Front bumper of a car is one of the most important exterior parts of a car. It must not only have sufficient strength and rigidity, but also serve as a buffer and protect car body in a collision accident. It must also pursue harmony and unity with car body shape, achieve its own lightweight. In order to achieve this purpose, current front bumper body of a car is made of plastic, commonly known as plastic bumper, which is injection molded by an injection mold.

Shape of front bumper of a car is similar to that of a saddle. Specific structure is shown in Figure 1. Material is PP+EPDM-T20, shrinkage rate is 0.95%. Among them, PP is polypropylene, which is main material of bumper. EPDM can improve elasticity of bumper cover, and T20 means that 20% of talcum powder is added to material, which can improve stiffness of bumper cover.
Characteristics of plastic parts are:
(1) Shape is complex, size is large, and wall thickness is relatively small. It is a large thin-walled plastic part.
(2) Plastic parts have many collisions and insertion holes, many reinforcing ribs, and high resistance to injection molding melt flow.
(3) There are three undercuts on the inside of plastic part, and it is quite difficult to pull core laterally at each point.

 

Figure 1 Structure of front bumper of car

Injection mold for main body of front bumper adopts an internal parting surface, passes through a hot runner, and is controlled by a sequence valve to enter glue. Inverted buckle on both sides adopts structure of large lifter sleeve, horizontal lifter and straight top, maximum size is 2500*1560*1790mm, and mold structure is shown in Figure 2.

 

 

Figure 2 Structure of front bumper injection mold
1. Fixed mold fixing plate; 2. Bearing plate; 3. Hot runner plate; 4. Positioning ring; 5. First-level hot nozzle; 6. Air valve; 7. Valve needle; 8. Second-level hot nozzle; 9. .Moving mold fixed plate; 10. Square iron; 11. Push rod fixed plate; 12. Push rod bottom plate; 13. Support column; 14. Dynamic mold square guide column; 15. Push rod; 16. Movable template; 17. Support Foot; 18. Fixed mold square guide post; 19. Fixed target plate; 20. Inclined push rod; 21, 29. Revolving pin; 22, 28. lifter; 23, 34. Guide rail; 24, 27. Movable mold insert ; 25, 26. Straight top; 30. Inclined push rod; 31, 39. Guard rod; 32, 33, 40, 41. Guide post; 35, 36, 37. Push rod; 38. Positioning block; 42. Wear-resistant Block; 43. side core pulling; 44. spring; 45. limit block

3D diagram of this mold is shown in Figure 3, advanced internal parting surface technology is used in mold design, as shown in Figure 4. Advantage is that parting clamp line is hidden on non-appearance surface of bumper, and cannot be seen after being assembled on car, so it will not affect appearance. But this kind of technology is more complicated than outer parting bumper in terms of difficulty and structure, and technical risk is also higher. Mold cost and mold price will be much higher than outer parting bumper, but because of its beautiful appearance, it is widely used in middle and high-end automobiles, as shown in Figure 3.

 

 

Figure 3 Three-dimensional view of front bumper injection mold
In addition, this plastic part has a large number of through holes, some of which are larger in area. Venting groove and avoiding groove are designed at collision point, insertion angle is greater than 8 °, which can increase service life of mold and prevent flash .
Front bumper injection mold molding parts and mold plate are integrated, and mold plate material can be pre-hardened injection mold steel P20 or 718.

 

Figure 4 Eight-point sequence valve hot runner system and cooling system

Mold gating system adopts an integral hot runner system. Its advantages include convenient assembly and disassembly, low processing accuracy requirements, no risk of glue leakage, reliable assembly accuracy, no need for repeated disassembly and assembly, low maintenance and repair costs.
Front bumper is an exterior part, surface is not allowed to have weld marks. Weld marks must be driven to non-appearance surface or eliminated during injection molding. This is one of key and difficult points of mold design. This mold adopts 8-point sequential valve hot runner gate control technology, namely SVG technology. This is another advanced technology used in this mold. It controls opening and closing of eight hot nozzles through drive of a cylinder, thereby achieving ideal effect of no weld marks on the surface of plastic part. Location of hot runner gate of front bumper injection mold is shown in Figure 5.

 

Figure 5 Analysis of injection mold flow of automobile front bumper design
SVG technology is a new hot runner molding technology developed in recent years to meet needs of automotive industry for large flat plastic parts and electronics industry for miniature thin-walled parts. Compared with traditional hot runner gate technology, it has following advantages.
①Melt flow is stable, holding pressure is more uniform and consistent, feeding effect is remarkable, shrinkage rate of plastic parts is consistent throughout, and dimensional accuracy is improved;
②Weld line can be eliminated, or weld line can be formed on non-appearance surface;
③Reduce clamping pressure and residual stress of plastic parts;
④Molding cycle is reduced, labor productivity of mold is improved.

 

Figure 6 is a simulation data diagram of glue injection of front bumper of a car using a hot runner sequence valve gate. Through mold flow analysis, it can be seen that under normal injection pressure, clamping force and mold temperature, melt flow is stable and quality of plastic parts is good, so service life of mold and product qualification rate can be fully guaranteed.
Figure 6: Analysis of injection mold flow of automobile front bumper design

Since front bumper adopts an inner parting surface, parting line at undercut of fixed mold A plate is located under lifter of movable mold side. In order to avoid risk of damaging mold during operation, core-pulling step during mold opening must be strictly controlled. For details, please refer to mold working process. This mold adopts a straight top and a lower lifter design, and a complex structure of a horizontal lifter (namely, a compound lifter) is designed in lifter. In order to pull core smoothly, there must be enough space between lifter and straight roof, contact surface of lifter and straight roof shall be designed with a slope of 3°～5°. Large lifter and large straight roof on both sides of inner parting bumper injection mold should be designed with cooling water channels. Side hole of fixed mold of inner parting bumper should be designed with a fixed mold elastic pin structure core-pulling, as shown in enlarged view at E in Figure 2. What I want to explain here is that internal parting bumper injection mold is different from general injection mold. When mold is opened, plastic part is not left in movable mold and then ejected, but is ejected synchronously by hook during mold opening process. Side core 43 of mold is ejected during mold opening process, plastic part will follow fixed mold for a certain distance, see "Mold Working Process".

