Rapidly developing new products that meet customer needs while maintaining leading productivity is goal that industry has racked its brains and made every effort to achieve. Understanding characteristics of thin shell molding, effectively using CAE simulation and prediction functions, eliminating problems before they occur (before mold opening), and finding problems after they occur (before mold repair) are the skills that a successful laptop computer industry must constantly practice.
Two cases are used to illustrate how to look at common problems and solutions for laptop computer shells from perspective of CAE.

Original design: Design with 4 gates and 11 cooling water channels
Figures 1 and 2 are distribution diagrams of cooling water channels and runner systems on male and female mold sides. Figure 3 is a diagram of melt wave front advancement. Color represents arrival time of melt wave front. Blue color is the earliest arrival place, and red color is the latest arrival place. Figures 4 to 6 show a series of short shots, and also show pressure distribution at the time. Red color is high pressure area, and blue color is low pressure area. It can be seen from figure that lower end is filled earlier than upper end, flow is unbalanced up and down. Injection pressure required for filling is 75.4Mpa. Figure 7 is melt temperature distribution diagram (281℃~245℃), red is high temperature area, blue is low temperature area. Figure 8 is fusion line distribution diagram. Figure 9 is wind distribution diagram. Figure 10 is volume shrinkage distribution diagram, red is high shrinkage area, blue is low shrinkage area, and color scale on the left shows the highest and lowest volume shrinkage at a specific time. The smaller difference between these two values, the smaller residual stress caused by non-uniform shrinkage, and the better design.
In order to accommodate internal assembly parts, keyboard cover is thinned on inner surface of cover to form four square grooves. Wall thickness of groove is 0.9mm and surrounding wall thickness is 1.8mm. Wall thickness is stepped from 1.8mm to 0.9mm, volume shrinkage difference is large, and appearance has obvious color difference. Figure 11 is temperature difference distribution diagram of male and female mold walls, and maximum mold wall temperature difference is 3.3℃. Figure 12 is a total shrinkage warpage distribution diagram, where black line represents original geometry of product and orange line represents geometry of product after deformation. C-MOLD can not only display deformation displacement caused by non-uniform shrinkage and unbalanced cooling, but also display deformation displacement caused by only non-uniform shrinkage or only unbalanced cooling. By comparing displacement caused by two factors together with displacement caused by each factor, C-MOLD users can find main cause of warpage. If culprit is non-uniform shrinkage, check filling system (including runners, runners, gates and mold cavities, shape and thickness distribution of product, number and location of gates are key). If culprit is unbalanced cooling, check cooling system. Figures 13 and 14 show that main cause of deformation is non-uniform shrinkage rather than unbalanced cooling. Figure 15 is a deformation displacement diagram in Z-axis direction.
Corrected design: Use 2 inlet gates and 11 cooling water channels.
Figure 16 is a diagram of advancement of melt wavefront. Figures 17 to 19 show a series of short shots and pressure distribution at the time. It can be seen from figure that upper and lower ends are filled at the same time, and injection pressure required for filling is 81.1Mpa. Figure 20 is a diagram of melt temperature distribution (282℃℃ ~ 248℃). Figure 21 is a diagram of fusion line distribution. Figure 22 is a diagram of accumulated wind distribution. Figure 23 is a diagram of volume shrinkage rate distribution. Modified design changes wall thickness from 1.8mm to 0.9mm to a gradual type. Figure 24 is a diagram of temperature difference between male and female mold walls, and maximum temperature difference between mold walls is 3.4℃. Figure 25 is a diagram of the total shrinkage warpage deformation distribution. Figures 26 and 27 show that main cause of deformation is uneven shrinkage, not uneven cooling. Figure 28 is a diagram of deformation displacement in Z-axis direction.
Table 1 Comparison table of original design and revised design

Original design: In order to accommodate some larger electronic components, wall thickness of laptop LCD back cover is thinned from 1.8mm to 1.2mm. Because of large difference in wall thickness, difference in volumetric shrinkage becomes larger, residual stress becomes larger, product is prone to warping and deformation.
Wall thickness near gate is thinner, and blushing occurs, affecting appearance.
Side wall of boss and side wall of rear cover overlap partially, making local thickness too large, and a sink mark appears.
As shown in Figure A, there is a 32mm diameter baffle tube (baffle) under sprue gate on male mold side, and baffle thickness is 2mm. Diameter of sprue gate on female mold side is 7.2mm, and there is no cooling collar outside sprue bushing. There is a cooling device (baffle tube) on male mold side, but not on female mold side, resulting in uneven cooling on both sides of male and female molds. Compared with the other cooling hole diameters of 8mm, baffle tube diameter of male mold side is 32mm, cross-sectional area is too large, flow rate is too slow, and cooling is poor. Gate of runner has a large diameter, plastic is concentrated, heat is difficult to release, and cooling time is long.

