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Ask ten engineers whether a part should be machined from Delrin or injection molded from POM, and at least a few of them will start comparing the wrong things. Delrin and POM are not two competing materials. Delrin is simply DuPont's brand name for POM homopolymer, the most widely used grade of polyoxymethylene (acetal). The real decision is between two very different manufacturing routes applied to the same base resin: CNC machining from solid stock, or injection molding with dedicated tooling. This guide breaks down how the two routes differ in cost, lead time, accuracy, and part performance, and how to pick the right one for your project.
POM, short for polyoxymethylene, is a semi-crystalline engineering thermoplastic also sold under names like acetal and polyacetal. It comes in two forms:
Both forms machine well and both can be injection molded. So when someone asks about "Delrin CNC machining versus POM injection molding," they are really asking a process question: should this acetal part be cut from solid rod or sheet, or molded in volume with a tool?
Delrin CNC machining is a subtractive process: milling, turning, drilling, and boring operations cut the part directly out of extruded POM rod, sheet, or block. There is no mold, no tooling investment, and no waiting for a toolmaker — programming starts as soon as the drawing is approved, and first parts typically ship in days.
Delrin is one of the friendliest engineering plastics on the machine. It produces clean chips, holds sharp edges, and its moisture absorption stays below 0.2%, so finished dimensions remain stable after the part leaves the shop. A general machine shop holds ±0.05 mm on acetal without much trouble; a precision shop working with stress-relieved stock and temperature-controlled inspection can reach ±0.01–0.02 mm on critical features.
The material does have one machining quirk worth knowing: its melting point is only around 175°C, and its low thermal conductivity traps heat at the cutting edge. Experienced shops compensate with sharp single-flute or polished carbide tools, high cutting speeds with moderate feed rates, and air blast cooling instead of flood coolant. Done right, the result is a smooth, near-metal surface straight off the machine.
Injection molding takes the opposite approach. POM pellets are melted and injected under pressure into a hardened steel or aluminum mold, the part cools and solidifies in seconds, and the cycle repeats. Once the mold is dialed in, the machine produces identical parts around the clock with very little labor per piece.
The trade-offs sit up front. A production mold costs anywhere from $5,000 to well over $100,000 depending on part size and complexity, and building it takes weeks. The design also has to follow molding rules: draft angles on vertical walls, uniform wall thickness, carefully placed gates, and no undercuts unless the mold gets slides or lifters. POM adds one more wrinkle — it is a highly crystalline resin with relatively high mold shrinkage, typically around 1.5% to 2.5% depending on grade and wall thickness, so the mold designer has to scale the cavity to compensate, and final dimensions still drift slightly with processing conditions.
| Factor | Delrin CNC Machining | POM Injection Molding |
|---|---|---|
| Tooling investment | None | $5,000–$100,000+ for the mold |
| Lead time to first part | Days | Weeks (mold fabrication and trials) |
| Cost-effective volume | 1 to roughly 1,000 parts | Thousands to millions of parts |
| Per-part cost | Higher, and roughly flat with volume | Very low once the mold is amortized |
| Dimensional accuracy | ±0.05 mm standard; ±0.01–0.02 mm achievable | Looser, limited by 1.5–2.5% shrinkage control |
| Design freedom | Undercuts, sharp internal features, no draft needed | Draft, uniform walls, and gate placement required |
| Cost of a design change | Edit the program, re-cut the part | Modify or rebuild the mold |
| Typical part risks | Centerline porosity in thick POM-H rod | Sink marks, weld lines, gate vestige, warpage |
Two parts with identical dimensions can behave differently in service depending on how they were made. A few differences matter most for acetal.
Extruded Delrin rod — especially in larger diameters — can carry microscopic voids along its centerline, a side effect of how the stock cools during extrusion. For pressurized fluid parts or critical structural cores, machinists either buy oversized stock and cut away the porous center, or switch to copolymer POM-C, which shows far less porosity. Molded POM parts avoid centerline porosity entirely, but they face their own defect family: sink marks over thick sections, weld lines where flow fronts meet, and warpage from uneven cooling.
Extruded stock carries residual stress from manufacturing, and removing material unevenly can make a machined part warp after it leaves the vise. Good shops manage this with annealed stock, balanced roughing passes, and a rest period before finishing. Molded parts lock in stress around gates and thin-to-thick transitions instead. Either way, POM's low moisture absorption helps: unlike nylon, an acetal part does not grow measurably when the weather turns humid.
A machined Delrin part never shrinks — the dimension you cut is the dimension you get, limited mainly by thermal expansion during cutting and measurement. That is why machining owns the tight-tolerance end of the market. Molding repeatability depends on shrinkage compensation, mold temperature control, and process discipline; even a well-run mold holds looser tolerances than a decent machining center, though it holds them across far larger quantities.
Machining costs behave like a straight line: programming and setup on the first order, then a fairly constant per-part price made of machine time and material. Molding costs behave like a cliff followed by a plateau: a heavy one-time mold investment, then pennies to a few dollars per shot. Where those two curves cross depends on part size, cycle time, and mold complexity, but in practice the crossover lands somewhere between a few hundred and several thousand parts — near 1,000 pieces for a simple part in a single-cavity mold, and past 10,000 when the mold is complex.
A practical rule: below a few hundred parts, machine it. Above several thousand with a frozen design, mold it. In the gray zone between, get both processes quoted — and remember that many programs start with machined prototypes and bridge production, then cut a mold once the design and demand are proven.
Choose Delrin CNC machining when:
Choose POM injection molding when:
Choosing between machining and molding is easier when your supplier runs both. ANOK Precision Manufacturing, an ISO 9001:2015 certified factory in Shenzhen operating since 2007, machines Delrin and the full acetal family alongside ABS, nylon, PEEK, PTFE, polycarbonate, and other engineering plastics on 3-, 4-, and 5-axis machining centers and CNC lathes. For programs that outgrow machining, the same team supports injection molding, 3D printing, and sheet metal under one roof, so the transition from prototype to production never changes hands.
On the machining side, ANOK's plastic CNC machining service routinely holds ±0.02 mm on Delrin components — gears, bushings, rollers, manifolds, and fixture components — with ±0.01 mm achievable on critical features, and every quote includes free DFM feedback on material grade selection, tolerancing, and whether machining or molding fits your volume. If you are comparing routes for an upcoming project, send your drawings for a quote and a process recommendation from a precision CNC machining team that works with acetal every day.
Delrin is DuPont's brand name for POM homopolymer (POM-H). All Delrin is POM, but not all POM is Delrin — copolymer acetal (POM-C) is sold under other trade names and behaves slightly better in hot water and strong chemicals.
CNC machining, by a wide margin. Machined Delrin holds ±0.05 mm as standard and ±0.01–0.02 mm on precision features, while molded POM parts are limited by 1.5–2.5% shrinkage control and typically land several times looser.
Yes, and it is common practice. Shops frequently add tight-tolerance bores, threads, and sealing faces to molded blanks as secondary operations, combining molding's unit cost with machining's precision where it counts.
There is no hard floor, but below roughly a thousand parts the mold cost rarely amortizes well unless the part is tiny and the tool is simple. For anything smaller, machining is usually the more economical answer.
Delrin CNC machining and POM injection molding are not rivals — they are two stages of the same material's lifecycle. Machining wins on speed, precision, and flexibility at low volume; molding wins on unit cost and consistency at high volume. The smart move is to match the process to your volume, tolerance, and design maturity, and to work with a supplier who can honestly quote both sides of the break-even line.