Polyacetals / Polyoxymethylene (POM)
Polyacetals (POM’s) are highly crystalline thermoplastics that have mechanical properties similar to nylon. They are among the stiffest and toughest thermoplastics[1]. There are two types of polyacetal readily used in injection molding: homopolymer and copolymer.
In general, both types of polyacetals have the following properties[2]:
- Highly crystalline; due to no large pendant groups on the chain.
- High crystallinity leads to excellent strength, stiffness, surface hardness, barrier properties, solvent resistance, and a sharp melting point.
- High crystallinity also leads to reduced toughness and they are somewhat notch sensitive.
- Good slip and wear resistancei
- Normally not sensitive to polar solvents; hence low water absorption.
- Acetal is attacked by acids and strong oxidizing agents.
- Due to low molecular polarity, they have low bonding tendency and low coefficient of friction.
- Acetal is UV sensitive; chalking and degradation of the polymer occurs with exposure.
Some comparative properties:
Homopolymer
Copolymer
Additives are available for polyacetals. Here is a short listing of some of the additives and their effects:
- Adding short fiberglass will enhance strength and toughness.
- Mixtures with fluorocarbons lead to good surface lubricity to prevent creakingi.
- Increase stiffness and strength; add 10-40% glass fiber, glass beads or other mineral fillers.
- For thin-walled parts – crosslinked rubber can be added to improve melt flow in the tool.
- Improved electrical conductivity and heat distortion point is obtained by adding powdered aluminum or bronze.
- Polyacetals have high mold shrinkage – warp[3]. Adding talc filler (or other heat sinking inorganic filler) will reduce the amount of warp; however, it is at the cost of reduced impact strength.
1 Osswald, et al, International Plastics Handbook, Hanser, 2006.
2 Strong, A. Brent, Plastics: Materials and Processing, Prentice Hall, 2000.
3 Campo, E. Alfredo, The Complete Part Design Handbook, Hanser, 2006.
