Is it important to control the cooling time of injection molding?

Author: MULAN -Plastic Molding Manufacturer

In the process of mold injection molding, the cooling time of plastic injection molding parts is basically 80% of the entire cycle. This also means the importance of cooling time, insufficient cooling will affect the dimensional stability of the product, usually leading to warping or surface defects. Reasonable arrangement of injection, pressure and cooling time can improve product quality and productivity.

The cooling time of a part is usually the time from when the plastic melt fills the injection mold cavity until the part can be opened and removed. Standards for when parts are removed by opening the mold are usually based on the part being fully cured and having a certain strength and stiffness. It will not deform and break when demoulding.

Even with the same type of plastic being molded, its cooling time will vary with wall thickness, temperature of the molten plastic, demoulding temperature of the molded part, and injection molding temperature. A formula that calculates cooldowns correctly 100% of the time has not been released in all cases, only ones calculated with proper assumptions. The calculation formula also depends on the definition of cooldown time.

Mold injection molding manufacturers usually use the following three standards as a reference for cooling time: ①The temperature of the central layer of the thickest part of the wall of the plastic injection molding part, the time required to cool below the thermal deformation temperature of the plastic; ②The temperature of the plastic injection molded part The average temperature in the cross-section, the time required to cool to the specified mold outlet temperature; ③The temperature of the central layer of the thickest part of the wall of the crystalline plastic molded part, the time required to cool below its melting point or reach the specified crystallization percentage required time. When solving the formula, the following assumptions are usually made: the plastic is injected into the injection mold, and the heat is transferred to the injection mold for cooling; the plastic in the cavity is in close contact with the cavity, and will not separate 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 time of contact. That is to say, when the plastic is filled into the mold cavity, the surface temperature of the workpiece is equal to the temperature of the mold wall; when the temperature of the mold cavity is equal to the temperature of the mold wall, the surface temperature of the workpiece is equal to the temperature of the mold wall. During the cooling process of plastic injection molded parts, the temperature of the surface of the injection mold cavity is always kept uniform; the degree of thermal conductivity of the surface of the injection mold is determined; (the melt filling process is regarded as an isothermal process, and the material temperature is uniform) Plastic orientation and thermal stress The effect on part deformation is negligible, and the size of the part has no effect on the solidification temperature.