POM / Acetal (Delrin) Injection Molding: A Buyer's Guide

If you’ve used a plastic gear, a snap-action mechanism, or a precision moving part that just works smoothly year after year, there’s a good chance it was acetal. POM is the engineering resin buyers choose when dimensional precision, low friction, and fatigue resistance matter most. This guide covers it from a buyer’s standpoint, as part of the material selection guide.

What POM / Acetal Is

POM (polyoxymethylene), commonly called acetal and known by the brand name Delrin, is an engineering thermoplastic prized for high stiffness, excellent dimensional stability, low friction, and outstanding fatigue resistance. It comes in two forms—homopolymer (often higher strength) and copolymer (often better chemical and thermal stability)—and is a default choice for precision mechanical parts.

Why Buyers Choose POM

Characteristic	What it means for your part
Excellent dimensional stability	Holds tight tolerances; low moisture absorption
Low friction / self-lubricating	Ideal for gears, bearings, and sliding parts
High stiffness and strength	Rigid, strong functional parts
Outstanding fatigue resistance	Survives repeated loading—springs, clips, mechanisms
Good chemical resistance	Resists fuels and many solvents

Compared with nylon, acetal’s big advantages are dimensional stability (it absorbs far less moisture) and low friction, which is why precision gears and mechanisms so often specify it.

Where POM Falls Short

• Prone to sink and harder to mold thick. Acetal crystallizes and shrinks significantly as it cools, making it prone to sink marks and voids in thick sections—uniform, modest wall thickness matters.
• Very difficult to bond and paint. Acetal’s chemical resistance cuts both ways: adhesives and coatings don’t stick well, so mechanical joining is usually preferred.
• Limited high-heat capability compared with high-performance polymers.
• Flammable and sensitive to thermal degradation if overheated during processing, which demands process control.

The molding behavior—high shrink, sink tendency, and a relatively narrow thermal processing margin—means acetal rewards a disciplined molder.

Common Applications

POM goes into gears, bearings, bushings, and cams; precision mechanical and snap-fit parts; springs and clips that flex repeatedly; fluid-handling components; automotive fuel-system and interior mechanisms; and consumer products with moving parts. Anywhere precision, low friction, and fatigue life are the priority, acetal is a leading candidate.

What Buyers Should Know About Molding POM

• Keep walls uniform and avoid thick sections. Acetal’s high shrink makes sink and voids a real risk in heavy areas—coring out thick sections and keeping walls even is the standard approach (see the DFM guide).
• Expect mechanical joining, not gluing. Design for snaps, screws, or press fits rather than adhesives.
• Process control matters. Overheating acetal can degrade it (and release gas), so a molder’s temperature and process discipline are relevant.
• Specify homopolymer vs copolymer. They differ in strength, chemical, and thermal behavior, so name the type or the requirement.

Typical Processing Window

POM has a narrower, less forgiving window than commodity resins—run too hot or held too long, it degrades and can release gas, so an experienced molder watches barrel temperature and residence time closely. The ranges below are illustrative for unfilled acetal:

Parameter	Typical range (unfilled POM)
Drying	~80 °C for 2–3 h (light)
Melt (barrel) temperature	~190–215 °C (narrow; avoid overheating)
Mold temperature	~60–105 °C
Mold shrinkage	~1.8–2.5% (high; thick sections are prone to sink and voids)

Illustrative ranges for unfilled grades, drawn from general industry processing references and a factory parameter table—not a substitute for the specific grade’s datasheet. Actual settings depend on grade, geometry, and machine, and filled grades differ (notably lower, more directional shrinkage). Confirm with the resin datasheet and your molder.

How POM Compares

POM and nylon are the two go-to resins for mechanical parts. Acetal wins on dimensional stability, low friction, and moisture insensitivity; nylon wins on toughness, heat resistance, and glass-reinforced strength. Against commodity resins, acetal is far more precise and durable at higher cost. Choose acetal when precision and low friction lead—see the material selection guide.

This is an independent buyer resource, not materials-engineering advice. Confirm homopolymer vs copolymer and the right grade with the datasheet and your supplier.

Buyer FAQs

What is acetal (POM) used for in injection molding?

Acetal is used for precision, low-friction, fatigue-resistant parts: gears, bearings, bushings, cams, snap-fit mechanisms, springs and clips, fluid-handling components, and automotive fuel-system and interior parts. Its dimensional stability and self-lubricating nature make it a default for moving and precision mechanical parts.

What is the difference between Delrin, acetal, and POM?

They refer to essentially the same material. POM (polyoxymethylene) is the polymer’s technical name, “acetal” is the common name, and Delrin is a well-known brand of acetal homopolymer. The meaningful distinction for buyers is homopolymer versus copolymer: homopolymer often offers higher strength, while copolymer often offers better chemical and thermal stability.

Why does acetal sink or form voids in thick sections?

Acetal is semi-crystalline and shrinks significantly as it cools and crystallizes. In thick sections, the surface solidifies while the interior keeps shrinking, pulling the surface in (sink) or creating internal voids. The fix is design: keep walls uniform and modest, and core out heavy sections rather than leaving thick mass—the same wall-thickness discipline that prevents sink in general.

Can you glue acetal?

Not easily. Acetal’s excellent chemical resistance also means adhesives and coatings bond poorly to it without special surface treatment. For assembly, mechanical joining—snap-fits, screws, press fits—is usually preferred and more reliable. If bonding is essential, discuss surface-treatment options, but designing for mechanical joining is the standard approach with acetal.