Substrate Secrets

In an earlier post I showed the poor stretching of an EVOH layer on OPP and discussed the impact that the quenching had on controlling the stretching uniformity. I have worked on several similar situations where stretching uniformity in thermoformed products have been the problem. In one case I presented a paper at SPE 2008 ANTEC “Reproducing the Film Structure Property Relationship” to describe the basic behavior of a Nylon coextrusion when thermoformed.  In this case the control of the stretching was again controlled by the quenching profile of the sheet and its impact on the resins in the combination. In this case it was “easy” to determine the controlling behavior of the thermoforming because I could measure the stress strain curves at the thermoforming temperature. This is important because the stress strain behavior is temperature dependent (as well as stretching rate dependent).

What was happening was at the percent elongations in the thermoformed products the stretched sections were in the elongation range for nonuniform stretching.  What do I mean by that? In earlier posts I have discussed the strain hardening point. That is the point on the stress strain curve where the elongation is high enough that the stress exceeds the yield point (See figure 1).  In figure 1 we see the region on the strain (elongation) axis titled the “poor gauge region” located between the yield point and the strain hardening point. As long as the elongation in the thermoformed part is located in this poor gauge region the thickness of the part will be non uniform. In order to have uniform stretching, the strain level has to exceed the strain hardening point. Between the yield point and the strain hardening point the stretched part will have thick and thin regions. For as the part stretches and yields, the thin sections elongate and reach the elongation of the strain hardening point and then become stronger than the thicker sections and they stop drawing. The thick sections then start to draw giving a non uniform thickness profile in the part. When all of the thick parts have stretched to the strain hardening point then the part will be of uniform thickness and further stretching proceeds at uniform thickness.

Therefore to obtain uniform stretching in thermoforming (or film orientation) you have to prepare the thermoforming sheet such that it has a stress strain curve at the stretching temperature which permits strain hardening below the elongation of the thermoforming. Ultimately this is controlled by the stretching temperatures and morphology of the various polymer layers in the sheet.