Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

PTFE machining considerations - tapping

Machining PTFE, as we have touched upon before, is never a straightforward process. Most machining handbooks will suggest that PTFE should be treated much like wood when it comes to machining, as this is the material it most closely behaves like when machined. And while this is a good starting point for tool selection and CNC programme settings, as we delve deeper into the aspects of machining PTFE, we see that it behaves much like it’s own material. So learning by doing becomes the only option – since PTFE is a niche product (when compared with other known polymers).

Recently, we faced an interesting issue when creating a rather complex part. The part is approximately 200 Grams in weight and machining it involved multiple operations including CNC turning, CNC milling, drilling and finally tapping. All in all, the drawing highlighted over 28 dimensions that needed to be within a strict tolerance and it took us the better part of a week to just get 10 prototypes ready.

We were pretty happy with the result: everything measured, as it should. We almost didn’t check the tapping – which called for an M3 tap in two places. The M3 taps used were brand new and the first tap was done on the VMC as part of the programme – so there was no way it could be an issue, we thought. But we were wrong.

The no-go gauge entered in the hole all too easily and we were pretty shocked to realise that even an M3 bolt was sitting loose in the hole. At first we though we had the wrong tap – which we didn’t. We then argued that the gauge would always enter – as it was designed mainly for harder materials and PTFE would yield all too easily, since it was much softer. To check this we used the same taps on a mild steel plate and confirmed that the no-go did not enter. But this still did not answer why the bolt itself was loose.

We searched extensively for an answer online, but there was very little information on tapping and even less on the issue we in particular were facing. We then decided to start experimenting with different combinations of taps and drill holes.

On the part, we had used a 2.2mm drill with all 3 taps. The first tap was done on our VMC, while the next 2 were done by hand. We tried the following combinations:

Drill Hole Tap 1 Tap 2 Tap 3 Result Remark

1.5

Y Y Y Reject Bolt loose

1.5

Y     Reject Bolt loose

1.5

    Y Reject Bolt tight

2

 Y Y Y Reject Bolt tight

2

    Y Reject Bolt loose

2.2

Y Y Y Reject Bolt loose

2.2

    Y Reject Bolt loose

In a couple of cases – where we used only the 3rd (finest) tap, the bolt was tight. However, none of the holes were answering to the no-go gauge, which passed equally easily in all the holes. We once again argued that this was a PTFErelated issue and that as long as the bolt was tight, it should not be a problem. But many of the consultants and experts I spoke with said that they had come across parts in PTFE that answered to the no-go gauge, and hence there must be a way to machine such a part.

The problem was finally solved when an engineer in our client’s side suggested we use a “Form Tap”. I had never heard of a form tap and when I searched it, it seemed to apply mainly to tapping soft metals (such a aluminium). There was no mention of applications to PTFE. Nonetheless, it was our last shot, so we tried it and were pleasantly surprised.

We eventually went with a 2.0mm drill and an M3 form tap to get a result that was both functionally good and which answered to the gauge.

The reason the form tap works, is because unlike a regular tap, it does not bore into the PTFE, taking out material as it does. Instead, it merely forms the tap profile within the drilled hole and as PTFE is soft, it yields quite easily. The result is that the tapped hole is much fuller than when a normal M3 tap is used – making it tighter and ensuring the pitch profile does not yield to the no-go gauge.

Surprisingly, this does again strengthen the PTFE-Wood similarity in machining. Tapping is unheard of in wood; a screw can be passed through a drilled hole and sit tight forever! In many ways, a form tap is nothing more than passing a screw/bolt into the PTFE to imprint its profile within the hole. Only that the form tap is possibly more exact and can ensure that the resulting tap is accepted when inspected with the correct gauges!

UHMWPE - the unknown polymer

One of the few good things to happen due to the unprecedented escalation of PTFE prices globally was that it allowed us to look at alternate materials and seriously gauge the feasibility of manufacturing them.

