Automobile rearview mirror is an important part of automobile exterior decoration. It is located on both sides of car and is one of the most important exterior decoration parts of car. Automobile rearview mirror is a device used to allow drivers to observe road conditions during driving. It can be said to be eyes of driver. Automobile rearview mirror is generally composed of mirror ring, base, mirror shell, bracket, rotating shaft and other parts.

 

Figure 1 Automobile rearview mirror structure diagram

 

Figure 2 Automobile rearview mirror shell parts diagram
Part shown in Figure 1 is rearview mirror of brand car Ford Mondeo. This article takes automobile rearview mirror shell as an example to analyze and introduce design points and experience of automobile rearview mirror parts mold. Automobile rearview mirror shell parts diagram is shown in Figure 2.

Figure 2 shows Ford Mondeo automobile rearview mirror shell parts diagram. Material is ABS, shrinkage rate is generally 1.005, plastic part is a left and right mirror part, and number of mold cavities is 1+1. Plastic parts are appearance parts and are one of the most important exterior decoration parts of a car. Size of plastic part is: 254.2*122.5*120.6mm. Characteristics of plastic part are as follows: (1) Appearance of plastic part is not allowed to have spots, gate marks, shrinkage depressions, weld marks, flash and other defects. (2) Plastic part is an exterior part. Appearance of high-end plastic part is a fine-grained part, and low-end plastic part is a painted part. Appearance requirements are extremely high. (3) Appearance of plastic part is complex and appearance requirements are extremely high. There is a large undercut on outer side of plastic part and 6 undercuts on inner side. Undercuts are large and area is large, which is a typical structure of automobile rearview mirror mold.

 

Figure 3 Structure diagram of injection mold of automobile rearview mirror shell
According to structural characteristics of mirror shell plastic part, mold preferably adopts a hot runner injection mold structure, using 1-point open + cold runner + banana gate (commonly known as bull horn). As shown in Figure 2, there are 7 undercuts on inner and outer sides of plastic part. Undercut area of this mold is large and there are many undercuts. Four undercuts on inner side of this mold all adopt structure of oblique push block core pulling. Because undercut area of undercut 5 and undercut 6 is large, structure of oblique push block + straight push block combination is adopted. For this type of mirror shell mold, plastic part can only be taken out manually after demolding, provided that there must be space for manual picking when taking out. Outer side of plastic part adopts structure of oblique slider core pulling because direction of ejection is inclined. Outer dimensions of this mold are: 800*600*647 (mm), total weight is about 0.9 tons, it belongs to a medium-sized injection mold. See Figure 3-Figure 5 for detailed structure.

 

Figure 4 Structure diagram of injection mold for automobile rearview mirror shell 2

 

Figure 5 Structure diagram of injection mold for automobile rearview mirror shell 3

Fixed and movable molds of this mold adopt mosaic type, and commonly used materials are: 2344. Fixed and movable molds are made of same material. When ordering materials, it is necessary to indicate that hardness of movable mold is about 2 degrees less than that of fixed mold to avoid same hardness, because same hardness of fixed and movable molds will make mold easy to wear and burn. Principle of molded part design is to minimize size while ensuring strength and life of mold to reduce manufacturing cost and production cost of mold. Service life of rearview mirror mold of this model is 300,000 times, so material is 2344.

This mold casting system adopts hot runner injection, and adopts a 1-point open hot runner + cold runner + banana-type gate injection method. Plastic material is ABS.
Because plastic part is ABS material, material fluidity is good. When designing runner, runner design range is wider, but runner length from hot nozzle to the edge of plastic part should be designed within 60mm as much as possible. If it is really not possible, try to control it within 100mm to avoid too long cold runner and avoid large pressure loss. Location of hot runner gate of injection mold of rearview mirror shell of automobile is shown in Figure 6.

 

Figure 6 1-point open hot runner control system

Lateral core pulling mechanism is one of the most important structures of this mold. For undercut 7 on outside of plastic part, mold adopts a core pulling mechanism with an inclined slider. Core pulling direction is shown in Figure 2 and Figure 3. This mold preferentially designs inclined slider on ground side. Due to its own weight, slider will fall to ground side when inclined guide column pulls the core, so there is no need to design a spring. Undercut 1-undercut 4 on inner side of plastic part all adopt structure of inclined push block core pulling. Due to large undercut area of undercut 5 and undercut 6, structure of inclined push block + straight push block combination is adopted. For this type of mirror shell mold, plastic part can only be taken out manually after demolding, provided that there must be space for manual picking when taking out.

