Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Precision Machining Polymers - The Challenges Are Plenty

Over the past decade we have seen a rapid shift from conventional machining to CNC machining. While CNC machined parts used to be required only in the most critical of applications earlier, they are now the mainstay, with even simple items like washers being churned out in this fashion, rather than depending on the perceived unpredictability of a manual system.

In the polymer space, while the shift to CNC has also been essential, there have been several complications that have arisen. We look at these here, in a bid to better understand the nuances of precision polymer machining and show that it is not always as straightforward as machining metals.

  1. Grades and varieties

    The first thing to realize is that the term “polymer” is both broad and vague. As a company deep rooted in PTFE (Teflon) as our core product, our experience into other polymers taught us that the differences in each make the process of CNC machining that much more unique. Let’s take a look at how some of the high-performance plastics behave:

    1. PA 6/ PA 66 (Nylon or Polyamide) – Nylon machines easily, but due to its low melting point, the feed rate and RPM need to be optimized to ensure that burrs do not melt and stick to the part. Furthermore, the high moisture absorption of Nylon implies that coolants can rarely be used, as these would ‘swell’ the component, causing dimensional deviations
    2. UHMWPE – like nylons, UHMWPE also suffers from having a very low melting point. Furthermore, as UHMWPE needs to be compression moulded, the orientation of the molecules within the part are not always predictable. Achieving high tolerances on UHMWPE is not always possible as a result
    3. PEEKPEI (Ultem)PI (Kapton) – these polymers are able to withstand high-temperatures and can therefore be run at higher speeds. However, due to the crystalline nature of the internal structures, the more stress applied during machining, the higher chance that the parts will crack. PEEK especially requires a special annealing process before it can be machined. In the event that multiple operations are required on a PEEK part, the part may be re-annealed between operations to ensure that the stress build up does not cause the part to crack later on.

      PTFE Radomes for Radar Applications

      Bellows – Virgin PTFE

      PTFE Radomes for High Precison Radar Applications

      PTFE Bobbins – Virgin PTFE – Tolerance of 0.04mm

      PEEK Adaptors for Aerospace

      PEEK Piston – Tolerance of 0.025mm

      PEEK Back Up Rings – 15% Carbon Filled

      PEEK Adaptors for Aerospace

      Nylon 66 Bobbins for Aerospace

      The above examples are just a few of the peculiarities that each polymer brings. With polymers such as PTFE (Teflon)DelrinPVDF (Kynar) and PVC, we have found the machining to be more straightforward. However, as the complexity of the part increases and the tolerances become tighter, the level of care needed increases, along with an increased need to understand the internal structure of the material.

  1. Tolerances and dimensions

    We are often approached by other companies also involved in some form of polymer machining, requesting whether we have any excess demand that they can support us with. Our first question is always “what tolerances are you able to achieve”?”. The answer is usually between 0.05mm and 0.1mm.

    From our perspective this is not adequate. While it is true that polymers do not lend themselves to the dimensional stability of metals (where tolerances of up to 1 micron are sometimes demanded), we have found that with the proper programming and handling, polymers can be machined to achieve a consistent tolerance of within 10-20 microns.

    It is in this endeavour that we have put a lot of our focus and effort. It is also why having CNC machines is alone not enough to ensure the parts would be of the highest possible precision. Knowing the material and understanding how the part needs to be handled – both during and after the machining process is complete, is critical to be able to get that extra 30-40 microns in tolerance.

    The other complexity on dimensions relates to the strength of the material. The longer the component, the tougher it becomes to attain close tolerances at the end – as the material starts to bend slightly, throwing the dimensions off. Again, knowing what the polymer is capable of and machining in a way that minimizes the deflection that the material would experience is key to ensuring a consistently machined component.

  2. Volumes

    While polymer machined parts have certainly found their foothold across industries, the volumes remain tiny when compared with metals, or even some injection molded polymer components.

    One of our concerns when shifting to CNC machining, was whether we could justify the expense against the low volumes of parts required. Keep in mind that apart from the machine cost itself, there are the added expenses of labour and special tooling.

