Guides / Machining

Why PEEK Parts Crack, Warp or Fail Inspection

September 26, 2026

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

Most PEEK parts that fail inspection do not fail because the machinist did something wrong. They fail because a property of the material, usually residual stress or moisture, was not accounted for on the drawing or in the process. Here are the defects that come up most, in the order a shop typically finds them.

Cracking

Cracking in machined PEEK is almost always a stress problem, not a material defect. Three situations produce it:

Interrupted or thin cuts on unfilled material. Unfilled PEEK is tough but not immune to a sharp stress concentration, and a thin wall next to a sudden tool exit is a classic crack initiation point.
Clamping force on filled grades. GF30 and CF30 are stiffer and more brittle than unfilled PEEK. A workholding setup that would be fine on unfilled stock can crack a thin filled-grade feature under the same clamping pressure.
Delayed cracking from released stress. This is the case buyers notice first, because the part passes inspection and cracks later, often at a corner or a thread root where stress concentrates. See annealing PEEK parts for when a stress relief cycle prevents it.

The fix in all three cases is the same direction: soften the transition (larger fillet, no sharp internal corners), reduce clamping force and increase contact area, and anneal before the feature is finished rather than after.

Warping and dimensional drift

A part that measures in spec at the shop and out of spec on your bench a week later has usually warped, not shrunk or grown uniformly. Warping comes from asymmetric stress release: material removed unevenly around a bore or a pocket lets the part relax unevenly, and it keeps moving for hours to days after machining as the stress finishes redistributing.

Thin walls, deep pockets machined off-center, and parts machined from one side of a plate rather than symmetrically are the geometries most exposed to this. The calculator and cycle guidance in annealing PEEK parts covers how to size a stress relief step to a specific wall thickness so the movement is caught before finishing rather than after.

Delamination on filled grades

GF30 and CF30 rely on fiber-to-matrix bond for their strength and stability. Heat built up from a dull tool or an aggressive feed can degrade that bond locally, and the surface can show fiber pullout or a slightly rough, delaminated texture rather than a clean cut. This shows up as a cosmetic defect first and a strength or sealing problem second, because a delaminated surface does not seal or bond as reliably as an intact one.

Sharp, polished tooling and conservative feeds on filled grades avoid this. It is a heat problem more than a hardness problem, because thermal conductivity in the fiber direction is much higher than across it, and a dull edge puts heat into the surface faster than the part can carry it away.

Poor surface finish

A rough or torn finish on PEEK is almost always a tooling or heat issue rather than a material limitation. Dull edges tear rather than shear, and the low thermal conductivity of the material means heat concentrates at the cutting edge instead of dissipating through the part. Sharp, polished carbine tooling and a finishing pass with reduced feed, not reduced speed, is the standard correction. Ra 0.8 µm is typical from a routine finish pass; Ra 0.4 µm needs a sharp edge and a light cut, and a genuinely mirror finish is not realistic on filled grades because fiber ends are exposed at the surface regardless of polish.

PEEK absorbs a small amount of moisture over time, typically well under half a percent by weight at equilibrium, but on a tight-tolerance part that is enough to matter. A part machined and measured dry, then stored or shipped in a humid environment, can measure slightly different on arrival. This is not a defect in the part; it is a measurement condition that was not controlled. Parts with tolerances tighter than ±0.05 mm should be measured, and ideally used, under a stated humidity condition, not just a stated temperature.

What to change on the drawing

Every defect above traces back to one of two gaps: a stress relief step that was not specified, or a tolerance that did not account for the temperature or humidity it holds at. Specifying both, alongside the grade and tolerance decisions covered in what to decide before PEEK goes on a drawing, removes most of the defect risk before the part is ever cut.

Send the drawing with the stress relief and tolerance callouts in place, and we match you with shops that catch the rest before it becomes a rejected part.

Frequently asked questions

Why does a PEEK part crack days after machining, not during it?

Residual stress released by machining does not always show up as a visible failure immediately. In a thin or highly loaded feature, that stress can propagate a crack over hours or days as the part sits, especially if it also picks up moisture or a temperature swing.

Is warping a sign of a bad shop or a bad drawing?

Usually neither on its own. It is more often a spec gap: the drawing did not call out stress relief and the part geometry, thin walls held to a tight tolerance, needed it. The fix is a process instruction, not a different shop.

Can a warped PEEK part be corrected after the fact?

Sometimes, with a re-anneal and a light finish pass, if the part has stock left to remove. A part already at final dimension when it warps usually has to be remade, which is the reason to catch the risk on the drawing rather than after inspection.

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