With development of automobile industry, consumers have higher and higher requirements for appearance quality of automobiles. One of important functions of automobile plastic parts is to play a surface decoration role, so surface quality requirements of automobile plastic parts are also more stringent. Shrinkage marks are local depressions formed on the surface of plastic parts, which are one of main appearance defects affecting surface quality of plastic parts. Reasonable mold structure design requires that there are no shrinkage marks on the appearance of plastic parts after molding. Due to limitations of traditional injection mold demolding mechanism and plastic part flange strength, anti-shrinkage mark design of flanges at both ends of automobile bumper has not reached ideal state, causing shrinkage marks generated by flanges at both ends of automobile bumper to become an industry technical problem. In order to make up for design defects of anti-shrinkage mark structure, it is necessary to increase number of injection mold gates and plastic part molding holding time, but this will lead to an increase in mold manufacturing costs and plastic part production costs, and anti-shrinkage mark effect is not ideal, as shown in Figure 1.

 

Figure 1 Shrinkage marks caused by flanges at both ends of bumper
In order to solve above problems, an anti-shrinkage mark mold mechanism was designed for a certain automobile plastic bumper, which met needs of reducing rubber on inner side of flanges at both ends of bumper to prevent shrinkage marks, reduce number of gates of bumper and shorten molding time, reduce mold manufacturing cost and plastic part production cost, improve appearance quality and flange strength of plastic part.

Plastic part is a front bumper of a certain car, made of PP-T20, and painted on the surface. Due to modeling and engineering requirements of plastic part, flanges need to be designed at the ends of plastic part and fender, angle θ between flange and appearance surface is less than 90°, as shown in Figure 2. If traditional bumper injection mold structure design is adopted, flange is formed by a combination of an outward push block and a large lifter, as shown in Figure 3. In order to prevent shrinkage marks on the exterior surface caused by flange, outer root of flange needs to be treated with reduced rubber. When using traditional bumper injection mold structure design, inner side of flange cannot be reduced in rubber. Reducing glue will form an undercut on inner side of flange, making it impossible to demold large lifter. Wall thickness a of flange root after reducing glue should not be greater than 0.33 times (empirical value) of main wall thickness b of exterior surface. Because flange has an assembly function, in order to ensure strength of flange, engineering requirements for flange wall thickness a should not be less than 0.5 times (empirical value) of main wall thickness b of exterior surface, which leads to a contradiction between design requirements for shrinkage marks on exterior surface and design requirements for flange strength. To solve this contradiction, on the basis of ensuring strength of flange, a gate is usually designed near flange and injection holding time is extended to reduce degree of shrinkage marks on exterior surface. However, this measure will increase mold cost and production cost of plastic parts, and shrinkage mark prevention effect is not ideal.

 

Figure 2 Flange structure at both ends of bumper

 

Figure 3 Combination of large lifter and push-out block
A fillet M is designed at connection between bumper flange and exterior surface (see Figure 3). According to experience, fillet has a stronger ability to resist shrinkage marks than flat surface. If rubber can be reduced from inside of flange so that shrinkage mark position "falls" on fillet M, wall thickness a at the root of flange after material is reduced is not greater than 0.65 times main wall thickness b of exterior surface, a better shrinkage mark prevention effect can be achieved, and requirements of project for flange strength can also be met. Take front bumper of car as an example: main wall thickness of front bumper appearance is 3 mm, and original wall thickness of root of flange at both ends is 2.5 mm. If traditional bumper injection mold structure design is adopted (rubber can only be reduced from outside of flange), without considering compensation measures, wall thickness of root of flange at both ends of bumper needs to be thinned to 1 mm (reduced by 1.5 mm) to achieve a better anti-shrinkage effect, which cannot meet engineering requirements for flange strength; if rubber can be reduced from inside of flange to prevent shrinkage, wall thickness of root of flange only needs to be thinned to 1.95 mm (reduced by 0.55 mm) to achieve a better anti-shrinkage effect, while meeting flange strength requirements. In order to reduce rubber from inside of flange, a new anti-shrinkage mold mechanism is designed for above-mentioned front bumper.

