Appearance
A manufacturer of medical instrument components (PEEK, unfilled/30% glass-filled/30% carbon-filled, Ø6 mm × 180 mm deep bores, Ra < 0.8 µm, no melting) initially used standard carbide gun drills at Vc = 60 m/min, f = 0.04 mm/rev, water-miscible coolant at 30 bar. Unfilled PEEK showed localised melting (0.05–0.10 mm resolidified layer, Ra 2–4 µm); carbon-filled PEEK caused rapid tool wear (VB = 0.2 mm at 3 m). Switching to polymer-optimised parameters — 90° point angle gun drill, polished rake face (Ra < 0.05 µm), PCD-tipped for carbon-filled, Vc = 25 m/min, f = 0.025 mm/rev, pressurised air with vortex cold-air gun (−20°C), peck depth 15 mm, 1-second dwell — eliminated melting, achieved Ra 0.3–0.5 µm, and extended tool life to 25 m (8×) in carbon-filled PEEK.
Polymer Drilling Fundamentals
Material Properties and Drilling Characteristics of Engineering Thermoplastics
| Polymer | Density (g/cm³) | Thermal Conductivity (W/m·K) | Melting / Softening Temp (°C) | Hardness (Shore D / Rockwell) | Tensile Modulus (GPa) | Elongation at Break (%) | Machinability Rating (1–5, 5 = best) | Primary Drilling Challenge | Moisture Absorption (% 24h) | Chemical Resistance |
|---|---|---|---|---|---|---|---|---|---|---|
| PEEK (unfilled) | 1.32 | 0.25 | 343 (melting) | Shore D 85–90 | 3.6 | 30–50 | 3 | Melting at bore surface from frictional heat; chip is stringy and tough | 0.1 | Excellent — resistant to most chemicals |
| PEEK (30% glass-filled) | 1.49 | 0.45 | 343 | Shore D 90–92 | 7.0 | 2–3 | 2 | Abrasive glass fibres cause rapid tool wear; surface finish rougher than unfilled | 0.1 | Excellent |
| PEEK (30% carbon-filled) | 1.41 | 0.95 | 343 | Shore D 90–92 | 13.0 | 1–2 | 1 | Highest tool wear (carbon fibres + PEEK matrix); electrostatically charged chips | 0.1 | Excellent |
| PTFE (virgin) | 2.16 | 0.25 | 327 (melting) | Shore D 50–65 | 0.5 | 200–400 | 3 | Extreme ductility — chips form long continuous ribbons that tangle; no melt but deforms plastically | 0.01 | Excellent — inert |
| PTFE (filled, 25% glass) | 2.2 | 0.35 | 327 | Shore D 65–70 | 1.5 | 50–100 | 2 | Abrasive glass filler + PTFE ductility combination | 0.01 | Excellent |
| UHMWPE | 0.93 | 0.42 | 130 (softening); no true melting point | Shore D 60–65 | 0.5 | 350–500 | 2 | Extreme ductility (500% elongation) — chips do not break; material deforms rather than shears; thermal expansion 200× steel | 0.01 | Excellent |
| Acetal / POM (homopolymer) | 1.41 | 0.31 | 175 (melting) | Shore D 80–85 | 3.0 | 15–40 | 4 | Gummy at high temperature; prone to local melting if coolant is insufficient | 0.2 | Good — avoid strong acids |
| Nylon 6/6 (dry) | 1.14 | 0.25 | 255 (melting) | Shore D 80–85 | 2.5 | 30–100 | 3 | Moisture content affects machining (dry nylon is brittle, moist nylon is ductile); hygroscopic expansion after drilling | 2.5 | Fair — absorbs water |
| PVC (rigid, Type I) | 1.40 | 0.17 | 80 (softening); 200 (decomposition) | Shore D 75–80 | 2.5 | 50–80 | 4 | Low thermal stability — decomposes at > 200°C releasing HCl gas; requires low cutting speed | 0.1 | Good |
| PMMA (acrylic) | 1.19 | 0.19 | 160 (softening) | Shore D 85–90 (M-scale 85–100) | 3.0 | 2–5 | 3 | Brittle — prone to cracking at drill entry and exit; transparent — surface finish must be optically clear for some applications | 0.3 | Fair — attacked by solvents |
| PVDF | 1.78 | 0.19 | 170 (melting) | Shore D 75–80 | 1.5 | 20–100 | 3 | Gummy chips; prone to melting; chlorine content requires corrosion-resistant tool coating | 0.04 | Excellent |
