Guides / Machining

Annealing PEEK: Why Parts Move After Machining

September 20, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

A PEEK part comes off the machine in tolerance, sits overnight and the bore is oval. Nothing was done wrong at the spindle. The stock was holding stress, and machining let it out.

This is the most common surprise for buyers moving from metal to high performance plastics, and it is the reason a PEEK quote sometimes includes an oven step that an aluminium quote never would.

Where the stress comes from

PEEK rod and plate are extruded or compression moulded, then cooled. The outside cools faster than the core, so the material sets with internal stress locked in. The stock looks fine and measures fine.

Machining removes material unevenly. The moment the balance is broken, the remaining material relaxes into a new shape. Thin walls bow, long parts curve, bores go out of round and flat faces cup.

Semi crystalline plastics also keep changing crystallinity slowly if they run hot. A part that is stable on the inspection bench can shift after its first hours at 180 °C in service.

What annealing does

Annealing takes the part above the glass transition temperature of 143 °C, holds it there long enough for the molecules to relax, then cools it slowly so new stress is not built in on the way down. The melting point of 343 °C is far above the soak, so the part keeps its shape.

Two moments matter:

Stock annealing. Good stock suppliers deliver rod and plate already stress relieved. Ask. It is the cheapest stress relief you will ever buy.

Intermediate annealing. The shop roughs the part, leaving finishing stock on critical features, anneals it, then finishes. Whatever movement the roughing released is machined away in the finishing pass. This is the step that saves tight bores.

When it is needed

Rules of thumb that shops use:

• Wall thickness under about 3 mm, or a wall much thinner than the rest of the part
• Deep pockets, or more than roughly half the block removed
• Long slender parts where a small bow ruins straightness
• Bores or slots held tighter than ±0.05 mm
• Parts that will run continuously above about 150 °C
• Any part cut from a large block down to a small finished size

A chunky bushing with open tolerances usually does not need it. A 180 mm wafer handling comb with 1.5 mm fingers absolutely does.

What it costs you

Oven hours, plus a second setup, plus a day or two of lead time. On a small batch it is a visible line in the quote, which is why it is worth flagging your thin sections in the RFQ. A shop that knows about them prices the cycle in. A shop that finds out later either eats the cost or sends you parts that move.

There is a second reason to mention it. Shops without an annealing oven have to send parts out, which adds handling and days. Knowing that up front changes who should quote your part in the first place. More on the rest of the price in what machined PEEK parts cost.

What to put on the RFQ

• Where the thin walls and deep pockets are
• Which bores and slots must hold, and to what
• The service temperature, continuous and peak
• Whether the part will be thermally cycled, for example autoclaved
• Whether you need the stock supplied stress relieved, with the certificate

Design changes that reduce the problem

• Even wall thickness wherever the function allows it
• A radius on internal corners instead of a sharp corner
• Fewer deep narrow pockets, more open features
• Rod instead of plate for round parts, so the grain of the extrusion runs with the part
• Tolerance only what has to fit

None of this is exotic. It is the same thinking as designing for casting: work with how the material was made, not against it. If you want the grade decision as well, see which PEEK grade should be on your drawing. For the failures that show up when this step gets skipped, see common PEEK machining defects.

Have a thin walled part and no idea whether it needs a cycle? Send the drawing. The shops we match will tell you, and the answer will be in the quote rather than in a surprise a month later.

Frequently asked questions

Keep reading

• Which Thread Method Works for a PEEK Part: Tap, Mill or Insert
• Specifying Swiss-Type Turned PEEK Parts Without Burrs or Runout