Outer structure and size of shell are shown in Figure 1. Shell material is 45% PC+ABS, and shrinkage rate is 0.5%~0.7%. There are 5 grooves and 4 bosses on shell, which require mass production. These factors affect and determine structure of injection mold. Shell molding process is analyzed as follows.

 

Figure 1 Shell shape and size
Note:  - boss;  - groove;  - plastic part profile has appearance requirements.
Dimensions of five grooves are: 15.2 mm*1 mm*20.48°*24.6 mm, 21 mm*21.1 mm*13.8 mm*21.6 mm*(20-13.7+1.3) mm, 6 mm*8 mm*6 mm *1 mm*(2*R0.5) mm*(2*R1) mm, 7 mm*4 mm*R1 mm, 4.5 mm*2.1 mm*R7 mm; dimensions of 4 bosses are: 0.8 mm* 2.5 mm*(2.1-0.9) mm*35.22°, 1 mm*(20+1) mm*18 mm, 1 mm*5.5 mm, 1.3 mm*0.9 mm*21.4°*13.7 mm*(21.6-18.1) mm ; No mold release marks are allowed on outer surface of shell.

According to analysis of shell molding process, two placement methods can be adopted when forming shell, the first is vertical placement, and the second is horizontal placement. Analysis of two injection mold structure schemes is as follows.
(1) Option 1: Parting surface Ⅰ-Ⅰ is shown in Figure 2(a), grooves ①, ②, ⑤ are formed by moving mold core inserts, grooves ③, ④ and bosses ① can be formed by internal core pulling mechanism of inclined push rod. Shell demoulding can first rely on push rods to push stripper plate to push shell and casting system condensate out of moving mold core. Since bottom of shell is a stepped surface, it will still stay in cavity of stripper plate, a secondary demolding mechanism is required to ensure that shell has no demoulding marks and deformation.
(2) Option 2: As shown in Figure 2(b), parting surface I-I is set at tangent point between R0.3 mm and R arc, which can avoid blocking of shell demoulding at arc height, but there are demoulding marks at parting surface I-I. Grooves ①, ②, and ⑤ are formed and core-pulled by external core-pulling mechanism of inclined guide column slider. Boss ① needs to be formed by machining grooves with corresponding shapes at moving mold core, while grooves ③ and ④ need to be formed by machining corresponding bosses or inserts at moving mold core. Casing can be demolded with a push rod. Since wall thickness of casing is only 1.6 mm, push rod will leave marks on casing and deform casing. Push rods and stripper plates can also be used for demoulding, but cores that form grooves and bosses must be installed on stripper plate. Although guide posts and guide sleeves are installed between stripper plate and movable platen, there is a gap between guide posts and guide sleeves, which will cause displacement of cores of forming grooves and bosses, resulting in out of tolerance. Core-pulling mechanism of core-pulling mechanism exceeds 65 mm, and core is a cantilever. Existence of gaps will cause wall thickness to not meet requirements when shell is formed. Since core-pulling distance exceeds 45 mm, in order to ensure rigidity of inclined guide column, a large mold base must be selected, and a hydraulic cylinder core-pulling mechanism must be used. So option 2 is not feasible.
After analysis, scheme 1 can ensure appearance requirements of forming shell and mold structure is simple, so scheme 1 is adopted to form casing. In order to improve processing efficiency of injection mold, mold adopts a structure of 1 mold and 4 cavities.

 

Figure 2 Shell injection mold scheme
Note:  - external core pulling of inclined guide column slider;  - inner core pulling of inclined push rod;  - machining groove on moving mold core;  - machining boss on moving mold core or adopting insert structure;  - represents secondary demoulding mechanism.

Injection mold structure is shown in Figure 3.

 

