Guides / Process

Annealing Machined Acetal: When Stress Relief Is Worth the Lead Time

September 21, 2026

An acetal part that passed inspection and then went out of tolerance on the shelf is almost always a stress problem. Annealing is the fix, and it is also the single most common reason two quotes for the same drawing differ by a wide margin.

Where the stress comes from

Extruded rod and plate cool from the outside in. The skin sets first, the core cools later, and the finished stock carries locked in stress as a result. Larger sections carry more.

Machining removes material unevenly and releases that stress unevenly. Take 8 mm off one face of a 25 mm plate and the plate bows, sometimes while it is still clamped, more often an hour after it comes off the machine. On a symmetric turned part the effect is small. On an asymmetric milled plate it can be several tenths of a millimetre.

What annealing does

Holding the part at an elevated temperature lets the polymer chains relax, then a slow controlled cool lets it set without building the stress back in. The part moves during the cycle, which is the point. It moves in the oven instead of on your assembly bench.

Typical practice for acetal is an air oven somewhere around 150 to 160 degrees C, held for a time that scales with wall thickness, commonly counted as about thirty minutes per 6 mm of section, then cooled slowly. Oil bath annealing is also used. Treat these as the shape of the process rather than as a recipe, because the resin supplier data sheet for the specific grade is the authority.

The slow cool is not optional. Pulling a part out to cool in room air puts stress straight back in.

When it is worth it

Annealing pays for itself when:

• The part is thick and a lot of material comes off one side
• Flatness or parallelism is tight over a long face
• The part is machined from plate rather than rod
• Several critical dimensions sit at plus or minus 0.025 mm or tighter
• The part will see a temperature cycle in service and must not move
• The part is a gear or a fit where post machining movement is a functional failure

It is usually not worth it for a small symmetric turned bushing, a spacer, a washer or any part where the tolerance is plus or minus 0.1 mm and nothing is clamped.

Where it fits in the sequence

The working order is:

1. Rough machine, leaving stock on critical features
2. Anneal
3. Finish machine
4. Let the part settle at inspection temperature
5. Inspect

For very tight parts, shops sometimes anneal twice, with a semi finish pass in between. That doubles the lead time contribution and should be discussed rather than assumed.

Annealing a finished part is a mistake that looks like a shortcut. The part relaxes and the finished dimensions go with it.

What it costs

Two things: oven time and calendar time. Oven time is modest. Calendar time is the real cost, because the slow cool and the batching of parts into an oven load mean a day is the practical minimum and more is common.

If a quoted lead time jumps by several days for what looks like a simple part, annealing is usually the reason. It is worth asking directly rather than assuming the shop is slow.

How to handle it on the drawing

You have two reasonable options.

Specify the outcome. State the tolerance, the inspection temperature and a note that the part must hold dimension after machining. Let the shop decide whether it needs an anneal. This is usually the better route because the shop knows its stock and its process.

Specify the process. Call out a stress relief cycle between roughing and finishing. Do this when you have a qualified process from a previous supplier and you need the new one to match it.

What does not work is specifying a tight tolerance with no note, getting parts that measured correctly at the shop, and finding out later that nobody agreed on when or at what temperature they were measured.

Have a part that moved after machining? Send your RFQ and describe what shifted. The shops we match will tell you whether a stress relief cycle belongs in the quote. Reply within 24 business hours.

Frequently asked questions

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