Bracket used to adjust height of headrest on the seat of a certain model is shown in Figure 1. External dimension is 35 mm*37.5 mm*121.3 mm, which is a slender plastic part. Material is PP, shrinkage rate is 1.6%, surface of cylinder has a snap for assembly, and end face of bracket has a skin pattern.

 

Figure 1 Plastic part structure
Bracket forming mold is a 4-cavity structure. Relevant parameters are: tie rod spacing is 430 mm * 430 mm, mold height is 160-450 mm, maximum mold opening distance is 800 mm, and push-out stroke is 350 mm. Mold parts are made of pre-hardened steel, heat treatment hardness is 30~33 HRC, hardness is uniform, it has mirror polishing and excellent machinability, etching processability and weldability.

Choice of parting surface is the key to mold design, which is usually affected by shape of plastic parts, appearance requirements, dimensional accuracy, number of mold cavities and other factors. In order to make designed mold have a variety of options in manufacturing, be easy to process, reduce manufacturing cost, feasibility analysis of plastic part data should be carried out before design, design for manufacture (DFM) report can be produced to make designed mold more suitable for processing and manufacturing requirements. In order to reduce follow-up problems, parting surface is a very important part of DFM report, subsequent mold design can only be carried out after obtaining customer's approval. Parting surface design is shown in Figure 2.

 

Figure 2 Parting surface design

In a multi-cavity mold, when runners are arranged, it is required that melt can enter cavity through runners at the same time, that is, distance from main runner to runners of each cavity is same. Mold is in the form of half-sliding Huff sliders, adopts a balanced arrangement of S-shaped runners, as shown in Figure 3. Cross-section of runner 1 is trapezoidal, which is arranged on fixed die; cross-section of runner 2 is circular, which are respectively arranged on two sliders.

 

Figure 3 Layout of shunt channel

Ejection of plastic parts is carried out by a push plate. Closed height of mold is 375 mm, which is less than maximum spacing of injection molding machine of 450 mm, which meets installation requirements of injection molding machine. Within mold opening stroke of injection molding machine, removal space requirements of manipulator should be met. In order to ensure precision of plastic parts, cylinder part is left on the side of movable mold, outer wall of cylinder is formed by Huff slider, inner hole of cylinder is formed by movable mold core, slider is set on push plate.
Seat headrest bracket is a thin and long deep cavity thin-walled plastic part. Push plate push has advantages of large and uniform push force, smooth movement, and no push out traces. Push plate design is shown in Figure 4.

 

Figure 4 Push plate design
1. Clamping buckle 2. Return spring 3. Spring guide rod 4. Push plate insert 5. Push plate insert
Push plate is inlaid and assembled. Push plate and mold base are made of same material as S50C. Slider inserts are made of pre-hardened steel PX4, heat treatment hardness is 30~33 HRC. Due to friction with push plate when slider moves, push plate insert 4 is made of pre-hardened steel 1.2738, heat treatment hardness is 28~32 HRC. In order to facilitate processing and maintenance, a push plate insert 5 is inserted into push plate insert 4, and material is same as 1.2738. For mold push plate with relatively high precision, 1.2738 can be used as a whole. Material of movable die insert is pre-hardened steel DH2F, and heat treatment hardness is 37~41 HRC.
In order to reduce friction between push plate and core insert during demoulding process, a gap of 0.2 mm should be left between push plate and core (see Figure 4). There is a clearance fit between push plate and moving model core. In order to prevent mold parts from being pulled, taper fit angle is 5°, an exhaust slot is set at I and leads to outside of mold. Taper fit can also play a role in assisting positioning. It can prevent eccentricity of moving die inserts from causing flashing. Set angle of push plate insert to 5°, angle of moving mold core insert to 4.83°, as shown in Figure 5, which can not only reduce friction between push plate and core, but also prevent occurrence of flash. At the same time, it plays role of exhaust and prolongs service life of mold.

 

Figure 5 Push plate angle setting
In order to prevent moving model core inserts from skewing, a step of more than 5 mm should be set at Ⅱ. When mold is closed, let push plate press moving model core inserts, at the same time, push plate and moving mold base plate are positioned by positioning block. In order to make push plate move smoothly, push plate guide sleeve adopts self-lubricating form, push plate needs to be equipped with a water circuit to strengthen cooling, so as to prevent sticking phenomenon due to excessive mold temperature during mass production.

Round hole at head of bracket is formed by fixed mold insert 1. Since movable mold insert for forming inner hole wall of bracket is thin and long, in order to avoid its inclination during molding, a tapered positioning is added to parting surface. Square hole of bracket head is formed by fixed mold insert 2, and hollow part of head is formed by slider insert. In order to enhance heat dissipation here, material of slider insert is Be-Cu. It can be seen from structure of plastic parts that fixed mold insert 1 needs to be inserted into movable mold insert through slider insert, and fixed mold insert 2 needs to pass through slider insert, as shown in FIG. 6 .

 

Figure 6 Formed Parts
Mold structure is shown in Figure 7. In order to ensure reliability of mold action, a sequential mold parting assembly is required. Working process of mold: when mold is opened, under action of mold clamp assembly 19 and polyurethane spring 24, fixed mold seat plate 20 and fixed mold plate 2 realize the first parting, fixed mold inserts 28 and 29 are separated from slider insert 1; mold continues to open mold, buckle of clamping device assembly 19 is released from function of clamping buckle 26, second parting is realized between fixed plate 2 and slider 4, under action of inclined guide column 3, sliding Block 4 is separated to complete core-pulling action; mold continues to open the mold. Under action of fixed-distance pulling plate, push plate 6 and movable mold seat plate 5 are separated to realize third parting and push out plastic parts, then manipulator takes out plastic parts, and mold opening is completed.. When mold is closed, push plate 6 is reset first under action of reset spring 11. Under action of clamping device 19, inclined guide column 3 drives slider 4 to move forward, fixed plate 2 and slider 4 are locked. Mold continues to be clamped, fixed mold insert 28 is inserted into movable mold insert 8 through slider insert 1, fixed mold insert 29 is inserted into slider insert 7 through slider insert 1, finally fixed mold seat plate 20 and fixed mold plate 2 are locked to complete mold closing action.

 

Figure 7 Mold structure
1. Slider inserts 2. Fixed mold plate 3. Inclined guide post 4. Slider 5. Moving die base plate 6. Push plate 7. Slider inserts 8. Moving die inserts 9. Slider positioning beads 10. Protection Foot 11. Return spring 12. Spring guide rod 13. Push plate insert 14. Push plate insert 15. Slider guide 16. Slider positioning block 17. Positioning block 18. Pull rod guide post 19. Clamp assembly 20. Fixed die seat plate 21. Guide sleeve 22. Slider pressure plate 23. Positioning block 24. Polyurethane spring 25. Inclined guide post fixing block 26. Mold locking buckle 27. Fixed die insert 28. Fixed die insert 29. Fixed die insert piece
Clamping device is one of key parts of mold, which can be customized. Material is SKD61, heat treated to 50~55 HRC, cut hook pin to required length, then process screw holes and positioning pin holes, and set 2 sets of components in the middle of mold.