Key points of heat resistant ABS resin injection molding

Reducing the internal stress of products is the key to the injection molding of heat-resistant ABS. In this paper, the author shared the processing points of heat-resistant ABS resin molding from five aspects, including mold design, material drying, molding temperature, holding pressure and time, and mold temperature.

The heat resistance of ordinary ABS is not good enough. Now, by adding ɑ- A series of heat-resistant ABS resins have been developed with heat-resistant agents such as methyl styrene copolymer and N-phenyl maleimide copolymer. The HDT is 90-120 ℃. For example, the Economic&Easy series heat-resistant ABS resin of Shanghai Jinhu Rili can meet the heat resistance requirements of different degrees at 95-109 ℃, and can be widely used in microwave ovens, rice cookers, electric heaters, hair dryers and other small household appliances; There are also series of products with higher heat resistance, which can withstand 109-120 ℃.

The molecular structure of heat-resistant ABS contains highly rigid benzene ring and N-phenyl group, which improves the heat resistance and also increases the steric hindrance effect of the molecular chain, reducing the relaxation speed of the molecular chain, which is easy to make the products have large internal stress, leading to defects such as stress cracking, stress whitening, and product brittleness. Therefore, reducing the internal stress of products is the key to the heat resistant ABS injection molding.

Product mold design

The product wall thickness shall be uniform. The wall thickness difference of ABS products shall be controlled within 25% to prevent excessive wall thickness difference from causing local stress concentration. The root of column with weak strength shall be added with R angle or reinforcement to prevent the column from breaking. For buckle design, the root of the buckle needs to be chamfered with an R angle. The size of the R angle depends on the wall thickness of the product. The R angle and the wall thickness ratio should be above 0.3. With the increase of this ratio, the internal stress gradually decreases, but the surface shrinkage of the product should also be considered. The design of mold cooling channel shall ensure the uniformity of cooling and avoid the internal stress caused by uneven cooling and contraction.

Material drying

Generally, the heat-resistant ABS resin will absorb moisture in the air during storage and transportation. The water absorption rate varies with the air humidity, generally between 0.2% and 0.4%. Therefore, the material must be fully dried to reduce the moisture content of the material to below 0.05%, preferably below 0.02%. Otherwise, water splash, silver wire and other surface defects may occur. The drying temperature of heat-resistant ABS resin is higher than that of ordinary ABS, usually 80-95 ℃, and the drying time is 3-4 hours.

Forming temperature

The molding temperature is a parameter that needs special attention during the processing of heat-resistant ABS. Its setting is to ensure that the heat-resistant ABS is fully plasticized as the benchmark. The temperature area above the middle value of the molding temperature recommended by the supplier should be used as much as possible. The increase of the molding temperature will significantly reduce the viscosity of the heat-resistant ABS, increase the fluidity of the resin, and make the flow distance longer to ensure that the material has sufficient filling capacity. For example, the heat resistant ABS HU600 of Jinhu Rili recommends a molding temperature of 240-260 ℃. In principle, when using the recommended upper limit of molding temperature, the residence time of the melt adhesive should be as short as possible to avoid high temperature degradation of the material.

Holding pressure and time

For heat-resistant ABS, whether the setting of holding pressure and time is reasonable or not directly affects the internal stress of the parts. The increase of holding pressure will reduce the molecular gap, reduce the range of chain segment activity, reduce the volume of melt, increase the density, and increase the force between molecules, which can improve the shrinkage and internal quality of the product, but will lead to greater internal stress. Therefore, lower holding pressure should be selected as far as possible on the premise of ensuring the appearance quality of the product.

The setting of pressure holding time is based on the time when the gate is cooled to complete its curing and the screw is pushed forward again and no longer exerts pressure on the molded products. If the pressure holding time is too long, it is easy to overfill the material, reduce the molecular gap and increase the internal stress; If the holding time is too short, the product is easy to shrink and the size is unstable. The best pressure holding time shall be the shortest time when the product weight does not change.

Mold temperature

The author suggests that the mold temperature machine should be used to control the mold temperature when forming heat-resistant ABS. The recommended mold temperature is 60-80 ℃. The higher the mold temperature, the better the flow, the higher the weld line strength and the lower the internal stress of the product will often be produced, but the molding cycle will be appropriately extended. If the mold temperature is lower than the recommended value, the internal stress of the product will be too high, which will reduce the performance of the product, and there may be product embrittlement, screw hole cracking or spray paint cracking and other undesirable phenomena.

Finally, each molding process element should be considered comprehensively and coordinated with each other.