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. PEEK, PEI (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), Delrin, PVDF (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.

2. 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.
   
3. 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 filled PTFE – A Useful Blend

The blending of polymers with additives is a common practice. There are very few materials that are used purely in their virgin form and PTFE is no exception here. Adding materials such a bronze, carbon and glass (to name but a few) have allowed us to augment the properties of PTFE to suit specific applications. In each case, we sacrifice some element of the original property of the PTFE, but enhance another. To take the case of bronze – the addition significantly increases the coefficient of friction of the PTFE and eliminates all electrical insulation properties. However, this is offset by a large and highly sought after increase in wear and hardness. Hence bronze filled PTFE is a preferred compound for a number of automotive and industrial applications.

In exploring what different additives do to the final properties of PTFE, we have found literature relating to materials such as those above, as well as less used additives such as molybdenum-di-sulphide, ekonol, stainless steel and graphite.

With the addition of PEEK, however, we find few sources with which to refer to on properties. While we do receive many requests for PEEK filled PTFE, the actual test data to support the compound is not easily obtained.

A 2006 paper titled: “A low friction and ultra-low wear rate PEEK/PTFE composite”, by David L. Burris, W. Gregory Sawyer, is all we have to refer to in this respect, but we will see that there are sufficient insights to help any OEM designer to assess the exact composition needed.

PTFE with PEEK fillers

We have come across a few applications where PEEK filled PTFE is the requested material. In most cases, what we receive is only a sample from the client. The light brown colour combined with the fact that the material “feels like”PTFE, is usually all we have to go with. Usually, the compound is used in sealing applications where high RPMs are involved.

Blending PEEK and PTFE

Unlike most other additives, PEEK blends with PTFE quite effortlessly. The lower particle size of PEEK (about 5microns against 25microns for PTFE) means that the grains of loose PEEK powder flow easily in between the PTFE grains and allow for a reasonably good blend. Further mixing is needed to ensure that the blend is uniform, but in our experience, it was less of a challenge to blend PEEK with PTFE than to blend pigments with PTFE.

Processing the material requires some minor fine tuning in the sintering cycle. However, when done properly, the resulting product is a very light brown that machines easily and offers some interesting properties.

Properties of PEEK filled PTFE

The paper by David L. Burris, W. Gregory Sawyer only looks into the wear and coefficient of friction of the blends of PEEK with PTFE. The paper looks at ratios (by weight) of 5%, 10%, 20%, 30%, 40%, 50% and 70%. The results obtained can be seen on the graphs below.

Coefficient of friction

Coefficient of Friction – PEEK filled PTFE

PTFE has a lower coefficient of friction than PEEK, so it would be reasonable to assume that the value keeps increasing with the addition of more PEEK. However, it is surprising to note that the coefficient is lowest at 50% of PEEK – at about 0.12.

It is important to mention than even at its lowest, the coefficient of friction is still much higher than for pure virgin PTFE(between 0.03-0.05). However, from a design standpoint, it is useful to know that adding a very small amount of PEEK is not the key to keeping the overall coefficient of friction as low as possible.

Wear resistance

Wear Resistance – PEEK filled PTFE

Again, given that virgin PEEK has better wear resistance when compared with virgin PTFE, we would assume that adding more PEEK keep improving this property. However, we again see that the best performing blend is PTFE+32% PEEK.

Conclusion

The above findings are useful from the point of view of grade selection. If an OEM wishes to design a seal using a combination that minimises the coefficient of friction and wear rates, they would be better off using a filler percentage close to 40%.

A Comparison of Dimensional Stability Among High-Temperature Polymers

In the world of engineering polymers, plastics capable of withstanding temperatures above 150°C come at a price. While Polyamides, POM (Delrin®), and PVDF (Kynar®) are all well suited to temperatures within this barrier, when we look beyond we find the options become rather expensive.

Polymers that can accommodate higher temperatures, such as PTFE, PEEK and Polyimides tend to be in the range of 3x-20x the price of lesser plastics. As a result, the cost implications of designing a system using high-temperature polymers are significant.

What do we mean by high-temperature polymer?

While the phrase seems fairly self-explanatory, high-temperature polymers need to be further evaluated to understand exactly how they behave. Usually, an OEM or product designer will look for the continuous service temperature to assure themselves that a part made using the polymer can withstand the conditions it will be subjected to.

Polymer	Common Brand Name	Glass Transition Temperature (°C)	Continuous Service Temperature (°C)	Melting Point (°C)
PTFE	Teflon®	-20	260	375
PEEK	Victrex®/Ketaspire®	150	250	340
Polyimide (PI)	Kapton®	400	450	NA
Polyetherimide (PEI)	Ultem®	220	185	250
Polysulfone (PSU)	Udel®	190	170	350
Polyphenylsulfone (PPSU)	Radel®	220	180	370

However, for the component manufacturer, the service temperature is less relevant than the melting point and the glass transition temperature of the material. This is because these are the temperatures that directly impact the production of the component – both in molding as well as machining. We will be focusing here on glass transition temperatures and trying to understand how this metric needs to be used in component design and manufacture.

