As shown in Figure 1, decoration of a certain automobile headlight (right) has a complex structure, a large height difference, a curved surface structure, and high appearance requirements. It is aluminum-plated on the surface and belongs to high-light plating parts. Defects such as black spots, ripples and flash are not allowed. Maximum size of plastic part is 633mm×216mm×70mm, average wall thickness is 2mm, and material is PC+ABS. It has super engineering material properties, high temperature resistance, flame retardancy, impact resistance, tensile strength, and good bending strength. Plastic part has two irregular large holes, one φ3.5mm round hole at S1; five φ4mm holes and one 6mm×4mm long hole at S2; buckle at S3 is convex and has one φ4mm hole, and there is a flange above buckle, so S3 buckle needs to be demolded by a side core pulling mechanism; there are two similar buckles at S4, each with a 5mm×15mm long hole, and buckle is inverted with Z axis of mold opening direction, so inner core pulling mechanism needs to be demolded.

 

Mold for automotive headlight decorative parts is shown in Figure 2, with a maximum size of 1150mm×1000mm×1050mm and a weight of 6.5×103kg. Mold uses a LKM mold frame, and fixed mold insert is made of German 1.2343ESR. After rough machining, it is quenched and hardness reaches 47~50HRC. After fixed mold core is finely processed, surface polishing is required to achieve a mirror effect. Movable mold has two large inner core-pulling sliders. In order to reasonably arrange and reduce difficulty of processing, a combination method is adopted to make mold structure compact, easy to disassemble and assemble, easy to repair and maintain. The whole mold has a total of 4 lateral core-pulling mechanisms, and mold side is also equipped with 2 water collectors for convenient connection of cooling water circuits.

 

1. Lifting ring 2. Moving mold base plate 3. Limiting pin 4. Push plate 5. Push rod fixing plate 6. Pad 7. Spring 8. Moving mold plate 9. Wear plate 10. Inner core pulling slider 11. Guide sleeve 12. Guide column 13. Push tube 14. Plastic part 15. Screw 16. Side core pulling slider 17. Oblique guide column 18. Integral wedge block 19. Locking block 20. Fixed mold base plate 21. Screw 22. Positioning ring 23, Cavity plate 24. Fixed mold plate 25, Plastic part 26, Push block 27. Push rod 28 Water collector 29. Core 30. Quick connector 31. Inner core pulling slider 32. Limiting column 33, Support column 34. Bullet 35, Push rod 36, Wear plate 37. Inclined guide column 38. Inner core-pulling slider 39. Screw 40. Tightening block 41. Foot column
Figure 2 Mold structure

Hot runner enables material to maintain good thermal stability during injection and can always keep plastic in a molten state. On the one hand, it can ensure stability of quality of molded plastic parts; on the other hand, it can reduce injection pressure and reduce generation of flash.
Mold gating system is designed with an open hot runner as main runner, as shown in Figure 3 (a). Runner is a common runner with a diameter of φ10mm and a U shape, attached to inclined surface of parting surface. Gate type is fan-shaped, and front end size is 14mm×1.5mm. In order to reduce injection pressure and shorten injection time, two gates are used on same side with a spacing of 30mm, and a 10mm long cold material hole is opened at the end of runner, as shown in Figure 3 (b). Gate position plays a key role in injection molding. Moldflow mold flow analysis software is used for analysis. As shown in Figure 4, the best gate position is A, followed by B. Because A is appearance surface, a gate cannot be set, and B is set as gate position. Injection molding analysis is shown in Figure 5. Injection pressure is less than 35MPa, filling time is 3.672s, material fluidity is good during injection process, filling molding is performed at a lower injection pressure to ensure molding quality and production stability of plastic parts.

 

Mold is a large injection mold, which uses 4 sets of positioning systems with high positioning accuracy, namely: ① φ60mm guide pillars are embedded on 4 corners of mold frame. Guide pillars are fixed to fixed mold plate and are 80mm higher than the highest surface of fixed mold, as shown in Figure 6 (a), to ensure safe clamping of movable and fixed molds. At the same time, matching guide sleeves are installed at corresponding positions of movable mold plate; ② In order to improve positioning accuracy of mold, pipe positions are designed at 4 corners of mold. Fixed mold is concave, and wear-resistant blocks are set on inclined surface to facilitate fitter to grind mold. Movable mold is convex, pipe position plays a role in locking and positioning; ③ 4 precise positioning blocks are set on mold parting surface for further precise positioning; ④ Interlocking with fixed mold is set at P, H, and K of movable mold plate, as shown in Figure 6 (b), to enhance positioning accuracy of mold. Finally, pressure plates and wear-resistant plates are evenly designed on the edges of cavity and core of parting surface to make pressure distribution more uniform during mold manufacturing, assembly and clamping, extend service life of mold.

