Abstract: According to structural characteristics of automobile rear bumper, a large thin-wall injection mold is designed by using advanced technology of external parting technology and sequential valve hot runner gate control. Mold forming parts, pouring system, lateral parting and core pulling mechanism, temperature control system, guide positioning system, demoulding system are analyzed and studied. Mold structure is advanced and reasonable, with many innovations, and a satisfactory molding effect is achieved.

Rear bumper of an automobile is one of the most important exterior parts of an automobile. Early automobile rear bumpers were made of metal, but modern automobile rear bumpers are made of plastic and are injection molded by molds. Plastic bumpers not only greatly reduce costs, but also have good buffering performance, are more elastic and can absorb impact force than metal, can automatically rebound and repair. However, because rear bumper is large in size, complex in structure, and has high appearance requirements, mold design is also very difficult. Following introduces a large thin-wall injection mold for rear bumper. It is hoped that colleagues engaged in mold design can use this to understand design points and advanced technologies of large automobile injection molds.

Figure 1 shows rear bumper of a car, which is similar to a bow or saddle shape. Material is PP+EPDM-T20, with a shrinkage rate of 0.95%. In blended material, T20 represents 20% talcum powder, which can improve rigidity of bumper. EPDM is ethylene propylene diene monomer rubber, which is a terpolymer of ethylene, propylene and non-conjugated dienes. Its main characteristics are excellent oxidation resistance, ozone resistance and corrosion resistance, and can improve elasticity of bumper.

 

Figure 1: Automobile rear bumper structure
Structural characteristics of rear bumper are as follows: ① Large size, flow length ratio of 200, belongs to a large thin-walled plastic part; ② Parting line has a large drop and a complex shape. There are 12 undercuts on the inside of plastic part. Lateral core pulling structure is focus and difficulty of mold design; ③ Outer surface requirements are high, and molding defects such as flash, shrinkage dents, and weld marks are not allowed.

Mold uses hot runners, order of feeding is controlled by a sequence valve, and external parting technology is used. Undercuts on both sides use core pulling structures of sliders, lifters and straight tops respectively. Maximum outer dimensions of mold are 2730 mm*1360 mm*1255mm, which is a typical large thin-walled injection mold. Detailed structure is shown in Figures 2 and 3.

 

Figure 2 Structure of rear bumper injection mold
1. Moving mold fixing plate 2. Pusher fixing plate 3. Slider 4. Hot runner frame plate 5 Fixed mold fixing plate 6. Pusher block 7. Secondary nozzle 8. Hot runner plate 9 Moving mold B plate 10. Positioning ring 11. Primary nozzle 12. Fixed mold A plate 13. Square guide column 14. Oil cylinder 15. Slider 16. Support plate 17. Support column 18. Ejector rod connecting pipe 19. Limit column 20. Nitrogen spring 21. Push rod

The first step in design of molded parts is to determine parting surface. There are two types of injection molds for automobile rear bumpers: external parting surface and internal parting surface. Structure of injection mold for external parting surface is simpler than that of injection mold for internal parting surface, but weld marks can be seen, which affects appearance of plastic part. After internal parting bumper is assembled, weld marks are hidden on non-appearance surface of automobile part, which has no effect on appearance, but structure of internal parting mold is more complex and mold manufacturing is more difficult, so its technical risk and mold cost are also higher than external parting bumper.
Although mold adopts external parting surface technology, it uses lateral slider to hide weld mark on invisible surface of plastic part, which not only simplifies mold structure and reduces cost, but also ensures appearance quality of plastic part, which is one of innovations of mold structure.
Mold forming parts are mainly composed of movable mold B plate 9 and fixed mold A plate 12. Mold forming parts of movable and fixed molds are all of an integral structure. Compared with inlay structure, it has advantages of smaller mold size, better strength and rigidity, and more compact structure.
Steel material of mold forming parts can be 718 or P20. For Japanese bumper molds, bumper mold is often cast in the form of cast steel FC250, which can reduce mold cost.

Rear bumper of car is a large thin-walled plastic part with difficult melt filling. Mold flow analysis and verification are required in the early stage of mold design. Casting system is another technical difficulty in mold design. Mold casting system uses 7 hot runner gates for feeding, 2 of which are in the middle of 2 lamp holes of plastic part, and the other 5 are below plastic part, as shown in Figure 3.

