Automotive side panel interior trim assembly includes door trim assembly composed of door trim panels, mechanism composed of upper and lower trim panels of A-pillar, B-pillar, C-pillar and D-pillar on both sides of door. Its rendering is shown in Figure 1. Left and right upper A-pillar trim panels are an important component of automotive interior parts. This article details design considerations and experience of large, thin-walled injection molds for these panels.

 

Figure 1: Assembly rendering of automotive side interior trim panel assembly

Left and right upper A-pillar trim panels are symmetrical. Figure 2 shows right upper trim panel part of a Honda SUV. Material is Shanghai SECCO polypropylene PP-K7926, with a shrinkage rate of 1.5%. Plastic parts are among the most important interior components in automobiles, with dimensional accuracy MT3 (GB/T 14486—2008). Maximum external dimensions of plastic part are 702.2 mm * 113.8 mm * 95.8 mm. Characteristics of plastic parts are as follows: (1) Plastic parts have complex shapes and high requirements for appearance lines. Appearance surface of plastic parts must be textured. Since texture is a rough texture, in order to prevent molded plastic parts from sticking to mold cavity or textured surface from being dragged during demolding, demolding draft angle of appearance surface shall not be less than 5°; (2) Parting line of plastic parts is a complex spatial curve, and parting surface is a complex spatial curved surface; (3) Left and right upper guard plates of car A-pillar are installed with other parts by overlapping and snap-fit positioning. There are 4 undercuts on inner side of plastic parts, namely S1, S2, S3 and S4; (4) Plastic parts are appearance parts, and surface is not allowed to have welding marks, shrinkage, gate marks, flash and spots and other molding defects.

 

Figure 2 Part of right upper guard plate of car A-pillar

Mold forms left and right guard plates of car A-pillar. According to its structural characteristics, mold adopts a hot runner injection mold structure, and adopts a 2-point sequential valve hot nozzle to cold runner + fan-shaped injection. Four undercuts of plastic part are all located on inner side of part, employing a side core-pulling mechanism of "inner core pulling + angled push rod". Mold's external dimensions are 1300 mm * 900 mm * 800 mm, with a total mass of approximately 4 t, classifying it as a large injection mold. See Figure 3 for specific structure.

 

1. Fixed mold plate; 2. Hot runner frame plate; 3. Fixed mold A plate; 4. Hot runner plate; 5. Hot nozzle; 6. Screw; 7. Runner insert; 8. Pressure block; 9. Square iron; 10. Limiting post; 11. Ejector fixing plate; 12. Ejector base plate; 13. Moving mold fixing plate; 14. Top ejector; 15. Guide post; 16. Moving mold B plate; 17. Guide sleeve; 18, 21, 32, 40. Inner core pull; 25, 29. Angled ejector guide rod; 20, 23, 30, 33. Angled ejector guide block; 19, 26, 28, 38. Angled ejector; 22, 24, 27, 34. Angled ejector base; 35. Angled ejector guide rod; 36. Ejector plate guide post; 37. Ejector plate guide sleeve; 39. Hydraulic cylinder.
Figure 3: Injection mold structure diagram of left and right upper guard plates of an automotive A-pillar

Both fixed and moving molds adopt an integral structure, effectively improving mold rigidity and reducing mold size. Fixed mold uses 718 mold steel, and moving mold uses P20 mold steel. To reduce friction and damage, draft angle of all parting surfaces is 5°, and draft angle of locating cone surface is 1°. 5° wear-resistant blocks are also designed on four peripheral bevels of moving mold B plate to prevent damage to mold parting surface during mold manufacturing due to use of a grinding machine. Mold parting surface is complex. To effectively ensure machining accuracy of CNC milling machine, prevent injection molding defects such as flash on parting surface, every part is made smooth without sharp corners or thin steel, all surfaces are sealed with flat or curved glue. Mating parts of ejector pins, inserts, and moving mold have chamfered R-angles or clearances at the root of stop, simplifying machining process and reducing workload, significantly lowering manufacturing costs. To improve mold lifespan and ensure uniform stress distribution, 12 pressure blocks are designed on moving mold parting surface, including 8 blocks measuring 122 mm * 62 mm * 12 mm and 4 blocks measuring 102 mm * 62 mm * 12 mm.
To enable automotive A-pillar trim to withstand impact forces, numerous reinforcing ribs are designed on inner side of plastic part. Since shrinkage marks are not allowed on the surface of plastic part, thickness of reinforcing ribs should be 1/2 to 3/4 of wall thickness. To prevent air trapping in mold cavity, reinforcing ribs with a depth greater than 8 mm are designed with inserts. For easy assembly and disassembly, these inserts are designed to be detachable from outside of parting surface. Based on the surface requirements of plastic part, mold cavity surface needs to be roughened. To prevent molded plastic part from sticking to mold cavity or scratching outer surface of plastic part, draft angle of cavity must be greater than 5°.

