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What is the basis for setting the cooling time of injection molded parts?

2019-10-11  291

In injection production, plastic injection molded parts cool down approximately 80% of the total injection production cycle. Poor cooling often results in warpage of the article or surface defects that affect the dimensional stability of the article. Properly arranging injection, holding pressure and cooling time can improve product quality and productivity.

The cooling time of the part usually refers to the period of time when the plastic melt is filled from the cavity of the injection mold to the time when the part can be opened. The time standard for the mold can be taken out, and the workpiece is fully cured, which has a certain strength and rigidity, and will not be deformed and cracked when the mold is ejected.

Even with the same plastic molding, its cooling time varies with the wall thickness, the temperature of the molten plastic, the mold release temperature of the molded part, and the temperature of the injection mold. The formula for calculating the cooling time correctly in all cases is not yet published, but only the formula for calculation based on appropriate assumptions. The calculation formula also varies depending on the definition of the cooling time.

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

1 The time required for the temperature of the central layer of the wall portion of the plastic injection molded part to be cooled below the heat distortion temperature of the plastic;

2 The average temperature in the section of the plastic injection molded part, the time required to cool to the mold temperature of the specified product;

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

When solving a formula, the following assumptions are generally made:

1 plastic is injected into the injection mold and transferred to the injection mold to be cooled;

2 The plastic in the molding cavity is in close contact with the cavity, and is not separated by cooling and shrinkage. There is no resistance to heat transfer and flow between the melt and the die wall, and the temperature of the melt has become the same at the moment of contact with the die wall. That is, when the plastic is filled into the cavity, the surface temperature of the workpiece is equal to the mold 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 degree of heat conduction on the surface of the injection mold is constant;

5 The influence of plastic orientation and thermal stress on the deformation of the part is negligible, and the size of the part does not affect the solidification temperature.