 

Figure 3 Hot runner gate location
Rear bumper is an appearance part. If an ordinary hot runner casting system is used, although melt can fill the entire cavity, there will definitely be weld marks on the surface. To solve this problem, mold design adopts sequential valve hot runner gate control technology, referred to as SVG technology, which is another innovative and advanced technology of mold. 7 hot nozzles in hot runner are all needle valve type, opening and closing sequence is controlled by cylinder and solenoid valve according to melt filling requirements, thereby achieving ideal effect of no weld marks on the surface of plastic part. SVG technology can not only eliminate weld marks on the surface of plastic part, but also eliminate residual stress inside plastic part and reduce molding cycle of mold.
In addition, mold adopts an integral hot runner system, which has advantages of easy assembly and disassembly, low processing accuracy requirements, no risk of leakage, reliable assembly accuracy, no need for repeated disassembly and assembly in the future, low maintenance and repair costs. Its structure is shown in Figure 4.

 

Figure 4 Integral 7-point sequential valve hot runner gating system

Rear bumper of car has undercuts at S1~S12. Lateral core pulling mechanism is quite complex and is core mechanism of mold. Three-dimensional diagram of lateral core pulling mechanism of undercuts S1~S10 is shown in Figure 5. They all use hydraulic core pulling and are composed of a cylinder, a slider, a guide block, a pressure block and a stroke switch. Although this lateral core pulling mechanism is complex, it is safe and reliable, is a classic structure in lateral core pulling mechanism.

 

Figure 5: Stereoscopic view of side core pulling mechanism of undercut S1-S10
Lateral core pulling mechanism of undercuts S11 and S12 has same structure. They both use an lifter lateral core pulling mechanism with an lifter angle of 3° to achieve lateral core pulling during ejection process. Lateral core pulling mechanism of undercut S11 consists of an inclined push block 24, an inclined push rod 22, an inclined push rod guide tube 23, an inclined push rod base 25 and an inclined push rod slider 26, as shown in Figure 6. Since upper surface of inclined push block 24 is an inclined surface, which forms an angle of 10° with bottom surface of mold, guide groove of inclined push rod base 25 must also slide along same angle and direction, otherwise top of rear bumper plastic part will be deformed or even cracked when lifter is pulled sideways.

 

Figure 6 Undercut S11 lateral core pulling mechanism

Temperature control system of rear bumper injection mold adopts a combined cooling method of "vertical water pipe + inclined water pipe + spacer water well", that is, water pipes are used first, followed by water wells. Among them, fixed mold uses 6 groups of water channels, and movable mold uses 8 groups of water channels, as shown in Figure 7. Mold temperature is fully cooled, which effectively guarantees molding cycle and plastic part quality of mold.

 

Figure 7 Bumper injection mold cooling system
Mold belongs to a large automobile part injection mold. Temperature control system is strictly followed in design of following principles:
(1) 3 m principle. Diameter of straight cooling pipe of a large automobile mold is generally φ15 mm, and the total length of cooling water channel cannot exceed 3 m. Because if it exceeds 3 meters, deep hole drill will not be able to process and drill nozzle is not long enough.
(2) Principle of priority for movable mold. Structure of movable mold is relatively complex, and heat is relatively concentrated, so it needs to be cooled intensively. However, cooling water channel must be kept at least 8mm away from push rod, straight top, lifter and other holes.
(3) Straight hole priority principle. If cooling water channel can be made into a straight hole, do not make an inclined hole. Inclined hole with an inclination of less than 3° should be directly changed to a straight hole. If inclined hole can achieve a good cooling effect, inclined hole should be used first.
(4) Palm effect principle. When arranging water channel of a large automobile injection mold, cooling water should flow in same direction, and intervals should be arranged like a palm. Distance between water channels should be controlled between 50 and 60mm, and water channel should be 25 to 28mm away from cavity surface. Water channel should be arranged along shape of cavity as much as possible to improve cooling effect.
(5) Length similarity principle. Lengths of different cooling water channels should not differ too much to ensure that mold temperature is roughly same.
In automotive parts injection mold, molding cycle of rear bumper mold was previously more than 120s, and some were even as high as 180s. At present, molding cycle of bumper mold is generally between 70 and 80s. Due to advanced and reasonable design of cooling system of this mold, cycle is shortened to 60s, and labor productivity of mold has been greatly improved, which has been highly recognized by customers.

Design of guiding and positioning system of injection mold of large automotive parts is very important. Difference from guiding and positioning system of other ordinary molds is that this mold uses 4 170mm×60mm×460mm square guide pillars and 1° precision positioning guide positioning, and specific positions are shown in Figures 2 and 8. This method is often used in large automotive parts injection molds. Square guide pillars can not only guide, but also have high positioning accuracy, subsequent maintenance and adjustment are more convenient, which effectively guarantees accuracy of plastic parts and service life of mold.

