【Abstract】Process characteristics of plastic part of rear headrest cover of a certain car seat are analyzed, structure design and working process of injection mold of plastic part are introduced. Because surface of plastic part has undercut and lug structure, and there is a concave formed by a square hole passing through undercut, it is difficult to demold. Demolding mechanism with movable mold slider and fixed mold slider is innovatively designed. Fixed mold slider is core-pulled before mold is opened and then attached to movable mold slider. When mold is opened, sliding force of inclined guide post is used to drive movable mold slider to complete lateral core pulling, orderly parting and distance of mold is realized through parts such as limit screws, springs, and nylon rubber plugs. In order to ensure safety of lower cylinder of slider, a first reset mechanism combined with a spring and a swing rod is set. Mold structure is reasonable and operation is reliable.

Headrest cover lock plastic part shown in Figure 1 is an important part of a car seat. Due to need for assembly and fixing of internal parts of equipment, surface of plastic part has not only undercut and lug structures (positions A, B, C), but also an inner recess formed by a square hole passing through undercut (position 4). These structures increase difficulty of molding. In this example, a core pulling mechanism with a movable mold slider and a fixed mold slider is provided to solve problem of molding and demolding of plastic parts.

Plastic material of rear headrest cover lock fastener of car seat shown in Figure 1 is PP+TD20 (polypropylene added with 20% talc), and its shrinkage rate is 1%. Outer dimension of plastic part is ϕ 45*118.5mm, average wall thickness of plastic part is 2.0mm, the thickest part is 4.8mm, and the thinnest part is 0.6mm at the rib. After molding, plastic parts must have a smooth surface, no burrs, no damage to lugs, a predetermined target value for clamping force, good mechanical strength, dimensional stability, and impact resistance. In Figure 1, positions A and C are assembly structure in same direction, which realizes molding of lateral structure by setting movable mold slider mechanism; position B is assembly structure in other direction, which is also formed by setting a movable mold slider mechanism; Taking into account structural stability of mold mass production and reduction of mold costs, position D prescription hole uses a fixed mold slider to first pull core and then attach it to movable mold slider to complete final core-pulling molding mechanism. 

 

Figure 1 Car seat rear headrest cover lock

Since arrangement of cavities is closely related to layout of gating system, mold is designed as one mold with two cavities, taking into account production batch size, external dimensions, and injection capacity of injection molding machine. Considering high quality requirements of main surface of plastic parts, no gate marks on outer surface are allowed, it is convenient to make core-pulling structure, mass production, specific structure and layout of mold, a single-point latent gate gating system as shown in Figure 2 is designed. Main runner adopts a cold runner system with a diameter of ϕ 6mm; branch runner adopts a common circular cross-section runner with a diameter of ϕ 5mm, and side gate end face size is ϕ 0.8*2.0mm. When mold is opened, condensate of gating system moves with movable mold, and gate is forcibly broken when plastic part is pushed out, thereby completing automatic separation from plastic part.

 

Figure 2 Latent gate

Molded parts mainly include movable and fixed mold cavities, cores, fixed mold sliders, movable mold sliders, movable mold inserts, sleeve sets, etc. Main part of movable and fixed mold cavity and core forming plastic parts are integrated into movable and fixed mold plates; fixed mold slider forms latent gate gating system as shown in Figure 2 is designed; movable mold slider forms buckle assembly structure at positions A, B, and C in Figure 1; end ribs of plastic part formed by movable mold insert (to facilitate glue position of front end of ribs to be processed by wire cutting in the core of movable mold); inner hole part of plastic part formed by sleeve. Mold design module of UG is used for parting. Final parting diagram is shown in Fig. 3, dynamic and fixed cavity and core diagrams are shown in Fig. 4.

 

Figure 3 Parting diagram of molded parts

 

Figure 4 Fixed and dynamic model cavity and core diagram
a — fixed model cavity b — moving model core
In addition, taking into account production batch, use and molding requirements of plastic parts, core, cavity, and movable mold inserts are selected from 738H mold steel, fixed mold slider and movable mold slider are selected from H13 mold steel, cylinder set is selected SKD61 die steel. German 738H tool steel is a pre-hardened plastic mold steel with good polishing, electro-erosion, welding repairability and cutting performance. It is used for large-scale long-life, high-surface finish, complex and possibly deformed injection molds. Choice of this material can ensure appearance quality of plastic parts and life of mold in mass production; H13 is a hot work mold steel, with high hardenability (hardenable in air), low heat treatment deformation rate, good toughness, wear resistance, cutting performance and excellent thermal cracking resistance, choice of this material can ensure wear resistance and thermal crack resistance of slider; SKD61 is a kind of hot work die steel with high strength, good cutting performance, high toughness and excellent wear resistance. Choosing this material can make sleeve have high strength, high toughness and high wear resistance.

