Investigation of process parameters for an Injection molding component for warpage and Shrinkage

8/22/2019 Investigation of process parameters for an Injection molding component for warpage and Shrinkage

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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013

ISSN: 2231-5381 http://www.ijettjournal.org Page 1237

Investigation of process parameters for an

Injection molding component for warpage and

ShrinkageMohammad Aashiq M

1, Arun A.P

1,Parthiban M

2

1 PGD IN TOOL & DIE DESIGN ENGINEERING-PSG IAS

2 ASST.PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING-PSG CT

Coimbatore 641004, India

Abstract The purpose of the research is to explore the

influence of different mold temperatures on the warpage &shrinkage of the injection molded components. The simulation

software MOLDEX 3D was used for this study, the simulationswere done by varying different mold temperatures and theircorresponding warpage & shrinkage were collected. It was foundthat the different mold wall temperature causes the asymmetrical

polymer flow in the cross-section due to which the asymmetricalstructure in the parts cross-section occurs and this was observedusing the flow analysis software. So it is required to assurehomogeneous mold wall temperature across the entire cavityduring the production of injection molded parts. This research

finally concludes that warpage and shrinkage decreases forincreased values of mold temperature.

Keywords: Injection molding, Warpage, Shrinkage, FlowAnalysis.

I. INTRODUCTIONInjection molding is a process where by a solid

thermoplastic material is heated until it reaches a state offluidity, is then transferred under pressure(injected) into a

closed hollow space (mold cavity),and then cooled in mold tillit reaches a solid state, conforming the shape of mold cavity.Assuring the proper mold temperature for a specified polymer

is a very important issue as well as keeping the temperatureconstant and equal across the cavity surface. Differences inmold temperature can lead to the problems with manufactured

parts like warpage [1-3]. The reason of this are stresses in theparts.Since polymer with higher temperature exhibits moreintensive shrinkage than in lower temperature, the temperature

differences created during part cooling in the Mold lead topart distortion. The deformation of a simple injection moldedpart due to less and high temperatures are shown in Fig. 1.Part becomes concave from the Mold hot side and convexfrom the Mold cold side. The main cause of warpage is thedifference in internal stresses produced in the material due to

differential shrinkage.It needs to be addressed early on duringthe molding process as the defects are hardly rectifiable later.

Fig. 1.Warpage of injection molded part due to different mold temperature.

The non-uniform mold temperature in the mold plate can bealso problematic in case of multi cavity Molds. If the parts areformed in different temperature they can differ in structure

and properties after manufacturing process [2].The differences in properties are significant for semicrystalline polymers that crystallize during solidification

process. The problem of different properties among the partsfrom one mold is especially important in case of moldingsmall parts in molds with many cavities. Moreover, the

different temperature can also affect polymer flow in runnersof multi cavitymold and this can lead to non-simultaneousfilling of the mold cavities [2].Computer simulation programs are very good tools forprediction of quality problems with injection molded parts [2].It is possible to predict polymer flow inside the cavity as well

as other physical properties distribution of melt across the

entire cavity like pressure, shear stress, shear rate, temperatureetc. Weldlines and air traps are also identified. Minimum

warpage optimization can be done by CAE analysis byprocessing parameters optimization [2]. Gate locationanalysis also can be done to achieve this. The warpage is also

dependent on presence of filler in the plastic as well as on thekind of filler. Particularly it can be different across the partdepending of the filler orientation [3]. The warpage is

dependent also on processing conditions. Computersimulations are helpful with warpage optimization.


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In case of thin-shell parts the melt temperature and holdingpressure are important parameters. The holding pressure isespecially important with minimizing the shrinkage ofsemicrystalline polymers, like POM.The shrinkage can be alsothe reason of warpage. To minimize warpage it is important to

assure the uniformity of the temperature across the part. It willalso result in preventing sink marks and different shrinkage in

the parts [2]. The cooling of the parts in injection moldingplays an important role. It is done by cooling channels in themold. These channels can be designed in many ways in thesame Mold but uniform mold temperature should be assured.

When there is only one and long cooling channel the coolantheats up in the mold and is not efficient. One of goodsolutions is to use special manifolds and design many coolantinlets and also many outlets in the mold [3]. Sometimesspecial injection molding techniques are used to avoid qualityproblems with parts. For example to avoid sink marks in parts

with thick walls polymer with foaming agent is used. It makesa possible manufacturing thick-walled part which is verydifficult in conventional injection molding because thick layer

of polymer is cooled very slowly and exhibits very highshrinkage. This leads to sink marks occurring and to thewarpage problems. Minimizing warpage is also possible when

using compression injection molding, which is especiallyrecommended for thin-walled parts and parts with applicationfor optics. The reason of minimal warpage after thismanufacturing process are very low internal stresses ininjection Molded parts [1]. The reason of warpage is usuallyunequal Mold temperature or differences in part thickness. In

addition, it is amplified by intensive shrinkage of fluid coreinside the part. If the fluid core is placed not symmetrically inthe middle of the part cross-section, one half of the part

contracts more because of intensive shrinkage . If melt

polymer flows inside a Mold channel (runner or gate) the flowfront is formed in a fountain flow [1]. The flow front is

symmetrical to axis of symmetry, placed in the half-distancebetween Mold walls, if only the conditions of flow are thesame near both Mold walls equal temperature, surface

roughness etc.

Fig. 2. The scheme of melt behaviour for less temperature

Fig.2. shows how a flow front of plastic material inside thecavity plate will be when the mold temperature is less(forexample :30C).The flow of plastic material will be different

for higher mold temperature.The flow (for example :60C) isshown in Fig.3.

The flow is symmetry since the mold temperature is high.

