Ask three molders what temperature to preheat POM to and you’ll get three different numbers. That’s not sloppiness. It’s because “POM preheating temperature” describes four separate steps in an acetal process, and only one of them has anything to do with moisture.
If you already run POM, you know the resin is unusually forgiving about water. It absorbs little and absorbs it slowly, so the sealed pellets on your floor often need no drying at all. You also know the one time moisture does cause trouble, it shows up as silver streaks and splay that no amount of temperature-dialing will fix.
This guide separates the four meanings of POM preheating, plus the neighboring step of post-mold annealing, and gives you the temperature, time, and moisture target for each. It also covers how to confirm the resin you buy will dry and run the same way lot after lot. Want to compare branded acetal grades side by side first? See the POM grades we stock.
POM Preheating Temperature: The Short Answer
POM resin is preheated to about 80°C. If the pellets have picked up moisture, pre-dry them at 80–90°C for 2–4 hours to a moisture content of ≤0.15%. Because POM absorbs very little water (roughly 0.25% at saturation), drying is optional for sealed, properly stored pellets. Whatever you do, never let the melt exceed about 215°C, where POM begins to degrade.
That paragraph answers the question most searchers arrive with. The rest of this section exists because the same phrase gets used for four different jobs on a shop floor.
The Four Things “POM Preheating” Can Mean
| Step | Typical temperature | Time | What it achieves |
|---|---|---|---|
| Resin preheat (dimensional stability) | ~80°C | Steady hopper setting | Uniform crystallinity, lower post-mold shrinkage and warpage |
| Pre-drying (moisture removal) | 80–90°C | 2–4 hours | Removes absorbed water; prevents silver streaks and splay |
| Nozzle/barrel startup preheat | Nozzle 5–10°C below front zone; barrel 180–205°C | At startup | Heats the melt path safely before the first shot |
| Metal-insert preheat | 100–150°C | Before insert loading | Reduces residual stress at the insert interface; prevents cracking |
| Post-mold annealing (not a preheat step) | 100–135°C | 2–8 hours | Relieves internal stress after molding |
If you’re setting up a process, the row you need is almost always the second one. If your parts warp out of tolerance, it’s the first. If you’re insert-molding, it’s the fourth.
Whichever step you’re tuning, it sits inside the wider set of engineering plastic temperatures that maps preheating, processing, and service temperatures across materials.
Does POM Need Drying Before Molding?
Usually not. That is the honest answer, and it is also why POM gets mishandled more often than genuinely wet resins like PA66.
POM’s Water Absorption (~0.2–0.5%) vs. Hygroscopic Resins
POM absorbs roughly 0.2–0.5% water at saturation, typically cited around 0.25%. Compare that with PA66 at 1.3–2.5% or polycarbonate, where hydrolysis during melt processing is a real risk above a tight moisture limit. Those materials require drying as standard procedure; PA66 in particular must be dried to well under 0.2% before every run.
POM sits at the other end of that spectrum. Pellets stored sealed and indoors generally run with no drying at all. Drying becomes necessary when something has changed: humid storage, condensation from temperature swings, or the addition of regrind.
A quick note on vocabulary, because it trips people up. In POM processing, “pre-drying” and “drying” mean the same thing: removing absorbed moisture before the resin reaches the barrel. For cross-material drying schedules, including where POM sits alongside PA66 and PC, see our engineering plastic processing temperature guide.
4 Signs Your POM Needs Drying
You don’t need a moisture analyzer to know a lot is wet. Four visible symptoms tell you almost every time:
- Silver streaks on the part surface, running in the direction of flow
- Bubbles or voids trapped inside the part
- Splay marks radiating outward from the gate
- A dull or blotchy surface bloom instead of the usual glossy acetal finish
The mechanism behind POM silver streaks is the same in every case. Water in the pellets vaporizes in the barrel, the melt flow stretches the vapor into thin filaments, and those filaments freeze in place as the part cools. The result looks like surface contamination but is actually moisture.