In an earlier post, we looked at the various properties of PTFE and compared them to the other polymers. And although the key takeaway from that exercise was that PTFE was an immensely versatile material which was difficult to replace, we did make mention of possible alternatives, provided the user was willing to compromise on some parameters.

A key polymer which struck us then and continues to feature prominently in our product offering today is UHMWPE. We would like to take a more detailed look at UHMWPE for two reasons:

  1. It does measure up against PTFE as a low-cost substitute (with certain limitations)
  2. It’s properties do not seem to be as widely known to end-users, resulting in a limited use in many applications where it would otherwise be ideal

 What is UHMWPE?

Sometimes referred to as just “UHMW”, UHMWPE or Ultra High Molecular Weight Poly Ethylene is an off-white polymer that exhibits superior strength while being both light-weight and possessing a low coefficient of friction.

While it is not entirely accurate to refer to it as an “unknown” polymer – our own analysis of search terms within Google tells us that a total of ~62,000 searches per month are done globally for UHMWPE and/or UHMW. This is tiny in comparison to searches for PTFE/Teflon (1,300,000 per month) or for Nylon (5,500,000 per month).

Comparison with PTFE

So how does UHMWPE compare with PTFE? In our own opinion – it compares rather well. In fact, if you take all the applications involving PTFE and remove the ones that call for heat resistance, UHMWPE is a very workable substitute.

Although a full comparison chart is given at the end of this article, we would like to look at some specific properties more subjectively.

  1. Temperature resistance: Let’s get this one out of the way, since we know that it is UHMWPE’s weakness. Having an operating temperature of only about 80°C compared with 260°C for PTFE, UHMWPE is automatically disqualified in a range of industrial applications where the temperatures surrounding the material are expected to be well in excess of it’s upper limit.
  2. Wear resistance: Before we were familiar with UHMWPE, we were asked to advice a cement plant on whether they could use Lubring sheets (PTFE+Bronze) in a wear application. We were confident that it would work and when they mentioned that they had tried UHMWPE and it had failed, we did not think it was worth looking into. But when we did compare the materials, we realized that if UHMWPE had failed, there was little chance PTFE would work – since the gap between the two materials on this parameter is quite wide.Keep in mind that PTFE+Bronze is the most wear resistance grade of PTFE available. So if we compare UHMWPE with plain PTFE, the rift is even wider.
  3. Coefficient of friction: It is difficult to beat PTFE on this parameter, although UHMWPE comes fairly close. While it remains true that the coefficient of friction between PTFE and polished stainless steel is the lowest between two known solids (0.03-0.05), UHMWPE is able to reach a somewhat respectable 0.1-0.15 on this metric. While this does put it out of range for many applications where the recommended coefficient cannot exceed 0.1 (eg: sliding bearings) – it is a useful substitute in components where smooth movement between parts is the only requirement.
  4. Dielectric strength: Both materials are pretty much neck and neck on dielectric strength. Where UHMWPE loses out is on its ability to be skived into thin tapes. While we regularly skive PTFE down to 0.04-0.05mm thicknesses, the same is more challenging with UHMWPE, since it lends a much higher wear on to the skiving blade, making it difficult to achieve long lengths of tape before the blade dulls out and breaks the tape. Nonetheless, thicknesses of 0.1mm and above are more than feasible, meaning that as an insulating pad or even a component used in high voltage applications, UHMWPE is more than suitable.
  5. Chemical inertness: PTFE is well known for it’s inertness and this allows it to lend itself to applications ranging from biotechnology to medical devices and chemical linings. While UHMWPE does not have quite the same extreme inertness as PTFE, it does find use in medical applications (it is used in parts for joint replacements) and can easily be used in both biotech and chemical applications, provided the exact nature of chemicals is known and compared against it’s capabilities.
  6. Weight: While weight has never been a consideration for PTFE in any of it’s applications, we would still like to highlight that UHMWPE is less than half the weight of PTFE (specific gravity of 0.95 vs. 2.15 for PTFE). The key difference this adds is in their respective cost cacluations. Not only is UHMWPE cheaper in resin form (roughly 1/4th the cost per Kg), but the fact that you consume only half the weight to get the same volume of finished product implies that the effective cost is 1/8th the cost of PTFE. This represents a significant saving.

