1）The influence of residual thermal stress on warpage deformation of the products
During the molding process of the plastic melt, due to the uneven orientation and shrinkage, the internal stress is uneven, so after the product is out of the mold, under the action of the uneven internal stress, warping deformation occurs. Therefore, the internal stress and warpage of the product are analyzed and calculated from a mechanical point of view. In some foreign literatures, warpage is considered to be caused by residual stress caused by uneven shrinkage.
During the cooling stage of injection molding, when the temperature is above the glass transition temperature, the plastic is a viscoelastic fluid with stress relaxation: when the temperature is below the glass transition temperature, the plastic becomes a solid.
In the uncured region, the plastic exhibits viscous behavior, described by the viscous fluid model, and in the cured region, the plastic exhibits viscoelastic behavior, described by the standard linear solid model, using the visco-elastic phase transition model and the two- dimensional finite element method to predict thermal residual stress and corresponding warping deformation.
2）Influence of filling and cooling stages on product warpage deformation
Under the action of injection pressure, the molten plastic is filled into the mold cavity, cooled and solidified in the cavity, which is the key link of injection molding. In this process, the interaction of temperature, pressure and speed has a great influence on the quality and production efficiency of plastic parts.
Higher pressures and flow rates create high shear rates that induce differences in molecular orientations parallel to and perpendicular to the flow direction, with a "freezing effect". The "freezing effect" will generate freezing stress, forming the internal stress of the plastic part.
3）The effect of temperature on several warp deformations is in the following ways:
The temperature difference between the upper and lower surfaces of the plastic parts will cause thermal deformation and thermal deformation;
The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas
The temperature state will affect the different shrinkage rates of plastic parts
4）The influence of the demoulding stage on the warpage and deformation of the product
Plastic parts are mostly glassy polymers as they come out of the cavity and cool to room temperature. Unbalanced demoulding force, uneven movement of ejection mechanism or improper ejection area can easily deform the product.
