PEEK selection and machining guide

Why engineers specify PEEK

PEEK is a high-performance thermoplastic, and it is specified when a part has to hold its shape and strength in conditions that would degrade a general-purpose plastic. The combination that makes it distinctive is that it retains useful mechanical properties at elevated temperature, resists a wide range of chemicals, and has low moisture absorption compared with nylons. It is also one of the few polymers that stands up to common sterilization methods, which is why it appears in medical-adjacent and analytical equipment designs.

None of that comes cheaply. PEEK stock costs many times more than POM or nylon, and it is harder on tooling. The right question is not whether PEEK is good, but whether the operating conditions actually require it. If the part runs at ambient temperature in a clean environment and does not need chemical resistance, a less expensive engineering plastic will normally do the job.

Unfilled and filled grades

PEEK is supplied unfilled and in compounds with fillers such as glass fibre or carbon fibre, and the filler changes the material substantially rather than slightly. Unfilled PEEK is the more ductile and more chemically resistant option and machines more easily. Filled grades raise stiffness, improve dimensional stability and reduce creep, at the cost of being noticeably more abrasive on tooling and, in some cases, of chemical resistance.

The specific grade, its filler content and the documentation that comes with it are confirmed per order, because suppliers differ and the certificate matters as much as the material. Tell us the grade on your drawing, or the operating conditions if the grade is still open, and we will confirm what we can supply.

How PEEK behaves in the machine

PEEK machines well with the right approach, but it is unforgiving of the wrong one. The two issues that cause most problems are heat and stress. PEEK does not conduct heat away from the cutting edge quickly, so excessive speed or a dull tool builds heat in the cut. Because the material is expensive, a scrapped part is a costly mistake rather than an inconvenience.

The second issue is internal stress. Extruded PEEK bar carries residual stress from its own manufacturing, and removing material releases it, which lets the part move after machining. On parts with tight dimensional requirements the usual answer is a stress-relief or annealing step before finishing, so the part settles before the final cuts rather than after. This is the single most common reason a PEEK part fails inspection, and it is a planning issue rather than a machining one.

Filled grades add a third factor: abrasion. Glass and carbon filled compounds wear tooling quickly and need sharper tools changed more often. They also tend to produce more dust, which is worth knowing if the part will be handled or cleaned afterwards.

Design details worth deciding early

• Wall thickness and section changes. Thick-to-thin transitions cool and shrink unevenly, and PEEK is less tolerant of that than POM.
• Sustained load. Thermoplastics creep under continuous load, so a part that is fine under a short test may relax over months in service. If the part carries permanent load, say so.
• Thermal expansion, which is much higher than for metals. A PEEK part that is dimensionally correct at 20 °C will not be the same size at operating temperature.
• Threads. Threads in PEEK are usually designed with deeper engagement than the metal equivalent, and thread forming is generally preferred to thread cutting where the geometry allows it.
• Sharp internal corners, which concentrate stress in a notch-sensitive material.
• Whether the part is machined from extruded bar or from a compression-moulded shape, because the starting stock changes the residual stress behaviour.

Where PEEK is normally considered

PEEK is typically evaluated for machined components in a few situations, and it is useful to know which one applies to your part.

• Parts that must survive repeated sterilization cycles and still hold dimension, where the alternative is a metal part that would corrode or a lower-grade plastic that would degrade.
• Components in analytical, semiconductor or vacuum equipment where outgassing and chemical compatibility matter and a metal part would contaminate the process.
• Electrical and thermal isolation, where a machined insulator is needed in a small envelope.
• Weight-sensitive parts that also need chemical resistance, where aluminum would corrode and stainless would be too heavy.
• Wear parts running against metal or against another polymer in an application where POM would not survive the temperature.

Whether PEEK is the right answer for a specific application is a decision for the customer's engineering team, and it depends on the service conditions. We machine to the material and specification on the drawing. If you are still selecting, tell us the conditions and we will tell you what we see from a machining and dimensional-stability point of view.

When another material is the better choice

• If the part needs precision and low friction but runs at ambient temperature, POM is easier to machine, holds tolerance well and costs far less.
• If the requirement is chemical resistance and electrical insulation rather than strength, PTFE is often the more suitable polymer.
• If the priority is impact toughness rather than temperature resistance, nylon absorbs energy better and costs less.
• If the part needs clarity or impact resistance at low cost, polycarbonate is the usual choice.
• If the part carries real structural load or needs to hold a tolerance over a wide temperature range, a metal will generally outperform any thermoplastic. See aluminum or stainless steel.

Finishing

PEEK parts are normally used as machined. There is no anodizing or plating equivalent, and the as-machined surface is usually the finished surface. That makes the machining strategy itself the finish, which is another reason to agree the critical dimensions and the stress-relief approach before production starts. If a specific surface condition is required, describe it on the drawing and we will confirm what the machining route can deliver.