So where can we use UHMWPE?

There are a range of applications where UHMWPE could and should be used. In many cases, we have tried to suggest to the end-user that we can offer them UHMWPE in stead of PTFE, but due to restrictions on standards and because changing specifications can be time consuming, very few have opted for the change.

Strangely, in many cases, clients have opted for suppliers offering reprocessed PTFE, but not UHMWPE. Given the highly diminished properties of reprocessed PTFE, this is functionally not a great trade-off in the medium to long term.

  1. Automotives: Most automotive applications use PTFE in high temperature environments, so UHMWPE does not fit the requirement. However, there are a number of applications where the parts operate at room temperature eg: car doors, seats, hand levers etc. and here UHMWPE can find a lot of use. We are aware that the wear strip used inside car doors employs UHMWPE. In general, UHMWPE wear strips offer a low cost and effective alternative to PTFE wear strips.
  2. Valves and seals: Typically, valves and seals require a low coefficient of friction with a good wear resistance.  UHMWPE is an excellent replacement for PTFE in these areas.
  3. Medical: UHMWPE is widely used in joint replacements due to its chemical inertness and light-weight.
  4. Infrastructure: Although regulatory restrictions prevent materials other than PTFE to be used POT bearings, there are many sliding bearing applications which do not fall under the government codes and are therefore potential areas where UHMWPE can be used. UHMWPE could be employed successfully in sliding bearings and as plain sliding pads.
  5. Electronics: Many components used in electronics have traditionally employed PTFE components for insulation. In a number of cases, we have successfully tested UHMWPE for these applications and convinced the client to shift.

Overall, there continues to be a resistance to employ a material like UHMWPE. Part of this is regulatory – drawings and specifications that call for PTFE cannot be changed over night. But mostly there is a genuine dearth of awareness about the material – which is equally difficult to change. While it is true that UHMWPE is a substitute for PTFE – we see it as more of a partner in application – allowing many end-users to find a competitive, low-cost solution where they would otherwise be unable to proceed with their development or manufacturing.

Comparison chart between PTFE and UHMWPE

  UHMWPE PTFE Units
Colour Off-white White  
Specific Gravity, 73°F 0.944 2.25  
Tensile Strength @ Yield, 73°F 3250 4000 psi
Tensile Modulus of Elasticity, 73°F 155,900 150,000 psi
Tensile Elongation (at break), 73°F 330 350 %
Flexural Modulus of Elasticity 107,900 145,000 psi
Compressive Strength at 2% deformation 400 1650 psi
Compressive Strength 10% Deformation 1200 2200 psi
Deformation Under Load 6-8% 2.5-5% %
Compressive Modulus of Elasticity, 73°F 69,650 79,750 psi
Hardness, Durometer (Shore “D” scale) 69 55-65  
Izod Impact, Notched @ 73°F 30 161 ft.lbs./in. of notch
Coefficient of Friction (Dry vs Steel) Static 0.17 .06-0.12  
Coefficient of Friction (Dry vs Steel) Dynamic 0.14 0.12  
Sand Wheel Wear/Abrasion Test 95 90 UHMW=100
Coefficient of Linear Thermal Expansion 11 6-7.2 in/in/°F x 10-5
Melting Point (Crystalline Peak) 135-145 380 °C
Maximum Service Temperature 80 260 °C
Volume Resistivity >1015 NA ohm-cm
Surface Resistivity >1015 NA ohm-cm
Water Absorption, Immersion 24 Hours Nil Nil %
Water Absorption, Immersion Saturation Nil Nil %
Machine-ability Rating 5 3 1 = easy, 10 = difficult

PTFE Pricing Again - is there another price hike in the offing?

