Process method for controlling warping deformation of long glass fiber plastic parts
Time:2025-01-20 08:37:53 / Popularity: 76 / Source:
1 Research object
Figure 1 shows typical structure of instrument panel skeleton, with a cross-section approximately U-shaped, an outer dimension of 1400mm*420mm*430mm, a basic wall thickness of 2.5mm, no appearance area, multiple holes in the middle, and dense reinforcement ribs on the back. Plastic parts are important structural parts, both rigidity and dimensional accuracy are required. Plastic part material is PP+LGF20, material grade is A422, long glass fiber content is 20%, mold design shrinkage rate is 0.45%, and recommended processing parameters are shown in Table 1.
Table 1 Recommended processing technology
Melt temperature/℃ | Mold temperature/℃ | Ejection temperature/℃ | Maximum shear stress/MPa | Maximum shear rate/s-1 | Material shrinkage/% |
230-250 | 40-70 | 124 | 0.25 | 100000 | 0.45 |
2 Determination of warping deformation optimization target
Warping deformation currently studied refers to the total deformation superimposed in three directions of X, Y, and Z in three-dimensional coordinate system. Maximum deformation of instrument panel skeleton CAE analysis of 10 projects that have been mass-produced is statistically analyzed, as shown in Table 2. Deformation of CAE analysis displayed later is maximum deformation after shrinkage compensation at a shrinkage rate of 0.45%.
According to statistics, actual production assembly is qualified when deformation is ≤10.77mm. Considering that maximum tolerance in GD&T (geometric dimension and tolerance) drawing is ±0.7mm, optimization target of warpage deformation is determined to be 10mm.
3 Preliminary determination of optimization direction of warpage deformation
Fundamental cause of warpage deformation of plastic parts is uneven shrinkage of plastic parts. If plastic parts shrink evenly in all directions, plastic parts will only shrink and will not warp. In injection process, three stages that cause uneven shrinkage of plastic parts are filling, holding pressure and cooling. Seven factors that affect these three stages are gate opening sequence, melt temperature, filling speed, holding pressure, holding pressure time, mold temperature, and cooling time. It is preliminarily determined that process optimization is carried out from these seven factors.
4 Determine gate scheme
According to experience, number of gates for dashboard skeleton is between 7 and 11, with a median of 9. Considering runner length and structure of plastic part, gate scheme is determined as shown in Figure 2.
5 Study on influence of different factors on deformation
5.1 Study and selection of gate opening sequence
In order to study influence of gate opening sequence on deformation (position of weld marks is not considered for non-appearance parts), representative gates are selected for simultaneous opening of all points, sequential filling (1 type, gate opening adopts flow front control, trigger position is gate node, and delay is 0.2s), single point opening first (9 types), from left to right (1, 9 open first), from right to left (5, 6 open first), from middle to both sides (3, 7, 8 open first), from front to back (6-9 open first) and from back to front (1-5 open first). There are a total of 15 gate opening sequences, and CAE analysis is performed under same other process conditions. CAE analysis results are shown in Table 3. Deformation data in Table 3 are calculated, and deformation range S1=3.464mm, average deformation T1=12.946mm, and S1/T1>10%. Under action of a single factor, gate opening sequence has a significant effect on deformation.
As shown in Table 3, V/P switching pressures of 15 opening sequences are all small, and all meet filling requirements. However, flow front temperature difference of 7 opening sequences of 3 first, 4 first, 7 first, 8 first, 3, 7, 8 first, 6-9 first, 1-5 first, and 1-9 at the same time is greater than 10℃, with obvious stagnation, which does not meet filling requirements.
As shown in Table 3, V/P switching pressures of 15 opening sequences are all small, and all meet filling requirements. However, flow front temperature difference of 7 opening sequences of 3 first, 4 first, 7 first, 8 first, 3, 7, 8 first, 6-9 first, 1-5 first, and 1-9 at the same time is greater than 10℃, with obvious stagnation, which does not meet filling requirements.
Since gate opening sequence is a discrete variable, after comprehensive consideration, order that meets filling requirements and has the smallest known deformation is selected for subsequent research.
5.2 Melt temperature study
Melt temperature recommended in material property table is 230~250℃. CAE analysis is performed with a gradient of 10℃ under same other process conditions. Analysis results are shown in Table 4. Calculation of data in Table 4 shows that extreme difference in melt temperature deformation S2=0.201mm, average deformation T2=11.289mm, and S2/T2<10%. Under action of a single factor, melt temperature is not a key factor affecting deformation, and no further research will be conducted.
Table 4 Melt temperature-deformation
Table 4 Melt temperature-deformation
Melt temperature/℃ | Deformation (after shrinkage compensation)/mm |
230 | 11.295 |
240 | 11.185 |
250 | 11.386 |
5.3 Mold temperature study
Mold temperature recommended in material physical property table is 40~70℃. CAE analysis is performed with a gradient of 10℃ under same other process conditions. Analysis results are shown in Table 5. Calculation of data in Table 5 shows that extreme difference in mold temperature deformation S3=1.812mm, average deformation T3=11.814mm, and S3/T3>10%. Under action of a single factor, mold temperature has a significant effect on deformation.
