POM Plastic (Acetal): Properties, Injection Molding, and When to Use It

For 17 years we have molded acetal valve components for a European medical-gas brand — plugs, stem guides, and check-valve seats, some so small they weigh just five milligrams — and POM is the one resin that holds the seal every time.

This is what nearly two decades of molding precision POM valve parts has taught us about the material: what it is, the properties that matter, the narrow processing window that makes or breaks a run, homopolymer versus copolymer, the defects to watch for, and when POM is the wrong material to specify.

POM Plastic Material at a Glance — Quick Answer

POM plastic material — short for polyoxymethylene, also called acetal or polyacetal — is a semi-crystalline engineering thermoplastic prized for low friction, high rigidity, and dimensional stability. It is the same family of resin sold under the Delrin® trade name, and it is not the same as acetate (cellulose acetate used in eyeglass frames).

Typical homopolymer properties: a 0.15 coefficient of friction (one of the lowest of any plastic), 0.2% water absorption, 62 MPa tensile yield strength, and a 172 °C melting point. Processing runs in a narrow window — melt temperature 190–210 °C (never above 230 °C, or POM decomposes and releases formaldehyde gas), mold temperature 80–110 °C, and a high 2.8–2.9% mold shrinkage.

POM is the default choice for gears, bearings, bushings, valve components, and precision sliding parts. It is not the right choice for strong acids (pH < 4), UV exposure, transparency, or continuous service above 100 °C.

What Is POM Plastic?

POM stands for polyoxymethylene. You will also see it called acetal, polyacetal, or simply POM plastic — they are all the same material. It is a semi-crystalline engineering thermoplastic built from repeating —CH₂O— units, and that highly ordered crystal structure is what gives POM its signature combination of stiffness, low friction, and dimensional stability.

One point of confusion is worth clearing up first. Acetal (POM) is an engineering plastic. Acetate — cellulose acetate — is a completely different material used for eyeglass frames and film. They sound alike and search engines blur them together, but they share almost nothing in performance. If you are sourcing a structural sliding part, you want acetal, not acetate.

You will also hear POM called Delrin®. Delrin® is a DuPont (now Celanese) trade name for homopolymer POM specifically. POM is the generic material; Delrin® is one branded grade of it. So when a drawing specifies "Delrin," it is calling out homopolymer acetal — which most molders, including us, supply under their own resin grade.

The chemistry explains the behavior. The tightly packed acetal backbone crystallizes to a high degree, which produces high rigidity, a self-lubricating low-friction surface, and very low moisture uptake. For the authoritative chemistry background, see the Wikipedia entry on polyoxymethylene.

What Are the Key Properties of POM Plastic?

POM is best pictured as a four-trait engineering plastic: rigid, low-friction, dimensionally stable, and self-lubricating. It is not the toughest or the most heat-resistant resin on the shelf, but for precision moving parts nothing matches its blend of stiffness and slip.

The table below lists representative values for the general-purpose homopolymer grade (M90-type) we run, drawn from our supplier datasheet:

Property	Value (M90 homopolymer)	Test Method
Density	1.41 g/cm³	ISO 1183
Tensile yield strength	62 MPa	ISO 527
Tensile modulus	2,700 MPa	ISO 527
Elongation at break	30–50 %	ISO 527
Flexural strength	61 MPa	ISO 178
Flexural modulus	2,400 MPa	ISO 178
Charpy notched impact	7 kJ/m²	ISO 179
Izod notched impact	7.5 kJ/m²	ISO 180
Rockwell hardness	M82 / R114	ISO 2039
Melting point	172 °C	ISO 3146
HDT @ 1.8 MPa	115 °C	ISO 75
Max continuous service temp	100 °C	Internal
Water absorption (24 h / sat.)	0.2 % / 0.7 %	ISO 62
Mold shrinkage (flow / cross)	2.8–2.9 % / 2.1–2.4 %	Internal
Coefficient of friction (vs steel)	≈ 0.15	—
Flammability	UL 94 HB	UL 94

A few numbers are worth memorizing if you are specifying POM:

• Coefficient of friction ≈ 0.15 — among the lowest of any unfilled engineering plastic, and it lubricates itself with no added grease.
• Water absorption 0.2% — roughly one-fifth that of PA66 nylon. This is why POM holds tolerance where nylon swells.
• HDT 115 °C, but 100 °C continuous — POM survives short heat spikes but should not be loaded continuously above 100 °C.
• Shrinkage 2.8–2.9% — high, and the mold has to compensate for it. More on that below.

For a broader framework on reading these numbers, see our guide to the key properties every injection molding engineer should know.

How Is POM Injection Molded?

POM molds well, but it is one of the least forgiving resins on melt temperature. Get it wrong and the material tells you immediately — with smell.

Does POM Need Drying?

Usually not. POM absorbs only about 0.2% moisture, so most batches mold straight from the bag. In humid weather we still dry at 80 °C for 2 hours as cheap insurance against surface streaking.

Why Is Barrel Temperature So Critical?

Homopolymer POM runs in a narrow window — a recommended melt range of 190–210 °C. The hard ceiling is 230 °C. Push past it, even for a few minutes of dwell, and POM thermally decomposes and releases formaldehyde gas. You get splay, bubbles, a sharp acrid smell, and mold corrosion. That is why our POM melt-temperature and purge discipline is stricter than for any other resin we run.

How Does Mold Temperature Affect the Part?

We run molds at 80–110 °C. A hotter mold raises crystallinity and surface gloss and reduces post-mold shrinkage — at the cost of a longer cycle. For a typical small valve part such as a safety-valve stem guide, cycle time lands around 32.7 seconds.

