Abstract Combining structural characteristics of an automobile interior trim gusset, a simplified three-plate injection mold is designed. Mold adopts a point gate + side gate compound gating system to improve filling performance of melt, a delay slider mechanism is designed to prevent side surface from being damaged when plastic part is demolded. Mold has been verified by production: molded plastic part meets design requirements, which has a certain reference effect on molding of similar plastic parts.

In injection production of automotive interior parts, difficulties in design of molding die are: ①Plastic parts have high appearance requirements, generally do not allow obvious casting marks, weld marks, parting lines, black spots, markings and other defects; ②Shape Irregular, with design requirements for parting surface, complex side core pulling and demoulding mechanism; ③Inner wall of plastic part has many and complicated installation structural features, such as buckles, undercuts, screw posts, avoidance grooves, etc., which requires a special inclined top block, push tube and other mechanisms with corresponding shapes and structures;④There are many ribs on inner wall of plastic part and wall is thin, corresponding cavity is difficult to fill with molten material. Narrow rib grooves on mold parts are difficult to process, more combined structure optimization designs must be designed. Now combined with injection molding of an automobile interior trim gusset, a three-plate injection mold is designed, which uses point gate + side gate to feed at both ends, delay slider mechanism is used to carry out side core pulling and demolding on a side wall of plastic part to be molded and 4 kinds of ejection elements to realize ejection of plastic part.

Plastic part is side cover of rear trunk of an SUV, and side cover is a self-elastic fastener, which is used to block electronic control unit combination box cabinet of taillight of car. Overall dimensions of plastic part are 350 mm*119 mm*20 mm. Central part of front end is a corrugated elastic buckle, rear end is an installation slot, both sides are arc-shaped box covers. Average wall thickness of plastic parts is 2.6 mm, maximum wall thickness is 3 mm, minimum wall thickness is 1 mm. Features that affect mold structure design on plastic part to be molded include: T1~T6 features and numerous stiffener features with a wall thickness of 1 mm. T1 is a side hole with a diameter of φ5 mm on stiffener, T2 is a tapered hole on inner wall, T3 is an oblique side hole, T4 is an oblique side groove, T5 is a side undercut, and T6 is a side hole on rib. Plastic parts are made of glass fiber modified plastic, modified composition is 30% GF+PP.

 

Figure 1 Side cover
Mold design plan of molded plastic part is mainly considered from following aspects.
(1) Length and width of plastic part are 350 mm*119 mm. No matter whether gate is set in the center or side of plastic part to be molded, cavity cannot be filled evenly. It is not suitable to set a single-point or multi-point gate on upper surface of plastic part to be molded. Point gates will leave gate marks and affect appearance of molded plastic part; if side gates are provided on front and rear sides of plastic part to be molded, it is not suitable. Combined with analysis of characteristics of front and rear sides, it can be seen that both sides need to be provided with a slider side core pulling mechanism, better pouring method is to open side gates on both left and right side end surfaces.
(2) T1, T2, T6, T7 features need to install inclined top block for side core pulling and demolding.
(3) T5 is characterized by side inverted buckle, and a side slider mechanism needs to be provided for core pulling and demolding.
(4) T3 and T4 are concentrated on upper oblique side wall. Side wall needs to adopt an oblique slider mechanism to achieve side core pulling and demolding, T3 and T4 features can only be demolded step by step, and a delay step is required. Core-pulling slider mechanism performs side core-pulling and demolding.
(5) There are many stiffeners on inner wall of plastic part. If mould parts are formed with integral inserts, forming grooves of stiffeners can only be processed by EDM, which will increase processing difficulty and manufacturing cost of mold parts. Better method is to use multiple inserts to form core. .

Shape of plastic part is relatively large, mold adopts a 1-cavity layout, as shown in Figure 2, cross-sectional structure is shown in Figure 3. In Figure 2 Q1~Q8 are core inserts; R1 and R2 are side gate runners; R3 is a point gate horizontal runner; C1~C16 are cooling water circuits; G1 and G2 are side gates; RP is a reset rod ; GP is a guide post; E1 is a round push rod; E2 is a square push rod; X1~X3 are inclined top mechanisms; S1, S2 are slider mechanisms; L1, L2 are pull rods; P1 is a die buckle. Cavity plate adopts mosaic type. Cavity is poured through two side gates G1 and G2, G1 and G2 gates are fed by two runners R1 and R2.

