Structure of plastic part is shown in Figure 1. It is annular as a whole, with a protruding handle on its side, and a circular protrusion (diameter φ47 mm, height 14 mm) at the center of upper surface of plastic part. There is a flat hole with a length of 14 mm next to protrusion. There are two inner and outer ring structures on inner surface of plastic part, of which outer ring has a diameter of φ132 mm and a height of 12 mm, inner ring has a diameter of φ114 mm and a height of 30 mm. There is a circle of ring buckles on outer cylindrical surface of inner ring.

 

Figure 1 Plastic part structure
Plastic part is a transparent part and belongs to appearance part. In order to prevent existence of push rod marks on plastic part, it cannot be pushed out with a push rod, and can only be pushed out with a push plate. Since there is a ring buckle on outer cylindrical surface of inner ring, a half slider structure is required for demolding. Inner ring core and core between inner and outer rings must be separated. Inner ring core is fixed on moving mold plate, core between inner and outer rings is located on push plate. If a one-time ejection mechanism is used, push plate pushes plastic part out of moving mold core, molded plastic part cannot be demolded from core between inner and outer rings. If no ejection mechanism is set on inner ring, and only push block is set at the edge of outer ring, ejection force is concentrated on the edge of outer ring, and clamping force of inner ring is large, which can easily damage plastic part during ejection. In order to solve above problem, a two-time ejection mold structure is adopted.

Main parting surface is designed at mouth of outer ring of plastic part. According to structure of plastic part handle, its parting surface adopts a pillow structure, which is a curved surface obtained by stretching parting line side of handle. Parting surface is higher than main parting surface. In order to enhance positioning accuracy of movable mold and fixed mold, a concave structure for positioning is set at four corners of movable mold (corresponding position of fixed mold is a convex structure), as shown in Figure 2.

 

Figure 2 Parting surface design

According to structure of plastic part, fixed mold cavity adopts an insert structure. Size of plastic part is small. In order to improve production efficiency of mold, a 2-cavity structure is adopted. In order to facilitate design of gate, handle of plastic part to be molded is placed outward, as shown in Figure 3. Convex positioning blocks are set at four corners of fixed mold insert to improve clamping accuracy of fixed mold and movable mold. Since plastic part is a transparent part and cannot be pushed out by a push rod, air in cavity cannot be discharged from around push rod during injection. An exhaust groove is set along contour of cavity, and it is connected to the outside through a branch exhaust groove.

 

Figure 3 Fixed mold cavity insert structure

There is a ring buckle on outer cylindrical surface of inner ring of plastic part. Buckle size is 7 mm wide and 1 mm deep. Demoulding is driven by inclined guide column. Slider and slider seat adopt an integral structure to enhance strength and rigidity of slider. Buckle width is only 7 mm. In order to enhance strength of slider and prevent slider from deforming, thickness of slider is set to 21 mm. Bottom surface of slider exceeds mouth of plastic part to be molded by about 12 mm. Slider structure is shown in Figure 4.

 

Figure 4 Slider structure

Since there are two inner and outer ring structures on plastic part, it is necessary to design a core with two inner and outer ring structures. Considering that plastic part is a transparent part, in order to avoid existence of push rod mark on molded plastic part, push plate is used to push plastic part out. Therefore, mold core is divided into two parts, inner and outer parts. Core of inner ring adopts an insert structure, as shown in Figure 5 (a). Core of outer ring is set on push plate insert and is an integral part of push plate insert, as shown in Figure 5 (b). There is a flat hole on plastic part, inner and outer surfaces of hole have transition fillets. Flat hole is formed by a butt-joint structure, core of flat hole is set on both movable and fixed molds. There is a vertical groove on outer cylindrical surface of inner ring of plastic part. In order to facilitate design of demoulding mechanism of vertical groove, a vertical groove is set on inner ring core, and vertical grooves of two cores are placed facing each other.

 

Figure 5 Core structure

Distance between top of annular groove on outer cylindrical surface of inner ring of plastic part and parting surface is only 17 mm, so distance between top of Huff slider and parting surface is also only 17 mm. In order to increase strength of push plate insert, thickness of push plate insert is designed to be 60 mm. In order not to affect structure of Haff slider, middle position of push plate insert is hollowed out and set as slide groove of slider (see Figure 5 (b)).

Push plate insert shown in Figure 5 (b) can only push molded plastic part out of inner ring core, but cannot push plastic part out of core between inner and outer rings. In order to smoothly demold plastic part, 4 push blocks are evenly arranged on push plate insert along outer ring contour of each plastic part to be molded, as shown in Figure 6. Plastic part is pushed out of core between inner and outer rings through push blocks. Since push rod that fixes push block needs to pass through Haff slider, in order to prevent push rod from interfering with slider, a flat hole is set on Haff slider, which serves to reserve movement space of slider.

 

Figure 6 Push block arrangement

Push mechanism of conventional mold has only one push plate. In order to realize two pushes in this mold, it is necessary to add another push plate at the bottom of push plate, which are push plate ①, push plate ② and push rod fixing plate, install auxiliary mold opening mechanism, as shown in Figure 7 (a), where push plate ② and push rod fixing plate are fixed with screws. Auxiliary mold opening mechanism consists of 4 components, among which part I is fixed on push plate ① with screws, part II is fixed on push rod fixing plate with screws, and part III is fixed on movable mold base plate with screws. Part IV is a movable part with wedge-shaped ends, located in slide groove of part I, can move left and right. Vertical distance between part IV and part III is 35 mm, as shown in Figure 7 (b).

