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Unlocking the full potential of injection moulding.will put you ahead.
Special Edition
CO2-GAIM. Fluid-assisted
injection technology is an
established method for
producing complex hollow
parts that cannot be imple-
mented with conventional
cores. The choice of fluid
has a significant impact on
part quality and cycle time,
and the quality of the inner
surface. What is not so well
known is that carbon diox-
ide, acting as injection
medium, offers huge potential to reduce part costs and energy input and to im-
prove part quality – without additional effort.
MARCEL OP DE LAAK ET AL.
Fluid-assisted injection technology(FIT) is an umbrella term for injec-tion molding methods in which an
injection fluid displaces a liquid polymerfrom the core of a molded part. Hobson[1] in a patent dating back to 1939 de-scribed the ways in which this can hap-pen. This basic idea spawned severalmethods in the 1980s that can be classi-fied by the manner in which the melt isdisplaced. The most important are:� the short shot process,� the full shot or spill-over process,� the pushback process, and� the floating core process.
The various types of melt displacementaside, experiments were also conductedon different injection fluids in the late1990s that led to water-assisted (WAIM)and the gas-water-assisted injection tech-nique (TiK-WIT process) [2, 3].Water it-self makes an extremely attractive injec-tion fluid for high-volume production ofparts, primarily because of its great cool-ing action and its low cost. However, wa-ter has drawbacks for the process, becausethe parts need to be drained or dried andbecause leaks can give rise to serious dam-age and costs.
This article will now present anotherfluid, which is also well known but hasnot yet established itself for fluid-assist-ed injection, namely carbon dioxide(CO2). It is the physical properties of CO2,which are rather atypical of gases and willbe discussed later, that make this gas so
interesting for the gas-assisted injectiontechnique (GAIM).
Benefits of CO2 Gas-assistedInjection
A comparison of which fluid is suitablefor which parts reveals that water and CO2
are equally favorable in terms of energyinput for fluid-assisted injection and at-tainable cooling or cycle times (Table 1). Inaddition, the GAIM process proceedsmore or less as readily with CO2 as it doeswith nitrogen (N2).
A look at the physical properties of CO2
clearly shows its benefits [4]. Its densitystarts to increase markedly only from150 bar on, approaching that of water asthe pressure continues to rise (Fig. 1). An-other interesting aspect is that CO2 canbe liquefied at room temperature simply
Untapped Potential in Gas-Assisted Injection Molding
Various handles made bythe gas-assisted injectiontechnique (photo: TiK)
2
I N J EC T I ON MOLD ING
© Carl Hanser Verlag, Munich Kunststoffe international 3/2013
Translated from Kunststoffe 2/2013Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111284
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I N J EC T I ON MOLD ING
by raising the pressure to 60 bar. Nitro-gen, however, cannot be liquefied at roomtemperature, and is a liquid only at cryo-genic temperatures (-196°C at atmos-pheric pressure).
CO2 is usually delivered as a liquid incylinders, cylinder bundles or tanks. Inthis physical state, the heat capacity, cp, ofCO2 (3.0 kJ kg-1 K-1) is three quarters thatof water (4.178 kJ kg-1 K-1) and almostthree times that of nitrogen (1.04 kJ kg-1
K-1). Another physical advantage of CO2
is the high heat of expansion which thegas extracts from its environment as thepressure falls. The totality of these phys-ical properties explains the huge coolingpotential of CO2, which can shorten thecooling time by half, compared with ni-trogen.
A practical advantage of CO2-GAIM isthat the gas has a very good cleaning ef-fect. Annular gap injectors virtually stopclogging up and can be thoroughlycleaned by simply purging with CO2 be-tween two production cycles. The processis more stable as a result because there isno danger that the injector will gradual-ly clog up.
Any assessment of the energy neededto compress the fluid must distinguish be-tween whether compression takes placein the liquid or the gaseous state. If theCO2 is in liquid form, the correspondingpumps have a very high flow rate and theamount of energy needed for pressuriz-ing is more than two orders of magnitudelower than that for compressing gaseousnitrogen. True, nitrogen can also be pres-surized in the liquid state, but only froma cryogenic liquid-gas tank, which slow-ly warms up over a long period and thusrequires large amounts to be drawn offcontinuously.
