Car door panel is an important part of car interior parts. It is located on the inside of car door. It is divided into front, rear, left and right. It varies according to car series, usually two-door and four-door. These parts are collectively called door panel series. This article takes left rear door panel of a car as an example to explain key points and experience in designing injection molds for car door panels.

Figure 1 shows a diagram of left rear door panel parts of a certain brand of car. Material is PP+EPDM. Shrinkage rate is generally 1.011. Where EPDM stands for ethylene propylene diene rubber, which is a copolymer of ethylene, propylene and a small amount of non-conjugated diene. It is a type of ethylene-propylene rubber. Because its main chain is composed of chemically stable saturated hydrocarbons and only contains unsaturated double bonds in side chains, it has excellent aging resistance such as ozone resistance, heat resistance, weather resistance, and can improve elasticity of door panel.
Door panel is an exterior part, with overall dimensions: 880*670.4*105.6mm. Its structural features are as follows:
1. Outer surface has high requirements, no spots or gate marks are allowed, and defects such as shrinkage dents, weld marks, and flash are not allowed.
2. Door panel is made of leather texture, and draft angle of exterior surface is at least 5°.
3. Door panel has a high surface finish, complex shape and structure, complicated parting lines, and many undercuts. There are 11 undercuts on inner and outer sides of plastic part (see S1 to S11 in 3D view of inner side of door panel in Figure 1), making demoulding difficult.

 

 

Figure 1 Car door panel parts

Due to large size and complex structure of door panel, mold adopts a hot runner pouring system. Four needle valve hot nozzles are controlled by sequence valves to enter mold cavity through ordinary runners and fan-shaped gates. There are 11 undercuts on inner and outer sides of door panel plastic part. Only S11 undercut is on outer side of plastic part. From perspective of mold reliability and processing, S11 adopts side core-pulling structure of "inclined guide pillar + slider", and other undercuts adopt side-extracted core structure of "inclined push rod + inclined push block".
The overall dimensions of this mold are: 1450*1400*985 (mm), the total weight is about 16 tons, and it is a large injection mold. Detailed structure is shown in Figure 2 plan view and Figure 3 perspective view.

 

a. Fixed mold arrangement diagram

 

b. Movable mold ranking chart

 

c. M-M

 

 

Figure 2 Structural diagram of automobile door panel injection mold

1. Fixed mold fixed plate; 2. Frame plate; 3. Fixed template; 4. Hot runner plate; 5. Positioning ring; 6. First-level hot nozzle; 7. Side core pulling 1; 8. Side core pulling 2 ; 9. Inclined guide column; 10. Locking block; 11. Slider; 12. Limiting block; 13. Moving template; 14. Support column; 15. Square iron; 16. Push piece fixed plate; 17. Push piece Bottom plate; 18. Fixed plate of movable mold; 19. Ejector; 20. Push rod; 21, 27, 31, 36, 40, 44, 48, 52, 56, 60. Inclined push rod; 22, 26, 30, 35, 39, 43, 47, 51, 55, 59. Sliding column; 23, 28, 32, 37, 41, 45, 49, 53, 57, 61. Inclined push rod guide bush; 24, 29, 34, 38, 42 , 46, 50, 54, 58, 63. Incline push block; 25. Wear-resistant block; 33. Screws

 

Molded parts and mold plate of automobile left rear door panel injection mold are all integrated, as shown in Figure 3. Using this form of injection molding, structure is more compact, strength is better, mold volume is relatively small, and processes such as frame opening, frame assembly, and wedge manufacturing are avoided.
Materials of fixed mold plate A and movable mold plate B are both P20 (2738 can also be used). Since mold is a large injection mold, fixed mold plate A and movable mold plate B adopt an internal mold positioning structure surrounded by four sides (see Figure 3). This structure allows moving and fixed molds to be integrated after mold is closed, which greatly improves molding accuracy of door panel and production life of mold.

