Article From: 6/1/2007 Modern Machine Shop, Derek Korn, Senior Editor
If you machine components for medical implant devices, then you’re either 1) already machining PEEK polymer or 2) soon will be. That’s because the medical industry has found medical-grade PEEK offers excellent strength, wear resistance and biocompatibility for components such as the dental healing caps, spiked washers and spinal implants shown to the right. PEEK is showing up in an increasing number of medical shops all over the country.
Craig Valentine is the technical manager for Invibio, a UK-based supplier of biocompatible PEEK polymers. Mr. Valentine provides the following material and machining information for medical shops that may have vast experience machining titanium and stainless, but are just setting their sights on machining polyetheretherketone (aka PEEK).
PEEK polymer is available in two basic grades: industrial and medical. Industrial-grade PEEK is a strong thermoplastic that retains its mechanical properties even at elevated temperatures. The flame-retardant material is abrasion resistant, has high impact strength and a low coefficient of friction. Industrial-grade PEEK components are used in the aerospace, automotive, chemical, electronics, petroleum, and food and beverage industries.
Medical-grade PEEK possesses those same physical properties in addition to biocompatibility, high chemical resistance and compatibility with several different sterilization methods. It is also naturally radiotranslucent when viewed using X-ray, MRI or computer tomography (CT). This provides doctors with an unobstructed view of tissue and bone growth around the PEEK implant during the healing process. Some medical-grade PEEK polymers, such as Invibio’s PEEK Classix, are suitable for applications requiring blood or tissue contact of less than 30 days. Other implantable-grade PEEK polymers, including Invibio’s PEEK Optima, have a bone-like stiffness and can remain in contact with blood or tissue indefinitely. Invibio has recently introduced a carbon-fiber-reinforced PEEK, which offers high wear resistance for components such as articulating joints.
Shops typically receive PEEK in the form of rods that can range from 6 mm to 150 mm in diameter. Before machining, the material must be stress-relieved through an annealing process. This decreases the likelihood that heat generated during the cuts will form surface cracks and internal stresses. PEEK components that will undergo long stretches of machining time may require additional intermediate annealing steps. For example, after initial annealing and rough machining, some shops will anneal the workpiece once more before taking finishing passes to prevent crack formation.
Industrial and most medical-grade PEEK polymers are machined similarly. However, medical-grade PEEK reinforced with carbon fiber must be approached differently. For instance, carbon-fiber-reinforced PEEK should be heated just prior to machining in order to soften the material and reduce tool wear. Table I on the following page lists cutting data Invibio offers to help shops dial in their machining processes for conventional medical-grade PEEK and carbon-fiber-reinforced PEEK polymers.
Invibio suggests shops use silicon carbide cutting tools for natural PEEK and diamond tools for carbon-fiber-reinforced PEEK. The best way to avoid adverse affects on the material’s biocompatibility is to machine dry. However, in many cases coolant is necessary to remove cutting heat that builds in the workpiece (PEEK doesn’t dissipate heat the way metals do.) Pure water serves as the best coolant because it is least likely to affect material biocompatibility. The chips produced from machining most medical-grade PEEK polymers can be collected and recycled for industrial applications.
As with all medical components, precautions must be taken to prevent surface contamination of PEEK workpieces. One precautionary measure is to dedicate the machine tool, fixturing and tools to machining only that material. Some shops also insist their employees use gloves when handling PEEK to keep oils off the part surface.