With development of automobile industry, demand for large-scale two-color automotive plastic parts continues to increase. Consumers' requirements for two-color automotive plastic parts continue to increase. Requirements of enterprises for mold injection efficiency are also increasing. Mold practitioners should understand how to design and manufacture two-color injection mold for large two-color automobile plastic parts. Two-color injection molds for automotive plastic parts have attracted wide attention from enterprises, and they account for an increasing proportion of plastic molds. Now, taking automobile air ducts as an example, injection mold design for two-color automotive plastic parts is introduced.

 

(A) First color material structure

 

(B) Second color material structure

Figure 1 Plastic part structure

Structure of plastic part is shown in Figure 1. External dimension is 795.5mm * 482.2mm * 158.9mm. It is a large and complex two-color plastic part and is produced using PPT20 and TPE-S materials. Inside and shape of plastic parts are complex and size is large, TPE-S soft rubber outside plastic parts has a wide distribution range. Average wall thickness of plastic part is 2mm, weight of first color PPT20 hard rubber is 898g, second color TPE-S soft rubber weight is 161g, and mold requires 1 cavity structure.

 

 

Figure 2 Details of plastic parts

Details of plastic parts are shown in Figure 2. Plastic parts of first color PPT20 have undercuts on three sides, second color TPE-S has undercuts around plastic parts, and there are undercuts in the middle. According to structural characteristics of plastic part, mold structure needs to design a fixed mold slider molding structure. After first color melt injection molding is completed, movable mold part of mold is rotated 180°, and then second color melt injection molding is performed.

 

Figure 3 Moldflow analysis of first color PP T20 material

In the early stage of mold design, Moldflow engineers participated in the design of mold gating system and performed detailed Moldflow analysis for plastic parts. Moldflow analysis results of first color PPT20 material are shown in Figure 3. Filling time is 2.6s, and maximum injection pressure is 34.9MPa, which is within injection requirements of PPT20 material.

 

Figure 4 Pressure curve of first color PP T20 material

 

Figure 5 First color material gating system

3577kN, pressure curve of first color PPT20 material is shown in Figure 4. First color PPT20 material gating system uses hot runners and 13 latent gates. Gating system is shown in Figure 5.

 

Figure 6 Moldflow analysis of second color TPE-S material

 

Figure 7 Pressure curve of second color TPE-S material

 

Figure 8 Second color material gating system

Moldflow analysis result of second color TPE-S material is shown in Figure 6. Filling time is 1.9s and maximum injection pressure is 83.56MPa. Injection pressure is too large, but number of hot nozzles and gates is relatively large. Within acceptable range, maximum clamping force is 3577kN. Pressure curve of second color TPE-S material is shown in Figure 7, second color TPE-S material pouring system uses hot runners and 14 latent gates, and second color material gating system is shown in Figure 8.

 

Figure 9 Balanced double-head injection molding machine

Due to large size and complex structure of plastic part, mould is also large in size, which is a large mould. Ordinary two-color injection molding machine is small and cannot be installed with this mold. Therefore, equipped two-color injection molding machine is a balanced double-head injection molding machine with a rotating table in the middle of injection molding machine, which is suitable for production of large two-color molds. ENGEL1700T double-head injection molding machine is shown in Figure 9. Maximum clamping force is 17000kN, which meets results of Moldflow analysis.

 

(A) First color material slider

 

(B) Second color material slider

Figure 10 Slider structure

Mold adopts a fixed mold slider to form plastic parts, and structure of slider is shown in Figure 10. Movable mold parts of first color material and second color material mold are designed to be same, two-color injection is realized by different structures of fixed mold slider and fixed mold cavity. According to characteristics of two-color injection molding machine, movable mold needs to be rotated in the middle of injection molding machine, that is, when injection molding of first color material is completed, middle turntable of injection molding machine drives movable mold to rotate 180°, and then second color material injection molding is performed. Structure is shown in Figure 11.

 

(A) Overall structure

 

(B) First color PP T20 material mold structure

 

(C) Second color TPE-S material mold structure

Figure 11 Mould structure

 

Figure 12 Plastic parts launch

After mold is installed on injection molding machine and debugged, first color PPT20 material injection molding is performed. After injection of first color PPT20 material is completed, injection molding machine opens mold for the first time. While mold is opened, 8 fixed mold sliders (see Figure 10) perform mold opening and core pulling actions under combined action of spring and hook. After mold is fully opened, middle rotating plate of injection molding machine drives movable mold of first color material and second color material to rotate 180°. During this process, plastic part rotates with first color movable mold, then mold is closed again for injection molding of second color material. After second injection molding is completed, mold is opened again, fixed-mold slide block performs mold-opening and core-pulling under combined action of spring and hook. Since fixed mold sliders 2, 3, 6 and 10 of second color material are large in size, hydraulic cylinders are designed on four fixed mold sliders to provide power. After mold is fully opened, ejection hydraulic cylinder of second color material mold starts to push out, driving second color material mold push rod fixing plate, push out plastic parts and aggregate through push rod, as shown in Figure 12, completing a cycle Injections, and so on.

 

Figure 13 Prototype of first debugging of mold

During first trial of mold, slider moves normally, movable mold rotates smoothly, and plastic part is pushed out smoothly. Except for slight flashing of second color soft rubber, plastic parts are good. Sample part at the first debugging is shown in Figure 13.

 

Figure 14 Difference between mold structure of first color material and second color material when penetration angle is small
When designing second color material injection, sealant design of second color material needs to be considered, and bump sealant is preferred. If structure of plastic part needs to be designed to wipe through sealant, design of wipe through angle should be greater than 7°. Ejection angle of  first color compound in E area of mold is 7°, so there is no need to design fixed mold slider structure in first color material mold. Ejection angle of second color compound in E area is only 2°, so fixed mold slider structure is designed in second color material mold to prevent mold from scratching plastic part when second color material is formed, as shown in E in Figure 14.

 

Figure 15 Reserved position of second color material slider
When designing second color soft rubber slider, it is necessary to reserve a width of 5mm and a height of 0.1mm on the slider and plastic part sealing position to ensure that second color material does not produce flash during injection molding, as shown in Figure 15. Show.

 

Figure 16 Structure of first color material fixed mold and second color material fixed mold slider structure
Manufacturing cost of mold needs to be considered during mold design, and mold structure should be simplified as much as possible. In first color material mold, there is no need to design fixed mold slider, fixed mold cavity plate and movable mold core are directly used for molding. Fixed mold slider is designed in second color material mold according to structure of plastic part to simplify overall mold design, reduce cost of mold manufacturing, as shown in Figure 16.

 

Figure 17 End of second color material flow

Second color molding material is TPE-S, which has good fluidity, but is prone to flash. When setting gate of second color TPE-S material, use multi-point gate for injection to shorten flow length ratio of material, flow length ratio cannot exceed 150. End of plastic material flow is shown in Figure 17.