Moving from metal to plastic

By Andrew Fletcher, TR Fastenings director of plastics and rubber

For certain applications, selecting a plastic material over metal for a fastening application can offer some key benefits. First is weight savings. A plastic part could weigh up to 80% less than the same part in steel or 50% less than in aluminium. This can offer substantial overall savings in applications where weight is a key factor in performance, emissions and energy efficiency.
Second, cost savings. In certain cases plastic parts can offer cost savings of between 25% to 50% over metal parts. These cost savings are mainly seen in the manufacturing processing and the ability to streamline the number of secondary operations associated with metal parts. This could be in assembly operations where multiple component features can be incorporated into plastic part design, or in secondary machining, welding, painting, plating, or finishing processes.
Third, speed of installation/application. Due to resilience and flexibility of certain plastics and the design freedom enabled by the manufacturing methods, key features like push-in and snap fits can be incorporated into the fastener’s design; offering substantial installation time savings over conventional threaded methods. Features can also be incorporated that enable fully-automated insertion, or simplified insertion eliminating the need for any special tooling.
Fourth, thermal and electrical insulation. In general, plastics have excellent electrical and thermal insulative properties, making them ideal for electrical and electronic applications.
Fifth, RF (radio frequency) transparency. RF transparency is highly important in applications requiring the unrestricted transmission of radio frequencies, for example radar and avionics equipment antenna. Many plastics have a high degree of RF transparency.
Sixth, chemical and corrosion resistance. Plastics do not corrode or cause electrolytic corrosion and are much less likely to suffer chemical attack, compared to metals.
Seventh, noise reduction. In general, the sound of plastic against plastic is less intrusive than metal against metal. In addition to this, dampening and compensating features in the part design can reduce rattle or noise transmission.
Eighth, product life span. In many applications, due to their high durability, the life span of a plastic component can be longer than its metal counterpart, thus reducing the product’s longer-term environmental impact.
There are, however, several key properties that need to be carefully considered with looking at making the switch from metal to plastic. First is strength. Advances in reinforcing methods and material for plastics is continually evolving. However, in most cases, a plastic fastener (of the same size) will be weaker than its metal counterpart (see table).

Table: Examples of some high-strength plastics, compared to steel

Special attention also needs to be given to the potential for the plastic fastener to ‘creep’ when the fastener is preloaded. This is where the polymer chains adjust position to reduce of integral stresses. If this occurs, the tension on the fastener will reduce. Common reinforcing materials used to produce high-strength plastics include glass or carbon fibres in various percentages of mix and fibre lengths (see also table).
Second, heat resistance. High heat can cause plastics to deform and degrade. Therefore, careful consideration needs to be given to both short- and long-term exposure to heat. With long-term exposure to elevated temperatures, mechanical and electrical insulative properties start to diminish. The addition of reinforcing materials help counter mechanical deformation with short-term exposure. Additives such as heat stabilisers can be added to help reduce degradation through long-term exposure.
Third, weathering/UV resistance. Ultraviolet light (for example, from the sun) can damage and degrade plastic, causing colour change, embrittlement and breakdown of surface finish. Again, additives (stabilisers, absorbers etc) can be added to help reduce or slow down this degradation.
Fourth, flammability. In the main, plastics, being carbon-based, will burn and give off gases and smoke. A plastic’s burning and emission behaviour needs to be carefully considered during the design process. Additives (flame retardants) are used to alter burning characteristics.
As energy efficiency and reduction in environmental impact are still front of the design engineer’s mind, the topic of metal to plastic conversion is still highly relevant. TR Fastenings is working with world leading manufactures specialising in the production of non-metallic fasteners. With a constantly expanding choice of high-performance plastics, now is an ideal time for OEMs to look more closely as the options available for metal to plastic conversion when it comes to fastening components.
For more information on TR Fastenings and our work on sustainability, please visit
www.trfastenings.com/sustainability