Left and right rearview mirror housing products are shown in Figure 1. Left and right plastic parts are completely symmetrical. Maximum external dimensions of product are 227.5 mm * 112.50 mm * 126.50 mm, average thickness of plastic parts is 3.36 mm, plastic parts are made of PP, and shrinkage rate is 3.36 mm. 1.016, weight of plastic part is 203.50 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilled injection molding, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product map of left and right rearview mirror housings
Left and right rear-view mirror products are necessary parts for cars, their shells are usually formed of PP materials and anti-aging additives. As can be seen from Figure 1, large opening of plastic part is installation part of mirror surface of rearview mirror, there are 3 screw posts for installation and fixation. A large opening on one side is installation part, there is a screw column inside for installation, and there is a small through hole on large plane on the side. Outer surface is all high-gloss surface, there should be no defects, gate marks, thimble marks, etc. Large opening on the side needs to be designed to pull slider core, and slider should cover ø7.15 small hole on the side.
Shape of mold is shown in Figure 2. Due to symmetry of plastic parts, mold design cavity ranking is 1+1, mold design ranking is that two cavities are arranged in a straight line, and two sliders are on same side. Mold base is a non-standard mold base CI5790. Mold is a large mold. Four inclined positioning blocks are designed on the edge of mold base, which is convenient for mold to resist lateral force of cavity during injection molding and maintain mold clamping accuracy. Positioning of mold base positioning block is usually called secondary positioning.

 

Figure 2 Outline drawing of left and right rearview mirror shell mold
Secondary positioning is precise positioning of front and rear mold bases, positioning accuracy of zero-degree positioning block is much higher than that of guide post and guide sleeve. Positioning block on mold base should be processed and assembled in mold base factory as much as possible to ensure matching accuracy of front and rear mold bases. Positioning block material is made of alloy steel heat-treated or copper plus graphite material, and there is a lubricating oil groove on friction surface of positioning block. Zero-degree positioning block has two functions of guiding and positioning, which can effectively protect weak penetration surface on mold core.

 

Figure 3 Parting surface and back mold diagram

 

Figure 4 Back mold slider
Three-level positioning is designed on the front and rear mold cores, that is, positioning tiger's mouth, which is designed at four corners of each mold core, and side angle is 5゜, which can effectively protect rubbing position in mold cavity. If rub-through angle of weak rub-through insert is small, angle of flank of tiger's mouth is taken as a small value. When designing mold, it is necessary to pay attention to make tiger's mouth of rear mold core protrude, and tiger's mouth of front mold core to be concave.
When designing mold, it should be noted that left and right rearview mirror shell molds are high-gloss molds, it is necessary to precisely control the molding conditions and design an adequate cooling circuit. Choose a steel suitable for high-gloss plastic parts. This kind of steel is mostly steel with high chromium content or high nickel content. Since there are many sharp corners in rear mold inserts of slider (see Figure 4), mold is easy to wear and tear. It is necessary to use heat-treated steel as a hard mold to effectively prolong service life of mold.
Plastic parts are ejected by thimble. For safety reasons, a reset mechanism is designed.
Lateral core-pulling mechanism is designed to be driven by inclined guide columns, and wear-resistant blocks are designed on inclined surface and bottom surface of slider for easy adjustment and replacement.
When designing mold, processing technology of mold parts must be considered. Determining benchmark is a very important part of processing of mold parts. One of the most realistic and common problems is that reference setting is not uniform in workpiece processing. Some engineers of same type of mold use center to bisect, while others are biased towards reference angle, which is very easy to make mistakes by on-site operators. Standardized manufacturing processes include process management standardization, process equipment standardization, and standardization of production processes from perspectives of process terminology, process documents, process regulations, and process elements. Datums and dimensions of a typical slider are shown in Figure 5.

 

Figure 5 Dimensional tolerances and requirements of the slider