In automobile manufacturing parts, for molded parts that often appear on hands, due to large wear and tear, general OEMs use in-mould deov (IMD) technology to ensure bright production state of produced parts. Process flow consists of two parts: one is pre-forming of diaphragm; the other is in-mold injection molding after pre-forming of diaphragm. In pre-forming, generally use ink printing to print function keys and other patterns on PC film, then preformed film is used to form a shell of a certain shape, and then peripheral residual material film is punched to obtain final pre-formed film for injection molding. When pre-formed membrane is injected, membrane is first placed in mold cavity by a manipulator or manually to position it tightly, injection mold is closed, final IMD plastic part is obtained after filling, holding pressure, and cooling. Aiming at in-mold injection molding of a certain automobile air duct, on the basis of obtaining pre-formed film, injection mold required for molding of plastic part was designed to provide a reference for design of similar plastic parts.

IMD pre-formed diaphragm of automobile air duct plastic parts is shown in Figure 1. Surface is in the style of metal-like wire drawing, thickness is 0.2 mm, and material is PC. Difficulty of diaphragm preforming lies in deformation control of rounded corners of diaphragm. Minimum rounded corners of edges should be controlled above R1.2 mm, and rounded corners of transition parts should be controlled above R3 mm.

 

Figure 1 Wind channel IMD pre-formed diaphragm
There are two methods for film IMD. One is to make printed film (film) into a circulating roller roll, install it in injection molding machine and injection mold; the other is to print film through molding machine ( forming), then placed in an injection mold after being cut for production. Diaphragm chooses second method to form, using IMD high pressure forming machine. Upper and lower molds of IMD high-pressure forming machine have temperature indicators, which can ensure stable formation of diaphragm, heating system consumables are cheap and durable; the whole machine is controlled by numerical control, control indicators include hydraulic pressure, position information of working parts, air pressure, molding cycle of hot press, etc. In production, it has advantages of stable molding, small internal stress of molded plastic parts, rapid molding, high-stretch molding, etc. IMD high-pressure forming machine is equipped with induction probes, 4 upper and lower molds, safety valves are installed on operating surface, cylinder drives iron net, and iron net is equipped with soft rubber to prevent impact, is equipped with a proximity switch.

Plastic part of pre-formed diaphragm after injection molding is shown in Figure 2. Material is modified alloy ABS+PC, and shrinkage rate is 0.53%~0.58%. There are upper and lower windows on plastic part, and maximum external dimension is 267 mm*159 mm*72 mm. There are 6 difficulties in injection molding of plastic parts: ① There are many rib features on front and back of plastic part. It also brings certain difficulties to processing of molded part,which requires use of insert splitting or EDM; ②There are 4 side holes on two sides of center of plastic part. It is necessary to design opposite core pulling mechanism in a small space, and there are 2 undercuts at the front end of plastic part, which requires use of sliders for side core pulling and demolding; ③There are two clamping slots on both sides of center of plastic part, clamping force of clamping groove is large, and a special demoulding mechanism is also required; ④Inner wall of plastic part has a lot of reinforcing ribs in A and B areas, packing force is large and it is difficult to form. It is necessary to use insert design separately; ⑤Corners of inner wall of plastic part C and D are difficult to form, and insert design is also required; ⑥Two pins at the front end of inner wall of plastic part are formed Difficult, also need to use insert molding.

 

Figure 2 Plastic part structure

Cavity uses a single parting surface for parting, and parting design is shown in Figure 3. Cavity is directly processed in fixed mold plate, that is, integral cavity plate, to ensure strength of cavity. Gate adopts a flat fan-shaped offset gate, which is opened on one side of cavity plate, cross-sectional size is 5 mm*1 mm. Runner uses a runner with a circular cross-section, and in order to shorten length of runner, a hot nozzle is used for main runner. In order to prevent diaphragm from wrinkling during injection, after edge of cavity is vented with point vents, venting groove is used to vent. In view of difficulty of forming front rib feature of plastic part, small inserts are used for forming.