A Process Flow Which Reducesthe Cycle Time
Before the injector forces the gas into theinjection mold and thus into the part, itis pressurized in the liquid state effective-ly from prevailing pressure (60 bar) in thecylinder bundle compresses the gas in theliquid state effectively and with a favor-able energy input from 300 to 400 bar. Asit is being injected, the CO2 expands intothe melt, displaces the liquid polymer andso forms the cavity in the part. During thesubsequent holding pressure time, the re-maining melt heats the CO2 so that, as aresult of this intense heat absorption, thegas passes into the supercritical state(Fig 2).
At the end of the pressure-holdingtime, the gas is discharged from the part.The drop in pressure causes a sharp dropin the temperature of the CO2, and a greatdeal of heat is extracted from the part.This enormous cooling effect can addi-tionally be utilized to specifically coolhotspots in the injection mold. To suc-cessfully employ CO2- GAIM, processorsmust observe a few important process de-tails. These concern � choosing the right injector, and � avoiding changes in the cross-section
of the supply line from the GAIM in-stallation to the injector.
The choice of injector is important for en-suring that gas-assisted injection pro-ceeds smoothly and that discharge isguaranteed [5]. True, this is also the casefor nitrogen but, because CO2 has a muchhigher density at a given pressure, it takeslonger to release it than to release N2 ifthe annular gap injectors are too small.The line from the GAIM installation tothe injector must not have any changes ofcross-section because these would lead toexpansion of the CO2 and the formationof dry ice, which would clog the line.
If users heed these tips, they can im-plement CO2-GAIM on most existingGAIM-injection molds that have beenpreviously operated with N2. Thus, as thefollowing case reports illustrate, the cycletime of many existing GAIM processescan be shortened by an average of morethan 25 % merely by replacing the gas andplant engineering (Fig. 3) – a joint devel-opment by Maximator GmbH, Nord-
Temperature
1,000
kg/m3
800
700
600
500
400
300
200
100
0
300
51
101
151201
251301
bar401
1
60100
140180
220°C
Pressure
Den
sity
900–1,000
800–900
700–800
600–700
500–600
400–500
300–400
200–300
100–200
0–100
Density of water
Fig. 1. The density of CO2 as a function of pressure and temperature. CO2 can be liquefied at roomtemperature by raising the pressure to 60 bar; as the pressure rises, its density approaches that ofwater (source: Maximator)
© Kunststoffe
Temperature
CO2Solid
(dry ice)
CO2Supercritical
CO2Gaseous
Gas-assisted injection
Gas release
Sublimation point-78.5 °C/1.013 bar
Triple point-56.6 °C/5.2 bar
Critical point31.0 °C/73.8 barWorking range of
cylinders/bundles
Working range oftank supply
CO2Liquid
104
103
102
101
100
10-1
bar
Pres
sure
-100 -80 -60 -40 -20 0 20 40 80 150 200 250 °C 300
Fig. 2. The phase diagram for carbon dioxide shows the thermodynamics of the CO2
gas-assisted injection technique where the gas is supplied from cylinders or cylinder bundles (source: Linde/Maximator/TiK)
© Kunststoffe
Kunststoffe international 3/2013 www.kunststoffe-international.com
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I N J EC T I ON MOLD ING
Kunststoffe international 3/2013 www.kunststoffe-international.com
hausen, and Linde AG, Munich, both inGermany.
Putting it Into Practice with Advantage
With the aid of the approach describedabove, i.e. simply by replacing the plantengineering and the injection gas, EngelFormenbau und Spritzguss GmbH, Sins-heim, Germany, shortened its cycle timefor manufacturing a refrigerator handlein the current series by 36 %. A thermalimaging camera is useful here because itenables the temperature distribution inthe part to be analyzed immediately afterthe handle has been automatically re-moved from the mold (Fig. 4). Direct com-parison shows that the handle producedwith carbon dioxide can be demolded ata much colder temperature than its coun-terpart produced with nitrogen. This notonly helps to shorten the cycle time, butalso improves the warpage behavior.