Aiming at shape and structure of rear headrest cover of car seat, 2D structure of designed mold is shown in Figure 5. In Fig. 5c, a core-pulling structure with a movable mold slider and a fixed mold slider is designed to realize problem of ejecting square hole at positions A, C, D shown in Fig. 1. Principle is that under force of spring when mold is opened, fixed mold slider first completes square hole core pulling at position D, then attaches to movable mold slider under restriction of limit screw, finally completes core-pulling action together with movable mold slider. This structure has a small proportion of structural space, smooth and stable operation, also improves design imagination of plastic part.
Core pulling structure of movable mold slider with fixed mold slider is mainly composed of oblique wedge 22, oblique guide post 24, fixed mold slider 25, limit screw 27, spring 28, slider insert 29, slider insert 30, oblique wedge 32, screw 33 and slider pressing block 60, as shown in FIG. 5 and FIG. 6.

 

 

Figure 5 2D structure diagram of mold
a — —Structural drawing of fixed mold side b — —Structure drawing of movable mold side c — —Mould structure drawing I d — —Mould structure drawing II e — —Mould structure drawing Ⅲ

1. Fixed mold base plate 2. Guide column 3. Guide sleeve 4. Fixed template 5. Fixed mold cavity 6, 22, 32. Inclined wedge 7. Inclined guide column 8. Moving mold slider 9, 29, 30. Slider Insert 10, 33. Screw 11. Plastic part 12. Movable mold insert 13. Screw plug 14. Movable mold core 15. Waterproof rubber ring 16. Quick nozzle 17. Guide sleeve 18. Division tube 19. Division tube needle 20 Screw 21. Pressure block 22. Oblique wedge 23. Pressure block 24. Inclined guide post 25. Fixed mold slider 26. Moving mold slider 27, 44. Limit screw 28, 43, 46. Spring 31. Wear-resistant block 34. Movable mold plate 35. Positioning pin 36. Mould foot 37. Support column 38. Ejector fixed plate 39. Ejector pad plate 40. Movable mold seat plate 41. Positioning ring 42. Pump nozzle 45. Reset rod 47. Guide column 48. Mandrel 49. Guide sleeve 50. Garbage nail 51. Nylon rubber stopper 52. Limit block 53. First reset lever 54. Stop block 55. Rotating pin 56. Swing lever 57. Fixed block 58. Side support column 59. 60. Slide block

Figure 6 3D schematic diagram of key components of core-pulling structure
22. Oblique wedge 24. Oblique guide post 25. Fixed mold slider 26. Moving mold slider 27. Limit screw 28. Spring 29, 30. Slider insert 32. Oblique wedge 33. Screw 60. Slider block
Under mold opening force of injection molding machine and action of spring 43, fixed mold seat plate 1 is separated from fixed mold plate 4 in Figure 5 (note: fixed mold plate 4 and movable mold plate 34 are connected together by action of nylon rubber plug 51), fixed mold seat plate 1 drives inclined wedge 22 away from fixed mold slider 25. At this time, fixed mold slider 25 generates an upward core pulling action under force of spring 28, slides in slider insert 29, then completes predetermined core pulling distance under restriction of limit screw 27, and attaches to movable mold slider 26; continue to open mold, complete predetermined separation distance between fixed mold seat plate 1 and fixed mold plate 4 under restriction of stop screw 44; continue to open the mold. Under strong action of mold opening force of injection molding machine, nylon rubber plug 51 temporarily loses its adhesive force, fixed mold plate 4 and movable mold plate 34 begin to separate, inclined wedge 32 also leaves movable mold slider 26 along with fixed mold plate 4. At the same time, inclined guide post 24 fixed on fixed mold plate 4 moves movable mold slider 26 with fixed mold slider 25, slider insert 29, and slider insert 30 to move backward along slider pressing block 60 until predetermined core-pulling distance is completed under restriction of the screw 33, core-pulling is completed.

