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

Polyoxymethylene (POM), universally known as acetal, is the standard engineering resin for gears, bushings, and precision moving components. It is specified when a part requires strict dimensional stability, natural lubricity, and fatigue resistance under continuous mechanical load.

This guide details acetal’s capabilities, its vulnerability to sink marks in thick sections, and its unique processing constraints, as part of the material selection guide.

What POM / Acetal Is

Acetal is a highly crystalline engineering thermoplastic, widely recognized by the brand name Delrin. It is manufactured in two primary variants:

• Homopolymer (e.g., Delrin): Offers higher tensile strength, stiffness, and fatigue resistance.
• Copolymer (e.g., Celcon): Offers superior chemical resistance, better thermal stability during processing, and improved resistance to hot water.

Unlike nylon, acetal absorbs almost no moisture from the environment. This makes its mechanical properties and dimensions highly predictable in service.

Why Buyers Choose POM

Characteristic	Engineering and Commercial Value
Excellent dimensional stability	Low moisture absorption ensures parts hold tight tolerances in humid environments.
Low friction	Inherent self-lubricating properties make it the default for gears and bearings.
High stiffness and strength	Supports continuous mechanical loads without yielding.
Outstanding fatigue resistance	Withstands cyclic loading in springs, clips, and snap-fits.
Broad chemical resistance	Resists fuels, oils, and organic solvents.

Where POM Falls Short

• Vulnerable to sink and voids. Acetal’s high crystallinity means it shrinks heavily during cooling. Thick cross-sections will almost certainly develop severe sink marks or internal voids. Uniform, thin walls are mandatory.
• Unbondable. Acetal is highly chemically inert. Adhesives and paints will not stick. Assembly requires mechanical fasteners or ultrasonic welding.
• Thermal limits. Acetal loses mechanical strength at high temperatures sooner than specialty polymers like PEEK or PPS.
• Processing degradation. Overheated acetal degrades rapidly in the barrel, releasing formaldehyde gas. It requires strict temperature control and residence-time management from the molder.

Common Applications

Acetal is heavily utilized in precision mechanical assemblies: gears, bearings, cams, fuel system components, lock mechanisms, zippers, seatbelt components, and medical device mechanisms. It is selected whenever friction must be minimized and tolerances must be held tightly over millions of cycles.

What Buyers Should Know About Molding POM

• Design out thick sections. If you design a thick gear hub in acetal, it will form internal voids that weaken the part. Core out the mass and use ribs for strength (see the DFM guide).
• Mechanical assembly only. Do not plan to glue acetal parts. Design snap-fits, use threaded inserts, or specify press fits.
• Process control is a safety issue. Acetal has a narrow processing window. If a molder leaves it hot in a stalled barrel, it degrades into formaldehyde gas, creating a severe pressure and safety hazard. Choose molders with tight process discipline.

Typical Processing Window

POM requires a disciplined molding environment. Running it hotter than specified to improve flow is a failure of process control that leads to material degradation.

Parameter	Typical range (unfilled POM)
Drying	~80 °C for 2–3 h (Recommended for cosmetics, not strictly required for structural integrity)
Melt temperature	~190–215 °C (Narrow window; do not overheat)
Mold temperature	~60–105 °C
Mold shrinkage	~1.8–2.5% (High; thick sections fail structurally due to voids)

These ranges represent general unfilled grades. Always confirm parameters against the manufacturer’s datasheet.

The Drying and Degradation Nuance

SABIC’s processing guide classifies acetal under “drying recommended” rather than “drying required.” Unlike polycarbonate, where moisture destroys the polymer chains (hydrolysis), moisture in acetal merely causes surface splay. SABIC notes 2–4 hours at 200 °F (93 °C) clears these volatiles.

Therefore, the primary molding risk with acetal is not moisture, but heat. Acetal degrades structurally from excessive barrel temperature or prolonged residence time. If a molder suggests raising the melt temperature to fill a thin wall, they are increasing the risk of structural degradation and gas generation. Push back, and verify their residence-time calculations.

How POM Compares

Acetal and nylon dominate the mechanical resin category.

• Acetal wins on dimensional stability (it doesn’t absorb water) and low friction.
• Nylon wins on absolute toughness and heat resistance, especially in glass-filled grades.

If the part operates in a wet environment or requires a precision gear tooth, use acetal. If the part must survive high-impact strikes or elevated temperatures, use nylon. See the material selection guide.

Buyer FAQs

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

Acetal is the default resin for precision moving parts: gears, bearings, cams, clips, fluid valves, and mechanical linkages. Its inherent low friction and resistance to dimensional change under moisture load make it ideal for dynamic mechanical assemblies.

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

They refer to the same material family. POM (polyoxymethylene) is the technical name, acetal is the common generic name, and Delrin is DuPont’s (now Celanese’s) brand name for acetal homopolymer. The most important buyer distinction is between homopolymer (stronger, stiffer) and copolymer (more chemically and thermally stable).

Why does acetal sink or form voids in thick sections?

As a highly crystalline polymer, acetal undergoes significant volumetric shrinkage as it cools from melt to solid. In a thick wall, the outer skin freezes against the mold steel while the molten core continues shrinking. This either pulls the skin inward (sink) or tears the interior plastic apart (void). The only solution is proper DFM: thin, uniform walls.

Can you glue acetal?

No. Acetal’s chemical resistance prevents adhesives from forming a bond. Design the part for mechanical joining (snap-fits, screws) or thermal welding. If bonding is completely unavoidable, specialized chemical etching or plasma treatments are required, adding significant cost.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Material behavior is grade-specific; confirm properties and suitability against the manufacturer’s datasheet and your supplier’s engineering review.