Similar success was achieved for a dip-stick guide tube produced by Gebr.Wielpütz GmbH & Co. KG, Hilden, Ger-many. The cycle time for this part wasshortened by 22 %. Made from a PA6.6GF30, the part has a very homogeneous(low) temperature of 75 to 80°C uponremoval, which is well below the normaldemolding temperature of 120 to 150°C(Fig 5). With this part, it is not the gaschannel which is crucial to shortening cy-cle times, but rather the machining stage,which the part needs following the injec-tion process.
Excessive shortening of the cycle timewould create hotspots at the thicker-walled areas of the holder and ribs. Thesehotspots are caused by small-sized slidersthat cannot be cooled with water. CO2
serving as a fluid for gas-assisted injectioncould be used to effect spot cooling, with-out the need for further process equip-ment and requiring only a slight modifi-cation to the mold; this would shorten cy-
cle times even more, thereby increasingthe molded part quality even further.
Synergy Effects for ProductionCosts and Quality Optimization
Once a strategic decision has been takenin favor of CO2 as gas injection fluid, fur-ther technical ways of lowering produc-tion costs and improving part qualityemerge:� spot cooling of injection molds,� dynamic mold temperature control,
and � the cleaning of injection molds and
plastic surfaces (as pretreatment forpainting).
In spot cooling (Fig. 6), liquid CO2 is passedthrough a flexible capillary tube, 1.6 mmwide at most, to that spot on the moldingwhich needs cooling. There, the CO2 ex-pands into a chamber, which needs to beprovided in the cavity,and effectively coolsthe region around the expansion cham-ber [6, 7]. A combination of CO2-GAIMand spot cooling can be used to supply theCO2 gas to the hotspot and cool it in thesame mold while the gas is being dis-charged in the GAIM process.
A further application for CO2 lies inthe extremely fast dynamic cavity tem-perature control of injection molds andmold inserts (Fig. 7). This process, devel-oped jointly by Linde AG, gwk Wärme
Fig. 3. The newly developed CO2-GAIM module installation with integrated liquid booster is suitable forCO2 and N2 applications (photo: Maximator)
Fig. 4. The cycle time for producing this refrigerator handle was shortened by more than a third. Thisthermal image, captured immediately after the process, shows the higher temperature of a handleproduced by N2 (left) compared with that for CO2-GAIM (right) (photos: Linde)
Process Easy tohandle Fluid costs Investment
in plantInvestment in
moldMaterial
costs
Process energyfor fluid-assisted
injectionInjector size Cycle time
CO2-GAIM ++ � 75 100 100 1 + 50
CO2-GAIM with flushing + – 75 105 100 3 � 40
GAIM ++ � 100 100 100 100 + 100
GAIM with flushing + – 100 105 100 300 � 80
TiK-WIT � + 150 120 130 1 – 50
FAIM cool � � 120 100 100 200 + 95
Table 1. Comparison of some fluid-assisted injection processes with different fluids. The percentages are expressed in relation to the nitrogen-GAIMprocess (= 100 %) (source: TiK)
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I N J EC T I ON MOLD ING
© Carl Hanser Verlag, Munich Kunststoffe international 3/2013
Kältetechnik mbH, Kierspe, Germany,and Iserlohn Kunststoff-TechnologieGmbH, was presented at Fakuma 2012.In it, CO2 is used to effect both heatingand cooling in the same mold insert. Toheat the cavity, gaseous CO2 is heated bymeans of a compressor and a turbo-
heater and passed through the confor-mal heating-cooling channels of themold insert. The gas is transported in aclosed loop. The mold insert is cooledagain by feeding liquid CO2 into thesame heating-cooling channels and re-laxing it. The cooling effect is similar to
the aforementioned spot cooling. Withthis technology, temperature gradientsof up to 20 K s-1 can be achieved forheating and cooling.
