Automobile door panels are an important part of automobile interior parts. They are located on the inside of car door and are divided into front, back, left and right. They vary according to car series, usually two doors and four doors. These parts are collectively referred to as door panel series. Below, editor takes left rear door panel of car as an example to explain design points and experience of car door panel injection mold.

 

(Figure 1) shows a left rear door panel part of a certain brand of automobile. Material is PP EPDM, and shrinkage rate is generally 1.011. EPDM is a copolymer of ethylene, propylene and a small amount of non-conjugated dienes, and 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 chain, it has excellent aging resistance such as ozone resistance, heat resistance, and weather resistance, and can improve elasticity of door panel. 
Door panel is an appearance part with an overall size of 880*670.4*105.6mm. Its structural features are as follows:
1. Outer surface has high requirements, and spots, gate marks, shrinkage depressions, weld marks, and flash are not allowed.
2. Door panel is a leather grain part, and demolding slope of appearance surface is at least 5°.
3. Door panel has high surface finish, complex shape structure, complex parting line, many undercuts, 11 undercuts on inner and outer sides of plastic part (see S1~S11 in 3D figure of inner side of door panel in Figure 1), which makes demolding difficult.

 

Figure 1 Automobile door panel parts

Due to large size and complex structure of door panel, mold adopts a hot runner pouring system. 4 needle valve hot nozzles are controlled by sequence valve to enter mold cavity through ordinary runner and fan gate in turn. There are 11 undercuts on inner and outer sides of door panel plastic part, and only S11 is undercut on outer side of plastic part. From perspective of mold reliability and processing, S11 adopts side core pulling structure of "oblique guide column slider", and the other undercuts all adopt side core pulling structure of "oblique push rod oblique push block".
Outer dimensions of this mold are: 1450*1400*985 (mm), with a total weight of about 16 tons, which is a large injection mold. Detailed structure is shown in Figure 2 plan view and Figure 3 stereogram.

a. Fixed mold arrangement diagram

 

b. Moving mold arrangement diagram

 

c. M-M

 

 

Figure 2 Structure diagram of automobile door panel injection mold

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

 

Molding parts and mold plates of injection mold of left rear door panel of automobile are all integrated, as shown in Figure 3. This form of injection mold has a more compact structure, better strength, relatively small mold volume, and avoids processes of opening frame, matching frame and manufacturing inclined wedge.
We recommend using standard P20 or 1.2738 plastic mold steel for fixed mold A plate and movable mold B plate. Since mold is a large injection mold, fixed mold A plate and movable mold B plate adopt an inner mold positioning structure with four-sided edges (see Figure 3). This structure makes movable and fixed molds integrated after molds are closed, greatly improving molding accuracy of door panel and production life of mold.

 

 

Figure 3 Stereoscopic diagram of injection mold of left rear door panel of car

Casting system of this mold adopts "hot runner + ordinary runner" glue injection form, in which hot runner adopts a hot runner plate plus 4 needle valve hot nozzles (see G1, G2, G3 and G4 in Figure 4). 4 needle valve hot nozzles are not injected at the same time, but are controlled by sequence valve to open in sequence according to shape and size of plastic part. Melt enters cavity through ordinary runner and finally through fan gate.
Since plastic part is made of PP EPDM material with good fluidity, length of ordinary runner can be controlled within 60-100mm. If ordinary runner is too long, it will cause excessive pressure and heat loss, affecting melt filling and molding quality of plastic part.
Door panel is an appearance part, and no weld marks are allowed on the surface. During injection molding, weld marks must be driven to non-appearance surface or eliminated. This is one of key points and difficulties in design of this mold. Although traditional synchronous multi-point pouring can make melt fill the entire cavity, it is difficult to achieve ideal product quality due to presence of weld marks. For this reason, this mold adopts a 4-point sequential valve hot runner gate control technology, which controls opening and closing of 4 hot nozzles through drive 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 sequential valve hot runner control system

Lateral core pulling mechanism is core mechanism of door panel injection mold. This mold has a total of 11 lateral core pulling locations, namely S1~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 block combined, which is safe and reliable. S1~S10 all adopt structure of "oblique push rod oblique push block". Its detailed structure and important dimensions are shown in Figure 2 (d) to (m).
In structural design of "oblique push rod oblique push block", inclination angle of oblique push rod should not exceed 12°, design of oblique push block should prevent plastic part from sticking to oblique push block during demolding, causing deformation and cracking of plastic part.

