Acetal (POM) Injection Molding

From machined part to moulded part

Acetal’s commercial position is straightforward: it is the material that turns a machined mechanical part into a volume part.

A gear, a bushing, a sprocket or a latch can be turned from rod one at a time. The same part can be moulded by the thousand once a tool exists. Acetal’s properties survive the transition — it is stiff for a thermoplastic, it has low friction, it wears well against steel, and its dimensional stability is good once the part has settled. So a moulded acetal gear runs quietly against a steel pinion, needs no lubrication, does not corrode and weighs a fraction of the bronze it replaced.

That is the same case the machined acetal page makes. The difference is entirely in the economics: moulding is a tool cost plus a very low piece cost, and machining is a higher piece cost with no tool cost. Below the break-even volume the part is machined. Above it, the tool is cut and the part is moulded.

The two characteristics that shape the tool

Shrinkage is high

Acetal is semi-crystalline, and that is the reason its shrinkage sits at roughly 1.6–2.5% — several times ABS’s. As the polymer cools, its chains pack into crystals, and the crystalline phase is denser than the melt, so the part contracts substantially as those crystals form.

Two consequences for the tool:

The mould is cut oversize. The cavity is larger than the finished part by the shrinkage allowance, and the allowance is agreed per drawing rather than read off a generic table, because wall thickness, gate location, packing and mould temperature all shift it within that band.

Shrinkage continues after ejection. Crystallisation is not finished when the part leaves the mould, so dimensions settle over hours to days. A part measured immediately after moulding can differ from the same part measured the next day. On a precision moulded gear or bushing, that settling period is part of the process, and the tool is sized to the settled dimension.

Homopolymer and copolymer grades shrink by different amounts, which is one of the practical differences between the acetal grades and a reason to name the grade on the drawing rather than specifying “acetal”.

The material is heat sensitive

Acetal has a genuine processing limit: overheat it, or leave it resident in the barrel too long, and it degrades, releasing formaldehyde.

That is not a hazard in normal operation — it is a discipline. The controls are standard:

• Barrel temperature is controlled precisely, because the window between “flows properly” and “degrades” is narrower at the top than for ABS or polypropylene
• Residence time is managed. A machine that stops with acetal in the barrel is purged rather than simply restarted
• Purging compounds are used when switching materials, because acetal does not mix well with some other polymers and a contaminated barrel produces defects

Under correct processing the material does not attack mould steel, and ordinary tool steels are adequate. It is worth knowing the constraint exists, because it is the reason an acetal job wants a disciplined moulder rather than a machine that is being pushed.

A hot mould is not optional

Acetal needs a mould temperature of roughly 80–100 °C, which is a hot tool by the standards of ABS or polypropylene.

The reason is the same crystallinity that drives the shrinkage. The crystalline structure forms as the material cools, and it needs time to form properly. A cold mould cools the part too fast, the structure is under-developed, and the part loses the stiffness, wear resistance and dimensional stability that acetal was chosen for — while post-mould shrinkage becomes larger and less predictable.

So a hot-oil or hot-water mould temperature controller is standard equipment on an acetal tool, and the cycle is longer than an equivalent ABS part because the mould has to be hot and the part has to cool enough to eject. That longer cycle is part of the cost of the material, and it is why the tool is sized and the cycle is estimated per drawing rather than assumed.

Design rules

The moulding design rules are the standard set, applied to acetal’s properties.

Keep the wall uniform. A thick section beside a thin one cools at a different rate and shrinks differently, producing sink marks over the thick region and warpage across the part. Where local thickness is needed — a boss, a rib, a mounting pad — core it out, or reduce it to roughly two-thirds of the nominal wall.

Draft the walls. 0.5° minimum, around 1° typical. Acetal releases reasonably well. A textured surface grips more and needs more draft.

Radius the corners. Real parts break at sharp internal corners; a radius of at least half the wall thickness is the usual rule.

Think about the gate. On a gear, gate location affects the weld line — and a weld line running across a tooth is a weak tooth. On a bushing, it affects roundness. Gate position is a functional decision on this material, not just a cosmetic one.

Design around the settling. Where a part has a tolerance that matters, the drawing should say which dimensions those are, so the tool can be sized to the settled condition and the inspection can happen at the right point in the part’s life.

Molding route

Plastic molding runs through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Mould tooling is quoted per drawing, and mould size, shot weight and gate count are reviewed before the tool is cut.

That division suits acetal parts particularly well, because the parts that suit acetal often need a metal feature — a threaded bush, a bearing bore, a hardened wear surface. Because the mould and the inserts come from the same drawing, the plastic-to-metal fit is planned before the tool is cut.

Moulding holds part-scale tolerances agreed per drawing in DFM; it does not match machined metal. Critical fits are carried by a machined insert rather than by the plastic, which is why a moulded acetal part with a precision bore normally has that bore in a metal bush rather than in the acetal itself.

Where molding acetal is the right choice

• Gears, sprockets and worm wheels produced in volume — see custom gears
• Bushings, bearings and thrust washers at volume — see bushings and sleeves
• Latches, clips, snap features and closures
• Rollers, cam followers and conveyor components
• Pump and valve components in volume
• Small precision mechanical parts where the tool cost amortises
• Parts that combine acetal with a moulded-in metal insert
• Any machined acetal part whose volume has grown enough to justify a tool

Where molding acetal is the wrong choice

The volume does not justify a tool. Below the break-even point, machining from rod is cheaper — and it also avoids the tool cost entirely for a part that may still change.

Tolerances tighter than moulding holds. Moulding has a tolerance envelope of its own, agreed per drawing, and it does not match machined metal. Where a fit is critical, it belongs in a machined insert.

The design is still moving. A tool cut for a design that then changes is money spent twice. Machining absorbs revisions far more cheaply.

The service temperature is above roughly 90–100 °C. Acetal softens and creeps. Polycarbonate and nylon both hold up higher, and the grades we run across every process are listed on the materials index.

Strong acids or bases are involved. Acetal is attacked by both.

The part must stay outdoors unstabilised. Acetal’s UV resistance is poor without a stabilised grade.

The part has to be bonded rather than mechanically assembled. Acetal’s low surface energy makes adhesives unreliable without surface treatment.

What to put on the drawing

Instead of	Write
Plastic	Acetal (POM), for a moulded mechanical part at volume
Acetal, grade	Name homopolymer or copolymer — shrinkage and properties differ
Acetal, tolerance	Say which dimensions matter — the tool is sized to the settled condition
Acetal, gear or bushing	Say what the functional surface is — it drives the gate position
Acetal, metal insert	Say so — the insert is machined on our floor and moulded in
Acetal, wall thickness	Specify the nominal wall and keep it uniform
Acetal, above ~90–100 °C	Wrong material — it softens and creeps
Acetal, low volume or still changing	Machine it instead — tooling is a fixed cost

The three lines that decide most acetal moldings are the volume against the tool cost, which dimensions are functional, and what the part meets in service. The first decides whether the part is moulded at all. The second sets the gate, the tool sizing and the inspection. The third decides whether acetal is the right material.

Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on shrinkage, wall, draft and gate position, and a comparison against machining the same part where the volume is borderline.