Factors influencing injection molding process
The influencing factors of thermoplastic plastic molding shrinkage is as follows: plastic varieties of thermoplastic plastic molding process because there are crystal taxiing on volume change, internal stress is strong, frozen inside the plastic residual stress is large, molecular orientation factors such as the gender is strong, so compared with thermosetting plastic shrinkage is bigger, wide range of shrinkage and clear direction, also after molding shrinkage, shrinkage rate after annealing or wet processing generally are larger than the thermosetting plastic.
The molten material contacts the surface of the mold cavity and immediately cools to form a low density solid shell.Due to the poor thermal conductivity of plastics, the inner layer of plastic parts is slowly cooled to form a dense solid layer with large contraction.Therefore, the wall thickness, slow cooling, high density layer thickness of shrinkage.In addition, the layout and quantity of embedded parts and embedded parts have a direct impact on the direction of material flow, density distribution and the size of contraction resistance, etc., so the characteristics of plastic parts have a significant impact on the size and directivity of contraction.
The shape, size and distribution of feed inlet directly affect the direction of feed flow, density distribution, compression and shrinkage and forming time.Large cross sections (especially thick ones) of direct feed inlet and feed inlet are smaller and more directional.Close to the feed inlet or parallel to the feed flow direction, the contraction is large.
The mold under the forming condition has high temperature, slow cooling of the molten material, high density and large contraction, especially for the crystallization material because of high crystallinity and large volume change, so the contraction is greater.The mold temperature distribution is also related to the inner and outer cooling and density uniformity of the plastic parts.In addition, maintaining pressure and time also have a greater impact on contraction.
High injection pressure, small viscosity difference of molten material, small interlaminar shear stress, large elastic rebound after demoulding, so the contraction can also be reduced moderately, high temperature, large contraction, but small directivity.Therefore, it is also possible to adjust the mold temperature, pressure, injection speed and cooling time during the molding process.The mold is designed according to the shrinkage range of various plastics, the thickness and shape of the plastic part, the form size and distribution of the feed inlet, the shrinkage rate of each part of the plastic part is determined according to experience, and then the size of the mold cavity is calculated.