu will be rotational] cores, does allow undercut. This is cesses, such a both the design or me
The soft polyvinyl -... forced out of a cavity. Tk tle or no undercuts. Polyethy
The shape of an undercul 1 1 1 ~
direction that the molded part ml the cavity. Right angled undercu encourage that part to deform in this
Refemng to the molded part sho L.. ,.. . .,. . avoided by positioning the mold's parting lint would require that Undercuts B, C, D and E be deformed cavity. All of these undercuts, except the inside Undercut E, Lvuru vb Lllllll
repositioning the mold's parting line to Location PL 2.
FIGURE 15 Rotationally mylded undercuts b d, .V <" - ' . <
It Ps interesting to notd%htt%b"sift polyvinyl chloride plastisols would allow all of these undercuts to be easily forced out of the cavity. At the same time, the adverse appearance of the parting line could be rninimhd by molding @s part in a one-piece electroformed cavity with the parting line located at PL 3. .bl:,.,"l
The Undercuts B and C can be stripped from the cavity if the pla$~~'*&ka will deform sufficiently to a c c o r n m ~ the depth of the undercut. Removal frod the cavity will be enhanced if the shape of the undercut encourages the inward deformation of Walls Band C.
When they must be specified, their depth must be kept to a minimu limited to flexible or semirigid plastic materials only.
The best rotational molds ari of a two-piece construction. However, loose cavity!! ;.'!'t, i' parts can be built into a rotational molding mold in order to ammmodat~ u*df,''i$ ' details, such as undercuts. side cored holes or molded-in inserts. These wrm &ty $ \ f b , !::+,,, j ' , , components add significantly to the mold" initial construction and ongoing n~&&-,~>,a ; : ,I
2.10 Holes
hole in wherever practical. Molder5 and tool makers have been resourceful in developing ways to provide
holes in rotationally molded parts. For example, all of the blind and through holes shown in Fig. 16, except B: are routinely being produced.
fi!< ., ' I .
c- \ 1 %
, . , #
@3W 16 Rotationally moldable holes $.7~ny ; - . i t + 4 . b
The inwardly projecting blind holes, A and F, are produced on cores that extend b €ha cavity. These tooling cores must be designed to cbduct enougb heat to the fie@ mfl of the core to ensure that they are coated with plastic materid.
TW wtmxdly projwting blind hole or Boss B1, c k be molded because it is sw- the material to flow properly. The out-
e made by rotational molding. This solid f;GPF the flow of material.
I
177
sucb polphyl chloride md att, as less susceptible to molding cycle variations than are crystalline ~ @ b high density polyethylene and
There are always some minor in tb bateh-to-batch u n i f d t y of the plastic as rmived fr;Qm tb -4 ~ttpp1let However, in more recent
other plastics promsing techniques, the best tolerance is the broad- will satisfy the end use fudonal requirement of the part.
limit to the level of
e P ' = s B w m '&re applied to the varkzus
design has been set
, . *
.025 -025 .@25 .m
.010. -810 .810 .005 .OQ5 .010
.010 .QlQ .a10 .008 .008 . : . ,
FIGURE 20 Rimensioairl
&fare cxmmew* my design able to d k dJ m b m t &tors. Two SP&
P@2T$ OF DESIGN I* . essentials are that the product will be fuaakm-
toohg should be kept wela bx ~t:ob&w at a later stage. The; @ding to be used may well have a lyarklg m
tLJdb, Thd-ib* with the .side p d : h a w
I
' 1, '1
' 8
I 1
! < , I ' I . '
J
I ' 193
1,
r ,
A 2 , . Metal irtserts are normally made in brass, ~ ~ ~ u ~ u I ~ I , or steel, those with a bettar ' aemal mductivity being preferred. They must be of thin enough section to
m u r e heat penetration and should not have sharp edges. There must be a means of make certain it is located in the comct position rel-
the moulding. Whilst the moulding will shrink onto o that it is "locked" into the material. (The male insert whilst the female insert has been knurled around the
When designing a product with inserts, thought must be given to its effect on the tooling Mative to de-moulding, since several inserts (on differing faces) will almost -1y increase the number of split lines in the tool.
