🧪 PEEK
Swiss Machining PEEK: Unfilled, Glass-Filled and Carbon-Filled
PEEK is the high-performance polymer that earns a place on Swiss screw machines normally reserved for metals, because medical, semiconductor, and aerospace buyers need small precision turned plastic parts that survive heat, chemicals, and sterilization that would melt ordinary plastics. Machining it well is less about cutting force, which is low, and more about heat and internal stress, since PEEK softens near its glass transition and remembers every bit of locked-in stress when you cut into it.
ISO 13485ISO 9001AS9100
Heat management and the annealing step that prevents cracking
PEEK is a semicrystalline thermoplastic with a glass transition around 143 degrees C and a melt point near 343 degrees C, which is high for a plastic but still means cutting heat must be controlled so the material does not soften, smear, or gum at the tool. Sharp tooling, high spindle speeds with moderate feeds, and good coolant or air blast keep the cut cool; dull tools generate friction heat that ruins the surface. The chips are stringy thermoplastic ribbons, so chip control and evacuation matter on a guide-bushing machine much as they do with gummy metals.
The defining issue, though, is internal stress. Extruded and molded PEEK stock carries residual stress, and machining away material unbalances it, causing parts to warp or even crack, sometimes hours after machining. The standard defense is stress-relieving the stock before machining and annealing parts between roughing and finishing for tight-tolerance work, using a controlled stepped heat cycle. Reinforced grades and thick-section parts especially benefit. A buyer should expect a quality PEEK shop to anneal as part of the process, and any part to a tight tolerance that skips proper stress relief is a candidate to move dimension or crack in service, which is why this step is non-negotiable for precision work.
Unfilled, glass-filled, and carbon-filled: choosing the grade
Unfilled (virgin) PEEK is the ductile, impact-resistant, biocompatible grade, and it is the one used for implantable and food-contact parts because fillers complicate biocompatibility. Medical-grade unfilled PEEK (such as the implantable Optima grades) dominates spinal cages, dental, and trauma components. It machines cleanly and takes a good finish, and it is the most forgiving of the three to cut.
Glass-filled PEEK (typically 30 percent glass) adds stiffness, dimensional stability, and creep resistance at the cost of impact strength and at the cost of tool wear, because the glass fibers are abrasive and dull tooling faster, so cutters are changed more often and carbide or PCD tooling is preferred. Carbon-filled PEEK (30 percent carbon fiber) goes further, adding the highest stiffness, strength, wear resistance, thermal conductivity, and electrical conductivity (it is no longer an insulator), and it is the most abrasive of the three on tooling. Carbon-filled grades suit bearings, wear parts, and structural components where stiffness and wear resistance matter. The grade choice is property-driven: unfilled for toughness, biocompatibility, and electrical insulation; glass-filled for rigidity and stability; carbon-filled for maximum stiffness and wear, accepting faster tool wear with the reinforced grades.
Tolerances, finish, and what PEEK costs
PEEK holds tighter tolerances than most plastics but not metal-level ones, because it is more thermally sensitive and slightly more compliant. A good shop holds roughly +/-0.001 to +/-0.002 inch on turned features in PEEK, and tighter on stable reinforced grades after proper annealing, but the +/-0.0002 inch realm of metals is generally not realistic given the material's thermal expansion (much higher than metal) and moisture and stress sensitivity. Designers should allow looser tolerances than they would for a metal part and avoid over-specifying. Surface finishes come off smooth with sharp tooling, and reinforced grades show fiber texture under close inspection.
PEEK is expensive raw material, dramatically more than common plastics and even more than some metals on a volume basis, and reinforced and medical-grade stock costs more still, so material is a significant fraction of part cost, unlike most plastics. Glass and carbon fillers add tool wear cost. There is no corrosion finishing as with metals, which simplifies the back end, but stress-relief and annealing add process steps and time. A small turned unfilled PEEK part at moderate volume might run a few dollars to well over ten dollars each depending on size, tolerance, and grade, with medical traceability and inspection adding more. Lead times run similar to metals, 3 to 5 weeks typical, with stock availability sometimes the gating item for specialty medical grades.
