There are many types of bent pipe plastic parts in automobiles, such as intake pipes and fuel tank connecting pipes, and structural designs of different brands are also different. Based on structural characteristics of automobile bent pipe joint plastic parts and their appearance and performance requirements, a complex injection mold was designed, including a fixed mold and moving mold core-pulling mechanism, a fixed-distance priority mold opening mechanism, an opening and closing sequence protection device, and a forced mold closing mechanism. In the early stages, relevant literature was reviewed to understand research conducted by many scholars on design of injection molds for bent pipe core-pulling and other related issues. For example, Xiao Guohua took demolding problem of inner wall of a 180° bent pipe plastic part as research object, dividing bent pipe core of inner wall of the 180° bent pipe into two groups, with three short arc forming blocks in each group, thus solving demolding problem of 180° bent pipe plastic part. Taking molding of three-dimensional bent pipes as an example, Meng Jian designed an injection mold with a two-stage single-opening and two-ejection mechanism, successfully solving problem of difficult demolding of inner wall of bent pipe. Wang Huaiao et al., using a 90° bend in automotive hydraulic hoses as their research object, designed two demolding mechanisms to solve product demolding problem: a rack and pinion arc rotation core-pulling mechanism and a cylinder-driven side slider forced demolding mechanism. This paper describes a complex injection mold designed for automotive bent pipe plastic part, taking into account its structural and appearance requirements. Article mainly introduces mold's innovative points and design key points: a forced mold closing mechanism, a fixed-distance priority mold opening mechanism, a mold opening and closing sequence protection device, a moving mold side parting angled core-pulling mechanism, and a fixed mold angled slider core-pulling mechanism.

Figure 1 shows structural diagram of bent tube plastic part. Part is tubular with a 30-degree bend that creates undercuts a and b in demolding direction. A 3.0 mm thick flange at the bottom serves as an assembly point. Surface of part requires texturing, must not have any scratches. Flange has four through-holes, no burrs are allowed inside. Meeting these conditions simultaneously increases difficulty of mold design. Part's dimensions are medium to small, measuring 140 mm * 123 mm * 122 mm, and it weighs approximately 130 g. Material is sodium fatty alcohol polyoxyethylene ether sulfate (AES), a resin with excellent weather resistance. This means that even long-term exposure to outdoor ultraviolet radiation, humidity, rain, sunlight, and ozone can maintain stable properties without coating, better preserving original fresh and bright color of product. Therefore, it is increasingly used in automotive parts and other consumer electronics products requiring long lifespan and reliable safety. Advantage of not requiring coating makes AES more competitive in price. In addition to avoiding common injection molding defects such as shrinkage and flash, special attention must be paid to surface scratches, trapped air burning, and control of parting line lines on the side.

 

Figure 1 Part Structure

Mold has three core-pulling mechanisms. One fixed mold inclined slider core-pulling mechanism disengages undercut a. Fixed mold inclined slider needs to be designed with a fixed-distance priority mold opening mechanism to ensure that fixed mold inclined slider core-pulling mechanism has priority in core pulling during mold opening. Large sliders on both sides of moving mold are side parting structures, disengaging from plastic part cavity to solve demolding problem of undercut b. To prevent collision between moving mold slider and fixed mold inclined core-pulling mechanism during mold closing, a forced mold closing mechanism is designed to ensure moving mold slider core-pulling mechanism has priority in reset (see diagram below for forced sequential mold closing protection device), then fixed mold slider is reset after mold closing. Mold adopts a one-mold-one-cavity layout, with a standard mold base size of 650 mm * 610 mm * 631 mm and a weight of approximately 1300 kg. Mold opens in two sequential steps to eject plastic part, and closes in two sequential steps to reset core-pulling mechanism. An opening and closing sequence protection device provides an extra layer of safety for mold opening and resetting. The overall mold structure is shown in Figure 2.

 

1. Ejector Panel 2. Plate A 3. Plate B 4. Square Iron 5. Ejector Pin Panel 6. Ejector Pin Base Plate 7. Base Plate 8. Angled Pin 9. Wear-resistant Block 10. Slider B 11. Limiting Block 12. Return Pin Spring 13. Cooling Water Channel 14. Plastic Part 15. Sprue Bushing 16. Moving Mold Core 17. Return Pin 18. Guide Pillar 19. Guide Bushing 20. Fixed Mold Core 21. Spring 22. Limiting Screw 23. Flange 24. Squeegee 25. Fixed Mold Angled Slider Base 26. Ladder Slot Structure 27. Fixed Mold Angled Slider 28. Pin 29. Rocker Arm Base 30. Rocker Arm Spring 31. Rocker Arm 32. Parting Surface Fixing Block 33. Ejector Plate Guide Pillar 34. Magnetic Mold Locking Device Assembly 35. Resin Opener/Closer 36. Fixed mold inclined slider and moving mold core guiding and positioning locking structure; 37. Moving mold slider pressure plate; 38. Slider guide block; 39. Fixed/moving mold core guiding and positioning locking structure; 40. Slider A
Figure 2: Mold Assembly Diagram

Wall thickness of bent pipe plastic part is 3.0 mm, with no particularly thick or thin sections, resulting in a uniform and reasonable design. Mold design opts for a single-point, large-gate injection method directly into surface of sprue at assembly location, and a sprue bushing is designed to extend sprue's lifespan. Advantages of this injection method are fast filling, easy injection control, low filling pressure loss, less burr and flash caused by high injection molding pressure. Disadvantage is that injection point needs to be treated separately afterward. However, since injection point is located on assembly position on appearance surface, processing requirements for injection point are not high and are relatively simple.

