Seat trim material is PP/PE-TD20, melt flow rate is 2 g/min, density is 1.05 g/cm3, tensile yield strength is 19 MPa, flexural modulus is 1 500 MPa, and melting temperature is 165 ℃ . PP material has characteristics of high flow rate and easy molding, but its shrinkage rate is large, it is easy to produce shrinkage holes, dents and deformation. PP has a large heat capacity, and if mold temperature is too high during injection process, it is easy to warp and deform molded plastic part; if temperature is too low, it may cause uneven gloss or defects such as weld lines. Recommended process parameters for plastic part molding are as follows: injection temperature 180~230 ℃, injection pressure 50~80 MPa, mold temperature 20~60 ℃.

Guard plate is shell, the overall dimensions are 586 mm * 160 mm * 114 mm, and main wall thickness is 2.5 mm. A triangle with a side length of 10 mm in global mesh is selected as basic element, data model of plastic part is divided into double-layer meshes, then a 3D solid mesh is generated, as shown in Figure 1. Generated 3D solid mesh requires a proper aspect ratio, and does not allow inverted tetrahedra (overlapping of adjacent tetrahedra), extreme inter-face angles of folded surfaces, insufficient thinning in thickness direction, or existence of internal long sides. Number of grids is 53 689, maximum aspect ratio is 14.75, average aspect ratio is 1.85, minimum aspect ratio is 1.15, and matching percentage is 90.4%, which meets analysis requirements of more than 90%.

 

Figure 1 Grid model

Considering size and material properties of plastic parts, combined with injection molding parameters, optimal gate scheme suitable for injection molding of seat trim was obtained through comparative analysis. Scheme 1 is designed for single-point feeding, and gate is designed on the side; scheme 2 is designed for double-gate feeding. In order to avoid fusion of two streams on the surface of molded plastic part, gate 2 is delayed for 2.6 s to open. Both schemes are rectangular gates, gate size is 20 mm * 1.2 mm, and corresponding gate positions are shown in Figure 2.

 

Figure 2 Number and location of gates in two schemes
According to recommended process parameters for plastic part molding, process conditions set in Moldflow software are: melt temperature 220 ℃, injection time 2 s, pressure holding time 12 s, mold surface temperature 25 ℃, and cooling time 25 s.

Filling time refers to time for molten plastic to fill the entire cavity. Filling time can be used to check filling of the entire cavity and whether there is insufficient filling or hysteresis. Speed/pressure switching is mainly the last stage (1%~10%) of filling process. When Moldflow analysis software switches between speed and pressure, pressure changes from the highest filling pressure to packing pressure (generally default is 80% of maximum filling pressure), and final filling of cavity is completed through packing pressure.
According to filling analysis, filling time of scheme 1 is 2.286 s and maximum filling pressure is 30.98 MPa; filling time of scheme 2 is 3.289 s and maximum filling pressure is 25.72 MPa, all of which meet molding requirements. Filling pressure is shown in Figure 3.

 

Figure 3 Filling pressure of 2 schemes

Temperature of melt flow front is intermediate flow temperature when plastic melt fills a node, which represents temperature at the center of section. If temperature difference is large, temperature distribution will be uneven, which will lead to uneven surface gloss of molded plastic part. After analysis, flow front temperature of scheme 1 is 210.3~220.3 ℃, temperature difference is 10 ℃; flow front temperature of scheme 2 is 205~220.4 ℃, and temperature difference is 15.4 ℃, as shown in Figure 4. Temperature of flow front of the two schemes is in recommended molding range of 180-230 ℃ [7], and the lower temperature of front is on back side, which does not affect surface quality of molded plastic parts. Temperature difference of scheme 1 is smaller, which is conducive to ensuring consistency of surface quality of molded plastic parts.

 

Figure 4 Filling time of two schemes

Weld line is formed by convergence of two melt flow fronts, and the more intersections of material flow, the more weld lines. Quality of weld line is related to confluence angle of the two streams and temperature of front. Generally speaking, when confluence angle of the two streams is greater than 135°, a weld line is formed, and when it is less than 135°, a weld line is formed.
From structural analysis of guard plate, there are two irregular square holes on the surface of plastic part, and it is difficult to avoid welding marks around square holes. Moldflow software was used to analyze weld lines. Lengths of the two weld lines in scheme 1 were 8.49 and 15.77 mm, respectively, lengths of the two weld lines in scheme 2 were 9.63 and 20.93 mm, respectively. As shown in Figure 5, weld line length of scheme 1 is shorter than that of scheme 2.

 

Figure 5 Weld line
From analysis of confluence angle of the two streams, minimum confluence angle of the two streams in Scheme 1 and Scheme 2 both occurs at edge of irregular square hole in guard plate. This position has no sharp corners or stress points on structure of plastic part, and weld line has little effect on strength of plastic part. Minimum converging angle of scheme 1 is 33.5°, and minimum converging angle of scheme 2 is 25.91°, as shown in Figure 6. Scheme 1 has a larger minimum converging angle, which is beneficial to improve quality of weld lines.

