Material · injection molding
POM (acetal) injection molding design guidelines
POM (acetal) is stiff, slippery and stable in size once molded: the usual choice for gears, bearings and snap fits. It is semi-crystalline and shrinks a lot, so it wants even walls.
This page gives the wall thickness PartCheck checks POM against, the rib, boss and corner sizes that follow from it, and the published processing figures. You can then check your own part, free and in your browser.
POM wall thickness
A POM part is checked against a nominal wall of 0.76 mm to 3.05 mm. Thinner, and the plastic may not fill the cavity or will be weak; thicker, and sink marks, voids and cooling time all climb.
POM is semi-crystalline. Semi-crystalline resins shrink more, and unevenly, so a wall that varies warps the part. PartCheck flags regions more than 15% off the part's nominal wall. Where the wall has to change, step it gradually; published guidance keeps neighbouring sections within about 3:1.
Ribs, bosses, corners and draft
These follow from the nominal wall rather than from the plastic, so here they are at the thinnest, middle and thickest wall in POM's range.
| Nominal wall | 0.76 mm | 1.90 mm | 3.05 mm |
|---|---|---|---|
| Rib ≤ 0.6× wall | 0.46 mm | 1.14 mm | 1.83 mm |
| Boss wall ≤ 0.8× wall | 0.61 mm | 1.52 mm | 2.44 mm |
| Inside radius ≥ 0.5× wall | 0.38 mm | 0.95 mm | 1.52 mm |
| Thick section from 2.0× wall | 1.52 mm | 3.81 mm | 6.10 mm |
- Ribs
A rib thicker than this at its root shows as a sink line on the face behind it. Make it taller for stiffness, not thicker.
- Bosses
A screw boss with a thicker wall shrinks harder than the wall it stands on and pulls a sink through to the face opposite.
- Inside corners
A sharp inside corner concentrates stress and makes the plastic turn too hard as it flows. Half the wall is the usual radius.
- Draft
At least 1° on faces along the direction the mold opens, so the part releases without dragging. Textured faces need more, roughly a degree for every 0.025 mm of texture depth.
POM processing figures
| Figure | Value |
|---|---|
| Grade the figures are for | POM copolymer, unfilled |
| Density | 1.41 g/cm³ |
| Melt temperature | 216 °C |
| Mold temperature | 93 °C |
| Ejection temperature | 129 °C |
| Mean cavity pressure | 59–60 MPa |
POM is also offered filled: glass fibre at 10–50% by weight. The filled grade's weight is worked out from the filler; its walls and the other figures are the unfilled resin's.
Check a part in POM
Open your STEP file in PartCheck's DFM tab, choose injection molding and POM, and every wall, rib, boss, corner and draft above is checked on your part, with each problem marked where it is. The full list of checks is in the injection molding DFM guide.
Questions
What is the minimum wall thickness for POM?
0.76 mm is the thinnest nominal wall PartCheck checks POM against. Thinner sections risk not filling and are weak.
What is the maximum wall thickness for POM?
3.05 mm. Above it, sinks, voids and cycle time climb; core the section out instead.
How thick should a rib be in POM?
At most 0.6× the wall it stands on: 1.14 mm on a 1.90 mm wall.
What draft angle does POM need?
1° is PartCheck's default for every plastic, and more on textured faces.
Is POM amorphous or semi-crystalline?
Semi-crystalline, which is why its walls are held within 15% of nominal.
Try it on a real part
The demo is a moulded enclosure lid in two revisions, drawn in CAD. It opens in the tool with no files needed, or you can open your own STEP files. Nothing is uploaded: the analysis runs in your browser.