Common problems with PEEK and how to avoid them
PEEK is a high performance thermoplastic that holds its properties at temperature and resists chemicals. It is used for parts that have to work where a metal corrodes or where weight and insulation matter. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, aerospace is used as the example: traceability and a first article before anything enters the batch.
The problems that actually occur
These are the failures that show up on a real job, listed in the order in which they are usually discovered.
- Machining it with a steel cutting geometry, which melts rather than cuts. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
- Measuring it straight off the machine, before it has cooled. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
What causes them
In most cases the cause is upstream of the operation, not in it. Machining it with a steel cutting geometry, which melts rather than cuts is usually a symptom of one of the three causes above rather than a problem in its own right.
How they are prevented
Prevention is duller than diagnosis. Use sharp polished tools and air blast rather than a flood coolant, and the result is confirmed by dimensional inspection at 20 °C after stabilising before the batch is released.
| Item | Note |
|---|---|
| Failure | machining it with a steel cutting geometry, which melts rather than cuts |
| Prevention | use sharp polished tools and air blast rather than a flood coolant |
| Tolerance to hold | ±0.05 mm, because of thermal expansion |
| Finish to check | Ra 0.8 µm achievable with sharp tools |
Applications
PEEK is used for parts that have to work where a metal corrodes or where weight and insulation matter, and in practice it appears most often in semiconductor handling, medical devices, pump wear parts, insulators.
How it fits into a machining route
PEEK is chosen at the drawing stage, and everything downstream follows from it: the tool geometry, the cutting data, the heat treatment and the certificate. Changing the grade later is a different part, not a different quote.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: material cost is high per kilogram. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is material certificate with the batch. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
Dimensional inspection at 20 °C after stabilising is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for PEEK?
±0.05 mm, because of thermal expansion is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
PEEK is the usual choice, supplied against material certificate with the batch.
How is the result verified?
Dimensional inspection at 20 °C after stabilising, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
Measuring it straight off the machine, before it has cooled.
How long does it take and what does it cost?
A realistic cycle is fast cutting, but with care on heat build up, and the cost driver is material cost is high per kilogram.
Related capability
If you are specifying PEEK on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to PEEK in the encyclopedia: Inconel 718, C3604 Brass, Splined Shafts. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
