Deep Hole Drilling of PEEK and High-Performance Thermoplastics: Process Parameters and Thermal Management

A manufacturer of PEEK spinal implant components (Ø12 mm × 120 mm, requiring Ø4 mm × 90 mm axial through-bore, Ra < 0.4 µm, tolerance ±0.015 mm) was using conventional twist drilling (HSS, point angle 118°, Vc = 30 m/min, f = 0.04 mm/rev, dry). The bore exhibited surface melting and re-solidification (Ra 3–6 µm), diameter shrinkage of 0.04 mm from thermal expansion/contraction, and 15% scrap. Switching to a specialised thermoplastic process — DLC-coated carbide gun drill (point angle 20°, helix 42°, mirror-polished flutes), Vc = 80 m/min, f = 0.025 mm/rev, cold air coolant at -10°C and 6 bar, peck cycle 5 mm — achieved Ra 0.2–0.35 µm, diameter tolerance ±0.008 mm, no melting, and scrap rate below 2%.

Material Properties and Drilling Challenges ​

Key Properties Affecting Deep Hole Drilling by Thermoplastic ​

Deep Hole Drilling Parameters for High-Performance Thermoplastics ​

Tool Geometry: Thermoplastic vs Metal Drilling ​

FAQ ​

Why is thermal management the most critical factor in deep hole drilling of thermoplastics? ​

Thermal management is the most critical factor in deep hole drilling of thermoplastics because the materials have thermal conductivity of 0.22–0.60 W/m·K — approximately 1/200th to 1/500th of steel — meaning cutting heat cannot dissipate through the workpiece and instead accumulates in the cutting zone. This accumulated heat causes three distinct problems: (1) Thermal softening — as the cutting zone temperature approaches the glass transition temperature (Tg) of the thermoplastic, the material transitions from a rigid, cuttable state to a rubbery, deformable state. For PEEK (Tg = 143°C), the cutting zone reaches this temperature within 2–5 seconds of continuous cutting at Vc = 60 m/min without coolant. In the rubbery state, the material does not shear cleanly — instead it deforms plastically and tears, producing a rough, smeared bore surface (Ra > 2 µm). (2) Melting and re-solidification — if the cutting zone exceeds the melting temperature (Tm = 343°C for PEEK), the material melts and flows rather than cutting. The molten polymer re-solidifies on the bore surface and tool flutes as a glassy, amorphous layer. This re-solidified layer has different mechanical properties (lower crystallinity, reduced strength) than the parent material and is unacceptable for medical implants or aerospace components. The re-solidified layer also bonds aggressively to the tool surface, causing chip adhesion that increases cutting forces and further elevates temperature — a self-reinforcing cycle that leads to tool seizure or workpiece damage within 10–20 mm of drilling. (3) Thermal expansion and contraction — thermoplastics have coefficients of thermal expansion (CTE) of 47–200 × 10⁻⁶/K, which is 4–15× higher than steel. During drilling, the bore wall expands from localised heating, creating a larger bore diameter than the tool. As the material cools after drilling, the bore contracts — potentially 0.02–0.05 mm for a Ø6 mm bore in PEEK — which can cause the bore to close below the specified tolerance. Conversely, if the material cools around the tool (during a pause or feed hold), the bore contracts onto the drill, causing seizure. The solutions to these thermal challenges are: (a) use a cooling method that actively removes heat from the cutting zone — cold air at -10°C or cryogenic CO₂ at -78°C is far more effective than room-temperature air; (b) use peck drilling cycles with short peck depths (2–10 mm, depending on bore diameter) to allow the cutting zone to cool between pecks; (c) maintain a minimum feed rate (f > 0.020 mm/rev) to ensure chip thickness is sufficient for the cutting edge to shear rather than rub; and (d) design the drill with a razor-sharp edge (< 2 µm radius) to minimise cutting forces and thus heat generation.

What tool coating is best for deep hole drilling PEEK and other thermoplastics? ​

DLC (diamond-like carbon) coating is the best tool coating for deep hole drilling PEEK and most high-performance thermoplastics. DLC provides three critical properties for thermoplastic drilling: (1) Low coefficient of friction — DLC has a friction coefficient of 0.05–0.15 against thermoplastics, compared to 0.4–0.6 for uncoated carbide and 0.3–0.5 for TiAlN-coated tools. The low friction reduces frictional heat generation at the chip-tool interface by 50–70%, which is the single most important factor in preventing thermal softening and melting. (2) Chemical inertness — DLC does not react with thermoplastic polymers at cutting temperatures. Uncoated carbide tools can chemically bond with the polymer's functional groups at elevated temperatures, creating adhesion sites that initiate chip welding. TiAlN and AlTiN coatings, while excellent for metal cutting, have higher surface energy than DLC, making them more susceptible to polymer adhesion. (3) Surface smoothness — DLC coatings replicate and enhance the surface finish of the underlying tool substrate. A DLC-coated drill with a mirror-polished substrate achieves a rake face roughness of Ra < 0.05 µm, which is essential for preventing tacky thermoplastic chips from adhering to the flute. Uncoated polished carbide (Ra < 0.1 µm) is the second-best choice and is acceptable for short production runs (< 500 bores) or for materials like PTFE that have minimal adhesion tendency. Diamond-coated tools (CVD diamond) provide even better wear resistance than DLC but are typically too rough (Ra 0.3–0.5 µm) for thermoplastic drilling — the rough surface promotes chip adhesion. TiN-coated tools should not be used for thermoplastic drilling because TiN has high surface energy and reacts with the polymer at cutting temperatures. For glass-fibre-reinforced thermoplastics (PEEK 30% GF, PPS 40% GF), the coating must also provide abrasion resistance against the glass fibres. For these materials, DLC remains the best choice, but thicker DLC (> 3 µm) is recommended to prevent coating breakthrough by the abrasive fibres. AlTiN coating can be used as an alternative for fibre-reinforced grades when DLC is unavailable, but the higher friction coefficient requires 15–20% lower cutting speeds to maintain acceptable cutting temperatures.

