Warpage with in-mold constraint effect using injection molding simulation

Abstract
In-Mold Constraint effect in injection molding is one of the concepts to improve the accuracy of warpage results in simulation. In Mold stage effect is suitable for complex part geometry with critical dimensions and multi cavity molds. Warpage is the results of uneven volumetric shrinkage in the part geometry and as a result of crystallization of part from molten state to solid state. Before the time reaches to ejection, the warpage has been developed inside the mold cavity. The effect of In-Mold stage in the different time period will improve the warpage results. The results of In-Mold constraint effect for warpage analysis depends on the In-Mold time period.

Introduction
Injection molding is one of the most versatile production techniques of plastic part manufacturing. In this process, plastic melt is injected through a sprue, runner and gate system into the mold cavity. The melt is then packed at high pressure and then cooled. The stress developed during crystallization under the packing stage. The stress can be either flow induced or thermal induced. Before the part is ejected, the warpage will be developed inside the cavity. Based on practical applications, warpage will be smaller when the In-Mold period is longer.

The warpage and shrinkage in molded plastic parts are influenced by material rheological properties, process parameters, and part geometry.

Prediction of warpage during the product development stage will reduce the process lead time. The conventional warpage analysis does not consider the In-Mold constraint effect. In the In-Mold stage, the contact behavior will be induced between mold steel wall and plastic part surface. This analysis approach has been applied to simulate the warpage with consideration of the In-Mold constraint effect for door cover part.

In-Mold constraint Analysis Approach

The analysis is performed with four cavity mold design in two stages. The first stage is without considering the In-Mold effect. The second stage is with considering the In-Mold constraint.               

The product design considered the nominal wall thickness of 2.5mm with snap lock and rib design. The product requirement was four cavity. The mold design concept was finalized with cold feed edge gate with four lifter per cavity. There was a mold design constraint to give a cooling in core side due to lifter construction as shown in Figure 1.

Part with undercutFigure 1.Part with undercut

In cavity side, 12mm diameter line cooling was considered. The volume of the cavity is about 28.88cm3. The material used for analysis is PP grade SABIC 579S. The mold material is P6. The melt temperature is 240°C and the mold temperature is 40°C. Considered ejection temperature is 90°C. The maximum cooling time of 50 sec in order to ensuring the sufficient In-Mold constraint effect.

Part with coolingFigure 2.Part with cooling

Figure 2 shows the part with cooling channels. The coolant (water) is used to maintain the mold temperature and flow rate of 120 cc/sec. For the above said condition, 50% cooling efficiency observed on cavity side. The part can be filled easily with a filling time of 0.73s as shown in Figure 3 and the filling temperature rise and drops observed inside the cavity as shown in Figure 4.

Filling Melt FrontFigure 3.Filling Melt Front (Fill Time is 0.73s)

Temperature Distribution at the end of fillingFigure 4.Temperature Distribution at the end of filling

After filling, packing starts in the cavity until the gate freezes. Then the holding pressure will be acting till the start of cooling time. During the holding time, stress in the cavity will be stabilized. The temperature observed in rib and snap area is higher after cooling as shown in Figure 5 which leads to more warpage after ejection.

Temperature at the end of coolingFigure 5.Temperature at the end of cooling

The predicted, maximum cooling time with In-Mold constraint is 8.0 sec per cavity as shown in Figure 6.

Predicted cooling time for single cavityFigure 6.Predicted cooling time for single cavity

The Total warpage displacement without In-Mold constraint effect is 1.146 mm as shown in Figure 7 where the warpage level is higher than the acceptable limit. Whereas, the In-Mold constraint effect total warpage displacement is less, comparatively as shown in Figure 8. The deformation is reduced with longer cooling period under In-Mold constraint effect.

Warpage without In-Mold constraint effectFigure 7.Warpage without In-Mold constraint effect

page with In-Mold constraint effectFigure 8.Warpage with In-Mold constraint effect

Results and Discussion
The total displacement without In-Mold constraint effect of the part when it is ejected from the mold and cooled had resulted in more warpage compare to with In-Mold constraint effect. The Figures 9 to 12 shows the difference in cooling time will minimize warpage levels and improve the part quality.

Warpage_ Maximum cooling time =8sFigure 9. Warpage_ Maximum cooling time =8s

Warpage_Maximum cooling time =12sFigure 10.Warpage_Maximum cooling time =12s

Warpage_Maximum cooling time =24sFigure 11.Warpage_Maximum cooling time =24s

Warpage_Maximum cooling time =50sFigure 12.Warpage_ Maximum cooling time =50s

Conclusion
The In-Mold constraint effect is the recent developed approach in the simulation results which represents real time phenomena. Whereas in conventional warpage analysis, the increased cooling time will not match with real time phenomena. Increasing cooling time has a significant effect on the final warpage and considering the In-Mold constraint effect will improve the warpage prediction.
References

  1. Yi-Hui Peng*, David C.Hsu and Venny yang. Core Tech System Co.Ltd., Hsin Chu, Taiwan 300,ROC
  2. Manzione (Ed.), ‘’Application of computer Aided Engineering in injection molding’’, Hanser (1987)
  3. L.Tucker III (Ed.), ‘’Fundamentals of Computer Modeling for Polymer processing’, Hanser (1989)
2019-01-14T09:07:52+00:00