Why POM Causes Mold Deposit & Practical Solutions for Injection Molding

POM (Polyoxymethylene) is widely used for gears, sliders, bearings and precision moving parts due to its low friction, high dimensional stability and good wear resistance. However, mold deposit is one of the most frequent headaches for molders. White or light brown residue builds up on mold surfaces, which leads to poor part surface, frequent mold cleaning, shutdown and higher scrap rate.

Many engineers simply increase mold cleaning frequency without fixing the source of the problem. Mold deposit on POM is not always a quality issue of the raw material. It comes from a combination of material formulation, molding parameters, venting design and mold condition.

Main root causes of POM mold deposit

1. High processing temperature & overheating decomposition POM has narrow processing temperature window. When barrel temperature is too high or material stays in the barrel for too long, formaldehyde decomposition happens. Volatiles condense and accumulate on the mold surface and form deposits.
2. Poor venting on the mold Trapped gas cannot escape from the cavity. Decomposed volatile gas gathers inside and sticks to mold walls. Thin-wall, deep-cavity and high-speed filling parts are more likely to have this issue.
3. Lubricant & additive precipitation Internal lubricants added in POM grades will migrate out under high shear and temperature. Some low-cost modified POM contains excessive lubricants, which worsen mold fouling.
4. Moisture contamination Although POM absorbs much less water than PA, damp storage or unsealed packaging can still bring moisture. Water vapor plus thermal decomposition accelerates residue formation.

   Practical adjustment steps you can test on site

   ✅ Optimize barrel temperature: Keep temperature within the recommended range from TDS, avoid overheating. Do not exceed the upper limit just for better flow.

   ✅ Reduce back pressure and screw speed: Lower shear heat to reduce material decomposition.

   ✅ Improve vent design: Widen and deepen vents, add extra vents at weld lines and end of fill. Regularly polish vent slots.

   ✅ Shorten residence time: Avoid long material hold-up inside barrel during machine stop. Purge the barrel properly when stopping production.

   ✅ Dry material if necessary: Even though POM does not require strict drying, dry at 80–90℃ for 2–4 hours if storage environment is humid.

   ✅ Switch to low-fouling POM grade: If mold deposit persists after all process tweaks, consider a POM grade optimized with low-volatile additive package.

   When to talk to your resin supplier

   If you have adjusted all molding parameters and mold structure but mold deposit still occurs consistently across batches, it is time to check whether the POM grade matches your application. Some general-purpose POM is not suitable for long-run high-volume production where mold maintenance time must be minimized.

   At Shanghai Xingshengyuan Plastic Products Co., Ltd., we supply virgin POM homopolymer and copolymer grades, including low-deposit POM suitable for continuous injection production. We provide complete TDS, processing guidance and molding troubleshooting support for global molders and OEMs.

   If you are facing POM mold deposit or other molding defects, share your part design, machine parameters and current grade with our technical team. We can help evaluate if a better POM grade is available for your project.

   👉 Contact us for grade recommendation and datasheet