In the injection production, plastic injection molding parts cooling time of the entire injection production cycle of about 80%. Poor cooling often results in warpage of the product or surface defects, affecting the dimensional stability of the product. Reasonable arrangement of injection, packing and cooling time, can improve product quality and productivity.
Part of the cooling time, usually refers to the plastic melt from the mold filled with mold cavity can be open to take out the part of the time. Can open the mold out of the time standard, often with the workpiece has been fully cured, with a certain strength and rigidity prevail, in the mold out of the top of the deformation will not crack. Even with the same plastic molding, its cooling time varies with wall thickness, molten plastic temperature, mold release temperature and injection mold temperature. The formula for calculating the cooling time on a 100% basis in all cases is not yet published, but only on the basis of the appropriate assumptions. The calculation formula also varies depending on the cooling time definition.
At present, usually the following three criteria as a cooling time reference:
① the temperature of the thickest part of the wall of the plastic injection molding wall, the time required to cool below the heat distortion temperature of the plastic;
(2) the average temperature in the section of the plastic injection molding section, the time required to cool the mold temperature of the specified product;
③ the temperature of the thickest part of the central wall of the crystalline plastic molding wall, the time required to cool below the melting point, or the time required to reach the specified percentage of crystallization.
In solving the formula, the following assumptions are generally made:
① plastic injection in the injection mold, and the heat transfer to the injection mold and be cooled;
② plastic molding cavity and the mold cavity in close contact with the cooling shrinkage and separation, melt and mold wall between the heat transfer and flow without any resistance, melt and mold wall contact with the moment the temperature has become the same. That is, when the plastic into the cavity, the workpiece surface temperature is equal to the mold wall temperature;
③ plastic injection molding parts cooling process, the injection mold cavity surface temperature is always uniform;
④ injection mold surface heat conduction degree must be; (melt filling process as isothermal process, and the material temperature uniform)
⑤ plastic orientation and thermal stress on the deformation of the parts can be ignored, the size of the pieces of solidification temperature does not affect.