What is glass transition?

Put simply, a material moves from crystalline to amorphous states beyond its glass transition temperature. All polymers, when in a crystalline state, have internal stresses that keep it dimensionally stable. These stresses are a culmination of the inherent molecular arrangement of the molded shape and further stresses lent to it during the machining stage. Heating the part above the glass transition point causes the molecules to realign, thereby relieving the stresses and causing dimensional changes to the part. As stress due to machining can be significant, most polymers are subjected to an annealing cycle prior to machining, to ensure that the stress build up does not cause the part to crack during the process. Polymers such as PEEK will crack under so little as a simple turning operation of not annealed beforehand.

The stresses are very relevant for machined components, as it ensures that machined parts subjected to temperatures within their glass transition point will not deviate dimensionally. However, it is equally true that in the event of higher temperatures, the deviation may result in part failure. This is typically the case for highly machined components.

Consideration for Dimensional Stability

Our experience with dimensional stability rests around the use of PTFE and PEEK. Both polymers behave very differently both during machining and after. We shall look at them one by one.

PTFE

Among high temperature polymers, PTFE is unique in that it has a glass transition temperature under 0°C. The implication of this is that PTFE is generally amorphous even at room temperature and therefore does not suffer the internal stresses that other polymers do. As a result, PTFE typically does not require annealing, although it is still done as a means to improve the hardness of the material. No internal stresses mean that the material undergoes minimal duress during machining and any cracking of the machined part is avoided.

The flip side of this property is that PTFE has a very weak dimensional stability when subject to applications where a high range in temperatures may be present. While PTFE can easily withstand high temperatures, close tolerances would need to be abandoned when subjecting it to these conditions as the material itself experiences an up to 3% deviation in linear dimensions between 0 and 100°C.

So although PTFE is capable of surviving the harshest of environments, a PTFE part machined with close tolerances is usually employed only in areas where the temperature, while high, must remain range bound within +/-15°C.

PEEK

In contrast to PTFE, achieving close dimensional tolerances in PEEK and difficult due to the constant build up of stress during machining. In our own experience, PEEK parts may sometimes need to be annealed multiple times to ensure that after each stage of machining, the internal stresses are adequately relieved so that the part does not crack/deform after the next stage.

Unlike PTFE, which constantly gives off heat as it is applied to it, PEEK needs external help in cooling it down. As a result, the use of a coolant is common in PEEK machining and helps reduce the extent of stress induced in the part.

Finally, while close tolerances of up to +/-0.01mm have been achieved on PEEK parts, there is no guarantee these tolerances will be retained should the part be subjected to a temperature above its glass transition point during application. In such an event, stresses induced during the final operation of machining will relieve themselves and cause the molecules within the PEEK material to realign slightly, causing dimensional deviations in the part.

So given the above hazards, why are PTFE and PEEK still so widely used? One reason is that there exist very few applications where strict dimensional stability in temperatures above 200°C are a co-requisite. Hence, we have applications of high temperature where the dimensional tolerances tend to be very lax and we have applications with tight machining tolerances, where the part may experience a maximum temperature of only 150-160°C.

PEEK Seals – Numerous Applications, Many Choices

As a polymer, PEEK is most often compared with PTFE. The two have multiple similarities including good temperature resistance, chemical inertness and dielectric strength. When it comes to pure physical strength however, PEEK moves ahead on two counts.

First – the absolute strength of the material is much higher. With a higher tensile strength and hardness, PEEK is preferred to PTFE in applications where dimensional stability over prolonged physical strain is required. Although PTFE does have fillers, such as glass and carbon, which allow for increased stiffness, it still does not compare with PEEK on this metric.

Second – PEEK has a lower specific gravity (1.35 against 2.25 for PTFE). As a result, in applications where the overall weight of the assembly needs to be minimized, PEEK emerges a winner.

One such application where PEEK is highly sought after is in the seals industry. Seals themselves include a huge range of polymers, elastomers and metals, each of which rely on the specific characteristics of the material being used to achieve effectiveness in its application.

Types of PEEK seals

Piston Ring Seals

Piston rings are used primarily to aid wear absorption on the outer diameter of the piston shaft. PEEK is hard enough to withstand the extensive wear induced within the piston, but not hard enough to damage the metal components themselves. The rings are usually machined from a PEEK bush and have different types of cuts, which aid in installation and performance.

Ball Valve Seats

Ball valve seats show a predominant preference for PTFE, as they require a soft material that yields easily to the shape of the ball valve. However, there are a significant number of PEEK seats being used in high-performance valves, where both the PTFE and the metal are machined to ensure a proper fit. Typically, we see these being used in valves employed on oil-rigs or power plants, where the high temperatures indicate a requirement for a polymer slightly tougher than PTFE.

