Automobile headlight housing is shown in Figure 1. Its structure is complex, height difference is large, and parting surface is irregular. There are two large holes in lamp housing plastic part. Mold is designed to be formed by a through-structure. There are multiple mesh reinforcement ribs and installation fixing columns inside plastic part. There are reinforcement ribs and fixed installation holes at A and B. Mold needs to be designed with a lateral parting core pulling mechanism to demold; hole at C adopts a through-molding structure, a lateral core pulling mechanism is adopted at D, E, F, and G. There is a positioning pin hole at K. Mold needs to be designed with an oblique push mechanism to demold; mesh at H is deep, molding and demolding are difficult. Multiple push rods need to be set around. Lamp housing material is modified engineering plastic PC+ABS, with a shrinkage rate of 0.5% and a wall thickness of 2 mm.

 

Figure 1 Automobile headlight housing

Automobile headlight housing is a bilaterally symmetrical part. Mold adopts a 2-cavity layout, and pouring system adopts an open hot runner. Maximum outer dimensions of mold are 1160 mm * 900 mm * 802 mm, and weight is 5000 kg, as shown in Figure 2. Mold structure is complex, with 12 lateral core pulling mechanisms and 2 oblique push mechanisms. Mold side is also equipped with 2 water collectors to connect cooling water circuit. φ70 mm round guide columns are designed at each of four corners of mold frame. Guide columns are assembled on fixed mold plate, and matching guide sleeves are installed on movable mold plate. In order to improve positioning accuracy of mold and extend its service life, pipe positions are also set at four corners of mold frame, and wear-resistant blocks are set on pipe position slope of fixed mold plate. Four precision positioning parts are set in the middle part around movable and fixed mold plates to play an auxiliary positioning role, as shown in Figure 3 (a). A symmetrical inclined surface is set at the middle k of moving template to interlock with fixed mold plate to enhance positioning accuracy of mold. At the same time, moving mold plate is also designed with 23 pressure blocks of different sizes, as shown in Figure 3 (b).

 

Figure 2 Mould structure
1. Limit pin 2. Fastening screw 3. Pad 4. Push rod 5. Moving plate 6. Water collector 7. Wear-resistant pad 8. Slider 9. Guide sleeve 10. Guide column 11. Fixed plate 12. Positioning ring 13. Fixed plate 14. Moving plate 15. Push plate 16. Push rod fixing plate 17. Limit column 18. Spring 19. Reset rod 20. Support column 21. Support column 22. Locking buckle 23. Lamp housing 24. Nozzle 25. Hot runner 26. Limit block 27. Stopper 28. Wear-resistant pad 29. Inclined guide column 30. Wear-resistant block 31. Slider

Figure 3 Fixed and moving mold structure
1. Guide column 2. Precision positioning part 3. Wear-resistant block 4. Pressure block 5. Pipe position 6. Water collector

Mold flow analysis was performed using Moldflow software. First, lamp housing was meshed, diagnosed and repaired. As shown in Figure 4 (a), mesh matching rate reached 92%, which is greater than recommended mesh matching rate of 85%, meeting requirements of mold flow analysis. Main runner is an open hot runner. Gating system design is shown in Figure 4 (b). Gate position plays a vital role in injection. If design is unreasonable, defects such as insufficient cavity injection are likely to occur. According to lamp housing structure and installation requirements and mold cavity distribution, a comprehensive analysis was conducted. Gate was set at step edge inside lamp housing, which did not affect its assembly requirements and was convenient for operators to trim gate condensate. Mold flow analysis results are shown in Figure 4 (c). Filling time is 2.782s, plastic melt flows smoothly, and filling is uniform. Filling end pressure analysis is shown in Figure 4 (d). Maximum pressure is 30.94 MPa. Weld line analysis is shown in Figure 4 (e). Weld line often appears at location where material flow reconverges after bypassing certain geometric shapes (such as holes) of plastic part. Weld line can be improved by increasing melt temperature, increasing filling rate, and increasing holding pressure. Cavitation analysis is shown in Figure 4 (f). When designing mold, exhaust grooves are set around cavity (see Figure 4 (g)), which is conducive to discharge of gas. At the same time, gas can also be discharged from gaps between inserts and pores of push rod.

