Analysis and calculation of cooling time of plastic injection molded parts

In injection production, the cooling time of plastic injection molded parts accounts for about 80% of the whole injection production cycle. Poor cooling often leads to warping and deformation of the product or surface defects, which affects the dimensional stability of the product. Reasonable arrangement of injection, pressure-holding, and cooling time can improve product quality and productivity.

The cooling time of the part is usually the period when the plastic melt is filled with the cavity of the injection mold and can be removed from the mold. Can open the mold to take out the time standard, often to the part has been fully cured, with a certain strength and rigidity shall prevail, in the opening of the mold ejected without deformation and cracking.

Even when using the same plastic 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. There are no published formulas for calculating the cooling time that is 100% correct in all cases, but only formulas that are based on appropriate assumptions. The formulae also vary depending on the definition of cooling time.

At present, the following three criteria are usually used as a reference for cooling time:
① The temperature of the center layer of the thickest part of the wall of the plastic injection molded part, the time required to cool down to below the heat deflection temperature of the plastic;
② the average temperature in the section of plastic injection molded parts, cooling to the specified temperature of the product out of the mold to the time;
③ the temperature of the thickest part of the center layer of the wall of the crystalline plastic molding part, the time required to cool to below its melting point, or the time required to reach the specified crystallization percentage.

In solving the formula, the following assumptions are generally made:
① The plastic is injected into the injection mold and is cooled by transferring heat to the injection mold;
② The plastic in the molding cavity is in close contact with the mold cavity and does not separate due to cooling shrinkage, and there is no resistance to heat transfer and flow between the melt and the mold wall, and the temperature of the melt has become the same at the moment of 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 temperature of the mold wall;
③In the cooling process of the plastic injection molding process, the temperature of the surface of the cavity of the injection mold always remains uniform;
④The heat conduction degree of the injection mold surface is certain; (the molten material filling process is regarded as an isothermal process, and the material temperature is uniform)
⑤ The influence of plastic orientation and thermal stress on the deformation of the part can be ignored, and the size of the part does not affect the solidification temperature.