MouW-in inserts should only be used for smaller diameter thread sizes; on larger sizes, consideration should be given to moulding-in the required thread form. AS in other aspects, the thread will be better formed ia one rnaterlal &m in mbr
" - but it is advisable to restrict moulded-in threads to a coarse or quare fgpe. /It will be appresilfed that with moulded-in thread1 a retmctable core a & MB to be designed into the tool, to facilitate moulding withdrawal. ?Pi# 4.3 Undercuts
. Where undercuts are essenw h the p~QduC% d@$gIl be applied to the placing of fie parting line(s) in tlue allel to the parting line can cause problems.
4 2 Inserfs practice to be e s l w d These can be "moulded in" a d a typical mde md femde insea are in F i r e 14.
+*n damage and/or some part deformation.
In cases where the depth of undercut is slight to a depth of say, 1.5rnrn) then in the m e of the %@ create any difficulty. This class of tmt~rial &ngh that are necessary. . ,
I
ing is a simple box as Figure 15a, then in most cases the initial Skuinkage of the material will enable withdrawal to be made even without any draft angle. Nevertheless, if aesthetically there is no objection to a draft angle being incorporated then this will always facilitate de-moulding (Figures 15b and 15~). I
m e n lwge, and particularly deep, mouldings are involved then one is certainly I avisable, in the order of 3 - So.
made of engravFg a tool to reproduce, say, a company I
h#m. The use of dry-fix transfersldecals (as a post moulding operation) is a way of &Wing attraction and "life" to a product (see Figure 16). I
A more merit and hereasing &valap&mt h the use of moulded-in graphics.
CHAPTER 9
Process Control for Rotational Moulding PJ. Nugent and R.J. Crawford
1 I
.i ' . , a t I'
1% , ,/ A 1
it;Pelf. .As 8 rmdt it is not realistie to to get precise process control based soleiy on the oven air t e m p e m . materials, such as polyethylene, there is a kink corresponding to the c~y"a-
point of the material (point 'D'). Latent heat absorbed during melm i9a =leasd and this maintains the internal air temperature at a constant kv& gk@ hngth 'of the plateau is directly related to the volume of material in @ % e d
polyethylene shows this kink very well. The position of this second plateau m&k%?$3 the melting region (A-B) can be linked with degradation due to the fa, 8M
deqradd material will crystallise at a lower temperature. 1
--r "... tue but this line provides veqy v in relation to the chain nf O ' - -- --- v.
events happeniag to the plastic it lwak5fes.k @%Wit's inside tfie rotating mould. ~ l w r enamination of the line Z @i.g. 1) thaw he - A d mlt using Fig. 2 which - - -- - - - - .- w- -, --y -- --- w...
illustX&% vfEious stages i ~ ~ v o i v d in t t ~ ~ier I fkg of &@ plmtic powder the firmion of a lceating on tim in* d m mda Gtage 6:
~t ,GF' there is another inflexion in the a w e and this is linked with the sad
s- 1: of solidifihation across the wall thickness of the moulding. The material then con- ~nues M m l , and close inspection of the trace shows a slight change in slope at point 8. This is linked with the plastic moulding pulling away hom the mould
. -.
*m I~~ tilt-llloua h' * d -a - ---- . wall. Ifimults in the plastic and the internal air cooling less quickly because they
m r r ~ ~ . his continues &w~&rmdrnOuld
becomg insulated from the cooled mould by an air (or vacuum) gap. The points D, hot fur ~~ to stick. E and ~ d a n be identified quite clearly if the difference in temperature between the
inner & and the inner surface of the: plastic is recorded as shown in Fig. 3. The process of cooling continues until the part can be handled or removed from
the mould. The raa of cooling is less critical to the quality of the moulding in these latter stagw but, as i n d i e above, rapid mling can be difficuIt if the part has separated firam the mould. Point 'G' on Fig. 3 shows a typical de-moulding point
# i!?#$'qk;.'r 1: . " ,. ) f fib.*& , 8
. . I > ,
--. At tbs s t a t of p l~xx3~4 ing , 'h &d a d pol- am add a d the powder tumbles dXaMbby. Upto po&%* lb~ ~f t$eb air &es steadilv aq hest
$a&? 3: At point '3' all the ~ c r w & ha8 &d to fo& a mdten &in with a nowdew i m w -
p+ratme continues ts rise as the melt wnmfidates solve. The h e r &rrface lx?mll~es smooth and m&
n tlw moald moves into the cooline bav ii-5
- - -- .. - - r - .' ---J -- surface; the inkd& air tsrnmmhlpe rises h m l v once mat.e as ho amit
taprature a c : M inside the m d d (Pa 'C") is w amd the time spent therein, hpwt studies have shown dimaly rdarrxlto the smgt l l dthe moula~ed prodm, q'G,t.,a
w rise sliglvtly &e to Tbebmpmtwethtafailsatttrrtate Ambient wling is slowest, &csd atr fastest. If the coaling rate is too fitst $v&& taa slow the proces;s is hat melted layer on the
200
The shape of the internal air t e m p e m profile has been shown (2) to be con- sistent across a wide range of moulding conditions including mould material, mould size, mould thickness, part thickness, part material and oven temperature. .