 

 

Figure 3 Three-dimensional view of injection mold of left rear door panel of car

This mold pouring system adopts the "hot runner + ordinary runner" glue feeding form, in which hot runner uses a hot runner plate plus 4 needle valve hot nozzles (see G1, G2, G3 and G4 in Figure 4), 4 needle valve hot nozzle does not feed glue at the same time, but are controlled by sequence valves to open sequentially according to shape and size of plastic part. Melt enters mold cavity through ordinary flow channel and finally through fan-shaped gate.
Since plastic parts are made of PP+EPDM material with good fluidity, length of ordinary flow channel can be controlled within 60 to 100mm. If ordinary flow channel is too long, it will cause excessive pressure and heat loss, affecting melt filling and plastic part molding quality.
Door panel is an exterior part, and welding marks are not allowed on the surface. During injection molding, welding marks must be moved to non-appearance surface or eliminated. This is one of key points and difficulties in design of this mold. Although traditional simultaneous multi-point pouring can fill the entire cavity with melt, it is difficult to achieve ideal product quality due to existence of weld marks. To this end, this mold adopts 4-point sequence valve hot runner gate control technology, which controls opening and closing of four hot nozzles through driving of oil cylinder, thereby achieving ideal effect of no weld marks on the surface of plastic part. Location of hot runner gate of door panel injection mold is shown in Figure 4.

 

Figure 4 4-point sequence valve hot runner control system

Lateral core-pulling mechanism is core mechanism of door panel injection mold. This mold has 11 lateral core-pulling locations, namely S1 to S11. Among these 11 lateral core-pulling mechanisms, S11 adopts structure of "inclined guide column + slider". Limit of slider adopts structure of limit clamp and stopper, which is safe and reliable. S1~S10 all adopt structure of "inclined push rod + inclined push block". Its detailed structure and important dimensions are shown in Figure 2(d) to (m).
In structural design of "inclined push rod + inclined push block", inclination angle of inclined push rod should not exceed 12°. Design of inclined push block should prevent plastic parts from adhering to inclined push block during demolding, causing plastic parts to deform and crack.

Design of temperature control system has a great impact on molding cycle and product molding quality. It is especially important for automobile door panel injection molds with high appearance requirements. One of design principles of cooling water channel is that distance from mold cavity surface should be approximately equal to achieve a roughly balanced temperature throughout mold cavity. Temperature control system of this mold adopts combination of "straight-through water pipe + inclined water pipe + water well", see Figure 5 (a) and (b) for details. In this combination, straight-through water pipes are preferred, supplemented by inclined water pipes, and water wells are used as a last resort. Its advantages are uniform cooling of plastic parts, short molding cycle, and high molding quality. It is suitable for molds with high requirements on appearance and performance.
In automotive mold design, similar to interior and exterior trim molds such as automotive front and rear bumpers, dashboards, central tunnels, grilles, and automotive decorative strips, cooling water channel layout is generally designed according to following rules:
(1) Direction of cooling water should be consistent with direction of material flow.
(2) Fixed and movable mold cooling water channels are preferably designed in the form of a cross grid, and cooling circuits intersect with each other to form an intertwined network of water channels to evenly cool plastic parts.
(3) When crisscross waterway cannot be designed, fixed and movable waterways are arranged alternately at gaps between each other.
(4) Try to design only four circulating water channels for each group of cooling water to avoid long water channels and affect cooling effect of plastic parts.
(5) Cooling water channel should be designed to be connected to another set of water channels by external water pipes to facilitate subsequent adjustments of plastic parts due to deformation, shrinkage and other phenomena. It solves defects of plastic parts through water channel adjustment and is widely used in automotive interior and exterior plastic parts molds.
(6) Distance between each cooling water channel should be controlled at 3.5-5 times diameter of water channel (generally around 50-60mm). Distance between cavity surface and cooling water channel is generally between 15-25mm, depending on size of mold. .
(7) Distance between cooling water channel and push rod, inclined push rod and insert must be ensured to be more than 8 to 10 mm. Because mold is large and water channel is long, it is easy to drill off. It is necessary to avoid water channel being too far away from cavity or other structures. Recently, cooling water leakage has occurred.
(8) In design of automobile injection molds, hot nozzle should be designed with a separate set of cooling water channels as much as possible and cannot be connected in series with other water channels to facilitate heat dissipation in hot nozzle area.