As shown in Figure 2(a), moving mold side parting angled core pulling mechanism has two sliders, A and B, which are side parting structures, and the entire cavity of plastic part is contained within them. Its structure is that moving mold slider pressure plate is fixed to plate B with screws. During mold opening, angled pins provide core pulling power to safely eject plastic part cavity and undercut b. Angled pins are made of SUJ2 high carbon chromium bearing steel, with two pins for each slider, each 25 mm in diameter, ensuring sufficient strength. Wear-resistant block structures are designed on locking surface and bottom sliding surface of sliders A and B during mold closing, made of medium carbon steel and heat-treated to a Rockwell hardness (HRC) of 52~56 degrees. Their function is to reduce slider wear, extend slider life, and facilitate later fitter maintenance. Plastic part has four through holes in its flanged structure. Surface has a 1-degree draft angle in core-pulling direction. To prevent damage to texture and the formation of burrs at through holes, bottom of slider has a 3-degree draft angle. When slider pulls core, it effectively increases draft angle, changing insertion of through holes into a push-pull action. Bases of both sliders are original structures within mold, ensuring a more precise and reliable fit. Limiting distance between sliders A and B is 76.5 mm, controlled by a limiting block. A spring maintains slider position, ensuring relative position of slider and base during mold closing, allowing for smooth reset and preventing mold compression. See Figure 3 for details.

 

9 Wear-resistant block; 10 Slider A; 11 Limiting block; 14 Plastic part; 16 Moving mold core; 37 Slider pressure plate; 38 Guide block; 42 Puncture point.
Figure 3: 3D structural diagram of moving mold slider side parting angle core-pulling mechanism
Forced mold closing mechanism prevents moving mold slider from colliding with fixed mold angle slider during mold closing. As shown in Figure 4, if fixed mold angle slider resets first, followed by moving mold slider, a collision will occur. Its working principle is to achieve priority closure of parting surface II between plates A and B through a mechanism of four rocker arms fixed on fixed mold panel. As shown in Figure 5, when mold is closed, rocker arms are positioned outside mold and are not associated with it. When mold is opened, after parting surface I is fully opened, parting surface II opens to a certain distance, and rocker arms are fully reset under action of spring. Boss on rocker arm hangs on the bottom of plate A and maintains this state. When mold is closed, boss on rocker arm will preferentially push plate A to close, allowing moving mold slide to reset first, until guide slope on rocker arm cooperates with guide slope on plate B to squeeze rocker arm out of mold, causing boss on rocker arm to detach from plate A. At this time, parting surface II between plates A and B has closed, and forced mold closure is achieved. Only then does parting surface I begin to close until mold is completely closed.

 

1. Panel 2. Plate A 3. Plate B 10. Slider B 13. Cooling Water Channel 16. Moving Mold Core 20. Fixed Mold Core 24. Pump Nozzle 25. Fixed Mold Inclined Slider Base 27. Fixed Mold Inclined Slider 32. Parting Surface Fixing Block 36. Fixed Mold Inclined Slider and Moving Mold Core Guiding and Positioning Interlocking Structure 43. Undercut Collision Position 44. Slider A and B Guiding and Positioning Interlocking Structure
Figure 4. 3D schematic diagram of collision state when mold core is reset first during mold closing.

 

1. Panel 2. Plate A 3. Plate B 28. Pin 29. Rocker Arm Base 30. Rocker Arm Spring 31. Rocker Arm 34. Magnetic Mold Locking Device Assembly 45. Rocker Arm Boss 46. Rocker Arm Stop 47. Rocker Arm and Plate B Angled Guide Mating Surface 48. Plate B Angled Guide Surface 49. Rocker Arm Angled Guide Surface
Figure 5. 3D schematic diagram of forced mold closing mechanism.

Bent plastic part has an inclined undercut a on fixed mold side. A mold angled slider core pulling mechanism is required to remove it. Fixed mold angled slider core pulling must be completed by fixed distance priority mold opening mechanism before other mold opening actions can be performed; otherwise, mold will be damaged. As shown in Figure 6, during mold opening, parting surface I opens first under action of fixed distance priority mold opening mechanism components spring 21, limit screw 22, resin opener/closer 35, and mold opening/closing sequence protection device. Mold opening distance of 60 mm is limited by limit screw 22. During this process, fixed mold angled slider is driven by stepped groove structure 26 on shovel base to complete core pulling and maintain its state, and undercut a on plastic part is safely removed. As mold continues to open, friction between resin opener/closer 35 and plate B 3, along with magnetic clamping force of mold clamping assembly, cannot withstand pulling force of injection molding machine. Parting surface II opens, and mold completes opening action.