Fig. 3. The scheme of melt behaviour for higher temperature

The differences in Mold temperature can be caused bydifferent cooling.The location of cooling channels is oftendetermined by shape of molded part cavity in the Mold. The

unequal cooling and flow asymmetry causes further problemswith molded parts first of all warpage. The Molded part is

warped in the way like shown in Fig. 1. Due to Temperaturedifference part is contracted. Therefore on this part wall thesurface is concaved. The theory presented in some

publications explains the reasons of the warpage. It is requiredto do more research in this field to study about warpage. . Thiscould provide some information helpful in more detaileddescription of warpage phenomenon.

II. PROCEDUREA.Materials

In this study, Acrylonitrile butadiene styrene (ABS) wasused as a polymer material. The grade of the ABS was LGABS AF-303 and its Melt Flow Index Value is MFI 50

g/10min at 230C.

B.Injection MoldingThe basic function of a mold is two fold. Firstly it should

impart shape to the part and secondly it should cool themolded part. Double cavity injection Mold is designed for

performing the tests. The specimens (bars: 55.5x20x2.95 mm)was used for the investigation. The layout of component isshown in Fig. 5. . The sketch of one of molding plates in the

Mold showing the layout of cavities and runners is presentedin Fig. 4. The mold base was designed in order toaccommodate cooling lines that are running around the

component.the mold base dimensions are(244x194mm). Theimportant aspect in injection Molding is cooling the part. Inthe case of experimental Mold used here the cooling system is

very simple one circuit in each Molding plate. The details ofcooling system in the Mold are sketched in Fig. 6. The entireMold consists of several plates (mounting plates, ejector

plates etc.) but cooling circuit is made only in two plates(Molding plates) that form the parts. One of these plates is flatthe plate is with sprue bushing mounted. In the second one the

cavities are milled together with the runners and edge gates, asit is shown in Fig4.


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Fig. 4. Sketch of the injection Mold used for the Analysis

. Fig. 5. Sketch of the injection mold component

C. Processing ParametersThe polymer used for investigation was LG ABS AF-

303.This material is used to manufacture parts like caps,

blocks and automotive components. Even though ABSplastics are used largely for mechanical purposes, they alsohave electrical properties that are fairly constant over a wide

range of applications. The temperature values were changed tocompare the warpage to highly deformed parts and to observethe asymmetry of flow front in the Mold. The Mold

temperature values were kept different for each run in FlowAnalysis (30C,40C,50C,60 C) to analyse the warpage.Allother parameters were kept constant for all the analysis.

Fig. 6.Scheme of mold cooling

Injection temperature: 230CMold temperature: 30 - 60C.Injection velocity: 30 cm/sInjection time: .37 sHolding pressure: 60 MPaHolding time: 5 sCooling time: 17 s

For the Mold temperature values that were kept different

for each run in Flow Analysis (30C, 40C, 50C, and 60 C)to cause the stress in the parts and, as the result, deformationof the parts(to analyze warpage). The temperature values were

changed to compare the warpage to highly deformed parts andto observe the asymmetry of flow front in the Mold.

III. RESULTS AND DISCUSSIONThe difference in value of warpage & shrinkage for different

mold temperature values were observed from the simulatedresults. The parts deformation were showed in the following(Fig.7.a,7.b,7.c,7.d.).

A. Observation of parts deformationThe parts after processing with different Mold temperatures at

30C, 40C, 50C, and 60 C are deformed. The variation inwarpage is noted along X-displacement, Y-displacement andTotal displacement. In practice the displacement along X andY can only be calculated using the Co-ordinate MeasuringMachine.So the Total displacement values are noted.The

shrinkage variations are also noted down.


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Fig. 7a.Warpage Displacement of Component at 30C

Fig. 7b.Warpage Displacement of Component at 40C

Fig. 7c.Warpage Displacement of Component at 50C


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Fig. 7d.Warpage Displacement of Component at 60C

The deformation measured with the results obtained fromFlow Analysis run clearly shows that warpage is caused due toMoldtemperature differences. The optimum Mold temperature helps toavoid the warpage in the plastic component. Here the optimum moldtemperature is considered as 60C.

TABLE I

EFFECTS OF PROCESSING PARAMETERS

Mold

Temperature(C)

Cooling time

(sec)

Shrinkage (%) Warpage (mm)

30 13 5.726 0.201

40 15 4.828 0.232

50 16 3.04 0.283

60 16 3.177 0.092

Fig. 8. Measured shrinkage [%] in total volume

Fig.9. Measured deformation [mm] in the parts length

IV. CONCLUSION If the Mold temperature is different, this leads to

asymmetry of melt flow. This, in turn, causes the

asymmetrical structure development in the part cross-section.

This study concludes that the warpage and shrinkagevalue decreases for higher mold temperature

In further research the effect of variation of moldtemperature on the warpage and shrinkage obtainedusing the simulated results will be verified by

experimental work.

REFERENCES

[1]

J.P. Beaumont, R. Nagel, R. Sherman, 2002, SuccessfulInjection Molding, Hanser

[2] J.P Beaumont., 2004 Runner and Gating Design Handbook.Tools for Successful Injection Molding, Hanser, Munich,inatti.Smorawiski,1989 InjectionMolding technology, WNT,Warsaw, ,460 (in Polish).

[3] T.A. Osswald, L-S.Turng, P.J. Gramann, 2001 InjectionMolding Handbook, Hanser Publishers, Munich.

[4] Practical Guide to Injection Molding, 2004 Edited by VanessaGoodship, Rapra Technology Limited and ARBURG Limited,,193- 94.

[5] http://www.innovateus.net/science/what-are-causes-warpage

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Investigation of process parameters for an Injection molding component for warpage and Shrinkage