What is POM, and what is it used for?

Polyoxymethylene (POM), also commonly known as Acetal, is a naturally white semi-crystallinethermoplastic. It is used to produce precision parts that require high resistance to abrasion and heat, low friction, good dimensional stability, resistance to water absorption, and high tolerance to organic chemical compounds (e.g. hydrocarbons). It is a very high tensile strength plastic with significant creep resistant properties that bridge the material properties gap between most plastics and metals. Typical applications include small gears, consumer electronics, plastic zippers, medical devices, and furniture components such as the plastic feet underneath a couch.

Now that we know what it is used for, let’s examine some of the key properties of POM. Polyoxymethylene is classified as a “thermoplastic” (as opposed to “thermoset”), and the name has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (175 degrees Celsius in the case of POM homopolymer plastic and 162-173 degrees Celsius in the case of POM copolymer). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like Polyoxymethylene liquefy, which allows them to be easily [injection molded] and then subsequently recycled.

By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.   Acetal is a semi-crystalline material, meaning that it exhibits highly ordered and repetitive structural characteristics at the molecular level with sharp melt points (as opposed to amorphous materials that are not arranged in regular arrays).

Acetal is a synthetic polymer which can fall into one of two different categories (homopolymer and copolymer). Synthetic polymers are by definition large molecules composed of repeating subunits (monomers). Homopolymers are composed of repeating monomers of a single molecular substructure while Copolymers are composed of several different monomer types in combination with one another.

There are plusses and minuses to both homopolymers (of which Delrin is the principal example) and copolymers (of which there are a large number of examples, see below). Here are a few to think about:

• Homopolymers are stiffer.
• Homopolymers have higher room temperature impact strength.
• Homopolymers have higher tensile strength.
• Homopolymers have a slightly higher operation temperature.
• Homopolymers are slightly harder and more slippery.

• Copolymers have better dimensional stability.
• Copolymers are more resistant to chemicals.
• Copolymers are less porous when extruded.

Why is Acetal used so often?

Acetal is an incredibly useful plastic for applications requiring low friction. At Creative Mechanisms, we use Acetal when we need a strong, slippery and/or flexible part (or perhaps a part with just one of those criteria). More often than not, we use Polyoxymethylene (Acetal) for the purposes of reducing friction. Acetal is inherently slippery, so it is great for sliding mechanisms and gears. Think about the feet on the bottom of a couch sliding across a wood floor. Most likely those “feet” are small injection molded Acetal parts.

What Else is Polyoxymethylene called?

POM is identified by a number of technical and industrial names (the most common of which is Acetal). Other technical names include the following:

• Polyacetal
• Polyformaldehyde
• Polymethylene glycol
• Polyoxymethylene glycol

Industrial names refer to slightly different formulas for essentially the same substance, and differ by the chemical company involved in their production. Examples include the following:

• Celcon®: Acetal Copolymer plastic made by Ticona (a Celanese subsidiary out of Florence KY, USA).
• Delrin®: Acetal Homopolymer plastic made by the DuPont Company, Wilmington, DE, USA.
• Duracon®: Acetal Copolymer plastic made by Polyplastics Taiwan Company, China.
• Kepital®-POM: Acetal Copolymer plastic made by the Korea Engineering Plastics Company (KEP), Seoul, Korea.
• Hostaform®: Acetal Copolymer plastic made by the Celanese Corporation, Ramtal, Irving TX, USA.
• Tecaform®: Acetal Copolymer plastic made by Ensinger Company out of Washington, PA, USA.
• Tepcon®: Acetal Copolymer plastic made by Polyplastics Taiwan Company, China.
• Ultraform®: Acetal Copolymer plastic made by BASF, Ludwigshafen, Germany.

We often refer to Acetal Homopolymer as Delrin, a DuPont trademarked name for their particular resin. Delrin is one of the most popular forms of Polyoxymethylene in the United States and is typically sold in rods or sheets. It is the only one of those listed that is a Homopolymer.

