Two Plastics, One Decision You Will Make Over and Over

Almost every mechanism that moves ends up with a nylon part or an acetal part in it. Gears, cams, bushings, slides, latches, rollers, thrust washers, lead-screw nuts. Both materials are self-lubricating, both machine well, both mold well, and both cost a fraction of a metal equivalent. That similarity is what makes the choice easy to get wrong. The two behave very differently once the part is in a real assembly with real humidity, real load, and real tolerances.

Acetal is polyoxymethylene, sold as POM, and you will see it under trade names like Delrin (homopolymer) and Celcon or Hostaform (copolymer). Nylon is polyamide, most often nylon 6 or nylon 6/6, with cast nylon, glass-filled nylon, and MoS2 or oil-filled grades as variants. Below is the practical comparison, not the datasheet one.

Moisture Absorption Decides More Cases Than Anything Else

This is the single biggest split between the two. Nylon 6/6 absorbs roughly 1.0 to 1.3 percent water at 50 percent relative humidity and up to about 2.5 percent at full saturation. That water is not cosmetic. It swells the part. A nylon component can grow 0.2 to 0.6 percent in linear dimension between a dry, freshly machined state and equilibrium in a humid environment. On a 4 in (100 mm) part that is 0.008 to 0.024 in of growth, which is far larger than the tolerance most designers put on the drawing.

Acetal absorbs roughly 0.2 to 0.8 percent and moves accordingly less. If you are holding a bore concentric to a shaft within a few thousandths, or cutting gear teeth that must mesh at a fixed center distance, acetal is the default and nylon is the material that will make you chase a phantom quality problem for months.

Two consequences. Inspect nylon parts after conditioning, not straight off the machine, or your first article data is meaningless. And a nylon part sized for Phoenix will not behave the same in Miami. Stacking those variations across an assembly is what tolerance stack-up analysis exists to catch.

Where Acetal Wins

  • Dimensional stability. Tight-tolerance bores, gear pitch diameters, valve internals, precision cams.
  • Stiffness and creep resistance. Flexural modulus around 400 to 450 ksi, and it holds load over time better than unfilled nylon, which matters for anything preloaded or spring-loaded.
  • Low, consistent friction dry. Coefficient of friction against steel around 0.2, with very little stick-slip. Good for unlubricated slides where a squeak is a warranty call.
  • Machinability. It cuts cleanly, holds a sharp edge, and does not fuzz. Machined acetal prototypes correlate well with molded production parts.
  • Fatigue in flexing features. Acetal is the classic material for living hinges and spring latches that cycle. If your part is a latch, read snap-fit design before you pick a wall thickness.

Where Nylon Wins

  • Impact and toughness. Nylon absorbs a hit that cracks acetal. Acetal is notch sensitive; a sharp internal corner is a crack starter.
  • Abrasion resistance. Under grit, sand, or repeated sliding against a rough counterface, nylon outlasts acetal.
  • Higher load capacity when lubricated. Oil-filled cast nylon carries a considerably higher PV limit than acetal in a greased bushing.
  • Chemical exposure to fuels, oils, and alkalis. Nylon handles them well. Acetal does not tolerate strong acids or chlorinated water, and chlorine exposure is a documented cause of acetal cracking in plumbing and pool equipment.
  • Bonding and finishing. Neither is easy, but acetal is worse. Its surface energy is so low that adhesives and paint barely hold without flame or chemical treatment. Design for mechanical fastening in acetal, and see threads in plastic parts for how to do that without stripping bosses.

Grades Matter As Much as the Family Name

Saying "nylon" on a drawing is not a specification. Cast nylon 6 comes in large stock shapes and machines to tighter tolerances than extruded rod. Nylatron and other MoS2-filled grades run better dry. Thirty percent glass-filled nylon roughly doubles stiffness but becomes abrasive to whatever it slides against, so never pair it against a soft mating part.

On the acetal side, homopolymer is stiffer and stronger but can have centerline porosity in large-diameter rod, which shows up as a leak path when you machine a fluid passage through the middle. Copolymer holds up better in hot water and caustic environments. Call out the specific grade and supplier designation, along with the surface finish of the mating part, in the drawing notes that go into your manufacturing data package.

Molding Behavior You Have to Plan For

Both materials are semi-crystalline and both shrink a lot. Acetal shrinks roughly 1.8 to 2.5 percent, nylon roughly 1.0 to 2.0 percent, and both shrink anisotropically depending on flow direction. On a molded gear that means the pitch diameter you get is not the one you drew unless the toolmaker steels the cavity to allow correction. Budget for a first tool iteration on any precision molded gear. Thick sections in either material sink badly, so the usual uniform-wall discipline in wall thickness for molded parts applies with no exceptions.

Nylon also has to be dried before molding, so ask a supplier how they dry the resin and what moisture level they mold at.

Pairing Rule: Do Not Run Like Against Like

Acetal sliding on acetal, or nylon on nylon, tends toward adhesive wear and galling under load. Pair dissimilar materials: acetal against steel, nylon against acetal, or a filled grade against an unfilled one. This one rule fixes a surprising share of wear complaints. When a mechanism keeps wearing anyway, the underlying cause is usually load or misalignment rather than material, which is the territory of material fatigue in product design.

Cost and Availability

Stock shapes run roughly $5 to $9 per lb for nylon 6/6 and $8 to $15 per lb for acetal, with cast and filled grades higher. At prototype quantities that difference is noise next to machine time, and in a molded part what actually drives cost is cycle time and tooling, as laid out in injection molding cost. Both are widely stocked in standard rod and sheet, so lead time is days unless you specify an unusual filled grade.

Get the Material Call Right the First Time

Projects House specifies and tests moving-part materials as part of mechanical design work, including wear testing on real counterfaces before a tool is cut. Send your part function, load, duty cycle, and environment through our contact form and we will tell you which grade the application actually needs.