Recommended Deep Hole Drilling Parameters for Engineering Thermoplastics
| Polymer | Point Angle (°) | Clearance Angle (°) | Rake Angle (°) | Cutting Speed Vc (m/min) | Feed f (mm/rev) | Coolant Type | Coolant Pressure (bar) | Recommended Tool Material | Peck Depth (× drill Ø) | Surface Finish Ra (µm) | Expected Tool Life (cumulative metres) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| PEEK (unfilled) | 90 | 12–15 | 10–15 (positive) | 20–40 | 0.02–0.04 | Compressed air + vortex cold air (−10 to −20°C); or water-miscible emulsion at 5–10°C | 6 (air); 20–30 (coolant) | PCD (preferred for production); uncoated micrograin carbide (K10) with polished rake face (Ra < 0.05 µm) | 3–4 | 0.2–0.5 (with PCD or polished carbide) | 50–200 (PCD); 10–40 (carbide) |
| PEEK (30% glass-filled) | 90 | 10–12 | 8–12 (positive) | 15–30 | 0.015–0.03 | Compressed air + vortex cold air; or water-miscible at low temperature | 6 (air); 30 (coolant) | PCD (mandatory — glass fibres wear carbide rapidly) | 2–3 | 0.3–0.6 | 20–80 (PCD) |
| PEEK (30% carbon-filled) | 90 | 8–10 | 5–10 (positive) | 12–25 | 0.01–0.025 | Compressed air + vortex cold air (carbon filled PEEK conducts electricity — water-miscible coolant may cause conductivity issues) | 6 (air) | PCD (mandatory) | 2–3 | 0.3–0.6 | 15–60 (PCD) |
| PTFE (virgin) | 130 | 15–20 | 15–20 (positive, polished) | 80–200 | 0.03–0.08 | Compressed air (cold air not required — PTFE has low friction coefficient and generates less heat) | 6 | Uncoated carbide with polished flutes (PTFE is not abrasive) | 2–3 | 0.5–1.0 | 50–200 (carbide) |
| PTFE (glass-filled) | 130 | 12–15 | 10–15 | 60–120 | 0.025–0.06 | Compressed air | 6 | PCD or diamond-coated (glass filler is abrasive) | 2–3 | 0.6–1.2 | 20–80 (PCD) |
| UHMWPE | 130 | 15–20 | 15–25 (high positive, polished) | 60–150 | 0.04–0.10 | Compressed air (cold air may cause material shrinkage — use ambient temperature air) | 6 | Uncoated carbide with polished flutes (UHMWPE is extremely soft — no abrasive wear) | 2–3 | 0.5–1.0 | 100–400 (carbide) |
| Acetal / POM | 90 | 10–15 | 10–15 (positive) | 60–120 | 0.03–0.08 | Compressed air; or water-miscible emulsion | 6 (air); 20–30 (coolant) | Uncoated carbide with polished flutes | 3–4 | 0.2–0.5 | 100–300 (carbide) |
| Nylon 6/6 (dry) | 90 | 10–12 | 10–15 (positive) | 40–80 | 0.025–0.05 | Compressed air (avoid water-based coolant — nylon absorbs water) | 6 | Uncoated carbide with polished flutes | 2–3 | 0.3–0.6 | 50–200 (carbide) |
| PVC (rigid) | 100 | 8–10 | 5–10 (positive) | 30–60 | 0.02–0.05 | Compressed air with vortex cold air (critical — PVC decomposes above 200°C releasing HCl gas) | 6 | Uncoated carbide (HSS is acceptable for short runs) | 3–4 | 0.3–0.6 | 100–300 (carbide) |
| PMMA (acrylic) | 90 | 8–10 | 5–10 (positive, polished) | 15–30 | 0.01–0.025 | Compressed air with vortex cold air (critical — PMMA softens at 160°C and melts at 200°C; heat must be minimised) | 6 | Uncoated carbide with polished flutes | 2–3 | 0.2–0.4 (optically clear if Ra < 0.2 µm) | 30–100 (carbide) |
| PVDF | 110 | 10–15 | 10–15 (positive, polished) | 30–60 | 0.02–0.04 | Compressed air with vortex cold air; or water-miscible at 5–10°C | 6 (air); 20–30 (coolant) | PCD or uncoated carbide with polished flutes | 3–4 | 0.3–0.6 | 30–100 (carbide) |
FAQ
Why is thermal management the most critical factor in deep hole drilling of plastics, and how does it differ from metal drilling?
