Plastics can be used to form functional components with diversified structures due to their high specific strength and good processing performance. They have been widely used in automotive industry, enabling functional components on automobiles to achieve integrated design of many application features. Due to need to install car lights and transparent lampshades, automobile lampshade shells have mostly curved surfaces, complex structures, and many assembly features. Due to needs of working environment, higher requirements for temperature resistance of product are put forward, which is a typical application of replacing steel with plastic. Combining mold design process and results of a certain automobile lampshade, a case reference is given for reference in mold structure design of similar plastic parts.

 

(A) Outer structure

 

(B) Inside structure

 

(C) Wall thickness analysis
Figure 1 Lampshade structure of a certain car
Figure 1 shows structure of an automobile lampshade. Material is modified polypropylene (PP-T20), brand is IsofilK20TYLO232, and shrinkage rate is 1.01%. Because of its good high temperature resistance, it can meet requirements of actual working conditions.
Outer dimension of car lampshade is 374mm*224mm*472mm. Volume of plastic part measured by UG10.0 is 497.6cm3. Average wall thickness of plastic part is 1.66mm, and maximum wall thickness is 6.31mm. Plastic parts are required to be free from defects such as bubbles, weld marks and silver streaks. In addition, due to needs of its assembly function, there are multiple thin-walled and positioning jack features in outer part of main body of automobile lampshade. These structural features eventually lead to a large number of side cores of mold, which increases design difficulty of overall structure of mold. In Figure 1, M1 and M2 represent thin-walled connectors; M3 represents positioning jacks; M4~M7 represent positioning slots.

 

(A) Gating system

 

(B) Filling analysis

 

(C) Cooling system
Figure 2 Design of gating system and cooling system
Car lampshade has a large external size. Due to features of multiple thin-walled and positioning jacks in peripheral part of main body, multiple side core-pulling mechanisms are required around it. Final cavity layout adopts structure of 1 cavity, casting method adopts hot runner + ordinary runner, latent gate feed, plastic parts and gate aggregate do not need to be separated manually. According to calculation and design experience, main dimensions of gating system are determined as follows: main runner length is 369mm, main runner taper is 2°, small end diameter is ϕ5mm; runner is designed to be circular with a radius of 5mm and a gate diameter of ϕ2mm. Final gating system design is shown in Figure 2(a).
Plastic parts were imported into MoldFlow software to simulate filling of car lampshade. Result analysis is shown in Figure 2(b). Time for plastic melt to fill cavity is 2.4s, filling effect is good, and there is no insufficient filling.
A reasonable temperature control system can realize rapid demoulding of plastic parts, further improve shaping quality and production efficiency of automobile lampshade shells. In design process of injection mold cooling system, there are mainly three structures: cooling water pipes, cooling water wells and heat transfer rods. Cooling water pipes are round holes that is drilled in mold, cooling water and other media are passed into round hole to take away heat.
Mold cooling water pipe arrangement is shown in Figure 2(c). There are 8 water channels on movable mold side and 6 water channels on fixed mold side. Diameter of mold cooling water hole is ϕ11.5mm and ϕ18mm. Contact surface of mold core, cavity plate and each mold plate is equipped with a sealing ring for sealing to prevent water leakage. Arrangement design of cooling system ensures uniform cooling of plastic parts, improves molding quality and production efficiency of molded plastic parts.

 

Figure 3 Launching mechanism design
1.φ8mm round push rod 2.φ12mm push tube 3.φ12mm round push rod 4.φ10mm round push rod
Design of demolding system needs to consider principles of stable launch, beautiful plastic parts, safety and reliability. Because round push rod has advantages of easy processing and low movement resistance, it is the most widely used, its diameter size needs to be selected according to different positions of plastic part. Due to large volume of car lamp housing, a large demolding force is required. Design should ensure that thrust point of push rod is set in the part with high rigidity and strength of plastic part, large diameter push rod should be used as much as possible to increase effect surface. Materials of parts of injection mold are all selected SKD61 mold steel, carbonitriding treatment is carried out to increase hardness and wear resistance of friction surface. Final demolding system design is shown in Figure 3, with 1 ϕ12mm push tube, 7 ϕ10mm round push rods, 9 ϕ8mm round push rods and 11 ϕ12mm round push rods. When pushing out, push tube and round push rod push plastic part out of cavity at the same time, which ensures balanced force when plastic part is pushed out.

