Design of Injection Mold for One-piece Shell of Rearview Mirror

Time:2020-11-16 11:39:22 / Popularity: / Source:

Car rearview mirrors are divided into exterior rearview mirrors and inner rearview mirrors. Exterior rearview mirrors of different models have different shapes and structures, and outer shell is an important appearance part of rearview mirror. In order to meet functional, appearance and economic requirements of automotive products, structure of rear-view mirrors tends to be beautiful, refined and integrated. One-piece shell reduces number of rear-view mirror assembly parts, number of molds required, and production costs, making it the first choice for rear-view mirror structure design. At present, there are more domestic researches on split housings, but less research on design of integrated housings. Now we introduce design of an injection mold for an integrated housing of an automobile rearview mirror.

1 Plastic parts structure and process analysis

Plastic parts structure 
Figure 1 Right rearview mirror housing
Housing of right rearview mirror is shown in Figure 1. Structure of left rearview mirror is symmetrical. External dimensions are 246.8mm*121.6mm*141.4mm, average wall thickness is 4.6mm, material is PA66+30%GF (glass fiber), shrinkage rate is 0.5%, material fluidity is poor, and structural characteristics of plastic parts are as follows.
(1) Thickness of side wall of concave part of plastic part is 19.5mm, appearance surface is not allowed to have spots or gate marks, and gate position needs to be selected reasonably.
(2) Shape of plastic part is complicated. There is a 1/2 circle groove along mirror installation part, a 1/4 circle rib, and a connection structure with bracket, which requires external core-pulling molding.
(3) Inner right side of plastic part has a large concave, which needs to be core-pulled, internal reinforcement ribs are more and deeper, exhaust problem needs to be considered. Difficulty of mold design is that inner recess on the right side of plastic part is wide and narrow on the outside, inner surface is a curved surface obliquely downward. Inside of one-piece shell integrates motor mounting seat structure, along core pulling direction, it affects layout of inner core pulling mechanism and limits its moving distance, making it difficult to form and demold.

2 Key points of mold design

01 Gating system design

mold design 
(A) Gate location
mold design 
(B) Sector gate and cavity pressure adjustment screw
Figure 2 Mold casting system
Material contains glass fiber and has poor fluidity. Use of ordinary runners can easily lead to underfilled plastic melt or excessive molding pressure. Use of a two-point hot runner gating system can reduce material loss, shorten molding cycle, and improve molding quality of plastic parts. In order to simplify mold structure and shorten molding cycle, a two-plate mold structure is adopted. Combined with mold flow analysis, fan-shaped gate is set at connecting part of rearview mirror with the thickest wall thickness and bracket. Gate is set on non-appearance surface, which can ensure aesthetics of surface of plastic part. Gate is at position of slider, and material is fed on slider. In order to ensure that gate condensate stays on the side of movable mold for easy removal, a pull rod shape similar to a latent gate is processed on slider, as shown in Figure 2 (a). When mold is opened, slider moves outwards to automatically separate gate condensate and improve production efficiency. An M6mm adjusting screw is installed at corresponding sector gate of upper mold. As shown in Figure 2(b), cross-sectional area of gate can be changed by adjusting screw depth to adjust flow balance of two cavities. An air needle is also arranged on oblique push rod on the side close to gate, gas-assisted molding is adopted, which is beneficial to enhance flow of plastic melt and ensure molding quality of plastic parts.

02 Forming and core pulling mechanism design

Main parting surface chooses position shown in Figure 1 to facilitate processing of mold parts. Plastic part requires both external core pulling and internal core pulling, adopts a slider and an inclined push rod core pulling structure. Among them, inner core pulling mechanism is a design difficulty.
Outer core pulling mechanism design
core pulling mechanism design 
Figure 3 Outer core pulling mechanism
Grooves, ribs and recesses connected to brackets along mirror surface require external core pulling. A four-sided slider mechanism is designed, using slider + inclined guide post mechanism and slider + T-shaped block mechanism for core pulling, as shown in Figure 3.
Design of inner core pulling mechanism
core pulling mechanism design 
(A) Top surface interference
core pulling mechanism design 
(B) Side interference
core pulling mechanism design 
(C) Inner core pulling and block structure
Figure 4 Internal core pulling mechanism
According to structural analysis of plastic part, if inner core-pulling part is formed by an oblique push rod, volume of oblique push rod is larger, which will interfere with top and side surfaces of plastic part when mold is opened, as shown in Figure 4 (a), (b). At the same time, motor mounting seat structure in core pulling direction in inclined push rod may also interfere with core pulling movement.
In view of above interference situation, overall oblique push rod is split into one push rod and two oblique push rods, as shown in Figure 4(c). Oblique push rods 1 and 2 are in contact with push rod through an inclined surface. Angle of inclined surface is designed with reference to angle between side and horizontal direction, which are 13° and 8° respectively.
When opening mold, oblique push rod moves along two oblique surfaces of push rod to lower left (upper) side to avoid interference with side. After integral oblique push rod is divided, push rod does not perform lateral core pulling when opening mold, which avoids interference with structure of mounting seat and shortens core pulling distance. In addition, avoid part that may interfere with inclined push rod and structure of mounting seat to ensure sufficient core pulling distance. Base of oblique push rod is designed as an inclined guide chute. When mold is opened, oblique push rod slides obliquely downwards and separates from inner surface of back of plastic part to avoid interference with top surface of plastic part when mold is opened, and to ensure quality of inner surface of plastic part. In order to avoid jamming caused by reverse torsion moment of oblique push rod when it is moving, angle of oblique push rod should not be too large, 6°~8° is the best, generally not more than 10°, and now 7°. Angle between guide chute and horizontal plane should be greater than angle between top surface of reference plastic part and horizontal plane (17.5°), which is 20°.