 

Figure 7 Push mechanism
When mold is in reset state, wedge at the end of part IV is engaged with concave buckle of part II. At this time, part III presses against the other end of part IV to prevent part IV from moving to the right. When mold is opened, ejector of the injection molding machine acts on push plate ①, driving push plate ② and push rod fixing plate to move upward. Since part Ⅱ is fixed to push rod fixing plate with screws, push rod fixing plate drives part Ⅱ to lift upward. At this time, since part Ⅳ cannot move to the right, part Ⅱ lifts part Ⅰ through part Ⅳ. Part Ⅰ is fixed to push plate ① with screws. Push plate ① will follow push plate ② and push rod fixing plate to move upward to achieve first push. When push distance reaches 35 mm, part Ⅳ reaches corner of part Ⅲ, and part Ⅳ will move to the right until it is separated from part Ⅱ. Part Ⅱ no longer drives part Ⅰ and part Ⅳ to move upward, then part Ⅰ and part Ⅳ stop moving upward. At this time, push plate ① stops moving, push plate ② and push rod fixing plate move upward together to achieve second push.

According to structure of plastic part, it is suitable to adopt a normal runner and latent gate casting system. A main runner is opened on parting surface. Cross section of main runner is circular (φ6 mm). In order to eliminate injection marks on the surface of plastic part, vertical runner is slightly extended, and two ends of main runner are slightly extended to exceed bifurcation position of latent gate. Extended part of runner acts as a cold material hole. Pouring system is shown in Figure 8 (a). In order to facilitate ejection of runner condensate, a runner insert is designed. Runner and latent gate are both opened on runner insert, as shown in Figure 8 (b). Runner insert remains stationary during ejection.

 

Figure 8 Design of pouring system

A cooling system with good cooling effect is conducive to rapid cooling and shaping of molded plastic parts, prevents deformation of plastic parts. Generally, main parts of mold need to be designed with cooling water channels. Fixed mold in this mold adopts an insert structure. Since cavity is relatively flat, cooling water channel of fixed mold adopts a straight water channel (φ8 mm), which is arranged in a rectangular shape and located at the bottom of cavity. Vertical distance from parting surface is about 30 mm, as shown in Figure 9 (a). Since inner ring core is cylindrical, it is suitable for use of a spacer-type water well water channel. Each moving mold core is equipped with 4 water wells arranged in a square. In order to maintain uniform mold temperature and facilitate processing, every 2 water wells form a group of water channels, and a total of 4 groups of water channels are set, as shown in Figure 9 (b).

 

Figure 9 Cooling system design

Mold adopts a two-plate mold structure and uses a latent gate for casting. According to structure of plastic part, a two-time ejection structure is adopted. Mold structure is shown in Figure 10.

 

Figure 10 Mold structure
(a) Sectional view along inclined guide column (b) Sectional view along plastic part handle and push block; 1. Moving mold base plate 2. Push plate ① 3. Push plate ② 4. Push rod fixed plate 5. Moving mold plate 6. Push plate 7. Half slider 8. Push plate insert 9. Inclined guide column 10. Fixed mold base plate 11. Fixed mold plate 12. Guide sleeve 13. Guide column 14. Reset rod 15. Push plate push rod 16. Pad 17. Auxiliary mold opening mechanism 18. Support column 19. Core 20. Fixed mold insert 21. Pull rod 22. Gate sleeve 23. Positioning ring 24. Runner insert 25. Push block 26. Slide strip 27. Push block push rod 28. Push plate guide pin guide sleeve
Mold working process: After injection is completed, ejector pin of injection molding machine drives movable mold of mold to move, mold is separated between fixed mold plate 11 and ejector plate 6. At the same time, inclined guide pin 9 drives half slider 7 to move, so that buckle position on outer cylindrical surface of inner ring of plastic part is demolded. When movable mold is completely separated from fixed mold, ejector pin of injection molding machine pushes on push plate 2 to drive ejection mechanism to do demolding movement. Under action of auxiliary mold opening mechanism 17, when ejection distance is 0~35 mm, push plate 2, push plate 3 and push rod fixed plate move upward together. At this time, push plate push rod 15 drives push plate 6 to move upward, and molded plastic part is ejected from core 19 to achieve the first ejection. At the same time, pull rod 21 pulls out runner condensate. When ejection distance is greater than 35 mm, under action of auxiliary mold opening mechanism 17, push plate 2 and push plate 6 stop moving, only push plate 3 and push rod fixing plate 4 move upward. At this time, push block 25 is driven upward by push block push rod 27 to push molded plastic part out of core in push plate insert 8, realizing second ejection. After taking out plastic part, injection molding machine ejector drives movable mold to start resetting, and resetting process is opposite to mold opening process. During resetting process, fixed mold plate 11 presses on the end face of reset rod 14 to reset ejection mechanism of mold (push plate 2, push plate 3 and push rod fixing plate). When mold is completely reset, next plastic part can be molded.