Elastic support plastic part of a bumper rack tray is shown in Figure 1, which is composed of two parts of tubular features: a tube structure with inclined holes and a tube structure with a spiral pattern on outer wall. Centerline of inclined hole forms a certain angle with centerline of bell mouth. Length of plastic part is 156 mm, width is 36 mm, and thickness is 30 mm. Average wall thickness is 1.8 mm. Plastic part is made of engineering plastic ABS with a shrinkage rate of 0.5 %~0.65%.

 

Figure 1 Elastic support of bumper tray
Mold for forming plastic parts requires 4 cavities to meet order requirements, and mold adopts a three-plate mold structure to reduce manufacturing costs. Based on design experience, difficulty in design of injection mold lies in: middle part of the two parts of plastic part is an empty tube, and cores forming inner wall form a certain cross angle, which needs to be core-pulled in sections; in addition, demoulding of outer wall part of plastic part with spiral pattern needs to be combined with specific mold structure layout to determine whether it needs to be equipped with a demoulding mechanism for demoulding. If cavity adopts a vertical layout, helical pattern on outer wall needs to be demoulded by means of core pulling on both sides, and core at the center of inclined hole needs to be core-pulled using an oblique core-pulling mechanism; if horizontal layout is adopted, it is only necessary to use two sections of cores on inner wall of plastic part to pull cores separately, and core pulling mechanism can be arranged on horizontal plane. In contrast, horizontal layout is more suitable for low-cost mold manufacturing required by this plastic part.

Mold structure is shown in Figure 2.

 

Figure 2 Die structure
1. Cavity plate insert 2. Core 3. Push-out block 4. Hydraulic cylinder 5. Guide rod 6. Core slider 7. Guide sleeve 8. Guide column 9. Core 10. Moving mold plate 11. Slider table Push rod 12. Support column 13. Push plate guide column 14. Moving mold seat plate 15. Pad block 16. Ejector rod 17. Travel switch 18. Push plate 19. Push rod fixing plate 20. Fixed mold plate 21. Stripping plate 22 .Fixed mold seat plate 23. Mold frame guide column 24. Fixed mold plate tie rod 25. Pull rod 26. Sprue sleeve 27. Side block 28. Inclined tube inner wall core 29. Stripping plate pull rod W1, W2, W3 , W4 cooling water circuit
(1) A single cavity adopts a horizontal layout, and is suitable for gating with a point gate + side gate composite gating system. Combined with molding requirements of bell-mouth tube structure, it is necessary to use a bell-mouth side core-pulling mechanism for core-pulling. In order to facilitate demoulding of spiral pattern features on both sides of outer wall, it is necessary to push plastic part from core 9 through core 2 of bell mouth, then pull core 2 out of plastic part, so mechanism S1 and S2 (see Fig. 3) that need to be designed include parts 2 to 8, push rod 11 of slider table, side stopper 27, push rod fixing plate 19 and pushing plate 18. Cavity plate insert 1 and core 9 constitute outer surface molding of plastic part, core 2 and inner wall core 28 of inclined tube constitute inner wall molding of plastic part. Core 2 is installed on core slider 6, core slider 6 is installed on guide rod 5 and can slide, core 2 is inserted into cavity formed by cavity plate insert 1 and core 9 after passing through push-out block 3 and side stopper 27, and its front end collides with core 28 on inner wall of inclined tube. After parting surface of mold L3 is opened, ejector rod of injection molding machine pushes push plate 18 and push rod fixing plate 19 to push out, pushing block 3 is pushed out the push rod 11 of slider table, parts 2, 4, 5, 6, and 7 on pushing block 3 are all pushed out synchronously, core 2 pushes plastic part out of core 9.  plastic part is blocked by side stopper 29, core pulling finishes, and plastic part falls off automatically. Mechanisms S1 and S2 are floating push-out side core-pulling mechanisms, mold base structure adopts a three-plate mold base structure, which is opened in 3 times, namely L1, L2, and L3. Mold opening control mechanism components are fixed mold base pull rod 24 and stripper plate pull rod 29.

 

Figure 3 Mold layout
S1, S2, S3, S4 core pulling mechanism; V1, V2, V3, V4 cavity number; G1, G2, G3, G4 side gate number; R1, R2 point gate runner
(2) Gating of cavity is poured using a point gate + side gate composite pouring system. Each cavity is provided with a side gate (G1, G2, G3, G4) separately, and feed of side gate uses a point gate. There are 2 point gate runners, namely R1 and R2, to facilitate layout of 4 cavities. Cavity layout is shown in Figure 3. Each cavity must be provided with a core 2, and 2 cavities share a bell mouth side core-pulling mechanism (S1, S2). Layout of mold is shown in Figure 3. Cores 28 on inner walls of inclined tubes of four cavities all need to be driven by side core pulling, inclined guide post slider mechanisms S3 and S4 are set. Mechanism S3 performs core-pulling drive for inner wall cores 28 of inclined tubes in cavities V3 and V4, mechanism S4 performs core-pulling drive for inner wall cores 28 of inclined tubes in cavities V1 and V2. In mold, set independent waterways W1, W2, W3, W4 (see Figure 2) for the four cavities for cooling.
(3) Slanted guide column slider mechanisms S3 and S4 are commonly used slanted guide column driven slider mechanisms, as shown in Figure 4, taking mechanism S3 as an example, inner wall core 28 of inclined tube is fixed on slider with inserts 31, driven by inclined guide column 30, locked with locking block 32 when closing mold, slider bead 33 is used for sliding guide of slider 31, and slider 31 uses positioning glass beads for positioning of core-pulling stroke. A cooling water channel is arranged in slide block 31 to strengthen cooling of core 28 on inner wall of inclined tube.

 

Figure 4 Slanted guide column slider mechanism
10. Moving mold base 20. Fixed mold base 27. Side block 28. Inclined tube inner wall core 30. Inclined guide post 31. Slider 32. Locking block 33. Slider bead 34. Pull rod
(4) Structure of main forming parts of mold is shown in Figure 5. Combined with mass production requirements of plastic parts, material of forming parts is made of pre-hardened mold steel, mainly medium carbon alloy steel with high hardenability, P20 is used for fixed mold plate 20, movable mold plate 10, 718 is used for core 2 and inner wall core 28 of inclined pipe, 2311 is used for cavity plate insert 1 and core 9, material is pre-hardened steel, which can be used directly.

 

Figure 5 Main parts of mold

Combined with mold structure shown in Figure 2, when mold is in use, action of its mechanical components is: mold is closed, injection process is completed by injection molding machine, mold is opened, movable mold is retreated, mold is first opened at L1 parting surface to complete separation of point gate runner condensate and side gate runner condensate. Movable mold continues to retreat, mold is opened at L2 parting surface, and gate runner condensate is demoulded. Movable mold continues to retreat, mold opens at L3 parting surface, plastic part comes out from cavity plate insert 1, inclined guide post 30 of mechanisms S3 and S4 synchronously drives slider 31 and inner wall core 28 of inclined tube to complete side drawing core. After parting surface of L3 is opened for a certain distance, moving mold stops moving backwards, ejector pin of injection molding machine pushes push plate 18 and push rod fixing plate 19 out, pushing out mechanisms S1 and S2. After pushing out for a certain distance, piston rods of hydraulic cylinders 4 of mechanisms S1 and S2 drive core 2 to pull out core to realize complete demoulding of plastic part. When resetting, push-out mechanism first drives mechanisms S1 and S2 to reset first, then mold resets in the order of L3-L2-L1 until it is completely closed and starts next injection cycle.