Moisture Target and Drying Parameters
Set the acetal drying temperature and time from the grade datasheet where one exists; the values below are the standard starting point. Target a POM moisture content of ≤0.15% before molding, and hold the hopper dryer temperature in the 80–90°C band.
| Parameter | Setting |
|---|---|
| Dryer type | Dehumidifying (desiccant) hopper dryer preferred; hot-air acceptable for short runs |
| POM drying temperature | 80–90°C |
| POM drying time | 2–4 hours |
| Target moisture content | ≤0.15% (some grades specify ≤0.1–0.2%) |
| Escalation if silver streaks persist | 100°C for ≥3 hours |
| Do not exceed | ~100°C for prolonged periods (oxidation risk) |
Two operational details matter as much as the numbers:
- Seal the hopper after drying and use the resin within about an hour, or moisture re-absorbs from ambient air.
- Don’t fix a streaking problem by simply raising the temperature. At 80°C, two to four hours does the job. Holding POM above 100°C for hours damages it, as the over-drying section below explains.
If you are chasing a streaking defect that survives a correct drying cycle, the problem may not be moisture at all, but the grade or the supplier. Send our technical team the details and we’ll help you isolate it.
POM Preheating for Dimensional Stability (~80°C)
Here is where the Chinese molding-practice literature and Western datasheets diverge, and where the ~80°C number most searchers have heard actually comes from.
Preheating POM resin to about 80°C is standard practice in high-volume Asian molding operations, and the reason is dimensional stability, not moisture removal. A warm, evenly conditioned melt crystallizes more uniformly as it fills the mold. More uniform crystallinity means less variation in post-mold shrinkage, which for unfilled POM runs roughly 1.8–2.5%. Hold that shrinkage range tight and your tolerance window survives the cooling stage.
Preheating the resin does not replace proper mold temperature. POM wants a mold at 80–100°C for most parts, and up to 120°C for precision, thin-wall, or geometrically complex parts. Running the mold below about 50°C to chase cycle time under-crystallizes the resin, producing hazy, weak parts with poor dimensional control. Resin preheat and mold temperature work together; neither substitutes for the other.
For the crystallinity and shrinkage mechanics in more depth, see our POM melting point and processing guide.
Nozzle and Barrel Preheat at Startup
Startup is where a small sequencing mistake can cost you a production shift. The rule is simple: preheat the nozzle first, then bring up the barrel zones.
Heating the barrel before the nozzle lets degraded material pool at the cold nozzle tip, where it forms a plug that later drools or shoots through as a black speck. Warming the nozzle first keeps the melt path clear from the start.
Once the process is running, keep the nozzle set 5–10°C below the front barrel zone to prevent drooling. Barrel setpoints typically land around 180–205°C for copolymer POM, with a hard ceiling near 215°C at the melt. Above roughly 220–230°C, POM degrades and releases formaldehyde, which corrodes ordinary mold steel. Two protections follow from that: run corrosion-resistant steels such as S136 or NAK80 in the mold, and purge the barrel with PE or PP whenever you switch from a material that ran hotter.
Barrel-zone profiles for other materials are covered in the cross-material processing temperature guide.
Preheating Metal Inserts (100–150°C)
Insert-molded POM parts, think automotive connectors, threaded bushings, and reinforced housings, fail in a specific and predictable way: stress cracks around the metal insert.
The cause is differential thermal expansion. The metal insert and the surrounding POM cool at different rates and pull against each other, concentrating stress at the interface. Preheating the insert to 100–150°C before loading closes most of that gap and prevents the cracking. It is a short, inexpensive step that English-language molding guides routinely omit.
Two related cautions. First, a colder insert pulls heat out of the local melt and can cause short shots or poor weld-line strength near the insert. Second, avoid annealing an assembly that contains a metal insert, because the annealing cycle reintroduces the very thermal mismatch you just designed out.
