Structure of top cover of AOS display of car audio of an electric vehicle is shown in Figure 1. It consists of two parts: shell and connector. Material is polycarbonate (PC), of which connector part is black PC and shell part is translucent PC. The overall size of plastic part is 66 mm×21 mm×14.5 mm, and average wall thickness is 0.8 mm. After injection, shell needs to be painted and lasered. When in a low-brightness environment, surface of shell is transparent through illumination of backlight, and relevant control information is clearly displayed.

 

Figure 1 Structure of top cover of AOS display of car audio
To meet use requirements, flatness of upper surface of shell needs to be controlled within 0.2 mm, and there should be no defects such as flow marks, warping, and depressions on the surface. At the same time, after AOS display top cover of car audio system is installed, indication information on translucent shell should be displayed completely and clearly. Therefore, during two-color injection molding, position accuracy of connector information display frame should be guaranteed, and defects that affect appearance quality of shell should be avoided.

Two-color injection molding uses two independent injection devices on same injection molding machine to inject in sequence, and obtain two-color or two-material plastic parts after two moldings. Generally, the first injection of two different materials in two-color injection molding is hard material, and second injection is soft material. Shell and connector in AOS display top cover of car audio system are made of same material PC, which are translucent and black materials respectively. Selection of injection sequence mainly considers flatness requirements of shell and display function requirements of indication information in display top cover.
(1) Option 1: The first injection is black PC, and second injection is translucent PC. Through injection molding simulation, it was found that when translucent PC was injected for second time, molten plastic had a higher temperature, which would cause secondary heating of molded connector; at the same time, the higher injection speed would cause scouring of molded connector surface, causing information display frame displayed on transparent shell surface to shift after assembly, affecting display effect of indication information in AOS display top cover of car audio.
(2) Option 2: Inject translucent PC for the first time and black PC for second time. Through injection molding simulation, it was found that when molten plastic of second injection entered cavity under high temperature and pressure, it first impacted molded shell. As injection pressure, injection speed and melt temperature increased, melt squeezed molded shell surface, causing surface deformation, which could not meet flatness requirements of shell surface.
By analyzing and comparing above two two-color injection schemes, problem of displacement of black connector frame caused by injection sequence of scheme one cannot be solved well; use of scheme two will cause shell surface to warp and deform, affecting flatness of upper surface of shell. Main reason is that cooling system of second injection adopts traditional linear cooling water channel, and cooling effect of connector is not ideal. As melt temperature of second injection increases, upper surface of molded shell is squeezed, causing surface to warp and deform.
Based on above analysis, two-color injection sequence of scheme two is finally adopted, but cooling water channel of connector needs to be improved during second injection to make plastic parts cool evenly, reduce deformation of shell surface, and make its flatness meet technical requirements.

In design of two-color injection mold structure, design points of two-shot injection are basically same as those of traditional single-color injection mold structure, but injection sequence selection, movable mold transposition method and gating system design need to be focused on.
P-type (parallel double-shot) injection molding machine is currently used, that is, two plasticizing devices of injection molding machine are arranged horizontally, and moving part of injection molding machine is equipped with a flat rotating turntable, which can drive movable mold part of two-color injection mold to rotate 180° in both positive and negative directions to achieve transposition of semi-finished product in two-color molding.

Shell and connector in the top cover of AOS display of car audio are both small-sized flat parts. According to design principle of parting surface, parting position is usually set at maximum contour of shape of plastic part to be molded. In two-color injection molding, semi-finished product of first injection molding needs to be transposed to second injection station before next injection. Therefore, parting surface of plastic part to be molded is required to remain consistent in two injections, and parting surface is set on common surface of the two to ensure that movable mold can fit with fixed mold parting surface after rotating 180°. Parting surface design is shown in Figure 2.

 

Figure 2 Parting surface design
Shell and connector are not large in size and have a simple structure. However, considering rotation and transposition requirements in two-color mold molding, too many cavities may cause interference in mold structure. In addition, the overall size of mold is limited by mold loading space of two-color injection molding machine and should not be too large. Therefore, mold adopts a 4-cavity layout, as shown in Figure 3.

