POM (Acetal) Injection Molding Guide

POM (Polyoxymethylene), also known as acetal, is a high-performance thermoplastic polymer derived from formaldehyde. It comes in two types:

• POM-H (Homopolymer)

• POM-K (Copolymer)

Both types are known for:

• High crystallinity and density

• Excellent mechanical strength and stiffness

• Outstanding wear and abrasion resistance

• Low friction characteristics

• Chemical resistance (excluding strong acids)

Key Molding Characteristics

• Crystalline polymer with a sharp melting point

• Rapid viscosity drop once melted

• Prone to thermal degradation if overheated or held at high temperatures too long

• Copper and its alloys catalyze degradation—avoid using copper in contact areas

1. Raw Material Processing

• Water Absorption: Low (0.2%–0.5%)

• Drying: Not always required unless material is wet

• Drying Temp: ≥80°C

• Duration: 2–4 hours (follow supplier guidelines)

• Regrind Usage: Typically up to 20–30%; in some cases up to 100% depending on application and part requirements

2. Injection Molding Machine Selection

• Standard injection molding machines can be used

• Screw design should avoid material stagnation zones

3. Mold and Gate Design

• Mold Temp: 80–90°C

• Runner Diameter: 3–6 mm

• Gate Design:

• Width ≥ 2 × wall thickness

• Depth ≈ 0.6 × wall thickness

• Edge gate length: ~0.5 mm

• Circular gate: Diameter ≥ 0.5–0.6 × wall thickness

• Rectangular gate:

• Draft angle (stripping slope): 40′ to 1°30′

4. Venting System

• POM-H: 0.01–0.02 mm thickness × 3 mm width

• POM-K: 0.04 mm thickness × 3 mm width

5. Processing Parameters

Melt Temperature

• POM-H: 190–230°C (ideal: 215°C)

• POM-K: 190–210°C (ideal: 205°C)
  Use air injection method to verify melt temp.

Injection Speed

• Moderate to moderately fast

• Too slow: surface ripple/corrugation

• Too fast: shear overheating, silver streaks, burn marks

Packing Pressure

• Should be relatively low, typically not exceeding 200 bar

Screw Dwell Time

• Avoid long retention to prevent degradation:

• POM-H @ 215°C: Max 35 minutes

• POM-K @ 205°C: Max 20 minutes

• POM-K @ 240°C: Max 7 minutes

If machine stops for >20 mins, reduce barrel temp to ~150°C and clean thoroughly.

6. Downtime Procedure

• Clean with PE or PP resin

• Lower barrel temp and turn off heaters

• Avoid any acidic residues or halogenated materials before running POM

• Improper additives, pigments, or glass-filled nylons can trigger POM degradation or explosions

7. Post-Processing (Annealing)

For parts under high stress or demanding conditions, annealing is recommended to relieve internal stress.

Stress Testing

• Immerse molded part in 30% HCl for 30 minutes

• Inspect visually for stress cracks

Annealing Methods

Summary of Best Practices

• Maintain stable melt temperature

• Avoid copper contact with molten POM

• Ensure minimal residence time in the barrel

• Clean barrel and screw thoroughly before and after processing

• Consider annealing for high-precision or load-bearing parts

• Use regrind cautiously depending on application requirements

If you have a project requiring POM injection molded components, our team at SWY MOULD has extensive experience in delivering high-precision acetal parts. Contact us today (shine@swymould or +86 15757668880) for a quote or technical consultation.