ACETAL POLYOXYMETHYLENE
Acetal plastic is an engineering thermoplastic. It is often used to make a variety of custom parts as well as acetal plastic screws, hex bolts, hex nuts as well as other parts. Polyoxymethylene or POM is another name for Acetal. POM is used in precision parts that require high stiffness, low friction, and dimensional stability. POM is known for its high strength when exposed to temperatures from -40 degrees C to 80 degrees C. Natural POM is an opaque white material. However, it is easy to dye it to any color.
It has a high crystalline composition. And Acetal Plastic is available as a homopolymer and a copolymer. Different chemical companies sell POM under different names. These include Delrin®by DuPont and Celcon® by Celanese. However, it is not UV resistant. POM does not perform well in the rays of the sun. Additives are required when the material is exposed to UV rays.
ACETAL COPOLYMER VERSUS ACETAL HOMOPOLYMER
Acetal copolymer and homopolymer are two types of POM. Although these are very similar, they are not exactly the same material. The difference is in the structure of each. Acetal homopolymers have chains made of one monomer. However acetal plastic copolymers have chains made of two different monomers. For many applications, either will perform well. This is because most properties of the two materials are within 10% of each other. However, there are important differences.
CHARACTERISTICS OF ACETAL POM
Both homopolymer and copolymer POM has the following characteristics:
Pros
- Creep resistant
- Excellent mechanical stability
- Good electrical insulating and dielectric properties
- Strong, rigid, and tough
- Excellent moisture resistance, low water absorption
- Low coefficient of friction, high lubricity
- Chemically resistant. Mostly unaffected by solvents and fuels.
- Low thermal expansion
- High stiffness and hardness, therefore a good alternative to metals
- FDA approved
- Easily machined and molded
- Easy to color
Negatives
- Acetal cannot be painted.
- Flammable and heat sensitive
- Does not adhere well to other objects due to high lubricity
- Acetal copolymer is a semi-crystalline thermoplastic. Importantly, the copolymer has the advantage of reduced centerline porosity. In addition, the copolymer has better chemical properties. It has good pH resistance from pH 4-9. It is hydrolysis resistant up to 60 degrees Celsius. The continuous service temperature in the air is 90 degrees Celsius. And the acetal copolymer exhibits higher out-gassing. It is available in natural and black. In addition, the copolymer is a little cheaper.
Copolymer
Acetal copolymer is a semi-crystalline thermoplastic. Importantly, the copolymer has the advantage of reduced centerline porosity. In addition, the copolymer has better chemical properties. It has good pH resistance from pH 4-9. It is hydrolysis resistant up to 60 degrees Celsius. The continuous service temperature in the air is 90 degrees Celsius. And the acetal copolymer exhibits higher out-gassing. It is available in natural and black. In addition, the copolymer is a little cheaper.
Homopolymer
Acetal homopolymer is a crystalline thermoplastic. Thus, it is the ideal material to replace metal parts. However, the negative side of the homopolymer is that it exhibits centerline porosity. However, generally the homopolymer has better mechanical properties:
- 15% higher stiffness and strength.
- 10% better creep resistance
- Higher Rockwell hardness rating
- Natural grade is FDA, NSF, and USDA compliant
- Higher tensile strength
- Significant impact resistance
Why Use Delrin® (Homopolymer) To Replace Metals
Delrin® has high tensile strength as well as low friction, and high wear resistance. In addition, it has low creep, is resistant to warping, and is overall tough and durable. Delrin® has low moisture absorption and is chemically resistant to hydrocarbons, solvents, and neutral chemicals.
While acetal plastic copolymers exhibit some of these characteristics, it is the unique combination of properties that makes the acetal plastic homopolymer the right plastic to replace metals in some applications.
This is not to say that Delrin® is as strong as metals. It is not. However, Delrin keeps on performing in applications such as pump and valve components where parts are exposed to moist or wet environments. Metals may fail while Delrin® keeps on performing.
APPLICATIONS
Typical applications for POM include high-performance parts such as gears, safety restraints, bearings, bushings rollers, fittings, and electrical insulator parts. Many parts such as acetal screws, hex nuts, and washers, hex bolts and many other custom parts are made from this plastic. Also, it is used to make consumer products such as eyeglass frames, ski bindings, guns, and knife handles. Additionally, the consumer electronic industry uses Delrin® widely. And it is preferred by the automotive industry for use on car interiors. POM has high impact strength.
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