 

Figure A. Water channel distribution of original design
Figures 29 and 30 are distribution diagrams of cooling water channel and runner system on male and female mold sides of original design. Figure 31 is a diagram of advancement of melt wave front. Color represents time when melt wave front arrives. Blue color is the earliest arrival place, and red color is the latest arrival place. Figures 32 to 34 show a series of short shots and also show pressure distribution at the time. It can be seen from figure that left end is filled earlier than right end, left and right flows are unbalanced. Injection pressure required for filling is 77.4Mpa. Figure 35 is a diagram of melt temperature distribution (225.8℃~252.6℃). Figure 36 is a shear rate distribution diagram. Shear rate near gate is 21,858 (1/s). Figure 37 is a volume shrinkage distribution diagram. Due to uneven wall thickness (1.2mm ~1.8mm), volume shrinkage rate is unevenly distributed. Volume shrinkage rate of unthinned area on the right is larger than that of thinned area on the left. Maximum volume shrinkage rate of 6.91% occurs in overlapping area of stud sidewall and rear cover sidewall. Figure 38 is a distribution diagram of sag index. Red area is more prone to sag, and blue area is less prone to sag. Maximum sag index of 4.7% also occurs in overlapping area of stud sidewall and rear cover sidewall. Figure 39 is a distribution diagram of temperature difference between male and female mold walls. Maximum mold wall temperature difference is 12.47℃. Figure 40 is a distribution diagram of total shrinkage warpage deformation. Black line represents original geometric shape of product, orange line represents geometric shape of product after deformation. Figures 41 and 42 show that main cause of deformation is uneven shrinkage, not uneven cooling. Figure 43 is a deformation displacement diagram in Z-axis direction.
Modified design: Product design should be considered as a whole, and wall thickness (1.8mm) should be uniform. Inner hole of stud that overlaps side wall of rear cover can use a copper pin of appropriate size to accelerate removal of heat from thick part and reduce degree of sinking.
Thickening of wall thickness near pouring point can reduce shear rate and avoid occurrence of blush.
Figure B shows that diameter of baffle tube is reduced to 15mm on male mold side to promote turbulent flow and improve heat transfer efficiency. On female mold side, a cooling sleeve is installed outside runner bushing to improve heat transfer around runner and balance cooling of male and female mold sides.

 

Figure B. Water channel distribution diagram of modified design
Figures 44 and 45 are distribution diagrams of cooling water channel and runner system on male and female mold sides of modified design. Figure 46 is a diagram of melt wave front advancement. Figures 47 to 49 show a series of short shots and pressure distribution at that time. It can be seen from figure that left and right ends are filled at the same time-flow balance. Injection pressure required for filling is 74.8Mpa. Figure 50 is melt temperature distribution diagram (224.5℃~251.6℃). Figure 51 is shear rate distribution diagram, and shear rate near gate is 15,000 (1/s). Figure 52 is volume shrinkage distribution diagram. Modified design uses a uniform wall thickness (1.8mm). Maximum volume shrinkage is 4.67%, which occurs in overlapping area of stud sidewall and rear cover sidewall. Compared with original design, it has been greatly improved. Figure 53 is sag index distribution diagram. Red area is prone to sag, and blue area is not prone to sag. Maximum sag index is 2.42%, which occurs in overlapping area of stud sidewall and rear cover sidewall. Figure 54 is temperature difference distribution diagram of male and female mold walls, and maximum temperature difference of mold wall is 7.93℃. Figure 55 is the total shrinkage warpage deformation distribution diagram. Figures 56 and 57 show that main cause of deformation is uneven shrinkage, not uneven cooling. Figure 58 is deformation displacement diagram in Z-axis direction.
Table 2 Comparison table of original design and revised design

The above two cases illustrate CAE can directly or indirectly find out causes of warpage, cooling time, dents, redness, color change, shrinkage, warpage, etc., then find solutions accordingly. If you can gather ideas from all sides at initial stage of design, test various controversial or questionable designs on computer with CAE, eliminate unqualified ones, then find the best one from qualified designs, you can avoid many unnecessary troubles during mold testing and production. Not only shortening delivery time, but also ensuring quality, reducing costs, and increasing profits are ways to survive and win in fiercely competitive market.