 

Figure 2 Layout of mold parts

 

Figure 3 Mould structure
1. Fixed mold base plate 2. Stripping plate 3. Fixed mold plate 4. S1 inclined guide column 5. S1 slider 6. S1 wedge block 7. S1 core 8. S1 spring 9. S1 wear plate 10. S1 Limit screw 11.S1 limit bead 12. Spacer 13. Movable mold seat plate 14. Push rod fixed plate 15. Push plate 16. Square push rod 17. S2 limit screw 18. S2 limit bead 19. S2 wedge Tight block 20. S2 slider 21. S2 inclined guide column 22. Push tube 23. Round push rod 24. Push plate guide column 25. X2 inclined top mechanism 26. Die buckle 27. L2 short pull rod 28. Pull rod 29 .Reset rod 30. Reset rod spring 31. Support column 32. Core insert 33. L1 long tie rod 34. Cavity plate insert 35. Pressure ring 36. Positioning ring 37. Sprue sleeve 38. Guide column 39.X1 Slant top mechanism 40.X3 Slant top mechanism 41. Slant top inserts;
In order to facilitate injection molding machine nozzle to feed G1 and G2 side gates, gating system of mold adopts a combination of three-plate point gate + two-plate side gate, that is, injection molding machine nozzle feeds three-plate mold horizontal runner R3, R3 runner feeds vertical runners at both ends, and vertical runners at both ends feed main runners of R1, R2 runners through their respective point gates, which facilitates selection and opening of G1, G2 side gate positions , G1 and G2 are opened on core.
Mold adopts a three-plate mold structure and is opened after three times of classification. Mold base is a simplified three-plate mold base. Guide mechanism is composed of 4 mold base guide posts GP. Mold base opening mechanism includes 4 long pull rods L2, 4 short pull rods L1 and 4 die buckles P1.
Mold cooling water circuit settings: 2 cavity plate inserts (C1, C2), 2 slider S2 (C3, C4), 4 slider S1 (C5~C8), 8 core inserts (C9~ C16).
Demoulding mechanism of molded plastic parts includes: X1~X3 inclined top mechanism, S1, S2 slider mechanism, among which S1 is a delay inclined slider mechanism, S2 is a smooth block mechanism, 18 round push rods E1, 16 square pushers Rod E2. Flow channel aggregates of point gate pouring system are automatically demolded by three-plate mold stripping plate, flow channel aggregates of two-plate side gate pouring system are pulled out of mold by draw rod.

Mold structure is shown in Figure 3.
(1) S1 time-delay slider mechanism is used for forming and demolding features of T3 and T4. Mechanism consists of parts 4 to 11, mechanism is an inclined guide post driven inclined slider mechanism. In this mechanism, S1 slider 5 is used for molding of side where T3 and T4 features are located, S1 core 7 is used for molding of T3 hole, in order to ensure that entire side of plastic part to be molded will not be pulled on the side of S1 slider 5 Strain, core pulling and demoulding with T3 and T4 features are carried out in two parts. Working principle of mechanism is: when mold is opened, S1 wedge block 6 releases locking of S1 slider 5, also releases locking of S1 core 7. S1 core 7 is pulled out under drive of S1 spring 8. At this time, S1 oblique guide post 4 does not touch oblique guide post hole of S1 slider 5, and there is a delay distance in the middle, so it can ensure that S1 core 7 is separated from molded plastic part first, front end of S1 slider 5 is still in a bonded state with molded plastic part, T3 feature is demolded first. With further opening of mold, S1 oblique guide post 4 contacts oblique guide post hole of S1 slider 5, and S1 slider 5 is driven to complete core pulling. When reset is closed S1 slider 5 resets first, and S1 core 7 resets later.
(2) S2 slider mechanism is a commonly used oblique guide column slider side core pulling mechanism, and mechanism components include parts 17 to 21.
(3) For screw column on plastic part to be molded, use push tube 22 to push out demoulding.
(4) In order to ensure that molded plastic parts can be pushed out evenly, a plurality of round push rods 23 are set.
(5) R4 is vertical runner of point gate pouring system, and R5 is main runner of side gate pouring system. Separation of the two is broken at point gate at the end of R4 runner, and demolding of side gate gating system aggregates is pulled out by pull rod 28.
(6) Mold uses 3 inclined top mechanisms to form side hole features of plastic parts, including X2 inclined top mechanism 25, X1 inclined top mechanism 39, and X3 inclined top mechanism 40.

Working principle of mold is same as ordinary simplified three-platen mold, specific working process is as follows.
(1) Mold clamping and injection. Mold is installed on injection molding machine, injection molding machine injects its cavity. After cavity is filled, pressure-maintained, and cooled, it waits for mold to open.
(2) PL1 is turned on. Movable mold is first opened at PL1 under drive of slider of injection molding machine, R4 runner condensate and R5 runner condensate are separated at point gate.
(3) PL2 is turned on. Movable mold continues to retreat, PL2 is opened, point gate runner condensate is pushed out of mold by stripper plate 2.
(4) PL3 is turned on. Movable mold continues to retreat, PL3 is opened, S1 slider mechanism and S2 slider mechanism complete side core pulling and demolding, plastic parts are left on core.
(5) Molded plastic parts are launched. As movable mold continues to retreat, ejector pin of injection molding machine abuts push plate 15 so that push plate 15 pushes ejection mechanism on ejector rod fixing plate 14 to push plastic part and runner aggregate from core, so as to realize demolding of plastic part and runner aggregate.
(6) Reset. Reset process is opposite to mold opening process.

Aiming at injection molding of automotive interior parts, a simplified three-platen mold is designed for its automated injection production. Mold has been verified by production: molded plastic part meets design requirements, which has a certain reference for molding of similar plastic parts.