Injection mold for left and right A-pillar guard plates of automobiles is a large mold with high requirements for appearance and dimensional accuracy of plastic part. Therefore, requirements for guiding and positioning system are very high. In addition to positioning cone surface and four-sided edge positioning on parting surface, a φ50 mm * 270 mm round guide post is designed at each of four corners. Guide posts are installed on moving mold side, their length must ensure that they extend 30 mm above the highest point of moving mold to ensure safe mold closing of molded parts of moving and fixed molds. Venting and inlet grooves should be provided above guide bushings of large injection molds to prevent air trapping during mold closing and vacuum during mold opening (which could prevent guide pillars from being removed). For safety, venting direction of venting grooves should not face operating side; ideally, they should be located above mold. Design of guide pillars should not obstruct robot's part removal. In automotive mold design, front end of round guide pillar should have a 5° slope on one side. Guide bushing length should be 1.5 times guide pillar diameter. Guide bushing is fixed with a pressure plate, made of CR12 hardened material, which can serve as both a pressure plate and a bearing plate.
Mold positioning system mainly uses four circumferential positioning cones with a 5° inclination angle. Positioning cones are mainly composed of six sets of 1° precision positioning blocks. These precision positioning blocks are easy to replace and maintain. A good parting surface positioning structure ensures mold positioning accuracy and dimensional accuracy of molded plastic parts. See Figure 4 for details of mold guiding and positioning system.

 

Figure 4. 3D view of injection mold for left and right upper guard plates of car A-pillar

Maximum size of upper guard plate of car A-pillar is 702.2 mm, with an average wall thickness of 2.6 mm. Ratio of maximum size to wall thickness is 270, classifying it as a thin-walled plastic part. Mold adopts a two-point hot runner to conventional runner gating system. Feeding sequence and feeding time of two hot nozzles are controlled by sequential valves, effectively solving problem of difficult melt filling. By adjusting injection time of two gates, influence of weld lines on strength of molded plastic part was successfully eliminated. Two-point sequential valve hot runner gating system is shown in Figure 5. Melt finally enters mold cavity through a fan-shaped gate. The thinnest part of fan-shaped gate is 1 mm, as shown in Figure 6.

 

Figure 5 2-point sequential valve hot runner gating system

 

Figure 6 Fan-shaped gate

Each plastic part of automotive A-pillar upper guard plate has four undercuts S1, S2, S3, and S4 on its inner side. Due to small inner space, a lifter inner side core-pulling mechanism is used for all of them. Undercuts S2 and S3 are similar, with a larger core-pulling distance, while undercut S1 has a larger area. Customers reported that because lifter of S1 is large, core-pulling force is also large, and with an inclination angle reaching 13°, lifter rods of injection mold for same vehicle model often bend and break during core-pulling, and sometimes cannot be completely reset during resetting. Cause of this failure is that lifter rod is too long and bears a large torque. To solve these problems, side core-pulling mechanism S1 adopts a double lifter rod lifter structure. Two inclined push rods have same inclination angle, sharing torque during inclined core pulling, greatly improving stability and safety of lateral core pulling mechanism. This effectively solves problem of easy deformation and breakage of inclined push rods during lateral core pulling, significantly increasing mold's service life (see Figure 3(b)). For S2 and S3, due to core pulling distance of 32 mm, inclined push rod inclination angle reaches 15°. Because of large core pulling force and inclination angle, bending and breakage of inclined push rods can occur, severely affecting mold's labor productivity. Therefore, a guide rod 25 was designed for inclined push rod 26, and a guide rod 29 was designed next to inclined push rod 28 (see Figure 3(b)). Guide rods share torque of inclined push rods, improving motion accuracy and stability of inclined push rod base. Inclination angle of guide rod must be equal to that of inclined push rod.
Inner surface of undercut area of automotive A-pillar trim is all inclined. To prevent deformation of molded plastic part during inner core pulling, all inclined push rod base guide grooves are designed with same inclination angle (see Figure 2). Mold employs a double-angled push rod and guide rod side-pulling structure, effectively solving problem of bending and breakage of angled push rods in large injection molds. This is core structure of mold and its innovative feature. A three-dimensional view of mold's inner side-pulling mechanism is shown in Figure 7.

 

Figure 7: Three-dimensional view of mold's side-pulling mechanism
Automotive A-pillar trim injection mold is a large mold. Angled push rods and guide rods should be designed to be detachable from bottom of mold, allowing for installation and removal of angled push rods without removing mold base plate. Angled push rod base is integral; to reduce wear, both angled push rod and its base are designed with anti-rotation positioning structures. Mold's angled push rods are relatively long; to prevent bending and deformation, in addition to using double-angled push rods and guide rods, all angled push rods have a diameter of 16 mm, are designed with guide blocks. Guide section, 119 mm long, is approximately 2/3 of angled push rod's length. Both angled push rods and inner side-pulling mechanism are made of SKD61, hardened.

