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Processing (Molding Powder S-PTFE)
PTFE is processed using techniques which are very similar to processing sintered materials. Due to its high melting viscosity PTFE cannot be processed like other high-performance thermoplastics.
At high temperatures (340 – 380°C) PTFE merely becomes highly viscous, thus precluding injection molding or normal extruding processes. Semifinished products are therefore manufactured by compression molding and sintering or by ram extrusion.
Compression Molding and Sintering
PTFE powder is inserted into a cylindrical mold and subsequently compressed under high pressure. It is important that the air enclosed in the mold can escape almost completely.
The powder is compressed either on hydraulic presses with speed, pressure and time control, on isostatic or automatic presses. After the compression molding process, the preformed parts are sintered in electrically heated ovens with recirculating air according to pre-defined programs. The compression and sintering parameters must be adjusted to the respective PTFE powder form or, in the case of compounds, the composition. This is a prerequisite for ensuring optimum material properties.
Ram Extrusion
Ram extrusion is a compression molding process allowing the manufacture of continuous profiles. The granulate is fed into a cylindrical extrusion tube via a metering device, compressed with a hydraulically operated ram and – inside the tube – moved to a zone heated to sintering temperature.
In the sintering zone the various metered batches “melt” together, creating a continuous profile.
Processing (E-PTFE, Paste Powder)
PTFE produced by emulsion polymerization can be processed by socalled paste extrusion. The powder
is mixed with a lubricant, for example naphtha, to create a paste, and then processed into simple profiles, such as strands, tubes or bands, at room temperature, using a piston press.
Extrusion is followed by drying to remove the lubricant. Afterwards the material achieves its ultimate properties by sintering at app. 350 – 380°C.
E-PTFE compounds may be processed by paste extrusion as well. |
