Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Over-moulding PTFE on to Stainless Steel

As polymers go, PTFE is not the easiest material to deal with. It behaves contrary to nearly every other plastic and requires special processes to create even the simplest of forms. Whether we look at the extrusion of PTFE tubes or the forming of PTFE films (both rather straightforward when we consider melt-processable polymers), the methods we need to employ for PTFE are a practically standalone and need to be understood and developed from first principles.

Similarly, over moulding a plastic onto a metal part is not a very complex task when we look at injection moulding. In such a process, the metal part is inserted within the injection moulding die and molten polymer is injected and cools around it.

However, there is limited scope to follow this process when trying to over-mould PTFE onto a metal object. The limitations are the following:

  1. As PTFE cannot be melted, there is no way to form the polymer around the metal part in any way that would be uniform and consistent using heat alone
     
  2. Since PTFE can only be compression moulded, the metal part would need to be kept within a compression moulding die and the dry PTFE powder would need to be packed around it. However, as PTFE is very sensitive to the amount of pressure being applied during compression, it is essential that the metal part does not have too many contours, as this would lead to an irregularity in the compression.

    It is possible, in the event of a more complex metal part, that isostatic moulding is used to ensure there is even pressure on the PTFE powder during compression. However, as isostatic moulding requires a lot of die and mould costs, such a process could only be justified if the volumes are significant.
     
  3. Even if we do manage to pack the PTFE around the metal uniformly, the final issue remains concerning the heat cycle.

    PTFE is sintered (cured) at temperatures of around 375°C over a period of anywhere between 15 and 100 hours – depending on the size of the part. This heat cycle means that within the oven, both the PTFE and the metal are subject to high temperatures. Here the challenge is to match the thermal expansion and contraction of the PTFE and the metal, so that mismatches in the rates of expansion do no cause the PTFE to crack.
     

Overcoming these challenges required a lot of R&D. Different combinations of pressures and heat cycles were employed until we were able to consistency achieve a part that would not crack. The moulded part also needed to be machined, which meant the part needed to be free from any internal irregularities as well.

Development of a high cross-section expanded PTFE (ePTFE) gasket profile

The best products are usually borne from testing the limits of what can be manufactured. As any company would testify, making a commoditised item has few benefits other than giving a good boost to volumes. The true joy in manufacturing comes from creating something specialised and knowing that the client not only appreciates the efforts involved, but that they would have no reason to take their business elsewhere.

One such product we were asked to develop was a unique and challenging profile in ePTFE (expanded PTFE). We should point out that at the time of taking on this development, our own understanding of ePTFE was still nascent and our knowledge of what our equipment could manage was also still developing. Nonetheless, we took on the challenge, because it seemed like a good one and because, if nothing else, the learning curve it would take us on would undoubtedly leave us more technically sound than when we started.

The challenge

 Put simply, the client had two basic parameters:

  • An expanded PTFE (ePTFE) sealing element with a width of 20mm, a thickness of 12.5mm and a step on one corner for fitment (see picture)
  • A specific gravity within 0.35


Both these metrics were challenging. For one, making such a large cross section would involve significant load on our equipment. Making ePTFE involves stretching, which needs to be done by gripping the top and bottom of the tape and pulling it. The higher the cross section, the more the load on the pulling mechanism.

Most expanded PTFE (ePTFE) tapes have specific gravities of between 0.55-0.75. Attaining 0.35 would mean stretching at an even higher rate (more stretching means softer, less dense tape) when the cross section is already very difficult to stretch.

Furthermore, the step makes it complicated. We were not sure how the profile would remain after stretching, since the tape tends to get squeezed in order to improve the grip and stretch the tape adequately.
 

The approach

We started by making a simple rectangular profile, without the cutaway. We needed to first assess whether we could even attain 0.35 on a cross section this size, considering the load involved.

At first, it seemed fine. The final dimensions were a bit off, but the tape coming off the machine seemed really soft. We tested a small piece and found it was just under 0.35. However, when we checked the same tape a day later, we found that it felt harder. Another test showed the specific gravity had increased to nearly 0.6!

This led us to our first learning – that because PTFE has memory, it will try and revert to its original form. The stretch rate we were giving was so high that the expanded PTFE was ‘breathing back’ as it was cooling, as it tried to reduce the tension within the material.

We were able to remedy this by using special spooling restraints, that prevented this ‘breathing back’ until the ePTFE had cooled to room temperature.

The second challenge was the step itself. We did not know to what extent the stretching process would deform the step. Making a step exactly as per the drawing would not work, as there was definitely some amount of deformation to be expected. Ultimately, we had to go with trial and error here, which involved a lot of die work, as the extrusion die needed to be modified and/or scrapped to change the final dimension.

 

The result

We were told by the client that they had approached 4-5 different manufacturers before they found us. All had been defeated by this development.

The resulting product is now a part of a sealing mechanism that has the potential to revolutionise the sealing method used in the client’s industry!

Understanding the Characteristics of Turcite ®

It is only rarely that a marketing push succeeds so much that the brand name becomes synonymous with the product itself. When Turcite® B was first introduced, PTFE itself had been around for quite a while. Yet, so effective was the branding of this variant of PTFE that everything from the properties, to the composition to the colour merged under a single umbrella and became known simply as “Turcite®”. The material itself has become a mainstay in the industrial goods market with machine tool builders, re-conditioners and bearing manufacturers demanding it for their applications.

