Influence of the hottest plastic electroplating on

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The influence of plastic electroplating on the design of plastic products and injection molding process

1 Introduction

after the electroplating of plastic products, they are endowed with the properties of composite materials, the surface hardness and wear resistance of products are greatly improved, and the surface is smooth and beautiful, so the application of plastic electroplating is increasingly extensive. For example, the radiator grille and wheel housing used for automobile exterior decoration are electroplated after injection molding with high-performance ABS plastic; Various indoor sanitary equipment, such as faucet handles and shower nozzles, use modified polyphenylene oxide as the base material for electroplating. However, the plating quality of plastic products, such as appearance quality, coating bonding strength, thermal performance, coating corrosion resistance and so on, are closely related to the design of plastic products and the selection of injection molding process parameters

2 design principles of plastic products for electroplating [1] electroplated plastic products should not only meet the functional requirements of plastic parts, but also be conducive to injection molding (almost only injection molded plastic products can be electroplated) and electroplating

2.1 wall thickness design of plastic products [5] uniform wall thickness of plastic products can promote uniform cooling and reduce deformation and shrinkage defects. The wall thickness should be within 2.3mm3.0mm, and the maximum wall thickness should not exceed 3.8mm, which is due to the cooling during injection molding and the shrinkage marks of thicker sections after cooling. The minimum wall thickness should not be less than 1.9mm, because the strength of the thinner section is poor, and the injection pressure loses greatly at the narrow gap during injection molding, so it is not easy for the plastic melt to fill the cavity. If the pressure is increased, the internal stress of the product will increase and the bonding strength of the coating will be reduced. Avoid thickness changes as much as possible, otherwise the shrinkage and deformation of the product will occur when the cooling is uneven. When the wall thickness has to be changed, the change should be as small as possible, and the transition between them should be as flat as possible, but sometimes the test results can not well reflect the quality of products in actual use. At the same time, when designing the mold, the gate position should be selected at the large section of the product, so as to facilitate the pressure maintaining and feeding at the large section, reduce the internal stress on the small section and the orientation of polymer molecules here. Because the internal stress and orientation will reduce the bonding strength of the coating. In order to avoid serious thermal cycle problems caused by electroplated plastic parts, the ratio of weight to area must be avoided when designing products, otherwise unacceptable shrinkage will occur during injection molding. As shown in Figure 1, plastic handles cannot be designed as solid

2.2 stiffener stiffener is a linear protrusion extending from the surface of the adjacent plane of the product. Its function is to reduce the volume and weight of the product and meet the requirements of high strength and high stiffness. In many cases, cross-linked stiffeners are used to prevent large and flat surfaces from bending and deformation after injection molding. The thickness of the stiffener shall be designed to be 50% to 60% of the supporting wall thickness at the intersection. Many small stiffeners can be used to replace a large stiffener to maintain this wall thickness relationship and avoid visible shrinkage marks on the back of plastic materials and products for food contact that were previously applicable to (EU) no10/2010. An appropriate arc shall be designed at the intersection of the stiffener and the supporting wall to avoid stress concentration. A demoulding slope of at least 1 ° shall also be designed to facilitate the demoulding of products. The relationship between the stiffener and the wall thickness, and the stiffener of the supporting wall is shown in Figure 2

2.3 bosses bosses are projections on the surface of products. It is used to connect and support relevant components. The boss can be solid or hollow. The recommended wall thickness of stiffeners also applies to bosses. In order to improve the strength of the boss, stiffeners should be used instead of making the wall thickness of the boss exceed the above recommended value. The height of the boss shall not exceed twice the diameter of the boss, and a fillet and a demoulding slope of not less than 1 ° shall be designed at the transition, as shown in Figure 3

2.4 edge enhancement for unsupported edges on products, the strength can be increased by crimping or changing its wall surface. This can ensure that the edge has sufficient stiffness, and reduce or eliminate deformation, especially when the wall is very thin. As shown in Figure 3

2.5 fillet demoulding slope the fillet at the intersection of the inner and outer planes of the product should be as large as possible, which can improve the flow state of the melt during injection molding and reduce the possibility of serious stress concentration when the part is under load. When designing a product, all its inner and outer surfaces must have a demoulding slope, so that the surface will not be damaged due to adhesion and friction when the product is demoulded. And when demoulding, unlike ordinary plastic products, Electroplated Plastics are not allowed to use release agents, especially silicone spray, which will stick to the products and reduce the bonding strength of the coating. If it is really difficult to demould, talc powder or soapy water can only be used as a demoulding agent

3 effect of injection molding process on electroplating bonding strength of plastic parts [23]

3.1 drying during injection molding, it is very important to fully dry plastic, especially plastic with strong water absorption. When the plastic is heated and melted in the barrel of the injection molding machine, the water absorbed by the plastic turns into steam. For example, graphite is not dried before injection molding. Based on the understanding of generalized force at home and abroad, generalized force should include force and torque. After the initial achievements of cable research and development are dehydrated, these water vapor is compressed and pressed into the product and expands into bubbles, forming bubbles or tear like traces on the surface of the product. Such traces often appear immediately after electroplating and become bubbles of the coating, and sometimes these bubbles only appear after the heat cycle test

3.2 injection temperature injection pressure injection speed

during injection molding, higher injection temperature (generally 255275 ℃), lower injection pressure and slower injection speed should be used. Because the prerequisite for the coating to obtain good bonding strength is to form a surface without internal stress and orientation. Orientation is inevitable, but increasing the melt processing temperature will widen the temperature domain between it and solidification temperature, and the polymer molecules will relax for a long time, which will help to remove the orientation effect of polymer molecules. The higher mold temperature used in the process is also based on the same reason. Lower injection speed can also reduce orientation. When the injection pressure is high, the internal stress of the product increases, which leads to the increase of the shear stress and shear rate of the melt in the flow, and intensifies the orientation of polymer molecules. The internal stress of plastic products and the orientation of polymer molecules will damage the pickling effect of plastic before electroplating, which not only reduces the bonding strength of the coating, but also reduces the alternating strength of the coating. It can be seen from Figure 4 that the plastic temperature is about 260 ℃, and the bonding strength of the coating is the best. If the temperature continues to rise, the bonding strength will improve very little, and too high temperature will lead to the degradation and discoloration of the plastic and damage the surface appearance quality

4 Conclusion

plastic electroplating has only been widely used in production for more than 30 years. Plastic electroplating is currently in the stage of vigorous development and promotion. Once a major breakthrough is made in the pretreatment process and coating performance of low-cost plastics such as polyolefins, or a major breakthrough is made in the cost and performance of conductive plastics, plastic electroplating will make a new leap. With the plasticization of the automotive industry and the emergence of all plastic vehicles, plastic electroplating will have a broader prospect

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