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

The cooling time of the part usually refers to the period from the time when the plastic melt fills the injection mold cavity to the time when the part can be taken out by opening the mold. The time standard for taking out the parts by opening the mold is often based on the fact that the parts have been fully cured and have certain strength and rigidity, so as not to deform and crack during opening the mold.

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Even if the same plastic is used for molding, its cooling time varies with the wall thickness, the temperature of molten plastic, the demoulding temperature of molded parts and the temperature of injection mold. At present, the formula to calculate the cooling time 100% correctly on all occasions has not been published, but only on the basis of appropriate assumptions. The calculation formula also varies according to the definition of cooling time.

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

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

② The average temperature in the section of plastic injection molded parts, and the time required to cool to the specified molding temperature of the product;

③ The temperature of the central layer of the thickest part of the wall of a crystalline plastic molded part, the time required to cool below its melting point, or the time required to reach the specified crystallization percentage.

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. There is no resistance to the heat transfer and flow between the melt and the mold wall. The temperature of the melt becomes 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 mold wall temperature;

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

④ The degree of heat conduction on the surface of injection mold is certain; (the melt filling process is regarded as an isothermal process and the melt temperature is uniform)

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