As a standalone material with no filler reinforcements, PEEK (PolyEtherEtherKetone) comfortably holds its own as one of the most robust polymers around. With the exception of Polyimide (aka Kapton), PEEK has no viable rivals on pure strength and high-temperature capabilities. A specific gravity of only 1.3 makes it a darling of the aerospace industry, while a chemical inertness that nearly matches that of PTFE makes it highly sought after in nuclear applications and refineries.

That said, there is always room for improvement. The same fillers that give materials such as PTFE or PPS enhanced properties also work well on PEEK. As such, with the aim being to maximise strength and durability, carbon is a key filler for PEEK.

Carbon (Coke) Powder vs Carbon Fibre

Before we proceed, a distinction must first be made between powdered carbon – which is effectively finely ground coal, and carbon fibre – which is a result of milling more complex carbon materials to yield miniscule, high-strength strands that go on to reinforce the polymer into which they are blended. While carbon fibre is far more expensive than conventional carbon powder, its impact too exceeds that of its poorer cousin by a significant margin.

Thus, when we refer to carbon filled PEEK, it should be understood here that we are speaking specifically about carbon fibre.

PEEK, PEEK+Carbon, and HPV PEEK

In this article, we will primarily be comparing three different grades.

Unfilled PEEK is the polymer in its virgin form. As stated above, even in this form – where the PEEK takes an even tan colour – the material is robust and capable of withstanding high loads and temperature.

PEEK+Carbon or Carbon-filled-PEEK is usually a blend of PEEK and either 15% or 30% carbon. For the purpose of this comparison, we will be looking at PEEK+30% carbon (CF30), as we have accurate data for the same.

PEEK HPV material is a special blend of PEEK popularised by the brand Ketron (Quadrant). HPV PEEK was designed to have a composition of 70% PEEK, 10% carbon, 10% graphite, and 10% PTFE. This unique blend offers a large boost to the strength of the base material – thanks to the carbon reinforcement – while also bringing better wear and friction properties due to the graphite and PTFE. HPV PEEK – also referred to as bearing grade PEEK – is both difficult to blend and prohibitively expensive in comparison to regular PEEK. However, given its fantastic properties, most OEMs are happy to pay the price.

Comparing Properties

As you can see from the table below, the addition of carbon and other fillers has an appreciable impact on the properties of the material. 

Apart from an increase in tensile strength of ~35%-207% for HPV and CF30 respectively, there are increases of anywhere from over 300% to over 500% on other metrics such as Young’s Modulus, Flexural Modulus, and Flexural Strength.

In addition to pure strength, what can also be observed are lower coefficients of thermal expansion and higher deflection temperatures under load – both indicative of a more dimensionally stable material in high-temperature applications.

HPV – which has a lower coefficient of friction and can go as low as 0.05 under lubricated conditions – is ideal for wear applications and dry-running applications.

	Unfilled PEEK	PEEK+30% Carbon	PEEK HPV	Unit	Test
Tensile Strength	97	201	133	Mpa	ASTM D638
Young's Modulus	3650	19700	11000	Mpa	ASTM D638
Flexural Modulus	3860	17500	10500	Mpa	ASTM D790
Flexural Strength	152	317	221	Mpa	ASTM D790
Coefficient of Linear Thermal Expansion	4.3 x 10-5	5.2 x 10-6	2.2 x 10-5	cm/cm/°C	ASTM E831
Deflection Temperature Under Load	162	315	291	°C	ASTM D648
Coefficient of Friction	0.35	-	0.25		ASTM D3702

Machining properties

As manufactures of high-precision machined components, a lot of our experience ultimately ends with understanding how well the part adheres to close dimensional tolerances.

Both bearing grade PEEK and Carbon reinforced PEEK are highly stable both during and post-machining. While special tools are needed to handle the material, tolerances of as low as 10 microns can be achieved on the machined parts. Further, the low coefficients of expansion allow for parts that are machined in one part of the world to travel elsewhere with no loss of dimensional integrity due to changing climates and environments.

Conclusion

While Carbon filled PEEK (CF30) is clearly the winner on pure strength, bearing grade PEEK (HPV) forms a decent middle ground between PEEK and CF30 and is an excellent choice for bearing applications. While it has traditionally been an expensive grade, in-house blending techniques have allowed for significant cost optimisations, allowing the price to rest only slightly above that of unfilled PEEK.

Whatever the requirement, it is evident that any application where strength is a key criteria would benefit from the reinforcement of PEEK with carbon.

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Read More

1. PEEK Components and Bearings - Durable, lightweight, and dependable

2. PEEK Seals - Numerous Applications, Many Choices

3. PTFE vs PEEK - A Comparison of Properties

With new developments being constantly introduced in the high-performance polymer space, exciting new products are always entering the market. However, as the scale remains low, most new plastics remain prohibitively expensive but for the niche applications for which they may have been created. Through all this, the erstwhile stalwarts - PTFE and PEEK - have retained much of their effectiveness as increased scale and breadth of application has allowed them to become more cost-effective and compete with existing medium-performance polymers on high-volume parts.