 

(a). Fixed mold cooling system

 

(b) Moving mold cooling system
Figure 5 Door panel injection mold cooling system
Characteristics of temperature control system of fixed and movable molds of this mold are: seven sets of water channels are designed for both fixed and movable molds. Both fixed and movable molds have seven inlets and seven outlets. Mold cooling water channels are designed to be consistent with material flow direction, and "vertical type" is given priority. Design form of "water pipe + inclined water pipe + spacer type water well" follows shape of plastic part. Distance between water inlet and outlet is approximately equal to length of water path, so that mold gets a good cooling effect and plastic part gets a good appearance quality.

Quality of design of guide positioning system directly affects accuracy of molded plastic parts and life of mold. In design of automotive injection molds, due to large mold, large batch of plastic parts, high appearance requirements and dimensional accuracy, design of mold's guide and positioning system is very strict.
This mold is designed with a square guide pillar and a round guide pillar at each of four corners, as well as four 1° precise positioning structures. See Figures 2 and 3 for details. Among them, size of four circular guide posts is ∅ 40*225mm (the longest guide post cannot exceed 10 times its diameter), and they are installed on fixed mold side. Since plastic part remains on movable mold side after mold is opened, it will not affect removal of plastic part. At the same time, four guide pillars also serve as support pillars when turning over mold, making it easier to fit mold.
Length of guide pillar of mold without slider must be 30mm higher than the highest point of fixed and movable mold; for mold with slider, ensure that guide bush is inserted 20mm before inclined guide post is inserted into slider, otherwise it will cause a lot of trouble in manufacturing and production of mold, and in serious cases, mold will be damaged.

Demoulding mechanism of this mold adopts a "push rod + inclined push block + push block + nitrogen spring" push-out structure. After fixed and movable molds are opened, mold relies on pusher to push out plastic part and runner. Push piece fixed plate is mechanically pushed by injection molding machine through K.O hole and reset under action of four reset rods. When designing demoulding system, pay attention to following points:
(1) Large molds (more than 1400mm*700mm in length and width) need to design 6 reset rods and 6 push rod plate guide posts.
(2) All automobile mold reset rods must be designed with a recovery block one level larger than reset rod. Recovery block is generally 45# (S50C) nitrided.
(3) Guide posts of push rod plate should be arranged near pushing parts with strong pushing force (such as oil cylinders, reset rods, etc.).
(4) All automobile molds need to design limiting columns, which should be prioritized above or near K.O hole.
(5) Push rods should be arranged at stress-bearing position close to R and at a position with large tightening force. Push rod design should be larger, push rod layout should be more, and push rods should be designed with same specifications as much as possible. This can avoid frequent replacement of drill bits, save processing time and processing costs.

In automobile mold design, design of exhaust system is very important. If exhaust design is unreasonable, it will seriously affect quality of plastic parts, resulting in injection molding defects such as insufficient filling, trapped air, and poor demoulding. If air is severely trapped, plastic parts will be scorched.
Car door panels are interior parts, and appearance of plastic parts has strict requirements. Reasonable design of exhaust is very important. When designing mold exhaust system, pay attention to following points:
(1) Exhaust should be prioritized at the end of material flow and corner of plastic part.
(2) Close to insert or the thinnest wall thickness, because weld lines are most likely to form here.
(3) It is best to set up on parting surface, because it is easiest to remove spills on parting surface. Exhaust of this mold is set in fixed mold.

When designing a mold based on size and structure of plastic part, strength and rigidity of mold must first be ensured. This is especially important for large automotive injection molds. A reasonable mold design concept should be: mold strength is reasonable and inexpensive, local materials are used, mold strength and cost are taken into consideration, optimal design plan and processing technology are selected. If strength of mold is too strong, it will be wasteful. If strength is too weak, service life of mold will be affected. Parting surface tube position of this mold is designed in fixed and movable mold. Four sides of fixed and movable mold are made with a 5° slope and wear-resistant blocks are made around them. This four-sided design method is widely used in molds such as car door panels and fenders.
Calculation method of two dimensions A and B shown in Figure 6 is: first add 50mm sealing position from the largest edge of plastic part (in automobile mold design, 30mm sealing position for small molds (within 5050), 40mm sealing position for medium-sized molds (5050-1010) , 50mm sealing position for large molds (above 1010).), then add 50-70mm of avoidance space (in automobile mold design, only sealing position is matched, and the rest are all spaced out, reducing workload of FIT mold. Avoidance space is also area to ensure strength of mold.), then add size of mold base to handle profile pressure plate, which is size of A.B. In this way, a mold can be designed that not only meets customer's mold strength requirements but also saves costs. Different plastic parts will have different values for different sizes at C. At least fixed mold with C size must ensure that the highest glue level of plastic part is more than 80mm away. Movable mold needs to be thickened accordingly due to high injection pressure, and minimum design is more than 100mm to ensure strength of mold. In short, in daily design, we use it flexibly according to different customers and factories to design molds that both meet customer needs and save costs.
Since parting surface of automobile plastic parts is often complex, positioning of movable and fixed molds on parting surface is very critical and is the first issue that mold designers must consider.