How do peck cycles and step feed strategies differ for deep hole drilling of thermoplastics versus metals? ​

Peck cycles for deep hole drilling of thermoplastics must be significantly more aggressive — with shorter peck depths and longer dwell times — than those used for metal drilling, because the purpose of pecking in thermoplastics is thermal management rather than chip evacuation. In metal drilling, the primary purpose of a peck cycle is to break and evacuate chips that would otherwise pack in the flute. A typical metal peck cycle for a deep hole might use peck depths of 10–30× diameter with no dwell time. The heat generated is conducted away through the workpiece, so thermal buildup is rarely the limiting factor. In thermoplastic drilling, the primary purpose of pecking is to prevent the cutting zone temperature from exceeding Tg or Tm. The thermoplastic's low thermal conductivity means that the tool cannot rely on heat dissipation through the workpiece — instead, the drill must be withdrawn periodically to allow the cutting zone to cool by convection with the coolant air. The recommended peck cycle for thermoplastic drilling depends on the material's Tg and the cutting speed: for PEEK (Tg = 143°C) at Vc = 80 m/min, f = 0.025 mm/rev, the peck depth should be 3–6 mm (approximately equal to the drill diameter for small bores, < 1× diameter for larger bores). The dwell time at retract should be 0.5–1.5 seconds, sufficient for the cold air coolant to reduce the cutting zone temperature below 80°C before re-entry. For polyimide (Vespel, Tg = 310°C), peck depths can be longer — 8–15 mm — because the higher Tg allows more continuous cutting before thermal softening. For PTFE (Tm = 327°C), peck depths of 10–20 mm are feasible because PTFE's low coefficient of friction generates less frictional heat. For UHMWPE (Tm = 135°C), peck depths must be very short — 2–4 mm — because the low melting point and high CTE (200 × 10⁻⁶/K) cause rapid thermal expansion and melting. The peck retract distance should be at least 20 mm or to the bushing face, ensuring the drill clears the bore completely for air circulation. Peck cycle optimisation for thermoplastics follows a simple rule: the cutting time per peck should not exceed the time required for the cutting zone to reach 80% of Tg (or 70% of Tm for semi-crystalline polymers). The number of pecks per bore increases cycle time by 30–60% compared to continuous drilling, but this is offset by the elimination of scrap from thermal damage. Some production systems use adaptive peck control, where the peck depth is automatically reduced if the spindle power exceeds a threshold (indicating thermal softening), providing maximum drilling speed without thermal damage risk.