Quality of temperature control system design has a great influence on molding cycle and product molding quality of mold, which is especially important for automobile door panel injection molds with high appearance requirements. One of design principles of cooling water channels is that distance from cavity surface should be roughly equal to achieve a roughly balanced temperature at all locations in mold cavity. Temperature control system of this mold adopts a combination of "straight-through water pipe inclined water pipe water well", see Figure 5 (a) and (b) for details. This combination is to give priority to straight-through water pipes, supplemented by inclined water pipes, and use water wells only as a last resort. Its advantages are uniform cooling of plastic parts, short molding cycle, high molding quality, suitable for molds with high requirements and high appearance performance requirements.
In design of automobile molds, similar to molds for interior and exterior trims such as automobile front and rear bumpers, dashboards, central channels, grilles and automobile decorative strips, layout of cooling water channels is generally designed according to following rules:
(1) Direction of cooling water should be consistent with direction of material flow.
(2) Cooling water channels of fixed and movable molds are preferably designed in a cross grid form, and cooling circuits form a water channel interweaving network to evenly cool plastic parts.
(3) When it is not possible to design a cross-shaped water channel, fixed and movable mold water channels are arranged alternately at gaps between them.
(4) Each group of cooling water should be designed with only four circulating water channels as much as possible to avoid long water channel distances that affect cooling effect of plastic parts.
(5) Cooling water channel should be designed in a way that can be connected to another group of water channels through external water pipes to facilitate subsequent adjustment of plastic parts due to deformation, shrinkage and other phenomena. Solving plastic part defects through water channel adjustment is widely used in automobile interior and exterior trim plastic parts molds.
(6) Distance between each cooling water channel should be controlled within 3.5-5 times diameter of water channel (generally about 50-60mm), distance between cavity surface and cooling water channel is generally between 15-25mm, which is determined by size of mold.
(7) Distance between cooling water channel and push rod, inclined push rod and insert should be guaranteed to be more than 8-10mm. Because mold is large and water channel is long, it is easy to drill off-center. It is necessary to avoid occurrence of cooling water leakage due to water channel being too close to cavity or other structures.
(8) In design of automotive injection molds, hot nozzle should be designed with a separate set of cooling water channels as much as possible, and it should not 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 fixed and moving mold temperature control system of this mold are: fixed and moving molds are designed with seven groups of water channels, fixed and moving molds are seven inlet and seven outlet. Design of mold cooling water channel is consistent with material flow direction. Design form of "vertical water pipe, inclined water pipe, and partition-type water well" following shape of plastic part is preferred. Inlet and outlet distances are roughly equal to length of water channel, so that mold has a good cooling effect and plastic part has a good appearance quality.

Quality of guiding and positioning system design directly affects precision of molded plastic parts and life of mold. In design of automobile injection molds, due to large mold, large batch of plastic parts, high appearance requirements and dimensional accuracy, design of mold guiding and positioning system is very strict.
This mold is designed with a square guide column and a round guide column at each of four corners, as well as four 1° precision positioning structures, as shown in Figures 2 and 3 for details. Among them, size of 4 round guide pins is ∅ 40*225mm (the longest guide pin cannot exceed 10 times its diameter), which is installed on fixed mold side. Since plastic parts are left on movable mold side after mold is opened, this will not affect removal of plastic parts. At the same time, 4 guide pins can also serve as support pillars when turning mold, which is convenient for FIT mold.
Length of guide pin of mold without a slider must be 30mm higher than the highest point of fixed and movable molds; mold with a slider must ensure that guide sleeve is inserted 20mm before inclined guide pin is inserted into slider, otherwise it will cause great trouble in manufacture and production of mold, and in serious cases, mold will be damaged.

Demoulding mechanism of this mold adopts "push rod oblique push block push block nitrogen spring" ejection structure. After fixed and movable molds are opened, mold relies on push piece to push out plastic parts and runners. Push piece fixing plate is mechanically pushed by injection molding machine through K.O hole and reset under action of 4 reset rods. When designing demoulding system, pay attention to following points:
(1) Large molds (length and width exceeding 1400mm*700mm) need to be designed with 6 reset rods and 6 push rod plate guide pillars.
(2) All automotive mold reset rods need to be designed with a return block that is one level larger than reset rod. Gud Mould recommends that return block be made of national standard 45# steel or S50C nitrided steel that we distribute.
(3) Push rod plate guide pillar should be arranged near ejection parts with large ejection force (such as cylinders, reset rods, etc.).
(4) All automotive molds need to be designed with limit columns, and limit columns should be arranged above or near K.O holes.
(5) Push rods should be arranged at force-bearing position close to R and at position with large clamping force. Push rods should be designed to be large and arranged more. Push rods should be designed to be of same specification as much as possible, so as to avoid frequent replacement of drill bits, save processing time and processing costs.

In automotive 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 defects such as insufficient filling, air entrapment and poor demolding. In severe cases, air entrapment will also burn plastic parts.
Automobile door panels are interior parts, and appearance of plastic parts is strictly required. It is very important to design exhaust reasonably. When designing mold exhaust system, pay attention to following points:
(1) Exhaust should be opened at the end of material flow and corner of plastic part.
(2) Close to insert or the thinnest wall thickness, because it is easiest to form a weld line here.
(3) It is best to open it on parting surface, because overflow on parting surface is easiest to remove. Exhaust of this mold is opened in fixed mold.

