Figure 1 shows a car dashboard skeleton. Material is PP+LGF20. Process requires physical foam injection molding. There are 4 inserts in airbag frame area of dashboard skeleton, which are placed in corresponding areas by a robot when mold is open. During production process of this plastic part, bulges often occur in corresponding area of insert. As shown in Figure 2, some parts are soft due to insufficient cooling, and are propped up by physical foaming to form bulges. In order to verify above point of view, cooling time was extended from 30s to 52s in terms of process, and bulging no longer appeared in corresponding area. Based on this, breakthrough direction for improving appearance defects of plastic parts is to improve cooling effect of corresponding area.

 

Figure 1 Plastic parts with appearance defects during molding process

 

Figure 2 Bulge in corresponding area of insert
Due to large output of plastic parts, extending cooling time solves problem of appearance defects of plastic parts, but reduces daily output and faces huge delivery pressure. By using Autodesk Moldflow, an analysis model identical to actual situation was built, and mold flow analysis was carried out based on process parameters of corresponding mold.

Use 3D software to check areas where bulges appear in plastic parts. It is found that due to shape of insert, thickness of plastic part in corresponding area is inconsistent. Main wall thickness of plastic part is 2.5 mm, and maximum material thickness in the area where insert is located is about 8.485 mm, which is exactly where bulge is, as shown in Figure 3. Through further observation, there is an area with a material thickness of approximately 7.899 mm in adjacent airbag mesh area, but there is no bulge in this area. Autodesk Moldflow was used to conduct cooling analysis on plastic parts and analyze temperatures of surface layer, core layer and insert at bulge location.

 

Figure 3 Area where bulge occurs and actual insert

Table 1 shows data summarized by Autodesk Moldflow simulated plastic part cooling analysis. When cooling time is set to 30 s, core temperature of plastic part in the area associated with insert is 158℃, while core temperature of plastic part in the area associated with mesh has reached 164 ℃, but core temperature of the latter is slightly higher but no bulging occurs. This shows that core temperature is not the cause of bulging, and it may be the difference in surface temperature of plastic part.

Table 1 Plastic part cooling analysis results
Surface temperature of plastic part in the area related to insert is 91.5 ℃, and surface temperature of plastic part in the area related to mesh is 62 ℃. It can be inferred that because surface temperature of area associated with insert is higher and surface plastic is softer, under action of physical foaming, surface of plastic part is lifted up by compressed gas to form a bulge; while surface temperature of plastic part in the area associated with mesh is only 62 ℃, at this time surface plastic in this area has hardened and cannot be lifted up by compressed gas. Since insert is made of metal and has obvious heat absorption properties compared to plastic, insert will conduct absorbed heat to adjacent plastic area, causing surface layer of corresponding area of plastic part to show obvious high temperature.
Based on above inference, temperature of insert is further analyzed. Table 1 lists insert temperature under different cooling time conditions. When cooling time is set to 30 s, insert temperature is 121℃, and a bulge appears in corresponding area of plastic part. When cooling time is set to 52 s, insert temperature is 100℃, and no bulging appears in the corresponding area of plastic part.

Insert in contact with bulge area of plastic part is a nut, and its area corresponds to a large inclined push block mechanism on mold. When mold is opened, insert is placed on insert of inclined push block by a robot, and then each mechanism is reset, mold is closed, and molding cycle is entered.. There is a margin of about 0.1 mm on one side between insert and inner hole of insert, and bottom surface of insert is in contact with inclined push block, as shown in Figure 4. Due to limited steel size of insert and inclined push block mechanism around insert, it is impossible to design a conventional waterway. Combined with above analysis, insert reflects heat storage effect and is root cause of bulge.

 

Figure 4 Corresponding position of problem point on mold

By looking at 3D structure of mold, a solution was proposed to optimize cooling on both outside and inside of area where insert is located. It is planned to use conformal water channels on the outside of insert to optimize cooling effect, and to cool insert on the inside to fundamentally eliminate hot spots.
Corresponding area of inclined push block cannot be equipped with conventional waterways due to steel space limitation. It is modified into a conformal waterway with the help of 3D printing technology, and peripheral water channels of inclined push block are connected in series with it. In order to maximize cooling effect of insert, beryllium copper-like, non-toxic and environmentally friendly high thermal conductivity material is specially selected, so that outside of insert is surrounded by cooling water channels to obtain the best cooling effect.
Figure 5 shows cooling effects obtained by using ordinary water channels and conformal water channels at a cooling time of 52 s. Table 2 summarizes improvement of core, surface layer and insert temperatures of plastic parts with the two water channels. It can be seen that when cooling time is set to 52 s, use of high thermal conductivity materials and design of conformal water channels have a better effect on improving temperature of plastic part, but at this time temperature of insert is still not improved. Under effect of "heat supply", there is still possibility of bulging after plastic part is pushed out, that is, cooling time cannot be compressed to 30 seconds.

 

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

Table 2 Comparison of cooling effects between ordinary water channels and conformal water channels
Figure 6 shows comparison of analysis results before and after cooling of insert. Cooling time is set to 20 s. When insert does not increase cooling, its temperature is as high as 120.8 ℃, and temperature of core of adjacent plastic part reaches 147.7 ℃; when insert increases cooling , its temperature dropped to 68.09 ℃, and temperature near core of plastic part dropped to 129.6 ℃. It can be seen that after adding cooling to insert, cooling effect is significantly improved, as shown in Table 3.

 

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

Table 3 Comparison of analysis results of increased cooling of inserts (20 s)

Based on above analysis results, structural optimization plan is further evaluated based on 3D structure of mold. For solution on the outside of cooling insert, a cooling insert is installed in corresponding area of original inclined push block. Insert is equipped with a 3D printed conformal water channel inside, as shown in Figure 7. For solution of cooling inside of insert, original positioning insert is drilled to make insert a "gas-assisted device", as shown in Figure 8; at the same time, an air path is set up in inclined push block mechanism to ensure implementation of air cooling solution, as shown in Figure 9.

 

Figure 7 Cooling inserts are installed in corresponding area of inclined push block.

 

Figure 8 Insert is modified into an air needle

 

Figure 9 Setting up air path in tilt push mechanism
After mold was optimized and modified, a mold trial was conducted to verify it. As shown in Figure 10, when cooling time was set to 19.6 s, no bulges appeared in plastic parts. 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, measured temperature of insert is 65.9℃. Compared with analysis result of 68℃ in Table 3, mold flow analysis result is accurate and has high precision.

 

Figure 10 Mold test verification after optimization