[Abstract] Through analysis of structural characteristics of hollow bracket, a reasonable mold structure plan was designed. Bracket is a hollow structure, mold adopts a lateral parting and core pulling structure, mechanism is composed of 12 sliding blocks, of which 8 sliding blocks are diagonal sliding blocks. Mold structure design is reasonable, which has a certain reference effect for mold design of this type of plastic part.

Plastic part shown in Figure 1 is a hollow bracket with 6 assembly holes on 6 corners and hollow structure on 8 sides. The overall dimensions of plastic part are: 30.4*30.4*37.6mm, material is PC, and color is black. From performance analysis, plastic part must have a certain comprehensive mechanical strength.

 

Figure 1 Plastic parts of hollow bracket

According to analysis of structure and molding process of plastic part, mold structure of 1 cavity is selected, 160t injection molding machine, S136 is selected for cavity and core material, heat treatment hardness is 50~52HRC.

Due to special structure of plastic part, main demoulding method adopts lateral parting and core pulling mechanism. 4 fixed die oblique drawing sliders, 4 movable die oblique drawing sliders, and 4 movable die assembly hole sliders are designed. Parting line is shown in Figure 2.

Since there is a hole in the center of plastic part, glue cannot be fed in the center. A platform is designed to feed glue at the top of plastic part. Way of feeding glue is shown in Figure 3. The entire length of gate material is 112mm. To ensure that gate material can be released smoothly, limit of fixed-distance tie rod is set to 130mm.

Plastic part is a hollow structure, and 12 sliders are designed, including: 4 fixed die oblique drawing sliders, 4 movable die oblique drawing sliders, and 4 movable die assembly hole sliders.

 

Figure 2 Parting line selection
a — —Fixed mold parting line b — —Moving mold parting line

 

Figure 3 Gating system

In order to ensure that plastic part can be demolded from fixed mold, fixed mold needs to be designed with a core-pulling mechanism. Because inner bevel angle of plastic part is not horizontal, movement of slider cannot be designed to be horizontal, and it must be designed to meet its demolding angle, as shown in the figure 4.

 

Figure 4 Fixed mold oblique drawing slider
Moving direction angle of fixed-mold oblique sliding slider=32.3°+6°/2=35.3° (see Figure 5), and core pulling distance> 5.41+3~5mm (see Figure 6). Oblique wedge of fixed mold slider is fixed on fixed mold base plate. During mold opening process, 112mm long gate material needs to be removed. Oblique wedge will be separated from slider. In design process, to ensure that slider can open and close mold smoothly, it is necessary to pay attention to design of positioning of slider. Positioning of slider is designed as a positioning ball, as shown in Figure 7.

 

Figure 5 Movement direction of oblique slide block Figure 6 Core pulling distance

 

Figure 7 Moving and fixed mold oblique drawing slider
a — —Moving and fixed mold oblique drawing slide block assembly drawing b — —Fixed mold slide core drawing distance calculation c — —Moving mold oblique drawing core drawing distance calculation
In order to ensure that oblique wedge can be pulled out of slider smoothly, angle of mating position between oblique wedge and slider must also be designed to be a more appropriate angle. As shown in Figure 7, angle of slider is designed to be 75°. After mold is opened, fixed mold wedge moves 32.6mm to drive slider to pull core 9mm in the direction of movement and then separate from slider.

Plastic part has a symmetrical style, design of movable mold oblique sliding slider is basically same as that of fixed mold slider, as shown in Figure 7. Since movable die slide wedge does not need to be separated from movable die slider, in order to meet ejection distance and reduce stress, matching angle of T-slot of tilt wedge and slider is perpendicular to slider movement angle of 54.7°. With function of limit screw on stripper plate, stripper plate drives movable mold wedge to move 18mm and then stops. At this time, core pulling distance in moving direction of slider is 10.4mm, which meets demolding requirements of plastic part.

Since accuracy of mounting hole of bracket needs to be ensured, sliding block adopts style of mounting pin and structure of insertion, as shown in Fig. 8, which is convenient for adjustment and replacement.

 

Figure 8 Movable mold assembly hole slider

Assembly holes at upper and lower ends are ejected using cylinder structure. As shown in Figure 9, cylinder jack does not directly participate in forming, and plastic part is ejected after sliding block slides.

 

Figure 9 Cylinder is ejected

According to parting line of plastic part, it can be seen that most of plastic part are on slider, fixed mold cavity has a small amount of glue position, movable mold core has no glue position, as shown in Figure 10. Mold core of this mold mainly plays role of sliding block guide and positioning, it is necessary to consider details of processing, difficulty of processing and assembly during design.

According to layout of cavity and size of each other, structure of mold base is DAI-3535-A100-B110-C120-L420. Due to actual production requirements, some adjustments were made to mold structure.

 

Figure 10 Fixed and moving model core and cavity
a — fixed model cavity b — moving model core

Mold structure is shown in Figure 9.

