Product consists of two different plastic parts, as shown in Figure 1 and Figure 2. Outer dimensions of plastic part I are 105 mm×109 mm×70 mm, wall thickness is 1.5 mm, surface is an irregular curved surface, and its inner surface has two φ2 mm oblique cylindrical holes with an inclination angle of 70°. There are three small holes at one end, the largest diameter is φ10 mm, and the other end is an arched notch. Outer dimensions of plastic part II are 105 mm×109 mm×68 mm, and wall thickness is 1.4 mm. Since two plastic parts form a product, materials of two plastic parts are ABS and sizes are basically same, in order to make output of two plastic parts consistent, to assemble and package two plastic parts after injection is completed, it is decided to mold two plastic parts in same mold.

 

Figure 1 Structure of plastic part I

 

Figure 2 Structure of plastic part II

Due to different structures of two plastic parts, parting surfaces must be designed separately. According to structural characteristics of two plastic parts, edge lines of mouth of plastic parts to be molded are used to create strip surfaces respectively. In order to save mold part materials, parting surface of plastic part II is set higher than that of plastic part I, as shown in Figure 3. In order to facilitate processing of mold parts, mold parts for molding two plastic parts are set as independent inserts, so there is no need to connect two parting surfaces with smooth surfaces; positioning is set on parting surfaces of two plastic parts to increase positioning accuracy of fixed mold and movable mold; in order to ensure strength of movable and fixed mold inserts, positioning protrusions of two parting surfaces are in different directions, where mold parts for molding plastic part I are positioned upward, and mold parts for molding plastic part II are positioned downward.

 

Figure 3 Parting surface design

Since structures and shapes of two plastic parts are different, and their parting surfaces have large fluctuations, in order to balance casting of two plastic parts, a three-plate mold is used, and each plastic part is set with a point gate, as shown in Figure 4 (a), where length of runner of plastic part I is 39.65 mm, and length of runner of plastic part II is 89.96 mm. After measurement, volume of plastic part I is 36 223 mm³, volume of plastic part II is 36 831 mm³, and volume of plastic part II is slightly larger than that of plastic part I. In order to balance casting of two plastic parts, gate diameter of plastic part I is set to φ7.5 mm, and gate diameter of plastic part II is set to φ8.5 mm. By adjusting gate diameter, casting of two cavities can be balanced. Gate positions of two plastic parts are designed as concave structures to facilitate trimming of gate condensate, as shown in Figure 4 (b).

 

Figure 4 Casting system design

According to structure of plastic parts, both fixed mold and movable mold are designed as insert structures. In order to facilitate processing of mold parts, independent movable and fixed mold inserts are designed to form plastic parts. A total of 4 inserts are designed, as shown in Figure 5. Positioning is set at 4 corners of each insert to increase clamping accuracy of movable and fixed molds. According to characteristics of mold structure, positioning directions of two plastic parts are different. Plastic part I is concave positioning of fixed mold and convex positioning of movable mold. Positioning direction of plastic part II is opposite.

 

Figure 5 Fixed mold and movable mold insert structure

Since edge of arched notch on reverse side of plastic part I is an arc transition, it cannot be formed by movable and fixed mold collision structure. Design is driven by an inclined guide column, as shown in Figure 6. Slider is designed as an integral structure to improve strength of slider; in order to avoid inclined guide column, slider is designed as a single-sided T-shape; a spring is set on the end face of slider, and spring assists inclined guide column to drive slider to move, ensuring smooth movement of slider.

 

Figure 6 Slider 1 structure
For three small holes of plastic part I, in order to prevent existence of slider marks on outer surface of plastic part, a fixed mold slider structure is used for demoulding, which consists of a slider seat, an insert, a pressure strip, a pressure plate, a spring and an inclined wedge, as shown in Figure 7. Inclined wedge is fixed on fixed mold seat plate, slider is set on fixed mold plate, and a spring is set on slider seat to assist inclined wedge in driving slider to move smoothly; in order to facilitate processing of mold parts, insert of slider is fixed to slider seat with a pressure plate.

 

Figure 7 Slider 2 structure

Inner surface of plastic part I has two φ2 mm cylindrical holes with an inclination angle of 70°. Due to large inclination angle, if inclined push structure is used for pushing, inclined push rod is easy to break. According to structure of plastic part, inner slider demoulding structure is designed, which consists of an inclined wedge and a slider. Inclined wedge is connected to slider through an inclined T-slot. In order to facilitate mold repair and modification, cylindrical hole core is designed as an insert, as shown in Figure 8. When mold is opened, inclined wedge remains stationary, moving mold core moves upward, slider is driven to move along axis of small hole to demold core.