The salient features remain the same with only the relative positions moving according to changes in conditions. It thus represents a consistent basis for quality control during rotational moulding. Measuring equipment such as Rotolog (3) is commercially available to record the required i n f o d o n .
2.1 Effect of Mould Matertal Figure 4 shows the effect of variations in the mould material. Differences in ther- mal properties and indeed wall thickness will have marked effects on the cycle
During moulding, the bmperme gradient across the wall of an aluminium .
mould will be shallower than for a thianer steel mould. This lower temperature gra-
23 Effect af Part Thicknm 5 shows how the internal air temperature is affected by increasing shot
weight. For thin park, the melting and cooling plateaus are shallow due to the small amounts of material involved. As the wall thickness increases, it takes longer to . melt the larger bulk af powder and so the temperature plateau during which melting occurs becomes longer and flatter. For very thick parts (in excess of 20mm) the plate= can be almost h h n t a l . The temperature rises very slowly to just below the mel- point of the mateaid. As shown in Figure 5, the point at wbich the powder disappears can wily be seen as the m i t i o n becomes more dramatie for thi~kex parts.
2.3 Materials Other than Polyethylene
FIGURE 6 External mould wall and internal air temperature profiles during moulding of polypropylene (3mm)
for 3mm thick polyethylene samples which had been produced ar a range of oven times for several oven temperatures. For each oven temperature there is a clearly defined time after which the impact strength of the material falls sharply. This is attributable to the degradation which occurs at the inner surface. Internal air tem- perature measurements taken at these ~ e a k times show that the air tem~erature
peratures measured were 222"C, 216°C and 210°C.
FIGURE I6 Internal air temperature profiles with points of maximum impact shown
3.2 MFI (Surface ind Within Wall) As the plastic material is heated, the flow properties change due to thermal break- down and oxidative degradation. Melt Flow (MFI) measurements taken at the inner surface of a part also exhibit a sharp downward change due to overturing. Figure 17 shows the way in which the MFI at the outer surface, inner surface and
*
combined cross-section change with oven time. The surface measurements were made by shaving off a thin layer of material from either face of the part whilst the cross-sectional tests used material right through the part. The MFI at the inner sur- face falls away to zero very rapidly after oxidative degradation occurs. The PvlFI at the outer surface falls slowlv as the material suffers only thermal degradation in I the absence of oxygen. The hF1 for the cross-section also falls sharp& but not SO dramatically as the inner surface.
3.3 Bubble Count Bubbles are a common problem for rotational moulding entraps air pockets between melting powder e c 1 a s . , ;.,- r>xT=,
208 209
improved cycle times; multi-layer (multiple shot) processes; control of blowing and ith the other parts. The hating crosslinking agents; liquid systems. by adding a water sP*Y-
4.1 Mould Balancing ~~. point at Moulders with many different products will often run several different moulds on the same arm of a machine. Where the part thicknesses and moulds are very similar greater savings If a fourth 'bal- the cycle times will be similar. However, it will be more likely that the moulds will be of different sizes, that the parts are of differing thicknesses or that the actual materials may be different. This can result in problems of different levels of cure; some parts may be properly cured at the expense of overcuring another. Using the processing curves for a range of parts under consistent moulding conditions allows the moulder to establish the best group of moulds to use together to gain the most even curing and most efficient oven use. This may allow, for example, the use of thin parts in thick aluminium moulds alongside thicker parts in thin steel moulds, or polypropylene parts alongside polyethylene parts.