As the market expanded, new manufacturers developed their own variants under different brand names (ours being Lubring), which all succeeded in their own way. However, to this day, clients will initially demand “Turcite®” – following which there will be a brief discussion about the fact that our material is equivalent to Turcite®, but that we brand it under a different name. Usually the client is happy as long as the properties match and that the colour of the material matches the turquoise-green shade developed specifically for Turcite®.

We want to take a closer look at this material, because despite it’s widespread usage, there are always questions from clients regarding the application and installation of this material. We will look at the following aspects:

  1. What is Turcite® (Lubring)
  2. Where can it be used? What are the possibilities and limitations?
  3. How must it be installed?

 

What is Turcite® (Lubring)?

Quite simply – Turcite® is PTFE impregnated with fillers and additives that serve to enhance the wear properties of the material. It is used, most often in a sheet form, in thicknesses ranging from 0.5mm (0.02”) to 4mm (0.16”), although in some applications, it is also used as a bush and in more rare applications it is used as a thick plate.

Around the world, Turcite® is identified by its distinct turquoise (blue-green) shade. While some may mark this as another branding tactic, the pigment used is not a random choice. Studies of the wear properties across different PTFE-pigment combinations show that the specific pigment used in Turcite® (Lubring) sees a spike in the wear resistance of the material. It is clear that there was a significant amount of R&D done before the specific shade we use today was selected.

When we look at terminology, there are a few variants used around the world. Other than “Turcite”, we have had clients refer to the material as “Turkite”, “Turquite”, and “Torquite”. Clearly the original name itself has spread far enough and for long enough to spawn organic variations in different regions of the world.

Being based on PTFE, the material cannot be extruded like a normal plastic sheet and instead needs to be “skived” – the process most commonly used to make thin PTFE sheets. Also, the material will not easily adhere to other surfaces – another feature resulting from its PTFE base. Therefore a chemical etching is required on one surface of the material, so the sheet can be bonded to other articles.

In a broad sense, Turcite® (Lubring) offers the following key advantages:

  • Very low friction for reduced power loss
  • No stick-slip for positional accuracy / control
  • Good specific bearing loads
  • Low wear for long life
  • Excellent chemical resistance / fluid compatibility
  • Unlimited shelf life
  • High temperature resistance
  • Absorbs vibration during machining

 

Applications of Turcite® (Lubring)

Most commonly, Turcite® (Lubring) has been used in the machine tool industry where it serves 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.

As mentioned above, Turcite® is also used as a bush – which needs to be specially moulded and machined as per the customer’s requirements. These bushes are usually replacements for metal bushes – especially in areas where the lubrication of the metallic bush is as issue. Turcite® – and in fact all PTFE grades – has self lubricating properties which means it can function deep within a sub-assembly taking enormous wear loads and does not need to be lubricated constantly to avoid damage.

However, the material is not an out-and-out replacement for metal. Being PTFE based, the material has a compressive strength limited to 150 Kgs per square cm (2,200 psi). This means that a single square foot of Turcite® can accommodate a load of up to 150 Tonnes – which is more than sufficient for most applications. However, it is also a soft material (Shore D Hardness: 50-60) – meaning that point loads and excessive squeezing of the material can cause deformation.

Another limitation is with regards to insulation. Although the material has excellent temperature resistance (up to 260 Degrees Celsius/ 500 Fahrenheit), it does not have any electrical insulation.

In all, the industries for which we have supplied Turcite® (Lubring) include:

  • Automotives
  • Machine tool
  • Infrastructure
  • Nuclear power
  • Casting and forging
  • Textiles
  • Pumps and valves
  • Pipe liners

 

Installation guidelines for Turcite® (Lubring)

Preparation: The metal surface to be mounted with Turcite® (Lubring) can be prepared by the normal machining methods such as, grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm. Once roughened the surfaces can be cleaned with Trichloroethylene, Perchloroethylene or Acetone.

Bonding: For bonding of Turcite® (Lubring) the following resin adhesive can be used: Ciba Geigy's Araldite - Hardener - HV 953U; Araldite AW106. The Araldite should be applied both to metal and Slideway and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the surface brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Hardening: After mounting the slideway a clamping pressure of between 30-35 Kg/cm2 is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening time for various temperatures is: 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing: After curing of the adhesive, the Turcite® (Lubring) can be machined by conventional means – if required. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of Turcite® (Lubring) use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open ’stone. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment. 

Oil Grooves: Turcite® (Lubring) pads can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the Turcite® (Lubring) Slideway. Oil grooves should be away from the edges by 6mm.

Metal Mating Surface: The metallic mating surface running against the Turcite® (Lubring) pad should be preferably Stainless Steel (SS) 304 with a grade #8 mirror finish. Against this material, Turcite® (Lubring) will have a coefficient of friction of between 0.1-0.12.

 

Conclusion

The recent surge in PTFE prices has obviously had a substantial impact on the price of Turcite® (Lubring). However, owing to the ambiguity surrounding the material’s composition (many clients know it simply as “Turcite®” and are unaware that it is PTFE based), there has been genuine confusion as to why the price has increased recently. While it takes time and patience to convince clients that the price of Turcite® is governed by the price of PTFE, it does serve as yet another reminder of how an effective branding campaign can truly give a product its own identity.

Turcite® is the registered trademark of Trelleborg Sealing Solutions