03 Launched institutional design

Top surface of rearview mirror housing has an obliquely downward arc-shaped inner surface. When ejected, oblique push rod and ejection mechanism are pushed out at the same time. Plastic part will pack forming part of oblique push rod and cannot be demolded smoothly. If forced demoulding is performed, inner surface of plastic part will be damaged. Therefore, a secondary push-out mechanism is designed, in which screw post of motor mounting seat is pushed out by a push tube. There are reinforcing ribs at screw column with a depth of 23mm, which is prone to trapped air. It is necessary to disassemble insert and use breathable steel for exhaust.
core pulling mechanism design 
Figure 5 Launch agency
1. Push plate 2. Push plate 3. Push rod fixing plate 4. Oblique push rod seat 5. Oblique push rod 6. Push rod 7. Push tube 8. Control hook
Secondary ejection mechanism is composed of inclined push rod, push rod, push tube, push plate, push rod fixed plate, secondary push limit mechanism, limit post, etc. Among them, push rod 6 and push tube 7 are fixed on push rod fixing plate 3, oblique push rod 5 and oblique push rod seat 4 are fixed on push plate 1, as shown in FIG. 5.
When mold is opened, ejector rod of injection molding machine pushes push plate 2 and drives push rod fixed plate 3 to move upwards. Push rod fixed plate drives push plate 1 and inclined push rod 5 through limit mechanism to push out once, with a stroke of 95mm. In this process, oblique push rod achieves inner core pulling. After plastic part is separated from core for a certain distance, hook is controlled to open, push plate 1 and inclined push rod on it will no longer move, push plate 2 and push rod fixed plate 3 continue to move upwards, push tube and push rod are used for a second push to make plastic part completely separate from forming part of inclined push rod, so that inner surface of molded plastic part will not be damaged, molding quality is guaranteed. Stroke of second push is 30mm.

3 Mould structure

mold structure 
Figure 6 Structure of injection mold for rearview mirror housing
1. Slider 2. Inclined push rod 3. Cavity plate 4. Core 5. Slider 6. Limit column 7. Push rod 8. Slider 9. Hot runner system 10. Air needle 11. Oblique push rod 12 .Slanted push rod seat 13. First reset mechanism 14. Second push out limit mechanism
Plastic part belongs to mass production. According to customer requirements, mold adopts a 1-mold 2-cavity structure, left and right rearview mirror housings are injection molded at one time. On the basis of LKM CI65100A220B160C330 mold base, a hot runner plate and a push plate are added. Gating system adopts hot runner and fan gate, core adopts mosaic method, cooling system is pipeline water-cooled, push-out system adopts secondary push-out mechanism of inclined push rod and push rod, exhaust system adopts parting surface and breathable steel exhaust. Injection mold structure of rearview mirror integrated housing is shown in Figure 6.
Main features of injection mold design for rearview mirror housing are as follows.
(1) Integrating advantages of side gates and latent gates, pouring on slider can realize automatic separation of gate condensate, high production efficiency, selection of screws to adjust flow rate is convenient to adjust balanced filling of two cavities.
(2) According to different demoulding requirements, different types of inner and outer core pulling mechanisms are designed, demoulding of complex structures is realized by adopting form of oblique push rods.
mold structure 
Figure 7 Left and right rearview mirror integrated housing
(3) Use of secondary push-out design ensures appearance quality of plastic parts and can realize automatic production of plastic parts. Plastic part has been mass-produced, and actual product is shown in Figure 7. Practice has proved that mold has a compact structure and a stable working process, all indicators of plastic parts have reached customer requirements, which can provide a reference for mold design of similar plastic parts.

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