Engineering plastics are not just soft metals — they are a different class of material with different drilling physics. Nylon absorbs coolant and swells during drilling. PEEK melts if the cutting speed is too high. Acetal chips and cracks at hole exit if the feed is not reduced. PTFE deforms under the cutting edge instead of shearing — producing a rough surface that looks like it was torn rather than cut. Drilling plastics requires understanding the material's thermal and mechanical behavior — not applying metal-drilling rules.
Material Characteristics
Plastic Properties Affecting Drilling
| Plastic | Melting Point (°C) | Thermal Conductivity (W/m·K) | Coefficient of Friction | Water Absorption (24 hr) | Hardness (Shore D) | Tensile Modulus (GPa) |
|---|
| Nylon 6/6 (PA66) | 255 | 0.25 | 0.2–0.4 | 1.2–2.5% | 80–85 | 2.8–3.5 |
| PEEK | 343 | 0.25 | 0.1–0.3 | 0.1–0.5% | 85–90 | 3.5–4.0 |
| Acetal (POM-H) | 175 | 0.30 | 0.15–0.35 | 0.2–0.5% | 80–85 | 2.8–3.2 |
| PTFE (Teflon) | 327 | 0.25 | 0.04–0.10 | < 0.1% | 55–65 | 0.4–0.8 |
| PET (polyester) | 255 | 0.20 | 0.2–0.4 | 0.1–0.3% | 80–85 | 2.5–3.5 |
| PVC (rigid) | 80–100 (softening) | 0.15 | 0.3–0.5 | 0.1–0.3% | 75–85 | 2.5–3.5 |
| Polycarbonate (PC) | 267 | 0.20 | 0.2–0.4 | 0.1–0.2% | 80–85 | 2.3–2.5 |
| UHMWPE | 135 | 0.40 | 0.1–0.2 | < 0.1% | 60–65 | 0.5–1.0 |
Drilling Challenges by Plastic
| Plastic | Primary Challenge | Secondary Challenge | Typical Applications |
|---|
| Nylon 6/6 | Water absorption — swells during wet drilling — diameter changes after drying | Stringy chips — built-up edge on drill | Bushings — gears — guide rails — insulators |
| PEEK | High melting point — heat concentrates at cutting edge — thermal damage | Expensive material — scrap cost high | Medical implants — aerospace — semiconductor — oil/gas seals |
| Acetal (POM) | Brittle at exit — chipping and cracking | Low thermal stability — heat causes degradation | Precision mechanical parts — valves — pump components |
| PTFE | Very soft — cutting edge pushes material instead of cutting | Low thermal conductivity — heat builds up — material deforms | Chemical processing — seals — gaskets — liners |
| Polycarbonate (PC) | Stress cracking — coolant chemical attack | Stringy chips — poor chip breaking | Transparent parts — medical — lighting — glazing |
| UHMWPE | Very soft — smearing — poor chip formation | Low melting point — chips melt and re-adhere | Wear strips — liners — food processing |
| Feature | Recommendation | Why |
|---|
| Cutting edge | Very sharp — 3–8 µm edge radius | Plastics cut by shearing — a sharp edge produces a clean cut — a dull edge pushes and smears |
| Rake angle | High positive rake — 15–25° | Reduces cutting forces — helps chip flow — reduces heat generation |
| Relief angle | 10–15° | Prevents rubbing — rubbing generates heat that melts or degrades plastic |
| Point angle | 90–120° (lower for soft plastics — higher for filled) | Lower point angle reduces thrust — reduces exit burr in soft plastics |
| Flute finish | Mirror polished — chrome plated or DLC coated | Prevents chip adhesion — plastic chips stick to rough flutes |
| Flute design | Wide, open flutes — large chip space | Plastics produce continuous stringy chips — need large flute volume |
| Helix angle | High helix — 30–40° | Improves chip evacuation — reduces cutting forces |
| Margin width | Narrow margins (0.1–0.3 mm) | Reduces friction against hole wall — reduces heat generation |
Coating Selection
| Coating | Friction Reduction | Wear Resistance | Best For | Not Recommended For |
|---|
| DLC (diamond-like carbon) | Excellent — lowest friction | Good | PTFE — UHMWPE — general plastic | Abrasive-filled plastics |
| Chromium plating | Excellent — smooth surface | Good | Nylon — acetal — general | High-temperature plastics |
| TiN | Moderate | Good | Filled plastics — abrasive grades | Soft plastics (BUE formation) |
| Uncoated carbide | Moderate | Good | Most plastics — general purpose | Sticky plastics (chip adhesion) |
