It is well known that there are frozen in stress an deformation in thermoplastic materials processed in the molten state. When they are cooled below the Tg or the melting point (in case of semi.crystalline polymers) these stresses get fixed and will cause warpage during use - unless we are very far from Tg. the question is somewhat different for rubbers (networks). In this case stresses should relax, unless they develop after reaching the gel point of the system. My estimate is that such things may occur due to
- anisotropic temperature distribution
- uneven crosslinking
- local sticking to the mold wall
What is the experience of colleagues working rubbers? Do they observe serious deformation after removing rubbery articles form the mold? What they usually do to prevent it?
Polymer melt viscoelasticity and uneven cooling of the part are the causes of frozen stress generation and warpage after mold extraction. First, we have to discriminate between thermoplastic and thermosetting rubbers
For thermoplastic polymers the passage from "mainly" viscous (the elastic component is always present in the fluid state) to "mainly" elastic behavior resides is the crystallization while for thermosetting polymers gelation is the critical point, which is usually defined when the tan delta reaches its minimum value during the cross linking process (then it increases again due to the increasing dissipative component in more tight cross linked systems). It has to be considered the molten polymer flowing in the mold (both for thermoplastic and thermosetting) maintains "memory" of the undergone shears during the mold filling (this is due to the viscosity elastic component) so, if the flow in the mold is not homogeneous or it takes some preferential routes, this will be transferred in the solidifying part as different frozen stress states that differently deform the component out of the mold. The solution for thermoplastic and thermosetting systems may be different (I could supply specific answers on request), and could interest injection points, temperature of the mold etc ....
Uneven cooling of the component with respect to the part or thickness symmetry plane could generate warping as it occurs for not symmetric and balanced composite laminates.
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