Can Delrin be machined?
Yes. Delrin is among the easiest plastics to machine. Sharp tools and low heat hold about ±0.05 mm on small parts; watch stress and thick-bar porosity.
Acetal is the plastic machinists reach for when a part must be stiff, stable and accurate. Homopolymer (Delrin) and copolymer (POM-C) both cut much like free-machining brass: chips break cleanly, burrs are small and the surface comes off the tool smooth. Acetal absorbs little water, about 0.2-0.25% in 24 hours and under 1% at saturation, so it holds dimensions far better than nylon. That is why it is the default for gears, bushings, rollers, valve seats, manifolds, insulators and inspection fixtures. With good practice, ±0.05 mm (0.002") is realistic on small features and ±0.1 mm (0.004") on general dimensions.
Heat is the enemy, not tool wear. Homopolymer melts at about 175 °C (347 °F) and copolymer at about 165 °C (329 °F), and a dull or rubbing tool gets there quickly and smears the surface. Use sharp, polished tools with high positive rake (uncoated carbide or HSS; coatings with honed, rounded edges tend to rub), high spindle speeds with enough feed to make a real chip, and air blast or a water-soluble coolant. Acetal expands about 100-120 µm/m·°C, roughly ten times steel, so a part measured warm off the machine shrinks as it cools; inspect at 20-23 °C.
Internal stress and porosity are the two stock issues. Extruded rod and plate carry residual stress, and heavy or one-sided material removal releases it, bowing plates and pulling bores out of round. For thin or flat parts, rough, let the part rest or anneal it per the stock supplier cycle, then take a light finish pass. Thick homopolymer rod and plate, roughly 50 mm (2") and up, can also have centreline porosity from the extrusion process: a line of small voids along the core that opens up when a bore or sealing face cuts into it. Copolymer stock is largely free of it, so specify copolymer or certified porosity-free stock for thick sections that seal, hold pressure or will be seen.
CNC Machining for Delrin parts: where to send them
What to specify
- Grade: acetal homopolymer (Delrin 150 or equal) or copolymer (POM-C), and whether "Delrin" means the brand or any acetal
- Stock to ASTM D6778 (POM) where a material specification is required, with certs
- Tolerances suited to plastic: ±0.05 mm (0.002") on small features, ±0.1 mm (0.004") general, measured at 20-23 °C
- Stress-relieved stock, or an anneal between roughing and finishing, for thin-walled and flat parts
- Copolymer or porosity-free stock for thick sections that seal or hold pressure
- Colour (natural or black) and an FDA-compliant grade for food or product contact
- Surface finish on sliding and sealing faces, typically 0.8-1.6 µm Ra (32-63 µin)
Pitfalls
- Centreline porosity in thick homopolymer stock showing up in a bored seal face; use copolymer
- Plates bowing after heavy one-sided pocketing; rough, relieve and finish
- Dull tools rubbing and melting the surface, leaving smear and burrs
- Steel-style tolerances on large parts; a 5 °C swing moves a 200 mm acetal part about 0.1 mm
- Press fits sized like metal; acetal creeps and relaxes, so use the resin maker interference data
- Planning to glue acetal; it bonds poorly, so design snap fits, screws or press fits instead
Frequently asked questions
- Is Delrin easy to machine?
- Yes, it is one of the easiest plastics to machine. Chips break cleanly, burrs are small, and it holds size better than nylon because it absorbs very little water. Keep tools sharp and avoid heat build-up.
- What tolerance can be held on machined Delrin?
- About ±0.05 mm (0.002") on small features and ±0.1 mm (0.004") on general dimensions. Tighter is possible on small turned parts with temperature control, but thermal expansion of about ten times steel limits large parts.
- Delrin or nylon for machined parts?
- Delrin for accuracy and dimensional stability: it absorbs under 1% water at saturation, nylon 6/6 up to about 8%. Nylon is tougher and cheaper and fine where size can drift a little.
