THE WARPAGE SIMULATION WITH IN-MOLD CONSTRAINT EFFECT IN INJECTION MOLDING

THE WARPAGE SIMULATION WITH IN-MOLD CONSTRAINT EFFECT IN INJECTION MOLDING
注塑成型模內干涉翹曲模擬分析

Yi-Hui Peng*, David C.Hsu and Venny Yang
CoreTech System Co.,Ltd., HsinChu, Taiwan 300, ROC
Rong-Yeu Chang
National Tsing-Hua University, HsinChu, Taiwan 30043, ROC

Abstract

This paper develops a new numerical approach to simulating the mold constraint phenomenon of warpage at the in-mold stage in injection-molded part of complex
geometry and improves the accuracy of warpage analysis. Warpage in injection molding is the result of unequal volumetric shrinkage of material throughout the geometry of the piece as it cools from a melt to a solid state. Before the part is ejected, the deformation of warpage has been developed inside the mold. The influence of in-mold constraint for warpage is different according to the time of in-mold period. This in-mold constraint effect of warpage analysis technique is proved to improve the accuracy of warpage analysis for 3D/2.5D model. Several industrial cases with complex geometry are also studied to illustrate the capability of the proposed methodology.

Introduction

Warpage in injection molding is the result of unequal volumetric shrinkage of material throughout the geometry of the piece as it cools from a melt to a solid state. Before the part is ejected, the deformation of warpage has been developed inside the mold. The
influence of in-mold constraint for warpage is different according to the time of in-mold period. In general the warpage will be smaller when the in-mold period is
longer. The conventional warpage analysis doesn’t consider the in-mold constraint effect. The part is free deformation after the melt fill the part. When the cooling period become longer, the conventional warpage analysis will lose more real phenomena. The warpage with in-mold constraint effect is complicated phenomena. The contact behavior will be induced between rigid mold and plastic part. The part can’t shrink and warp freely at in-mold stage due to the rigid mold. Besides, this contact behavior also induces the stress of part. This paper develops a numerical approach to simulating the in-mold constraint effect for warpage analysis in injection-molded part of complex geometry.

Analysis Approach

Besides, the analysis approach of in-mold constraint effect combines part and mold model. Assume a contact interface exists between part surface and mold wall. The combination of part mesh and moldbase mesh correctly simulate the contact behavior between part surface and mold wall. The numerical method adopts FEM, the displacements and rotations are six degree of freedom for each node. The procedure of warpage analysis is separated into two processes. The first process is mold-constraint deformation from EOP to EOC. It considers the effect of in-mold constraint. The second process is free deformation after the part is ejected and cools to air temperature. The total warpage is the sum of them.

Results and Discussion

In order to validate the proposed numerical model, the injection molding in a thick rib plate with 3D methods, as shown in Figure 1. This model has cooling circuit system below the bottom. The gate is at the side of plate. The volume of cavity is about 10(cc). The used resin is ABS. The mold material is P6. The melt
temperature is 230oC, and the mold temperature is 70oC. The cooling time is 300 sec in order to ensuring the the filling melt front. The Figure 3 shows the temperature at the end of filling. After the cooling analysis simulates, the analysis results show the
temperature on the side of ribs are higher, as shown in Figure 4. It will induce the ribs shrink close to each other. And the predicted cooling time of cooling analysis is about 70 sec, as shown in Figure 5. So the warpage is almost developed at the in-mold tage due to long cooling time (300 sec). The Figure 6 and Figure 7 show the warapge results without in-mold constraint effect. The ribs deform close to each other. The Figure 8 and Figure 9 show the warapge results with in-mold constraint effect. The results are different from the results without in-mold constraint effect. The ribs can’t shrink close to each other due to the resistance of rigid mold wall. Here, the predicted warpage patterns with in-mold constraint effect, deflection and deformation obtained by models are in fairly good agreement. The in-mold constraint effect is related to the
cooling period. The long in-mold stage will induced more the in-mold constraint effect, the part will restrain from rigid mold wall. On the other hand, the short in-mold period reduce the in-mold constraint effect, the part almost deform freely. The figure 10 hows the different warpage with different cooling time. The figure 11 shows the relation between cooling time and warpage for the top of rib. The total deformation means the deformation as the part is ejected and cools to air temperature. The in-mold deformation means the deformation before the part is ejected. The total deformation becomes smaller as the cooling time becomes longer. However, the in-mold deformation becomes larger as the cooling time becomes longer. The warpage will be stable after the melt inside the cavity cools to a solid state. This relation is in good agreement with real world. Sevral industrial parts of complex geometries are analyzed to demonstrate the capabilities of proposed in-mold constraint approach. Figure 12~17 show the results of other industrial cases. Figure 12~14 are 3D
thick model. Figure 15~17 are 2.5D thin model. They present the mold constraint induced difference for warpage analysis. The total deformation becomes smaller
as the cooling time becomes longer. However, the in-mold deformation becomes larger as the cooling time becomes longer. The deformation will be stable after the melt inside the cavity cools to a solid state.

Conclusions

The conventional warpage analysis doesn’t consider the in-mold constraint effect. The part is free
deformation after the melt fill the part. When the cooling period become longer, the conventional warpage analysis
will lose more real phenomena. This developed approach considers the effect of mold constraint. It presents more
sufficient in-mold constraint effect. The Figure 2 shows real phenomena. This new warpage analysis technique is
proved to improve the accuracy of warpage analysis for 3D/2.5D model.

Acknowledgements

The authors would like to thank the support from CoreTech Co. and NTHU through the Molding Technology Fundamental Research Project (Project No.0002182).

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Key Words

Warpage Analysis, In-mold Constraint, Injection Molding

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