 

 

Figure 6 Automobile door panel mold strength reference

Melt passes through nozzle of injection molding machine, enters ordinary runner through hot runner, and then enters mold cavity through fan gate. After melt fills mold cavity, is pressure-maintained, cooled and solidified until it is sufficiently rigid, injection molding machine pulls movable mold fixing plate 18 of mold, mold is opened from parting surface I, and plastic part leaves fixed mold cavity. At the same time, locking block 10 leaves slider 11. Under movement of inclined guide column, slide block drives lateral core pulling 7 and 8 to complete lateral core pulling of undercut S11. After mold opening distance reaches 500mm, injection molding machine oil cylinder pushes pusher fixing plate 16 and pusher bottom plate 17, then pushes all ejector pins and oblique push rods. In this process, oblique push rods 21, 27, 31, 36, 40, 44, 48, 52, 56 and 60 respectively push inclined push blocks 24, 29, 34, 38, 42, 46, 50, 54, 58 and 63 to complete lateral core pulling of undercut S1~S11, push plastic part away from movable mold core.
After plastic part is taken out by robot, injection molding machine oil cylinder pulls push piece and its fixed plate to reset. Then injection molding machine pushes movable mold to close mold, and mold continues next injection molding.

This mold adopts demoulding system of "push rod + inclined push block + push block + nitrogen spring" and temperature control system of "straight-through water pipe + inclined water pipe + water well". The former has been discussed in detail in article. Following focuses on temperature control system of this mold.
Temperature control system is crucial in automobile mold design and has a great impact on molding cycle and molding quality of plastic parts. Reasonable temperature control has a profound impact on improving quality of plastic parts and shortening molding cycle. For design of cooling system for large automobile molds, following points should be followed:
(1) Three-meter principle. The total length of cooling water path cannot exceed 3 meters, because if it exceeds 3 meters, cooling effect is poor, and it must be avoided that cooling water has not come out after mold is opened. In addition, length of a single cooling water channel must consider length of drill bit. If hole is too deep and drill bit is not long enough, drilling will not be possible. Diameter of straight-through cooling pipe of large automobile molds is generally ￠15mm. If fixed and movable mold cooling water channels of door panel injection mold are designed to conform to shape (arranged according to shape of plastic part), they should be designed to be drilled at both ends.
(2) Palm effect. Design water channel for large automotive plastic parts molds. Arrange water channel to flow in one direction, and arrange it at intervals like palm of your hand. Distance between water channels is controlled between 50 and 60 mm.
Automotive injection mold temperature control system mainly has following two combinations:
(1) First combination form: vertical straight-through water pipe + inclined water pipe + spacer-type water well;
(2) Second combination form: vertical water pipe + spacer well + inclined water pipe.
Difference between these two forms is that between inclined water pipes and spacer-type water wells, the former gives priority to inclined water pipes, while the latter gives priority to spacer-type water wells. These two combinations have different focuses and therefore different effects.
For automotive plastic parts, naturally conforming cooling water channels are beneficial to cooling of plastic parts and life of mold. Strict European and American molds do not even allow or minimize use of cooling water wells and sealing rings. Because diameter of water wells is large, too many will affect strength of mold, thereby shortening life of mold. Sealing rings are prone to aging and failure, so their use must be minimized in design. (For other design considerations of door panel molds, please refer to Door Panel Mold Design Issues and Design Key Points).