Factors Affecting Injection Molding Process (Part 3)

The fluidity of various plastics also varies due to various molding factors, and the main influencing factors are as follows:

① The flowability of materials increases with high temperature, but different plastics also have differences. The flowability of plastics such as PS (especially those with high impact resistance and MFR values), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA, etc. varies greatly with temperature. For PE and POM, temperature changes have little effect on their fluidity. So it is advisable to adjust the temperature during molding to control the fluidity of the former.

② As the pressure of injection molding increases, the molten material is subjected to greater shear force and its flowability increases, especially for PE and POM which are more sensitive. Therefore, it is advisable to adjust the injection pressure during molding to control flowability.

③ The form, size, arrangement, cooling system design, and flow resistance of the molten material (such as surface smoothness, channel section thickness, cavity shape, exhaust system) of the mold structure pouring system directly affect the actual flowability of the molten material in the cavity. Any factors that promote the molten material to lower temperature and increase flow resistance will result in a decrease in flowability.

When designing molds, a reasonable structure should be selected based on the fluidity of the plastic used. During molding, factors such as material temperature, mold temperature, injection pressure, and injection speed can also be controlled to adjust the filling situation appropriately to meet the molding needs. Crystalline thermoplastic plastics can be divided into two categories: crystalline plastics and amorphous (also known as amorphous) plastics, based on the absence of crystallization during condensation. The so-called crystallization phenomenon refers to a phenomenon in which the molecules of plastic move independently and are completely in an unordered state from a molten state to a state where they stop moving freely, move in slightly fixed positions, and have a tendency to align into a regular model.