Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE Bellows - Machining Challenges & Durability Tests

Bellows are among the most complex components that can be machined from PTFE (Teflon). Owing to the nature of PTFE, there exist several limitations to making the bellow by conventional, melt-processable means. However, the properties of PTFE make it invaluable in certain applications, requiring the bellows to be machined from solid PTFE.

For the most part, high-performance bellows are made using rubber or equivalent elastomers. The limitations of rubber include high temperatures and resistance to corrosion. It is likely that PTFE is only used in very specialised circumstances both because of cost and complexity.

The challenges for machining PTFE bellows include:

  1. The bellow often needs to be machined out of a solid rod or bush. The process can result in wastage of up to 80% in some cases
     
  2. The process of creating the convolutions needed to form the bellow requires special tooling and high-precision CNC machining. The smallest deviation in the machining process can result in a complete loss of the whole component. In cases where the final part is large, this represents a high risk of loss due to wastage and rejection
     
  3. The raw material needed for making bellows needs to be of the highest purity and the final rod or bush that is moulded would need to be completely free from any micro-faults, such as cracks, inclusions or discoloration. For the best quality bellows, grades such as DuPont NXT or 3M TFM are used. More recently, Inoflon has come out with a resin grade – M690, which also works well in making bellows. These grades are modified grades, having a high purity and capable of more flexibility
     
  4. The final produce needs to have total uniformity and be completely free from any irregularities in colour or dimension. Even the smallest deviation in wall thickness or the slightest micro-crack in the walls of the bellow can result in complete failure during operation.
     
  5. The design of the bellow itself needs to accommodate the properties of PTFE. For instance, it may not be able to have a wall thickness that goes below 0.5mm. As a result, the maximum compressibility of the bellow is limited when compared with an elastomeric bellow. Not only are elastomers more elastic, but their melt processability allows for much thinner cross sections, meaning more compressibility

Despite these limitations, when applications require high corrosion or temperature resistance, the only option is to use PTFE (Teflon). Hence, designers work around the limitations of the material to create a bellow that would work in their conditions.

If properly manufactured, a PTFE (Teflon) bellow can withstand up to 2,000,000 cycles. Since most requirements call for anywhere between 500,000 and 1,000,000 cycles for the life of the bellow, this metric allows for a sufficient cushion.

Once manufactured, a bellow is subjected to a series of tests to confirm its long-term durability during application.

  1. Burst pressure test
    The burst pressure of a PTFE bellows shall be at least four times the design pressure given by the manufacturer, after it has been subjected to 2000 cycles at 10 cycles per minute between its maximum axial extensions. The pressure and related temperature at which cycling is carried out shall be selected from the pressure/temperature graph supplied by the manufacturer. The pressure to produce failure shall be applied uniformly at such a rate that failure occurs within 5 minutes. As a minimum two tests shall be carried out; one at ambient temperature and the other at 180 °C.
     
  2. Cycle testing
    No failure shall occur when a PTFE bellows is subjected to 100 000 cycles at 10 cycles per minute between its maximum axial extension, or a combination of axial and lateral extension, at a pressure and temperature selected from the pressure/temperature graph supplied by the manufacturer.
     
  3. Temperature test
    The bellows shall be bolted to a mating flange and held at a temperature of 260 °C for 2 hours. After cooling, the PTFE flange face shall be examined to ensure no deformation or damage.
     
  4. Dimensional inspection
    PTFE bellows shall be subjected to dimensional inspection, visual examination, liquid penetrant examination, holiday testing (if necessary) and a hydrostatic pressure test. A burst pressure test and a pressure shock test shall be applied if the bellows ordered are to be used for critical applications (this to be specified by the Principal).

Because of the extensive testing and care needed during manufacture, the cost of a PTFE (Teflon) Bellow does tend to be many times that of its elastomeric counterpart. As a result, these bellows are only used sparingly, in applications where there is no option but to use PTFE.

 

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