Injection molding machine injection molding process heat sensitive plastic and easy to hydrolyze plastic
Thermosensitive means that some plastics are sensitive to heat, when the heating time is long at high temperature or the cross section of the feed port is too small, and when the shearing effect is large, the temperature of the material is increased, and the tendency of discoloration, degradation and decomposition is easy to occur. Plastics are called heat sensitive plastics. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene and so on. Thermally sensitive plastics produce by-products such as monomers, gases, solids, etc. during decomposition, and in particular, some decomposition gases have irritating, corrosive or toxic effects on human bodies, equipment, and molds. Therefore, mold design, selection of injection molding machine and molding should pay attention to the screw injection molding machine, the casting system should have a large section, the mold and barrel should be chrome-plated, there must be no * angle stagnation, the molding temperature must be strictly controlled, plastic Stabilizer is added to reduce its thermal performance.
Some plastics (such as PC), even if they contain a small amount of water, decompose under high temperature and high pressure. This property is called easy hydrolysis and must be preheated and dried.
Injection molding machine injection molding process stress cracking and melt fracture
Some plastics are sensitive to stress, and are prone to internal stress during molding and are brittle and fragile. The plastic parts are cracked under the action of external force or under the action of solvent. To this end, in addition to adding additives to the raw materials to improve the crack resistance, the raw materials should be dry, reasonable selection of molding conditions to reduce internal stress and increase crack resistance. And should choose a reasonable plastic shape, it is not appropriate to set inserts and other measures to minimize stress concentration. When designing the mold, the draft angle should be increased, and a reasonable feeding port and ejection mechanism should be selected. The material temperature, mold temperature, injection pressure and cooling time should be properly adjusted during molding to avoid mold release when the plastic parts are too cold and brittle. After molding, the plastic parts should also be post-treated to improve the crack resistance, eliminate the internal stress and prohibit contact with the solvent.
When the polymer melt of a certain melt flow rate exceeds a certain value when passing through the nozzle hole at a constant temperature, a significant lateral crack occurs on the surface of the melt, which is called melt fracture, which impairs the appearance and physical properties of the plastic part. Therefore, in the selection of polymers with high melt flow rate, the nozzle, runner, and feed port sections should be increased to reduce the injection speed and increase the material temperature.