Frequently Asked Questions
Because PEEK carries residual internal stress from extrusion or molding, and removing material during machining unbalances that stress, causing parts to warp, distort, or even crack, sometimes hours after the part comes off the machine. The standard defense is to stress-relieve the raw stock before machining and to anneal parts between roughing and finishing on tight-tolerance work, using a controlled, stepped heating and cooling cycle that lets the stress relax without distorting the part. This is most critical for thick sections, reinforced grades, and any part with tight tolerances or thin walls. A quality PEEK shop treats annealing as a standard process step, not an option, because a precision part that skips proper stress relief is a real candidate to move dimensionally or crack later, sometimes in service. The annealing steps add process time and cost but are essential for dimensional stability. Buyers specifying tight-tolerance PEEK parts should confirm the shop's stress-relief and annealing practice, since it is one of the biggest differentiators between a part that holds tolerance and one that fails.
Unfilled (virgin) PEEK is ductile, impact-resistant, biocompatible, and an electrical insulator, and it is the easiest of the three to machine, taking a clean finish. It is the grade used for implantable and food-contact parts because fillers complicate biocompatibility, so it dominates spinal cages, dental, and trauma components. Glass-filled PEEK, usually 30 percent glass fiber, adds stiffness, dimensional stability, and creep resistance but reduces impact strength, and the glass fibers are abrasive, so they wear tooling faster and call for carbide or PCD cutters changed more often. Carbon-filled PEEK, typically 30 percent carbon fiber, delivers the highest stiffness, strength, wear resistance, and thermal conductivity, and it becomes electrically conductive rather than insulating; it is also the most abrasive on tooling of the three. The choice is property-driven: pick unfilled for toughness, biocompatibility, and insulation, glass-filled for rigidity and stability, and carbon-filled for maximum stiffness and wear resistance, accepting that the reinforced grades cost more in tool wear and require tougher tooling.
PEEK holds tighter tolerances than most plastics but does not reach metal-level precision. A good shop typically holds about +/-0.001 to +/-0.002 inch on turned features in PEEK, and somewhat tighter on stable reinforced grades after proper annealing, but the +/-0.0002 inch range routine in metals is generally not realistic. The limits come from the material itself: PEEK has a thermal expansion coefficient much higher than metal, so it grows and shrinks more with temperature, it absorbs a little moisture, and it is sensitive to residual stress, all of which make sub-thousandth tolerances hard to guarantee in service. Designers should allow looser tolerances than they would for an equivalent metal part and avoid over-specifying, since unnecessarily tight callouts drive up cost and scrap without benefit. Proper stress-relief and annealing of the stock and parts is essential to hitting and holding whatever tolerance is specified, because un-relieved stress will move the part after machining. Surface finishes come off smooth with sharp tooling, with reinforced grades showing some fiber texture under close inspection.
The dominant cost driver is the raw material. PEEK is one of the most expensive engineering thermoplastics, costing dramatically more per pound than commodity plastics and more than many metals on a volume basis, and reinforced grades and certified medical-grade stock cost more still, so unlike most plastics where material is cheap, PEEK material can be a large fraction of the part cost. On top of that, glass-filled and carbon-filled grades are abrasive and accelerate tool wear, adding tooling cost and favoring carbide or PCD cutters. The required stress-relief and annealing steps add process time. The offset is that PEEK needs no corrosion finishing like metals, which simplifies the back end. A small turned unfilled PEEK part at moderate volume might run a few dollars to well over ten dollars each depending on size, tolerance, and grade, with medical traceability and inspection pushing it higher. Lead times are similar to metals at 3 to 5 weeks typical, though availability of specialty medical grades can occasionally be the gating factor on schedule.
Related Pages
Last updated: July 2026
Find PEEK Swiss Machining Suppliers
Search verified shops that handle PEEK swiss machining.
No logins. No email gates. Just results.