Elringklinger - Giving Cold Flow the Cold Shoulder
PTFE is a popular material for use in valves & fittings applications due largely to its universal chemical resistance.
The significant properties of PTFE such as high temperature resistance, universal chemical resistance and low friction makes this material ideally suited for use in fittings and process valves. Typical application examples are V-ring packings, spring-energized seals, guide rings and guide tapes as well as customized design elements such as valve diaphragms and PTFE bellows. In the case of valve seats for example which are subject to high loads, the maximum compressive stress of PTFE is often exceeded. High media pressures generate greater contact pressures between the valve seat and sealing element. In order to counteract the cold flow of PTFE especially at elevated temperatures, valve seats require a complex, fully confined design to be used. Alternatively, filled PTFE compounds can be used however depending on the filler, these may have lower chemical resistance, higher permeation or be more abrasive to the counter surface.
Alternatives to PTFE
To overcome some of the dissadvantages of PTFE compounds in high load applications, the use of thermoplastic materials with high temperature resistance can be considered. Frequently, materials such as PEEK (polyether ether ketone) are used which can be sufficient in terms of chemical resistance whilst offering advantages regarding creep, permeation and heat deflection temperature. When using PEEK as a material for seat rings, low ultimate elongation and high elastic modulus leads to an extremely hard and inflexible material. In the final application, this has to be accomodated in the form of enhanced surface textures, tighter tolerances and closer controlled geometric features, all of which have a negative impact on cost and product consistency especially where large temperature ranges need to be accomodated.
As the heat deflection temperature of PEEK is about 3 times higher than that of unfilled PTFE, using a Such a compound exhibits enhanced mechanical properties however, since PTFE is a sintered material, the structure compared to a melt-processable material is relatively inhomogeneous. The result of which can be inferior surface quality and a reduction in permeation resistance, both of which may have a negative effect on the sealing efficiency of the finished component (Figure 2).combination of PTFE and PEEK is an obvious choice.
In Pursuit of a Suitable Combination
An ideal valve seat material therefore is one that combines the advantages of both PTFE and PEEK to deliver a low creep, chemically resistant, temperature stable material with low permeation and good elongation. The problem with combining melt-processable and sintered materials is that an inhomogeneous structure occurs (Figure 2) which has a negative impact on the materials mechanical properties, permeability and machinability. Only the combination of PEEK with meltprocessable Moldflon PTFE can deliver a material with properties that are perfectly alighned to the performance requirements of the final product.
With Moldflon MF40002, ElringKlinger has managed to develop a material with a higher heat deflection temperature than PTFE whilst exhibiting significantly higher elongation and flexibility than virgin-grade PEEK or traditional PTFE/PEEK compounds. In addition to the ultimate elongation being nearly ten times higher (see Figure 3), the structure is significantly more homogeneous than that of alternative PTFE/PEEK compounds. Today, ElringKlinger is able to offer Moldflon MF40002 in sizes up to diameter 850mm and as the material is both machinable and melt-processible, it is suitable for use in small scale as well as large volume manufacturing processes.
Tel: 01642 492492
Published in Valve User Magazine Issue 43
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