1, Effect of residual thermal stress on Warpage of products
In the process of plastic melt forming, due to the non-uniform orientation and shrinkage, the internal stress is not uniform, so after the product is out of the mold, under the action of the uneven internal stress, warpage occurs. Therefore, the internal stress and warpage of the products are analyzed and calculated from the mechanical point of view. In some foreign literatures, warpage is considered to be caused by residual stress caused by uneven shrinkage.
In the cooling stage of injection molding, when the temperature is higher than the glass transition temperature, the plastic is viscoelastic fluid with stress relaxation phenomenon: when the temperature is lower than the glass transition temperature, the plastic becomes solid.
The liquid-solid phase transition and stress relaxation of plastics during cooling have great influence on the accurate prediction of residual stress and deformation. The phase transition and stress relaxation behavior of plastics from liquid to solid in cooling stage.
For the uncured region, the plastic presents viscous behavior. The viscous fluid model is used to describe the plastic. The standard linear solid model is used to describe the plastic. The viscoelastic phase transformation model and two-dimensional finite element method are used to predict the thermal residual stress and the corresponding warpage deformation.
2, Effect of filling and cooling stages on Warpage of products
Under the effect of injection pressure, the molten plastic is filled into the mold cavity, and the process of cooling and solidification in the cavity is the key link of injection molding. In this process, the coupling of temperature, pressure and speed has a great impact on the quality and production efficiency of plastic parts.
Higher pressure and flow rate will produce high shear rate, which will lead to the difference of molecular orientation parallel to and perpendicular to the flow direction, and produce "freezing effect". "Freezing effect" will produce freezing stress and form internal stress of plastic parts.
The influence of temperature on warpage is reflected in the following aspects:
The temperature difference between the upper and lower surfaces of plastic parts will cause thermal stress and thermal deformation;
The temperature difference between different regions of plastic parts will cause uneven shrinkage among different regions;
Different temperature conditions will affect the shrinkage of plastic parts.
3, Influence of demoulding stage on Warpage of products
Most of the plastic parts are glassy polymers when they leave the cavity and cool to room temperature. It is easy to deform the products due to unbalanced demoulding force, unstable movement of the ejection mechanism or improper ejection area.
At the same time, the stress frozen in the plastic part during the filling and cooling stages will be released in the form of deformation due to the loss of external constraints, which will lead to warpage deformation.
