keypoint of heat-resistant ABS injection molding

key point of heat-resistant ABS injection molding

The ordinary ABS is not good at heat resistance . 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 heat deformation temperature (HDT) is 90-120 ℃.

Because the molecular structure of heat-resistant ABS contains highly rigid benzene ring and N-phenyl group, it not only improves the heat resistance, but also increases the steric effect of molecular chain, reduces the relaxation speed of molecular chain, and is easy to cause large internal stress in the product, resulting in defects such as stress cracking, stress whitening and brittleness of the product. Therefore, reducing the internal stress of products is the key of heat-resistant ABS injection molding.
Product mould design
The product wall thickness shall be uniform, and the wall thickness difference of ABS products shall be controlled within 25% to prevent local stress concentration caused by excessive wall thickness difference. The R reinforcement shall be added at the root of the column to prevent the strength of the weak column from increasing.
In the buckle design, the root of the buckle needs an inverted R angle. The size of the R angle depends on the wall thickness of the product. The ratio of R angle to wall thickness should be more than 0.3. With the increase of this ratio, the internal stress will gradually decrease, but the shrinkage of the product surface should also be considered. The design of mold cooling water channel should ensure the uniformity of cooling and avoid the internal stress caused by uneven cooling and shrinkage.
Material drying
Generally, heat-resistant ABS resin will absorb moisture in the air during storage and transportation, and the water absorption varies with the air humidity, generally between 0.2 ~ 0.4%. Therefore, the material must be fully dried to reduce the moisture content of the material to less than 0.05%, preferably less than 0.02%, otherwise there may be surface defects such as water spray and silver wire. 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 based on ensuring the full plasticization of heat-resistant ABS. Try to use the temperature area above the middle value of the molding temperature recommended by the supplier. The increase of the molding temperature will significantly reduce the viscosity of heat-resistant ABS, increase the fluidity of resin and prolong the flow distance, To ensure that the material has sufficient filling capacity.
Holding pressure and time
For heat-resistant ABS, whether the setting of holding pressure and time is reasonable directly affects the internal stress of the workpiece. 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 products, but will lead to the increase of internal stress. Therefore, try to choose a lower holding pressure on the premise of ensuring the appearance quality of products.
The setting of pressure holding time is based on the time until the solidification of the gate is completed with cooling, and the screw is pushed again, and the pressure is no longer applied to the molded product. 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 pressure holding time is too short, the product is easy to shrink and the size is unstable. The setting of pressure holding time shall take the shortest time when the product weight does not change as the best pressure holding time.
Mold temperature
When forming heat-resistant ABS, the mold temperature machine is used to control the mold temperature. The recommended mold temperature is 60-80 ℃. Higher mold temperature will often produce good flow, higher weld line strength and less internal stress of the product, but the forming cycle will be appropriately prolonged.
If the mold temperature is lower than the recommended, the internal stress of the product will be too high, which will reduce the performance of the product, and adverse phenomena such as product embrittlement, screw hole cracking or painting cracking may occur.