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.

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.

Delrin – the machinist’s best friend

In a world of specialized plastics requiring immense tensile strength or high wear resistance or minimal coefficient of friction, Delrin holds its own against the more versatile polymers such as PEEK and PTFE.

Our own experience with Delrin began with the PTFE price increases in 2010-2011, as we scrambled to find substitutes for PTFE to offer clients, without compromising too much on properties. As we have already stated in earlier articles – finding a true substitute for PTFE was futile. However, despite our attempts to push UHMWPE and PA66 as replacements (materials we were more familiar with), it was ultimately Delrin which clients were most comfortable in adopting.

What is Delrin?

Delrin (brand name of DuPont) is also commonly referred to as POM (Polyoxymethylene), polyacetal, or simply acetal. The names all refer to a polymer that is characterized by a high tensile strength, high stiffness, low coefficient of friction and excellent dimensional stability. In addition to its properties – Delrin is a relatively inexpensive material compared to PTFE and even PA66. This makes it a sought after choice in machined component development – as the parts are dimensionally very stable and significant trials can be done without being too expensive.

Properties of Delrin

• Delrin is characterized by its high strength, hardness and rigidity to ~40 °C
• In its natural form, it is a white (opaque) plastic, although it is easily pigmented and often available in a variety of colors
• Delrin has a specific gravity of 1.410-1.420 g/cm3
• As a homopolymer it is 75-85% crystalline with a melting point of 175°C, while as a copolymer has a slightly lower melting point of 165–175°C
• It has a relatively low coefficient of friction of 0.2 – much higher than PTFE, but still suitable for a wide number of engineering applications
• Delrin is resistant to a wide variety of chemicals including alcohols, aldehydes, esters, ethers, hydrocarbons, agricultural chemicals, and many weak acids and bases. This ability is even more impressive when we consider that even under harsh chemical environments, Delrin does not lose its dimensional stability
• Electrically, Delrin rates slightly below PTFE, but is nonetheless a very useful substitute. Its dielectric constant (~3.5) is only slightly higher than PTFE (~2)

Advantages of Delrin:

• High mechanical strength and rigidity
• Toughness and high resistance to repeated impacts
• Long-term fatigue endurance
• Excellent resistance to moisture, gasoline, solvents, and many other neutral chemicals
• Excellent dimensional stability
• Good resilience and resistance to creep
• Natural lubricity
• Wide end-use temperature range
• Good electrical insulating characteristics

Due to its versatility, Delrin finds uses in a number of applications including:

• Automotives
• Industrial equipments
• Consumer goods
• Medical equipments
• Electrical equipments

Machining Delrin

As a machined item, Delrin is particularly easy to work with. With PTFE, we need to consider the softness of the material and also its sensitivity to temperatures, with nylons and UHMWPE, we need to be careful of the part melting during machining, with PEEK, the tool itself can break, if we do not control the RPM. However, Delrin is surprisingly accommodating as the part retains its stiffness, but is still soft enough that the tool is able to work through the plastic. In addition, the dimensional stability post machining is also excellent. While we have had instances of PTFE parts being under tolerance when shipped to colder climates, the same is not an issue with Delrin parts.