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Effect of plastic injection molding temperature on shrinkage

Time: 2019-03-29

Plastic molding is also known as injection molding, it is a kind of injection and molding molding method. Injection molding method has the advantage of production speed, high efficiency, can realize automation operation, breed of design and color is much, can shape by Jane to numerous, can from big to small size, and precise in dimension and product upgrading, can into complex shape parts, injection molding is suitable for mass production and complex shape product processing area, but the linyi plastic products wholesale about the temperature of the injection molding of plastic products has enormous influence on the performance.

Above the viscous flow temperature, the relationship between the viscosity and temperature of polymer with the increase of temperature, the melt of the free volume will increase, intermolecular forces will weaken each other, can increase the liquidity of the polymers, the melt viscosity decreases with increasing temperature, in the form of index, because in polymer injection molding processing to improve the filling mold ability of the melt, the temperature is a primary means to adjust viscosity.

Both from the mechanism of crystallization and orientation of the polymer and from the principle of thermal expansion and cold shrinkage, it is believed that the shrinkage of the product during the viscosity setting phase of the polymer should increase with the increase of temperature, but some experiments have reached the opposite conclusion.

This situation can be explained as follows: after the temperature of the material rises, the melt viscosity will decrease. If the injection pressure and holding pressure remain unchanged at this time, the freezing speed of the gate will be slowed down, indicating that the holding time will be prolonged, the role of filling will increase, and the density will increase with the increase, so the shrinkage rate will decrease.

It can be seen from the above analysis that the effect of material temperature on the forming shrinkage is the result of the combined effect of crystal shrinkage, orientation shrinkage, thermal shrinkage and pressure-retaining shrinkage. On the contrary, the shrinkage rate will decrease with the increase of temperature when the pressure holding and filling action is larger.

Mold temperature

After the thermoplastic melt is injected into the mold cavity, a large amount of heat is released and solidified. At this temperature, will be most conducive to the molding of plastic parts, molding efficiency is the highest, the internal stress and the minimum warpage deformation.

Mold temperature is the main factor to control the cooling and shaping of the products, and its influence on the shrinkage of the molding is mainly reflected in the process before the mold release after the sprue is frozen. Before the sprue freezes, the mold temperature rise tends to increase the thermal shrinkage, but it is the higher mold temperature that prolonging the freezing time of the sprue, increasing the influence of injection pressure and retaining pressure, and increasing the retraction and negative shrinkage.

Therefore, the total shrinkage is the result of the combined effect of two reverse contractions, and its value does not necessarily increase with the increase of mold temperature. If the sprue freezes, the influence of injection pressure and pressure retaining pressure will disappear. With the increase of mold temperature, the cooling setting time will be prolonged, so the shrinkage rate of products after demolding will generally increase.