Inner door panel of an automobile is an important component of automotive interior parts. Located on the inside of car door, it provides excellent convenience, safety, comfort for vehicle and is an essential part of modern automobiles. This article introduces detailed structure and design experience of injection mold for inner door panel of a certain new energy vehicle from Guangzhou Honda.

Figure 1 shows a part drawing of inner door panel of a certain new energy brand automobile. Material is PP/EPDM, with a shrinkage rate of 1.2%. PP has good fluidity, is a high-performance thermoplastic synthetic resin. Main chain of EPDM is composed of chemically stable saturated hydrocarbons, and side chains contain unsaturated double bonds. It has excellent weather resistance, heat resistance, aging resistance, and ozone resistance, can effectively improve rigidity and elasticity of door panel, belonging to reinforced plastics. Characteristics of molded plastic parts are as follows: (1) Maximum dimensions of inner panel are 1042.3 mm * 497.5 mm * 89.8 mm, with an average wall thickness of 3 mm, which is a large thin-walled plastic part. (2) Molded plastic parts are appearance parts, and surface quality requirements are high. There should be no welding marks, spots, shrinkage depressions, flash, or other molding defects, as well as gate marks. (3) Surface of plastic parts is imitation leather texture, and draft angle of appearance surface should not be less than 5 (°). (4) Structure of plastic parts is complex, with 13 undercuts on the inner side, making demolding difficult.

 

Figure 1 Car door inner panel part drawing

According to customer's requirements, left and right car door inner panels are molded in one mold. Due to high precision of plastic parts, large size, and flow length ratio of about 170, they belong to thin-walled injection molding. In order to ensure molding quality, mold adopts a hot runner gating system. Injection sequence and injection time are controlled by a needle valve type sequence valve. Inner side of plastic part has 13 undercuts, and mold adopts an inner core-pulling structure of "lifter + slanted push rod".
2D structure diagram of mold is shown in Figure 2. Its maximum dimensions are 1780 mm in length, 1750 mm in width, and 1342 mm in height, with a total weight of approximately 23 tons, classifying it as an ultra-large injection mold.

 

1-Fixed mold panel; 2-Fixed mold core; 3-Hot runner plate; 4-Compressor valve; 5-Fixed mold A plate; 6-Positioning ring; 7-First-stage hot injection nozzle; 8-Second-stage hot injection nozzle; 9-Ejector block; 10-Pressure-resistant block; 11-S12 angled ejector block; 12-Moving mold B plate; 13-Ejector rod; 14-S12 angled ejector rod; 15-Square block; 16-S12 angled ejector base; 17-First set of ejector fixing plate; 18-First set of ejector base plate; 19-Second set of ejector fixing plate; 20-Second set of ejector base plate; 21-Moving mold base plate; 22-Limiting post; 23-Moving core ejector rod; 24-Plastic part ejector rod; 25-Moving core; 26-Frame plate; 27-Second set of ejector plate guide sleeve; 28-Ejector plate guide post; 29-First set of ejector plate guide sleeve; 30-Reset rod; 31-Hydraulic cylinder; 32-Ejector tube core I 33-S4 sloping top; 34-S5 sloping block; 35-S11 sloping block; 36-S6 sloping block; 37-S10 sloping block; 38-S10 sloping push rod; 39-S6 sloping push rod; 40. 41, 42, 44, 47 sloping rod base; 43-S11 sloping push rod; 45-S5 sloping push rod; 46-S4 sloping push rod; 48-push tube core 2; 49-push tube.
Figure 2. Structure diagram of an injection mold for an automotive door inner panel
Fixed and moving molds adopt an integrated structure, meaning cavity is directly formed on moving and fixed mold plates. This significantly reduces mold size while greatly improving its rigidity. Molding parts are made of P20 mold steel. Fixed mold plate A and moving mold plate B use a four-sided edge positioning structure. In design of injection mold for automotive door inner panel, design of horn-shaped mesh hole molding part is one of challenges. In this mold, horn hole is formed by moving mold's movable core 25. To ensure smooth venting of horn hole, core is assembled using an inlay method. Horn hole core is made of ventilated steel, which, although expensive, provides excellent venting performance. For ultra-large injection molds, designing molding parts to ensure both mold strength and rigidity while minimizing manufacturing and production costs is extremely challenging. To achieve this, we designed a 1310 mm * 1150 mm square conical positioning structure (commonly known as inner mold tube) on mold parting surface. Fixed mold protrudes, moving mold is recessed, and taper is 5°. Eight wear-resistant blocks are designed on protruding surface of fixed mold, as shown in Figure 3. In figure, "E" is distance from the deepest part of fixed mold cavity to bottom surface of fixed mold plate A, and E must be greater than 80 mm; "F" is distance from moving mold cavity to bottom surface of moving mold plate B. Due to high injection pressure, dimension B needs to be thickened accordingly, ensuring B > 100 mm. Method for determining dimensions C and D is as follows: first, add 50 mm to outermost edge of plastic part for sealing, then add 50~70 mm for clearance, and dimensions of pressure plate at parting surface of mold base. These are dimensions of C and D. Different customers often have different requirements for dimensions of molded parts, as shown in Table 1.

