Polyoxymethylene (POM), often known as acetal resin, is a high-performance engineering thermoplastic valued for its combination of strength and stability. This semi-crystalline polymer is utilized in precision parts that require excellent mechanical properties, low friction, and high dimensional accuracy. POM frequently serves as a lightweight replacement for metals in demanding applications without sacrificing performance. This capability has positioned POM as a preferred material across various industrial sectors.
The Chemical Makeup of POM
The fundamental structure of Polyoxymethylene is based on repeating units of the formaldehyde molecule. This simple linear chain structure results in a high degree of crystallinity, which is responsible for the material’s inherent rigidity and strength. There are two primary variants: homopolymer (POM-H), often recognized by the trade name Delrin, and copolymer (POM-C).
The difference between these two types lies in how the polymer chain is stabilized during manufacturing. Homopolymers are created by polymerizing a single monomer, resulting in a highly regular, crystalline structure. Copolymer acetals incorporate a second monomer, which interrupts the regular chain and leads to a more amorphous structure with lower crystallinity. Homopolymers exhibit about 15% higher hardness, stiffness, and tensile strength. The copolymer structure provides superior resistance to chemical attack, particularly from hot water, strong alkalis, and solvents, and is less prone to centerline porosity.
Key Physical and Mechanical Properties
POM is selected for precision applications because it offers a unique blend of physical and mechanical attributes that mimic or exceed those of some metals. It has high stiffness and strength, with tensile yield stress falling around 62 MPa, enabling it to withstand significant pulling forces. The material also exhibits high flexural fatigue strength, meaning parts can endure repeated bending and flexing over long periods without failure under cyclic stress.
The excellent dimensional stability of POM results from its high crystallinity and low moisture absorption. This helps it retain its shape and size across a range of temperatures and humidity levels, which is essential for parts that must maintain tight tolerances. POM possesses outstanding self-lubricating qualities due to its low coefficient of friction (0.2 to 0.3). This low friction, combined with high wear resistance, makes it suitable for moving parts and sliding applications, reducing the need for external lubrication.
The material’s resistance to creep allows it to maintain its form under prolonged mechanical stress over time. This makes POM a reliable choice for load-bearing applications like gears and bearings that must function consistently. It also maintains its strength and rigidity down to temperatures as low as \(-40^\circ\text{C}\), demonstrating its suitability for harsh environments.
Common Applications and Processing
The favorable properties of POM have led to its adoption across numerous high-performance applications in diverse industries. In the automotive sector, it is used for fuel system components, such as valves and filter housings, due to its resistance to hydrocarbons and high dimensional stability. Its wear resistance and low friction are leveraged in small precision mechanical components, including gears, bearings, bushings, and snap-fit assemblies in consumer electronics and machinery.
In the medical field, POM’s ability to withstand sterilization and its low extractable content make it suitable for surgical instrument handles and drug delivery systems. Other common uses include:
- Zippers
- Conveyor belts
- Window regulators
- Components in household appliances, such as pump impellers
To manufacture parts from POM, the material is supplied in granulated form.
The most common method for shaping POM is injection molding, where the material’s excellent flow properties allow for the creation of intricate shapes with fine details and tight tolerances. Extrusion is also a widely used process, especially for producing stock shapes like rods and sheets that can be further modified. POM is also valued for its excellent machinability, enabling manufacturers to easily create complex, custom components using standard machining techniques.