Patent GB-707966-A (GB)
Title: Improvements in or relating to articles consisting essentially of a polymer of tetrafluoroethylene and to the manufacture thereof
Inventor: SQUIR ALAN THOMAS BARRINGTON P
IPC: B29C43/00
Filed: 1950-01-26
Abstract: A thin sheet-like article of polytetrafluoroethylene &c. is produced by making a preform by applying moulding pressure through a sheet of flexible resilient material, e.g. rubber, uniformly to a charge of dry moulding powder in a mould, and then sintering the moulded preform.  The article may be a sheet, disc, diaphragm or gasket, and may be reinforced by one or more layers of metal.  The articles may be moulded one at a time, or a plurality may be moulded simultaneously.  Thus, in moulding simultaneously a plurality of unreinforced articles, moulding powder is screened to remove fibrous particles, a charge thereof sufficient for one article is spread evenly on the bottom of the mould, the charge is consolidated by a pressure of 6 tons per sq. inch for 1 minute at room temperature (or down to 500 pounds for longer time and/or higher temperature), a separator of linen tracing paper is placed over the consolidated charge, another charge is put in and consolidated and covered with a separator, and so on.  A rubber sheet is placed over the uppermost charge and the whole assembly is then pressed at 6 tons per sq. inch for 10 minutes at room temperature, the pressure being gradually built up over 2 minutes (or down to 500 pounds for longer time and/or higher temperatures).  The preforms are removed from the mould, separated, sintered at increasing temperatures to 390 DEG C., and cooled rapidly to below 327 DEG C. either by placing them between cold metal plates or by quenching in cold water.  Moulding reinforced articles:-The reinforcement is one or more layers of metal such as gauze or wires.  The metal should be one that is not attacked by P.T.F.E, e.g. stainless steel or chromium-coated phosphor bronze. Where there is more than one layer of reinforcement, the wires in one layer preferably extend in different directions from the wires of the adjacent layer or layers.  A reinforcing layer may be only partly embedded in the P.T.F.E., so that free metal projects from the latter, this giving a rough surface useful when the article is to be clamped between a pair of members.  Each charge of powder is divided into two portions.  One portion is spread evenly in the mould, then the reinforcement is put in, then the other portion.  The reinforcement has only a small clearance from the mould walls.  One charge may thus be built up, or a plurality of superposed charges separated by linen tracing paper. The subsequent procedure is as above.  Laminated articles:-One or a plurality of superposed charges are built up, each consisting of alternate layers of P.T.F.E. and stainless steel gauge discs, which are inserted in the mould seriation and subjected to gradually-applied pressure up to 6 tons per sq. inch at 20 DEG C. (or at 200 DEG C.), a rubber sheet being inserted between the charge or charges and the pressing platen.  The articles are then sintered and quenched.  The Provisional Specification states that if the thickness of the mass being pressed is less than about 0.15 inch, the flexible resilient sheet may be dispensed with; and refers to an electrical heating element comprising moulded sintered P.T.F.E having embedded therein one or more electrical conductors.