The characteristics of injection molds are determined by the properties of the raw materials, primarily involving two aspects: the flow behavior of the plastic melt as it fills the mold during injection, and the shrinkage behavior of the plastic as it cools and solidifies within the cavity. These factors dictate the unique nature of injection molds and the complexity of their design.
As plastic melts are viscoelastic materials, their viscosity varies with shear stress and shear rate during flow, and macromolecules tend to align along the direction of flow. Once the cavity is filled, the melt undergoes partial compression. The subsequent cooling and solidification process involves complex shrinkage behavior; shrinkage rates vary across different locations and directions within the cavity, and differ significantly between various types and grades of plastic. Even for the same grade of resin or plastic, variations in formulation can lead to differences in mold-filling flow characteristics and shrinkage rates during processing.