Figure 19 shows a trial which examined four different moulds simultaneously. All four were apparently similar in wall thickness and cycle time. However, during . processing it immediately became obvious whilst using Rotolog that one of the moulds (line 4) was lagging behind the other parts. The cycle had been set to allow this part to be properly cured which meant that the other three parts were over- cured, reaching internal temperatures in excess of 220°C. To balance the arm prop- erly would have required a fourth mould with a similar performance to the first three moulds. However, as a temporary measure the shotweight in the fourth part
ltaoeously - revised cycle
0 i ~ ~ p e c & u p within the mould is vital where additives are
- ._XI @es of materials have a cocktail of heat and W stahiisers b --A RW mr~~rino that the is not o v e ~ h the maul-
y #gra@3. These will typically require activation tem- 180-m°C. 'Ib ensure that these compounds have been
&&& camsistent tesults, the moulder requires a knowledge of
the d~p'sd blow-
4.3 Mdti-layer Technology Multiple layers in a moulding offer many potential advantages such as: increased stiffniss where a solid skin and foam are combined, improved barrier properties
sider&Iy. Figure 21 shows how the b$md temperature &s high and falls nm!e sbwly at the onset of ~~.
F'rgure 22 shows the internal air profile for a cmsslinlring grade of mwriad. C h s s W n g agents iqmve $ 4 ~ i?tm$tb and sti&es:s of standard p o l y ~ t h y ~ ~ and produce mat&& with imptwed pfxmeati~n mistance. During gvscessing, the cros- tkm mt ocem until affl.und lBPC (depending on the grade used) aad produceis no major a&rm duriag reaction. Thus the inter- nal & temperature curve exhibits no major diffdmnce froq a standard grade. The idomation mpird by the muider 3s to enensum that the activation temperature has been exceeded consistently for all mouldi~gs.
a d penneation resistance by using a thin inner or outer layer of low permeability rnntarid: cost savings where an ex~ensive material can be laid down on a substrate .a*- "---- - - - -
of cheaper standardYmateria1. ~ e v i l o ~ m m t s in multilayer pats such as these have heen hakbered not only by the physical difficulties of adding a second shot of kiterid !$t also by not-knGwing&xisely when to add it. This can now be deter- mined easily from observations of the internal temperature of the mould, so remov- ing mu&,of the trial and error previously involved. . Figurq23 shows a two stage process where a skin of standard polyethylene hnterial h combined with an inner layer of another material. The first shot of mate- v------ -- - -
sal moulds in tbe normal fashion wi$ a plateau indicating the points at which the material first adheres to the mould and when it has finished adhering. At point 'P", when &is layer has cured, the mould is removed from the oven and &e second sbot of material added, This has the effect of cooling the inside of the part as shown at
0 6 1 2 1 8 2 4 I 3 6 4 2 U
(-1 NGURE 21 Internal air temperahre prom for MDPE premixed with Mowing age&
- - -
~~d%k!Vi $48 ~ a a &&w& air wm @lie during a two shot moulding p m
heating is resur moulder is able
ned, the? second to cuntrol the i
layer fuses in the sam nternzll temperature to
wm properties in the second layer (point 'C'). When prohucing very t&& p a le moulder must be careful not to thermally degrade the outer ,@pm at the Kpense of optimising the properties of the inner layer.
' -fpid Systems are obvious differences between the behaviour of liquid and powdered
These mainly lie in the rmmn.er in which material is distributed and the
Other t~~@&&-hti~t tfeea;18 W- fix . m* C- quite rapidiyonw&
this.-. The sing rewtion
213
the mould is moved to a second oven stage. Initidly the second had been set to 190°C. After a short second heating period, the
was added and this was distributed and allowed to react as d rotated in the cooling bay. The reaction inside the mould should ideally
test for the same part. The first oven cycle is consider- is now hot before entering the fmt oven. The second
been reduced to 170°C to reduce the heat input for the ote also that, after assessing 5-6 parts, the target temperature
C al was reduced to 140°C.
seveml more vmi-
was m l at xt 3 5 - m . was r&jW to bring the mouM ts bke target
of snatdal is added d then rlrres a an exothermic reaction , i *
I - *
.. 320 1 - - 1
LwnDmnnml I I I I I
improving rapidly over the last few years with major and cooling bay efficiencies. These
machine improvements without fad- lastic inside the mould. Real h provide vital data on the effects
varies across an oven, on how the been opened, on how the cooling
are related and on how the cooling bay media can provide valuable guidelines
ced close to the outer surface of the mould to examine the mould experiences as it moves about the oven md CC~Q-
areas. This am show how the f h t w k s across ttn oven as each side of the mould ptsses in turn thw& f8me Wt'& inlet to coalex areas d the oven. I3qw~lw oft the size of ovm at@, &BY nsmch k space the mould takes q within this, tlm flu-$ em be ras$. . w +I- W C aroustd the oven set point (see Figure 26). Improved air flow cod1 gawkace a mot?(: homQpneous oven environment.
