Preforming and sintering

We have done most of our studies on granular PTFE by compacting it into a preform and sintering to make a shape matching ASTM D-1708-66 for tensile testing. Orientation measurements by IR dichroism require thinner specimens, and for this purpose we used dispersion-grade material, commercially cast and processed into film (Dilectrix Corp., Farmingdale, NY). These films were used as received without further heat treatment. ... 

Resin powder particles are separated by air which is removed during preforming and sintering. Figure... 

Figure 5.4Schematic diagram of preforming and sintering sequence with polytetrafluoroethylene.Pi... 

Expanded structures can be formed from PTFE by a number of other means.t l For example, preforming and sintering cycles can be controlled to leave voids in the molded part. Another approach is to stretch tapes of PTFE uniaxially or biaxially to create expanded structures that contain pore/voids with controlled size. PTFE compounded with thermally unstable compounds that produce gases such as carbonates can also produce foamed structures. Carbonates decompose and generate carbon dioxide gas when PTFE is heated during the sintering cycle. Carbon dioxide expands in the void space and expands the molten PTFE. 

To improve homogeneity, the preformed article is heated to 370—390°C. The time required for heating and sintering depends on the mold dimensions cooling, which affects the crystallinity and product properties, should be slow. 

For ordinary materials and higher production rates, P/M forging can be used (26,28). After parts are compacted and sintered to medium density, they are reheated, lubricated, and fed into a hot-forming or P/M-forging press. The part is formed by one stroke of the press in a closed precision die. A typical hot-forming press setup includes die sets, automatic die cooling and lubrication, transfer mechanism, an induction heating unit for preforms, and controls. 

Dispersion polymer, which leads to products with improved tensile strength and flex life, is not easily fabricated by the above techniques. It has, however, been found possible to produce preforms by mixing with 15-25% of a lubricant, extruding and then removing the lubricant and sintering. Because of the need to remove the lubricant it is possible to produce only thin-section extrudates by this method. 

A simple variant of the various methods described above is based on metal salt infiltration into porous ceramic preforms, followed by reduction and sintering under controlled atmosphere. This method skips the more complicated stages of calcining, secondary milling, and sieving. 

Metal powder processing techniques in which a preform is molded and sintered are used to process PTFE. Compression molding may also be used to fabricate PTFE parts. Its dispersions are applied by similar techniques to other coatings. Paste extrusion in which PTFE is blended with a hydrocarbon, prior to molding a preform, is used to continuously fabricate PTFE into tubes, tapes, and wire insulation. The hydrocarbon is vaporized before the parts are sintered. PVF is dispersed in a polar latent solvent such as dimethyl acetamide and is melt-extruded as a plastisol, followed by solvent removal by drying. 

Relatively simple equipment is used for billet molding. It consists of a stainless steel mold and hydraulic press for fabrication of the preform, and an oven for sintering. A lathe and skiving blades are required for preparation of film and sheet. 

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