 

Figure 3 Molded Parts

 

Table 1 Reference Dimensions for Mold Strength of Automotive Door Inner Panels (mm)
Because plastic material is PP/EPDM, which has good fluidity, but mold cavity is narrow and belongs to thin-walled injection, ensuring rapid filling of cavity without weld lines, poor filling, or other molding defects is a challenge in gating system design. Therefore, mold adopts a hot runner gating system, with 5 secondary hot nozzles for 5-point feeding in each cavity. Opening and closing times of 5 hot nozzles are intelligently controlled by a sequence valve; this technology is referred to as SVG technology. By controlling feeding sequence and feeding time of 5 hot nozzles, melt filling is effectively controlled, achieving ideal effect of no molding defects on the surface of plastic part.
Two left and right door inner panels molded by mold are mirror images, using a total of 10 feeding points, positions of which are shown in Figure 4. G3 and G8 directly enter cavity, while the others use a combined gating system of "hot runner + cold runner + arc gate feeding". Length of runner from hot runner nozzle on the outside of cavity to edge of plastic part is generally 60-100 mm. An excessively long cold runner will lead to excessive pressure and temperature loss, increasing flow resistance of melt. Additionally, a gate must be designed near flared hole of door panel, as shown in G1 and G7 in Figure 4, to facilitate melt filling and injection pressure holding.

 

Figure 4 10-point hot runner gating system for mold
Inner door panel has large dimensions, thin walls, and many lateral undercuts. Each cavity requires 13 lateral core-pulling mechanisms. Due to large mold size and fact that all undercuts are on inner side, space for lateral core-pulling is limited. Therefore, each of 13 undercuts uses an inner core-pulling mechanism with 13 "lifter + slanted push rod" elements. This lateral core-pulling structure facilitates manufacturing and maintenance. See Figures 2 and 5 for details on core-pulling direction and angle. Due to numerous lifter blocks and deep undercuts in injection mold for car door inner panels, molded plastic parts often stick to lifter blocks during demolding, causing deformation. To address this issue, with customer's consent, we designed a locating post near each lifter block without affecting part assembly. This successfully solved problem. Locating post is a hollow post, 12 mm long, with an outer diameter of φ8 mm and an inner diameter of φ4 mm, as shown in Figure 5.

 

Figure 5: Mold "lifter + Angled Push Rod" Side Core-Pulling Mechanism
Car door inner panel is an important interior part of an automobile, requiring high surface quality. Mold temperature control system must not only quickly remove heat from melt from mold but also ensure that temperature throughout cavity is approximately equal, with a temperature difference not exceeding 10 ℃. Otherwise, molded plastic parts are prone to warping and deformation. For this purpose, 12 sets of crisscrossing water channels were designed for both fixed and moving molds. Each water channel consists of a straight water pipe, an inclined water pipe, and a partitioned water well. Diameter of cooling water pipe is ϕ13 mm, and diameter of water well is ϕ30 mm. Temperature control system of mold is detailed in Figure 6. Melt flow resistance is high at flared hole of plastic part; therefore, water channels cannot be designed for core of flared hole, otherwise weld lines or even poor filling may occur. Hot nozzle has a high temperature, so a separate water channel was designed near each hot nozzle to ensure rapid heat dissipation from hot nozzle area. To ensure that temperature is approximately equal throughout mold cavity, based on design principle of conformal water channels, we tried to make distance between each cooling water pipe and cavity surface approximately equal, while also ensuring that cooling water channels of mold are roughly in same direction as material flow. This balanced and rapid cooling system increased labor productivity of mold by 10%, and precision of molded plastic part reached MT3 (GB/T14486—2008), an improvement of one level.

 

Table 4. Effect of Paint Sludge Content on Thermal Aging Performance of PE-HD Waterproof Board Without Coupling Agent
For ultra-large injection molds, an unreasonable guide and positioning system design can make it difficult to guarantee mold's accuracy and lifespan, and may even directly lead to mold design failure. Based on this, we designed one round guide post at each of four corners of mold and one square guide post on each side of mold, meaning guide system adds four square guide posts to conventional four round guide posts. Round guide posts have a specification of ϕ80 mm * 500 mm, and square guide posts have a specification of 280 mm * 100 mm * 60 mm. Square guide posts serve both guiding and positioning functions. For positioning, in addition to square guide posts, a 1320 mm * 1150 mm (large end size) square 1° conical precision positioning surface is also designed on parting surface, as detailed in Figures 2 and 7. Since plastic part remains on moving mold core after mold opening, we installed both round and square guide pillars on fixed mold plate to facilitate part removal. This also facilitates mold manufacturing and handling.

