As shown in Figure 1, front features of plastic part have 1 rectangular side hole, 1 deep rib, 2 side grooves, 2 undercut grooves, and 2 triangular ribs. These features have a great influence on mold structure design. The overall dimensions of plastic part are 389 mm*187 mm*94 mm, material is PP+20TALC, and shrinkage rate is 1.012.

 

Figure 1 Shape of plastic parts

According to structural characteristics of plastic parts, following schemes are proposed to design mold structure: ① Gating system with multi-point gates of ordinary runners is adopted, gate form is point gate, and number is set to more than 3; ② Main molding parts of mold are designed in the form of inserts, and molding parts with local features are also in the form of inserts, which not only saves materials, but also reduces processing costs and processing difficulty; ③ Plastic parts have large dimensions, are prone to warping and deformation, cooling of formed parts needs to be strengthened. Through CAE analysis and experience reference, it is more appropriate to use a cooling pipe diameter of φ12 mm, or to use a cooling water well to enhance cooling; ④ Difficulty in demoulding molded plastic part lies in 1 rectangular side hole, 1 deep rib, 2 side grooves, 2 1 undercut groove and 2 triangular ribs. Molded parts of these features are all located on the side of cavity surface of fixed mold. Among them, 2 undercut grooves can only be demolded by method of core-pulling the fixed mold first. 2 side grooves, 1 A rectangular side hole can be demolded by core-pulling on the side of movable die, 2 triangular ribs and 1 deep rib can be demolded by reinforced push-out from movable die side; complete demoulding of plastic part is pushed out by a round push rod.
In summary, mold adopts a 1-mold 1-cavity layout and a multi-point gate three-plate mold structure. Mold needs to be equipped with a fixed mold core-pulling mechanism and a fixed-die side core-pulling mechanism to complete demoulding of local features of plastic part, then use push rod to push out plastic part to completely demold plastic part.

Combined with above-mentioned overall design ideas of mold, main structure of designed mold is shown in Figure 2.

 

Figure 2 Mold structure design
Main molding parts of mold are divided into two first-level molding main inserts by PL parting surface, fixed mold and movable mold. Combined with demoulding needs of two undercut grooves, two secondary inserts are set in fixed mold insert: I1, I2. In order to reduce processing difficulty of movable mold insert, 4 secondary inserts are set in movable die insert: I3, I4, I5, I6; combined with demoulding needs of 1 side slot, a secondary insert I7 is set in movable mold insert; combined with demoulding needs of 1 side groove and 1 rectangular side hole, a second level insert I8 is set in movable mold insert; edges of 1 deep rib and 2 triangular rib features are pushed out by multiple flat push rods, complete demoulding of plastic part is pushed out by multiple φ6 mm push rods.
In mold gating system, three point gates G1, G2 and G3 are used for pouring. In order to enhance cooling of mold cavity, cooling water channels C1~C8 are set up in fixed mold for cooling. There are many moving mold inserts, and cooling water channel is difficult to arrange. Therefore, only one water channel C9 is designed for cooling, but cooling enhancement measures are taken for this water channel, and multiple cooling water wells are added to ensure that cooling water can penetrate into top of core to cool inner wall of plastic part as much as possible.
Material of main forming inserts and local small inserts is P20 steel, and the overall layout of mold is shown in Figure 3.

 

Figure 3 Overall layout of mold

The overall structure of mold is shown in Figure 4. Mold base uses LKM standard mold base FCI 4060, thickness of fixed platen is 110 mm, thickness of movable platen is 140 mm, and height of spacer is 120 mm. When plastic part is taken out from mold, it is opened in three times, three parting surfaces are P1, P2, and P3 respectively.

 

Figure 4 Overall structure of mold
1. Guide column 2. Stripping plate 3. Fixed mold plate 4. Moving mold plate 5. Moving mold insert 6. Push plate 7. Side slot slider mechanism 8. Sprue sleeve 9. Cavity plate insert 10. Short tie rod 11. Stripping plate spring 12. Long pull rod 13. Positioning block 14. Limiting block 15. Rectangular side hole slider mechanism
Mold is opened in three times: P1 side is opened, runner waste of gating system is separated from plastic parts at point gate, and at the same time, two fixed mold core-pulling mechanisms are driven to pull core; P2 side is opened, stripping plate will separate runner waste from pulling rod to ensure that runner waste is demolded first, and at the same time, two fixed mold core-pulling mechanisms are driven to complete core-pulling action (see Figure 5); P3 side is opened, plastic part is released from fixed mold insert and left on movable mold insert. When P3 side is opened, rectangular side hole slider mechanism 15 and side groove slider mechanism 7 simultaneously complete side core pulling action. Final demoulding of plastic part is pushed out by push rod on which push plate 6 pushes plastic part from movable mold insert.

 

Figure 5 Fixed die first core pulling slider mechanism
16. Front fixed mold slider mechanism 17. Rear fixed mold slider mechanism 18. Side lock 19. Push rod
In mold, opening sequence of three parting surfaces is controlled by nylon buckle, short tie rod 10 , stripper spring 11 and long tie rod 12 . When P1 side is opened, stripper plate 2 and fixed platen 3 are first opened by stripper plate spring 11, P2 side is opened by fixed platen 3 pulling long tie rod 12, then pulls stripper plate 2 and short tie rod 10, to ensure second sequence opening of P2 side, opening distance is limited by short tie rod 10. After P2 side is opened, due to fixed distance limitation of short tie rod 10, nylon buckle on movable mold plate 4 is forcibly pulled out from hole in fixed mold plate 3, so as to realize separation of fixed mold plate 3 and movable mold plate 4, realize third open sequentially.
Rectangular side hole slider mechanism 15 and side slot slider mechanism 7 have similar structures, both use a slider mechanism in the form of a long inclined guide column drive. Slider of this structure is provided with an auxiliary push-out spring at the bottom and front, a limit screw at tail end to ensure that after slider slides out, inclined guide post can be accurately inserted into inclined guide post hole of slider during reset, so as to ensure accurate reset of slider.
Secondary inserts I1 and I2 are set in the form of a fixed die slider mechanism, and core-pulling action is completed before mold is opened. On die base plate, secondary inserts I1 and I2 are driven through inclined T-shaped slot, as shown in Figure 5.
During operation of mold, in order to ensure accurate reset when fixed mold plate 3 and movable mold plate 4 are closed, a positioning side lock is set around mold plate to ensure that fixed mold inserts and movable mold inserts that have been used for a long time are closed and reset accurately.

Working process of mold is as follows: ①Mold is closed for injection, cavity is filled, pressure-maintained, and cooled, then mold is opened; ②P1 surface is opened, waste of runner is separated from plastic part, rectangular side hole slider mechanism 15 and side groove slider mechanism 7 begin to pull core; ③P2 surface is opened, rectangular side hole slider mechanism 15 and side groove slider mechanism 7 complete side core pulling, waste in runner is scraped off pulling rod by stripping plate 2 and automatically falls off; ④P3 side is opened, plastic part is released from fixed mold insert, rectangular side hole slider mechanism 15 and side groove slider mechanism 7 complete side core pulling synchronously; ⑤Ejector rod of injection molding machine bears against push plate 6, pushes push rod to push out plastic part from movable mold insert 5, so as to realize complete demoulding of plastic part, then mold is reset.