 

Figure 9 Mould structure diagram
1. Positioning ring 2. Sprue sleeve 3. Limit screw 4. Big tie rod 5. Fixed distance tie rod 6. Fixed mold inclined wedge 7. Fixed mold cavity 8. Fixed mold slider 9. Limit block Ⅰ 10. Rubber plug 11. Moving die slider 12. Limiting spring block 13. Moving die wedge 14. Limiting block Ⅱ 15. Division tube 16. Fixed die seat plate 17. Unloading plate 18. Fixed template 19. Locking block 20. Reset rod pull block 21. Assembly hole slider 22. Slider insert 23. Movable template 24. Take off template 25. Ejector fixed plate 26. Ejector backing plate 27. Square iron 28. Movable die seat plate

Due to multiple slide structure of this mold, in order to ensure normal opening and closing of mold, it needs to be divided multiple times, as shown in Figure 10.
The first parting: when injection molding machine is opening mold, there is a rubber plug between movable and fixed mold plate, stripping plate and fixed mold plate are only connected by guide post. At this time, under action of pulling force, first parting surface Ⅰ -I is separated first, plastic part and gate material are pulled apart, fixed mold plate moves with movable mold plate for a distance of 130mm. At this time, four fixed-mold oblique wedges pull fixed-mold slider to move, upper part of plastic part is ejected, then fixed-mold oblique wedge is separated from fixed-mold slider.
The second parting: gate material is pulled due to inverted sprue sleeve and does not move with fixed mold plate. When fixed mold plate moves to set distance of 130mm, it is blocked by fixed distance pull rod. Because fixed mold plate moves with movable mold plate under action of rubber stopper, fixed-distance pull rod drives unloading plate to continue to move a distance of 6mm limited by limit screw, second parting surface Ⅱ-Ⅱ is separated. At this time, gate material pulled by inverted buckle is forcibly pulled out by discharge plate, and gate material is released.

 

Figure 10 Mold opening movement
The first step: remove fixed mold slider to make gate material separate from plastic part; the second step, gate material and sprue sleeve are reversed and forcefully released, gate material is removed; the third step: remove assembly hole slider; the fourth Step: slide block of movable mold; the fifth step: plastic part is ejected from cylinder .
The third parting: After gate material is smoothly pulled out, fixed mold slider is also pulled out, movable mold continues to retreat with fixed mold plate. Because movable mold plate and stripper plate limit mechanism are limited and fixed (limit block 1, limit block 2, elastic block), when pulling force of movable template of injection molding machine is greater than locking force of rubber plug, the third parting surface Ⅲ-Ⅲ will be separated first, assembly hole slider will move backward to escape plastic part assembly hole.
The fourth parting: After third parting surface Ⅲ-Ⅲ is separated, movable mold part then moves backwards. Due to function of limit mechanism, limit block 1 is stuck in elastic block position, mold is opened 50mm between movable and fixed mold plates. When tensile force of movable mold plate of injection molding machine is greater than elastic force of spring in elastic block, limit block 2 pushes elastic block and then moves backward along with stripper plate. The fourth parting surface Ⅳ-Ⅳ separates, stripper plate moves backward to a limited distance of 18mm, movable mold oblique wedge pulls movable mold slider to move, and plastic part is released. Due to action of reset rod pull block, relationship between ejector pin fixing plate, ejector backing plate, cylinder and movable template does not change. Stripper plate, square iron, movable mold seat plate, and cylinder needle move backward under pulling force of injection molding machine. At this time, when limit block Ⅱ pushes projectile block until projectile sinks into movable mold plate, limitation of limit block I and projectile block is released, movable and fixed mold plate is completely separated. Finally, ejector pin on injection molding machine pushes ejector plate upward by 30mm, cylinder pushes out plastic part.
When mold is closed, ejector pin fixing plate and ejector pad plate are actuated by compression spring to drive cylinder to reset; stripper plate is closed with movable mold plate, movable mold slider is reset; then movable mold plate pushed fixed mold plate and stripping plate to reset. Fixed mold slider and assembly hole slider are reset, rubber plug is locked, and a cycle is completed.

When I just got this set of hollow bracket plastic part models, I felt that I couldn’t get started. It was fundamentally different in structure from plastic parts that I had touched before. The other molds designed were mainly formed in a dynamic and fixed cavity. This mold mainly relies on sliders to form. With many sliders, it is difficult to determine a lot of data and parameters during design. It is ambiguous. After searching for data, calculations and 3D simulation motions, many modifications have been made to gradually become reasonable and perfect. In the process of designing mold, fully considering use of space and convenience of processing and assembly, 12 sliders are designed in mold in an orderly manner; in order to meet demolding movement of these sliders, some limit mechanisms need to be set up, movement of each board and part interference caused b change of the movement sequence that may occur need to be considered.
Mold has not been officially put into production due to strategic reasons, some institutions may need to be adjusted through production verification, but its special structural characteristics reflect professionalism and flexibility of mold design. Design of mold is to find out the best plan in continuous analysis, comparison and summary, then find some easily overlooked problems through production. Design of lateral parting and core-pulling mechanism, movement of parting surface of mold are all key links in design of this mold. Through detailed analysis, comparison and review by related engineering and manufacturing personnel, this mold has been basically confirmed to be ready for production.