POM Annealing Temperature (Post-Mold Stress Relief)
Annealing is not a preheat step, but searchers conflate the two constantly, so it belongs in this guide.
Typical POM annealing runs at 100–135°C for 2–8 hours in air or an oil bath, followed by slow cooling. Shop-floor rules of thumb are easier to apply than a fixed range: run roughly 10 minutes per 1 mm of wall thickness, or set the oven 10–20°C above the part’s service temperature. If you’d rather anchor the number to a thermal property, set it 10–20°C below the material’s heat deflection temperature; our HDT vs melting point vs glass transition guide explains how to read those three values correctly.
Should you anneal every POM part? No. POM’s flexible polymer chains and very low glass transition temperature (around -60°C) let internal stress relax naturally at room temperature. Annealing is for parts where precision or thick walls make that natural relaxation too slow or too uneven. Applied by default, it adds cost and cycle time for no benefit.
Do not over-anneal. Excessive temperature or time discolors the part, decomposes the surface, and can blister it, all while you are trying to improve it.
POM Preheating Temperature and Drying by Grade
The temperatures above hold for standard POM, but the exact schedule depends on whether you’re running homopolymer or copolymer and on what the compounder added.
Homopolymer vs. Copolymer
Copolymer grades such as Ultraform®, Hostaform®/Amcel®, DURACON®, KOCETAL®, and FORMOCON® offer better thermal stability and a slightly wider drying and processing margin. That margin is why they dominate automotive underhood and plumbing applications, where process variation is a fact of life.
Homopolymer grades of the Delrin® type deliver higher crystallinity, strength, and surface hardness, which suits precision gears and structural parts. The trade-off is tighter discipline: less tolerance for high temperature and long residence time in the barrel.
Representative Branded Grades
| Grade family | Polymer | Representative drying schedule | Notes |
|---|---|---|---|
| Delrin® (Celanese) | Homopolymer | 80°C / 2–3 h | Verify against grade datasheet; tighter melt discipline |
| Ultraform® (BASF) | Copolymer | 80–90°C / 2–4 h | Wider processing margin |
| Hostaform® / Amcel® (Celanese) | Copolymer | 80–90°C / 2–4 h | Automotive and industrial grades |
| DURACON® (Polyplastics) | Copolymer | 80–90°C / 3–4 h, ≤0.15% | Precision and automotive |
| KOCETAL® (Kolon) | Copolymer | 80°C / ≥3 h; 100°C / ≥3 h if mold deposit | Escalate on visible deposit |
| FORMOCON® (Formosa) | Copolymer | 80–90°C / 2–4 h | General-purpose grades |
Glass-filled, lubricated, and UV-stabilized grades dry on the same base-polymer schedule but flow and cool differently, so their melt and mold settings diverge. Always verify the drying parameters against the specific grade’s datasheet, such as a published POM datasheet, rather than a generic table, including this one.
Why Preheating Consistency Depends on Resin Quality
You can set a perfect drying protocol and still get inconsistent results, because the variables that shift are usually in the resin, not the machine.
Regrind and Off-Spec Resin Are the Real Moisture Risk
Sealed prime pellets rarely need drying. Regrind, humid storage, and unverified compound frequently do. Regrind has more surface area, more exposure history, and often more absorbed moisture than virgin pellets, so a blend that ran clean on prime resin can streak the moment regrind enters the mix.
Off-spec or repackaged lots add a second variable: a shifted melt flow index. A drying protocol you validated on prime resin may not transfer, because the resin itself is different. And in high-volume precision parts, counterfeit or misrepresented “branded” acetal is a genuine sourcing risk.
