How to set the cooling time of injection molded parts
After the injection raw material is injected into the mold core, a cooling time is generally required to facilitate the molding of the plastic part. This cooling time is very important. It is a guarantee of the quality of the plastic part and a key factor to maintain the size of the plastic part. Reasonable setting of 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 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 taking out the part by opening the mold is often based on the part that has been fully cured, has a certain strength and rigidity, and will not be deformed and cracked when the mold is opened and ejected.
The setting of the cooling time is based on the characteristics of the raw materials, and also on the structure of the mold. For the same raw material, the thickness of the mold is different, the cooling time is also different. How to set the cooling time of the plastic parts, Mainly based on the following standards as a reference:
① The time required for the temperature of the center layer of the thickest part of the wall of the plastic injection molding part to cool below the thermal deformation temperature of the plastic;
② The average temperature in the section of the plastic injection molding part, and the time it takes to cool down to the mold release temperature of the specified product;
③ The temperature of the center layer of the thickest part of the wall of the crystalline plastic molding, the time required to cool down to below its melting point, or the time required to reach the specified crystallization percentage.
When calculating the solution 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;
② The plastic in the molding cavity is in close contact with the mold cavity, and is not 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 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 molding parts, the temperature of the surface of the injection mold cavity remains uniform;
④The degree of heat conduction on the surface of the injection mold is certain; (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, and the size of the part has no effect on the solidification temperature.