Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Inspection Metrics for Complex Polymer Profiles

When manufacturing machined components, the key challenge is to ensure the consistency and repeatability of the dimensions and tolerances. With metals, this challenge is not so acute. There is a wealth of industry experience around the machining, punching and shaping of metals and therefore enough data to support how the material will behave when subjected to external forces. Metals also adhere to much closer tolerances. Precision metal part manufacturers will frequently cite tolerances of as low as one micron (1µm), which are unheard of in the polymer space. In addition to this, polymers are not as well traversed as metals and manufacturers who develop specific methods to ensure close tolerances will usually keep these as proprietary. Furthermore, each polymer behaves in its own manner and as such throws up its own limitations to what can be allowed by way of dimensional accuracy. A few such examples are given below:

  1. PEEK – tends to absorb stress during machining and can deform post machining (sometimes after days of being kept on the shelf), if not treated properly
  2. PTFE – does not absorb heat from the tool during machining, but can experience thermal expansion and its softness, can make it difficult to achieve close tolerances
  3. Nylons – can be affected by the heat from the tool, causing melting at the part’s surface. At the same time, coolant is not an option as the material tends to absorb moisture and swell post-machining. Methods need to be developed to balance the machining RPM with the feed rate to ensure tolerance and part finish are both achieved

The above examples are only a few to illustrate how each polymer behaves. There are literally thousands of polymers grades and each would come with its own set of properties to influence how it must be handled.

Non-contact inspection

One feature of most polymers is their softness when compared with metals. This is especially true when considering a thin cross section, where the part will deform even when held by a human hand.

For such thin sections, traditional inspection tools – such as callipers, micrometres and probes – do not work. The mere act of holding the part and applying the pressure of the tool will throw out the dimension, rendering the process useless. To remedy this, non-contact methods, such as profile inspection and vision inspection were developed.

Vision inspection is not only useful for non-contact inspection, but can also be used to inspect radii, chamfers, angles and other unique profiles that gauges and tools cannot be easily developed for. In addition to inspection, the process also allows for the dimensions to be captured on the part and a visual to be created to be send to the client or end-user as proof of the dimensional accuracy.

Inspection of complex profiles

More recently, we encountered a set of parts which can only be inspected by sectioning the final component and using the vision inspection system to capture the dimensions. Obviously, once the part is sectioned (cut through), the part itself is destroyed.

Given the size of the parts, this kind of destructive testing can be cost prohibitive. Furthermore, as the parts were made from an especially expensive PTFE-based compound, the challenge to inspect and approve the FAI parts before beginning manufacture was proving to be an expensive one. To be absolutely sure that the dimensions did not deviate during bulk manufacture, it would be optimal to perform this destructive test at the beginning, the middle and the end of the machining cycle. As such, a batch of say 50 such parts would require the destruction of 3 parts to ensure the dimensions were within tolerance. This represents a wastage of 6% - which is high and very costly.

To remedy this, we made three suggestions:

  1. We machine the parts needed for destructive testing from a polymer that is inexpensive. When making both PTFE and PEEK components, we usually machine a few parts in POM (Delrin) first. POM is dimensionally very stable and mirrors the behaviour of PTFE and PEEK to a high extent. Once we are happy with the dimension, we shift over to the more expensive polymer
  2. The second option was to machine the FAI part in pure virgin PTFE, which would most closely match the behaviour under machining of the PTFE compounded material. This would work out more expensive than option 1, but still cheaper than destroying the compounded parts
  3. The final option was to ask the client to specify the percentage of parts they needed tested and work this back into the commercials. In such a scenario, we would need to define a minimum batch quantity – say 40 parts – and come to an agreement that 1 out of the 40 would be checked, leading to a 2.5% increase in the costing

Eventually, because of the sensitivity of the part and the stringent need for quality, it was decided that option 3 would be used.