Design of temperature control system for injection mold of front bumper has a great influence on molding cycle of mold and quality of product. Mold temperature control system adopts form of "straight cooling water pipe + inclined cooling water pipe + cooling water well", as shown in Figure 2.
Design points of cooling water channel of this mold are as follows:
① Structure of movable mold is more complicated, heat is more concentrated. Cooling should be emphasized, but cooling channel must be kept at least 8mm away from push rod, straight top and lifter hole.
②Distance between water channels is 50-60mm, distance between water channels and cavity surface is 20-25mm.
③If cooling channel can be made straight holes, oblique holes should not be made. Oblique holes with an inclination less than 3 degrees can be directly changed to straight holes.
④Length of cooling water channel should not be too different to ensure that mold temperature is roughly equal.

This mold is a large-scale thin-walled injection mold. Design of guiding positioning system directly affects accuracy of plastic part and life of mold. This mold adopts square guide pillars and 1 ° precise positioning guide positioning. Among them, movable mold side adopts 4 square guide pillars 80*60*700 (mm), movable mold and fixed mold adopt 4 square guide pillars 180*80*580 ( mm). Location is shown in Figure 2 and Figure 3.
In terms of parting surface positioning, mold adopts two conical surface positioning structures (also known as internal mold tube positions) at both ends, inclination angle of conical surface is 5°.

Plastic parts are large thin-walled parts, demoulding must be stable and safe. Middle position of mold adopts straight top and ejector pin, diameter of ejector pin is 12mm. Because contact area is small, it is difficult to return,it is easy to cause thimble to collide with cavity surface of fixed mold, so inner parting bumper is designed to be straight as possible, thimble is used less.
Due to large number of push parts, demolding force and pusher resetting force are relatively large, so demolding system uses 2 hydraulic cylinders as power source. Position of oil cylinder is shown in Figure 7. Size L in figure is distance to be delayed, which is related to size of fixed mold undercut, generally 40~70mm.
Due to uneven surface of movable mold core, all fixed ends of thimble and cylinder are designed with anti-rotation structures.

 

Figure 7 Position of cylinder

Since this bumper injection mold uses internal parting technology, parting line of undercut position of A plate is located under lifter of movable mold side. In order to avoid risk of damage to mold during operation, working process of mold is very strict. Let's talk about steps and precautions starting from mold clamping.
①Before closing mold, ejector plate is at a distance of 50mm from bottom plate of mold, so as to ensure that undercut of A plate does not touch horizontal small lifter protruding from large lifter, to ensure that A plate can be smoothly reset by pressing Lever to complete clamping action. See Figure 8. In figure, A1 indicates that parting surface of A plate presses 6 reset rods to reset, A2 is 6 reset rods, A3 is 8 red springs, which are used to ensure that push rod plate is pushed out by 50mm.

 

②Fixed mold A plate returns push rod plate and oblique top pressure, as shown in Figure 9.

 

③Before opening mold, it is necessary to pre-apply pressure to ejection cylinder to ensure that entire ejection system and A plate can be opened synchronously. When opening mold, A plate and ejector plate are first opened 60mm synchronously to ensure that plastic parts and horizontal small lifter are all separated from inverted surface of A plate. See Figure 10.

 

④Fixed mold A plate continues to open mold, ejector plate in movable mold keeps ejection state of 60mm unchanged to achieve effect of separating A plate from straight top, as shown in Figure 11.

 

⑤After fixed mold is opened to required space, ejector plate in movable mold continues to be pushed out to 164mm. At this time, horizontal small lifter guide rod reaches inflection point of changing angle of guide rail, inverted buckle surface of plastic part leaves mold. If plastic part has a small lifter at this time, directly pull plastic part out of mold by hand, otherwise, ejector plate will continue to push out to final position of 210mm, as shown in Figure 12.

 

⑥If product is slightly sticky to small lifter, it will be ejected when it is pushed out to 164mm. Remove product and directly cycle to step ① to prevent product from being pulled back by horizontal small lifter, resulting in failure to remove product.

 

⑦If plastic part does not stick to horizontal small lifter, ejector plate continues to be pushed out to 210mm, after demolding is completed, plastic part is removed, cycle goes to step ①, as shown in Figure 14.

 

1. This mold adopts internal parting technology to ensure beautiful appearance of plastic parts.
2. Mold adopts secondary core-pulling structure of "composite lifter", which solves problem of lateral core-pulling in complex parts of plastic part.
3. Mold adopts an eight-point needle valve type sequence valve hot runner gating system, which solves melt filling problem of large thin-walled plastic parts.
4. Mold uses hydraulic pressure as power of demolding system, which solves problems of large demolding force and difficult resetting of the pusher.
Practice has proved that mold is advanced and reasonable in structure and accurate in size. It is a classic among automobile molds. Since mold was released from production, lateral core-pulling action has been coordinated and reliable, quality of plastic parts has been stable, meeting customer requirements.