 

50 Square guide post; 51 Round guide post; 52 Guide sleeve; 53 Buckle rod; 54 Limit switch; 55 Spring; 56 Movable block; 57 Toggle block.
Figure 7. Mold guiding and positioning system and secondary ejection mechanism
Generally, ejector blocks are used around flared holes in automotive door inner panels. Due to large number of flared holes, clamping force on core is significant, easily leading to deformation of molded plastic part during demolding. To solve this problem, we made flared hole core 25 a movable structure, mounted on ejector plate. Mold uses a double (set) ejector plate secondary ejection mechanism. During demolding, flared hole core 25 moves 20 mm simultaneously with molded plastic part. After molded plastic part detaches from moving mold, flared hole core 25 stops moving, while ejector part continues to move, safely and undamagedly pushing molded plastic part out of mold. This structure solves both problem of difficult ejection of flared holes and problem of difficult venting of flared holes. This is an innovation of this mold.
Secondary ejection mechanism consists of two sets of push plates. Ejection parts such as push rods and angled push rods are assembled on first set of push plates, while movable core 25 for forming flared hole is assembled on second set of push plates, as detailed in Figures 2(c) and 7(a). Ejection process of molded plastic part is divided into two steps, as detailed in mold operation process section.
Ejection system of car door inner panel injection mold consists of push rods, push tubes, angled pushers, and push blocks. Ejection parts are driven by four symmetrically arranged hydraulic cylinders 31, as shown in Figure 2. After fixed and moving molds open, mold ejects plastic part and runner using push parts. Push part fixing plate is pushed by injection molding machine through hydraulic cylinders and reset by action of four reset rods. Car door inner panel injection mold is an ultra-large mold. To ensure safe and stable operation of ejection parts, push part fixing plate is designed with 6 reset rods, 6 guide pillars, and 6 limit pillars. Mold also features limit pillars located above or near K.O. holes. All push rods have a diameter of 16 mm, with 24 rods per cavity, for a total of 48 rods.
Automotive door inner panel plastic parts are large, flat components. During injection molding, a large amount of gas inside cavity must be expelled promptly. Based on past experience, inner panel plastic parts are prone to injection molding defects such as poor filling, weld lines, air bubbles, deformation cracking near flared holes. Increasing injection speed can cause localized burning of plastic part, severely affecting quality of molded part. Main cause of these defects is poor mold venting and air intake. Therefore, in this mold design, in addition to using secondary ejection to improve venting and air intake, a full ring of secondary venting grooves was designed on parting surface of each cavity. Secondary venting grooves are connected to cavity through 40 primary venting grooves, and directly to outside of mold through 16 tertiary venting grooves (see Figure 8). Besides venting grooves on parting surface, ejector pins, ejector tubes, inserts, and cores also provide venting. During trial molding, no molding defects occurred in plastic parts due to poor venting and air intake.

 

Figure 8 Mold venting system
(1) First step: Melt enters primary hot nozzle 7 through injection molding machine nozzle under action of screw, then enters hot runner plate 3, secondary hot nozzle 8 and mold cavity in sequence; (2) Second step: After melt fills cavity, injection pressure of injection molding machine barrel is changed to holding pressure to prevent melt from flowing back. Then it cools and solidifies; (3) Third step: After molded plastic part cools to a sufficient rigidity, mold opens from parting surface I under drive of injection molding machine, molded plastic part is separated from fixed mold cavity, with a mold opening distance of 500 mm. (4) Fourth step: Demolding cylinder 31 drives ejection of part, under action of fixed distance parting mechanism buckle rod 53 and movable block 56, two sets of push plates move simultaneously. Molded plastic part and moving mold core 25 of molded horn hole move simultaneously, plastic part is separated from moving mold plate B 12, but not from core 25. Simultaneously, after ejecting 20 mm, pusher block 57 in base pushes movable block 56 to disengage from pull hook 53, second set of push plates stops moving, while first set of push plates continues to move forward, pushing molded plastic part away from horn hole core and completing inner core pulling. Second ejection distance is 150 mm. After plastic part is removed by robot arm, injection molding machine cylinder drives pusher and its fixing plate to reset. Then, injection molding machine pushes moving mold to close, movable core 25 and all lifters accurately reset, and mold begins next injection molding.

(1) Each mold adopts 12 sets of crisscrossing "straight-through water pipe + inclined water pipe + water well" cooling water channels. This naturally conforming cooling water channel not only benefits mold's lifespan but also significantly improves quality of molded plastic part and mold's labor productivity. Precision of plastic part meets design requirements of MT3 (GB/T14486—2008), and molding cycle is reduced by 10%.
(2) Mold adopts a secondary ejection mechanism, assembling movable core 25 of horn hole onto push plate. This solves both problem of difficult venting of horn hole and difficulty of ejecting horn hole.
(3) Due to numerous lifters and deep inner undercuts in door panel mold, molded plastic part experiences a large clamping force against lifters. In actual production, plastic part often follows lifters. To address this, we designed a positioning post near each undercut of plastic part without affecting assembly, effectively solving this problem.
(4) Mold trial was successful, and after production, it operates safely and stably. All indicators of molded plastic part meet design requirements.