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POM (Polyoxymethylene / Acetal) CNC Machining Material Manual

Cnc plastic materials
Last updated: May 23, 2026

POM (Polyoxymethylene / Acetal / “Saigang”) — CNC Machining Material Manual

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Rating legend — ★★★★★ best · ★☆☆☆☆ worst. For machinability/wear/heat resistance more stars = better; for cost, fewer stars = cheaper.

📋 Material Quick-Reference Card

┌──────────────────────────────────────────┐
│  Material Name: Polyoxymethylene (POM)     │
│  (Acetal / commonly "Saigang/Delrin")      │
│  Category: Crystalline engineering plastic │
│            (polyacetal resin)              │
│  Density: 1.41~1.42 g/cm³                  │
│  Tensile Strength: 60~70 MPa               │
│  Flexural Strength: 90~100 MPa             │
│  Hardness: 80~90 (Rockwell M)              │
│  Melting Point: 165~175 ℃                  │
│  Machinability: ★★★★★ (best among plastics)│
│  Wear Resistance: ★★★★★ (self-lubricating) │
│  Chemical Resistance: ★★★★☆               │
│  Cost: ★★★☆☆ (moderate, high value)      │
│  Keywords: "steel of plastics", wear-      │
│            resistant, dimensionally stable,│
│            low friction                    │
└──────────────────────────────────────────┘

1. Material Overview

1.1 Introduction

POM (Polyoxymethylene) is a high-rigidity, highly wear-resistant, dimensionally stable crystalline engineering plastic. Because its mechanical properties rival those of metals, it is known as the “steel of plastics” (the nickname “Saigang” means “surpasses steel”). It is the most machinable plastic for CNC, producing smooth surfaces and extremely high precision.

  • English Name: Polyoxymethylene / Acetal / POM
  • Common Nicknames: Saigang, Acetal, Polyacetal, Formaldehyde resin
  • Famous Brand Names: Delrin (DuPont), Hostaform, Ultraform

1.2 Two Main Types ⭐ Important

Type Full Name Characteristics
Homopolymer POM POM-H (e.g., Delrin) Higher strength, hardness, and rigidity; higher crystallinity
Copolymer POM POM-C Better thermal stability, superior chemical resistance, more stable machining ⭐ Common for CNC

💡 Copolymer POM (POM-C) is more commonly used in CNC machining because it resists cracking during processing and offers better dimensional stability.

1.3 Raw Material Forms

Common forms for CNC machining:

  • POM Rod (round bar): turned parts, shafts
  • POM Sheet/Plate: milled parts, flat components
  • Common colors: natural white, black

2. Composition & Physical Properties

2.1 Material Composition

POM is polymerized from formaldehyde, with a molecular backbone composed of repeating —CH₂—O— units. Its high crystallinity (over 70%) is the source of its high rigidity and wear resistance.

Type Molecular Structure
Homopolymer POM Pure polyacetal backbone, higher crystallinity
Copolymer POM Backbone incorporates a small amount of C—C bonds, better thermal stability

2.2 Physical Properties

Property Value
Density 1.41~1.42 g/cm³
Melting Point 165~175 ℃
Heat Deflection Temp. 110~136 ℃
Long-term Service Temp. -40~100 ℃
Thermal Conductivity 0.31 W/(m·K)
Water Absorption 0.2~0.25% (low)
Coefficient of Thermal Expansion 110×10⁻⁶ /℃

💡 POM has low water absorption, giving it excellent dimensional stability. It is unlikely to deform from moisture after machining (superior to nylon).

3. Mechanical & Chemical Properties

3.1 Mechanical Properties

Property Value
Tensile Strength 60~70 MPa
Flexural Strength 90~100 MPa
Elastic Modulus 2600~3100 MPa
Elongation 25~40%
Hardness 80~90 (Rockwell M)
Impact Strength Good (some toughness)
Coefficient of Friction 0.2~0.35 (low, self-lubricating)

⚠️ POM is highly rigid but its impact resistance is inferior to PC. For high-impact applications, choose PC or nylon.

3.2 Chemical Resistance

Medium Resistance
Weak acids, weak bases ✅ Good
Organic solvents, fuels, alcohol ✅ Excellent
Strong acids ❌ Poor
Strong oxidizers ❌ Poor
UV light (long-term outdoor) ⚠️ Fair, requires UV stabilizers

3.3 Notable Characteristics

  • Self-lubricating, low friction: can function as a sliding part without lubrication
  • Fatigue resistant: withstands repeated stress (e.g., snap-fits, springs)
  • Dimensionally stable: low water absorption, low creep
  • Good electrical insulation

4. CNC Machining Process ⭐⭐ Core

4.1 Machinability Rating

★★★★★ Best machinability among plastics — POM is hailed as “the most CNC-friendly plastic”:

  • Light, smooth cutting with good chip evacuation; chips curl off without sticking
  • Surface finish is mirror-like, often requiring no polishing for good appearance
  • High dimensional accuracy, achievable to ±0.01mm
  • Minimal burring, clean edges
Item Recommendation
Tool Material Carbide (HSS also acceptable)
Cutting Edge Sharp edge, polished flutes to prevent sticking
Rake Angle Large rake angle (15°~20°)
Helix Angle Large helix angle for chip evacuation
Flutes 2~3 flutes (large chip pockets)
Operation Spindle Speed (RPM) Feed Rate (mm/min) Depth of Cut (mm)
Rough Milling 5000~10000 1500~3000 1~4
Finish Milling 8000~15000 800~1500 0.1~0.5
Turning 1500~3000 0.1~0.3/rev 0.5~2
Drilling 1000~3000 50~200 —

📌 Parameters are for reference only; adjust based on machine rigidity, tool diameter, and part geometry.