What quality control methods are specific to thermoplastic deep-drilled bores? ​

Quality control for thermoplastic deep-drilled bores must address defects that are unique to polymer machining — thermal damage, dimensional relaxation, and moisture-related changes — in addition to conventional dimensional and surface finish inspection. The key methods are: (1) Surface finish measurement — for thermoplastic bores, contact profilometry can damage the relatively soft surface. Non-contact optical methods (white light interferometry or confocal microscopy with a borescope probe) are preferred. The critical surface finish parameter for thermoplastic bores is not only Ra but also the material ratio (Rmr) — the bearing area curve — because a smeared or re-solidified surface may have acceptable Ra but poor Rmr (indicating a surface with flattened, melted peaks and deep valleys). For medical implant bores, Ra < 0.4 µm with Rmr(c) > 80% at c = 0.5 µm is typically required. (2) Thermal damage detection — the most critical quality check for thermoplastic bores. Thermal damage is detected by: visual inspection at 20–50× magnification (look for glossy, glassy areas indicating re-solidification, or discolouration indicating degradation); solvent etch test (immersion in isopropanol for 30 seconds — thermally damaged areas appear white or cloudy due to differential solvent absorption); or micro-hardness measurement (thermally damaged PEEK shows a 10–30% reduction in micro-hardness measured by Knoop indenter at 25 g load). The acceptance criterion is zero visible thermal damage for medical and aerospace components. (3) Dimensional relaxation measurement — thermoplastic bores change dimension over time after drilling due to viscoelastic relaxation and thermal contraction. The bore diameter must be measured at a standardised time after drilling — typically 24 hours at 23°C and 50% relative humidity. The dimensional change from immediately after drilling to 24 hours can be 0.01–0.05 mm for unfilled PEEK and up to 0.10 mm for UHMWPE. The drill diameter must be oversized to compensate: for a target Ø4.000 mm bore in PEEK, the drill diameter should be 4.025–4.040 mm, depending on the specific material batch crystallinity. (4) Moisture content verification — for hygroscopic polymers (PA66 absorbs up to 2.8% water, PPSU absorbs 0.37%), the moisture content affects dimensional stability and mechanical properties. The workpiece should be dried before drilling (typically 4 hours at 120°C for PEEK, 16 hours at 80°C for PA66) and stored in desiccated packaging after machining. The moisture content should be verified by Karl Fischer titration or by weighing before and after drying. (5) Crystallinity measurement — for semi-crystalline polymers (PEEK, PPS, PA66), the drilling process can alter the surface crystallinity, which affects chemical resistance and wear performance. Differential scanning calorimetry (DSC) on a sample coupon drilled under the same conditions can verify that the surface crystallinity remains within specification (typically 30–35% for standard PEEK). (6) Biocompatibility verification — for medical implant components, the drilled bore must be verified to be free of machining residues (coolant, polymer degradation products) that could cause adverse tissue reactions. Fourier transform infrared spectroscopy (FTIR) of a solvent rinse of the bore surface can detect residual hydrocarbons or degradation products. The acceptance criterion is no peaks outside the reference spectrum of the virgin polymer.

When should deep hole drilling be avoided in favour of alternative methods for thermoplastic components? ​

Deep hole drilling should be avoided for thermoplastic components in several situations where the process limitations make alternative methods more practical or economical. The specific scenarios are: (1) Bore diameters below 2 mm — gun drills below Ø2 mm are extremely fragile for thermoplastic use. The low elastic modulus of the polymer (3–4 GPa for PEEK) provides insufficient support for the drill, leading to deflection and breakage. The recommended alternative for small bores is laser drilling (for through-holes, Ø0.1–2 mm, L/D up to 30:1 in thin sections) or micro-moulding with core pins (for diameters > 0.5 mm, unlimited depth, but requires tooling investment). (2) Bore depths beyond 50× diameter — the chip evacuation challenge for stringy thermoplastic chips becomes insurmountable beyond L/D = 50:1, even with optimised flute geometry and air coolant. The chips pack in the flute, generate frictional heat, and cause melting or drill seizure. The alternative for deep, small-diameter bores in thermoplastics is to mould the component with a core pin, or to design the component as an assembly of two separate halves with a groove (bonded or welded together to form the bore). (3) High aspect ratio bores in glass- or carbon-filled grades — fibre-reinforced thermoplastics (PEEK 30% CF, PPS 40% GF) are highly abrasive, and tool wear for deep hole drilling is rapid (50–200 bores per tool for Ø4 mm × 100 mm). At high production volumes (> 5,000 components per year), the tooling cost for deep hole drilling exceeds the amortised tooling cost for injection moulding with a core pin. The economic crossover point is typically 3,000–8,000 components per year, depending on bore geometry. (4) Bores with tight concentricity (< 0.05 mm TIR) in semi-crystalline polymers — the non-uniform crystallinity distribution in extruded or compression-moulded thermoplastic stock causes differential thermal expansion during drilling, making it difficult to maintain concentricity below 0.05 mm TIR. For applications requiring tighter concentricity (e.g., precision valve seats), moulding with a precision-ground core pin (concentricity ±0.01 mm achievable) or post-drilling reaming with a diamond-tipped reamer is recommended. (5) Very soft thermoplastics (Shore D < 70) — materials such as UHMWPE, PTFE, and thermoplastic elastomers (TPEs) are too compliant for precision deep hole drilling. The elastic deformation ahead of the cutting edge produces oversized entries, undersized exits, and poor surface finish. For these materials, moulding, waterjet cutting (for through-holes in thin sections), or machining from rod stock with conventional drilling (shallow holes, L/D < 5:1) are preferred. (6) High-volume production (> 20,000 components/year) — for any thermoplastic component with a bore, injection moulding with a core pin is more economical than drilling at volumes exceeding 10,000–20,000 parts per year, regardless of bore geometry. The mould tooling investment ($15,000–50,000 for a multi-cavity mould) is amortised over the production volume, yielding a per-part cost of $0.10–1.00 for moulded bores compared to $1.00–5.00 for drilled bores. For prototype or low-volume production (< 500 components), deep hole drilling is generally the most economical option because it avoids tooling investment and allows design changes without mould modification.

This article provides an overview of deep hole drilling of PEEK and high-performance thermoplastics. Material selection, process parameters, tool design, and quality control methods depend on the specific polymer grade, fillers, component geometry, and regulatory requirements. Process validation trials are recommended for new thermoplastic drilling applications, particularly for medical and aerospace components. The technical data presented here reflects published research and documented case studies as of 2026.