 

Figure 8 Inner slider structure
In order to drive inner slider to move, a support plate is set at the bottom of moving mold plate, and an auxiliary mold opening mechanism is set at both ends of mold frame, which consists of a top block, a stopper and a limit block. Upper surface of stopper and inner surface of limit block are inclined surfaces, as shown in Figure 9. Top block is fixed on push plate, limit block is fixed on support plate, stopper is fixed on moving mold plate, and support plate is fixed on moving mold base plate with screws. When mold is in reset state, spring on the back of stopper pushes stopper out, and top block resists lower surface of stopper. When mold is pushed out, push plate, top block, and stopper drive moving mold plate, moving mold core and inner slider to move upward. Since inclined wedge is fixed on support plate and remains stationary, inner slider moves along T-slot on inclined wedge to demold. After being pushed out for a certain distance, inclined surface of limit block contacts inclined surface on stopper, driving stopper to shrink into moving mold plate. When stopper is completely retracted into moving mold plate, top block no longer presses against bottom of block, and no longer drives moving template to move upward. Inner slider mechanism stops demoulding, then push plate drives ejection mechanism of mold to do demoulding movement.

 

Figure 9 Ejection mechanism

Inner surface of plastic parts I and II each has two reinforcing ribs, with a wall thickness of about 1 mm and a height of about 37 mm. If cavity is not well vented, during injection, top of reinforcing rib will accumulate high-temperature and high-pressure gas, resulting in burning or incomplete injection. In order to overcome this phenomenon, core of molded reinforcing rib is designed as an insert, as shown in Figure 10. During injection, high-temperature and high-pressure gas on the top of reinforcing rib can be discharged through gap around insert. For cylindrical hole (φ2.0 mm×10 mm) on inner surface of plastic part, due to small inner diameter and deep depth, core is easy to break. In order to facilitate mold repair and modification, a push rod kit is used for ejection.

 

Figure 10 Small insert structure

During injection, in order to cool and shape molded plastic parts as quickly as possible and prevent deformation of plastic parts, cooling system should be designed according to plastic parts and mold structure. Fixed mold adopts a water channel that combines straight-through and water wells, as shown in Figure 11 (a). Fixed mold of plastic part I adopts a straight-through water channel with a water pipe diameter of φ8 mm. Height of plastic part is relatively large. In order to make mold temperature uniform, water channel is divided into two layers, upper and lower. Each layer is a separate circulation water channel. One group of water channels is set in the middle of plastic part, and one group of water channels is set at the top. Fixed mold of plastic part II adopts a straight-through + spacer type water well water channel. Two water wells are set with a diameter of φ20 mm and connected by a straight-through water channel. Movable mold adopts a straight-through water channel. Movable molds of plastic parts I and II are designed with separate water channels, as shown in Figure 11 (b).

 

Figure 11 Cooling system

To mold two plastic parts with different structures in one mold, point gate feeding is used to fully pour two cavities. Since it is necessary to drive inner slider of molded plastic part to demold, a support plate is set at the bottom of moving mold plate, and auxiliary mold opening mechanisms are set at both ends of mold base. Mold structure is shown in Figure 12.

 

Figure 12 Mold structure
1. Limiting column 2. Reset spring 3. Support column 4. Ejector block 5. Stopper 6. Limiting block 7. Limiting screw 8. Fixed mold plate 9. Fixed mold insert 10. Stripper plate 11. Mold foot 12. Positioning ring 13. Small insert 14. Push rod kit 15. Push rod 16. Guide sleeve 17. Guide column 18. Moving mold base plate 19. Push plate 20. Push rod fixing plate 21. Pad 22. Support plate 23. Moving mold plate 24. Slider 25. Moving mold core 26. Inclined guide column 27. Fixed mold base plate 28. Inclined wedge 29. Fixed mold insert 30. Inner slider 31. Inclined wedge 32. Fixed mold slider
Mold working process: Before injection, mold is in reset state. After injection, mold is first separated between fixed mold base plate 27 and stripper plate 10, with a separation distance of 10 mm. At the same time, inclined wedge 28 drives fixed mold slider 32 to perform demolding movement, so that three small hole cores on plastic part are demolded, and PL1 mold opening is completed. Then mold is separated between fixed mold plate 8 and movable mold plate 23, and inclined guide column 26 drives slider 24 to perform demolding movement, completing PL2 mold opening. When fixed mold and movable mold are completely separated, injection molding machine slider drives mold ejection mechanism to perform demolding movement. When ejection distance is 0~35 mm, top block 4 and stopper 5 drive movable mold plate 23 to separate from support plate 22 together. Since inclined wedge 31 is fixed on support plate 22, inclined wedge remains stationary, so that inner slider 30 performs demolding movement along T-slot on inclined wedge. When distance between movable plate 23 and support plate 22 is 35 mm, limit block 6 pushes stopper 5 back into movable plate, top block 4 disengages from stopper 5, and top block 4 no longer drives movable plate 23 to move. Movable plate 23 stops moving, and PL3 mold opening is completed. Mold ejection mechanism continues to move, and molded plastic part is ejected from movable model core 25. After plastic part is taken out, injection molding machine slider drives mold part to reset, and reset process is opposite of mold opening process. When mold is completely reset, next plastic part can be molded.