Automotive door panels are an important part of automotive interior components, located on the inside of car door. To meet requirements of lightweighting, aesthetics, and safety, automotive door panels are made of plastic and are injection molded. Automotive door panels are produced in large batches, are large in size, have complex structures. Automotive door panel injection molds are typical large, thin-walled, precision, complex, long-life injection molds, making them difficult to design and requiring a long design cycle. This article introduces advanced structure, innovative technology of injection mold for left front door panel of a certain new energy vehicle.
1 Plastic part structure and molding process requirements
Figure 1 shows a part diagram of left front door panel of a certain new energy vehicle. Material is PP/EPDM, with a shrinkage rate of 1.1%. EPDM is a copolymer of ethylene, propylene, and a small amount of non-conjugated diene, a type of ethylene propylene rubber, which can improve impact resistance and elasticity of door panel. Structural characteristics and molding process requirements of plastic part are as follows: (1) Dimensions of plastic part are 804.8 mm * 721.3 mm * 102.4 mm, with an average wall thickness of 2.6 mm and a flow length ratio of approximately 200. It is a typical large thin-walled plastic part, making molding difficult. (2) Plastic part has a complex shape, with a total of 6 undercuts (S1~S6) on inner and outer sides that are inconsistent with demolding direction, making demolding difficult. (3) Outer surface of plastic part (commonly known as A-side) has a textured surface, and draft angle is greater than or equal to 5°. (4) Based on position of car door, outer surface of molded plastic part is not allowed to have defects such as flash, shrinkage depressions, weld lines, black spots, or gate marks.

 

Figure 1. 3D Schematic Diagram of Glue Injection

Since left front door panel of an automobile is a large flat part, a hot runner gating system is used in mold to improve melt filling. However, since gate marks are not allowed on the surface of plastic part, molten plastic finally enters mold cavity from side through a fan-shaped gate. There are a total of 6 undercuts on the inner and outer sides of plastic part. S6 is inner undercut, with a large undercut area and an undercut depth of 10 mm. Mold uses an inner core-pulling mechanism of "lifter + slanted push rod". S1, S2, S3, S4, S5 are outer undercuts, and mold uses a core-pulling structure of "slider + slanted guide post". See Figure 2 for detailed structure of mold.

 

1. Fixed mold plate; 2. Frame plate; 3. Fixed mold A plate; 4. Fixed mold insert; 5. Hot runner plate; 6. Secondary hot injection nozzle; 7. Positioning ring; 8. Primary hot injection nozzle; 9. Moving mold B plate; 10. Limiting post; 11. Ejector fixing plate; 12. Ejector base plate; 13. Moving mold fixing plate; 14. Square block; 15. Ejector rod; 16. Support column; 17. Limit switch; 18. Ejector cylinder; 19. Slider; 20. Stop block; 21. Locking block; 22. Angled guide post; 23. Moving mold insert; 24. Guide post; 25. Ball bearing guide sleeve; 26. Reset rod; 27. Elastic rubber; 28. Pressure-resistant hard block; 29. Angled guide post; 30. Slider; 31. Locking block; 32. Stop pin; 33. Spring; 34. Wear-resistant block; 35. Runner tie rod; 36. Plastic part ejector rod; 37. Lifter. 38 - Angled push rod guide sleeve; 39 - Angled push rod; 40 - Push plate guide post; 41 - Guide sleeve; 42 - Support post; 43 - Angled push rod base.
Figure 2. Structural Diagram of Injection Mold for Left Front Door Panel of Automobile
External dimensions of this mold are: 1400 mm * 13000 mm * 995 mm, with a total weight of approximately 10 t, classifying it as an ultra-large injection mold.

To improve mold rigidity and reduce its overall dimensions, fixed and moving mold forming parts are integrated with mold plate, meaning cavity is directly formed on fixed mold plate A and moving mold plate B. Fixed mold plate A is made of P20 mold steel, and moving mold plate B is made of 718 mold steel. Fixed mold plate A and moving mold plate B use a four-sided conical positioning structure, as shown in Figure 3 (mold perspective view).