The client was not comfortable with option 1. This is understandable, since as we have described above, no two polymers behave exactly alike. While we may be comfortable using POM as an approximation for PTFE and PEEK for internal dimensional verification, the same cannot always be considered accurate for bulk manufacture.

In order to better map the behaviour of POM with PTFE and PEEK, we will be undertaking a correlation study to be done over 3 months across 200-300 parts. Once the data is collected, it would put us in a better position to recommend option 1 to clients in the event that cost restrictions limit the level of destructive testing allowed.

The effects of Carbon Nanotubes on PTFE Properties

It has long been known that the properties of PTFE, while diverse and highly sought after, can be further enhanced with the addition of fillers. Given the high-melting point of PTFE (>350°C), most such fillers need to be stable at high temperatures in order to maintain their integrity during the sintering (curing) process that takes PTFE from a raw state into its final, workable form.

Over the years, fillers such as glass, bronze, stainless steel and carbon have become commonly used. Each of these fillers – used anywhere between 5% and 60% by volume - adds something to the property of the PTFE, while also taking away. For example, the addition of bronze greatly enhances the wear resistance of the material but increases the coefficient of friction and also renders the PTFE unsuitable for any electrical applications.

Over the past few months, we have been working extensively with carbon filled PTFE, in an attempt to improve the anti-static properties of the material. Being an excellent electrical insulator, PTFE tends to allow static charges to build up on its surface, rather than allow any vent for the charge to dissipate. While this property is useful in applications where high voltage loads need to be contained, it poses some problems in areas where a spark – caused when the static charge reaches some critical mass - can cause other materials around the PTFE to ignite. In order to dissipate the static charge through the material, carbon fillers are usually added. The resulting material retains much of the dielectric properties of PTFE but gives just enough conductivity within the material to allow static charges to dissipate. Such anti-static materials have become popular, calling for both machined components and PTFE tubes to be made using the compound.

The quantum and variants of carbon used in such compounds can offer differing values for the surface resistivity of the material. The standard grade of PTFE+Carbon employs a 25% content of the filler by volume. By most accounts, this is rather high. In addition to being a little too conductive, the compound also tends to “shed” some quantum of carbon in the form of microparticles. This can cause big problems in precision equipment, where clean-room levels of non-contamination are called for both within and around the apparatus. As a result, compounds have been developed using Vulcan.

Vulcan is a form of specialty carbon black which can be used in percentages as low as 2% to induce conductivity in PTFE. The resulting material is a lot softer and smoother than PTFE+25% Carbon – which, apart from being less ‘clean’, is also tougher to the machine – and is ideal for electrical applications calling for anti-static properties in sanitized environments.

Carbon nanotubes

We recently used Vulcan to manufacture anti-static tapes but were informed that the tests were unsuccessful. The client’s application called for a surface resistivity of 1 x 104 Ohms, whereas our material was only able to offer 1 x 103 ohms. The failure led us down a path of discovery to see what other materials could be added to further improve the properties as required.

Many new studies have pointed to the addition of carbon nanotubes to achieve the surface resistivity required.

Carbon nanotubes have been around for a while and its property as a high-strength to weight material has made it the stuff of the future. In addition to its impressive load bearing properties, the material also acts as a semiconductor. Although the applications into PTFE are relatively new, studies have prescribed its addition as a substitute for carbon black. According to these studies, the addition of as little as 1% of carbon nanotubes into PTFE can offer a surface resistivity of 1 x 104 Ohms, as required by the application.

In addition to the electrical properties, fillers of 7-10% have been shown to improve the wear properties of the material significantly. Another study looks at a composition of PTFE+20% Ekonol+1% CNT, claiming that at this composition, the tensile properties and elongation reach an optimum.

The mixture of carbon nanotubes to PTFE as a dispersion allow for enhanced corrosion resistance and deliver a superhydrophobic layer.

In the wider scheme of things, both PTFE and carbon nanotubes are relatively new materials. We are still discovering properties of each and there is no doubt we will experiment extensively with different compositions to see what effects arise.

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.