Acetal, an extremely popular engineering thermoplastic also known as POM (polyoxymethylene), comes in two basic types: homopolymer (POM-H) and copolymer (POM-C). While either one will work in over 95% of the applications, it’s important to know the differences.
Historically, homopolymer (widely known by its DuPont tradename “Delrin®) was the preferred type in North America; in Europe, it was copolymer. The difference was with sourcing – here, we “grew up” with DuPont, in Europe they had Hoechst-Celanese and BASF.
Copolymer traditionally had cost and processing advantages over homopolymer, advances in extrusion technology in the early 1990’s led to North America’s transition to copolymer.
So, what are the actual differences in the materials?
Functionally, either will work satisfactorily in most applications. Both are FDA compliant, machine very well and have similar properties, though POM-C has better chemical properties. Homopolymer’s more basic structure gives it higher physical properties, making it the correct choice for such applications as gears and “keels” (structural support) for artificial feet
|Tensile strength PSI||11,000||9,500|
|Flex Strength PSI||13,000||12,000|
|Compressive Strength PSI||15,000||13,500|
|Heat Deflection Temp (⁰F) 264 PSI||250||220|
Click to expand
Despite POM-H’s advantage in physical properties, its disadvantages are significant:
- Centerline porosity (low density area due to the way the more crystalline POM-H cools) – this often requires buying oversize material to prevent porous surface exposure in final form)
- Formaldehyde outgassing – POM-H’s backbone is anhydrous formaldehyde, and the odor generated during machining & occasionally in service is very noticeable and can be an irritant
- Size Limitation – due to the difficulty in processing POM-H, it cannot be made in very large cross-sections rod or plate, and not in tubes at all
- Cost – the resin is 10%-15% higher in cost than POM-C, and costs more to make shapes
Copolymer’s primary advantages:
- Uniform density, eliminating the centerline porosity issue (in food service, porosity = mold)
- Better steam / Hydrolysis resistance – up to 180F
- Less formaldehyde outgassing – most of the formaldehyde is transformed into trioxane.
- Less inherent stress – takes less pressure to process
- Size Availability – rod up to 24” diameter, plate up to 10” thick, and extruded tubing; it can be made by extrusion and compression molding
- Medical Applications – certain POM-C resins have USP approval for medical uses such as orthopedic trial implants
- Colors – an extension of medical needs, POM-C is readily available in standard & custom colors
NOTE: POM-H stock shapes are also available in modified versions, including three PTFE (“Teflon®”) filled grades, glass filled, UV stabilized and several different viscosity grades.
Have questions? We can help! The experts at WS HAMPSHIRE are ready to assist you to find the right material for your requirements! Call us – you’re in the right place!
Tom Connelly is a self proclaimed “Street Engineer” with 40+ years in the plastics industry.