Since lateral parting and core-pulling distance of structure at position B shown in Fig. 1 is not large, design adopts method of lateral parting and core-pulling by sliding slider by inclined guide post. As shown in Figure 5c, when mold is opened, sliding force of inclined guide post 7 is used to force sliding block 8 to move backward along sliding block 59 until predetermined core-pulling distance is completed under restriction of screw 10, core-pulling is completed. .

Mold uses parting surface, sleeve and sleeve hole, sliding block and mold core, insert and mold core to exhaust air.

In order to ensure that mold temperature is always in temperature range specified by production process during use, mold adopts a circulating cooling water circuit arrangement on fixed mold plate 4 and fixed mold cavity 5, movable mold core 14 and movable mold plate 34, as shown in FIG. Mold cooling system can ensure uniform cooling throughout cavity, short forming cycle, and high quality plastic parts.

In order to ensure that movable mold slider 8 and movable mold slider 26 do not interfere with cylinder 18 during mold clamping, a mechanical swing lever first reset mechanism as shown in FIG. 5e is designed. Principle is that in mold clamping process, reset rod 53 first hits pendulum rod 56 before inclined guide posts 7, 24 are inserted into movable mold sliders 8, 26, then pendulum rod 56 rotates downward through rotating pin 55, hits fixed block 57, uses force of spring 46 to force ejector rod fixing plate 38 to return to position first, thereby driving cylinder 18 fixed on ejector rod fixing plate 38 to return to position.

Mold is a three-plate structure, designed mold structure is shown in Figure 5. Its working process is as follows: When mold is opened, under action of mold opening force of injection molding machine and spring 43, fixed mold base plate 1 is separated from fixed mold plate 4, fixed mold base plate 1 drives inclined wedge 22 away from fixed mold slider 25. At this time, fixed mold slider 25 generates an upward core pulling action under force of spring 28, slides in slider insert 29, then completes predetermined core pulling distance under restriction of limit screw 27,    and is attached to movable mold slider 26; continue to open mold, complete predetermined separation distance between fixed mold seat plate 1 and fixed mold plate 4 under restriction of set screw 44; continue to open the mold, under strong action of opening force of injection molding machine, nylon rubber stopper 51 temporarily loses its adhesive force, fixed mold plate 4 and movable mold plate 34 begin to separate, inclined wedge 32 is also separated from movable mold slider 26 with fixed mold plate 4. At the same time, inclined guide post 24 fixed on fixed mold plate 4 moves movable mold slider 26 with fixed mold slider 25, slider insert 29, and slider insert 30 to move backward along slider pressing block 60 until predetermined core-pulling distance is completed under restriction of the screw 33, core-pulling is completed (Note: At the same time, movable die slider 8 also completes core-pulling action under action of driving force of inclined guide post 7 and restriction of screw 10 ). Continue to open mold until it reaches predetermined position set by injection molding machine. Then ejector rod of injection molding machine moves upward against ejector pad 39, cylinder 18 fixed on ejector pad 39 follows and pushes plastic part 11 to move upward; spring 46 sleeved on reset rod 45 is compressed upward under action of ejection force; ejection continues until ejection is completed under restriction of limit block 52, then plastic part 11 is taken out. When mold is closed: in mold opening state, ejector rod on injection molding machine is recovered, spring 46 in compressed state is released and rebounds, ejector rod fixing plate 38 follows return position, drives cylinder 18, reset rod 45, ejector pin 48 and other components fixed on it to return. In this process, before inclined guide posts 7, 24 are inserted into movable mold slider 8, 26, reset mechanism 53, 54, 55, 56, 57, force ejection mechanism to fully return to its position first, continues mold clamping until parting surface is closed and an injection molding cycle is completed.

Due to need for assembly and use functions of rear headrest cover lock plastic parts of car seat, there are local structures inside and on both sides, which increases difficulty of forming. Structural characteristics of plastic part forming mold are as follows:
(1) Latent gate gating system is adopted, gate is forcibly broken when plastic part is pushed out, thereby completing automatic separation from plastic part.
(2) Aiming at complex plastic part structure in which square hole passes through undercut to form an inner concave, a core-pulling mechanism with a movable mold slider and a fixed mold slider is designed. This structure occupies a small proportion, movement runs smoothly and stably, which also increases design imagination space of plastic parts.
(3) In view of complicated problem of cylinder ejection mechanism under slider, a mechanical swing rod first reset mechanism is designed so that the two will not interfere during mold clamping, so as to achieve purpose of smooth mold clamping.
(4) Mold has been tested in production practice, mold structure is simple and reasonable, produced plastic parts meet design requirements.