Spence and RJ. Crawford
RGURE 28 Oven and internal air tern* profiles for pmt m l e d under ambient conditions followed by water
been cured to a standard level. This type of documentation would be a major boost to the general image of rotational moulding, particularly for moulders working to. . meet technically demanding markets.
It is also evident that the next stage of process control in rotational moulding will be direct machine control from the temperature inside the mould. This kas
will thus have moved into the high techology procesdi m&ml p v i associated with injection moulding and extrusion blow moulding.
earchers have made a serious attempt at REFERENCES
(1992).
Raa and Throne used this model ta explain features such mod&@. They suggested that mar because the vo I enough to,overeome the surfam @miom fe,m required to pull the sutface voii away fiom dre mould surfam an$ iato ol bwWh Elwas k c m o w Eh&b)tt;that surf- porosity is inwinsic to the pnwess. .,
Ten years later, Progelbof et dm fmrhm deve1oped the powder densification theory. 'I%& updated theory s ~ e s t d tha QS the powder is heated, the particles becow sticky sind adbere ka eateh other, md ppon further heating, the particles fuse 1' together or dens& to form a witid auEFure. From observinn hot date exwti-
b i d e the buWb has
reduced an$ this chain
e insignificant. They also stated that the trapped air will di£fuse into the g polymer mass, and produced measurements to support this. The iniw e bubble has a significant effect on the rate at which it dissolves, as b mm ea-to-volume ratio is inversely proportional to the diameter. They found iW size was dependent on the size of the particles, and that the numbrid b*
was dependent on the particle size distribution and the IvWI of &daE ~wford and Xu[61 complemented Scoet's earlier work on bubble d y & ~ ~ 'I'by &wd a semi-empirical relationship between bubble diameter ratia, tempmw & . b e . From this relationship the bubble diameter was predicted M a ¬ion t$
;)and melt temperature. ,,,wefore, it can be concluded h t bubbles form due to h e encapsulation of air
between powder partides as they melt and fuse together (see Figure 1). +s pinrlting process continues, these bubbles remain stationary, due to the $igh a- @ty of the molten polymer. The bubbles then slowly diminish in size and may
scosity of the material, the heating cycle time and tbe bubble formation stage is critical in that of
uenced by the powder's characteristie am$ bg dw
1.
I
:c Powder Trapped air narticies \ I
@FFECT OF THE MOULDING MATERIAL ON BUBBLES most ioaslencing factors when considering bubbles in xoko41ouldedJ
i s ffhe &mk& Z>e'me used. A cornon feeling in the indust -
u & &&. &&.- &gether? Tbe answer lies with tfbel
/
220
different MFI'sril. The hot plate &st provides a means by which the formation and removal of bubbles can be obsemd in various powdered materials while they are being heated on a metal plate.
3.1 Mdt Rhealogy
bubbles contained in a more viscous tmkxial
part. For coloured parts this is all that is necessary to produce what appears to be a bubble free product. Additives which have a higher melting temperature than the .
polymer tend to be not so effective.
3.3 Powder Characteristim Bubbles which form in rotationally rnoukhl pgts occur as a nsult of air trappj between individual powder partkles. Tkmfok, it is reasonable to suggest that the powder's characteristics (such 13s partick shape, size and. distribution) will affect the formation and size of bubbles, and mfwe pores in particular. The effect of p ~ c l e size was investigated[lj by series gf nauldimgs with varying percentages , of fine (> 3 0 0 ~ ) and & (300-5001un) prtidep. Aftar moulding, the surface -
porosity of each moulded part was o b s e d using sn image analyser, which provided information such as the quantity of bgbbles, the size of bubbles and the percentage area of the wa3l cross-section with bubbles.