Rotary Shaft Seals

We have developed compounded grades of PTFE with PEEK to cater to the rotary shaft seals market. The combination of PTFE and PEEK is a powerful one. The PTFE provides a boost to the self-lubrication properties, while the PEEK adds strength. Although they work well together, specific applications do call for pure PEEK. The purpose is similar to that of the piston ring, except here the shaft moves radially. PEEK again serves the purpose of being able to withstand wear at high RPMs, while being soft enough not to damage the metal in the event of misalignment or seal failure.

Ball and Butterfly Valve Seats

A number of different materials are used in this application, including PTFE, Delrin and UHMWPE. PEEK finds acceptance specifically in applications with high pressures and temperatures. Butterfly valves are an integral part of any fluid regulatory system, including hydroelectric power plants, oil and gas refineries and shipping.

Manufacturing process

PEEK seals and seats are made primarily via machining. It is possible to injection mould the components directly, but this involves extensive tooling. Furthermore, the precision needed on the part’s dimensions would dictate the need for further machining. Hence, unless the volumes are vast, it is most likely machined from a bush.

The bush itself may be either extruded or compression moulded. Extrusion offers higher productivity and longer length parts, but is again dependent on the correct type of tooling being available. Compression moulding is cost effective and allows the dies to be modified easily, so that the moulded part is made with minimal excess material (a very key criterion when dealing with an expensive material like PEEK). The issue with compression moulding is that it is a slow process with very limited productivity.

So looking at the trade-off between productivity and tooling cost, an OEM can accordingly decide which method to adopt, depending on the volumes.

Variants in PEEK

While most specifications call for pure, virgin (unfilled) PEEK, there are requirements for filled variants also. Most commonly, PEEK is used with a 30% Glass or Carbon filler to aid properties such as creep, dimensional stability and flexural strength.

As mentioned above, PEEK also does well with PTFE. More specifically, compression moulding best-practices sometimes recommend the addition of 5% PTFE into the PEEK mould, as this allows for better self lubrication of the material, while letting it maintain its superior strength.

Another polymer well suited to blending with PEEK is Polyimide. Although the blend is not nearly as proven as the regular filled variants, initial studies show that the addition of Polyimide allows PEEK to maintain its flexural modulus over a much high temperature range as against unfilled PEEK.

It is difficult to combine too many other polymers with PEEK, simply because the temperatures needed to process PEEK far exceed the melting points of most of these polymers.

A word on PEK

PEK or PAEK has recently emerged as a competitor to PEEK. Industry experts have observed that while PEK does match PEEK on most metrics, it’s long-term effectiveness in maintaining its properties is still being tested.

We recently received a failed seal from an OEM, asking us to analyse whether it was PEEK. After testing it in a lab, it was found that the part was made using PEK. The end-user claimed that the part had only survived a few months in his valve assembly, before failing. This may have been a one-off incident, or could also point to the improper processing of the PEK part. However, it is useful to keep in mind.

Conclusion

PEEK is well known as a versatile polymer. Seals and seats are one more application where this material finds application. The product, however, requires precise dimensional tolerances that not all processors are able to offer. In addition to this, the availability of variants both within PEEK and amongst competing polymers makes the choice of material an exercise that the OEM must take very seriously, before committing one way or another.

PTFE vs PEEK – A Comparison of Properties

Although both PTFE and PEEK are well established within their respective fields, there are frequently questions around which would better suit a given application. OEMs typically have to make a choice based on technical suitability and hence need to be better informed as to how these materials match up against each other.

Below is a short comparison on properties between these two polymers and can be used a guide to aid new product development.

Parameter	PTFE	PEEK	Preferred material
Price	Moderately expensive	Very expensive	PTFE
Tensile Strength	25-35 Mpa	90-100 Mpa	PEEK
Elongation	350-400%	30-40%	PTFE
Compressive Strength	30-40 Mpa	140 Mpa	PEEK
Flexural Modulus	495 Mpa	3900 Mpa	PEEK
Coefficient of Friction	0.03-0.05	0.35-0.45	PTFE
Temperature resistance	Up to 250°C	Up to 250°C	NA
Dielectric strength	50-150 Kv/mm	50 Kv/mm	PTFE
Chemical resistance	Virtually inert	Affected by Sulphuric acid	PTFE
Coefficient of linear thermal expansion	14 x 10-5/K	5 x 10-5/K	PEEK
Machine-ability	Good	Very good	PEEK

In a nutshell, applications requiring strength and low levels of deformation would usually employ PEEK, whereas those requiring resistance to voltage or chemicals utilize PTFE.PTFE also rates highly in that it is self-lubricating. This makes it a preferred choice in high wear applications.

The biggest disadvantage of PEEK remains the price. It is roughly 10 times the price of PTFE and as a result has remained a niche polymer, used only in applications where it is absolutely necessary.