Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE pricing revisited ? inevitabilities in long term supply and demand

When we started this blog, our aim was two-fold:

1. To inform and educate readers about PTFE, it’s applications and derived products
2. To serve as a platform for clients and other end-users to understand PTFE better and make informed decisions regarding their applications

However, it seems to have been the articles on pricing which have brought most of our traffic as regardless of how important the applications of PTFE are, it is – understandably – on pricing that most questions currently centre.

We therefore want to look at pricing again, just to see if any new information gleaned over the past couple of months helps us understand the situation any better than we did earlier.

Since our last blog on the impact of Fluorspar on PTFE prices, we have – like all other processors – been praying for stability. We haven’t been praying for a reduction in prices – that would be optimism to the point of pure irrationality. However, if prices could simply stabilize – even for a few months, it would give us some time to re-group, re-assess and possibly resume normal operations.

However, the pricing fluctuations have been coupled with lesser-known events in the background and together, these effects are causing the stabilization process to take much longer than earlier assumed.

We would like to take a look at some of the news floating in the market at present. We can vouch that since these are from rather reliable sources, we are inclined to believe them and therefore base our outlook on their implications:

1) Fluorspar shortage is no longer an issue

A supplier who regularly sources semi-finished PTFE from a Chinese manufacturer told us this anecdote: The supplier approached the manufacturer with the offer to supply R22. The proposed arrangement was that the PTFE manufacturer could then supply the resin manufacturer with R22 (assumed to be in very short supply) and in return procure resin at a discounted price. The supplier was shocked to hear that the resin manufacturer declined – saying that they had ample R22 to meet their production needs. This does lead us to believe that although the Fluorspar story may have started the PTFE price frenzy, it is now not playing as significant a part.

2) European resin manufacturers have re-allocated resources away from PTFE

This was partly confirmed by a representative from DuPont, who stated that their company was slowly coming out of PTFE resin manufacture, as long-term competition against Chinese suppliers was not feasible for them. The resulting effect, we hear, was that most of the European resin manufacturers have sub-contracted their PTFE business to Chinese resin suppliers. Since the realization for PTFE resins in Europe is much higher – the European price has become the new acting price across the global market.

This impact does throw some light on why the PTFE prices have increased so drastically all over the world. On the one hand, we have a supply constraint, as European manufacturers no longer compete in the market. At the same time, you have a huge supply-demand mismatch as European demand for resins stays the same and this drives up prices.

3) Russian suppliers are in a state of flux

From what we have heard, Russia has two main companies who manufacture PTFE resins, one of which acquired the other. The combined company is said to be undergoing some transition issues and management is also contemplating moving away from PTFE and into ETFE. The result has again been to constrain supply, impacting prices in the process.

4) Pricing set to stabilize within the next 2-3 months

Obviously, many are hoping that things will settle down sooner than this, but considering the extent of changes occurring across the market, one might expect that it would take no less than a few months to stabilise.

Our own local the supplier – who increased prices by another 30% in the month of July 2011, assured us that this would be the last-but-one, if not the last, price revision from their side. The current price we are getting is US$26.5 per Kilo for virgin PTFE resin. From what information we have from our European counterparts, it appears that local rates there are around the same price – so it does look like some sort of balance has been reached.

For those thinking about the long term implications of all this, we can infer the following from what data we have already collected:

  1. High prices are here to stay. If there is one thing that all this has shown us, it is that the demand has stayed strong enough despite the price escalation. This has justified the price hike for resin manufacturers from a business standpoint
  2. Long term, quality will improve. Although it looks like Chinese companies will be doing most of the manufacturing of PTFE resins, if they are supplying through companies like DuPont and 3M, the quality controls will most probably be more stringent.
  3. Volumes in PTFE will shrink. Although we have not seen a significant amount of substitution away from PTFE, there are murmurs of new materials and possible replacement materials in some areas. For the most part, we continue to believe that as a material, the extensive spectrum of properties offered by PTFE makes it a difficult material to shift out from. However, we do expect that at least 15-20% of the volumes in PTFE would slowly shift to other polymers such as PA66 and UHMWPE. Nonetheless, we can take comfort in the fact that a 15-20% fall in volumes when combined with a 100-200% increase in prices still implies an overall growth in the industry in value terms.
  4. Repro is here to stay. Not that anyone though that reprocessed PTFE would go away, but we do believe that the acceptance of recycled material in many applications (due to the price implications) would bring about some regularization in the market, with manufactures offering transparency on the extent to which reprocessed PTFE is used and possibly on the properties it could be expected to exhibit. Again – this would be a good thing from a quality standpoint, as buyers of semi-finished PTFE would at least know exactly what they were getting.

