Figure 1 shows structure of instrument cluster cover. It is an elliptical cylinder as a whole, with uneven flanges on outside. The overall dimensions are 370 mm * 170 mm * 130 mm, basic wall thickness is 2.5 mm, and surface treatment is fine leather texture. Inside of plastic part is a high-visibility area, back has features such as buckles and reinforcing ribs. Material is PC+ABS with a shrinkage rate of 0.5%. Recommended processing parameters are shown in Table 1.

 

Figure 1 Combination instrument cover structure

Table 1 Recommended processing parameters
Box in Figure 2 shows color difference problem of instrument panel cover. Color of local area is inconsistent with surrounding area.

 

Figure 2 Color difference problem

Shape of part of color difference on surface A (appearance surface) of plastic part is basically same as position of reinforcement on it. Structure of plastic part may be one of influencing factors, as shown in Figure 3.

 

Figure 3 Local structure of plastic part with 1 color difference defect
Color difference shape of part of A side of plastic part is consistent with shape of slider molding area on mold. As shown in Figure 4, molded plastic part has no other features here. Slider here is installation point of molded plastic part and cannot be canceled. Cancellation is not possible. Inclined guide pillars were removed from mold and slider became an insert. There was no change in color difference problem, indicating that color difference has nothing to do with movement of slider. Due to small size of slider and no cooling water path designed, temperature of slider is inconsistent with surrounding area when molding plastic parts. Local temperature difference may be one of influencing factors. In actual molding process of plastic parts, it was also found that holding pressure, holding time and mold temperature also have an impact on color difference problem.

 

Figure 4 Partial structure of the mold with two chromatic aberration defects

After preliminary analysis, it was determined that there are 5 key factors affecting color difference of molded plastic parts, namely plastic part structure (A), local temperature difference (B), holding pressure (C), holding time (D) and mold temperature (E). According to recommended molding process parameters and actual production conditions, high and low levels of key design parameters are defined as shown in Table 2.

Table 2 High and low levels of process parameters
Create a 5-factor 2-level design of experiment (DOE) analysis in Minitab software. Results of experiment are shown in Table 3. Figure 5 shows a photo of test process. Scoring value is a subjective evaluation, with 1 being the best on a 10-point scale. This test only focuses on color difference. Defects such as weld marks are not within scope of evaluation and must be comprehensively considered during actual production.

Table 3 Test results

 

Figure 5 Sample photos and evaluation results
Analyze test results and draw conclusion based on Figure 6: Plastic part structure has no impact on color difference problem. Local temperature difference, holding pressure, holding time and mold temperature are significant factors. Order of influence is local temperature difference > holding pressure > holding time > mold temperature.

 

Figure 6 Pareto diagram
Regression equation obtained from experiment is shown in Equation (1). According to regression equation, functional relationship between color difference and four factors can be known. By substituting known values of each factor into function, color difference value can be obtained. Using regression equation, optimal solution of factor can be inferred based on set target color difference value without actual operation, thereby saving manufacturing costs. 
α=4.968 8+1.468 8B+1.156 3C+1.093 8D-0.906 2E+0.281 2B×C×E-0.2812B×D×C (1)
Further analysis using response optimizer is used to obtain optimal solution as shown in Figure 7. According to optimal solution in Figure 7, it can be seen that under premise of material physical properties, mold structure and actual production, the smaller local temperature difference, the better, the smaller holding pressure, the shorter holding time, the better, the higher mold temperature, the better.

 

Figure 7 Optimal solution

Optimization of local temperature differences is achieved by optimizing cooling system. In order to ensure uniform temperature in all areas of mold, cooling water paths of moving and fixed molds must be adequately arranged. Focus is on cooling water path design of slider. Cooling water path design of small sliders is easily overlooked. Slider in visible area must be designed with a cooling water path. If slider space is insufficient, a cooling water path in the form of a "water well" can be used, as shown in Figure 8.

 

Figure 8 Slider cooling water path

If you only adjust injection process parameters and keep other parameters unchanged, reducing holding pressure and shortening holding time will produce weld marks and other defects, resulting in unqualified molded plastic parts. Final optimization direction is to achieve effect of high holding pressure under low holding pressure without causing other defects. This requires simultaneous optimization from both wall thickness of plastic part and pouring plan.

Focus of optimizing wall thickness of plastic parts is to prevent weld marks. Main body wall thickness must be uniform to avoid sudden changes. Thickness of reinforcement ribs must be less than 0.4 times main body wall thickness. For important installation structures, consider reducing root thickness and demoulding through core pulling mechanism, as shown in Figure 9.

 

Figure 9 Plastic part wall thickness optimization

Focus of optimization of pouring plan is to reduce pressure loss of pouring system, shorten length of runner, and avoid premature solidification of melt in runner that affects feeding. Therefore, number of hot runner gates should be sufficient, gate size should be designed as large as possible, cross-sectional area of ordinary runner should also be designed as large as possible, and length of runner should be shortened as much as possible. Optimized pouring scheme is shown in Figure 10. In order to avoid shortcomings of traditional scheme, optimized scheme adds a hot runner, shortens length of ordinary runner to 50 mm, increases gate depth to 0.5 times main body wall thickness, and shortens the overall runner length to less than 250 mm.

 

Figure 10 Optimization of pouring plan

Focus of mold temperature optimization is to increase mold temperature, so hot water is used for cooling, and normal temperature water (around 25℃) cannot be used for cooling. Mold water pipe needs to be replaced with a high-temperature resistant water pipe. During production, a mold temperature controller is connected and mold temperature is set according to actual needs.

Appearance quality of plastic parts optimized according to above plan has been greatly improved, with no obvious color difference defects and meeting quality requirements, as shown in Figure 11. In actual production, mold temperature is set to 75℃, cooling water is passed through slider, holding pressure is set to 50 MPa, and holding time is set to 3 s, which is consistent with DOE test analysis results.

 

Figure 11 Optimized effect