From the results illustratd in Figure 5, it can b e . 8 ~ &at as the percentage of fine particles incre&es, then the sunimum, average md median pore size decrease: This can be explained by the fact that her panicles tny, smaller packets of air, which result in smaller pores. Tho most significant chaoge in the average pae sLe and the median pore size occurs when the percentage of fine particles is between 0 and 10%. This is due to the sieving w o n which takes place during rotational moulding. As the powder tumbles inside the mould, the fine particles tend to be
I . ' . . 4 - ' .
adiacent to the thinner area nf the mnnlrl The mnr~lrl thirhreec aIcn R , - , ~ , ~ - . +h
-- -. - J - - - - - -- --w--- --' --r--- --- --- -"-.a"- y--IV..U U
time. This giv& i e bubbles and pores more time to diffuse through a less viscou m melt- Therefore, variations in mould thickness, or variations in the thermal conduc tivity of specific areas of the mould, will lead to hot or cold spots and this is like1 to result in variations in the surface porosity of the moulded part- = 5. BUBBLE REMOVAL USING PWSURE Pressurising the inner atmosphere of the mould provides the rotomoulder with an alternative method to remove bubbles and surface pores. By introducing a smrl"-- positive pressure into the mould, after the polymer has melted, bubbles and su&- pores can be removed. Once the bubbles have formed in the melt, the introduction of pressure to the mould will act as a compressive force causing the gas m o l e c u l e i contained in the bubble to diffuse through, and out of the polymer (see Figure 8) This process may take only a few seconds to remove all bubbles, depending on th~ level of pressure applied and the characteristics of the material.
~ U R E 8 Pressurisation process - The effect of ~ressurisation was initiallv investi~at~A[ll with the aid nf a
---- ' ----- ---- ,-- ..- --I--.-- that there was little or no difference experienced in the diffisiorl rates of bubbles in atmospheric pressure, ~0m~ared to the diffusion of bubbles in a ~ m r i s e d atmo- I
J -0- ----- \-- - -*- The reason why these initial pressure tests had no effect on bubbles or pares
became apparent after considering the formation of bubbles in unpr=Wkd t atmosphere. When bubbles form under normal conditions. thev GW due fib'
QIlte to the air above the melt (atmos~hefic ~ressum). ' l%eeeb these b fbey I
I ' I ' / I
t&vdi&w tkwu& $IW =It. such
235
seen during testing and are illustrated at xl the tensile test, the bubbles elongate, incr the sample to break. If pressure is used to remove B&
16, then the sample has no points of wealmw* In contintlous and the sample does not break.
237
Fessure levels ranging from 0-3 bar (043.5 psi) were applied for 10 minutes and w n relieved. Following rotational moulding, some mechanical properties were
113bvestigated (impact and tensile). '1 From the impact test results (-20°C), it may be seen that all three applied # F s U e levels removed bubbles and pores completely, and hence increased the hghness of the mouldings. However, there was no noticeable increase in impact
nced for each increasing pressure level. But the tensile properties the applied pressure level (see Table 3). As the applied pressure
ased, the tensile strength and tensile modulus both increased. The tensile improved because increasing the pressure level consolidates the polymer
and improves its resistance to the externally applied forces.
I ' , Table 3 Tensile Data for Increasing Pressure Levels
/
New Cycle T i e - Original Cycle Time
Time (mins)
FIGURE 17 Rot01
242
[81 Barnes, HA, Hutton, JF and Walters, K "An Inwduction to Rheology", Oxford: Elsevier, 1989. [9] Heath, RJ "A Review of the Surface Coating of Polymeric Substrates", Progress in Rubber and Plastics CHAPTER 11
Te~hnology, pp369-401, 1990, V01.6. [lo] "Metals Handbook 9th Edition" Ohio: American Society for Metals, 1985. [Ill Evaos, UR "Corrosion and Oxidation of Metals", London: Edward Arnold Ltd, 1960.
Rotational Moulding or ~ iqu id Polymers [12} Crawford, RJ and Nugent, PJ "Impact S-fh of Rotationally Moulded Polyefhylenc Articles", Plastics,
Rubber & Composites Processing &Applications, pp33-41, 1992, Vol. 17. E. Harkin-Jones and R.J. Crawford
ACKNOWLEDGEMENTS The authors are grateful to the Science and Engineering Research council, Lin Pac Rotational Moulders and Neste Chemicals (now B o w s ) for the financial suppoa of this work. The work benefited greatly by the regular inputs from the staff at the sponsoring companies and from Dr Bob Pittillo of the Polymer Engineering Group. The authors are also indebted to Dr Jovita Oliveira and Dr Jose Comas from the Universidade de Minho, Portugal for assistma with the bubble analysis and 7
measurement of melt rheology at low ,shear rates. Thanks are also due to Dr Steve -
DISCLAIMER The effects of pressure and v a c u d described in this chapter are know to be safe - when used correctly. However, it is up to the moulder to check that mould&,gre sufficiently strong to withstand the forces which arise when pressure or vacur$ are applied. Great care should be taken to ensure that excessive forces are not generated in the mould.
