POM (Acetal) Injection Molding: Thermal Stability and Tooling

What POM is for

Polyoxymethylene — acetal, POM — is a highly crystalline engineering plastic with an unusual combination: high stiffness, low friction, excellent fatigue resistance, very good machinability, and dimensional stability that nylon cannot match because POM absorbs almost no water.

That last property is why POM wins so many precision parts. A POM gear holds its dimensions from the day it is moulded to the day it is scrapped, in any humidity. A nylon gear does not. For anything that must fit, run quietly and stay in tolerance for years, POM is usually the right answer.

Grades

Table 1 — POM grade families. The homopolymer / copolymer choice is the first decision.

The homopolymer versus copolymer distinction matters more than it does for most materials. POM-H is about 10 – 15 % stiffer and stronger, and has better creep resistance. POM-C is more resistant to thermal degradation, more resistant to hot water and weak acids, and has a wider processing window. For a part that will be moulded in a difficult tool or run on an old machine, POM-C is the safer choice.

Properti

Table 2 — POM properties. The equilibrium water absorption of 0.5 – 0.8 % is 3 – 4× lower than nylon.

That last row is the commercial argument for POM over nylon in precision parts. Nylon grows 0.3 – 0.6 % in service; POM grows 0.1 – 0.2 %. On a 50 mm part that is the difference between 0.25 mm of movement and 0.06 mm — the difference between a tolerance that holds and one that does not.

Thermal stability: the failure mode that matters

POM decomposes by unzipping — the polymer chain depolymerises from the end, releasing formaldehyde, and the formaldehyde catalyses further decomposition. Once it starts, it accelerates. A barrel left hot with POM in it over a weekend can be destroyed, and the gas is flammable and toxic.

Table 3 — POM stability failures. Every one of these is avoidable with discipline.

The practical rules that come out of this table:

• Size the machine to the tool. A shot that uses less than 40 % of the barrel capacity leaves material in the barrel for too long. This is the most common cause of POM problems in job shops.
• Purge before every stop. Natural PE or PP is the standard purge; there are also commercial POM purge compounds.
• Never let the barrel sit hot and idle for more than 15 minutes. If a run pauses, either keep cycling or purge and shut down.
• Ventilate. Formaldehyde is a respiratory irritant and a suspected carcinogen. Extraction at the nozzle and at the mould is normal practice.

Processing window

Table 4 — POM processing. The 220 °C ceiling is not a suggestion.

Mould temperature is the quality lever. At 60 °C a POM part has a thin crystalline skin over a less crystalline core, and it will wear faster and creep more. At 100 °C the part is uniformly crystalline, dimensionally stable and has the wear resistance the datasheet promises. The cycle is longer — 25 – 35 % longer than a cold mould — but for a gear or a bearing, it is the difference between a part that lasts and one that does not.

Design rules

Table 5 — POM design rules. The 2 % shrinkage is the number toolmakers must not guess at.

Tooling: steel, gates and hot runners

Table 6 — POM tooling. Stainless steel is not a luxury — it is a maintenance requirement.

POM as a machined material

POM is one of the best engineering plastics to machine: it produces a continuous chip, holds tolerance, takes a fine finish, and does not need coolant for most operations. That makes it the standard choice for prototypes and low-volume precision parts before a tool exists.

Table 7 — Machining POM. The tap oversize note saves a lot of broken taps.

Defects and causes

Table 8 — POM defects. Any smell of formaldehyde means stop and purge.

Where POM wins over nylon

Table 9 — POM versus nylon. The hot-water row is decisive: nylon degrades in continuous hot water, POM-C does not.

Mould temperature and the cycle-time trade

POM is the clearest case in injection moulding where a longer cycle buys a better part. The mould temperature controls crystallinity, and crystallinity controls wear resistance, creep and dimensional stability.

Table 9 — Mould temperature versus crystallinity for POM. A 50 % cycle increase buys roughly double the wear life.

Whether that trade is worth it depends entirely on the part. For a cosmetic clip, a cold mould is correct — the part is not loaded and the cycle matters. For a gear running at 3,000 rpm in a power tool, a 90 – 100 °C mould is the difference between a gearbox that lasts five years and one that whines and fails in eighteen months. Both parts might come off the same machine on the same day, and they need different moulds.

What tolerance POM can actually hold

Table 10 — Realistic POM tolerances. These are production figures, not best-case.

The reason POM holds tolerance better than most semi-crystalline materials is that it does not absorb water — the dimension you measure a day after moulding is the dimension it will have next year. That stability is worth more than the absolute tolerance figure, because it means a part that passes at incoming inspection still passes at assembly, six months later, in a humid warehouse.

POM versus other wear materials

Table 11 — Wear material comparison. POM wins on friction plus stability plus machinability, which is a combination nothing else matches.

The temperature column is where POM loses. Above 100 °C continuous, POM softens and creeps, and a gearbox running at 110 °C will fail even though the material was correct at room temperature. That is the point at which PBT or PPS becomes necessary — and the point at which the part stops being machinable and the tooling cost goes up.

Frequently asked questions