What is the difference between thermoplastics and thermosets?
When choosing a plastic for a machine part, you will usually look at properties such as strength, wear resistance, friction or temperature resistance. Before that, however, there is a more fundamental question: is it a thermoplastic or a thermoset?
That distinction may seem mainly theoretical, but it has major consequences in practice. It determines how a material reacts to heat, whether it can be recycled and, above all, which processing operations are possible. A plastic that you want to bend or weld, for example, needs a completely different structure than a plastic that has to remain hard and dimensionally stable under all circumstances.
In this blog we explain what thermoplastics and thermosets are, how they differ and what that means for your application.
Two families of plastics
Plastics are synthetic materials made through chemical processes. They can roughly be divided into two groups: thermoplastics and thermosets. The difference lies in the chemical structure of the material and in how that structure reacts to heat.
Plastics consist of long molecular chains, known as polymers. How those chains are connected to each other determines which family a plastic belongs to.
Thermoplastics: formable under the influence of heat
In thermoplastics, the molecular chains are held together by relatively weak forces. When the material is heated, those forces release: the plastic becomes soft, can be shaped and eventually melts. On cooling, the material becomes solid again and retains its new shape.
No chemical reaction takes place during heating. The structure of the material remains the same, which means thermoplastics can be heated, shaped and cooled several times. This makes them exceptionally versatile.
The vast majority of the engineering plastics used in mechanical engineering are thermoplastics. Think of:
- POM, PA and HMPE for guides, bearings and wear parts
- PETP for dimensionally stable precision parts
- Polycarbonate and acrylic for transparent machine covers and guards
- PVC, PP and PE as construction plastics for chemically resistant applications
- High-performance plastics such as PEEK
Thermoplastics are widely used because of their formability, machinability and recyclability.
Thermosets: once cured, always hard
Thermosets work in a fundamentally different way. During production, the material undergoes a chemical reaction in which the molecular chains form strong cross-links with one another. This creates a rigid, three-dimensional network.
That process is irreversible. A thermoset that has cured does not soften or melt when it is heated again. At very high temperatures the material eventually decomposes, but it can never be reshaped.
This property makes thermosets suitable for applications where hardness, stiffness and dimensional stability are essential, such as electrical insulation, electronic housings, aircraft components and composite materials.
Well-known thermosets are epoxy and phenolic resins. Among engineering plastics, laminated fabric and laminated paper (phenolic laminates) are the best-known examples. In laminated fabric, cotton fabric is impregnated with resin and cured under high pressure and temperature. Laminated paper is built up in the same way, but with layers of paper as the base. The result is a very stiff, layered sheet or rod material that is used for, among other things, bushings, rings and insulating parts.
What does the difference mean for processing?
For anyone having a plastic part made, this is the most important practical difference. The structure of the material determines which processing operations are possible.
Bending is only possible with thermoplastics. In heat bending, a bend line is heated locally so that the material becomes plastic and can be bent into the desired shape. This is done, for example, with machine covers made of polycarbonate or acrylic. A thermoset does not soften when heated and will break when bent.
Welding is likewise reserved for thermoplastics. In plastic welding, two parts are fused together using heat and a welding rod made of the same material. Materials such as PVC, PP and HMPE are well suited to this, for example for watertight drip trays. Because thermosets do not melt, they cannot be welded.
Milling and turning are both possible with either family. With thermoplastics, the material's sensitivity to melting is taken into account, for example through the choice of tooling and cutting speed, so that the material does not get too hot. Thermosets such as laminated fabric, on the other hand, machine very finely, so good extraction of the dust that is released is important. Parts made from thermosets are therefore almost always milled or turned from sheet or rod material.
Joining thermosets is done by gluing or mechanical fastening. With thermoplastics, gluing, welding and mechanical fastening are all possible, depending on the material and the application.
Is a thermoset always more heat-resistant?
A common misconception is that thermosets are by definition better at withstanding high temperatures than thermoplastics. That is not entirely true.
It is true that a thermoset does not soften or melt. But there are also thermoplastics that have been specifically developed for long-term thermal load. High-performance plastics such as PEEK and PAI retain their mechanical properties up to approximately 250°C in continuous use, combined with high strength and wear resistance.
What matters in practice is not just the maximum temperature, but above all whether a material retains its properties under prolonged heat, load and the conditions of your application. We go into this in more depth in our blog Which type of plastic is most resistant to heat?
Sustainability and recycling
There is also a clear difference when it comes to sustainability. Thermoplastics can be melted down and processed again, giving leftover material a second life. More and more thermoplastics are therefore also available as recycled plastic.
With thermosets this is much more difficult. Because the network in the material cannot be reshaped, a cured thermoset cannot be recycled in the same way. This is one of the reasons why thermoplastics are preferred in many applications.
Thermoplastic or thermoset: which suits your application?
In most mechanical engineering applications, a thermoplastic is the logical choice. These materials offer a wide range of properties, from dimensional stability and low friction to impact strength, chemical resistance and heat resistance. They can also be processed in many ways, from sawing and milling to bending and welding.
A thermoset comes into the picture when a part has to remain hard, stiff and dimensionally stable under all circumstances, or when electrical insulation plays an important role. Think of insulating parts, bushings and rings made of laminated fabric or laminated paper.
Ultimately, as with any material question, the right choice starts with the application. What load will be placed on the part? Does heat play a role? Will it come into contact with moisture or chemicals? And does the part need to be bent, welded or glued?
Not sure which plastic best suits your part? Lakwijk supplies both thermoplastics and thermosets, as semi-finished products or as machined end products. Contact us for advice on material selection and processing.