Table 5 Mold temperature-deformation
Table 5 Mold temperature-deformation
Mold temperature/℃ | Deformation (after shrinkage compensation)/mm |
40 | 11.179 |
50 | 11.185 |
60 | 11.902 |
70 | 12.991 |
5.4 Filling speed study
The faster filling speed, the shorter filling time. According to experience, filling time of instrument panel skeleton is about 4s. Research range is expanded to 1~7s, and CAE analysis is performed under same other process conditions. Analysis results are shown in Table 6. Calculation of data in Table 6 shows that filling speed range S4=0.846mm, average deformation T4=12.275mm, and S4/T4<10%. Under action of a single factor, filling speed is not a key factor affecting deformation, and no subsequent research will be conducted.
5.5 Holding pressure study
According to experience, holding pressure of instrument panel skeleton is 30~60MPa. Now research range is expanded to 20~80MPa, and CAE analysis is performed with a gradient of 10MPa under same other process conditions. CAE analysis results are shown in Table 7. Calculating data in Table 7, extreme difference of holding pressure S5=1.195mm, average deformation T5=11mm, S5/T5>10%. Under action of a single factor, holding pressure has a significant effect on deformation.
5.6 Holding time study
According to experience, holding time of instrument panel skeleton is 1~7s. Now range is expanded to 1~9s, with 1s as gradient, and CAE analysis is carried out under same other process conditions. Analysis results are shown in Table 8. Calculating data in Table 8, extreme difference of holding time S6=5.916mm, average deformation T6=12.965mm, S6/T6>10%. Under action of a single factor, holding time has a significant effect on deformation. The longer holding time, the smaller deformation.
5.7 Cooling time study
According to experience, cooling time of instrument panel skeleton is 20~25s. Range is now expanded to 15~45s, with 10s as gradient, and CAE analysis is performed under same other process conditions. CAE analysis results are shown in Table 9. Calculation of data in Table 9 shows that cooling time range S7=0.007mm, average deformation T7=11.814mm, and S7/T7<10%. Under action of a single factor, cooling time is not a key factor affecting deformation, and no subsequent research is conducted. Through above research, four key factors with a greater impact on deformation are screened: gate opening sequence, mold temperature, holding pressure, and holding time.
Table 9 Cooling time-deformation
Table 9 Cooling time-deformation
Cooling time/s | Deformation (after shrinkage compensation)/mm |
15 | 11.809 |
25 | 11.816 |
35 | 11.816 |
45 | 11.816 |
6 Experimental design analysis
Among four key factors screened, gate opening sequence is a discrete variable and cannot be quantitatively studied. Current optimal result is substituted into subsequent research. Remaining three key factors are mold temperature A, holding pressure B, and holding time C. According to recommended molding process parameters and actual production conditions, levels of key design parameters are defined as shown in Table 10. A 3-factor (2-level, 1-center) design of experiment (DOE) analysis was created in Minitab software, and deformation results obtained through CAE analysis are shown in Table 11.
Table 10 High and low levels of process parameters
Table 10 High and low levels of process parameters
Key design parameters | Mold temperature/℃ | Packaging pressure/MPa | Packaging time/s |
High level | 70 | 60 | 10 |
Central point | 55 | 45 | 5.5 |
Low level | 45 | 30 | 1 |
Experimental results were analyzed by Minitab software. As shown in Figures 3 and 4, P values of holding pressure, mold temperature * holding pressure, mold temperature * holding time, and holding pressure * holding time are greater than 0.05, which are insignificant items and need to be screened out. Screened-out items do not have no effect on deformation, but they have less effect than other factors under multi-factor interaction. Remaining mold temperature and holding time are significant factors, and influence ranking is holding time > mold temperature.
Regression equation obtained after screening out insignificant items is shown in Formula (1). According to regression equation, functional relationship between deformation α and two factors can be known. Substituting known values of each factor into function can obtain deformation. Regression equation can be used to reverse optimal solution of factors according to set target deformation without actual operation, saving manufacturing costs.
α=12.996+0.0643A-0.6015C(1)
Further analysis was performed using response optimizer, and expected deformation target was set to 10mm. Optimal solution (mold temperature 47℃, holding time 10s) was obtained as shown in Figure 5. According to Figure 5, under premise of material properties, mold structure and actual production, the lower mold temperature, the better, and the longer holding time, the better.
α=12.996+0.0643A-0.6015C(1)
Further analysis was performed using response optimizer, and expected deformation target was set to 10mm. Optimal solution (mold temperature 47℃, holding time 10s) was obtained as shown in Figure 5. According to Figure 5, under premise of material properties, mold structure and actual production, the lower mold temperature, the better, and the longer holding time, the better.
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