 

1. Wear-resistant block 2. Pressure-bearing block 3. Precision positioning block 4. Fixed model core 5. Guide column 6. Guide sleeve 7. Pipe position
Figure 6 Design of guide positioning system

Layout of side core pulling mechanism and plastic part to be molded in mold is shown in Figure 7 (a). There are 4 side core pulling mechanisms in total. Outer core pulling mechanisms are D1 and D2. Due to compact mold structure, D1 and D2 wedge blocks 5 are integrated into one, with an inclination angle of 18° and a material of mold steel P20, as shown in Figure 7 (b). Diameters of inclined guide pins 3 and 7 are both φ20mm, effective length of core pulling is 85mm, and inclination angle is 16°. Core pulling distance L1=85sin16°=23.43mm of side core pulling sliders 2 and 8 is greater than height of S3 boss (20.5mm) in Figure 1, meeting core pulling requirements. D3 and D4 in Figure 7 (a) are inner core pulling mechanism, which is a left-right mirror structure and has a large overall structure. Slider 8 is made of German 1.2343ESR, and hardness reaches 52~55HRC after heat treatment. The overall dimensions are 585mm×145mm×220mm, and there is a cooling water channel inside, as shown in Figure 7 (c). When designing, it is necessary to consider convenience of fitter research and extension of mold service life. Wear-resistant blocks are set around slider. Wear-resistant blocks are also set on the sides of wedge block 15. Material is mold steel P20, and inclination angle is 18°. Diameter of inclined guide column 11 is φ30mm, effective length of core pulling is 80mm, and it has been quenched. Inclination angle is 16°, and core pulling distance L2=80sin16°=22.4mm, which is greater than the S4 buckle height (18.6mm) in Figure 1, meeting core pulling requirements. Since push rod passes through slider, slider is set with a long hole to avoid it, and end of slider is built with a butterfly spring of φ30mm×80mm to assist slider to slide to limit position when opening mold.

 

1. Limit block 2. Side core pull slider 3. Oblique guide column 4. Resistance block 5. Wedge block 6. Wear block 7. Oblique guide column 8. Side core pull slider 9. Limit block 10. Wear block 11. Oblique guide column 12. Inner core pull slider 13. Screw 14. Wear block 15. Wedge block 16 Wear block 17 Plastic part
Figure 7 Design of side core pull mechanism

Mold cooling system has a great influence on molding quality of plastic parts. It is necessary to try to make plastic parts cool evenly to prevent defects such as warping of plastic parts caused by uneven cooling. Mold adopts a grid-cube water channel layout of "straight-through water pipe + water well". Water channel diameter is φ12mm, water well diameter is φ19mm, and there is a partition in the middle of water well. Spacing between each water channel is about 50mm. Due to complex structure of plastic part and large height difference, 8 groups of circulating water channels are designed for fixed mold, as shown in Figure 8 (a). Middle group of water channels is used as hot runner cooling water channel; outermost group of water channels is fixed mold plate cooling water channel, which plays the role of overall cooling; remaining 6 groups of water channels are fixed mold core cooling water channels, which are distributed in three layers of high, medium and low. Advantage of this design is that deformation of plastic part can be controlled by adjusting cooling water channels. 7 groups of circulating water channels are designed for movable mold, as shown in Figure 8 (b). Left group of water channels is cooling water channel for two inner core-pulling sliders, which are connected by a hose to form a loop; remaining 6 groups are cooling water channels for movable mold, which are also distributed in three layers of high, medium and low. Cooling effect of mold is good, and temperature difference is controlled within 4℃, as shown in Figure 8 (c). There are many water channels in mold. In order to facilitate quick connection of water channels and arrangement of water channel pipes, a water collector is installed on the side of mold, and a quick connector is provided on water collector (see Figure 2).

 

After injection molding, ejection mechanism ejects plastic part from mold cavity to separate plastic part from mold. During demolding process, plastic part shrinks during pressure holding and cooling process in mold, and punch is tightly held, making demolding more difficult and increasing ejection force. When ejection force is unbalanced, it will cause molded plastic part to warp or whiten. Ejection mechanism design is shown in Figure 9. Ejection system consists of 2 push tubes, 16 round push rods, 4 flat push rods and 14 sets of "push rod + push block" components. Both push rods and push blocks are made of SKD61 steel, and hardness reaches 50~54HRC after heat treatment. Since ejection surface of plastic part is an inclined surface, push rod is equipped with an anti-rotation device, a grid is engraved on the end surface to increase friction and prevent sliding. There are 4 φ30mm guide pillars and 6 reset rods on push rod fixing plate, and an elastic element is set under reset rod. Ejection mechanism is reasonably arranged and ejection force is balanced, so that molded plastic parts can be demolded smoothly and steadily.

 

1. Elastic element 2. Guide column 3. Limit column 4. Guide block 5. Spring 6. Push rod 7. Push rod 8. Reset rod 9. Push block 10. Plastic part 11. Push block
Figure 9 Push mechanism

Mold obtains intact plastic parts during injection molding process and is related to cavity exhaust. If gas cannot be discharged from cavity in time, it will cause incomplete filling or burnt spots due to trapped gas. Therefore, exhaust system design should be reasonable. Exhaust system design is shown in Figure 10. Fixed model cavity is set with a three-level exhaust system. The first-level exhaust groove is 5mm wide and 0.015mm deep; second-level exhaust groove is arc-shaped, 5mm wide and 0.5mm deep, and is arranged around cavity; third-level exhaust groove is same size as second-level exhaust groove, which is connected to second-level exhaust groove to discharge gas out of mold. In addition, some gas is discharged from joints of mold inserts, push block joints, and slide block joints.