4.4 Machining Challenges & Solutions

Challenge Cause Solution
Thermal deformation Poor thermal conductivity, concentrated cutting heat Control speed, ample cooling or air blast
Internal stress release distortion Residual stress in rod/plate stock Anneal before machining, remove material symmetrically
Slender part deflection Material elasticity Reduce depth of cut, add support
Thin-wall chatter Insufficient rigidity Optimize clamping, reduce feed
Burrs (minor) Edge toughness Sharp tools, proper toolpaths, deburring

4.5 Annealing Recommendation ⭐

To reduce machining distortion and relieve internal stress, annealing is recommended before/after machining precision parts:

Reference Annealing Process:
• Temperature: 140~160 ℃
• Time: approx. 30~60 min per 25mm of wall thickness
• Cooling: slow furnace cooling (avoid rapid quenching)

💡 For high-precision parts, the rough machining → annealing → finish machining workflow greatly improves dimensional stability.

4.6 Cooling Methods

  • Air cooling: most common; POM machining relies mainly on chip evacuation for heat dissipation
  • Water-soluble coolant: used for precision work; watch for moisture absorption
  • ❌ Avoid cutting fluids containing strong solvents

5. Surface Treatment

POM has low surface energy and strong chemical inertness, so surface treatment options are limited — this is its main shortcoming:

Process Feasibility Notes
Polishing ✅ Excellent Naturally smooth, can achieve mirror finish
Mechanical texturing / sandblasting ✅ Feasible Improves appearance and grip
Laser marking ✅ Feasible Commonly used for labels, logos
Screen printing ⚠️ Requires pretreatment Mediocre adhesion; needs flame/plasma treatment
Painting / electroplating ❌ Difficult Inert surface, poor adhesion, not recommended
Dyeing ⚠️ Limited Usually colored at the raw material stage (black/white)

💡 If rich surface treatments (e.g., electroplating, painting) are needed, choose ABS or PC instead. POM is mostly used in its natural color (white/black) + polishing/laser marking.

6. Applications & Material Selection

6.1 Typical Application Industries

Industry Application Parts
Mechanical transmission Gears, racks, sprockets, cams
Precision mechanisms Bearings, bushings, sliders, guide rails
Automation equipment Fixtures, locating parts, conveyor components
Automotive Fuel system parts, window/door mechanisms, clips
Consumer electronics Structural parts, sliding parts, buttons
Medical devices Precision structural parts (sterilizable)
Food machinery Contact parts (food-grade POM)

6.2 Pros & Cons Summary

✅ Advantages ❌ Disadvantages
Excellent CNC machinability, high precision Few surface treatment options (hard to paint/plate)
Self-lubricating, wear-resistant, low friction Limited heat resistance (long-term ≤100℃)
Dimensionally stable, low water absorption Poor resistance to strong acids/oxidizers
High rigidity, fatigue resistant Fair UV resistance
Resistant to organic solvents, fuels Impact resistance inferior to PC
Good value for money Flammable, drips when burning

6.3 Material Selection Guide

✔ Recommended for POM:

  • Moving parts requiring high wear resistance and self-lubrication (gears, bearings, sliders)
  • Precision parts requiring high dimensional accuracy and stability
  • Parts in contact with fuels and organic solvents
  • Elastic snap-fit parts subjected to repeated stress

✘ Not recommended for:

  • High impact resistance → choose PC or nylon
  • High temperature (>120℃ continuous) → choose PEEK, PTFE, or PI
  • Strong acid / strong oxidizer exposure → choose PTFE or PVDF
  • Outdoor / UV-critical parts without stabilizers → choose UV-stabilized grades or PVC
  • Parts needing painting or electroplating → choose ABS or PC

⚠️ Safety & Handling Notes

Hazard Detail Precaution
Thermal decomposition Overheating POM (>240℃, e.g. dull tools, no cooling) releases formaldehyde gas — pungent and toxic Keep tools sharp, control cutting heat, ensure ventilation/extraction
Dust inhalation Fine machining dust may irritate the respiratory tract Use dust extraction; wear a mask for prolonged dry machining
Flammability POM is flammable (UL94 HB) and drips while burning Keep away from open flame; no self-extinguishing behavior
Static Standard POM can accumulate static charge Use antistatic (ESD) grades for electronics applications
Storage Low water absorption, but store dry and away from prolonged UV Keep stock indoors, off the floor, away from direct sunlight

⚠️ Never let POM smolder in the cut. The biggest practical safety issue is local overheating from blunt tooling or inadequate chip evacuation, which produces formaldehyde fumes. Good chip control = safe machining.

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