Reducing internal stress is the key to heat - resistant ABS injection molding. In this paper, from the mold design, material drying, molding temperature, holding pressure and time, mold temperature and other five aspects, sharing the author of heat-resistant ABS resin molding processing points.
Ordinary ABS heat resistant performance is not good enough, now, by adding ɑ - and N - phenyl methyl styrene copolymer heat resistant agent such as maleic imide copolymer developed a series of heat-resistant ABS resin, thermal deformation temperature (HDT) 90-120 ℃. Such as Shanghai kumho Japan Korea double yi (Economic&Easy) series of heat-resistant ABS resin can meet the demand of 95-109 ℃ heat resistance of different level, can be widely used in small home appliance such as microwave oven, electric cooker, electric heaters and hair dryers; And higher heat resistance of series of products, can heat 109-120 ℃.
As the molecular structure of heat-resistant ABS contains highly rigid benzene ring and n-phenyl group, it improves the heat resistance, increases the steric hindrance effect of the molecular chain, reduces the relaxation rate of the molecular chain, and tends to cause greater internal stress in the products, resulting in the defects of the products such as stress cracking, stress whitening and product brittleness. Therefore, reducing the internal stress is the key to heat - resistant ABS injection molding.
Product mold design
The wall thickness of products shall be uniform, and the wall thickness difference of ABS products shall be controlled within 25% to prevent excessive wall thickness difference from causing local stress concentration. At the root of the column with weak strength, R Angle or additional reinforcement shall be added to prevent the column from breaking. The size of R Angle depends on the wall thickness of the product. The ratio between R Angle and wall thickness should be above 0.3. As this ratio increases, the internal stress gradually decreases, but the problem of surface shrinkage of the product should also be considered. The design of mold cooling channel should ensure the uniformity of cooling and avoid the internal stress caused by uneven cooling and different shrinkage.
The material is dry
Generally, heat-resistant ABS resin will absorb moisture in the air during storage and transportation, and the water absorption rate will vary 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 below 0.05%, and preferably below 0.02%, otherwise there may be water spray, silver wire and other surface defects. Heat-resistant ABS resin drying temperature is higher than ordinary ABS, usually is 80-95 ℃, the drying time 3 to 4 hours.
Mold temperature
Mold temperature is heat-resistant ABS process needs to pay special attention to one parameter, the setting is to ensure that the heat-resistant ABS plastic into a benchmark in full, should try to use the intermediate value of molding temperature is recommended supplier the temperature on the partial area, molding temperature increase will significantly lower the viscosity of the heat-resistant ABS, increase liquidity in the resin, the flow distance is longer, to ensure that the material has enough ability to fill. The heat-resistant ABS HU600 kumho, for example, Japan Korea suggested molding temperature is 240-260 ℃. In principle, when using the recommended molding temperature upper limit, melt adhesive retention time should be as short as possible to avoid high temperature degradation of materials.
Holding pressure and time
For heat-resistant ABS, whether the setting of holding pressure and time is reasonable directly affects the size of the internal stress of the product. The increase of the holding pressure will make molecular gap narrowed segment range decreases, and the volume of the melt and density change, forces between molecules increase, can improve the shrinkage and internal quality, but can lead to internal stress, therefore, on the premise of guarantee the quality of product appearance as far as possible choose low pressure maintaining pressure.
The pressure holding time is set according to the time until the screw is no longer pressed on the formed product when the gate solidifies after cooling. The pressure holding time is too long, which can easily lead to excessive material filling, smaller molecular gap and larger internal stress. Pressure holding time is too short, the product is easy to shrink, size is not stable. The best pressure holding time should be the shortest time when the weight of the product does not change.
Mold temperature
The author suggest when molding heat-resistant ABS use to control mold temperature, mold temperature machine suggested that mold temperature is 60-80 ℃. Higher die temperature tends to produce better flow, higher weld strength and less internal stress
Finally, the molding process elements should also be considered and coordinated.
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