\
Table 1 Adrantages to be gained when moulding liquid polymers
1. In general, liquid polymers are processed at much lower temperatures than those required for powders (PVC phtstiwls me the exception). A number of polyurethanes and epoxides can be pmessed at room temperature.
12. It is possible to employ a greater range of mould mateds. With maw liquid!
I expensive, light, @ass fibre moulds. I I 3, Cycle times can be as short as two minutes ibr materials such as reactive liquid
nylon. Cycle times are also gmally independent of part sizehall thickness. I A / 4. Liquid polymers give exDdlmt rqroduaion of s u r b detail, threaded h s a s ac. I l l 15. Because lower mould t e m p t m a are genera@ required and exothermic heats ofl 1
particles and the system increases in viscosity to beeom a dry &Wd mass. On further heating, fusion of the polymer mol@m
(ii? Reactive liquid polymers such as poly-e and n consist of two or more components whichrh, When (some systems do not reauire the addition of bat), causing -an increase in l i h d viscosity md Cooling is not generally reqnired in such reduee handling temper&&.
wflaoBRhislQrriSmythencoat&e
mes a rotat-
insight into the reasons for the different moulding behaviours of these materials. Figures 1 and 2 show the viscosity changes in each material during the moulding process. It can be seen that Nyrim bas the lowest viscosity of any of the materials tested. Even when re-pooling due to heating occurs in the other materials, the reduced viscosity is never as low as that of Nyrim. Having a low initial or mini- mum viscosity is in itself not bad if low rotation speeds are employed to prevent sloshing of the liquid in the mould. However, when a low initial viscosity is com- bined with a rapidly increasing viscosity during cure, as is the case for NHm, the result is a moulding with poor uniformity of wall thickness.
0 50 100 150 200 250 300 350 400 450 500
Time. (sec.) FIGURE 1 Material viscosity profiles for biaxial rotation tests carried out at 5.8 rprn
0 100 206 300' 4afl 500 600
Time, (set.)
Hyperlast 7853184 has the next lowest initial viscosity. It is next in line to NyTim in terms of poor uniformity of wall thickness at rotation speeds of 5.8 rpm. Its moulding behaviour improves dramatically, however, as the rotational speed increases. This improvement in part uniformity with increasing rotation speed is more apparent for Hyperlast 7853184 than for Nyrim due to the fact that the polyurethane has a higher initial viscosity. Increasing speed therefore allows the mould to lift a greater liquid layer thickness than is the case with Nyrim. The amount of liquid remaining in the pool when the viscosity begins to increase rapidly will thus be less for Hyperlast 7853184 and the final part uniformity will be better than that for Nyrim.
The PVC plastisols have viscosity profiles that increase as rapidly as Nyrim EXMI more rapidly than Hyperlast 7853184 yet they produce better parts than both of these mteW. This can be attributed to the fact that the minimum viscmity attained by the plastisoh during the moulding process is never as low as that for Nyrim and Hyperlast 7853184. The plastisols will not therefore have a large pool of liquid to dissipate ontb the mould walls once the viscosity begins to increase since most of the liquid pool has been lifted anto the mould wall early in the prcr- cess. This will result in a more uniform part wall thickness.
The Hyperlast 7850506 mouldings have the best part wall thickness uniformity of the materials under consideration. This is due to a combination of two fmtom: (1) a high initial viscosity and a small degree of re-pooling, which means that most of the liquid pool will coat the mould walls after the fust few rotations of the mould and, also, very little mat5:M will re-pool due to heating; (2) a lcelatively slowly increasing viscosity profile which allows the remaining pool to k pdual ly lifted by the rotating wall to allow a uniform coating of the mould.
It must be noted that Hyperlast 7850506 does not perform as well at a mould temperature of 45°C as it does at 7Q°C. Although it has a more gently increasing vis- cosity profile at 45°C its minimum viscosity during proceesing is never sufftciently low to allow the material picked up in a heavy layer at the start of mtation to spread out evenly over the mould walls. In fact, at a rotation speed of 13.3 rprn part of the mould will remain untouched by material altogether because the high initial liquid vis- cosity combined with a high rotation speed means that the pool is lifted by one part of the mould wall ahd tends to cure in this configuration. It is therefore necessary when moulding high viscosity malerials such as Hyperlast 7850506 to use a low rotation speed and an initial mould temperature which is sufficiently high to allow re-pooling of the liquid .which will then result in more even coverage of the mould.