    Getting high-volume parts that also demand the criticality that we offer remains a crucial challenge.

    Overall, the intricacies of polymer machining make it a rewarding experience. To be able to attain industry leading levels of tolerance across a whole range of polymers is something we are very proud of. So while CNC machining technologies certainly helped us move ahead, what set us apart was the ability to take the precision machining of polymers up a notch.

PEEK: The Superman of Polymers

If you deal in polymers and have not come across PEEK – it’s probably because its one of those materials which does not surface unless really needed. When it is needed – there’s little else that can be used in its place and this often confuses OEMs, because even among expensive, high-end engineering polymers PEEK sits at a price point that causes the client no small amount of shock.

It is important to talk about the price of PEEK before all its other characteristics, as this is usually the first thing the client wants to discuss. Invariably, they come knowing that they need this mystery polymer (PEEK), but knowing little else. They expect the price to be similar to Polyacetal or, at the very worst PTFE. When they find out that it is close to 10 times the price of PTFE, it comes as a huge surprise.

Why PEEK is expensive is not fully known. Perhaps it is because it has not yet reached the global scale of manufacture of more commoditized polymers, or perhaps the technology is so unique that it allows resin suppliers to charge a huge premium – knowing that alternatives are not available. As processors, we know only so much:

  1. The resin is 5-8 times more expensive than PTFE
  2. Processing PEEK is time consuming and expensive in comparison to PTFE
  3. Machining PEEK is tricky in comparison to other polymers

Since the resin prices are not in our control, we would like to look at points 2 and 3 and discuss them in more depth. But first, let’s get a better idea of what PEEK offers.

High tensile strength

In the polymer space, it would be tough to find something tougher than PEEK. It is so strong, in fact, that machining guidelines for PEEK need to follow the same as those for metals.

This strength allows PEEK to be used in applications such as gasketing and auto components – especially where metals cannot be used, but where a metal-like durability is required

High temperature resistance

PEEK melts at about 400 Degrees Celsius and is capable of running in environments of 300-325 Degrees without deforming. While PTFE can withstand up to 250 Degrees, any pressure/ load on PTFE at this temperature will invariably cause deformation. In the case of PEEK, its hardness allows it to be in a high-load-high-temperature environment without loss of dimensional properties.

High wear resistance

Again, while both PTFE and UHMWPE can take a significant amount of wear, PEEK exhibits a high PV value and can withstand wear effects even under harsh physical and chemical conditions.

Chemical resistance

While not on the same level as PTFE for pure chemical inertness, PEEK exhibits resistance to many harsh chemicals, allowing it to be used in corrosive environments, under heavy loads
In a nutshell, PEEK’s ability to stay dimensionally stable under harsh environments makes it a highly sought after polymer. OEMs who use PEEK do so knowing well that for the properties offered, PEEK is unique and therefore expensive.

Processing PEEK

We will not delve very deep into the processing of PEEK (as this is a proprietary process unique to each processor), but we will point out the key differences between PEEK and PTFE processing (which has been looked at earlier). It should be noted that here we are referring only to compression moulding, and not injection moulding.

The main difference is that while PTFE is cold compression moulded and then loaded in batches into a sintering oven, PEEK needs to be sintered during compression itself.  Furthermore, post sintering, PEEK needs to go through an annealing process, which is time consuming. This leads to a few complications:

  1. Batch processing is difficult. Since the total heating cycle for a single piece can take up to 8 hours, and since heaters are expensive, PEEK is normally moulded a few pieces at a time. So unlike PTFE, where a batch of 8-10 large pieces can be moulded in series and then put in the oven for a single cycle, PEEK will offer only a few pieces in the same amount of time.
  2. Since PEEK is heated under pressure, issues of flash can arise as the resin becomes molten, but has pressure being applied on it. Furthermore, the pressure and temperature have to be balanced very carefully, since the temperature makes the PEEK molten, allowing it to reach its desired shape, but the pressure is responsible for vacating air bubbles from the material, so that there is no porosity.
  3. Batch processing the PEEK parts for annealing is possible, but takes about 24 hours

So overall, the productivity in moulding PEEK is far below that of PTFE. This does answer, in part, the question of why the price of the finished material is so expensive.