 

Figure 3: Mold Perspective View
Design challenge of this mold forming part design is design of door panel horn mesh forming structure. Due to large number of horn mesh holes, drawing them all in mold design would severely impact computer processing speed. Therefore, when designing door panel horn mesh holes, it is not necessary to design every single horn mesh hole in 3D. Only a small number of reference horn mesh holes need to be designed, then different colored horn mesh hole distribution lines are designed on parts to represent distribution of horn mesh holes using 2D lines, as shown in Figure 4.

 

Figure 4. Design of horn mesh in the left front door panel of a car (Part 1)
This example uses height of perforated mesh as a reference. Long steel is used for sealing, and several horn mesh holes are cut out locally as reference for mesh size design. If perforated and non-perforated mesh holes are not of equal height, then we need to create a realistic 3D model for all mesh holes of different heights. This design method only applies to circular mesh holes; other shapes of mesh holes require realistic 3D modeling. Perforated and non-perforated mesh holes should be distinguished in 3D design, preferably by color (see Figure 5). Because perforated holes provide significant resistance to plastic melt, a gate should be designed near horn mesh holes to improve melt filling, enhance pressure holding, and eliminate weld lines (see Figure 6). Draft angle of fixed mold cavity for horn mesh in car door panel should be as large as possible. A 0.1~0.15 mm step should be designed at contact point between fixed and moving molds (see Figure 7). Reinforcing ribs should be designed at corresponding position of moving mold to prevent sticking to fixed mold.

 

Figure 5. Design of horn mesh in the left front door panel of an automobile (Part 2)

 

Figure 6. Gate design near horn mesh

 

Figure 7. Draft angle design for mesh
To avoid air trapping, horn mesh insert uses powder metallurgy permeable steel. Although permeable steel is expensive, it has good venting effect. If P20 mold steel is used, venting pins or inserts need to be designed on mesh insert, and venting grooves also need to be designed at the bottom of mesh insert. However, these venting grooves are easily blocked by glue powder and oil, resulting in poor venting and affecting melt filling, which will lead to higher production costs.

Mold guiding and positioning system is crucial to success of design of large, precision, and long-life injection molds. Injection mold for left front door panel of an automobile is designed with 4 round guide pillars and 4 square guide pillars. Round guide pillars are arranged at 4 corners of mold, with dimensions of ϕ80 mm * 300 mm. Square guide pillars are positioned at the center of four sides of mold, measuring 50 mm * 150 mm * 300 mm. These square guide pillars serve both guiding and positioning functions, as detailed in Figures 2 and 3. All four round and four square guide pillars are mounted on fixed mold plate A. This structure facilitates removal of molded plastic part and mold manufacturing. During mold assembly, four guide pillars can also serve as support feet.
In five sets of "slider + inclined guide pillar" lateral core-pulling mechanisms, each slider is pushed back into position by a locking block. To reduce wear between locking surfaces, guide post 24 must be inserted into ball bearing guide sleeve 25 20 mm before each locking surface contacts.

Considering that left front door panel is a large, complex, thin-walled flat plastic part with numerous horn mesh through-holes, mold adopts a "3-point hot nozzle + hot runner plate" gating system. Three-point injection inevitably produces multiple weld lines, which not only affect aesthetics but also reduce strength of door panel. To eliminate weld lines or move them to non-appearance surfaces, injection time and sequence of three hot nozzles are controlled by a sequence valve. Sequence valve is driven by a hydraulic cylinder, its opening and closing sequence and timing are controlled by mold intelligent system to achieve optimal molding quality. Since there can be no gate marks on door panel surface, a combined "hot runner + cold runner" gating system is used. Melt flows through hot runner and cold runner, finally entering cavity from side through three fan-shaped gates. Gate locations are shown in Figure 8, where G1, G2, and G3 are hot nozzle locations. Runner length from hot nozzle to edge of plastic part should not exceed 50 mm; an excessively long cold runner will lead to excessive heat and pressure loss.