Figure 3 Injection mold structure
1. Moving mold base 2. Moving mold core 3. Sprue sleeve 4. Fixed mold core 5. Fixed mold base 6. Fixed mold base seat plate 7. Positioning ring 8. Screw 9. Screw 10. Push rod 11. Screw 12. Spacer 13. Push rod fixing plate 14. Push plate 15. Moving mold seat plate 16. Moving mold backing plate 17. Spring 18. Reset rod 19. Guide post 20. Guide sleeve 21. Moving mold core 22. Housing 23. Step screw 24. Push tube 25. Cylindrical pin 26. Push rod 27. Spring 28. Inclined guide column 29. Slider 30. Screw plug 31. O-ring 32. Cooling water joint 33. Cylindrical pin 34. Push plate 35. Step Screw 36. Oblique push rod 37. Step screw 38. T-slot slider 39. Screw plug 40. O-ring seal 41. Cooling water joint 42. Cylindrical pin
(1) Design of reset mechanism. Reset mechanism is composed of a push rod fixing plate 13 , a push plate 14 , a reset rod 18 and a spring 17. After ejector rod of injection molding machine is retracted, initial reset is performed by spring 17, and then reset rod 18 , push rod fixing plate 13 , push plate 14 and slider 29 , push plate 34 and step screws 23 , 35 and 37 are pushed by fixed plate 5 to reset accurately when fixed mold and movable mold are closed. Function of mechanism is to realize mold reset after shell and gating system condensate are demolded, and its structural arrangement is conducive to automatic cycle of injection processing.
(2) Design of gating system. Gating system is composed of main runner in gate sleeve 3, movable mold core 2, fixed mold core 4 runners and point gates. Plastic melt enters cavity through main runner, runner and point gate, fills it, then forms a shell after cooling. Trace of point gate is small, which meets requirements of shell appearance.
(3) Design of cooling system. During injection molding process of melt, heat of melt is transferred to mold parts. As molding continues, heat of mold parts continues to rise, causing plastic to become overheated and brittle, so a cooling system needs to be set up to reduce mold parts. Temperature, cooling system can be divided into fixed mold cooling system and movable mold cooling system.
1) Fixed mold cooling system design. Cooling water channel is machined in fixed mold core 4 and fixed mold plate 5, groove for installing O-ring seal is machined at junction of fixed mold core 4 and fixed mold plate 5, and pipe threaded hole is machined at the end of cooling water channel to install screw plug 39. Cooling water joint 41 is installed at the end of cooling water channel of fixed mold plate 5 by machining pipe threaded hole.
2) Design of cooling system of moving mold. In same way, cooling water channels and pipe threaded holes are processed in movable mold plate 1 and movable mold core 2, and screw plugs 30, O-rings 31 and cooling water joints 32 are installed. It flows out from water outlet of cooling water joint 32 through cooling water channel to take away heat of mold parts.
(4) Design of moving and fixed cores. Due to influence of thermal expansion and contraction of plastic, design size of fixed mold core and movable mold core should be size of shell + size of shell * average shrinkage rate of 0.6%. In addition, dimension of profile with demolding direction should be set with a demolding slope of 1.5°.

In order to avoid demoulding marks on molded shell, a stripper plate is used for demoulding. When mold is opened, lower end of shell stays in cavity of stripper plate, so it is necessary to use a push tube for secondary demoulding. .

(1). Moving and fixed molds are closed. As shown in Figure 4(a), movable and fixed molds are closed, after core-pulling mechanism and demolding mechanism in inclined push rod are reset, a cavity is formed between moving mold core 3, mold core 5, fixed mold core 6 and inclined push rod, plastic melt enters cavity through gating system and cools molding shell 9 .

 

Figure 4 Secondary demoulding state of shell

(a) Closed state of movable and fixed molds (b) The first demoulding state of fixed mold and shell (c) The second demoulding state of shell;

1. Moving mold plate 2. Moving mold plate 3. Moving mold core 4. Stripping plate 5. Core 6. Fixed mold core 7. Fixed mold plate 8. Fixed mold plate 9. Shell 10. Step screw 11. Push Tube 12. Cylindrical pin 13. Push rod 14. Spring 15. Inclined guide column 16. Slider 17. Moving die base plate 18. Push plate 19. Push rod fixing plate 20. Pad
(2) The first demoulding of fixed mold and shell. As shown in Figure 4(b), when fixed mold part is opened, ejector rod of injection molding machine pushes push plate 18, push rod fixing plate 19, step screw 10 and stripper plate 4, and stripper plate 4 separates shell 9 from movable mold core 3 . At this time, push rod 13 starts to contact slope of slider 16 .
(3) Shell is demolded for second time. As shown in Figure 4(c), ejector rod of injection molding machine continues to push push plate 18, push rod fixing plate 19, step screw 10 and stripper plate 4, stripper plate 4 pushes shell 9 and gating system condensate away from movable mold core 3 again. But at this time, push rod 13 pushes push tube 11 along inclined surface of slider 16, slider 16 slides in dovetail groove of mounting plate, pushes shell 9 out of cavity of stripper plate 4 to realize secondary demoulding of shell 9, condensate of gating system is also separated from the runner of stripper plate 4. When ejector rod of injection molding machine is retracted, ejection mechanism is reset under action of reset mechanism. Since push rod 13 and push tube 11 are connected by cylindrical pin 12, under action of spring 14 on push rod 13, push tube 11 is also reset.
Shell 9 is demolded for the first time through stripper plate 4, and second demoulding is realized through push tube 11, which not only ensures smooth demoulding of shell 9, but also ensures that its appearance has no traces of gate and demoulding. .

Since there are bosses and grooves on the sides of vertically placed plastic parts to be formed, cores for forming side bosses and grooves will hinder demoulding of shell 9. Core of forming side boss and groove adopts movement of core pulling and demolding in inclined push rod, which can eliminate blocking effect of core of boss and groove on demoulding..
As shown in Figure 4, working process of inner core pulling and demolding mechanism of inclined push rod for forming side boss of casing and groove core is as follows: injection molding machine push rod pushes push plate 18, push rod fixing plate 19, step screw 10 and stripper plate 4, while stripper plate 4 pushes shell 9 and gating system condensate to achieve the first demoulding, core of forming boss and groove on inclined push rod 36 (see Figure 3) is restricted by inclined groove of movable mold plate 2, and upper end of inclined push rod 36 performs demoulding movement on the one hand. On the other hand, core pulling force is generated, and lower end of inclined push rod 36 can slide in groove of T-shaped groove slider 38 (see FIG. 3 ). Second demoulding of mold enables shell 9 to be released from cavity of core 5 and stripper 4 smoothly.