Gear is shown in Figure 1. Number of teeth z is 33, modulus m is 1.48 mm, pressure angle α is 29°, displacement coefficient X is -0.971, tip circle size da=48.60 mm, root circle size df= 42.60 mm, using Delrin 100 NC010 grade POM as test material, material parameters are shown in Table 1.

 

Figure 1 Gear structure

 

Table 1 Material parameter table

Use industrial CT to scan formed gear to obtain a grayscale attenuation image, as shown in Figure 2, input VG studio software and reconstruct visualized solid image through three-dimensional image reconstruction algorithm (feldkamp-davis-kress, FDK), define pores with a size above 0.2 mm3 as shrinkage holes according to industry requirements. Scanning uniformly from one side of plastic part to the other side, it can be seen from Figure 2 that shrinkage holes are concentrated in the middle layer of gear web, and there are significant large-size shrinkage holes in the middle layer of hub. Microstructure of shrinkage holes adopts SEM Observation, as shown in Figure 3, proportion of shrinkage cavities is counted and used as response value of shrinkage crater process improvement.

 

Figure 2 CT scan slice-by-slice detection map

 

Figure 3 Defect microstructure

Combined with processing experience and Moldflow forming analysis, processing parameters are determined to be set as: melt temperature 210~220 ℃, mold temperature 80~120 ℃, cooling time 20~40 s. Plastic parts have been inspected by industrial CT, and there are shrinkage holes of different sizes.

Full factorial experimental design can estimate main effects of all factors and interaction effects of each order. Before full factor test, process parameters in injection process (holding pressure, mold temperature, injection speed, cooling time, melt temperature, pressure holding time) were designed and initially screened by Taguchi test. Through range analysis, it was found that holding time had no significant effect on proportion of shrinkage cavities. Due to complex interaction between various factors in injection molding process, combined with Taguchi test results and actual processing experience, main influencing factors are selected as holding pressure, injection speed, cooling time and mold temperature, they are used as main variable factors to conduct a full factorial experimental design to explore main effects and interaction between factors.
Full factorial experiment is designed with two levels for each factor, and experimental level is shown in Table 2. Repeated tests were carried out at high and low levels according to experimental design requirements such as randomization and division into blocks, and three repeated tests were carried out at the center point. Test settings and response value of shrinkage cavity ratio test results are shown in Table 3.

 

Table 2 Factor level table

 

Table 3 Test scheme and results
Full factorial experimental design distinguishes degree of influence and interaction of four factors, including holding pressure (A), mold temperature (B), melt temperature (C), and injection speed (D), on proportion of shrinkage cavities. From normalization effect shown in Figure 4, it can be seen that significant effects on proportion of shrinkage cavities are holding pressure (A), melt temperature (C), holding pressure & injection speed (AD), mold temperature & injection speed. (BD), holding pressure & mold temperature (AB), holding pressure is main factor affecting proportion of shrinkage cavities, and other significant second-order interactions are also inseparable from holding pressure. Combined with Figure 5 and Figure 6, it can be seen that holding pressure (A), holding pressure & injection speed (AD) and holding pressure & mold temperature (AB) have obvious negative effects on proportion of shrinkage cavities, and melt temperature (C) has a significant positive effect on proportion of shrinkage cavities.

 

Figure 4 Normalization effect

 

Figure 5 Factors and Indicator Trends

 

Figure 6 Normality of standardized effects
The larger holding pressure makes material fully compacted, compresses shrinkage cavity space, and inhibits internal shrinkage of material. Increasing melt temperature, the higher melt temperature increases temperature difference between core layer of plastic part and outer layer of plastic part during cooling process of same time, plastic part shrinks unevenly, resulting in more and larger shrinkage holes, and proportion of shrinkage holes will increase. Test results also show that increasing injection speed will increase proportion of shrinkage cavities, and increasing injection speed will lead to an increase in melt temperature, but effect melt temperature is not significant.
In specific analysis of interaction between influencing factors, it can be seen from interaction of factors in Figure 7 that holding pressure & mold temperature (AB), holding pressure & injection speed (AD), mold temperature & injection speed ( BD), melt temperature & injection speed (CD) interaction mean lines are not parallel, indicating that interaction between factors is significant; while the other two groups have good parallelism, indicating that there is no strong interaction between factors, no need to set parameters. In actual material forming process, while changing one factor, consider another factor that interacts significantly with it. Experimental study shows that there is no significant interaction between holding pressure and melt temperature. According to results reflected in Figure 5, high holding pressure can be set. Pressure pressure and low melt temperature are used as process improvement measures to suppress shrinkage crater defects.

 

Figure 7 Factor interactions

In order to verify main effect results of test analysis, a single factor experimental design was carried out on holding pressure of significant influencing factor. Considering that when holding pressure was increased to 130 MPa during actual mold trial process, it appeared that plastic parts were difficult to demould, and holding pressure increased from 70~120 MPa, evenly valued at intervals of 10 MPa. In order to ensure complete filling of plastic parts to be molded and take into account degradation characteristics of polyoxymethylene material, combined with requirements of full factor test results, melt temperature was set to 210 ℃; combined with interaction between mold temperature and injection speed, selected injection speed was set to a low level 36 mm/s; mold temperature was set to a high level of 120 ℃ considering interaction between holding pressure and mold temperature. When other factors are set to fixed values, with increase of holding pressure, proportion of shrinkage cavities decreases significantly. As shown in Figure 8, test results are consistent with main effect holding pressure trend results in full factor test, which verifies reliability of experimental design for consideration of main effect, illustrates necessity of considering interaction between injection processes in process setting.

 

Figure 8 Single factor effect
Results of full factorial experimental scheme with interaction considerations were compared with optimal scheme in initial screening Taguchi design: mold temperature 120 ℃, melt temperature 210 ℃, holding pressure 120 MPa, injection speed 72 mm/s, cooling time 20 s , pressure holding speed is 18 mm/s for comparison. Proportion of shrinkage cavities in Taguchi experiment was optimized from 0.49% to 0.07%, and proportion of shrinkage cavities was optimized from 0.7% to 0.06% in interaction analysis added in full factor experiment. In contrast, optimization effect was more obvious