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If you deal in polymers and have not come
across PEEK – it’s probably because its one of those materials which does not
surface unless really needed. When it is needed – there’s little else that can
be used in it’s place and this often confuses OEMs; because even among
expensive, high-end engineering polymers PEEK sits at a price point that causes
the client no small amount of shock.
It is important to talk about the price of PEEK
before all it’s other characteristics, as this is usually the first thing the
client want to discuss. Invariably, they come knowing that they need this
polymer (PEEK), but knowing little else. They expect the price to be similar to
Polyacetal or, at the very worst PTFE. When they find out that it is close to
10 times the price of PTFE, it comes as a huge surprise.
Why PEEK is expensive is not fully known.
Perhaps it is because it has not yet reached the global scale of manufacture of
more commoditized polymers, or perhaps the technology is so unique that it
allows resin suppliers to charge a huge premium – knowing that alternatives are
not available. As processors, we know only so much:
- The resin is 5-8 times more expensive than PTFE
- Processing PEEK is time consuming and expensive in comparison to PTFE
- Machining PEEK is tricky in comparison to other polymers
Since the resin prices are not in our control,
we would like to look at points 2 and 3 and discuss them in more depth. But
first, let’s get a better idea of what PEEK offers.
High tensile strength
In the polymer space, it would be tough to find
something tougher than PEEK. It is so strong, in fact, that machining
guidelines for PEEK need to follow the same as those for metals.
This strength allows PEEK to be used in
applications such as gasketing and auto components – especially where metals
cannot be used, but a metal-like durability is required
High temperature resistance
PEEK melts at about 400 Degrees Celsius and is
capable of running in environments of 300-325 Degrees without deforming. While
PTFE can withstand up to 250 Degrees, any pressure/ load on PTFE at this
temperature will invariably cause deformation. In the case of PEEK, its
hardness allows it to be in a high-load-high-temperature environment without
loss of dimensional properties.
High wear resistance
Again, while both PTFE and UHMWPE can take a
significant amount of wear, PEEK exhibits a high PV value and can withstand
wear effects even under harsh physical and chemical conditions.
Chemical resistance
While not on the same level as PTFE for pure
chemical inertness, PEEK exhibits resistance to many harsh chemicals, allowing
it to be used in corrosive environments, under heavy loads
In a nutshell, PEEK’s ability to stay
dimensionally stable under harsh environments makes it a highly sought after
polymer. OEMs who use PEEK do so knowing well that for the properties offered,
PEEK is unique and therefore expensive.
Processing PEEK
We will not delve very deep into the processing
of PEEK (as this is a proprietary process unique to each processor), but we
will point out the key differences between PEEK and PTFE processing (which has
been looked at earlier). It should be noted that here we are referring only to
compression moulding, and not injection moulding.
The main difference is that while PTFE is cold
compression moulded and then loaded in batches into a sintering oven, PEEK
needs to be sintered during compression itself. Furthermore, post sintering, PEEK needs to go through an
annealing process, which is time consuming. This leads to a few complications:
- Batch processing is difficult. Since the total heating cycle for a single piece can take up to 8 hours, and since heaters are expensive, PEEK is normally moulded a few pieces at a time. So unlike PTFE, where a batch of 8-10 large pieces can be moulded in series and then put in the oven for a single cycle, PEEK will offer only a few pieces in the same amount of time
- Since PEEK is heated under pressure, issues of flash can arise as the resin becomes molten, but has pressure being applied on it. Furthermore, the pressure and temperature have to be balanced very carefully, since the temperature makes the PEEK molten, allowing it to reach its desired shape, but the pressure is responsible for vacating air bubbles from the material, so that there is no porosity.
- Batch processing the PEEK parts for annealing is possible, but takes about 24 hours
So overall, the productivity in
moulding PEEK is far below that of PTFE. This does answer, in part, the
question of why the price of the finished material is so expensive.
Machining PEEK
As discussed above PEEK machines more
like a metal than like a polymer. It is hard and has a significant impact on the
tool. The same tool that might churn out 3000-4000 PTFE parts may struggle to
churn out a few hundred PEEK parts. Again – this adds to the cost of the
finished product significantly.
More importantly for machining
though is that if the PEEK is not annealed properly, the part will behave
erratically during machining as different areas within the material react differently to the stress being placed by the tool. Thus, cracks can develop
during machining and the dimensional stability across a batch of components can
vary significantly.
As a result, PEEK machining is a
difficult process and there are few who are willing to take on the risks of
machining such an expensive item, knowing that the rate of rejection could be
very high.
In conclusion – PEEK has remained a
largely niche polymer mainly due to its prohibitively high price. If it were cheaper –
say around the price of PTFE – there are chances that it could steal a
significant chunk of the PTFE market. PTFE still rates much higher than PEEK on
characteristics like coefficient of friction and dielectric strength, but where
it is a question of sheer strength, PEEK stands unmatched amongst polymers.