 

Figure 3 Parting design

In view of difficulty of molding and demolding plastic parts, a corresponding mechanism is set up, as shown in Figure 4. For 4 side holes on 2 sides of center of plastic part, oblique push mechanism X1 and X2 are respectively set for demolding; for two undercuts at the front end of plastic part, use oblique guide pillar drive type slider mechanism S1 and S2 for side core pulling and demolding respectively; aiming at difficulty of forming and demolding many ribs on both sides of window on reverse side of plastic part, push rod push block push mechanism D1~D6 is used; In view of difficulty in forming ribs on the two sides of window on reverse side of plastic part, inserts c, d, e, and f are set for split molding to reduce difficulty of molding. For C and D areas at the end of plastic part, inserts a and b are used for molding.

 

Figure 4 Mechanism design
Use 10 water channels to uniformly cool cavity plate, as shown in Figure 5, to prevent uneven shrinkage of plastic part and cause surface wrinkles. Water channels W1~W5 are used to cool surface of plastic parts, water channels W6~W9 are used to cool core.

 

Figure 5 Cooling water circuit design

Mold adopts a two-plate mold structure, as shown in Figure 6, a layout of 1 cavity. Gating system adopts form of hot runner + ordinary runner. Integral cavity is directly processed in fixed mold plate 2, and core is in the form of inserts. A number of wear-resistant positioning plates 3 placed at a certain slope are arranged around core insert 20 to ensure a stable fit with cavity plate, cooling water channel uses PT1/8 standard joints. Components of push-out mechanism are push rod 18, inclined push rod 14, push block 10, and pull rod. Among them, inclined push rod 14 must be provided with a wear-resistant rod sleeve to prevent wear. Ejection mechanism is powered by two hydraulic cylinders 19 arranged on the side to achieve balanced ejection. Push rod fixing plate 11 and push plate 12 are guided by push plate guide post 16. In order to ensure service life and strength of movable mold plate 4, four support columns 15 are arranged on movable mold seat plate 8 to increase strength of bottom surface of movable mold plate 4. Push rod fixing plate 11 and push plate 12 are pushed back into position by reset rod 17, and accurate reset information is measured by the two travel switches 9 at both ends. Heating control line of hot nozzle 22 is controlled by plug socket 21. Fixed mold plate 2 and movable mold plate 4 are positioned by tiger mouth, that is, four positioning frustums are arranged around four-corner guide pillars to ensure accuracy of resetting and closing when mold is closed. Each time mold is opened and closed, counter 6 must be used to ensure reliability of data on number of injections of mold.

 

Figure 6 Mould structure
1. Fixed mold base plate 2. Fixed mold plate 3. Wear-resistant positioning plate 4. Movable mold plate 5. Fine positioning block 6. Counter 7. Pad block 8. Movable mold base plate 9. Travel switch 10. Push block 11. Push rod Fixed plate 12. Push plate 13. Oblique push rod fixed plate 14. Oblique push rod 15. Support column 16. Push plate guide column 17. Reset rod 18. Push rod 19. Hydraulic cylinder 20. Core insert 21. Insert wire Block 22. Hot nozzle
Material of plastic parts is ABS+PC alloy, and material of mould forming parts can use alloy steel S136, NAK80, DC51. Corrosive gas is generated when ABS+PC material is injected. Therefore, molded parts are required to have certain corrosion resistance and wear resistance. Surface of material needs to be hardened. Above-mentioned steel after hardening can meet requirements of mold. Fixed mold plate 2 and core insert 20 use alloy steel S136, push block and inclined push rod use 8407. Hardened DC51 is used for wear-resistant parts, which can make service life of mold reach 1 million strokes.

(1) Mold is closed. When mold is in open state, pre-formed membrane shown in Figure 1 is placed in cavity of fixed mold plate 2 by manipulator, mold is closed under push of slider of injection molding machine, waiting for injection.
(2) Injection. Injection molding machine injects molten plastic into cavity, opens mold after filling, pressure holding, and cooling are completed.
(3) Open mold. After injection is completed, mold is opened at PL, and slider mechanism completes side core pulling action.
(4) Demoulding. After mold is opened, hydraulic cylinder 19 drives push rod fixing plate 11 and push plate 12 to move, plastic parts and runner aggregates are completely pushed out through 4 kinds of pushing parts.
(5) Reset. Before resetting, hydraulic cylinder 19 acts to drive push rod fixing plate 11 and push plate 12 to reset first, then movable mold and fixed mold are closed by slider of injection molding machine.