| CVD diamond | Very low friction | Excellent | Highly abrasive filled plastics | Unfilled plastics (overkill) |
| Tool Material | Best For | Notes |
|---|
| Micro-grain carbide (0.5–0.8 µm) | All engineering plastics — standard choice | Sharp edge possible — good wear resistance — best all-around |
| Ultrafine-grain carbide (0.2–0.5 µm) | Filled plastics — abrasive grades | Better edge retention — can produce very sharp edge |
| HSS (M42, T15) | Short runs — soft plastics — low production | Lower cost — sharp edge — but rapid wear in filled grades |
| PCD (polycrystalline diamond) | High-production — abrasive filled plastics | Very long tool life — excellent surface finish — expensive |
Drilling Parameters
Recommended Parameters by Plastic
| Plastic | Cutting Speed (m/min) | Feed Rate (mm/rev) | Coolant Pressure (bar) | Coolant Type | Expected Tool Life (m) |
|---|
| Nylon 6/6 (unfilled) | 80–150 | 0.05–0.15 | 20–50 | Water-based coolant 5–7% or mist/air | 50–200+ |
| Nylon 6/6 (glass-filled) | 40–80 | 0.04–0.10 | 30–60 | Water-based coolant 5–7% | 10–40 |
| PEEK (unfilled) | 40–80 | 0.03–0.08 | 40–80 | Water-based coolant 5–7% — high lubricity | 15–50 |
| PEEK (glass/carbon-filled) | 20–50 | 0.03–0.06 | 50–80 | Water-based coolant 7–8% — EP additives | 5–20 |
| Acetal (POM) | 100–200 | 0.05–0.15 | 20–50 | Water-based coolant 5–7% or air | 100–300+ |
| PTFE (unfilled) | 50–120 | 0.03–0.10 | 20–40 | Water-based coolant 5–7% or air/mist | 30–100 |
| PTFE (filled) | 30–80 | 0.03–0.08 | 30–50 | Water-based coolant 5–7% | 10–40 |
| Polycarbonate | 60–120 | 0.04–0.12 | 30–50 | Water-based coolant 5–7% — avoid aggressive chemicals | 30–80 |
| UHMWPE | 80–150 | 0.05–0.15 | 20–40 | Water-based coolant 5–7% or air/mist | 50–200+ |
Parameter Adjustment Guidelines
| Condition | Adjustment | Reason |
|---|
| Melting or smearing on drill | Reduce cutting speed 30–50% — increase coolant flow | Heat generation too high — plastic melts at cutting edge |
| Rough surface finish in hole | Increase cutting speed — reduce feed — check tool sharpness | Smearing from dull tool — or feed too high for clean shear |
| Chipping at hole exit (acetal, polycarbonate) | Reduce feed to 30–50% for last 3–5 mm — support exit | Brittle plastics fracture at breakout if feed is too high |
| Oversize hole | Check thermal expansion of plastic — measure at room temperature | Plastic expands during cutting — contracts after cooling — measure after stabilization |
| Undersize hole (nylon, moisture-sensitive) | Check water absorption — measure after drying if required | Nylon swells when wet — bore shrinks after drying |
| Chip packing in flutes | Increase coolant pressure — use polished flutes — reduce depth-to-diameter ratio | Plastic chips are stringy and sticky — need high pressure and smooth flutes |
Coolant Requirements
| Plastic | Coolant Recommendation | Coolant Concentration | Special Considerations |
|---|
| Nylon 6/6 | Water-based semi-synthetic | 5–7% | Nylon absorbs water — dimension changes after drying — consider dry drilling or mist if dimensional stability critical |
| PEEK | Water-based semi-synthetic — high lubricity | 5–8% | PEEK has low thermal conductivity — adequate coolant flow essential — mist cooling may be insufficient for deep holes |
| Acetal (POM) | Water-based semi-synthetic or air/mist | 5–7% | Acetal is sensitive to chemical attack — verify coolant compatibility — avoid high-pH coolants |
| PTFE | Water-based semi-synthetic or air/mist | 5–7% | PTFE is chemically inert — compatible with most coolants — coolant mainly needed for chip evacuation |
| Polycarbonate | Water-based synthetic only | 5–7% | Polycarbonate stress-cracks in contact with alkaline solutions — avoid high-pH coolants — use neutral pH synthetic |
| UHMWPE | Water-based or air/mist | 5–7% | Low melting point — coolant essential for heat removal — mist may not be adequate |
Quality Considerations
| Quality Issue | Plastic-Specific Cause | Mitigation | Inspection Method |
|---|
