Method of stress-relieving free-sintered polytetrafluoroethylene

There are some types of PTFE material which can be free-sintered in large sizes without objectionable poros ity or imperfections. One such material is known as" Teflon 7'which is a very finely divided powder-like material prepared for such use. Sintered billets of this material are used for forming thin sheets by skiving them therefrom. First a billet is preformed in a mold, say in the range of 10 to 12 inches long by about 10 inches outside diameter and with a core hole of about 4 inches diameter, under a high pressure in the range of 2000 p. si. Such a billet may weigh 50-75 lbs. It is removed from the mold and sintered free of confinement or re straint in an oven. It is found that after sintering the billet has changed shape considerably and that tape which is skived there from has residual stresses which produce objectionable re sults in the use of the tape. If the tape is treated to relieve these stresses and thereby stabilize it …