What is Acetal Plastic? — POM Plastics for Flow Control Parts

Last updated on November 12, 2019

Why Acetal or POM plastic might be the best material choice

What is Acetal Plastic?
Acetal plastic, also called polyacetal and polyoxymethylene (POM), is a general purpose, semi-crystalline, engineered thermoplastic. Acetal is commonly used for parts that need to be very stiff, have low surface friction and good dimensional stability. Dimensional stability is the ability of a plastic part to maintain its original dimensions when it is exposed to changes in temperature and humidity.

What is an engineered plastic?
Engineered plastics have better mechanical properties and handle heat better than commodity plastics. This makes them tougher and more suitable for extreme environments. Commodity plastics are simply not as tough. Some familiar commodity plastics are PVC, polyethylene and polypropylene.

What are semi-crystalline plastics?
Semi-crystalline plastics have a very organized molecular structure and sharp melting points. They will not gradually soften as their temperature increases. Instead, they stay solid until their melting point is reached. When they are hot enough, semi-crystalline plastics quickly change from solids into low viscosity or thin liquids that flow easily.

Typically, semi-crystalline thermoplastics melt and flow at temperatures ranging from about 275º to 335º F (135º to 335º C). The specific melting point depends on the type of plastic.

Drawing of amorphous versus semi crystalline thermoplastics.

Amorphous vs semi-crystalline 

Amorphous vs. Semi-Crystalline Thermoplastics at Redwood Plastics and Rubber.

What are thermoplastics?
Thermoplastics are useful for making products because they can be heated until they melt and flow. After cooling, thermoplastics become hard again. This cycle of melting, reforming and cooling can be repeated again and again. Injection molding is a production process that takes advantage of this. It can be used to quickly and economically produce large numbers of consistently high-quality thermoplastic parts.

Copolymer and homopolymer acetal plastics
Acetal is made in slightly different formulation variations sold under various trade names. Each trade name acetal plastic is also usually made in a range of recipes that are adjusted to improve specific properties. One thing that all acetal plastics have in common is that they are either a copolymer acetal or a homopolymer acetal. The differences between copolymer and homopolymer acetal plastics are relatively small but they are measurable.

Some of the better-known acetal plastic trade names:

  • Acetron® - copolymer and homopolymer acetals made by Mitsubishi
  • Celcon® - copolymer acetals made by Celanese
  • Delrin® - homopolymer acetals made by the DuPont™
  • Duracon® POM - copolymer acetals made by Polyplastics
  • Hostaform® - copolymer acetals made by the Celanese
  • Kepital® POM – copolymer and homopolymer acetals made by KEP (Korea Engineering Plastics)
  • Sustarin® C – copolymer and homopolymer acetals made by Röchling
  • Tecaform® - copolymer and homopolymer acetals made by Ensinger
  • Tenac™-C - copolymer and homopolymer acetals made by Asahi Kasei
  • Tepcon® - copolymer acetals made by Polyplastics
  • Ultraform® - copolymer acetals made by BASF 

    …and more

Copolymer and homopolymer acetal plastic differences
One of the most important differences between homopolymer acetal (POM-H) and copolymer acetal (POM-C) is porosity. Acetal homopolymer may contain a lower-density or porous center. Porosity in a plastic means it may contain small bubbles or voids. These allow gases and liquids to seep into the plastic. Copolymer acetals have little or no porosity at their centers. This makes them the preferred acetal type for food contact or medical applications.


Homopolymer vs copolymer

  • Copolymers have less outgassing
  • Homopolymers have better creep resistance
  • Copolymers have better dimensional stability
  • Copolymers are less porous in extruded shapes
  • Homopolymers have higher Rockwell hardness ratings
  • Copolymers have slightly better overall chemical resistance
  • Homopolymers have about 10% to 15% higher tensile strength
  • Homopolymers have slightly higher operating temperature limits
  • Homopolymers are stiffer at room temperature and high temperatures
  • Homopolymers have higher impact strength at room temperature and low temperatures

Get more detail about homopolymer and copolymer acetal plastic differences at the Difference Between Homopolymer and Copolymer page at Pediaa.

Food grade acetal plastics
Acetal (POM) copolymers and homopolymers are available in formulations suitable for contact with food. These include compliance with FDA, USDA, NSF, Canada AG and 3-A Dairy material standards. While most acetals used for these applications are natural (white) color, there are compliant colorant additives available that can provide color options.

There are also acetal plastics with metal detectable additives. These are made for the food processing and food packaging industries. Metal detectable additives makes it easier to spot plastic particle contamination using conventional metal detection systems.

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Glass-filled and glass-reinforced acetal plastics
One other type of acetal plastic that is sometimes used to make flow control parts is glass filled or glass reinforced acetal. The glass used in glass filled and glass reinforced acetal plastics is actually chopped glass fibers. While the terms glass filled and glass reinforced are often used in the same way, there are actually some significant differences between the two.

For glass filled acetals, the glass fibers act as a filler and make the parts stiffer but not necessarily stronger. Fiber reinforced acetals use glass fibers that have been sized and chemically treated to help them stick to acetal plastic. Glass reinforcement provides both stiffness and strength.

Glass filled acetals

  • Glass filler adds stiffness but not strength
  • Intended for general industrial applications
  • No chemical bonding of the glass fibers with the acetal plastic

Glass reinforced acetals

  • Glass fibers provide high stiffness and strength
  • For parts requiring high or very high stiffness and strength
  • Glass reinforced acetal is always stronger than glass filled acetal
  • Requires chemical bonding or coupling of the glass fibers with the acetal plastic

Get a copy of our Acetal (POM) Chemical Compatibility Chart.

Get the chart

Steven C. Williams headshot March, 2018.

About the author
Steven C. Williams my LinkedIn profile link button.
Steven C. Williams, BS, is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.       

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