 

Figure 6 Weld line convergence angle
From temperature analysis of material flow front, temperatures of the two weld lines in scheme 1 are 220.2 and 220.1 ℃ respectively; temperatures of two weld lines in scheme 2 are 213.2 and 216.6 ℃ respectively, as shown in Figure 7. Compared with the scheme 2, temperature drop of flow front at position of weld line is smaller and more stable, which is beneficial to improve quality of weld line.

 

Fig.7 Schematic diagram of flow front temperature

Two gate schemes were simulated and analyzed by Moldflow, filling time, filling pressure, flow front temperature, and weld line size were obtained, as shown in Table 1.

Table 1 Comparison of molding parameters of the two schemes
By comparing molding parameters, filling time of scheme 1 is 1 s shorter than that of scheme 2; temperature difference of flow front of scheme 1 is 5.4 ℃ lower than that of scheme 2; length of weld line of scheme 1 is 1.1~5.2 mm shorter than that of scheme 2, minimum confluence angle is 7.6° larger than scheme 2, and temperature of flow front is 4~7 ℃ higher than scheme 2, quality of weld line formed by scheme 1 is better; in addition, single-gate structure is more economical. Therefore, single gate design of scheme 1 is more reasonable.

Cavitation is one of common defects in molding process of plastic parts, which not only affects appearance, but also affects performance of plastic parts. This defect can be solved by improving exhaust system [9]. There are air pockets in reinforcement rib area on the back of plastic parts formed by Scheme 1 and Scheme 2, and there is a phenomenon of poor exhaust near each air pocket, as shown in Figure 8. Poor exhaust will lead to lack of material in back reinforcement during molding process, which will affect strength of plastic part. Rib overlaps on frame to fix and support frame. If strength of rib is insufficient, it will affect mechanical properties of guard plate and cause abnormal noise when guard plate is pressed on seat frame. Therefore, it is necessary to design a movable insert at position where air pockets are generated to quickly discharge air and reduce filling resistance. At the same time, an exhaust groove of 0.01-0.02 mm should be designed around insert to make gas easier to discharge, as shown in Figure 9.

 

Figure 8 Cavitation distribution

 

Figure 9 Exhaust insert

During injection molding process, surface temperature of mold cavity affects mechanical properties and appearance of plastic part. If temperature difference on the surface of plastic part is too large, a large internal stress will be generated during cooling, resulting in warping deformation. A reasonable waterway layout can make mold temperature more uniform and stable. Layout of cooling waterway depends on shape and wall thickness of plastic parts. Number of waterways should be as many as possible and evenly arranged around molded plastic parts.
There are 12 groups of water paths in seat guard mold, including 7 groups of movable mold water paths and 5 groups of fixed mold water paths. Diameter of each group of water paths is φ12 mm, and they are arranged along surface shape of molded plastic parts. Distance between water channels is 50 mm, and distance between water channels is 30 mm from surface of plastic parts.

 

Figure 10 Cooling water circuit layout

After cooling water circuit is optimized, Moldflow simulation analysis results are shown in Figure 11. Cooling time is 30.32 s, which meets requirements and ensures molding cycle of plastic parts. After plastic part is cooled in mold, gate condensate is removed, and surface temperature of plastic part is below 60 ℃, which ensures dimensional stability of plastic part when it is demolded.

 

Figure 11 Optimization effect of cooling water circuit

According to analysis results of Moldflow, gate scheme was determined by comparing molding parameters, inserts and exhaust grooves were designed according to distribution of back air pockets, and cooling system was optimized. Optimized mold structure is shown in Figure 12.

 

Figure 12 Mold structure
The overall dimensions of mold are 1 430 mm * 860 mm * 980 mm, the overall structure is a two-plate mold structure with one mold and one cavity layout. In order to reduce temperature loss and ensure temperature balance, a heat insulation plate is added to fixed mold side. Actual structure of mold is shown in Figure 13.

 

Figure 13 Actual mold
In actual production process, raw material used is PP/PE-TD20, grade is C322T-FNS, model of injection molding machine is MA6000II/2950, screw diameter of injection molding machine is φ70 mm, and clamping force is 6 000 kN. Parts are shown in Figure 14. Three-coordinate measurement of plastic parts is carried out, and measurement results are shown in Figure 15. Maximum deviation of comparison digital-analog is 0.326 mm, which meets requirement of ±0.5 mm for deformation.

 

Figure 14 Actual plastic part

 

Figure 15 Three-coordinate detection results
Use gloss meter and colorimeter to test actual production plastic parts. Measurement position is shown in Figure 16, and measurement results are shown in Table 2. Measurement results show that gloss of plastic parts meets standard requirements of 2.0±0.2, and color meets standard requirements of 2.0±0.2. |dL|<0.35, |da|<0.25, |db|<0.25 standard requirements.

 

Figure 16 Detection position

 

Table 2 Gloss and Color Measurement Results