It is obvious from the preceding information that not only is the rate at which viscosity- increases during procesSing important but so also is the initial material viscosity and the minimum viscosity attained during processing. Another factor which must be considered when moulding liquids is the presence of bubbles in the finished part. Of the materials considered here, Nyrim tends to produce mouldings with the greatest number of bubbles while Ijyperlast 7850506 has the least number. These are respectively the l~wes t and highest viscosity materials under considera- tion. Bubble formatiom is therefore a h ~ t i g n of material viscosity. The higher the viscosity the more likely the liquid b to adhew to the mould wall as it approaches
250
This is a problem with low v i d t y lk@& lea& to bubble formation.
a l=m INFLUENCE OF PRO C~XWMTIONS ON THE WALL
MOULDEDPhRTS
4.1 Rotation Speed Figwe 3 shows th~ influenix of primary airm mtatim speed on .part wall thickness uniformity (standard deviation from a3,) af sfl&gs mttdie from Nyrim. The greater the rotation sped, the grwtm the t . h i h s of liquid layer that can be piked up by the rotating wd. 'Ibis mswes that only a midl pool of liquid remains in the mould base to k lifted mho the walls once the l i e d viscosity begins to h e m e rapidly. There appem ta b liae bmfl t in in-ing the speed above 7 rpm, and lower rotation speeds & d d be en~~rmgixd in order to mkhise build- up of material at mould oslners (&e to meentgal effwts) and also to tfLinimise bubble formation (see Figure 4).
I . J 1.. ,
FIGURE 3 The effect of primary rotatima spsa8s an m m
L
w; ,
252
nxpird quantity of The rotation is then
tkdcbws unibmity of the finiaEmed part L imtpoved d tkw bubble content is , c : I.ed\aced. Note in X 7 i p 5 tbf a 2.4 & d d @ will have its well thieliness sm-
1,
~ d e v i a d o n i i r o E l g ~ 1 1 3 g s ~ 1 1 ~ R s m l . M ~ b O A m m ~ i t i s m a d e b y a two l o t pinc@B a5iq 1.2 kgp3 &&.
5 BuZWLDWG @QWMENI'
Table 6 The effect of mould shape on the uniformity of part wall thickness in Hyperlast 7850506 mouldings
Method of Improving Moulding Quality Standard Deviation Of Part Wall Thickness From The Mean (mm)
Cube mould and single shot 0.27
Smoothly contoured mould and 0.27 single shot
6. SOME RECOMMENDATIONS FOR THE ROTATIONAL MOULDING OF LIQUID POLXMERS --. \
It should be apparent from the preceding discussion that the rotational moulding/ , , .
liquid polymers is a highly complex process. It is possible, however, to malce p e ; following recommendations[q for minimising the number of problems likely to Pe encountered when processing such materials.
then rapidly increasing). For ekunple, wh& moulding -PVC p . h s I ~ - diisodecyl adipate plasticiser will praduce a less rapidly
'
than dipropylene glycol dibenzoate. I 2. Operate at the lowest possible mtation speed in order t~ minimis @ m-
bance and bubble formation. This will also lainimise the the entire pool of a high viscosity material on ow part of the m0M Wall&- ing the first rotation. - - - . ,? a * L ' I
!.I 7,: 11 i 3. Use moulds with generous corm% (25 t -?"
Transparency 237 Turret machines 1 13,116
Undercuts 3,169,193 Unicast process 243 Uniformity (of thickness) 159 Union Carbide 33 Urea formaldehyde 243 W stabilisers I I
Vacuum 239 Venting 15,72,139,149 Vertical style machines 110 VICAT softening point 40 Vinyl plastics 9.1 10 Viscoelasticity 19 Viscosity 34,57,218,222,232,244,245,254 Viscous material 220 Voids 218 Volume resistivity 39
Wall thickness 158,188,200 Warpage 57,79,147,161,185 Water absorption 65,70,8 1 Waxes 223 Weatherability 6,10 Welding 126
Young's modulus 19
Ziegler 33
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