| Hole diameter variation — wet vs dry | Nylon absorbs water — swells when wet — shrinks when dry | Measure after drying if part will be used dry — or control moisture content | Air gauge — measure at controlled humidity |
| Surface finish — rough or torn | Drill edge not sharp enough — plastic smears instead of shearing | Use sharper edge — increase cutting speed — verify edge condition | Profilometer — Ra target < 1.6 µm |
| Hole exit chipping | Brittle fracture at breakout — acetal — polycarbonate | Reduce feed at exit — support exit face — use lower point angle | Visual — microscope |
| Thermal damage — white ring around hole | Heat from cutting degrades plastic — PEEK — acetal | Reduce cutting speed — increase coolant flow — check drill wear | Visual — hardness test near hole |
| Burr at hole exit | Soft plastic pushed out instead of cut — PTFE — UHMWPE | Reduce feed at exit — use back-up support — sharp edge | Visual — deburring if needed |
| Hole taper — entry larger than exit | Drill deflection in soft plastic — tool pressure pushes drill away | Reduce feed — use guide bushing — stiffer drill — shorter overhang | Bore gauge — entry vs exit diameter |
| Stringy chip wrapping around drill | Continuous chip — not breaking | Increase feed — use chip breaker geometry — high coolant pressure to evacuate | Chip form observation |
| Stress cracking — polycarbonate | Coolant chemical attack — stress concentration at hole | Verify coolant compatibility — use stress-relieved material — reduce feed | Visual — crack detection |
FAQ
What is the best drill geometry for drilling plastics?
The best drill geometry for plastics features: very sharp cutting edge (3–8 µm edge radius — a sharp edge shears the plastic cleanly — a dull edge pushes and smears — producing a rough surface and generating excessive heat). High positive rake angle (15–25° — reduces cutting forces — helps chip flow — reduces heat generation — plastics have low thermal conductivity and heat buildup is damaging). High helix angle (30–40° — improves chip evacuation — essential for continuous stringy plastic chips — standard twist drill helixes of 20–25° are inadequate for deep holes in plastics). Polished flutes (mirror finish or DLC/chrome coated — plastic chips adhere to rough flute surfaces — polished flutes prevent adhesion and improve chip flow). Narrow margins (0.1–0.3 mm — reduces friction against the hole wall — reduces heat generation from rubbing). The critical difference from metal-drilling geometry is the cutting edge sharpness — a drill that works well for steel will seem dull and produce a torn, smeared surface when used on plastic. Dedicated plastic-drilling tools with sharper edges and polished flutes are essential for quality results.
What cutting parameters should I use for deep hole drilling PEEK?
PEEK requires careful parameter selection because it has a high melting point (343°C) but low thermal conductivity (0.25 W/m·K) — heat concentrates at the cutting edge and cannot escape through the workpiece. Recommended parameters for unfilled PEEK: cutting speed 40–80 m/min (start at 50 m/min and adjust upward — if the hole surface shows signs of melting or a white heat-affected zone, reduce speed 20%). Feed rate 0.03–0.08 mm/rev (higher feed within range for roughing — lower feed for finishing — consistent feed is critical — don't let the tool dwell). Coolant pressure 40–80 bar (water-based coolant at 5–7% concentration — adequate coolant flow is essential — mist cooling is not sufficient for deep holes in PEEK — the heat generated cannot be removed by mist alone). For glass or carbon-filled PEEK grades: reduce cutting speed to 20–50 m/min (the filler is abrasive — higher speed accelerates tool wear — the filler also reduces the plastic's tendency to melt). Tool life for unfilled PEEK: 15–50 m of hole length per drill — for filled grades: 5–20 m. Use DLC-coated or uncoated micro-grain carbide with sharp edge — and replace the drill at the first sign of surface finish degradation.
