Valve cover of forklift truck is shown in Figure 1. Features that affect design of mold structure are: ① 4 inverted buttons C1~C4 formed on cavity side; ② 3 holes H1~H3 on cavity side, of which hole H1 is an inclined hole for bidirectional core pulling and demoulding; ③ 5 inverted buttons K1~K5 formed on core side; ④ 5 threaded posts B1~B5 formed on core side.

 

Figure 1 Forklift valve cover
In view of above characteristics, corresponding measures that need to be taken in mold design are as follows: ①Inverted buckles C1~C4 need to be demolded by side core pulling; ②Hole H1 needs to be demolded with a two-way inclined hole core pulling and demoulding mechanism,  holes H2 and H3 only need to be set with cylindrical inserts; ③ Invert K1~K5 use inclined top mechanism to demold; ④ Threaded column B1~ The B5 uses a push tube to push out mold.
The overall dimensions of plastic parts are 200 mm*175 mm*81 mm, volume is 346.025 mm3, weight is 347.735 g, material is polypropylene (PP), and accuracy tolerance is MT5 level. Shape of plastic part is not a revolving body, and wall thickness is 2.29~2.6 mm. Side core-pulling structure and inclined top structure are required to demould molded plastic part. Outer surface of plastic part is required to be beautiful and smooth, and there should be no defects such as weld lines. Outer surface roughness is Ra0.8 µm, and inner surface roughness is Ra1.6 µm.

To sum up, inversions C1~C4 on cavity side of molded plastic parts require side core-pulling and demoulding. Three fixed-die straight core-pulling mechanisms are designed, of which undercut C1 and C4 each use a single mechanism, undercut C2 and C3 use a combined mechanism, as shown in picture 2. Cavity side inclined hole H1 needs to be demolded by a bidirectional inclined hole core pulling mechanism, that is, a movable mold inclined core pulling mechanism, holes H2 and H3 only need to be formed with cylindrical inserts. Five inverted buttons K1~K5 on core side of molded plastic part use five universal lifter mechanisms with same structure for core pulling and demoulding, threaded columns B1~B5 on core side are pushed out and demoulding with a push tube. Multiple round and square hole features on cavity side and core side of remaining plastic parts are formed with small inserts to reduce processing difficulty of formed parts.

 

Figure 2 Layout of demolding mechanism
T-slot locking blocks are used to drive upward and downward sloping cores of two-way inclined hole core-pulling mechanism; T-shaped guide rail type drive connection is adopted between core and downward inclined core. This design can effectively reduce mold space required by core-pulling mechanism, thereby reducing size of mold structure.

Cavity forming parts are shown in Figure 3, cavity plate inserts are made of 718H steel, and core inserts are P20 steel. 718H steel is a pre-hardened mirror-surface acid-proof plastic mold steel. After pre-hardening, material is uniform and cleanliness is high. It has good polishing performance, photolithography, hardenability, good electrical processing performance and skin texture processing performance. Hardness of mold cavity plate insert is 48 HRC.

 

Figure 3 Design of formed parts

Cooling water circuit design is shown in Figure 4. A total of 10 water circuits are set for cavity plate insert and core insert to cool cavity, so that temperature of plastic part is lowered and adverse effects due to uneven heating are prevented. Pipes W1~W2 are used for cooling of cavity plate inserts, W3~W10 are used for cooling of core inserts, and diameter of cooling water channel is φ8 mm. In order to ensure cooling effect on core side, in addition to cooling water circuit, there is also a cooling water well, diameter of which is φ12 mm and thickness of water barrier is 1.5 mm.

 

Figure 4 Design of cooling water circuit

Mold is a multi-plate mold structure. As shown in Figure 5, due to fixed mold core pulling mechanism and use point gate, mold adopts a layout of 1 cavity. Mold opening is divided into 4 times. Two openings on fixed mold side are used to drive fixed mold core pulling mechanism, remove condensate in runner, the two openings on movable mold side are used to drive movable mold release mechanism. Stripper plate 2 and fixed platen 3 are movable plates on fixed mold side, movable platen 4 is movable plate on movable mold side. To drive inclined core pulling mechanism on movable mold side to pull core and push inclined ejector rod 10 to realize demoulding of plastic part. Mold is matched with a 1 600 kN injection molding machine with a maximum injection pressure of 150 MPa.

 

Figure 5 Mold structure
1. Fixed die base plate 2. Stripping plate 3. Fixed formwork 4. Moving formwork 5. Moving die pad 6. Push rod fixing plate 7. Pad 8. Moving die base plate 9. Inclined top seat 10. Inclined top Rod 11. Core insert 12. Cavity plate insert 13. Up inclined core 14. T-slot locking block

Mold is assembled, repaired and installed on injection molding machine, and its working principle is shown in Figure 6.

 

Figure 6 Working principle of mold
2. Stripping plate 3. Fixed mold plate 4. Moving mold plate 6. Push rod fixing plate 8. Moving mold base plate 9. Inclined top seat 10. Inclined top rod 11. Core inserts 12. Inclined core 13. Upper T-slot locking block 14. Lower inclined core 15. Lower T-slot locking block
(1) Clamping injection. Four opening surfaces of mold are closed, and each core-pulling mechanism is reset to mold-closing state. Injection molding machine begins to inject melt.
(2) PL1 side is opened. Injection molding machine drives movable mold to retreat, mold is first opened on PL1 side, gate condensate at point gate is separated from plastic part, and at the same time, four fixed mold core pulling mechanisms on fixed mold side start to pull core.
(3) PL2 side is opened. When PL2 surface is opened, stripper plate 2 pushes out runner condensate from mold. After it is fully opened, fixed mold plate 3 will no longer follow moving mold plate 4. At the same time, core pulling of four fixed mold core pulling mechanisms on fixed mold side is completed, that is, upper T-slot locking block 15 pulls upper inclined core 13 to complete core-pulling, and plastic parts remain on core insert 11 .
(4) PL3 side is opened. Plastic part is released from fixed mold and remains on the side of movable mold.
(5) PL4 side is opened. After movable mold continues to retreat for a certain distance, ejector rod of injection molding machine bears against push plate and push rod fixing plate 6, so that movable mold plate 4 and movable mold plate 4 no longer follow movable mold to retreat, PL4 surface is opened, and movable mold base plate 8 continues to retreat. During this process, lower T-slot locking block 17 of movable die oblique core-pulling mechanism pulls lower oblique core 16 to complete core-pulling.
(6) INTRODUCTION. Lifter mechanism and push tube push out plastic part from core insert 11 to realize complete demoulding of plastic part.
(7) Reset. Reset process is opposite of mold opening process, which is carried out in sequence.