The complexity of plastic material performance determines the complexity of plastic injection molding process. The properties of plastic materials vary greatly with varieties, brands, manufacturers and even batches. Different performance parameters may lead to completely different molding results.
The melt flows into the cooling cavity and loses heat due to heat conduction. At the same time, heat is generated due to shear action, which may be more or less than heat lost by heat conduction, mainly depending on injection conditions. The viscosity of the melt decreases with the increase of temperature. In this way, the higher the injection temperature, the lower the viscosity of the melt and the smaller the required filling pressure. At the same time, the injection temperature is also limited by thermal degradation temperature and decomposition temperature.
The lower the mold temperature, the faster the rate of heat-loss due to heat conduction, the lower the melt temperature and the worse the fluidity. This phenomenon is especially noticeable when the injection rate is low.
The influence of injection time on injection molding is shown in three aspects:
(1) To shorten the injection time, the shear strain rate in the melt will also increase, and the injection pressure required to fill the cavity will also increase.
(2) The injection time is shortened and the shear strain rate in the melt is increased. Due to the shear thinning characteristic of the plastic melt, the viscosity of the melt is reduced and the injection pressure required to fill the cavity is also reduced.
(3) The injection time is shortened and the shear strain rate in the melt is increased. The greater shear heat, the less heat lost due to heat conduction. Therefore, the melt temperature is higher and the viscosity is lower, the injection pressure required to fill the cavity should also be reduced.
The results of three conditions above make the injection pressure curve of filling cavity present a "U" shape. In other words, there is an injection time when the required injection pressure is the minimum.