Today's KNOWLEDGE Share:molecular orientation in molded parts

Today's KNOWLEDGE Share

I often get questions in my seminars about molecular orientation in molded specimens.People often get a bit confused about the layers that could possibly show some degree of molecular orientation, potentially observed as birefringence for transparent grades.

- The very skin is coming directly from the fountain flow and freezing, so it is not very oriented at all since it never experienced much flow in the main direction.

- The "frozen skin" below it is extremely oriented since, by definition, it freezes UNDER FLOW. As a result it is very oriented, almost regardless of the polymer relaxation time / molecular weight.

- Other layers below, approaching the center line can show orientation induced during the second stage of the molding process, the packing phase. This flow stress is active at later times, thus acting on deeper layers.

- Although thick parts should show little orientation below the frozen skin, high molecular weight materials could still show significant orientation if their relaxation time exceeds the overall cooling time of the specimen. Try molding a blow-molding grade and see for yourself !

- Strictly speaking, the very center line should not show orientation since the shear-rate and shear-stresses are zero at all times during the process.

A quick test to assess the degree of average orientation (which really works perfectly for opaque crystalline polymers) is to examine the degree of (post-)shrinkage in the flow direction when annealing the specimen at the appropriate temperature (just below Tg for amorphous, just below Tm essentially for semi-crystalline).

Orientation is not strictly speaking a "residual stress" given the weakness of the entropic force at play. Upon reheating or softening with a solvent (PVC and acetone for instance reveal flow induced orientation), such tiny force can produce this significant post-shrinkage.

source: Vito leo