Written by Barbara Gerard, CEO
Craftech Industries, Inc.
Metal has long been the material of choice for mechanical fasteners. However, it’s no longer the go-to option for all design engineers. Depending on the application, the use of plastic is gaining popularity and offers several material advantages including strength, longevity, and cost-effectiveness.
Plastic fasteners are now more frequently used in high-pressure and critical environments. Much like with metals, plastics offer different properties and options — such as nylon, polyvinyl chloride (PVC), polyoxymethylene (POM), and others. The choice should be based on the demands of the application and its environment.
For example, nylon is a popular choice and works fine for everyday wear and tear. PVC is often ideal when corrosion and weather-resistance are required. For high-performance, however, you’ll want to consider POM.
Polyoxymethylene is an engineering material used in parts that require precision. Applications for POM include high-performance components, such as fasteners (screws, nuts, washers, etc.).
The material is ideal for parts that require high strength, stability, durability, and low friction. It’s also used in a variety of other applications, such as for zippers, ski bindings, insulin pens, coffee spigots, and even as a solvent for perfumes and as a synthetic flavoring ingredient.
POM is a thermoplastic material also known as acetal. It’s a molecule containing the functional group of carbon bonded to two –OR groups and was first discovered by a German chemist, Hermann Staudinger, who won the 1953 Nobel Prize in Chemistry.
Staudinger had studied this material in the 1920s but found it to be thermally unstable. This thermal instability meant the material was not useful, at least not commercially.
Eventually, however, a heat-stable POM homopolymer was discovered by the chemist, Dal Nagore. He realized that by reacting the hemiacetal ends with acetic anhydride, he could readily depolymerize hemiacetal into a thermally stable and melt-processable thermoplastic.
Soon after, the chemical company DuPont synthesized a version and filed for patent protection of the homopolymer. In 1960, it built a manufacturing plant to produce Delrin — its version of the POM homopolymer, which is still offered today. Around the same time, the chemical company Celanese completed its study of the copolymer and started production of Celcon.
Other manufacturers soon followed with their versions of this material.
It’s important to note that different manufacturing processes are used to produce the homopolymer and the copolymer versions of POM.
To make the homopolymer, first anhydrous formaldehyde is necessary. The formaldehyde is then polymerized by anionic catalysis and the resulting polymer is stabilized by the reaction to acetic anhydride.
The water that’s produced must be removed from the reaction. The homopolymer has excellent creep resistance. For example, DuPont’s Delrin is typically made this way. However, the copolymer of POM requires that the formaldehyde is converted to trioxane, which can be done by acid catalysis.
This is followed by purification of the trioxane by distillation or extraction to remove the water and other active impurities containing hydrogen.
The benefits of thermoplastic parts include an ability to provide strength and corrosion resistance, typically lowering maintenance and repair costs and extending longevity. This is why design engineers are now often choosing plastics over metal parts.
Plastic is more resistant to normal weather conditions and some extreme environments, resulting in less maintenance and fatigue over the long run. Not only will plastic fasteners last longer, for example, but they also offer higher-temperature thresholds, are less subject to vibration, and provide enhanced operational efficiency and reliability.
What’s more is metal parts tend to weigh more than plastic parts, which can make it more difficult to transport and install when compared to plastic.
Polyoxymethylene demonstrates the following properties:
• High stiffness
• Low coefficient of friction
• Hardness and durability
• High abrasion resistance
• High heat resistance
• Low water absorption
• Good electrical and dielectric properties
• Excellent dimensional stability
• More creep resistance than nylon
• Low smoke emissions
• A high-gloss surface
• Highly crystalline
There are drawbacks to the use of POM, depending on the application. Although this material offers high heat resistance, it’s unavailable in a flame-retardant grade. It also has poor resistance to acids and a high shrinkage rate.
The key is to choose the right material for your fastener application. Fortunately, there is more than one type of plastic available — some can be machined, others molded, and some are as strong as stainless steel. The type selected is critical because it will affect the function, longevity, appearance, and cost of the project.
As with any important purchase, choosing a knowledgeable supplier for plastic components is critical. Look for one with insight into the different materials available for fasteners that can offer strong technical support.