FIGURE 5. Variable residual stresses arise and the part deforms as layers of different frozen-in specific volume interact with each other
Process-induced vs. in-cavity residual stress
Process-induced residual stress data are much more useful than in-cavity residual stress data for molding simulation. Following are definitions of the two terms, along with an example that illustrates the difference between them.
Process-induced residual stress
After part ejection, the constraints from the mold cavity are released, and the part is free to shrink and deform. After it settles to an equilibrium state, the remaining stress inside the part is called process-induced residual stress, or simply, residual stress. Process-induced residual stress can be flow-induced or thermal-induced, with the latter being the dominant component.
In-cavity residual stress
While the part is still constrained in the mold cavity, the internal stress that accumulates during solidification is referred to as in-cavity residual stress. This in-cavity residual stress is the force that drives post-ejection part shrinkage and warpage.
The shrinkage distribution described in Warpage due to differential shrinkage leads to a thermal-induced residual stress profile for an ejected part, as shown in the lower-left figure below. The stress profile in the upper-left figure is the in-cavity residual stress, in which the molded part remains constrained within the mold prior to ejection. Once the part is ejected and the constrained force from the mold is released, the part will shrink and warp to release the built-in residual stress (generally tensile stress, as shown) and reach an equilibrium state. The equilibrium state means that there is no external force exerting on the part and the tensile and compressive stresses over the part cross-section should balance with each other. The figures on the right side correspond to the case with a non-uniform cooling across the part thickness and, thereby, causing an asymmetric residual-stress distribution.
FIGURE 6. In-cavity residual stress profile (top) vs. process-induced residual stress profile and part shape after ejection (bottom).
Reducing thermal-induced residual stress
Conditions that lead to sufficient packing and more uniform mold-wall temperatures will reduce the thermal-induced residual stresses. These include:
– Proper packing pressure and duration
– Uniform cooling of all surfaces of the part
– Uniform wall-section thickness