Milling Delrin and Acetal: The Easiest Engineering Plastic to Cut

Acetal (polyoxymethylene, POM) has a combination of properties that make it nearly ideal for milling among plastics: it is rigid, dimensionally stable, low in moisture absorption, and has a low coefficient of friction with good wear resistance. Critically for machining, it produces well-broken chips rather than the stringy swarf that gums up softer plastics, and it cuts with low force to crisp edges and smooth finishes using standard sharp tooling. Shops can run it fast with air blast or no coolant, and it does not melt and smear the way many thermoplastics do if speeds are reasonable. The main thing to respect is heat and thermal expansion. Acetal has a relatively high coefficient of thermal expansion, roughly 10 times that of steel, so parts grow and shrink with temperature more than metal and a part cut warm will measure differently cool. Aggressive cuts or dull tools can also locally soften the surface since acetal melts around 165-175 C. But compared to nylon, PEEK, or polycarbonate, acetal is forgiving: low moisture absorption means it does not swell and drift dimensionally after machining the way nylon does, which is a major reason it is preferred for precision parts.

The two families have a meaningful difference that affects part selection. Delrin is the homopolymer (Delrin 150 is a common general-purpose grade), which has slightly higher strength, stiffness, hardness, and a better surface finish, making it the choice for the most demanding mechanical parts like gears, bearings, and precision components. Its one quirk is a tendency toward centerline porosity in thicker rod, a small void down the core of extruded stock, which matters if you machine into the center of a thick part and need a flawless surface there. Acetal copolymer (such as the Hostaform and Celcon families) trades a little mechanical performance for better long-term chemical resistance, especially to hot water and alkaline environments, more consistent quality without the centerline porosity issue, and better resistance to thermal degradation. For parts exposed to hot water, certain chemicals, or where the centerline porosity of homopolymer rod is a concern, copolymer is the better pick. Both machine almost identically and beautifully; the choice is about the service environment and whether the part needs the homopolymer's slightly higher mechanical edge or the copolymer's better chemical and porosity behavior.

Acetal holds the best tolerances of the common machinable plastics; +/-0.002 in is routine and tighter is achievable, helped by its rigidity, dimensional stability, and low moisture absorption. Finishes come out clean and glossy with sharp tooling. The realistic limits are the same thermal-expansion and stiffness constraints all plastics share, so true metal tenths are not the target, but for a plastic, acetal is exceptionally repeatable, which is why bearings, gears, manifolds, and precision insulators are commonly milled from it. Cost is favorable: acetal is a mid-priced engineering plastic, far cheaper than PEEK, and its fast clean machining keeps part cost low, so it is often the value choice for precision plastic parts. Lead times are short because it machines quickly and rarely needs stress-relief cycles. Where acetal is the wrong choice: it has poor resistance to strong acids and limited high-temperature capability (continuous use around 80-90 C), it is not the best for structural load at temperature, and it cannot be solvent-bonded easily or painted well, so assemblies relying on adhesives may need a different material. For high-heat or aggressive-chemical service step up to PEEK or PTFE; for low-cost non-precision parts, nylon or UHMW may serve. But for everyday precision milled plastic parts, acetal is usually the right and economical default.