 

Figure 4 Mold flow analysis

There is a 10 mm protruding rear cover mounting ring around two large circular holes at the bottom of lamp housing (see Figure 5 (a)). If mold parts adopt an integral structure (see Figure 5 (b)), it is difficult and inefficient to process them in a CNC machining center, and it is difficult to polish them after processing. If an inlay structure is adopted (see Figure 5 (c)), insert is embedded in mold and then fixed with screws. Advantage is that processing technology is good and CNC processing can be used directly, turning internal shape processing into external shape processing. Processing is convenient and efficient, it is easy to ensure accuracy of processed parts and facilitate subsequent polishing. During injection production, gas can be discharged from gap between inserts, effectively solving defect of insufficient cavity injection due to poor exhaust. If insert is damaged, it is easy to replace and repair.

 

Figure 5 Structural analysis
1. Moving template 2. Insert 3. Screw

Mold is designed with 12 side core pulling mechanisms. Inclined guide column is made of SK3 with a heat treatment hardness of 60 HRC; slider is made of 718 with a heat treatment hardness of 33.35 HRC; locking surface of slider is equipped with a wear-resistant block, and bottom surface is equipped with a wear-resistant pad. Wear-resistant block and wear-resistant pad are made of Cr12 with a heat treatment hardness of 60 HRC to prevent mold accuracy from being affected by sliding wear during production.
There are many side core pulling mechanisms. It is necessary to ensure that core pulling mechanism moves smoothly and avoid mutual interference, and to minimize mold size as much as possible to make structure compact. Design of inclined guide column is shown in Figure 6 (a). Diameter of inclined guide column is φ25 mm, inclination angle is 18°, and effective length of inclined guide column core pulling is L=S/sin18° (S is sliding distance of slider). One end of inclined guide pin is a boss, and the other end is hemispherical, with two large semicircular gaskets to press guide sleeve to fix it on fixed mold.
Side core pulling mechanism is shown in Figure 6 (b), in which sliders are all designed on movable mold side, slider adopts an integral locking method, and slider locking angle is 20°. Core pulling mechanisms ① and ② are same, and structure is shown in Figure 6 (c). Slider head has a large sealing surface. In order to make molded plastic parts cool evenly, slider is designed as a φ6 mm cooling water channel inside, as shown in Figure 6 (d). A pull rod 9 is provided at the tail of slider to ensure that slider can slide smoothly to stopper 10 when mold is opened, and is limited by positioning block. Side core pulling mechanisms ③, ④, ⑤, and ⑥ are same, and structure is shown in Figure 6 (e). Size is small, and slider is not provided with a water channel. A spring is provided inside. When mold is opened, spring pushes slider to assist slider in completing demolding. When inclined guide pin completes core pulling, slider slides to limit block under action of spring force. In order to prevent spring from fatigue failure and avoid slider from hitting mold parts when resetting, a limit block is provided at the bottom of slider to limit slider.

 

Figure 6 Side core pulling mechanism design
1. Fixed mold plate 2. Guide sleeve 3. Oblique guide column 4. Gasket 5. Lamp housing 6. Slider 7. Wear block 8. Oblique guide column 9. Pull rod 10. Block 11. Spring 12. Spring sleeve 13. Slider 14. Cooling water channel 15. Spring 16. Wear pad 17. Slider 18. Wear block 19. Oblique guide column 20. Pressure strip