Why Does POM Keep Shrinking After Ejection?

This is the trap with POM. Flow shrinkage is high at 2.8–2.9%, and the part keeps shrinking for up to 24 hours after it leaves the mold as crystallization finishes. For a tight ±0.05 mm valve part, we let parts stabilize for 24 hours before final dimensional inspection, and we compensate the steel for the full shrinkage up front. We size that compensation through experience-based DFM review using SolidWorks 3D CAD and three decades of family-shop tooling judgment.

What About Shutdown Discipline?

Two rules are non-negotiable. Any stop over 10 minutes means purge the barrel and drop the temperature below 180 °C. And POM never shares a machine or a hopper with PVC, for the reason in the safety note above.

POM Homopolymer vs Copolymer — Which Should You Use?

POM comes in two backbones, and the difference matters for demanding parts.

	POM-H (Homopolymer)	POM-C (Copolymer)
Crystallinity	Higher	Slightly lower
Tensile / stiffness	Higher	Slightly lower
Melting point	~172 °C	~10 °C lower
Long-term heat / chemical	Slightly weaker above ~100 °C	Slightly better
Processing window	Narrower	Wider
Best known as	Delrin®	Celcon® / Hostaform®

In short, homopolymer buys you the highest stiffness, strength, and melting point, while copolymer trades a little of that for better long-term thermal and chemical stability and an easier processing window.

Where we sit: Weilin runs homopolymer POM (a general-purpose M90-type grade) as our single mainstay for valve and sliding components — the same grade documented in the datasheet above.

In the interest of an honest capability boundary, here is what we do not do with POM:

• No glass-fiber-reinforced POM (the gain is small and rarely specified).
• No PTFE-modified ultra-low-friction POM grades.
• No food- or drinking-water-contact-certified POM batches (no NSF/KTW lots).

For a wider material-selection view, see our injection molding material selection guide.

POM vs Other Plastics — When Should You Choose POM?

POM lives in the "low-friction + dimensionally stable + rigid" sweet spot. Whenever one property gets extreme — heat, UV, transparency, strong acid, or ultimate slipperiness — a more specialized resin wins.

You are choosing between	Choose POM if…	Choose the other if…
POM vs Nylon (PA)	You need dimensional stability, low moisture uptake, low friction, creep resistance (gears, bearings)	You need higher toughness, impact, or heat resistance (PA66)
POM vs ABS	You need a load-bearing, wear-resistant, precision engineering part	You need a cosmetic housing, easy coloring, or lower cost
POM vs PC	You need rigidity, wear, low friction, solvent resistance	You need transparency, extreme impact, or service > 100 °C
POM vs PP	You need stiffness, precision, and wear resistance	You need strong-acid resistance, a living hinge, or lower density/cost
POM vs PTFE/UHMW	You need stiffness and strength in a structural sliding part	You need ultimate low friction or chemical inertness (non-load-bearing)

The core rule: POM is the three-in-one of low friction, dimensional stability, and stiffness. Any single extreme requirement should send you to a dedicated resin.

For the nylon comparison in depth, see our PA6 vs PA66 nylon guide. For the cosmetic-housing side, see our guide to ABS plastic injection molding.

Common POM Molding Defects (and How to Fix Them)

Most POM defects trace back to the same handful of root causes — and nearly all of them start with melt temperature. The table below summarizes the patterns and the corrective actions that work in practice.

Defect	Root Cause	Corrective Action
Splay / bubbles / acrid smell	Melt above 230 °C — thermal decomposition releasing formaldehyde	Drop melt below 210 °C, shorten dwell, purge the barrel
Part shrinks undersize after ejection	High-crystallinity post-mold shrinkage (continues for 24 h)	Stabilize 24 h before inspection; compensate the mold for ~2.8%
Warpage	Uneven mold temperature plus high crystalline shrinkage	Balance mold temperature; adjust gate and holding pressure
Sticking in the mold	Mold too cold; shrinkage grips the core	Raise mold temperature; check draft angles
Surface streaks / silver	Material degraded from dwell, or trace moisture	Lower temperature, purge, pre-dry at 80 °C if needed
Weak weld line	Cold flow front	Raise melt (still < 230 °C) or mold temperature; adjust gate

For the economics behind scrap, regrind, and yield, see our injection molding cost guide.

Our POM Injection Molding Capability

We are an OEM injection molder based in Ningbo, China, and POM is a core resin in our valve-component work.

For over 17 years, we have supplied acetal valve components to a leading European medical-gas equipment OEM — a family of parts that includes safety-valve plugs, stem guides, check-valve seats, and diaphragm plates. These are micro-precision parts: the smallest weigh as little as ~0.005 g, and they depend on POM's 0.15 coefficient of friction for a sealing surface that slides clean and holds pressure. POM is also used in industrial sliding and structural parts such as diaphragm plates.

Our annual POM consumption runs around 3 tons, almost entirely in black — the standard color for these valve components.

Production Capacity

• In-house Ningbo facility — 5 injection molding machines, 100 T to 150 T clamp tonnage, optimized for small-to-medium precision parts.
• Hefei partner facility — 11 Yizumi presses from 126 T to 1,000 T (partner facility; certifications held by the partner, not by Weilin directly).

Tolerances and Quality

• Standard tolerance: ± 0.05 mm
• Tight tolerance on request: ± 0.02 mm
• ISO 9001 certified production
• Resin: a general-purpose homopolymer POM (M90-type) — datasheet available here

To learn more, explore our injection molding capabilities and our medical-gas component OEM capabilities. For tight-tolerance work specifically, see our precision injection molding guide.