After reducing rubber by 0.55 mm on inner side of flanges at both ends of front bumper (wall thickness of flange root before and after rubber reduction is 2.5 and 1.95 mm respectively), there is a side concave at N (see Figure 4) in demoulding direction Z1 of large lifter. In order to realize demoulding of side concave at N, traditional large lifter and push-out block combination mechanism is optimized, and an inner push block mechanism is designed on combination mechanism to prevent shrinkage marks, as shown in Figure 5.

 

Figure 4 After reducing rubber on inside, there is a side concave in direction of movement of large lifter

 

Figure 5 Anti-shrinkage mark mechanism
During mold opening process, inner push block moves 5.55 mm along Z2 direction (0.55 mm side concave depth + 5 mm safety distance) to remove side concave on inner side of plastic part flange in Z1 direction of large lifter demolding direction; outer push block moves 5.5 mm along Z3 direction (2.5 mm side hole depth + 3 mm safety distance) to remove side hole on outer side of plastic part flange; large lifter moves 39.89 mm along Z1 direction (34.89 mm side concave depth + 5 mm safety distance) to remove side concave of plastic part flange in main demolding direction of mold.

Anti-shrinkage mark mechanism of bumper injection mold is mainly composed of inner push block mechanism, outer push block mechanism, and large lifter mechanism.

Push block mechanism is shown in Figure 6. Push block is fixed on push block guide rod. Push block guide rod passes through push block guide rod guide sleeve and large lifter. Push block guide rod is installed on push block guide rail and can slide along track of guide rail. Push block guide rod guide sleeve is fixed on large lifter. Push block guide rail is fixed on mold fixed plate. When large lifter moves upward, push block, push block guide rod, and push block guide rod guide sleeve move upward under thrust of large lifter; push block guide rail is fixed, and push block moves along track of push block guide rail while moving upward; push block guide rod and large lifter demolding direction Z1 are at an angle of 25° (relationship between push block and large lifter is similar to relationship between lifter and core). During movement, push block is out of side concave on inner side of bumper flange in Z1 direction. Push block mechanism mainly forms side concave structure (side concave formed after reducing rubber) on inner side of flanges at both ends of bumper in demolding direction Z1 of large lifter.

 

Figure 6 Push block mechanism
1. Water channel connection block 2. Push block 3. Push block guide rod guide sleeve 4. Push block auxiliary rod guide sleeve 5. Push block auxiliary rod 6. Push block guide rod 7. Push block auxiliary rod 8. Push block guide rail 9. Bumper
Push block auxiliary rod is fixed to push block by screws, and push block auxiliary rod guide sleeve is fixed on large lifter. Two auxiliary rods play an auxiliary guiding role in movement of push block, increasing balance of push block movement.
Because contact area between push block and plastic part is large, push block receives a lot of heat during injection process. In order to prevent push block from overheating and expanding, resulting in poor movement, a "U"-shaped water channel is designed inside push block, as shown in Figure 7. Water channel is connected to external cooling water channel of mold through water channel connection block.

 

Figure 7 Internal water channel of inner push block

Outer push block mechanism is shown in Figure 8. Outer push block is fixed on outer push block guide rod. Outer push block guide rod passes through outer push block guide rod guide sleeve and large lifter. Outer push block guide rod is installed on outer push block guide rail and can slide along track of guide rail. Outer push block guide rod guide sleeve is fixed on large lifter, and outer push block guide rail is fixed on mold fixed plate. When large lifter moves upward, outer push block, outer push block guide rod, and outer push block guide rod guide sleeve move upward under thrust of large lifter; outer push block guide rail is fixed, and outer push block moves along track of outer push block guide rail while moving upward; outer push block guide rod and large lifter demolding direction Z1 are at an angle of 8° (relationship between outer push block and large lifter is similar to relationship between lifter and core). During movement, outer push block slips out of side hole on the outside of bumper flange. Outer push block mechanism mainly forms side hole structure on the outside of flange at both ends of bumper.

