Is PTFE Anisotropic?

Rubberman,

There are numerous sources of anisotropy in skived PTFE sheet.
1) The billet created for skiving will be axially oriented a little, due to the compression molding process.
2) During the sintering process of the billet, commonly it is stood vertically in the oven and while the billet is hot, the compressive stresses in the lower part of the billet are higher than those in the upper part. A portion of those stresses will be seen in the cooled billet. Basically it has sagged under its own weight, down the long billet axis.
3) Prior to skiving into 1/8" sheet, it is common to heat the billet. Often this is erratic and, when combined with the ever-reducing diameter of the remaining stock as skiving proceeds, guarantees that bending stresses during skiving and sometimes rewinding, will vary down the skived sheet axis.
4) Skiving itself gives rise to reaction stresses at the skiving blade edge. The unsupported outer annular skins will deform permanently into free space, while the central section, with symmetrical support from neighboring zones, will not so much.
5) If sold as sheet, a heated press often is used to flatten the sheet, giving rise to a whole new opportunity for anisotropy.

It's a complex array of processes that each introduce their own form of distortion.

Mark Bingham
Relativity PL