When designing a mold according to size and structure of plastic part, strength and rigidity of mold must be guaranteed first. This is especially important for large automobile injection molds. Reasonable mold design concept should be: mold strength is reasonable and not wasteful, local materials are used, mold strength and cost are balanced, the best design scheme and processing technology are selected. For mold strength that is too strong, it will be wasteful, and too weak will affect service life of mold. Parting surface pipe 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. This four-sided surrounding design method is widely used in molds such as automobile door panels and fenders.
Calculation method of the two sizes A and B shown in Figure 6 is: first add a 50mm sealing position from maximum edge of plastic part (in design of automobile molds, small molds (within 5050) have 30mm sealing, medium molds (5050-1010) have 40mm sealing, and large molds (above 1010) have 50mm sealing.), then add a 50-70mm avoidance position (in design of automobile molds, only sealing position is matched, and the rest are all avoided to reduce workload of FIT mold. Avoidance position is also the area to ensure strength of mold.) Then add size of pressure plate of parting surface at mold base to get size of A.B, so that a mold that meets customer's mold strength requirements and saves costs can be designed. C size of different plastic parts will have different values. At least C size of fixed mold should 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. It is used to ensure strength of mold. In short, in daily design, according to different customers and factories, it is flexibly used to design molds that meet customer needs and save costs.
Since parting surface of automobile plastic parts is often more complicated, positioning of movable and fixed molds on parting surface is very critical and is the first issue that mold designers must consider.

 

 

Figure 6 Reference for strength of automobile door panel mold

Melt passes through nozzle of injection molding machine, enters ordinary runner through hot runner, then enters mold cavity through fan gate. After melt fills the cavity, it is pressurized, cooled and solidified until it is sufficiently rigid. Injection molding machine pulls movable mold fixing plate 18 of mold, and mold is opened from parting surface I. Plastic part leaves fixed mold cavity. At the same time, locking block 10 leaves slider 11. Under push of inclined guide column, slider drives lateral core pulling 7 and 8 to complete lateral core pulling of undercut S11. After mold opening distance reaches 500mm, injection molding machine cylinder pushes pusher fixing plate 16 and pusher bottom plate 17, then pushes all ejectors and inclined push rods. In this process, inclined push rods 21, 27, 31, 36, 40, 44, 48, 52, 56 and 60 push inclined push blocks 24, 29, 34, 38, 42, 46, 50, 54, 58, and 63 respectively, completing lateral core pulling of undercuts S1~S11 and pushing plastic part away from movable mold core.
After plastic part is taken out by robot, injection molding machine cylinder pulls pusher 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, and following focuses on temperature control system of this mold.
Temperature control system is crucial in design of automobile molds. It has a great impact on molding cycle and molding quality of plastic parts. Reasonable temperature control has a far-reaching impact on improving quality of plastic parts and shortening molding cycle. Following points should be followed in design of cooling systems for large automobile molds:
(1) Three-meter principle. The total length of cooling water channel cannot exceed 3 meters. If it exceeds 3 meters, cooling effect will be poor. It is necessary to avoid mold opening before cooling water has come out. In addition, length of a single cooling water channel must take into account drill length. If hole is too deep and drill length is not enough, drilling will not be possible. Diameter of straight-through cooling pipe of a large automobile mold is generally ￠15mm. If cooling water channel of fixed and movable molds of door panel injection mold is designed in a contoured manner (arranged according to shape of plastic part), it should be designed to be drilled at both ends as much as possible.
(2) Palm effect. When designing water channel design of a large automobile plastic part mold, water channel should be arranged to flow in one direction, and intervals should be arranged like a palm. Distance between water channels should be controlled between 50 and 60mm.
There are two main combinations of temperature control systems for automotive injection molds:
(1) The first combination: vertical straight-through water pipe, inclined water pipe, and spacer-type water well;
(2) The second combination: vertical water pipe, spacer-type water well, and inclined water pipe.
Difference between these two forms is that between inclined water pipes and spacer-type water wells, the former prefers inclined water pipes, while the latter prefers spacer-type water wells. These two combinations have different focuses and thus different effects.
For automotive plastic parts, cooling water channel designed naturally conforms to shape, which is beneficial to cooling of plastic parts and life of mold. European and American molds with strict requirements do not even allow or try to use cooling water wells and sealing rings as little as possible. Because diameter of water well is large, too many of them will affect strength of mold and thus shorten life of mold. Sealing rings are prone to aging and failure, so their use must be minimized in design. (For other design considerations for door panel molds, refer to design issues and design points of door panel molds).