In injection production, the cooling time of plastic in […]
In injection production, the cooling time of plastic injection molded parts accounts for about 80% of the entire 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, holding pressure, and cooling time can improve product quality and productivity.
The part cooling time usually refers to the period from when the plastic melt is filled with the injection mold cavity to when the part can be opened and taken out. The time standard for the part that can be opened and taken out is usually based on that the part has been fully cured, has a certain strength and rigidity, and will not be deformed and cracked when the mold is opened and ejected. Even if the same type of plastic is used for molding, its cooling time varies with the thickness of the wall, the temperature of the molten plastic, the demolding temperature of the molded part, and the temperature of the injection mold. The formula for calculating the cooling time 100% correctly in all situations has not yet been published, but only formulas for calculations based on appropriate assumptions. The calculation formula also differs depending on the definition of the cooling time.
At present, the following three standards are usually used as the reference basis for cooling time:
①The temperature of the center layer of the thickest part of the plastic injection molded part, and the time required to cool to below the thermal deformation temperature of the plastic;
②The average temperature in the section of plastic injection molded parts, and the time required to cool down to the mold temperature of the specified product;
③The temperature of the center layer of the thickest part of the crystalline plastic molding wall is 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:
①The plastic is injected into the injection mold, and the heat is transferred to the injection mold to be cooled;
②The plastic in the molding cavity is in close contact with the mold cavity, and will not be separated due to cooling shrinkage. There is no resistance to heat transfer and flow between the melt and the mold wall. The temperature of the melt becomes the same when it contacts 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;
③During the cooling process of plastic injection molded parts, the temperature of the surface of the injection mold cavity is always uniform;
④The degree of heat conduction on the surface of the injection mold is certain; (the melt filling process is regarded as an isothermal process, and the material temperature is uniform)
⑤The influence of plastic orientation and thermal stress on the deformation of the part can be ignored, and the size of the part has no effect on the solidification temperature.