Preforming behavior of polytetrafluoroethylene paste

Due to its high melting point, polytetrafluoroethylene (PTFE) is processed by a number of unusual techniques including paste extrusion. The paste extrusion process is carried out at ambient temperature and is usually followed by sintering. It is necessary to preform the PTFE powder–lubricant mixture before extrusion to ensure paste densification. In this work, the preforming behavior of a series of homopolymer PTFE pastes was studied. It was found that preforming pressure is an important variable to be considered to ensure uniform densification of paste. Inadequate preforming pressure will cause variation in transient pressure during extrusion, which may potentially lead to a variation in the properties of the final product. It was also found that the required pressure level may quantitatively be correlated with the standard specific gravity or molecular weight of the resin. Furthermore, longer prefoming duration was found to increase the overall preform density, although this increases the extent of lubricant migration. Finally, it was found that an increase in the lubricant content increases the extent of the uniformity of preform density.