To conclude – we have, like most other processors, been trying to make the best out of a situation that has been completely out of our hands. We have faced a rather torrid 15-18 months, so if the end were 2-3 months away, we would look forward to that. In the mean time we would recommend planning one day at a time, because there is no telling what might happen during the next week or month.

Cantilever Load Considerations for PTFE Sliding Bearings

As a manufacturer of sliding bearings in India, one of the challenges we face is that there does not exist an official book of guidelines specifically for this type of bearing. The closest we have is the IRC:83 – which is the code book for POT-PTFE bearings, and which details the specifications of the materials to be used and also the testing parameters for POT bearings. As such the IRC:83 does provide some guidelines – but does not address the finer design requirements for sliding bearings. Consequently, during design and testing of the bearings, customers rely on the Quality Assurance Plan (QAP) we provide them, with the option to question, refute and even modify the requirements as they see fit.

 

Although the BS:5400 and AASHTO standards do make specific recommendations for PTFE sliding bearings, customers are not always willing to accept their parameters as they may sometimes vary from those specified in the IRC – in places where the codes overlap. This can result in technical stand-offs, between the customer and manufacturer, as each tries to convince the other regarding a certain process or parameter, without any official rule book, to back up their view point.

Among the more interesting technical debates we have had recently has been regarding the thickness of the top plate (upper sole plate) in bearings with high movement requirements.

Given a specified vertical load, a PTFE sliding bearing will be required to have a PTFE pad with side determined by the compressive strength of PTFE (usually taken at 100-200 Kgs per sq. cm). This side in turn determines the side of the lower sole plate. Give these two dimensions, the movement of the bearing (specified usually by the client/ contractor) will give us the side of the Stainless Steel sliding element, which in turn will give us the side of the upper sole plate.

Our designing of PTFE sliding bearings began with taking standard designs for specified loads and movements (such as C&P) and recommending them to clients. As we became better versed with PTFE sliding bearings, we started designing from scratch – using the parameters of movement, load and rotation to design customized PTFE bearings for specific project requirements. As many of the original standard drawings recommended sole plates of 12-15mm thickness for bearings up to loads of 50-65 Tonnes, we too continued with this recommendation. After all, the constraint of a bearing in taking a vertical load depends purely on the PTFE – as when compared with steel, PTFE has a much lower compressive strength (100-200 Kg per sq. cm versus steel’s 14000-15000).

It came as a surprise to us therefore when a client expressed concern over the top plate thickness – citing that the bending moment caused by the vertical load would be in excess of what the top plate could accommodate, given the overhang of the top plate over the bottom. This led us to revisit a lot of the standard designs – only to find that for similar load-movement parameters, other bearing designs did not recommend a thicker sole plate that our customer was insisting on.

We therefore went back to theory to gauge exactly how much of a bending moment would be caused by our given load and assess whether there was a case for changing our top plate design.

The formula for calculating the bending moment is as follows:

M= (P x L2)/2

Where:

P = Pressure (Kg/cm2)
L = Length of overhang (cm)
M = Maximum Bending Moment (Kg)

Once we know M, we divide by the Maximum Allowable Bending Stress (S) of the material (1650Kg/cm for steel) – to derive the thickness of the steel plate required.

In the example in question, the value of M obtained was 3941 Kgs, which when divided by S gave a thickness of 3.78cm – or 38mm.

This was a shocking revelation, as our recommendation was for 15mm – clearly insufficient for the load in question. Nonetheless, we wanted to dig deeper to find out how most standard bearing designs only called for a 12-15mm thickness – when the theory clearly showed that this was inadequate.

In most of our discussions with consultants and industry experts, the value of 38mm was ratified and we were told that this was in fact the thickness needed. They were unable to explain, however, why the standard designs did not tally with the theoretical calculations. Our question was finally answered when we spoke with a contractor who studied the drawings and design details and confirmed that while a thickness of 38mm was indeed required, the thickness of the insert plate also needed to be taken in to account.

