CF-PEEK and CF-PPS: Carbon-Fiber Polymer Machining

CF-PEEK and CF-PPS: Engineering Polymers Reinforced with Carbon Fiber

Carbon-fiber-reinforced PEEK (CF-PEEK) and carbon-fiber-reinforced PPS (CF-PPS) represent the high-performance tier of machinable engineering polymers. CF-PEEK, typically containing 30% short or continuous carbon fiber by weight, offers tensile strength exceeding 200 MPa, a service temperature of 250°C continuous, and inherent flame retardancy. CF-PPS, usually 40% glass or carbon filled, provides similar chemical resistance at a lower cost point with a continuous use temperature of 220°C. Both materials are machined into aerospace brackets, semiconductor wafer handling components, medical device housings, and oil-and-gas downhole tool components.

The carbon fibers that give these polymers their strength are also what make machining challenging. The fibers are extremely abrasive (Mohs hardness approximately 7), causing rapid tool wear, and the matrix polymer is susceptible to thermal damage, delamination, and fiber pull-out if cutting parameters are not optimized.

Tooling Selection for CF-PEEK and CF-PPS

Tool wear from carbon fiber abrasion is the dominant concern. Recommended tooling:

• Turning: PCD (polycrystalline diamond) tipped inserts are the gold standard, offering 20-50× the life of carbide. CVD diamond-coated carbide inserts provide a cost-effective alternative for moderate volumes. Uncoated carbide (ISO K10-K20) can be used for prototypes but expect rapid flank wear.
• Milling: Solid carbide end mills with diamond-like carbon (DLC) or CVD diamond coating. Three-flute geometry with high helix angle (40-45°) for efficient chip evacuation. Bur-style (compression) cutters for through-cutting sheet stock to prevent exit-side delamination.
• Drilling: Diamond-coated solid carbide drills with a 90°-118° point angle and polished, high-helix flutes. Brad-point geometry reduces entry delamination.

Turning Parameters for CF-PEEK (30% CF)

With PCD-tipped inserts on CF-PEEK rod stock:

• Cutting speed (Vc): 200-400 m/min (650-1,310 SFM)
• Feed rate (fn): 0.10-0.20 mm/rev (0.004-0.008 IPR)
• Depth of cut (ap): 0.5-2.0 mm (0.020-0.080 in.)
• Coolant: Compressed air blast or mist coolant; avoid flood coolant which can cause PEEK to absorb moisture and swell

With CVD diamond-coated carbide inserts, reduce speed to 150-250 m/min to prevent thermal softening of the PEEK matrix at the cutting zone. The goal is to shear the fiber cleanly rather than plowing it out of the matrix.

Milling CF-PEEK and CF-PPS

For profile milling and pocketing with a 10 mm diamond-coated solid carbide end mill:

• Cutting speed (Vc): 180-300 m/min
• Feed per tooth (fz): 0.05-0.10 mm/tooth
• Axial depth (ap): Up to 1.5D (15 mm for a 10 mm tool)
• Radial depth (ae): 0.1-0.3D for profiling, full width for slotting
• Spindle speed (10 mm tool): Approximately 5,730-9,550 RPM

Ramp entry rather than plunging directly into the material to avoid delamination at the entry point. For through-cuts, place a sacrificial backer board beneath the workpiece or use a compression cutter to prevent exit-side fiber tear-out. Climb milling produces cleaner fiber shearing and reduces delamination compared to conventional milling.

Drilling and Hole Making

Drilling is the most common operation on CF-reinforced polymer parts, and it is also where delamination damage is most prevalent. For a 6 mm hole:

• Drill type: Diamond-coated solid carbide, brad-point or 90° point
• Cutting speed: 100-180 m/min (approximately 5,300-9,550 RPM)
• Feed rate: 0.03-0.06 mm/rev; reduce feed by 50% for the final 2 mm before breakthrough
• Peck cycle: Recommended for holes deeper than 3×D to clear dust-like chips

Delamination at the exit side of the hole is the most common defect. Solutions include using a sacrificial backer plate, reducing feed rate near breakthrough, and employing a specialized step drill that scores the exit surface before the main cutting edges penetrate. Orbital drilling (helical interpolation) with a smaller diameter end mill is an alternative that virtually eliminates exit delamination in automated production.

Dust Extraction and Machine Protection

Machining CF-PEEK and CF-PPS generates fine carbon fiber dust that is both abrasive and electrically conductive. This dust can infiltrate machine tool guideways, ball screws, and electrical cabinets, causing premature wear and short circuits. Essential precautions include:

• Install high-volume dust extraction at the cutting zone (minimum 1,000 CFM for a standard VMC).
• Seal machine way covers and electrical enclosures.
• Use air blast rather than flood coolant to avoid creating a fiber-laden slurry.
• Schedule regular machine cleaning intervals; vacuum rather than blow off accumulated dust.
• Provide operators with PPE including respirators rated for fine particulates (P100 or FFP3).

Surface Finish and Dimensional Control

With sharp PCD or diamond-coated tools and optimized parameters, CF-PEEK can achieve Ra 0.8-1.6 µm on turned surfaces and Ra 1.6-3.2 µm on milled surfaces. Fiber ends protruding from the matrix create a microscopically rough texture even when the cutting parameters are correct. For applications requiring smoother surfaces, light abrasive blasting or vapor polishing (for PEEK matrix only, not exposed fibers) can improve finish to Ra 0.4 µm.

Dimensional tolerances of ±0.025 mm are achievable in production machining of CF-PEEK, but thermal management is critical. Both PEEK and PPS have relatively high CTE values (8-20 × 10−⁶/°C in-plane, higher through-thickness), so heat buildup during machining can cause significant thermal growth. Light cuts, air cooling, and allowing the part to return to ambient temperature before final measurement are standard practices.

Summary

Carbon-fiber-reinforced PEEK and PPS demand diamond-class tooling to survive the abrasive fibers, high cutting speeds to shear fibers cleanly, and disciplined dust management to protect both operators and equipment. Shops that invest in PCD tooling and dust extraction infrastructure gain access to the rapidly growing market for lightweight, high-strength polymer components in aerospace, semiconductor, and medical device manufacturing.