Acetal C and Acetal H are both engineering plastics more properly known as polyoxymethylene (POM) and are members of the thermoplastics family. POM is available in 2 different resins: Acetal Copolymer (POM-C / Ertacetal C®) and Acetal Homopolymer (POM-H / Ertacetal H®). One of the most commonly used Acetal H brands is DuPont™ Delrin®
Similarities / POM Properties
As Acetal C and Acetal H are both essentially polyoxymethylene they share many of the same POM properties:
- One of their most popular properties in the engineering plastics community is their ease of machining. Compared to various other engineering plastics, such as HDPE, UHMW and Nylon, both Acetal C and Acetal H tend not to deflect away from or grab machining tools and they also chip nicely, making them ideal if an application requires the material to be machined.
- High mechanical strength, stiffness and hardness makes them a good alternative to metal if the full strength of metal is not required but other properties of plastic are.
- Low thermal expansion.
- Low water absorption.
- Good creep resistance. This means they are good at maintaining their shape over long exposures to loads and maintain a tighter fit in snap-fit applications.
- Resistant to chemicals and mostly unaffected by solvents and fuels etc. However, they do not react well with chlorine, so would not be ideal for swimming pool applications.
- High lubricity which contributes to their high abrasion resistance and very low coefficient of friction, which makes them appear self-lubricating. However, this means that they are not good at adhering to things.
- Good electrical insulating and dielectric properties.
- Flammable and heat sensitive. They burn low and blue, with a sometimes invisible flame and emit formaldehyde gas when burning.
So, those are the similarities, but what makes Acetal C and Acetal H different? Well, as mentioned previously, Acetal C is a copolymer whilst Acetal H is a homopolymer. Polymers are formed by linking a large number of units called monomers through chemical reactions otherwise known as polymerisation. The main difference between copolymers and homopolymers is that homopolymers are created by using one monomer whilst copolymers are produced by using two types of monomer. This means that Acetal C and Acetal H have slightly different properties.
Acetal C has better chemical properties
Significant centreline porosity
Good chemical resistance in PH range 4-9
Hydrolysis resistance up to 60 degrees Celsius
Continuous allowable service temperature in air: 90 degrees Celsius
Reduced centreline porosity
Good chemical resistance in PH range 4-13
Hydrolysis resistance up to 85 degrees Celsius
Continuous allowable service temperature in air: 100 degrees Celsius
Acetal H has better mechanical properties
15% higher strength and stiffness
10% better creep resistance
Higher Rockwell hardness rating
Less stiff, more ductile
One of the biggest differences between Acetal C and Acetal H is centreline porosity, which is a characteristic of H but not C. Centreline porosity is caused by gasses trying to escape during the cooling process after the manufacturing process of extrusion or compression. It can appear as small bubbles in thicker rods or a white line down the middle of each cut edge of a sheet.
There are other slight differences between Acetal C and Acetal H:
- Acetal H has better mechanical properties than Acetal, including higher strength and stiffness, better creep resistance and higher Rockwell hardness rating.
- However, Acetal C has better chemical properties than Acetal H, including reduced centreline porosity, better chemical resistance, better Hydrolosis resistance, higher continuous allowable service temperature in air and less outgassing.
Typical Material Applications
Due to their various properties such as lubricity, strength, chemical resistance and ease of machining, both Acetal variants can be used in thousands of different applications. Common uses include:
- Conveyor moving parts
- Automotive and fuel related parts
- Sliding and guiding parts
- Electrical applications e.g. electrical components and insulation
- Winter sports equipment
- Pump and valve parts
Which One Should I Choose?
For most applications it doesn’t really matter whether Acetal C and Acetal H is used, as many properties are within 10% of each other. However, one of the biggest issues to consider is centreline porosity, which can be undesirable as it compromises structural integrity, has potential routes for leakage, has an unappealing aesthetic appearance of inconsistent colour and has areas where bacteria can grow, which is especially important in food processing applications. If centreline porosity is not an issue and, for example, the centre of the rod is going to be machined out, then Acetal H does have slightly better mechanical properties. However, if it is an issue, the centre of the rod will not be machined out, or the slight differences between the two will not matter for the application end use, then Acetal C tends to be cheaper and has better chemical properties.