The insert plate is installed at site and is simply a 25-30mm thick steel plate grouted/ cast along with the concrete substructure and/or superstructure. The bearing is welded or bolted to this plate and the load on the bearing is transferred through the plate as well. Thus, a bending moment will act through the layers of both the upper sole plates and the insert plates – meaning that even with a sole plate thickness of 15mm, the total thickness through which the load acts is 40mm – which is more than sufficient in our example.

We went back to our client with this, but were informed that they were not using an insert plate in this particular project and that the sole plate thickness needed to be at least 40mm as per our calculations.

The exercise was an eye-opener, because we had never before been questioned on the bending moment for sliding bearings, nor had we come across any design calculations for this in the various codebooks. However it is a critical point in the design of a PTFE sliding bearing – as there is always an overhang of top plate over the bottom. As a rule, one needs to verify that an insert plate is being used and of what thickness it is. Adding this thickness to the sole plate thickness, the designer needs to verify whether the bending moment is being accommodated. If not, the thickness of the upper sole plate must be re-worked.

PTFE Wear Plates: Misconceptions and Applications for Heavy Equipments

Although PTFE is used extensively for its wear resistant properties in a range of different products, its application as a wear resistant plate remains restricted and not widely known in certain areas where it would be ideal.

In most cases, the preferred composition for PTFE wear resistant material is PTFE with bronze (along with some friction reducing additives). As discussed in our earlier article (see: PTFE Compounds and their effects), this composition improves the PV value and wear rate for the material and although the coefficient of friction does increase, over-all it performs superbly as a replacement to metal bearing parts that require frequent lubrication.

Currently, some of the main applications for PTFE as a wear resistant material for which we supply include:

  1. Slideway bearings: Commonly referred to by the brand name “Turcite” (Poly Fluoro brand name: Lubring), PTFE slideway bearings are used widely in the machine tool industry, where they serve to either replace or reinforce standard phosphor-bronze LM guideways. The material was earlier used primarily to recondition old machines in which the guideways had worn out. However, increasingly it is incorporated in new machines as well – owing to the higher life and lower maintenance required in comparison to metal guideways
  2. Wear stripsPTFE wear strips are used either in running lengths or punched into flat components, which are used in sub-assemblies, like shock absorber struts and pistons. Usually the tolerance on thickness for such wear strips is very low – implying the requirement of a high precision skiving machine. In most case, where we supply these items, a tolerance of +/-0.02mm is maintained on thickness.
  3. Piston rings: Here, thin bands of PTFE wear material are machined and fitted on the piston shaft to absorb the wear resulting from a constant back-forth movement. As this process is wasteful (and therefore expensive) due to the machining involved, sometimes customers prefer to buy wear strips and bond them around the shaft. However, bonding PTFE is usually only recommended when there is minimal shear force being applied on the item – so this method is usually unsuccessful.
  4. Bushings: PTFE can either be machined into a solid bush, or be used as a layer on a metallic bushing (commonly called DU bushings). Again – the idea here is to create a self-lubricating bush, which can be installed within a sub-assembly and allowed to run without the constant need for lubricants.
  5. Wear plates: Used in more heavy duty applications, wear plates are usually employed in thicknesses exceeding 10mm and often require milling on the surface to create oil-grooves and holes for bolting. In most cases, their function is similar to that of a slideway bearing, however we have noticed that many OEMs remain apprehensive to employ PTFE wear plates into their equipments. In an attempt to clarify certain points regarding PTFE wear plates, we are going to be looking at 2 aspects of their usage:

 

1) The common pitfalls clients experience when using these bearings and misinformation regarding the same

2) Our own experience in the Die Casting Industry, where the success of these plates has led us to aggressively recommend it to OEMs

Issues hindering the adoption of PTFE wear plates

  1. Installation: We find that most people adopting PTFE wear plates do so because they have some prior experience with installing slideway bearings. Consequently, they assume the installation methods would also be the same. However, as slideway bearings are much thinner (going up to no more than 5-6mm) and because following installation, they remain subjected to very little shear loads, they can be bonded to the metal bed and this bond is likely to survive over a long period of time.In the case of wear plates, bonding is not an option as it is likely that there is some shear load which will get applied which, when coupled with the thickness of the plate would weaken the bonding and cause the plate to come loose in the medium to long term.