Thermal management is the most critical factor in deep hole drilling of plastics because the thermal conductivity of engineering thermoplastics (0.1–0.5 W/m·K) is 100–500× lower than that of steel (50 W/m·K) and 20–100× lower than that of aluminium (200 W/m·K). In metal drilling, the heat generated at the cutting edge (from plastic deformation and friction) is conducted into the workpiece (the metal acts as a heat sink), and the chip carries away 60–80% of the heat. In plastic drilling, the low thermal conductivity means that the workpiece cannot conduct the heat away from the cutting zone; 90–95% of the heat remains in the chip and at the tool tip, and the temperature at the cutting edge can reach 200–400°C within 0.1–0.5 seconds of the start of cutting — which exceeds the melting or softening point of most thermoplastics (PEEK melts at 343°C, acetal at 175°C, PMMA softens at 160°C, PVC decomposes at 200°C). The result is localised melting of the bore surface, which resolidifies as a rough, wavy layer (Ra 2–4 µm) that is unacceptable for most applications and can contain voids and porosity from trapped gas bubbles. The melted layer also smears across the bore surface and the tool, causing the drill to bind in the bore and potentially snap.
The thermal management strategy for plastic drilling must be fundamentally different from metal drilling. In metal drilling, the coolant is primarily a lubricant (reducing friction and heat generation at the tool-chip interface) and secondarily a coolant (removing heat from the cutting zone). In plastic drilling, the coolant is primarily a coolant (preventing the workpiece temperature from exceeding its melting/softening point) and secondarily a lubricant (reducing friction and heat generation). The coolant must have a high heat transfer coefficient (water-based or compressed air with vortex cooling) and must be delivered at sufficient flow rate to maintain the bore surface below the material's melting point. The recommended coolant for most thermoplastics is compressed air at 6–10 bar combined with a vortex cold-air gun, which produces an outlet temperature of −10 to −30°C at 500–2000 L/min. The vortex cold air provides a heat transfer coefficient of 100–300 W/m²·K (comparable to water-miscible coolant at low pressure) without the contamination or moisture absorption issues that water-based coolants cause for nylon and other hygroscopic polymers. For polymers with the lowest thermal stability (PVC, PMMA, PVDF), vortex cold air is mandatory — even water-miscible coolant at 5–10°C may not provide sufficient cooling because the heat is generated faster than the coolant can remove it at the cutting speed required for productivity.
The second thermal management strategy is peck depth control — the peck depth for plastic drilling should be limited to 2–4× the drill diameter (compared to 5–10× for steel drilling). The shorter peck depth limits the cutting time per peck to 0.5–3 seconds, which limits the total heat input per peck to a level that the coolant can remove during the retract dwell (0.5–1.5 seconds). The retract dwell is essential — during the dwell, the coolant flows into the bore and removes the heat from the bore surface before the next peck begins. The minimum dwell time for adequate cooling is approximately 0.5 seconds per 10 mm of peck depth. The third strategy is cutting speed reduction — the recommended cutting speed for plastics (20–80 m/min for most engineering thermoplastics) is 40–70% of the speed for steel drilling at the same feed rate, because the heat generation rate is proportional to the cutting speed, and the maximum allowable temperature is determined by the polymer's melting point rather than the tool's wear resistance. The speed should be set so that the steady-state bore surface temperature remains at least 30–50°C below the material's melting or softening point.
What tool geometry is optimal for deep hole drilling of plastics, and how does it differ from metal drilling tool geometry?
The optimal tool geometry for deep hole drilling of plastics differs from metal drilling in four key parameters: point angle, clearance angle, rake angle, and rake face surface finish. The point angle for plastic drilling depends on the polymer's ductility: for soft, ductile polymers (PTFE, UHMWPE, nylon), a large point angle of 120–140° is used to distribute the cutting forces over a larger section of the cutting edge, reducing the localised pressure that causes plastic deformation and smearing. For hard, brittle polymers (PMMA, PVC, filled PEEK), a smaller point angle of 80–100° is used to concentrate the cutting force and achieve clean shearing of the chip. For semi-crystalline polymers (PEEK, acetal, PVDF), a point angle of 90–110° provides the best balance of cutting action and heat distribution. The general rule: the softer and more ductile the polymer, the larger the point angle. PTFE (200–400% elongation) requires 130–140°, while PMMA (2–5% elongation) requires 85–95°.