Apart from PTFE and PEEK, it is also important to look at FEP and PFA. Both these high performance polymer variants were an offshoot of PTFE. Indeed, few realise that the trade name “Teflon”, which is used so interchangeably with PTFE does in fact cover PFA and FEP as well.

The reason for developing PFA and FEP was quite simple: PTFE has a very low melt flow and hence cannot be injection moulded. This limitation makes PTFE a material that can only be machined, which in turn makes complex parts and high-volume parts a difficult prospect when using PTFE. FEP and PFA both have lower melting points and have melt flows which allow for injection moulding. However, it is important to note that in this trade-off, both polymers surrender various properties, making PTFE the superior material in terms of absolute performance and versatility.

The below table offers some key comparisons between these four polymers, in order to offer an understanding of each one’s advantages, disadvantages, and applications.

 

About Polymer Bearings

In an already crowded bearing space, polymer bearings have made their mark for a host of different reasons. As a result, an area that was once dominated by steel and phosphor bronze is increasingly giving way to high performance polymers such as PTFE, PEEK, POM, and Nylons, where the sheer breadth of grades and fillers allows for a whole range of properties tailored to match the end-application and offer a solution that far exceeds what metal bearings were able to hitherto provide.

The advantages and disadvantages of polymer bearings against metals can be shown on the chart below:

Polymer Bearing
Advantages	Disadvantages
Lightweight	Limited load capability
Fully customisable	Can be expensive
Self-lubricating	Limiter temperature range
Easy to replace

 

As shown above, metallic bearings are typically preferred where the loads and possibly the temperatures are much higher. Here too, however, certain polymers such as PEEK and Polyimide (Kapton), can bear enormous loads and remain functional in temperatures of 300°C+. However, such polymers come at a price and are therefore limited in applications such as aerospace and medical, where cost may not be a key criterion.

However, for many applications, polymer bearings find that their advantages are highly sought after. Key among this is the ability to self-lubricate. Self-lubricating polymers such as PTFE, POM, and UHMWPE - to name just a few – offer dry-running capabilities which greatly reduce the need for external lubrication. This is especially valuable in consumer goods, where the structure of the device or appliance is such that the user will not have access to the moving parts. Similarly, in certain industrial applications, self-lubrication ensures minimal down time and greatly reduces the wear and load due to the build-up of friction.

Types of Polymer Bearings

Polymer bearings come in various shapes and sizes and can be either machined from a drawing or reverse-engineered from an existing part. Some of the typical bearings offered by Poly Fluoro Ltd. include:

1. Flange bearings
Flange bearings are designed to handle both axial and radial loads. In some designs the flange is also used as a locating mechanism to hold the sleeve in place.

Flange bearings can be machined either from stock rods or moulded. Polymer grades used would include PTFE (usually with a glass or bronze filling), PEEK (virgin or carbon filled), PPS (usually with a glass filling), and POM.

Flange bearings require a little more machining to the housing but can solve the unique load conditions of a shaft and some type of thrust surface.
 

2. Mounted bearings
Mounted bearings are machined with a double flange in order to sit within a pillow block. These bearings can be fabricated using several different plastic bearing materials to improve wear and reduce or eliminate lubrication.
 

3. Thrust bearings
Put simply, thrust bearings are washers made from any number of materials such as PTFE, PEEK, PPS, POM, Nylons, or Polyimides. They are generally thin, easy to install and prevent metal on metal contact in any thrust load conditions. They are easy to use and do not require lubrication of any kind in most conditions.

Although the design is simple, there is a need to machine the part so that the surfaces are perfectly parallel. This is where Poly Fluoro excels.
 

4. Sleeve bearings
These are the most common bearings, with a simple ID, OD, and length. However, as with the washers, care needs to be taken to ensure the tolerances are tight. Where most manufacturers would only offer a 100 Micron tolerance on linear dimensions, Poly Fluoro is able to go down to as low as 10 Microns in some cases.

The bearings are designed to carry linear, oscillating, or rotating shafts. The key to successfully designing a plastic sleeve bearing is paying attention to temperature, P, V and PV ratings for the material and match it with your application.
 

5. Spherical bearings
Spherical bearings are designed to allow for shaft misalignment, as they can rotate in two directions. Spherical bearings typically support a rotating shaft in the bore that calls for both rotational and angular movement.

Using self-lubricating polymers with very low static coefficients of friction, Poly Fluoro is able to ensure that even minor variations in alignment are immediately accommodated by the bearing to allow for non-stop performance.

While the above bearings are most common, application engineers are constantly finding new areas in which to apply the bearing properties of polymers. Ultimately, any application with repeated motion will benefit from a polymer bearing as it offers an unmatched ability to reduce wear and friction over a very long period of running time.

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