 

Figure 8 Rear bumper injection mold 3D structure

Large plastic parts have large clamping force and long ejection distance, must be ejected smoothly and safely. Mold demoulding mechanism adopts a "push block + push rod + inclined ejector" combined ejection mechanism. Since contact area between push rod and plastic part is small (push rod diameter is φ12mm), plastic part is easily deformed locally, so rear bumper injection mold uses as many push blocks as possible when designing. Ejection surface shape of push block can be determined according to shape and size of plastic part. In view of uneven surface of moving mold core, all push rods and push tubes must be designed with a rotation stop structure at fixed end.
Since mold has many ejection parts, demoulding force and ejection part reset force are both large, so demoulding mechanism uses 4 nitrogen springs as power source. Nitrogen spring structure is shown in Figure 9. Mold-specific nitrogen spring is a new type of elastic component with high-pressure nitrogen as working medium. Its advantages are long stroke, stable operation, small size, large elastic force, precise manufacturing, long service life (up to one million times), gentle elastic curve, and no need for pre-tightening. Disadvantages are high cost and inconvenient maintenance. Ejection distance of plastic part is 120mm, which is controlled by limit column 19 (see Figure 2). Depth of circular hole in Figure 9 is 122mm to avoid 2mm of air space to prevent interference.

 

Figure 9 Structure of nitrogen spring in injection mold of rear bumper

Mold working process can be divided into following eight stages:
(1) Melt filling stage: Plastic melt enters hot runner plate 8 through the first-level hot nozzle 11, then enters ordinary runner through second-level hot nozzle 7, and finally enters mold cavity through side gate.
(2) Pressure holding and cooling stage: After melt fills mold cavity, it is pressure held, cooled, and solidified. When it is solidified to sufficient rigidity, injection molding machine pulls mold movable mold fixed plate 1 to open mold.
(3) Mold opening stage: Mold opens from parting surface I, and molded plastic part is separated from fixed mold cavity. Mold opening distance is 1200mm and is controlled by injection molding machine.
(4) Lateral core pulling stage: After mold opening stroke is completed, oil cylinders in lateral core pulling mechanisms S1~S10 (see Figure 6) of molding undercut are started at the same time, and mold begins to pull core sideways. Lateral core pulling distance is 40mm, which is controlled by adjusting stroke switch.
(5) Demolding stage: After lateral core pulling is completed, injection molding machine pushes nitrogen spring 20 on pusher fixing plate 14 through K.O. hole on movable mold fixing plate 1. Nitrogen spring pushes push rod, push block and lifter, while performing inner side core pulling, and pushes rear bumper plastic part away from movable mold core.
(6) Part removal stage: Start robot to take out plastic part.
(7) Reset stage: Nitrogen spring pushes pusher fixing plate, then pushes push rod, push block and lifter to reset, and oil cylinder pushes slider in each lateral core pulling mechanism to reset.
(8) Mold closing stage: Injection molding machine pushes movable mold to close mold, and mold starts next injection molding.

(1) Movable and fixed molds adopt an integral structure, which greatly reduces mold's external dimensions, greatly improves mold's rigidity and life.
(2) Pouring system successfully eliminates weld marks and internal residual stress on rear bumper's exterior surface by adopting sequential valve hot runner gate control technology (SVG technology), greatly improving dimensional accuracy and molding quality of plastic part.
(3) Clever use of lateral sliders successfully solves problem of external parting leaving marks on the surface of plastic part. In addition, although lateral core pulling mechanism S1~S10 adopts hydraulic cylinder core pulling, which increases mold cost, it greatly simplifies mold structure, reduces mold failure rate, and ultimately reduces production cost of rear bumper.
(4) Mold temperature control system successfully reduces molding cycle of rear bumper to 60 s by adopting 14 sets of "vertical water pipes + inclined water pipes + spacer water wells" cooling water channels, greatly improving mold labor productivity and economic benefits of enterprise.
(5) Demoulding system uses nitrogen springs for ejection and reset, successfully solving problem of large thin-wall injection molds with large ejection force, long ejection distance, and easy deformation of plastic parts during demoulding.
In actual production process, mold production is stable, and all quality indicators of molded plastic parts meet design requirements. A complex, precise, large and long-life injection mold was successfully designed.