The Basis For Setting The Cooling Time In Injection Molding
In injection production, the cooling time of plastic injection molding parts accounts for about 80% of the entire injection production cycle. Poor cooling often leads to warpage or surface defects of the product, affecting the dimensional stability of the product. Reasonable arrangement of injection, holding pressure and cooling time can improve product quality and productivity.
What is the cooling time in injection molding?
The cooling time usually refers to the time from when the plastic melt fills the cavity of the injection mold until the part can be opened and taken out.
The time standard for mold opening and taking out the parts:
It is often based on the fact that the parts have been fully cured and have a certain strength and rigidity. What’s more, it will not deform and crack when the mold is opened and ejected.
How to calculate the cooling time in injection molding?
Even if the same plastic is used for molding, its cooling time varies with the following points:
- wall thickness
- temperature of the molten plastic
- demolding temperature of the molded part
- temperature of the injection mold
A formula to calculate the cooldown time 100% correctly in all cases is currently unpublished, but only one based on proper assumptions. On the other hand, the calculation formula also varies depending on the cooling time definition.
Reference basis for cooling time
At present, the following 3 standards are usually used as the reference basis for cooling time:
① The time required for the temperature of the thickest part center layer of the wall of the plastic injection molding part to cool below the thermal deformation temperature of the plastic.
② The time it takes for the average temperature in the section of the plastic injection molded part to cool down to the mold release temperature of the specified product.
③ The time needed for the temperature of the thickest part center layer of the wall of a crystalline plastic molding to cool below its melting point, or to achieve a specified percentage of crystallization.
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 is not separated due to cooling shrinkage. Besides, there is no resistance to heat transfer and flow between the melt and the mold wall. Then, the temperature of the melt and the mold wall has become the same at the moment of contact. 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 surface temperature of the injection mold cavity remains uniform.
④ The degree of heat conduction on the surface of the injection mold is certain. Also, the molten material 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. While the size of the part has no effect on the solidification temperature.