 

Figure 8 Push-out block mechanism
1. Push-out block guide rail 2. Push-out block guide rod 3. Push-out block guide rod guide sleeve 4. Push-out block auxiliary rod 5. Push-out block auxiliary rod guide sleeve 6. Bumper 7. Push-out block 8. Push-out block auxiliary rod
Push-out block auxiliary rod is fixed to push-out block by screws, and push-out block auxiliary rod guide sleeve is fixed to large lifter. Two auxiliary rods play an auxiliary guiding role in movement of push-out block, increasing balance of push-out block movement. Because contact area between push-out block and plastic part is small, heat received by push-out block during injection process is small, and no cooling water channel is designed inside push-out block.

Large lifter mechanism is shown in Figure 9. lifter block guide bar is fixed on mold fixing plate, lifter block is installed on lifter block guide bar and can slide along guide bar. lifter block is fixed on lifter rod, which passes through lifter rod guide sleeve and mold fixing plate. Root of lifter rod is fixed to the slide of universal slide by screws. lifter rod guide sleeve is fixed to mold fixing plate, and universal slide is fixed to push rod fixing plate. lifter rod can slide along lifter rod guide sleeve or follow slide on universal slide. One end of auxiliary rod is fixed to mold fixing plate, and the other end is fixed to mold movable mold base plate through fixed block. Auxiliary rod is connected to lifter rod through slide. During process of mold ejecting molded plastic part, push rod fixing plate moves upward, universal slide pushes lifter rod and lifter block to move upward. Angle between center axis of lifter rod and main demolding direction of mold is 14°. During upward movement (ejection distance is 160 mm), lifter block gradually escapes from side concave of flanges at both ends of bumper in main demolding direction of mold. Lifter auxiliary rod plays an auxiliary guiding role in movement of lifter rod, reducing risk of lifter rod movement being stuck. The large lifter mechanism mainly forms side concave structure of flanges at both ends of bumper plastic part in main demolding direction of mold.

 

Figure 9 Large lifter mechanism
1. Bumper 2. Ejector block guide strip 3. Ejector block 4. Ejector rod guide sleeve 5. Ejector rod 6. Slide 7. Fixed block 8. Universal slide 9. Ejector auxiliary rod

The overall structure of anti-shrinkage mark mechanism is shown in Figure 10. When mold is completed and mold is opened, it is first pulled and deformed (for internal parting bumper mold, deformability of plastic part is used to pull end of plastic part inward through push block to separate fixed mold cavity from side convexity of end of plastic part) and move synchronously. Synchronous movement is divided into two steps: in the first step, lifter block first moves backward (in Z1 direction mentioned above) to make room for bumper to be deformed; in second step, lifter block continues to retreat, inner push block and outer push block drive two ends of bumper to move backward along their respective guide rail B sections (see Figure 11), pull out side bulges of bumper in main demolding direction of mold due to inner parting, completing deformation action; then fixed mold is separated from bumper, lifter block continues to move backward relative to bumper, inner push block and outer push block push two ends of bumper forward relative to lifter block along their respective guide rail C sections. Because there is a certain inclination angle between inner push block guide rod, outer push block guide rod and lifter block, inner push block and outer push block gradually break away from inner side concave and outer side hole of flange at both ends of bumper during this movement (same principle as lifter breaking away from side concave of plastic part), then mold continues to perform subsequent ejection action to push molded plastic part out and enter next cycle. Mold and plastic parts are shown in Figure 12. After shrinkage mark prevention mechanism is adopted, shrinkage marks will no longer appear on flanges at both ends of plastic parts.

 

Figure 10 Shrinkage mark prevention mechanism

 

Figure 11 Segmented guide rail structure

 

Figure 12 Mold and plastic parts