Metal Replacement with PEEK

There is virtually no industry that does not strive for reductions in weight of the final product. In some cases, higher weight implies higher costs of the product itself; in some, weight reduction leads to improved efficiencies of the equipment.

Advancements in polymer science over the past few decades have allowed us to develop formulations that come close to mimicking the strengths offered by metals. While it is fair to say that on average, the strength of a polymer lags significantly against the toughness of metals, there are several applications where the polymer offers more than adequate strength, whilst bringing a host of other benefits that make it a more viable option.

We were recently approached by a client inquiring whether PEEK parts could replace the existing metal parts being used. This led to some research as to the compositions that would be needed to augment the tensile and yield strengths of PEEK beyond what it offered in its natural (virgin) state.

PEEK is a crystalline material that ranks as having amongst the highest tensile strengths within the polymer space. It is also a light-weight polymer, which makes it highly sought after in applications where weight reduction is crucial. In addition to this, the introduction of certain fillers allows the strength of PEEK to be enhanced significantly.

	SS 304	Virgin PEEK	Reinforced PEEK	Units
Ultimate Tensile Strength	515	95	586	MPa
Specific Gravity	8	1.3	1.52
Youngs Modulus	190	3.65	57	GPa
Elongation at Break	55	60	1.6	%
Service Temperature	900	300	300	°C
Cost Index (SS=1)	1	32	20
Adjusted Cost over SS	–	5.2	3.8

 

The table above shows the improvements that can be made when adding reinforcements to PEEK when compared to Stainless Steel 304.

1. It is apparent that we can exceed the tensile strength of steel, although this comes at the cost of very low elongation.
2. The Young’s Modulus (or Tensile Modulus) of stainless steel remains significantly higher than that of reinforced PEEK. When combined with the data on elongation, this tells us that steel still has a higher tolerance to loads and will deform when the loads increase. Reinforced PEEK, however, will most likely crack when pressures of over 57 Gpa are applied
3. At elevated temperatures, PEEK would still lag stainless steel, or indeed most metals. However, with a temperature rating of 300°C, PEEK is still sufficiently capable of handling most industrial applications
4. PEEK is over 30 times the cost of stainless steel. Even with reinforcement – which brings the price of PEEK lower as the fillers are not as expensive as virgin PEEK – the cost is 20 times that of steel. However, when you factor in the reduction in densities, the numbers fall to 5.2x and 3.8x respectively
   The implication of this however is that replacing steel with PEEK would bring a four-fold cost increase. Hence the decision to replace cannot be based on cost savings.
5. In addition to the basic properties listed above, PEEK offers a range of other benefits such as lower coefficients of friction, lower thermal conductivity, higher resistances to chemicals and electrical insulation.

The above data tells us why, while PEEK can certainly match up to stainless steel in some regards, the replacement is done in only very specialised fields.

• Aerospace
  
  PEEK is used extensively in this industry, replacing steel and even aluminium in many cases. Since the cost of replacement is one-time, but the benefits if weight reduction are continuous, PEEK is the preferred material of choice in aerospace.

PEEK is also a safer material to use, since its low conductivity allows for lower heat build-up. Furthermore, if a part does break off and/or release debris – the resulting damage is minimised as the material is not hard enough to jam other metallic moving parts.

• Healthcare and Pharma

Being chemically inert, PEEK is preferred to metals in areas where chemicals may be present and the reaction to metals may not be fully understood or predictable.

Medical devices may also use PEEK as the equipment may benefit from being light weight. Certain scanning devices shun metallic parts, as they may affect the readings. Here again, PEEK is used to ensure the device is physically durable but does not have too much metal in its structure

• Oil and Gas

PEEK is tough enough to withstand some of the harshest forces, while offering a resistance to corrosive fluids and high temperatures. In most cases, oil & gas equipment is placed deep underground, where the forces and chemicals are not fully predictable.  Oil and gas equipment frequently employs PEEK in seals and valve plates to ensure that there is no failure once the equipment is installed.

• Precision Instrumentation

PEEK offers an elevated level of structural rigidity, while providing electrical insulation. Connector covers, transducer covers, and cable channels are made using PEEK because it does not interfere with the electrical signals being passed through the wires within the device. Again, as weight reduction is a key criterion in this industry, PEEK is preferred even though it is expensive.

• Food processing

Several factors make PEEK the material of choice in food processing. For one, the material does not react with or impart any flavour to the food. For example – coffee machines that traditionally used aluminium valves replaced them with PEEK, because the aluminium imparts a metallic taste to the coffee. In addition to this, at high RPMs, PEEK offers minimal heat build-up. In food processing appliances, where the body may be made with a low-temperature plastic and the designs tend to be compact, it is essential that moving parts do not become so hot as to warp/damage the body of the equipment. Finally, since liquids are often present in food preparation, PEEK is preferred as it is less likely to corrode and/or cause an electrical short circuit within the device.

It is apparent that PEEK is not a low-cost alternative to metals. However, applications that look for efficiency and performance enhancements can benefit tremendously from using PEEK and not worry that its strength would not be sufficient. Since the replacement of metal with PEEK is a one-time expense in most cases, the consequent savings from the improved efficiency of the system would need to be weighed in to get a true measure of the overall cost savings.