Plastic Moulds For Automotive Rearview Mirrors

1. Forming components: including the punch (core) and die (cavity), which directly determine the shape, size and surface quality of the rearview mirror plastic part.

2. Gate system: The channel that introduces the molten plastic from the injection machine nozzle into the mold cavity, usually composed of the main runner, sub-runner, gate and cold slug well, etc. To improve the uniformity of material feeding and reduce weld lines, some rearview mirror molds adopt hot runner systems.

3. Guide mechanism: The mechanism used to ensure that the moving mold and the fixed mold can accurately align when closing, preventing mold misalignment. It is typically composed of guide pins and guide bushes. Guide pins are usually installed on the moving mold or the fixed mold, while guide bushes are installed on the corresponding fixed mold or moving mold. Their fit accuracy is high, ensuring smooth and stable opening and closing movements of the mold.

4. Side parting and core-pulling mechanism: Due to the complex structure of rearview mirrors, which often have undercuts and side holes, side parting and core-pulling mechanisms are needed to achieve the molding and demolding of these parts. Common ones include inclined slider core-pulling mechanisms, inclined pin core-pulling mechanisms, and slider core-pulling mechanisms. For instance, in a certain rearview mirror housing mold, the external undercut is core-pulled with the aid of gravity using an inclined slider, while the internal undercut is core-pulled using inclined push blocks or a combination of inclined and straight push blocks depending on different situations.

5. Ejection mechanism: The device used to remove the plastic part from the mold cavity after molding. Common ejection mechanisms include pin ejection, plate ejection, and block ejection, and multiple methods can also be combined. For example, some rearview mirror molds use a combination of pins, blocks, and inclined push blocks for ejection, relying on the oil cylinder of the injection machine to push the ejection plate to achieve the ejection and demolding of the plastic part.

6. Cooling System: To enhance production efficiency and plastic part quality, the mold needs to be cooled to ensure that the plastic melt cools and solidifies uniformly in the cavity. The cooling system is usually composed of cooling water channels. During design, it is necessary to arrange them reasonably according to the mold structure and the shape of the plastic part to ensure uniform cooling. For example, multiple water channels are designed on the front and rear molds of the rearview mirror mold to focus on cooling the heat-intensive areas such as the inclined slider and straight top.

1. Mold Steel: Commonly used mold steels include P20, 718, 2344, etc. P20 steel has good processing and polishing properties and is suitable for general requirements of rearview mirror molds; 718 steel is an improvement on P20 steel and has higher hardness and strength, meeting the requirements of medium-batch production; 2344 steel has excellent thermal strength, toughness and wear resistance, and is often used for high-precision and long-life rearview mirror molds.

2. Other Materials: Besides mold steel, some components of the mold may use other materials, such as guide pins and guide sleeves, which are usually made of copper alloys or bearing steel with good wear resistance; pins and reset rods are made of high-quality carbon steel or alloy steel and undergo heat treatment processes such as quenching and tempering to increase their hardness and toughness.

1. Parting Surface Design: It should be selected at the maximum contour of the plastic part to facilitate the ejection of the plastic part and ensure the appearance quality of the plastic part. At the same time, the shape and position of the parting surface should be considered to affect the mold structure and molding process. Try to simplify the mold structure and reduce the number of parting surfaces.

2. Splitting Mechanism Design: According to the structural characteristics of the plastic part such as reverse fitting and side holes, choose the type and quantity of the splitting mechanism reasonably, and precisely calculate parameters such as splitting stroke and splitting force to ensure the smooth operation of the splitting action and the intactness of the plastic part.

3. Ejection Mechanism Design: To ensure that the plastic part does not deform or get damaged during ejection, the ejection force should be evenly distributed on the plastic part. According to the shape, size and structure of the plastic part, select an appropriate ejection method and ejection parts, and calculate and verify the ejection force.

4. Cooling System Design: The layout of the cooling water channels should be adapted to the shape, wall thickness of the plastic part and the flow direction of the plastic. The cooling speed of each part of the mold should be uniform to avoid defects such as warping and deformation of the plastic part. At the same time, pay attention to optimizing parameters such as the diameter, length and spacing of the cooling water channels to improve cooling efficiency.

1. Mechanical Processing: Using equipment such as CNC machining centers, lathes, milling machines, and grinding machines, the mold parts undergo rough and fine processing to ensure the dimensional accuracy and surface quality of the parts. During the processing, attention should be paid to the selection of tools, optimization of cutting parameters, and the arrangement of processing sequences to improve processing efficiency and accuracy.

2. Electrical Discharge Machining: For some complex-shaped parts that are difficult to be processed by mechanical methods, such as deep holes, narrow slots, and countersunk parts, electrical discharge machining can be used for shaping. Electrical discharge machining utilizes the high temperature generated by the pulsed discharge between the electrode and the workpiece to gradually remove the material of the workpiece, achieving the processing goal.

3. Surface Treatment: To enhance the wear resistance, corrosion resistance, and demolding performance of the mold, surface treatment is necessary. Common surface treatment processes include nitriding, hard chrome plating, and PVD coating. Nitriding treatment can form a layer of high hardness and wear-resistant nitride layer on the mold surface; hard chrome plating can increase the hardness and smoothness of the mold surface, facilitating the demolding of plastic parts; PVD coating can form a layer on the mold surface with high hardness, high wear resistance, and good lubricity, thereby increasing the service life of the mold.

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