    The correct method of installation is bolting – although valid apprehensions exist with regards to this. For one: the plate needs to be milled with a stepped hole to allow the bolt to rest within the piece. Care needs to be taken to ensure that the bolt head does not rest above the surface of the PTFE plate. As an added measure, PTFE discs can be bonded to the head of the bolt to ensure that in the event that any extra pressure squeezes the PTFE plate, the contact between the bolt and the moving plate is not damaging. Furthermore, tightening the bolt too much can cause the PTFE plate to get crushed (a common reason cited by OEMs for not using a soft material like PTFE). Hence the correct method would be to use a metal bush to ensure the bolt is not tightened beyond a point (see below).

    The purpose of the bolt is to ensure the PTFE wear plate does not slide away during operation. As long as this is ensured, the plate will perform properly.

  2. Load bearingA common misconception relating to the load bearing capacity of PTFE leads many machine tool builders to write-off PTFE as a wear pad material. The assumption is that phosphor-bronze, being a metallic material, is the only option strong enough to take the load of heavy moving parts.

    In truth – PTFE has a compressive strength of at least 135-140Kg per square cm. This implies that a 100mm x 100mm plate would be able to withstand 13.5-14 Tonnes of vertical load. In most heavy-duty equipments, maximum loads of 5-6 Tonnes are present, meaning that the load bearing is not an issue at all. Furthermore, the coefficient of friction of PTFE against another surface only reduces with the application of pressure – implying that apart from taking the load, the effectiveness of the wear plate in ensuring a smooth functioning of parts is greatly enhances.

  3. MachiningClients who are looking to convert to PTFE wear pads frequently express two concerns pertaining to machining.

    The first is on tolerance: as the thickness on phosphor-bronze wear pads can be grond to within a few microns. In the case of PTFE – a maximum tolerance of 50 micros is possible – which we have found is acceptable in most industries.

    The other concern is around specific grooves and the exact positioning of holes. As PTFE can be milled (we use a CNC vertical milling centre) – any groove pattern and hole dimensions can be machined on to the surface of the wear plate.

  4. EnvironmentFinally – we have heard concerns over the conditions in which the equipment is used and whether PTFE will be able to withstand the same in the long term.

    Firstly – PTFE has the ability to withstand temperatures of up to 250 Degrees Celsius. In most industries we know, the actual heat generation never causes the surrounding temperature to go about 80 Degrees, so clearly there is no issue in using PTFE.

    The other concern is on the build up of dirt and whether grit and other hard particles will damage the surface of the PTFE plate. While the recommended option here would be to make a seal around the PTFE to ensure that dirt does not get accumulated between the PTFE and the other moving plate, it should also be noted that in case a particle does get lodged between the plates, PTFE has the unique ability to absorb the same so that it does not hinder the movement of the assembly.

 

Case Study: PTFE wear plates in the Die-casting industry

A client who was consulting on technical metrics with various companies engaged in aluminum die-casting approached us, a while back. The problem they were facing was that the wear plates that had been installed on as bearings between the platens was wearing out every 2-3 months, with the result that there was significant down time on the machines every time these plates needed to be replaced.

The plates being used were a fiber enforced resin plates and it was easy to see that a few months of usage had significantly worn out the plates leading to deformation and even cracks.

We offered them PTFE wear plates and these were installed on a few machines as a trial. The machines were run normally for a period of 3 months and the PTFE plates were analyzed with the following results:

  1. Wear out was minimal: In fact, the PTFE wear plates were much the same dimension as when they were installed. The customer felt that the load of 2.5 Tonnes being applied on the plate would compress the plate and lead to a deformation on thickness – but this was not the case.
  2. Lubricity was greatly enhanced overall: The plates had become completely smooth due to the constant sliding across its surface and this smoothness translated into the more efficient operation of the equipment. The customer also reported that while earlier there was some amount of “jerkiness” in the motion of the platens was no longer an issue.
  3. Improved cycle time: Apart from the fact that the down-time of the machine was no longer an issue as the plates were not worn out, the overall cycle time of the machine during production was also improved. This was mainly because there was no longer a need to continuously monitor the level of lubrication on the wear plate.

 

Following the successful trial of the PTFE wear plates, the material was adopted in all machines of the client and we are now working with a number of clients in the die-casting industry to replace their resin plates with PTFE plates.