In injection production, the cooling time of plastic injection molded parts accounts for about 80% of the entire injection production cycle. Poor cooling often results in product warpage or surface defects, which affects the dimensional stability of the product. Reasonable arrangement of injection, holding pressure and cooling time can improve product quality and productivity. part cooling time usually refers to the period of time from when the plastic melt is filled with the injection mold cavity to when the part can be opened and taken out. The time standard for opening the mold to take out the part is usually based on the fact that the part has been fully cured, has a certain strength and rigidity, and will not deform and crack when the mold is ejected. Even if the same type of plastic is used for molding, its cooling time varies with the wall thickness, the temperature of the molten plastic, the demolding temperature of the molded part and the temperature of the injection mold. The formula to calculate the cooling time 100% correctly in all situations has not yet been published, but only formulas that are calculated on the basis of appropriate assumptions. The calculation formula also differs depending on the definition of the cooling time. At present, the following three standards are usually used as the reference basis for cooling time: ① the temperature of the center layer of the thickest cap mould/mold part of the plastic injection molded part, the time required to cool to below the thermal deformation temperature of the plastic; ② within the section of the plastic injection molded part Average temperature, the time required to cool to the mold temperature of the specified product; ③ The temperature of the center layer of the thickest part of the crystalline plastic molding wall, the time required to cool below its melting point, or the time required to reach the specified percentage of crystallization time. When solving the formula, the following assumptions are generally made: ① The plastic is injected into the injection mold, and the heat is transferred to the injection mold to be cooled; ② The plastic in the molding cavity is in close contact with the mold cavity and does not separate due to cooling shrinkage, and the melt There is no resistance to heat transfer and flow between the mold wall, and the temperature of the melt becomes the same when it comes into contact with the mold wall. That is, when the plastic is filled into the mold cavity, the surface temperature of the part is equal to the mold wall temperature; ③ During the cooling process of the plastic injection molded part, the temperature of the injection mold cavity surface always remains uniform; ④ The heat conduction degree of the injection mold surface is certain; ⑤ The influence of plastic orientation and thermal stress on the deformation of the part can be ignored, and the size of the part has no effect on the solidification temperature.