 

Figure 8: Three-point sequence valve hot runner

Side parting and core pulling mechanism is one of the most complex structures in this mold. Molded plastic part has six undercuts that are not aligned with mold opening direction. Undercuts S1-S5 are on the outer side of part, and mold uses a side-pulling mechanism consisting of a slider, inclined guide post, and spring stop pin. Undercut S6 is an inner undercut with a depth of 10mm and a larger area, mold uses an inner-pulling mechanism consisting of an lifter and inclined push rod. Details of mold's side-pulling mechanism are shown in Figures 2(b), (c), and (e). Due to large core-pulling distance of undercut S6, to prevent molded part from sticking to lifter during side-pulling, which could lead to deformation or even breakage, draft angle of side reinforcing rib must not be less than 2°. Furthermore, since undercut direction of S6 is not perpendicular to demolding direction, it is an inclined inner-pulling mechanism. Therefore, direction of sliding guide rail in base of inclined push rod must be consistent with inclination angle of inner-pulling mechanism; that is, an inclined guide groove must be used. Otherwise, reinforcing rib will break during inner-pulling, as shown in Figure 9.

 

37 - Lifter; 38 - Inclined push rod guide sleeve; 39 - Inclined push rod; 43 - Inclined push rod base
Figure 9. S6 Inner Core Pulling Mechanism
Due to large clamping force of molded plastic part on lifter, design of lifter in mold of left front door panel of an automobile must prevent plastic part from sticking to lifter during lateral core pulling, thus avoiding lifter from tearing plastic part.

Cooling time of molded plastic part accounts for about 80% of molding cycle. Poor filling and warpage of molded plastic part are usually caused by unreasonable temperatures in various parts of mold cavity. For left front door panel of an automobile, which is one of the most important large interior parts, design principle of temperature control system is to enable mold to cool evenly and quickly. Therefore, distance between cooling channels must be reasonable, distance between cooling channels and cavity surface should be approximately equal.
Based on above principles, temperature control system of left front door panel mold of an automobile uses 8 sets of cooling water channels for moving mold, 10 sets of cooling water channels for fixed mold. Each cooling water channel consists of a straight water pipe and a partitioned cooling water well, as shown in Figure 10. Different colors in figure represent different cooling water channels. Mold's cooling water channels are arranged at equal intervals along the cavity, crisscrossing in a grid pattern. Cooling water flow direction is roughly same as material flow direction, inlet and outlet water channel lengths are approximately equal, spacing between channels is maintained between 50 and 60 mm, distance between cooling water channels and cavity surface is between 20 and 25 mm, and cooling water channels maintain a distance of at least 8 mm from ejector pins, ejector blocks, and other ejector hole designs. This mold incorporates mesh inserts in both fixed and moving molds, with separate cooling water systems for mesh inserts in both moving and fixed molds, as well as lifters. This rapid and balanced cooling system significantly improves molding quality and mold productivity. Dimensional accuracy of molded plastic parts has been improved by one grade, reaching MT3 (GB/T 14486—2008), and injection cycle time has been shortened by approximately 8%.

 

Figure 10 Mold Cooling System Design

Demolding system for left front door panel mold of an automobile includes all conventional ejection parts such as ejector rods, ejector tubes, and ejector blocks. Positions of each ejector are detailed in Figure 11. In figure, pink represents ejector tube (i.e., ejector sleeve), green represents ejector block, and red represents position of ejector pin. These ejection parts are fixed on ejector fixing plate 11, which is driven by four ejection cylinders 18. Their function is to eject demolding parts and pull them back to their original positions. Accuracy of reset is controlled by limit switch 17. Because it is an extra-large mold, ejector fixing plate must be designed with six guide pillars and guide sleeves, and six limit pins are designed below ejector base plate. A return block 10 mm larger than return rod is designed in the area where fixed mold A plate contacts return rod. Return block is made of oil-resistant DF2 (cold work tool steel) and has undergone quenching heat treatment. Mold is designed with six balanced limit pillars, and ejection distance is controlled at 110 mm. Ejector pins are positioned where clamping force is high, with a maximum diameter of ϕ20 mm.

