Figure 1 shows a car dashboard frame made of PP+LGF20. Process requirement is physical foam injection molding. Dashboard frame airbag area has four inserts, which are placed in their corresponding areas by a robot when mold is open. During production of this plastic part, bulges frequently occur in corresponding areas of inserts, as shown in Figure 2. These bulges are formed in areas that are not sufficiently cooled and become soft, due to physical foaming process. To verify this, cooling time was extended from 30 seconds to 52 seconds, bulges in corresponding areas no longer appeared. Based on this, breakthrough direction for improving appearance defects of plastic parts was determined to be improving cooling effect in corresponding areas.

 

Figure 1: Plastic part with appearance defects during molding

 

Figure 2: Bulges in corresponding areas of inserts
Due to large production volume of plastic parts, while extending cooling time solved appearance defect problem, it reduced daily output and created significant delivery pressure. An analytical model identical to actual situation was constructed using Autodesk Moldflow, and mold flow analysis was conducted based on corresponding mold's process parameters.

Bulging areas were examined using 3D software. It was found that due to shape of insert, thickness of plastic part in corresponding area was inconsistent. Main wall thickness of plastic part was 2.5 mm, and maximum thickness in insert area was approximately 8.485 mm, which was precisely location of bulge, as shown in Figure 3. Further observation revealed an area with a thickness of approximately 7.899 mm in adjacent airbag mesh area; however, this area did not exhibit bulging. Cooling analysis of plastic part was performed using Autodesk Moldflow, analyzing surface layer, core layer, and insert temperature at bulge location.

 

Figure 3. Bulging Area and Insert Component

Table 1 shows data summary of Autodesk Moldflow simulation of plastic part cooling analysis. When cooling time was set to 30 s, core temperature of plastic part in insert-related area was 158 ℃, while core temperature in mesh-related area reached 164 ℃. However, the latter, with a slightly higher core temperature, did not produce bulging. This indicates that core temperature was not cause of bulging; difference might be due to variations in the surface temperature of plastic part.

Table 1. Plastic Part Cooling Analysis Results
Surface temperature of plastic part in insert-related area was 91.5 ℃, and surface temperature in mesh-related area was 62 ℃. Therefore, it can be inferred that: due to higher surface temperature in insert-related area, surface plastic is softer, and under action of physical foaming, surface of plastic part is pushed up by compressed gas, forming a bulge; while surface temperature in mesh-related area was only 62 ℃, at which point surface plastic in this area had hardened and could not be pushed up by compressed gas. Because insert is made of metal, it has a significant heat absorption characteristic compared to plastic. Insert will conduct absorbed heat to adjacent plastic area, resulting in a noticeably high surface temperature in corresponding area of plastic part.
Based on above inference, temperature of insert was further analyzed. Table 1 lists insert temperature under different cooling time conditions. When cooling time is set to 30s, insert temperature is 121 ℃, at which point bulging occurs in corresponding area of plastic part. When cooling time is set to 52 s, insert temperature is 100 ℃, at which point no bulging occurs in corresponding area of plastic part.

Insert in contact with bulging area of plastic part is a nut. Its location corresponds to a large inclined push block mechanism on mold. During mold opening, insert is placed on insert of inclined push block by a robot arm. Then, all mechanisms reset, mold closes, and molding cycle begins. A clearance of approximately 0.1 mm is left between insert and inner hole of insert. Bottom surface of insert contacts inclined push block, as shown in Figure 4. Due to limited steel dimensions of insert and surrounding inclined push block mechanism, conventional water channels could not be designed. Based on above analysis, insert, acting as a heat storage element, is root cause of bulging.

 

Figure 4: Location of problem point on mold

By examining 3D structure of mold, a scheme is proposed to simultaneously optimize cooling on both outer and inner sides of area where insert is located. A conformal water channel is planned to be used on outer side of insert to optimize cooling, while inner side will cool insert, fundamentally eliminating hot spots.
Due to space constraints of steel in area corresponding to inclined push block, conventional water channels could not be installed. Using 3D printing technology, a conformal water channel was modified, connecting outer water channel of inclined push block in series with it. To maximize cooling effect of insert, a beryllium-like copper material that is non-toxic and environmentally friendly was selected, ensuring that outer side of insert is surrounded by cooling water channels for optimal cooling.
Figure 5 shows cooling effects of using a standard water channel and a conformal water channel at a cooling time of 52 s. Table 2 summarizes improvement in temperature of core, surface, and insert of plastic part by two water channels. It is evident that when cooling time is set to 52 s, using a high thermal conductivity material and designing a conformal water channel has a good effect on improving temperature of plastic part. However, temperature of insert is still not improved at this time. Under "heating" effect of insert, plastic part may still bulge after ejection, meaning cooling time cannot be compressed to 30 s.

 

Figure 5 Cooling effect of ordinary water channel and conformal water channel

Table 2 Comparison of cooling effect of ordinary water channel and conformal water channel
Figure 6 shows comparison of analysis results before and after insert cooling. Cooling time was set to 20 s. Without added cooling, temperature of insert reached 120.8 ℃, and temperature of core of adjacent plastic part reached 147.7 ℃. When insert was cooled, its temperature dropped to 68.09 ℃, and temperature of core of adjacent plastic part dropped to 129.6 ℃. It can be seen that adding cooling to insert significantly improves cooling effect, as shown in Table 3.

 

Figure 6 Comparison of cooling effect of insert (20 s)

Table 3 Comparison of analysis results of added cooling of insert (20 s)

Based on above analysis results, structural optimization scheme was further evaluated in combination with 3D structure of mold. For cooling insert's outer side, a cooling insert was installed in corresponding area of original inclined push block. This insert has a 3D-printed conformal water channel inside, as shown in Figure 7. For cooling insert's inner side, original positioning insert was drilled to become an "air-assisted device," as shown in Figure 8. Simultaneously, an air path was installed in inclined push block mechanism to ensure implementation of air-cooling solution, as shown in Figure 9.

 

Image 7: Cooling insert installed in corresponding area of inclined push block

 

Image 8: Insert modified into an air needle

 

Image 9: Air path installed in inclined push mechanism
After mold optimization and modification, trial molding was conducted. As shown in Figure 10, when cooling time was set to 19.6 s, no bulging occurred in plastic part. Based on above test results, mold cooling time limit can reach 20 s, which fully meets production cycle optimization target value of 30 s. At this time, insert temperature was measured at 65.9 ℃, compared with analysis result of 68 ℃ in Table 3, indicating that mold flow analysis result is accurate and has high precision.

 

Figure 10: Optimized trial molding verification