 

1-Panel 2-A Plate 3-B Plate 12-Return Needle Spring 13-Cooling Water Channel 14-Plastic Part 16-Moving Mold Core 17-Return Needle 20-Fixed Mold Core 21-Spring 22-Limit Screw 23-Flange 24-Pump Nozzle 25-Fixed Mold Angled Slider Base 26-Staircase Structure 27-Fixed Mold Angled Slider 28-Pin 29-Rocker Base 30-Rocker Spring 31-Rocker 34-Magnetic Mold Locking Device Assembly 35-Resin Opener/Closer 36-Fixed Mold Angled Slider and Moving Mold Core Guide Positioning Locking Structure
Figure 6: 2D structural diagram of fixed mold inclined slider core-pulling mechanism and fixed-distance priority mold opening mechanism
Magnetic clamping assembly is a protective device for mold opening and closing sequence. It plays an auxiliary role in both fixed-distance priority mold opening and forced mold closing. During mold opening, it clamps parting surface II, allowing parting surface I to open first and complete fixed mold inclined slider core-pulling. During mold closing, when parting surface II reaches magnetic field range, it increases mold closing force between plates A and B, accelerating closing of parting surface II and prioritizing reset of moving mold slider.

In addition to its own machining accuracy, precision of mold requires a well-designed guiding and positioning structure to ensure smoothness and stability of mold during operation, thereby obtaining high-quality molded plastic parts. In addition to guide pillar and guide sleeve structure of mold base itself, this mold set adds a locking structure (parting surface positioning block) between A plate and B plate. A locking mechanism with a jaw clamp is installed on fixed mold core and moving mold core to ensure accurate positioning of fixed and moving molds during mold closing. A jaw clamp locking positioning structure is also installed on the left and right side slides, and a guide block is added to middle of bottom of each slide to ensure that parting surfaces of left and right slides do not misalign, thus affecting surface finish of plastic part. Ejector plate guide pillars are installed on moving mold ejection mechanism to ensure smooth ejection and reduce risk of ejection pin burn-out.

Quality of mold venting is crucial to quality of molded plastic part and is an indispensable part of mold design. As shown in venting system diagram in Figure 7, cavities of plastic part in this mold set are all on slides on both sides, so a full-circle two-stage venting system is installed on the top and bottom parting surfaces of slides. Secondary venting groove is 8 mm wide and 0.3 mm deep; primary venting groove is 4 mm wide and 0.02 mm deep; this allows gas generated by molten material to be fully discharged from mold cavity during filling, thereby improving filling efficiency, preventing trapped gas and burning, and giving plastic part excellent appearance quality.

 

Figure 7 3D schematic diagram of venting system

(1) Mold is closed and machine is used to complete first injection molding. (2) Mold is opened. Parting surface I will open first under action of spring 21, resin opener 35 and magnetic clamping assembly 34. Mold opening distance is 60 mm. Fixed mold inclined slider completes core pulling, and a undercut is safely removed; mold continues to open. Friction force of resin opener 35 and plate B 3 and magnetic attraction force of magnetic clamping assembly cannot withstand pulling force of injection molding machine. Mold opening and closing sequence protection mechanism fails, parting surface II opens, and mold completes mold opening action. (3) During opening of parting surface II, rocker arm resets; sliders A and B complete core-pulling action under force of inclined pin, side parting opens, and the entire cavity of plastic part is ejected, solving demolding problem of undercut b. Plastic part remains on moving mold core. (4) Ejection system performs ejection action, either automatically dropping or robot arm picks up part. (5) After part is picked up, ejection mechanism retracts, injection molding machine closes mold, rocker arm mechanism intervenes, pushing plate A to close mold first, completing reset of moving mold sliders A and B. When distance of magnetic mold clamping assembly to parting surface II reaches magnetic field range, it begins to generate attraction, accelerating closing of parting surface II. Parting surface I simultaneously begins to close mold, allowing fixed mold inclined slider to reset, until injection molding machine completes mold clamping, forming a complete injection molding cycle. Repeating above steps allows for production.

(1) First key technical feature of this mold is solution to problem of opening and closing sequence of multiple parting surfaces by setting a forced mold closing mechanism, a fixed-distance mold opening priority mechanism, a mold opening and closing sequence protection device, thus ensuring safety and stability of mold production;
(2) Side parting angled core-pulling mechanism of moving mold slider is second key technical feature of this mold design. Addressing problems of burrs easily generated on moving insertion surface of core-pulling structure and insufficient draft angle of plastic part cavity easily causing texture damage, parallel side core-pulling is changed to angled core-pulling, which is equivalent to changing insertion surface to contact surface, greatly reducing possibility of burrs. In addition, angled core-pulling indirectly increases draft angle to solve problem of texture damage;
(3) Excellent venting system and guiding positioning structure are third key technical features of this mold design. By setting a good venting system, problems of air marks and trapped air burning on plastic part's appearance surface are solved; stringent guiding positioning structure makes side parting line nearly perfect, achieving excellent molding appearance quality.