How is POM made?

Polyoxymethylene, like other plastics, starts with the distillation of hydrocarbon fuels into lighter groups called “fractions” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation). You can read about the process in more depth here.

POM for Prototype Development on CNC Machines and 3D Printers:

We asked Michael Creighton, design engineer at Creative Mechanisms, to weigh in on the feasibility of acetal as it relates to its use in both CNC and FDM machine applications. Here’s what he had to say:

- Michael on Machining Acetal on CNC Machines:

“We usually make parts out of Acetal by machining them on our CNC machine; typically limited to black and white colors. The natural color of Acetal is more or less white, so that is often what is used. Acetal can also be injection molded or extruded. There really isn’t any limitation to color here as colorants can be added.”

- Michael on Printing Acetal on a 3D Printer (specifically an FDM machine):

“We need to be careful when it comes to talking about 3D printing of Acetal, or any other material. There are really two major realms of 3D printing: the professionals and the at-home hobbyists. The quality of the machines used is extremely different, and therefore the quality of the parts and materials is equally variable.  Yes, some guy out there might be 3D printing with Acetal, Polypropylene (PP), chocolate, ice, glue, mud, cement, rubber, or whatever. Technically, as long as you can extrude a material through a nozzle, you can 3D print with it. A lot of this is done by adjusting the hardware and software settings on a consumer grade 3D printer. You could spend a week modifying a consumer machine to print with Acetal. But now that machine will no longer print in ABS (Acrylonitrile Butadiene Styrene) or PLA (Poly Lactic Acid), two very common FDM materials.

For professionals, however, we need the ability to print a part without having to go in and fine tune a bunch of machine parameters before our part will build. The machines we use are considered 3D Production Machines (rather than 3D printers). There is never any doubt that the part will print properly, the material will always behave as advertised, and we spend basically no time making adjustments to the machine. We like to say that our FDM machine is so easy to use that it is very similar to printing a document from Microsoft Word. It’s not really that simple, but it’s only a few more mouse clicks.”

Currently neither Stratasys nor 3D Systems are 3D Printing in Acetal, which means that the material is likely difficult to print with. That said, both of these companies are routinely coming out with new materials to print with. As those are the two major players in the market you can look to them to be leading indicators of future advances in such a capability. POM (Acetal) is frequently used in consumer products, i.e. there is a demand for its use. Typically, when there is a demand for such a product, eventually technology and innovation will rise to meet it.

Is POM toxic?

In solid form, no. In fact, Acetal is often used in food processing equipment and manufacturing. Acetal can be toxic if inhaled and/or absorbed into the skin or eyes as a vapor or liquid. Be careful and closely follow handling instructions for molten polymer in particular.  

What are the Disadvantages of Acetal?

The disadvantage to Acetal (at least from a model making perspective) is that is does not bond well. For example, a lot of times we need to make adjustments or repairs to prototypes. To do this kind of work we typically use Cyanoacrylate super glues. Unfortunately, in the case of Acetal (as compared to other plastics), it doesn’t bond very well (the same problem is evident with other glues/adhesives). As a consequence we try to avoid gluing with Acetal if at all possible.

Of note, the reason that Acetal doesn’t glue well is because it is so chemically resistant. This of course can be a compelling reason to use POM in a great number of applications.

What are the Properties of POM?

Property	Value
Technical Name	Polyoxymethylene (POM)
Chemical Formula	(CH2O)n
Melt Temperature	182-218 °C (360-425 °F) ***^1
Typical Injection Molding Temperature	79 - 107 °C (175 - 225 °F) ***
Heat Deflection Temperature (HDT)	160 °C (320 °F) at 0.46 MPa (66 PSI) **
Tensile Strength	63 MPa (9100 PSI) ***
Flexural Strength	90 MPa (13000 PSI) ***
Specific Gravity	1.41
Shrink Rate	2.1 - 2.9 % (.021 - .029 in/in) ***