 

Figure 3 Two-color mold cavity layout

In two-color injection molding, there are usually two sets of movable molds and two sets of fixed molds. Fixed mold part is installed on fixed mold fixing plate of two-color injection molding machine, that is, connected to injection molding machine nozzle side; movable mold part is installed on movable mold rotating plate, that is, ejection side of injection molding machine. Using two independent injection devices of injection molding machine, two colors or different materials are injected into two molds for molding. Shapes of two sets of movable molds are consistent. During injection molding, two different (or same material but different colors) molten plastics are injected into mold cavity respectively. When mold is opened after the first injection, movable mold rotating plate drives two sets of movable molds to rotate 180°. At this time, ejection mechanism of the first injection does not work, and semi-finished product remains on movable mold side; after mold is closed again, semi-finished product is rotated to cavity of second injection and is injected through another set of injection devices of injection molding machine for injection molding. After that, ejection mechanism on barrel side of second injection starts to work and ejects complete two-color plastic part.
In summary, according to design scheme of parting surface of shell and connector, in order to smoothly realize two-color molding, shape of movable mold of two injections should be consistent, while shape of fixed mold should meet molding requirements of shell and connector respectively. Therefore, mold adopts an inverted structure, that is, pouring system and ejection mechanism are set on fixed mold side.

Material of plastic part is PC of different colors, with good molding performance, good fluidity, heat resistance and strength. Since two-color mold adopts an inverted structure, runner size of pouring system is too long. If traditional ordinary runner structure is used, runner condensate will increase. During two-color molding, shell of first injection molding must be transferred to cavity of second injection with movable mold. Therefore, it is generally required that pouring system of the first injection should have function of automatically separating runner condensate from plastic part, and a latent gate or point gate structure is often used. In order to simplify mold structure, inverted mold structure adopts method of converting hot runner to ordinary runner. Two hot nozzles are set on fixed mold side of each injection, and hot nozzle is used to convert ordinary runner. U-shaped split runner and side gate are used, and pouring system adopts a balanced layout, as shown in Figure 4.

 

Figure 4 Casting system structure
1. Hot runner for the first injection 2. Hot nozzle for the first injection 3. Ordinary runner for the first injection 4. Hot runner for second injection 5. Hot nozzle for second injection 6. Ordinary runner for second injection
Since the first injection adopts side gate form, runner condensate cannot be automatically separated from shell after mold is opened, and will be transferred to cavity of second injection with shell. In order to avoid runner condensate from interfering with second injection molding, branch runner of the first injection is set in movable mold part, and gate is set on fixed mold side by overlapping, and a single-point feeding method is adopted, as shown in Figure 5 (a). At the same time, branch runner of second injection is set in fixed mold part to avoid interference between runner condensate of two injections. Second injection adopts a 6-point feeding form, as shown in Figure 5 (b). This design layout of branch runner not only solves problem of interference of the first injection runner solidification material on second injection, but also facilitates flow balance of plastic melt in mold cavity, thereby improving molding efficiency of plastic part.

 

Figure 5 Ordinary runner structure

Melting temperature of PC is generally between 260 and 300 ℃, mold molding temperature is usually between 50 and 90 ℃. Therefore, mold needs to be equipped with a cooling system to adjust temperature of molding area. During injection molding process, core is surrounded by melt with high temperature and poor thermal conductivity, and it is inconvenient to connect with outside of mold. Setting of cooling water channel is difficult, so heat dissipation of core is more critical than cavity plate.
Since translucent shell has been molded by the first injection, when black connector is molded by second injection, shell surface will be greatly deformed and cannot meet flatness requirements. Through mold flow analysis simulation, it is found that by improving cooling water channel structure of second injection, surface deformation of shell can be better controlled. A double-layer cooling water channel is set on backing plate on fixed mold side of second injection to cool hot runner; at the same time, a conformal cooling water channel is set on core insert on fixed mold side. Cross section of water channel is rectangular, which can not only improve cooling effect of connector and control deformation ofthe shell surface, but also solve structural interference problem caused by insufficient water channel space on core, as shown in Figure 6.

 

Figure 6 Structure of cooling water channel for second injection

Fixed mold structure of final designed two-color injection mold for AOS display top cover of car audio is shown in Figure 7. Inverted structure is adopted to meet molding requirements of two-color mold that movable mold structure is same but fixed mold structure is different.

 

Figure 7 Fixed mold structure
Mold trial and actual production show that mold has good production stability, surface flatness of molded display shell is good, and flatness tolerance is controlled within 0.2 mm. Actual object is shown in Figure 8. After display top cover is assembled, information display frame exposed on the surface of transparent shell is in a good position to ensure normal display of indication information.

 

Figure 8 Actual plastic part