Machining PEEK

As discussed above PEEK machines more like a metal than like a polymer. It is hard and has a significant impact on the tool. The same tool that might churn out 3000-4000 PTFE parts may struggle to churn out a few hundred PEEK parts. Again – this adds to the cost of the finished product significantly.

More importantly for machining though is that if PEEK is not annealed properly, the part will behave erratically during machining as different areas within the material react differently to the stress being placed by the tool. Thus, cracks can develop during machining and the dimensional stability across a batch of components can vary significantly.

As a result, PEEK machining is a difficult process and there are few who are willing to take on the risks of machining such an expensive item, knowing that the rate of rejection could be very high.

In conclusion – PEEK has remained a largely niche polymer due to its prohibitively high price. If it were cheaper – say around the price of PTFE – there are chances that it could steal a significant chunk of the PTFE market. PTFE still rates much higher than PEEK on characteristics like coefficient of friction and dielectric strength, but where it is a question of sheer strength, PEEK stands unmatched amongst polymers.

PTFE Prices - taking a step back to leap forward?

So we’re back to pricing – because until they fully stabilize, we need to be on our guard. Considering the data below, one might be allowed to assume that things are finally easing out and that the sector is slowing reaching an equilibrium of sorts, coming off the highs seen in mid-2011 to rest at about US$24/Kg. But we would rather still be wary.

Since prices spiked in July 2011, there has been a decrease of about 13% in prices – which has been gradual. There are a number of reasons one can point to for this decrease – most of which we have already touched upon in our last article on pricing. To list them out:

  1. Re-entering of China and Russia into European and Indian markets at competitive rates
  2. Easing out of Fluorspar supplies due to opening of new mines and reduction in China’s domestic consumption

However we remain wary for 2 very specific reasons:

  1. China’s summer approaches.In our very first article on pricing, we specifically highlighted the impact that the Chinese summer was having on PTFE prices. Summer months spike domestic demand for refrigerators and air conditioning and consequently cause R22 to be diverted from PTFE and into these products. This creates the shortage in R22 and was one of the root causes for the price escalations seen last year. However, we also postulated that once summer passes, the prices would ease out – which they have. But what now? Summer is about 2 months away and there is nothing to suggest that the rest of the world’s fluorspar mines can support the industry as yet. Our own sources indicated that it would be at least 2 years before the re-opening of mines in Mexico and South America eased the supply side constraints on fluorspar.
  2. The PTFE industry is far from efficient.In finance, we always assume that if an event (like China’s summer) is imminent and the effects of that event are known – then the prices of goods linked to the event should already reflect this information. In other words, if processors were aware that prices are going to spike during the Chinese summer, they would already have stockpiled raw materials to avoid against it, implying that there would be less demand during the summer and prices would not escalate again. However, this is unlikely to have happened since, (1) there are mixed opinions on whether the prices will go up or keep going down and (2) processors have already had to triple their working capital in order to keep up with the price increase in raw materials and it is unlikely that too many would have funds to stockpile materials for a full quarter. Therefore we remain nearly as exposed as we were last year.

But the news may not be all that bad. For one, China has been seriously implementing the R22 phase-out and as of August 2011 was even awarded a grant to speed up the efforts. Whether this phase-out sees any immediate impact on domestic demand remains to be seen and would possibly define the price of PTFE for the next few years. Secondly, there would be good reason to believe that PTFE resin manufacturers have hedged against such a scenario – even if the processors themselves are unable to do so. In India, our local producer has not only augmented PTFE resin capacities, but also become one of the foremost global producers of R22.

In a nutshell, the state of the future depends largely on the balancing of the Chinese summer against the precautions taken by resin manufacturers to safeguard against a further spike. I do not believe processors have any real part to play in all this – we remain, for the most part, price takers. If there is a fluctuation in prices, we would need to absorb it much the same as we did last year.