There is a φ6 mm × 4 mm installation positioning pin hole on the back of lamp housing fixing buckle, as shown in Figure 7 (a). Mold uses an oblique push mechanism for demoulding, and adopts "oblique push block + oblique push rod + T-shaped seat" combination design, as shown in 7 (b). Guide sleeve of inclined push rod adopts style of avoiding air in the middle of two ends of tube. Material is bronze and fixed to moving mold plate with a round gasket. Material of inclined push rod is H13 and nitrided. T-shaped seat is made of 40Cr material. According to structure of plastic part, inclined push rod is designed to be 6°, mold ejection stroke is 50 mm, and demolding distance L=50tan6°=5.3 mm, which is greater than height of positioning pin hole by 4 mm, meeting demolding requirements. In order to make mold structure compact, T-shaped seat does not interfere with moving mold when mold is ejected, and is fixed to the bottom of push plate, as shown in 7 (c). When mold is opened, inclined push mechanism starts to move, and push plate drives T-shaped seat to push inclined push rod. Inclined push rod and inclined push block move along oblique direction to complete demolding movement.

 

Figure 7 Design of oblique push mechanism
1. Oblique push block 2. Guide sleeve 3. Oblique push rod 4. T-shaped seat 5. Moving mold plate 6. Pad 7. Moving mold seat plate 8. Lamp housing 9. Push rod fixing plate 10. Push plate

Injection mold uses cooling water circuit to adjust temperature to prevent defects such as warping and deformation of molded plastic parts due to uneven cooling. Layout of mold cooling water circuit is in the form of "straight-through water pipe + inclined water pipe + water well", which is three-dimensional and criss-crossed. Water channel diameter is φ12 mm, water well diameter is φ19 mm, and there is a partition in the middle of water well. Distance between each water channel is 30.40 mm, and distance from plastic part is 15.20 mm. Due to complex structure of plastic part and large height difference, design of mold cooling water circuit is difficult. Six sets of circulating water circuits are designed for fixed mold, and four sets of circulating water circuits are designed for movable mold, as shown in Figure 8 (c). Cooling water circuit of mold is analyzed by Moldflow software, and temperature difference of water circuit is within 5 ℃, which meets requirements of cooling effect. There are many cooling water circuits in mold. In order to facilitate quick connection of cooling water circuit and neat arrangement of water pipes, a water collector is installed on the side of mold to achieve segmented adjustment of cavity temperature, which is conducive to ensuring molding size or deformation adjustment of plastic part.

 

Figure 8 Cooling water circuit design

Ejection mechanism consists of a push rod, a push tube, a push plate, a push rod fixing plate, a reset rod and a pull rod. Warping deformation of plastic parts is mostly caused by imbalance of ejection, so ejection mechanism is an important part of mold design. Ejection mechanism design is shown in Figure 9. Six push plate guide pillars and six φ30 mm reset rods are designed. A φ60×200 mm butterfly spring is set on reset rod so that push plate can be reset smoothly after being ejected. Push plate is equipped with 20 φ8 mm push rods, 34 φ12 mm push rods, 6 φ10 mm push tubes and 2 oblique push mechanisms. There are 6 limit columns on push plate, limit pins under push rod fixing plate, and support columns in the middle round hole of push plate.

 

Figure 9 Push mechanism design
1. Push plate 2. Push rod fixing plate 3. Oblique push rod 4. Support column 5. Lamp housing 6. Lamp housing 7. Reset rod 8. Spring 9. Push rod 10. Limit column 11. Push plate guide column

After mold is installed on injection molding machine, it is positioned and locked by positioning ring. Injection material is PC+ABS, and relevant injection parameters are set. Injection molding machine starts to inject. When plastic melt fills the cavity, it is first pressure-maintained and then temperature control system is adjusted for cooling. When injection reaches set parameters, mold is opened by piston rod of injection molding machine, movable and fixed molds are separated. All slides of movable mold perform core-pulling movement driven by inclined guide pins. After core-pulling is completed, ejection mechanism pushes molded plastic part out of mold, and plastic part is separated from movable mold. After plastic part is taken by robot, ejection system is reset first, and at the same time, inclined guide pin resets slide to complete mold closing, and injection molding machine performs next injection molding.