In injection production, the cooling time of plastic injection molded parts accounts for approximately 80% of the entire injection production cycle. Improper cooling often leads to product warping, deformation, or surface defects, which affect the dimensional stability of the product. Properly arranging injection, pressure holding, and cooling times can improve product quality and productivity.

        The cooling time of a workpiece generally refers to the period from the time when the plastic melt fills the injection mold cavity to the time when the workpiece can be opened and taken out. The time standard for taking out the parts during mold opening is often based on the fact that the parts have been fully cured and have a certain strength and rigidity, which prevents deformation and cracking during mold opening and ejection. Even if the same type of plastic is used for molding, its cooling time also varies with the wall thickness, temperature of the molten plastic, demolding temperature of the molded part, and injection mold temperature. The formula for accurately calculating the cooling time in all situations has not yet been published, so it is only calculated based on appropriate assumptions. The calculation formula also varies depending on the definition of heating time.

At present, the following three standards are generally used as reference for cooling time:

① The temperature of the center layer at the thinnest part of the plastic injection molded part wall, and the time required to cool to below the thermal deformation temperature of the plastic;

② The average temperature within the cross-section of plastic injection molded parts, and the time required to cool to the specified exit temperature of the product;

③ The temperature of the center layer in the thinnest part of the wall of a crystalline plastic molding component, the time required to cool to below its melting point, or the time required to reach the specified crystallization percentage.

When solving formulas, the following virtual assumptions are usually made:

① Plastic is injected into the injection mold and transfers heat to the mold, resulting in cooling;

② The plastic inside the molding cavity is in close contact with the mold cavity and never separates due to cooling shrinkage. There is no resistance to heat transfer and flow between the melt and the mold wall. At the moment of contact between the melt and the mold wall, the temperature has become similar. If the surface temperature of the workpiece is equal to the mold wall temperature after the plastic is filled into the mold cavity;

③ During the cooling process of plastic injection molded parts, the surface temperature of the injection mold cavity remains uniform;

④ The degree of heat conduction on the surface of injection molds is certain; (The filling process of molten material is considered an isothermal process, and the material temperature is uniform);

⑤ The influence of plastic orientation and thermal stress on the deformation of the workpiece can be ignored, and the size of the workpiece has no effect on the solidification temperature.