What to Check on the COA (Moisture + MFI)
The Certificate of Analysis is how you confirm a lot will behave before it reaches the dryer. Check these fields on every batch:
- Grade designation and manufacturer, matched to the grade you specified
- Melt flow index, typically measured at 190°C under 2.16 kg
- Moisture content, per ISO 15512 or ASTM D6869
- Density and filler content for filled grades
- Lot or batch number, for traceability back to the producer
- RoHS and REACH statements, for compliance documentation
Sourcing Branded Prime POM from China
Consistency is ultimately a supply-chain question. Branded prime grades with traceable lots and a COA documenting both moisture and MFI keep a validated drying protocol stable, batch after batch. Suzhou Yifuhui supplies Ultraform®, Hostaform®, DURACON®, KOCETAL®, and FORMOCON® from a stocked warehouse, with COA, MSDS, commercial invoice, and packing list issued for every order.
Standard terms are straightforward: 25 kg minimum order, FOB Shanghai, and a 7–14 day lead time to major international ports. Located minutes from the Port of Shanghai, we handle first-time international orders through to customs clearance. Request a POM resin quote and we’ll respond within 24 hours.
Frequently Asked Questions
What is the preheating temperature for POM?
POM is preheated to about 80°C. If the resin has absorbed moisture, pre-dry it at 80–90°C for 2–4 hours to a moisture content of ≤0.15%. Sealed, properly stored pellets are barely hygroscopic and often need no drying at all.
Does POM need to be dried before injection molding?
Usually not, because POM absorbs only about 0.25% water at saturation. Drying is required when the pellets have been exposed to humid storage, condensation, or regrind, or when parts show silver streaks, bubbles, or splay.
How long should POM be dried and at what temperature?
Dry POM at 80–90°C for 2–4 hours. If silver streaks or mold deposit persist after a correct cycle, escalate to 100°C for at least 3 hours, and check the grade and supplier if the problem continues.
What happens if you over-dry POM?
Over-drying, meaning prolonged exposure above about 100°C, oxidizes the resin. The symptoms are yellowing or discoloration, surface degradation, and a loss of mechanical properties. A higher drying temperature is not “safer”; it is a failure mode of its own.
What causes silver streaks in POM parts?
Silver streaks come from moisture in the resin. Water vaporizes in the barrel, the melt flow stretches the vapor into filaments, and they freeze in the part as silvery lines. Dry the resin to ≤0.15% moisture and confirm the lot on its COA.
What is the annealing temperature for POM?
POM is annealed at 100–135°C for 2–8 hours in air or an oil bath, then slow-cooled. A common shop rule is roughly 10 minutes per 1 mm of wall thickness, or 10–20°C above the part’s service temperature.
Should metal inserts be preheated before POM molding?
Yes. Preheat metal inserts to 100–150°C to reduce residual stress at the insert interface and prevent the stress cracking that differential thermal expansion causes. Avoid annealing assemblies that contain metal inserts.
What moisture content should POM be before molding?
Aim for ≤0.15% moisture, though some grades specify ≤0.1–0.2%. Measure it per ISO 15512 or ASTM D6869 when the drying cycle matters to part quality.
Is POM preheating the same as drying?
No. Preheat for dimensional stability holds the resin at about 80°C to improve crystallinity and shrink control; drying removes absorbed moisture at 80–90°C for 2–4 hours. They overlap in temperature but serve different purposes.
Conclusion
The practical summary fits in a few lines:
- Preheat POM at about 80°C for dimensional stability.
- Dry it at 80–90°C for 2–4 hours to ≤0.15% moisture, but only when it has actually picked up water.
- Preheat metal inserts to 100–150°C.
- Anneal at 100–135°C only where precision demands it, not by default.
- Keep the melt under about 215°C at all times.
The part most shops miss is the last one: consistency. A drying protocol is only as reliable as the resin it runs on, and regrind, humid storage, and unverified lots are what break it. Getting the POM preheating temperature right is half the job. Sourcing branded prime acetal with a COA that documents moisture and MFI is the other half.
Want POM that dries and runs the same batch after batch? Request a quote for branded prime acetal from Ultraform®, Hostaform®, DURACON®, KOCETAL®, or FORMOCON®, with COA-documented MFI and moisture. 25 kg minimum order, FOB Shanghai, response within 24 hours.