 

Figure 11 Mold Demolding System
Since lifter 37 relies on fixed mold plate A 3 for push-back, to prevent repeated impacts that could cause indentation in fixed mold plate A 3 and affect reset accuracy of lifter 37, a reset rod pre-reset mechanism is designed in mold. This involves placing an elastic rubber 27 below large end of reset rod, as shown in Figure 2(f). After mold opening, elastic rubber 27 pushes reset rod 26 forward by 4 mm. During mold closing, fixed mold plate A 3 contacts reset rod 26 4 mm earlier, thus pushing lifter 37 back to its original position earlier, significantly reducing impact force between lifter 37 and fixed mold plate A 3.

Left front door panel of car is large, and a large amount of gas needs to be discharged from mold cavity during injection molding. Poor venting will seriously affect quality of molded plastic part, resulting in molding defects such as poor filling, trapped air, and even scorching of plastic part. In addition, contact area between molded plastic part and cavity is large. When mold is opened, external gas must enter cavity in time, otherwise a vacuum will be formed, causing mold cavity to stick or making demolding difficult. Injection mold for left front door panel of an automobile is a flat part. Mold mainly vents through parting surface. Venting grooves on parting surface are arranged along cavity and consist of a primary venting groove, a secondary venting groove and a tertiary venting groove. Venting grooves are opened on parting surface on fixed mold side, as shown in Figure 12. Depth of primary venting groove is 0.04 mm, depth of secondary venting groove is 0.5 mm, depth of tertiary venting groove is 1 mm, width of venting grooves is 10mm. Venting grooves on parting surface are easy to process, easy to clean, have a good venting effect.

 

Figure 12 Mold venting system design

(1) Injection filling: Melt passes through primary hot nozzle 8, hot runner plate 5, secondary hot nozzle 6 in sequence and enters cold runner between parting surfaces. Finally, it enters mold cavity through fan-shaped gate. It is divided into three stages: flow filling, pressure holding and material replenishment, and backflow prevention. (2) Cooling and solidification: Under action of temperature control system, melt cools and solidifies. (3) Mold opening and outer core pulling: After molded plastic part has solidified to a sufficient rigidity, injection molding machine drives moving mold to open mold. During mold opening process, 5 inclined guide pillars move 5 sliders to pull molded plastic part from outside. Mold opening distance is 500 mm, controlled by injection molding machine. (4) Demolding and inner core pulling: Injection molding machine ejector cylinder 18 pushes ejector fixing plate 11, while pushing ejector rod, ejector tube and ejector block to push molded plastic part away from moving mold, pushing lifter rod 39 and lifter 37 to pull inner core. (5) Mold closing and reset: After plastic part is taken out by robot, ejector cylinder 18 pulls ejector fixing plate 11 to reset, ejected part and lifter rod are finally accurately reset under action of reset rod 26, limit switch 17 is opened, injection molding machine pushes moving mold to close mold, and mold begins next injection molding.

(1) Horn mesh molding part of left front door panel of automobile was made of powder metallurgy permeable steel, which improved venting effect, improved melt flow, successfully eliminated molding defects such as poor filling of horn mesh and weld lines. Mold adopted an inclined guide groove, which effectively solved problem of demolding with inclined inner undercut.
(2) Three needle valve hot nozzles of mold adopted sequential valve hot runner intelligent control technology (SVG) to accurately control flow rate and pressure of nozzles, successfully solving problem of melt filling of large thin-walled plastic parts and greatly improving molding quality.
(3) 18 sets of water channels in moving and fixed molds are arranged in a grid pattern, effectively achieving temperature balance throughout mold cavity. This temperature control system demonstrates significant effectiveness, shortening injection cycle by approximately 8% and achieving dimensional accuracy that meets design requirements of MT3 (GB/T14486-2008).
Mold design concept is correct, structure is advanced and reasonable, providing valuable reference for design of large, thin-walled, precision, and long-life injection molds. Mold was successfully put into production, operating safely, and all indicators met design requirements.