Figure 1 shows a car key shell plastic part with an outer dimension of 65 mm*36 mm*8 mm. After analyzing appearance characteristics and technical requirements of plastic part, there are three main technical difficulties in plastic part molding: ① Key hole wall is thin and cavity is deep. Key hole size is 2.5 mm*10 mm*56 mm. The thinnest part of inner wall thickness of plastic part is only 0.8 mm, there are sealing requirements inside and outside; ② Plastic part has a high waterproof grade requirement. Due to particularity of plastic part, upper and lower covers are assembled without screws. A circle of inverted buckle structure is designed inside plastic part (see Figure 1 (b)), which is matched with sealing ring and upper cover to achieve fixation and sealing of upper and lower covers; ③ Appearance is not allowed to have injection defects, because key hole is thin and deep, and core will be impacted by material flow. Plastic part may be distorted up and down, resulting in holes or uneven wall thickness. According to above technical difficulties, material of plastic part is ABS+PC, which has a shrinkage rate of about 0.4%~0.8%, good fluidity, and is not prone to incomplete filling. If injection pressure is increased, flash is prone to appear. In addition, material has good impact resistance, high hardness, and is not easy to wear.

 

Figure 1 Car housing key

Shell appearance surface cannot have injection defects, and mold parting surface cannot be set on positive surface of plastic part. Combined with comprehensive analysis of plastic part structure, parting surface is considered to be set on edge line of plastic part, as shown in Figure 2. Mold adopts a two-plate mold structure, hydraulic cylinder slide core is pulled, and push rod is pushed out twice.

 

Figure 2 Parting surface selection

Considering customer requirements, plastic part structure and economic benefits, it is more reasonable to arrange mold cavities in a "2+2" manner, which not only ensures symmetry of injection material flow, ensures that molten plastic enters each cavity at the same time, but also balances core pulling on both sides. As shown in Figure 3, mold adopts hydraulic cylinder core pulling, and mold frame needs to be appropriately widened to leave enough space for external hydraulic cylinder slider system. Because of secondary ejection mechanism, pad also needs to be raised by 10 mm.

 

Figure 3 Cavity layout

Appearance requirements of plastic parts are high, and material is ABS+PC. High temperature and high pressure environment during injection process will cause small core of slider core pulling to deform up and down, which will leave obvious "reflection" or welding marks on outer surface of plastic part at the least, and will cause direct hole in plastic part at the worst. Refer to mold flow analysis to determine gate position, as shown in Figure 4. A process column is designed on plastic part for latent feeding. Molten plastic flows along core-pulling small core, so that core-pulling small core is subjected to downward pressure to prevent core-pulling small core from warping upward.

 

Figure 4 Feed position

Considering that there is a 0.5 mm undercut on inner circumference of plastic part structure, as shown in Figure 5, mold adopts a forced demoulding method, but structural strength of undercut part is weak, and it is necessary to leave a deformation space for forced demoulding part. Forced demoulding part is designed as a movable insert, and a limit mechanism is designed at the bottom to allow movable part to stop after push rod is pushed out 10 mm. After forced demoulding part has a deformable space, push rod moves forward again, and internal push rod continues to push outward to realize forced demoulding of plastic part. In addition, push rod cannot be avoided at keyhole core-pulling part. Push rod position is shown in Figure 6. Slider seat is designed to press one of push rods (return rod) to prevent push rod from being damaged.

 

Plastic parts have thin walls, light weight, and high appearance requirements. Middle of movable mold is hollowed out to design movable parts, and there are many movable parts. Therefore, it is difficult for cooling water channel to pass directly through cavity plate and can only be set on periphery. Water channel diameter is designed to be φ10 mm to improve cooling effect. According to number of batch injections of mold at one time (the more times, the higher mold temperature rises), mold temperature is controlled by mold temperature controller to ensure that appearance of plastic parts has no defects such as flow marks, and at the same time, service life of movable parts is extended.

Considering output and cost, surface of fixed mold cavity is a fine texture of a specified model as required by customer. Fixed mold is selected with a heat treatment hardness of 58~62 HRC to prevent fine lines of fixed mold cavity from being worn with increase in number of injections. Movable mold needs to use a material with better wear resistance because of movable parts. Material with a hardness of 48~52 HRC can be used. Movable insert can also be made of same material as movable mold. Small core of slider forming keyhole is a key component. Its material must have a certain strength to withstand impact of high-temperature molten plastic without deformation. At the same time, small core is subjected to long-distance friction during core pulling, it must also have high wear resistance and high hardness. Based on above and based on experience, after multiple tests and verifications, white steel (high-speed steel) with strength, wear resistance and hardness is selected.

Mold structure is shown in Figure 9. Mold opening action: First, movable mold starts to move, parting surface opens, and slider starts to pull core. Since a part of slider is pressed on return rod, return rod can only be pushed out after slider slides out to its position. Injection molding machine pushes push plate, and push plate starts to push out. Since plastic part has an undercut structure, plastic part tightly embraces movable insert and is pushed out with push rod (movable insert is designed with a 10 mm movable stroke). When it is pushed out 10 mm, movable insert stops pushing out to expose undercut part of plastic part, so that plastic part has an outward deformation space for demolding. Push rod continues to push out. Under action of push rod, undercut part of plastic part deforms outward and is forced to detach from movable insert, and is pushed out with push rod until plastic part falls off, completing demolding of plastic part.

 

Figure 7 Cooling water circuit

 

Figure 8 Mold forming parts

 

1. Fixed mold base plate 2. Fixed mold sleeve plate 3. Push rod fixing plate 4. Push plate 5. Moving mold base plate 6. Screws 7. Limit screws 8. Guide column spring 9. Slider seat 10. Slider 11. Moving mold movable insert 12. Core 13. Moving mold plate 14. Fixed mold plate
Figure 9 Mold structure
After demolding, injection molding machine push rod is retracted, push plate begins to retreat under pressure of spring, and movable insert is pulled back to its original position by push rod fixed on push plate. Mold closing action: After mold is opened, push plate returns to its original position under pressure of spring, and return rod also moves into place. When mold is closed, slider starts to slide first (if return rod has not returned to its original position, it will block slider from returning to its original position to prevent slider from breaking push rod), then movable mold starts to close mold. As mold is closed, slider core-pulling small core also returns to injection position, entering next round of molding cycle.