What is the basis for cooling the injection product?
In injection production, the cooling time of plastic injection molded parts accounts for about 80% of the entire injection production cycle. Poor cooling often results in warpage of the product or surface defects, affecting the dimensional stability of the product. Reasonably arranging injection, injection product holding pressure and cooling time can improve the quality and productivity of products produced by injection molding machines.
The cooling time of the part usually refers to the period from when the plastic melt fills the cavity of the injection mold to when the part can be opened and taken out. The standard for the time when the product can be taken out by opening the mold is often based on the part being fully cured, having a certain strength and rigidity, and not deformed and cracked when the mold is ejected and ejected.
Even if the same plastic is used for molding, its cooling time varies with the wall thickness, the temperature of the molten plastic, the demolding temperature of the molded part, and the temperature of the injection mold. The formula to calculate the cooling time 100% correctly in all cases has not yet been published, but only formulas that are calculated on the basis of 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 the reference for the cooling time:
① The temperature of the center layer of the thickest part of the wall of the plastic injection-molded part, the time required to cool below the thermal deformation temperature of the plastic;
② The average temperature in the cross section of the plastic injection-molded part, the time required to cool down to the specified product exit temperature;
③ The temperature of the center layer of the thickest part of the wall of the 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 Plastic is injected into the injection mold, and 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 when it contacts the mold wall has become the same. That is, when the plastic is filled into the mold cavity, the surface temperature of the product 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 the injection mold is certain; (the melt filling process is regarded as an isothermal process, and the material temperature is uniform);
⑤ The 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.
So the cooling time of injection product is very important