Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Over-moulding PTFE on to Stainless Steel

As polymers go, PTFE is not the easiest material to deal with. It behaves contrary to nearly every other plastic and requires special processes to create even the simplest of forms. Whether we look at the extrusion of PTFE tubes or the forming of PTFE films (both rather straightforward when we consider melt-processable polymers), the methods we need to employ for PTFE are a practically standalone and need to be understood and developed from first principles.

Similarly, over moulding a plastic onto a metal part is not a very complex task when we look at injection moulding. In such a process, the metal part is inserted within the injection moulding die and molten polymer is injected and cools around it.

However, there is limited scope to follow this process when trying to over-mould PTFE onto a metal object. The limitations are the following:

  1. As PTFE cannot be melted, there is no way to form the polymer around the metal part in any way that would be uniform and consistent using heat alone
     
  2. Since PTFE can only be compression moulded, the metal part would need to be kept within a compression moulding die and the dry PTFE powder would need to be packed around it. However, as PTFE is very sensitive to the amount of pressure being applied during compression, it is essential that the metal part does not have too many contours, as this would lead to an irregularity in the compression.

    It is possible, in the event of a more complex metal part, that isostatic moulding is used to ensure there is even pressure on the PTFE powder during compression. However, as isostatic moulding requires a lot of die and mould costs, such a process could only be justified if the volumes are significant.
     
  3. Even if we do manage to pack the PTFE around the metal uniformly, the final issue remains concerning the heat cycle.

    PTFE is sintered (cured) at temperatures of around 375°C over a period of anywhere between 15 and 100 hours – depending on the size of the part. This heat cycle means that within the oven, both the PTFE and the metal are subject to high temperatures. Here the challenge is to match the thermal expansion and contraction of the PTFE and the metal, so that mismatches in the rates of expansion do no cause the PTFE to crack.
     

Overcoming these challenges required a lot of R&D. Different combinations of pressures and heat cycles were employed until we were able to consistency achieve a part that would not crack. The moulded part also needed to be machined, which meant the part needed to be free from any internal irregularities as well.

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