The clearance angle for plastic drilling should be 8–20° — significantly larger than the 5–10° used for metal drilling. The larger clearance angle reduces the friction between the drill's clearance face and the bore wall, which is critical because the low thermal conductivity of the plastic means that any frictional heat from the clearance face (which is 5–10% of the total heat in metal drilling but can reach 30–50% in plastic drilling due to the higher coefficient of friction against polymers) will not be conducted into the bore wall and will instead accumulate at the tool-workpiece interface. The clearance angle should be at the upper end of the range (15–20°) for soft, gummy polymers (PTFE, UHMWPE, nylon) and at the lower end (8–12°) for hard, filled polymers (carbon-filled PEEK, glass-filled PTFE). The rake angle should be highly positive (10–25°) for plastic drilling, compared to 0–10° for metal drilling. The positive rake angle reduces the cutting force by helping the chip flow across the rake face, reducing the frictional heat generation. The rake face must be polished to a mirror finish (Ra < 0.05 µm) — a polished rake face reduces chip adhesion (built-up edge) by 80–90% compared to a ground carbide surface (Ra 0.2–0.5 µm), and chip adhesion is the primary cause of heat buildup and melting in plastic drilling. The rake face can be polished by diamond paste (3 µm abrasive, followed by 1 µm) on a felt wheel, or by a PCD-tipped tool that has a naturally smooth rake face.
How does the selection of coolant differ for plastic deep hole drilling compared to metal drilling, and when should each coolant type be used?
The selection of coolant for plastic deep hole drilling is determined primarily by the thermal management requirement (keeping the bore surface below the material's melting/softening point) and the chemical compatibility of the coolant with the polymer (some polymers absorb water, some are attacked by solvents in the coolant, and some require a non-contaminating coolant for medical or optical applications). The three coolant types used for plastic deep hole drilling are compressed air (with or without vortex cold air), water-miscible emulsion (low concentration, 2–5%), and water with a wetting agent (0.5–1% additive). Compressed air (6–10 bar, 500–2000 L/min) is the default coolant for most engineering thermoplastics because it is chemically inert (no reaction with any polymer), does not cause moisture absorption (critical for nylon, which absorbs 2.5 wt% water in 24 hours from water-based coolant), and leaves no residue (essential for medical and optical components). The cooling capacity of compressed air can be enhanced by a vortex cold-air gun, which uses the Ranque-Hilsch effect to split the compressed air into a hot stream (exhausted) and a cold stream (directed at the cutting zone) at −10 to −30°C. The vortex cold air increases the heat transfer coefficient by 2–3× compared to ambient air and provides a constant-temperature coolant (−20°C) that prevents the bore surface from exceeding the polymer's melting point.
Water-miscible emulsion (2–5% concentration, 10–15°C) is used for polymers that have high thermal conductivity (filled PEEK at 0.45–0.95 W/m·K, where the water-based coolant can remove the heat faster than the polymer can conduct it from the cutting zone), for high-speed drilling of PEEK and acetal where the heat generation rate exceeds the cooling capacity of compressed air, and for unfilled PEEK where no moisture absorption issue exists. The emulsion must be at low concentration (2–5% instead of the 5–10% used for metal drilling) to reduce the viscosity and improve penetration into the drill-bore annulus. The emulsion temperature must be below 15°C to maintain the bore surface temperature below 60°C (the softening point of many thermoplastics). Water-miscible coolant should not be used for nylon, PVC (the plasticisers in PVC can be extracted by water), or any hygroscopic polymer. Water with a wetting agent (0.5–1% surfactant in deionised water) is used for polymers where the thermal demand requires water-based cooling but the polymer is chemically compatible with water (PEEK, acetal, PVDF). The wetting agent reduces the surface tension of the water, improving its ability to penetrate the narrow drill-bore annulus (0.1–0.3 mm) and increasing the heat transfer coefficient by 10–20% compared to pure water. The water must be deionised to prevent mineral deposits on the bore surface. Dry machining (no coolant) is used only for PTFE (which has an exceptionally low coefficient of friction — 0.04 — and generates very little heat) and for short drilling cycles (< 5 seconds total cutting time) where the heat generation is below the thermal threshold.
The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified manufacturing engineers, polymer suppliers, and tooling manufacturers for specific plastic deep hole drilling applications. Data and recommendations are based on published research and industry experience as of 2026.