Conclusion
Using carbon dioxide as a fluid for thegas-assisted injection technique andslightly modifying the plant engineeringis a simple way for injection moldingshops to shorten cycle times and lowerpart costs. Since the process can be han-dled just as simply as the conversion fromnitrogen to CO2,CO2-GAIM can improveboth current and future GAIM applica-tions. The cleaning effect of the CO2 onthe injectors stabilizes the productionprocesses in the long term. In addition,CO2 offers an effective and inexpensiveway to solve temperature control tasks ininjection molding.�
REFERENCES
1 PS-US 2331688: Method and apparatus formaking hollow articles of plastic material (1939)Hobson, R.
2 DE 103 39 859 B3: Verfahren und Vorrichtung zurHerstellung eines Kunststoff-Bauteils, welcheseinen Innenhohlraum hat (2003) Op de Laak, M.
3 Op de Laak, M.; Rupprecht, V.: Die Qual der Wahl.Kunststoffe 96 (2006) 9, pp 115-120
4 A.N.Other: Eigenschaften der Kohlensäure.Fachverband Kohlensäure-Industrie e.V., 1997
5 Eyerer, P.; Elsner, P.; Knoblauch-Xander, M.; vonRiewl, A.: Gasinjektionstechnik. Hanser Verlag,Munich 2003
6 A.N.Other: Technisches Handbuch Toolvac, Fir-menschrift der AGA Gas GmbH, 1995
7 Berghoff, M.: Kühlen kritischer Bereiche imWerkzeug mittels CO2-Temperierung. Diplomar-beit, Märkische Fachhochschule Iserlohn 1999
THE AUTHORS
DIPL.-ING. MARCEL OP DE LAAK, born in 1970, isa managing director of TiK-Technologie in KunststoffGmbH, Teningen, Germany; [email protected]
DIPL.-ING. AXEL ZSCHAU, born in 1965, is a man-aging director of TiK-Technologie in Kunststoff GmbH,Teningen; [email protected]
DIPL.-ING. ANDREAS PRALLER, born in 1966,works for the Linde Gas division in Unterschleißheim,Germany, where he is project leader with responsibil-ity for developing and launching gas-based technolo-gies for the plastics-processing industry;[email protected]
DIPL.-ING. (FH) MADS RASMUSSEN, born in1977, works in the Sinsheim technical office of Maxi-mator GmbH, where he provides plastics processorswith support on technologies for gas and water injec-tion technology and physical foaming;[email protected]
Molded part
Capillarytube
CO2 CO2
CO2
Expansionchamber
Fig. 6. Schematicstructure of an injec-tion mold with spotcooling [6, 7]. The CO2
cools the regionaround the expansionchamber effectively(figure: Linde)
© Kunststoffe
Fig. 5. At the end of the cycle, the temperature of a dipstick guide tube made from PA6.6 GF30 byCO2-GAIM is much lower than the normal demolding temperature for such materials, as shownhere by the thermal image captured immediately in the 2-cavity mold after the process (photos: Linde)
Fig. 7. The installa-tion for dynamic moldtemperature controlcombines rapid heat-ing and coolingchanges at tempera-ture gradients of upto 20 K s-1 (photo: gwk)
Maximator GmbH
Technisches Büro SüdPostfach 17 6574877 SinsheimGermanywww.maximator.de
Matthias WipflerPhone +49.7261.9454-16Fax +49.7261.9454-20Mobile [email protected]
Dipl.-Ing.(FH) Mads RasmussenPhone +49.7261.9454-18Fax +49.7261.9454-20Mobile [email protected]
Linde AG
Linde Gas DivisionCarl-von-Linde-Strasse 2585716 UnterschleissheimGermanywww.linde-gas.com
Andreas PrallerPhone +49.89.31001-5654Fax +49.89.31001-5585Mobile [email protected]
Mikael OrsénPhone +46.40.283835 Fax +46.40.933918Mobile [email protected]
Contact.