Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Case Study - Development of a 4-axis PEEK Valve

At Poly Fluoro Ltd, we have always tried to abide by the rule: Build capability, not just capacity.

In a world where manufacturers are constantly looking to scale up operations as quickly as possible, this may sound rather counterintuitive. Scale allows for many benefits, such as lower input costs and allowing fixed expenses to be spread more thinly.

However, when it comes to niche polymers, scale is never guaranteed. In most cases, the demand for complex machined polymer components only runs into the thousands. With limited scale, the challenge becomes one of achieving higher realisations for the same time spent. In this endeavour, adding capabilities helps a company move up the value chain and scale up on value, even while volumes may remain small.

A while back, we engaged in the development of a very complex valve made from PEEK. By the analysis done by our engineering, we knew the part required a 5-axis milling machine and even though we really wanted to add the part to our portfolio, our milling machines only had 3-axis functions. We therefore needed to build this capability.

We initially stepped out to see whether a vendor could take the part up for us. However, every vendor we spoke with either gave up upon seeing the drawing (the part is very complex!) or said that they did not have capacity to take up the job. We were intrigued. Not only was the part truly a challenge – which is something we love – but the lack of capacity in the market meant that acquiring our own equipment was probably a good idea in the long run.

However, given that a 5-axis milling machine can be rather expensive, we were hesitant to jump into something requiring such a large investment, when the value of the order in question was small in comparison. We then came across a new equipment vendor offering 5-axis machines at a fraction of the cost that was being asked by the larger manufacturers. We gave them the part to develop, assuring them that if successful, we would pay the advance on a new machine immediately.

Sadly, the PEEK valve was tougher than they anticipated (I did say it was complex) and after struggling for a few weeks, they abandoned the project with nothing to show. We were back to square one.

We sat down to review the part and – in what would surely be a major loss of face – decide how we were going to inform the client that we had failed and that we could not take the development further. During our review, it was commented that in truth, we only needed a 4th axis to give us the extra dimension needed to machine the part. This caused us to stop and think. We already had 3-axis machines; what would it cost to add only one more axis? Was such an extension even possible?

After speaking with our existing machine’s vendor, we were excited to learn that they had a 4th axis attachment suited to our equipment. What’s more, it would cost a third of what we would have paid to the supplier to whom we had given our part for development. 

Within two weeks the new attachment was in place, although the part’s development still took a few weeks more (did I mention the part is very complex?).

Our obsession with making the part led us down a path of discovery that culminated in a successful outcome. However, in pushing for this outcome, we managed to add a 4-axis capability to our machining repertoire, giving us the option to take on new parts that we may have otherwise had to regret. It is this determination to constantly push what seems infeasible that leads us into newer, more interesting avenues of our operations.

And if we do come across a part more complex than what we can handle? Well, there’s always that elusive 5th axis we still need to get!


Read More

1. PEEK - The Impact of Carbon Fibre Fillers on HPV Bearing Grades

2. PEEK - Robust Enough for Nuclear Applications

3. The Effectiveness of PEEK Compressor Valve Plates

PEEK - The Impact of Carbon Fibre Fillers on HPV Bearing Grades

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 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.

HPV PEEK 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 – 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 HPV and CF30 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 CF30 is clearly the winner on pure strength, 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 addition of carbon to PEEK.


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

A Comparison of High-Performance Polymers

With new developments being constantly introduced in the 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 breath 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 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.

 

