Air valve structure used in traditional pneumatic ejection plastic parts is a cone-shaped valve of 60° to 120°. Process of ejecting plastic part by injection mold is: after mold is opened, air valve is opened, compressed air pushes valve open, and at the same time spring is compressed , high-pressure air enters gap between core and plastic part to force plastic part to be pushed out. After compressed air is closed, valve is reset under action of spring. This structure has disadvantages of troublesome disassembly and assembly with high manufacturing cost. Since air valve is completely reset by spring, spring often fails in fatigue during production process and needs to be replaced, reducing production efficiency of plastic parts.
Pneumatic pushing is often used to push out large, deep cavity, thin wall or soft plastic parts. Due to large undercut inside tire of toy car, there are many rubber columns that regularly protrude on outer circumferential surface, general push rod can not be used to demold. Direction of rubber columns arranged regularly on outer surface of toy car tire is radial, size of each rubber column is divided into two sections, and root is one level larger. Advantage of using pneumatic structure is that root of plastic part will not break when it is forcedly pushed out. Now referring to relevant design information and design method of injection mold, injection mold for toy car tire based on pneumatics is designed.

 

Figure 1 Plastic parts structure

 

Figure 2 Three-dimensional structure

Figure 1 shows a tire of a toy car. Material is PVC60°. There are regularly arranged rubber columns on outer surface of tire and they are radial. According to structure of plastic parts and characteristics of materials used, pneumatic demoulding is used to force demoulding during demoulding. Three-dimensional structure of plastic parts is shown in Figure 2.

 

Figure 3 Arrangement of cavity

 

Figure 4 Core structure

Toy car tires are produced in long-term mass production. In order to reduce production cost of plastic parts and improve economics of production, mold adopts a 4 cavity structure. Cavity layout design requires that total injection pressure be distributed to each cavity to ensure that melt in molten state is evenly filled in mold cavity at the same time to obtain 4 plastic parts with uniform internal quality and consistent appearance. Layout of cavity is shown in Figure 3, and structure of core is shown in Figure 4.

Since both ends of plastic part are through holes, it is not possible to seal gas, upper end surface of small through hole in the middle of plastic part is sealed with condensate during injection, so that plastic part is forcedly demolded from core by pneumatic force. Mold has a structure of 4 cavities, and each cavity must be injected in the center, adopting form of a ring point gate. Large end diameter of main runner of gating system is φ 7.54mm, nozzle diameter of SZY-300 injection molding machine is φ 4mm, small end diameter of main runner is larger than nozzle diameter of injection molding machine (0.5~1) mm, diameter of small end of main runner is designed to be φ 4. 5mm, taper of main runner is generally 2°~6°.
After plastic parts are demolded, ring-shaped condensate used for air sealing on plastic parts needs to be cut off with a special jig. Cutting method used in production is to place formed toy car tire in a special fixture, punch enters from larger end of middle hole of plastic part, and condensate at small end hole is cut off. When designing gating system, position of ring gate must be set at the end of plastic part with a smaller aperture, otherwise aggregate on plastic part will not be cut off on special fixture.
In order to be able to use the materials with good performance to manufacture mold parts and heat treatment later, bushing can be designed to be detachable, high-temperature and friction-resistant materials are used to process mainstream bushing. T8A and T10A are commonly used as materials for processing mainstream bushings during processing. Hardness of mold parts after heat treatment is 53~57HRC.

 

Figure 5 Parting surface

Parting surface is related to overall structural form of mold and manufacturing process. Whether parting surface design is reasonable will affect production and manufacturing of mold, demolding, forming quality of plastic parts. It can be seen that rational design of parting surface is an important part of mold design. In order to facilitate injection molding, demoulding of plastic parts and processing of mold cavity, parting surface is designed at the largest shape in the middle of outer circumference of plastic part, such as Figure 5 shows.

When injection mold cavity is continuously filled with plastic melt, air in gating system and cavity, gas generated when plastic is heated and melted must be exhausted in time to prevent molded plastic parts from defects such as insufficient filling due to gas in cavity, welding marks on the surface, or high temperature caused by pressure of gas, causing plastic parts to scorch. Injection mold structure can use parting surface exhaust and valve exhaust. Through study of mold cavity, matching gap between parting surface and mold plate (gap value is 0.03mm) is selected for exhaust.

 

Figure 6 mold structure

1. Sprue bush 2. Screw 3. Fixed mold seat plate 4. Pressure plate 5. Screw 6. Push runner plate 7. Fixed template 8. Nylon plug 9. Screw 10. Moving template 11. Support plate 12. Pad 13 .Push rod fixing plate 14. Push plate 15. Limit nail 16. Limit post 17. Retaining ring 18. Air blocking rod 19. Screw plug 20. Push rod 21. Sealing ring 22. Movable core 23. Moving mold insert 24. Fixed mold insert 25. Seal ring 26. Fixed distance rod 27. Washer 28. Screw 29. Spring 30. Fixed distance rod 31. Seal ring 32. Guide post 33. Guide sleeve 34. Guide sleeve 35. Guide sleeve 36. Pull rod
Structure of mold is shown in Figure 6. Material used for forming plastic parts is PVC60°, which is a heat-sensitive plastic. It will emit corrosive gas when melted, cause corrosion to mold parts. Therefore, molded parts must be made of corrosion-resistant steel. The biggest feature of mold is that driven mold insert, fixed mold insert, and core are forced to push out plastic part three times. Working process of mold is as follows:
(1) Injection mold filling. Melted PVC 60° melt in the barrel of injection molding machine flows through main runner in sprue bushing 1 through sub runner, and quickly enters cavity through ring gate.
(2) Cool and solidify. After PVC60° melt fills cavity, it is kept under pressure, cooled and solidified.
(3) Open mold. After molded toy car tire is kept under pressure, cooled and solidified, slider of injection molding machine drives movable mold to open mold. Under action of spring 29, nylon plug 8, and screw 9, push channel plate 6 and fixed plate 7 are opened, condensate is pulled off. Under action of fixed-distance tie rod 30, push channel plate 6 and fixed mold seat plate 3 are opened again, condensate automatically falls off. Under action of fixed distance tie rod 26, fixed mold plate and movable mold plate are opened at an opening distance of 65 mm, and plastic part is forced out from fixed mold insert 24. After mold is fully opened, ejector of injection molding machine pushes push plate 14 and pushes movable core 22 through push rod 20. At this time, plastic part is forcibly pushed out from movable mold insert 23. In this process, push rod 20 pushes upward, gas blocking rod 18 does not move, gas blocking rod 18 moves backward relative to push rod 20, and detaches from air blocking hole of movable core 22, air blocking hole becomes a through hole. Safety door is opened by operator, gas valve is manually opened. Compressed gas enters cavity of push rod from push plate channel, and then enters gap between movable core 22 and plastic part. Finally, compressed gas forces plastic part away from movable core 22.
(4) Mold clamping. After plastic part is demolded, injection mold is closed and next injection molding is started.