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PTFE, or PRODUCING TERRIFIC FINISHED EFFECTS

Part two of a two-part series on PTFE

by Rick Hudson

In our last issue, I took a closer look at the form and function of a truly remarkable material: polytetrafluoroethylene (PTFE, best known by the trade name Teflon®). I also outlined the most common PTFE fillers. This time around, I’ll be moving on to the processing techniques used to produce finished PTFE parts. I’ll also survey some of the many different types of applications in which PTFE parts are used.

FABRICATION TECHNIQUES Raw PTFE polymer is available as a powder, and this powder must be processed to produce a useful product. There are three main techniques used to process PTFE: compression molding, net molding, and extrusion.

Compression molding may be accomplished by either of two methods. The first of these is hydraulic compression molding. In this method, PTFE granules are poured between two metal tubes. A third tube with a slightly smaller outside diameter (O.D.) than the largest tube’s inside diameter (I.D.) and a slightly larger I.D. than the smallest tube’s O.D. is inserted between the tubes. The assembly is placed in a hydraulic press, where pressure is applied to the middle tube, thus compressing the PTFE. The compaction is typically 4 to 1. In order to achieve uniform compaction and billet density, the maximum recommended length for a hydraulic compression molded billet is 7". The billet must be sintered at high temperature to develop PTFE’s optimum properties.

Sintering involves slowly heating the parts in an oven to approximately 700° F (371° C) until the PTFE reaches its "gel state." The temperature is maintained for a period of twelve to twenty-four hours (depending on the mass of the part), then the temperature is reduced slowly. Cooling the PTFE slowly allows the polymer chains to align in a crystalline pattern. This crystallinity gives the finished part greater tensile strength and better elongation properties.

The other type of compression molding is isostatic compression molding. Isostatic molding is accomplished by compressing PTFE powder between a metal tube and a rubber bladder. The rubber bladder is inflated with very high hydraulic pressure, causing the PTFE powder to be compressed radially. Compression takes place between the I.D. and the O.D. of the billet rather than from the ends (as with hydraulic compression molding). Again, the billet must be sintered to achieve maximum properties.

Net molding involves the formation of PTFE parts in a hydraulic press. The parts must be relatively simple and have uniform wall thickness. After the parts have been formed and removed from the mold, they are said to still be in a "green" state. That is, the parts are still very fragile and can be broken with light hand pressure. They must be sintered to achieve maximum properties. Net molded parts have about one-half to two-thirds of the physical properties achieved by machining a part from a compression-molded billet. Net molded parts usually exhibit an "orange peel" surface appearance.

Extrusion is the third main fabrication method. During the extrusion process, PTFE powder is heated and forced under high pressure through a series of dies to produce either a rod or a tube. Because the PTFE is heated to its gel state, the extruded product does not need to be sintered (as required with compression molded and net molded products). One of the disadvantages of extruding is that the hot PTFE cools quickly. The result is a more amorphous structure with less tensile strength and poor elongation properties.

FINISHED PARTS Because of its resistance and versatility, PTFE can be used to produce a wide variety of parts.

O-rings may be machined from virgin (unfilled) PTFE. Because PTFE has poor memory, PTFE O-rings undergo high compression set and are usually rendered ineffective after a few thermal cycles.
Back-up rings may be machined from a variety of grades of unfilled and filled PTFE. Virgin and glass-filled PTFE back-up rings are primarily used in medium duty applications. Higher-pressure environments demand improved extrusion resistance, making bronze-filled and PPS-filled PTFE parts more desirable.
Piston rings are generally either glass-filled or bronze-filled PTFE and are widely used in hydraulic cylinder applications. They must be energized by an O-ring or lathe-cut ring (square or rectangular).
Capped T-seals are composed of a glass-filled or bronze-filled PTFE cap ring, a rubber T-section expander, and two back-up rings (usually made of nylon). Capped T-seals are intended for heavy-duty applications.
Buffer seals are O-ring energized PTFE seals used in conjunction with hydraulic rod seals. Buffer seals are not intended to be zero leakage seals; rather, they are designed to reduce the effects of pressure spikes on the rod seals.
V-packing is most commonly used in valves. PTFE V-packing offers excellent chemical and thermal resistance.
Spring-energized U-cups are used in rotary and reciprocating applications. Operating with very low friction, they also offer excellent resistance to chemicals, high and low temperatures, and high pressure.
Rotary shaft seals (lip seals) made of PTFE are able to handle higher pressures, temperatures, and surface speeds than elastomeric shaft seals. They also offer increased chemical compatibility. The best PTFE fillers for rotary applications are carbon / graphite and polyimide.

FINAL THOUGHTS As you can probably guess, we here at RL Hudson appreciate the usefulness of PTFE products. Whether you need to reduce friction, increase chemical resistance, or expand the workable temperature range, a PTFE part can often help. If you need assistance with a PTFE part, call us at 1-800-722-6766. We’ll be more than happy to help!

Go to part one of the series