Name of the Polymer   POLYETHER -ETHERKETONE POLYTETRAFLUOROETHYLENE POLYTETRAFLUOROETHYLENE GF 25 % PERFLUOROALKOXY ETHYLENE PERFLUOROALKOXY ETHYLENE GF 20 % FLUORINATED ETHYLENE - PROPYLENE FLUORINATED ETHYLENE - PROPYLENE 20 % GF
Trade name/ Typical name   PEEK PTFE PTFE 25 % GF PFA PFA 20 % GF FEP FEP20 % GF
Type of the Polymer   Thermoplastic Thermoplastic-Thermoset Thermoplastic Thermoplastic Thermoplastic Thermoplastic Thermoplastic
Advantages   PEEK is a high performance thermoplastic with the characteristics common to this group - strong, stiff, hard, high temperature resistance, good chemical resistance , inherently low flammability and smoke emission. It is pale amber in colour and usually semi-crystalline and opaque, except thin films are usually amorphous and transparent. It also has very good resistance to wear, dynamic fatigue and radiation Outstanding chemical resistance. Low coefficient of friction. High continuous use temp. 180 Cº . Very high Oxygen index. Higher modulus and surface hardness than PTFE. Improved creep resistance, dimensional stability and wear compared with PTFE. Melt processable, has similar chemical resistance to PTFE combined with the highest temperature resistance of melt processable fluoro plastics. Self-extinguishing. Retains room temperature stiffness and strength at elevated temperatures better than FEP. Excellent toughness. Significant increase in HDT and moderate increase in tensile strength compared with unmodified grades of PFA. Very high impact strength. Excellent high frequency electrical properties. Melt processable. Good weathering resistance. Significantly increased tensile strength, HDT and flexural modulus compared with unmodified grades of FEP. The mechanical properties of moulded components can be anisotropic.
Disadvantages   It is difficult to process and very expensive. Low strength and stiffness. Cannot be melt processed. Poor radiation resistance. Lower impact strength, lower tensile strength and more expensive than unmodified PTFE. Stiffness and strength similar to those of PTFE at room temperature. More expensive than PTFE. Decreased elongation at break and notched izod impact strength compared with unmodified grades of PFA. Very expensive, with the lowest strength and stiffness of all the fluoro plastics. Low HDT at c 50°C ( 120°F ) accompanied by poor wear resistance. Elongation at break and notched izod impact strength are reduced compared with unmodified FEP. The mechanical properties of moulded components can be anisotropic.
Applications   Applications include flexible printed circuit boards (film), fibres and monofilaments, injection moulded engineering components and items used in aerospace and radiation environments. Filled grades, including ones designed for bearing-type applications, are also used. Bearings, Chemical vessels linings, pipe and valve linings, gaskets, diapharms, piston rings, high temp. electrical insulation. As a coating of non stick applications. Wear pads, piston rings, and microwave oven rotating platforms. Heater cables, chemically resistant linings for pumps and pipes etc. that require a higher temperature resistance. Chemical plants. Coatings, protective linings, chemical apparatus, wire coverings, glazing film for solar panels. Valves, electrical components and equipment for chemical plants.
PROPERTIES UNIT              
Density g/cm³ 1.26 - 1.32 2.15 2.25 1.6 2.2400000000000002 2.1 2.2000000000000002
Surface Hardness RR M 99 [Rockwell] SD 63 SD72 SD60 SD 68 RR45 RR65
Tensile Strength Mpa 70-100 25 17 29 33 14 40
Flexural Modulus Gpa   0.7 1 0.7 0.7 0.6 5.5
Notched Izod Imapact strength kJ/m 0.85 0.16 0.12 A.06+ 0.7 1.06+ 0.2
Linear Expansion /Cº x 10?5   15 12 21 13.5 5 5
Elongation at Break % 50 400 250 300 4 150 2.5
Strain at Yield %   70 N/Y 85 N/Y 6 N/A
Max. Operating Temp. 250 180 180 170 170 150 150
Water Absorption % 0.1 - 0.3 0.01 0.01 0.03 0.04 0.01 0.01
Oxygen Index % 35 95 95 95 95 95 95
Flammability UL94 V 0 @ 1.5 mm V0 V0 V0 V0 V0 V0
Volume Resistivity log ohm.cm 10¹5-10¹7 18 15 18 18 18 14
Dielectric Strength MV/m 19 @ 50 ?m 45 40 45 40 50 13
Dissipation Factor 1kHz 2.9999999999999997E-4 1E-4 3.0000000000000001E-3 2.0000000000000001E-4 1E-3 2.0000000000000001E-4 5.0000000000000001E-4
Dielectric Constant 1kHz 3.2-3.3 @ 50Hz-10Khz 2.1 2.8 2.1 2.9 2.1 2.5
HDT @ 0.45 Mpa › 260 121 125 74 160 70 260
HDT @ 1.80 Mpa 160 54 110 30 150 50 158
Material Drying hours @ Cº 4-6 HOURS @ 200° NA NA NI NA NA NA
Melting Temp. Range 360-420 NA NA 360-420 360-420 340-360 350-380
Mould Shrinkage % 0.8 - 1.5 NA NA 4 0.8 2.5 0.4
Mould Temp. Range 175 - 200 NA NA 50-250 50-250 50-200 50-200