"Slider + inclined guide post" lateral parting and core pulling mechanism is usually used in core pulling mechanism outside movable mold and core pulling mechanism inside movable mold. Among them, outer core pulling mechanism of movable mold is the most commonly used, which has characteristics of reliable action, compact structure, and convenient manufacturing. It is suitable for occasions with small core pulling distance and low pulling force.

 

Figure 4 Layout of side core pulling mechanism
N1. Side core pulling mechanism І N2. Side core pulling mechanism ІІ N3. Side core pulling mechanism ІІІ N4. Side core pulling mechanism IV N5. Side core pulling mechanism Ⅴ
Mold selects surface with the largest profile of plastic part as parting surface. Based on above parting design principles, after setting main parting surface of plastic part, further considering that side slider core pulling mechanism is set on the side of fixed mold will make mold structure more complicated, and a lampshade needs to be assembled outside lampshade. Surface with high roughness requirement is set on the side of movable mold to meet quality requirements. Finally, side slider is set on the side of movable mold and inclined guide column is set on the side of fixed mold. In view of features existing on periphery of main body, a total of 5 core-pulling mechanisms on side slider side of inclined guide column are provided, as shown in Figure 4. When cavity is opened, inclined guide post fixed on the side of fixed mold drives sliding seat to move in direction of 7-shaped bead to complete demolding of local features. Among them, N1 is formed with M4 positioning slot, N2 is formed with M3 positioning socket, N3 is formed with M7 positioning slot, N4 is formed with M1 and M6 positioning slot, N5 is formed with M2 and M5 positioning slot.

 

(A) Moving mold (b) Fixed mold (c) Overall structure of mold
Figure 5 Three-dimensional structure of mold
Through above design and analysis, using UG10.0 design software to complete three-dimensional structure of mold as shown in Figure 5.

 

Figure 6 Two-dimensional structure of mold
1. Movable mold plate 2. Spring 3. Oblique guide column 4. Fixed mold plate 5. Screw plug 6. Fixed mold seat plate 7. Inclined guide column 8. Spring 9. Cushion block 10. Push rod fixed plate 11. Push plate 12. Movable mold seat plate 13. Push tube 14. Spring 15. Oblique guide column 16. Insert needle 17. Oblique guide column 18. Spring 19. Push rod 20. Reset spring 21. Reset rod 22. Hot nozzle 23. Hot runner junction box 24. Inclined guide post 25. Spring 26. Insert pin 27. Slide 28. Slide 29. Limit stop 30.7-shaped pressure strip 31. Slide 32. Pressure strip positioning pin 33. Slide 34. Slide 35. Guide sleeve 36. Guide post 37. Runner insert 38. Inclined guide post installation block
Mold base is a two-plate mold base with a size of 1060mm*800mm*960mm. Two-dimensional structure of mold is shown in Figure 6. Basic working process of mold: during injection, high-temperature melt enters ordinary runner system through hot runner system, then enters cavity through three latent gates at the same time, molded automobile lampshade shell is cooled by holding pressure and mold is opened. Relying on mold opening force of injection molding machine, oblique guide pillars set on fixed mold side drive sliding seats 27, 28, 31, 33, and 34 to move in direction of each matched 7-shaped bead to complete demoulding of local characteristics. The limit device is guaranteed by limit stop and spring.
Because movable mold side is provided with multiple core pulling mechanisms and polishing of  cavity wall of fixed mold ensures that plastic part can stay on movable mold side due to effect of packing force. Subsequently, movable mold side continues to make mold opening movement. Ejector rod of injection molding machine pushes push rod fixed plate to drive push rod and push tube to move at the same time, plastic part and gate aggregate are pushed out of movable mold cavity together with a distance of 100mm. After mold is closed, ejection mechanism is reset by reset rod and reset spring, sliding seat is driven by inclined guide post to complete reset of side core pulling mechanism. After closing mold, prepare for next injection molding.
Surface of car lamp shell is mostly curved, and many assembly features on periphery of main body are not conducive to demolding. Final mold is set up with a 1-cavity layout, gating system selects hot runner to ordinary runner + latent gate to form plastic part. In order to ensure smooth demolding of plastic parts, in view of local characteristics of car lamp housing, mold is equipped with 5 inclined guide pillar side slider core pulling mechanisms. 1 push tube and 27 round push rods are used to ensure force balance when plastic part is pushed out. Verified by actual mold trials, mold runs smoothly and quality of molded plastic parts meets customer requirements, which can provide a reference for design of injection molds for structural plastic parts on automobiles.