In the injection production, the plastic injection molding parts cooling time accounts for about 80% of the entire injection production cycle. Poor cooling often leads to warping of the product or surface defects, affecting the dimensional stability of the product. Reasonable arrangements for injection, pressure maintenance and cooling Time, improve product quality and productivity.
The cooling time of a part usually refers to the period of time from when the plastic melt fills the cavity of the injection mold to when the part can be removed from the mold. The time standard for taking out the part from the mold can be opened, and often the part is fully cured. With a certain degree of strength and rigidity, it will not be deformed and cracked when the mold is ejected.
Even when molded in the same plastic, the cooling time varies depending on the wall thickness, the temperature of the molten plastic, the mold release temperature and the mold temperature. The cooling time must be calculated 100% correctly in all cases. The formula has not been published yet, and only formulas based on appropriate assumptions are calculated. The formulas also vary depending on the definition of cooling time.
Currently, the following three standards are usually used as reference for cooling time:
1 The temperature of the central layer at the thickest part of the plastic injection molded part is cooled to the time required for the plastic's heat distortion temperature;
2 The average temperature in the cross-section of the plastic injection-molded workpiece, and the time required for cooling to the die temperature of the specified product;
3 The temperature of the center layer of the thickest part of the wall of the crystalline plastic molded part, the time required for cooling to below its melting point, or the time required to reach the specified crystallization percentage.
When solving formulas, the following general assumptions are made:
1 The plastic is injected into the injection mold and heat is transferred to the injection mold and cooled;
2 The plastic in the molding cavity is in close contact with the mold cavity, and does not separate due to cooling shrinkage. The heat transfer and flow between the melt and the mold wall do not have any resistance, and the temperature of the melt and the mold wall has become the same at the moment of contact. When the plastic is filled into the cavity, the surface temperature of the part is equal to the die wall temperature;
3 During the cooling process of plastic injection molded parts, the temperature of the surface of the injection mold cavity is always uniform;
4 The thermal conductivity 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 effect of plastic orientation and thermal stress on the deformation of the part can be ignored. The size of the part does not affect the solidification temperature.