Mirror shell of rearview mirror of the car is an exterior decoration with extremely high appearance requirements. Therefore, quality of temperature control system design has a great influence on molding cycle of mold and molding quality of plastic part. One of design principles of cooling water channels is that distance from cavity surface should be roughly equal to achieve 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 channel + water well" (see Figure 7 and Figure 8). This combination is to use straight-through water pipes first, followed by inclined water channels, and finally water wells. Its advantages are uniform cooling of plastic parts, short molding cycle, high molding quality, is suitable for molds with high requirements and high appearance performance requirements.

 

Figure 7 (a) Fixed mold cooling system

 

Figure 8 (b) Moving mold cooling system
Layout of automotive mold cooling water channels is designed according to following rules: 1. Cooling water direction should be consistent with material flow direction. 2. Fixed and moving mold cooling water channels are preferably designed in a cross grid form, and cooling circuits form a water channel interlaced network to evenly cool plastic parts. 3. When it is not possible to design a cross-cross water channel, fixed and moving mold water channels are arranged alternately at gaps between each other. 4. Each group of cooling water should be designed with only four circulating water channels as far as possible to avoid long distance of water channels affecting cooling effect of plastic parts. 5. Cooling water channel should be designed to be connected to another group of water channels through an external water pipe to facilitate subsequent adjustment of plastic parts due to deformation, shrinkage and other phenomena. Adjusting plastic part defects through water channels is widely used in automotive interior and exterior plastic parts molds. 6. Interval between each cooling water channel is controlled at 3.5-5 times diameter of water channel (generally about 50-60mm), distance between plastic part glue surface and water transport 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 must be guaranteed to be more than 8-10mm, because mold is large and water channel is long, which is easy to drill off. 8. In design of automotive molds, hot nozzle should be designed with a separate set of water channels as far as possible, and it cannot be connected in series with other water channels to facilitate heat loss in hot nozzle area.
Temperature control system of this mold is as follows: 4 sets of water channels are designed for both fixed and movable molds, 2 sets of water channels are designed for each cavity, both fixed and movable molds have 4 inlets and 4 outlets. Design of mold cooling water channel is consistent with direction of material flow, and adopts design form of "straight-through water pipe + inclined water channel + water well". Distance of water inlet and outlet is roughly equal, so that plastic parts have a good cooling effect and appearance quality. Inclined slider of this mold and 5 side core pulling projection areas of undercut are large, and heat is concentrated. Therefore, cooling water is designed on inclined slider and straight push block, glue position at inclined slider and straight push block is cooled intensively.

This mold is a "push rod + push block + inclined push block" structure. After fixed and movable molds are opened, mold relies on push piece to push out plastic parts and runner. Push piece fixing plate is pushed by injection molding machine through oil cylinder and reset under action of 4 reset rods. When designing demoulding system, pay attention to following points: 1. For large molds (length and width exceeding 1400mmX700mm), 6 reset rods and 6 push rod plate guide columns need to be designed. A recovery block one level larger than reset rod should be designed on reset rod of all automobile molds. Recovery block is generally nitrided with 45# (S50C). Push rod plate guide column should be arranged near ejection component with large ejection force (such as cylinder, reset rod, etc.). 2. All automobile molds need to be designed with limit columns, and limit columns should be arranged above or near KO hole first. Push rods should be arranged at force-bearing position close to R and at position with large clamping force. Push rod design should be large, and push rod arrangement should be more. Push rods should be designed to same specifications as much as possible, and do not design many specifications. This can avoid frequent replacement of drill bits, save processing time and processing costs.

Melt passes through injection molding machine nozzle and enters mold cavity through hot nozzle 23. After melt fills the cavity, it is kept under pressure, cooled and solidified until it is sufficiently rigid. Injection molding machine pulls movable mold fixing plate 14 of mold, and mold is opened from parting surface PLⅠ. After mold is opened for 300mm, injection molding machine cylinder pushes pusher fixing plate 12, and pusher fixing plate pushes push rod 30. Then cylinder continues to work and continues to push out 60mm, pushing all pushers to push molded plastic parts away from movable mold. After plastic parts are taken out, injection molding machine cylinder pulls pusher and its fixing plate to reset, then injection molding machine pushes movable mold to close mold, and mold starts next injection molding.

This mold adopts a demolding mechanism of push rod + push block + oblique push block, which is a typical structure of automobile rearview mirror shell mold. When designing this type of mold, it should be noted that this structure must be manually taken out. Manually taking out plastic parts requires manually breaking out plastic parts when taking out parts. For this type of plastic parts, plastic parts must have a swinging space. This kind of plastic parts that are bent out by hand are also common in automobile mold design, such as automobile steering columns, throttle covers, etc.