How do I prevent hole exit chipping when drilling acetal?
Acetal (POM) is prone to chipping and cracking at the hole exit because it is more brittle than many other plastics — especially at room temperature. To prevent exit chipping: reduce feed rate at exit (program the last 3–5 mm of hole depth at 30–50% of the drilling feed rate — this reduces the thrust force at breakout — the lower force reduces the tendency for brittle fracture). Support the exit face (clamp a backing plate against the exit face — this supports the material around the hole and prevents the crack from propagating). Use a lower point angle (a 90–100° point angle reduces the thrust force compared to a standard 118–130° point — lower thrust = less exit pressure). Use sharp cutting edge (a sharp drill shears through the last material cleanly — a dull drill pushes and bends the material before breaking through — the bending action causes chipping). Pre-warm the material (acetal at 40–50°C is less brittle than at room temperature — if practical, warm the workpiece — this reduces chipping tendency significantly). If chipping still occurs after these measures, add a chamfer at the hole exit before drilling through — the chamfer removes the fragile edge that chips — creating a clean breakout.
Standard water-based metal-cutting coolants can be used for most engineering plastics but verify compatibility: Nylon — compatible with standard semi-synthetic coolants at 5–7% — but nylon absorbs water — parts will swell slightly during wet drilling — if dimensional stability after drying is critical, consider dry drilling or using mist coolant. PEEK — compatible with standard semi-synthetic coolants — adequate coolant flow is essential (PEEK generates significant heat). Acetal — verify coolant pH (acetal is sensitive to high-pH coolants — avoid coolants with pH > 9.5 — some coolants contain chemicals that attack acetal — check with coolant supplier). Polycarbonate — use only neutral-pH synthetic coolants (polycarbonate stress-cracks in contact with alkaline solutions — standard semi-synthetic coolants with pH > 8.5 can cause cracking — use a synthetic coolant formulated for plastics). For all plastics: avoid coolants containing aggressive biocides that may attack the plastic — check with the coolant manufacturer for plastic compatibility. For many short-hole applications in unfilled plastics, air or mist coolant is sufficient — chip evacuation is often the primary function of coolant in plastic drilling, not cooling (because the plastic itself does not conduct heat well — the coolant removes heat by convection).
Why do holes drilled in nylon change size after drilling?
Nylon absorbs water from water-based coolant — the nylon swells as it absorbs moisture, increasing the bore diameter. After drilling, if the part is dried (either by ambient drying or by heat treatment), the nylon shrinks as it loses moisture — the bore diameter decreases. The diameter change can be significant: nylon 6/6 absorbs 1.2–2.5% water in 24 hours of immersion — this translates to approximately 0.3–0.6% linear expansion. For a 10 mm hole: the bore could change by 0.03–0.06 mm between the wet (as-drilled) and dry (after storage) condition — significant for precision applications. To manage this: measure drilled holes after the part has reached equilibrium with its service environment (if the part will be used dry, dry the part after drilling and measure the bore in the dry condition). If the part will be used in a humid environment, condition the part to the expected humidity before final measurement. For precision bores in nylon, consider drilling slightly undersize (0.03–0.05 mm smaller than target for a 10 mm hole) to allow for swelling if the part will be used wet — or oversize if it will be used dry. The same effect occurs to a lesser degree in other moisture-absorbing plastics (nylon, some polyurethane grades) — PTFE, PEEK, acetal, and polycarbonate have very low water absorption and negligible dimensional change from moisture.
Drilling engineering plastics requires a different approach than drilling metals. Use sharp cutting edges (3–8 µm radius) with high positive rake angles and polished flutes. Run higher cutting speeds (40–200 m/min depending on plastic) with moderate feeds — reduce speed if melting occurs. Use coolant for chip evacuation and heat removal — but verify coolant compatibility with the specific plastic (polycarbonate requires neutral-pH synthetic coolant — acetal is sensitive to high pH — nylon absorbs water and swells). Manage exit chipping in brittle plastics by reducing feed at breakout. For nylon bores, account for dimensional change from moisture absorption — measure in the service condition. With the right tool geometry and parameters, deep hole drilling in plastics produces clean, accurate holes. This article reflects industry practice as of 2026.
