POM Injection Molding: Key Considerations for Quality Parts

Polyoxymethylene (POM) is a high‑crystallinity engineering plastic with excellent mechanical properties and wear resistance. However, its thermal sensitivity and high shrinkage require careful process control. Below are the key points to consider during POM injection molding.

1. Machine and Screw Selection

POM is typically processed on a screw‑type injection molding machine. A standard single‑start, fully threaded screw is recommended. The nozzle should be a large‑bore, straight‑through type to minimize flow resistance and shear heating.

2. Part Design Guidelines

The wall thickness of POM parts is generally 1.5–5.0 mm and should be as uniform as possible. Sharp corners and notches should be avoided, and fillets should be used at all corners.

3. Mold Design Requirements

• Draft angle: 40 minutes to 1.5 degrees to facilitate demolding.

• Avoid dead corners in the mold where material could stagnate and degrade.

• Provide proper venting (vent holes or venting slots).

4. Metal Inserts

If metal inserts are used, preheat them to 100–150°C. This reduces residual stress around the insert and prevents cracking caused by external forces or temperature changes.

5. Barrel and Nozzle Temperatures

Front (nozzle side): 190–200°C

Middle: 180–190°C

Rear (feed zone): 150–180°C

Nozzle: 170–180°C

For thin‑wall parts, temperatures may be raised slightly — but do not exceed 210°C.

6. Mold Temperature

General range: 80–100°C

For thin‑wall, long‑flow‑length, or complex parts: up to 120°C

Higher mold temperature improves melt flow, reduces cooling‑related defects, and increases impact strength — but it also increases shrinkage.

7. Injection Pressure

Small gate, thin wall, long flow, large area: 120–140MPa

Large gate, thick wall, short flow, small area: 40–80MPa

General parts: around 100MPa

Properly increasing injection pressure improves melt flow and surface quality. However, excessive pressure may deform the mold and cause flash.

8. Holding Time

POM has high crystallinity and large volume shrinkage. Sufficient holding time is essential to prevent voids and sink marks. In general, thicker parts require longer holding time.

9. Injection Speed

Thick‑wall parts: Use slow injection to avoid jetting, which harms surface and internal quality.

Thin‑wall parts: Use fast injection to prevent premature solidification and short shots.

10. Heat Treatment (Annealing)

To eliminate residual stress and reduce post‑shrinkage, POM parts are usually annealed.

Medium: Air or oil

Temperature: 120–130°C

Time: Approximately 10 minutes per 1 mm of wall thickness

11. Thermal Degradation and Safety

POM is heat‑sensitive. If overheated or held too long at processing temperatures, it decomposes and releases harmful formaldehyde gas. This can:

• Degrade part quality

• Corrode the mold

• Harm human health

• In severe cases, cause gas expansion and even explosion inside the barrel

Precautions:

• Strictly control molding temperature and residence time

• If a strong pungent odor or yellow‑brown streaks appear, stop immediately

• Purge the barrel by injecting into the air, then clean with PE (polyethylene)

• Only resume production after the system is clean and stable

12. Startup Procedure

Before starting POM molding:

• Preheat the nozzle first, then heat the barrel.

• If the barrel contains any material that exceeds POM’s processing temperature, clean it thoroughly with PE.

• Lower the temperature to POM’s processing range, then clean once more with PE.

• Only then can POM be loaded and molding begin.

13. Material Contamination

Certain materials and additives — such as PVC or halogen‑containing flame retardants — can accelerate the degradation of POM. These must be strictly separated and not allowed to mix with POM.