The adage “Only the paranoid survive” has held very true in the PTFE industry these past 20 odd months. You see, while it is difficult to imagine worst case scenarios and constantly plan along their likelihood, it is usually the only way to make sure that one is not blind-sided by bad news when it does arrive. And since bad news has been arriving thick and fast, being mentally and commercially prepared for the price hikes in PTFE resins are what have allowed many PTFE processors to survive this period of turbulence.

So we choose to re-look at pricing again because despite the fact that PTFE prices have been stable since July 2011, the last thing we can afford to do is assume all will be well from now on and be rudely shocked if and when another price hike does come around.

But rather than subject ourselves to speculation, we have been looking at trends, hearing out rumors and gleaning information from various sources to gauge what might actually be the future of PTFE pricing in the near term. As always, there are various factors at play, but together they do suggest that another price hike is unlikely and that there may even be some easing out of prices in the offing.

1. Rate contracts

Our sources in Europe tell us that there is an increasing push by PTFE resin suppliers to enter into rate contracts for the coming quarter. In an environment where the suppliers have enjoyed increasing prices month-on-month, rate contracts suggest that the scenario may be changing and that an easing out of prices is expected.

Furthermore, many companies are rejecting the rate contracts, since there is a general feeling that prices will reduce in the near term.

2. China and Russia back in the game

In an earlier article we mentioned how both Chinese and Russian resin suppliers were experiencing capacity constraints due to a number of reasons ranging from Fluorspar reallocation to maintenance shutdowns and internal restructuring of capacities.

Now our sources tell us that China and Russia are once again making supplies into Europe and that the pricing is highly competitive in comparison to other suppliers.

Even locally, the marketing push by Chinese companies to try and sell resins into India has accelerated. We receive more mails every day from China and have even been approached by some sourcing agents, asking if we would be interested in entering an agreement to buy resins.

It was always our belief that prices may never come down, since the value growth due to pricing has more than compensated the volume reduction. However, we also know that China has always preferred higher volumes rather than higher margins (a strange strategy, but one that has allowed them to aggressively expand and build scale). So it is unlikely that they will join the rest of the world’s resin suppliers in keeping prices stable and highly probable that they will induce a price war of some sort – forcing prices to reduce.

A quick look at the global price benchmarks we have obtained show that while the rest of the world (India, USA, Europe) had stabilized around a price of US$25-27 per Kg, this rate was sustainable only as long as China and Russia were not supplying globally.

3. Anti-dumping duty no longer effective

 

When the anti-dumping duty on Chinese and Russian resins was first imposed in India, it effectively increased PTFE resin prices by US$3.3 per Kg. Given the local price of PTFE resins was US$7-8 per Kg at the time, this acted as a serious deterrent for processors buying from China and Russia. At US$20-25 per Kg, the US$3.3 duty ceases to be effective, as the landed cost of the resin would still be below the local rates being availed in India.

It does remain to be seen whether local resin manufacturers are able to bring about a further increase in the duty amount, but even this will take time, so it is likely China and Russia will re-enter India and put pressure on prices.

4. Re-opening of Fluorspar mines

 

An obvious trend – looking at the past year, may be that China is only able to supply PTFE resins now because there would be an easing out of domestic demand for R22 in refrigeration. We had in an earlier article suggested that in the medium term, as winter approached, there would be an increase in R22 available for PTFE resins and this would ease out prices. However, there is no guarantee that the same pattern would not repeat next year – with supply constraints forcing prices up again.

In the mean time, there are reports that mines in Mexico and South Africa have been re-opened, although it would take at least another 12-18 months for them to be operational. This suggests that prices may again increase during summer 2012 – although if processors stock up on raw materials prior to this, it would not allow the prices to escalate in the same manner as they did in 2011.

Up until last week, the local buzz was that PTFE resin prices were being hiked by up to 30% in January 2012. This has coincided with other news that points to the contrary – rate contracts, China and Russia, capacity expansion. It could be that we have missed out some key information and as a result, our own analysis is wrong. It remains to be seen whether there will be a price hike, but for now we’re staying paranoid – because it seems the safer option currently.