Left and right A-pillar trim panels of automobiles have a large area, require aesthetically pleasing spatial design, and have high appearance requirements. For safe and damage-free demolding, ejection area should be as large as possible, and ejection force must be balanced. Because ordinary round ejector pins are prone to whitening and cracking of plastic part, this mold adopts a combined demolding mechanism of "straight ejector + lifter + hydraulic cylinder". Straight ejector, due to its large ejection area, is less prone to whitening. After mold opening, hydraulic cylinder drives straight and lifters to complete side core pulling and demolding of molded plastic part. After demolding, hydraulic cylinder pushes straight and lifters back to their original positions. However, since liquids are not rigid, accurate resetting of ejector pins and lifter pins also relies on four reset rods. Four hydraulic cylinders are arranged in parallel, driving and resetting simultaneously to ensure demolding balance.

Temperature control system for mold's fixed mold adopts a combination of "straight-through water pipes (commonly known as linear water supply) + inclined water pipes". Due to presence of numerous ejector pins, lifters, push blocks in moving mold, a combination of "straight-through water pipes + partitioned water wells" is employed. Both stationary and moving molds are designed with three sets of water channels, as detailed in Figures 8 and 9. Hot runner areas of this mold require focused cooling, and each hot runner has its own dedicated water channel.

 

Figure 8: Temperature Control System of Mold

 

Figure 9: Simplified Diagram of Temperature Control System
Cooling water channels in injection mold for left and right A-pillar trim panels of an automobile are aligned with material flow direction. Diameter of cooling channels is 10 mm, distance between channels is 5-6 times channel diameter, distance from cooling channels to cavity surface is 2-3 times channel diameter. Cooling area reaches over 60% of surface area of plastic part (excluding areas outside plastic part). Mold achieves uniform and sufficient cooling, resulting in a balanced temperature, significantly improving precision of molded parts and labor productivity of mold. After mold was put into production, precision of molded plastic parts reached MT3 (GB/T 14486—2008), with a molding cycle of 30 seconds. Compared with injection molds for similar plastic parts, precision was improved by one level, and molding cycle was shortened by 10%.

Injection mold for left and right A-pillar guard plates of automobiles is a large mold. During injection molding, a large amount of gas inside cavity must be vented in a timely manner, and external gas must enter promptly when mold opens. Otherwise, problems such as incomplete filling, trapped air, and difficulty in demolding will occur. Severe trapped air can even burn plastic parts. This mold mainly uses venting grooves between parting surfaces for venting, including primary, secondary, and tertiary venting grooves, as detailed in Figure 10. This not only facilitates processing but also provides good venting effect, and venting grooves are easy to clean when blocked by glue powder or debris. Distance between primary venting grooves is uniform and reasonable, controlled between 60 and 80 mm. All tertiary venting grooves are connected to outside of mold, preventing closed-loop gas circulation.

 

Figure 10. Mold Venting System Design

PP melt enters hot runner plate through primary hot nozzle, and finally enters mold cavity through secondary hot nozzle controlled by sequence valve. After injection filling and cooling, mold opens from parting surface I under drive of injection molding machine, with an opening distance of 300 mm, controlled by injection molding machine. After mold opening stroke is completed, ejector cylinder 39 pushes ejector fixing plate 12, which pushes straight ejector 14 and lifter rods 19, 25, 26, 28, 29, 35, and 38, simultaneously pulling inner core and ejecting molded plastic part from mold. Under action of limit post 10, plastic part is ejected a distance of 95 mm and removed by robot. After demolding, hydraulic cylinder 39 pulls ejector rods, lifter rods, and their fixing plates to reset. Finally, injection molding machine drives moving mold to close, and mold continues next injection molding.

(1) Mold adopts a double push rod and guide rod lifter inner core-pulling mechanism, which effectively solves problem of easy bending and breakage of inclined push rod in large injection molds, ensuring smooth demolding of left and right A-pillar guards of automobiles, improving service life of mold; this is core structure of this mold and also innovation point of this mold structure;
(2) Mold adopts a combination of "straight ejector + lifter + hydraulic cylinder ejection" demolding mechanism, ensuring safe and undamaged demolding of molded plastic parts;
(3) Mold adopts a combination of partition-type water well, inclined water pipe and straight water pipe. This temperature control system with a near-conformal water channel controls mold molding cycle within 30 seconds, reducing it by about 10%, and dimensional accuracy of plastic parts reaches MT3;
(4) Mold structure is novel and practical, easy to manufacture, mold runs safely and stably during molding process, enabling fully automated production. All indicators of plastic parts meet design requirements.