For transparent parts, especially products with optical uses, such as transparent covers for monitors, light transmission requirements of finished product are relatively high. If product has residual stress, its light transmittance and service life may be affected. Therefore, it is necessary to design appropriate meat thickness and moderate filling parameter molding to reduce risk of residual stress.

Product description: Dimensions of monitor transparent cover (mm) are length 98 * width 98 * height 54, average thickness 5, product volume 26cc, number of holes 1, product and flow channel are shown in Figure 1.
Problem focus: This product has encapsulation problems and structural strength problems caused by residual stress.
Materials used: PC Lexan_940A

 

Figure 1: Plastic injection molded parts

Through filling analysis, we can determine reasons for encapsulation and residual stress in original mold design, and improve it by adjusting product thickness.
Fill flow analysis
Figure 2 shows thickness distribution of product and runner. It can be seen that thickness of side edge of cover is 0.28mm greater than thickness of main body. This thickness difference will cause flow competition. Figure 3 shows results of flow wave front. It can be seen that when melt flow reaches 60%, melt flow on the side of cover has exceeded main body with thin flesh. When it reaches 80% state, core temperature is higher in thicker places, flow will be faster. At this time, a wave front in the form of a backpack has been formed. When reaching end of flow, flow stagnation in the middle becomes more serious, speed on both sides accelerates, resulting in shear and temperature rise, and eventually encapsulation problems occur.

 

Figure 2: Distribution of product and runner thickness

 

Figure 3: Results of flow front 20%~99.9%

As shown in Figure 4, the overall injection pressure of product is 112Mpa, of which flow channel end pressure is 28Mpa and product pressure is 84Mpa. As shown in Figure 5, it can be seen that side of product near gate has a high shear rate. It can also be seen through stress polarizer that product has severe residual stress.

 

Figure 4: Filling pressure distribution

 

Figure 5: Shear rate results and actual stress distribution of product
It can be judged from above results that due to difference in meat thickness of product, flow is unbalanced and there is an encapsulation problem. Since flow on both sides of product is faster, a larger shear rate is generated, which will subsequently affect strength of product structure.

Fundamental reason for encapsulation and high shear rate of this product comes from thickness difference at rounded corners at tail end. Therefore, this design change is to correct thickness and round uneven thickness, as shown in Figure 6.

 

Figure 6: Product thickness design changes

It can be seen in Figure 7 that when flow reaches 40%, after changing thickness, flow speed of melt on both sides has slowed down. When flow front reaches 60% (Figure 8), two sides of original design have exceeded middle. After changing thickness, the two sides and middle area still maintain a flat push state. Figures 9 and 10 show results of 80% and 95% of flow wave front respectively. It can be seen that after changing thickness, flow is relatively more balanced. As can be seen from Figure 11, encapsulation problem can be solved after changing design.

 

Figure 7: 40% comparison of flow before and after design changes

 

Figure 8: 60% comparison of flow before and after design changes

 

Figure 9: 80% comparison of flow before and after design changes

 

Figure 10: 95% comparison of flow before and after design changes

 

Figure 11: Encapsulation position before and after design change

After changing thickness, shear rate on both sides of product (Figure 12) is significantly reduced.

 

Figure 12: Comparison of shear rate results before and after improvement

Through CAE analysis and comparison results, it was found that flow was more balanced after design change, and encapsulation problem was finally solved. Since flow is balanced, velocity gradient on both sides is not large, so shear rate is also improved. Subsequent products have been produced normally after mold repair.