To quickly explain the difference between thermoplastic and thermosetting polymers, let’s make an omelet (see Figure 1). We start with an egg, a slice of cheese, and a warm pan. The egg begins as a liquid (a colloid, actually, but let’s not quibble) that, when dropped in a warm pan, becomes a solid. The cheese, on the other hand begins as a solid, but when heated (but not overheated) becomes a viscous liquid. After heating the egg you can cool it or reheat it but it will never return to its liquid state; it remains solid, just as thermoset polymers do. But if you cool the melted cheese it regains its solid form; reheat it and it flows again, just like a thermoplastic.
Cross-linking determines many of the characteristics of the thermosets. It makes them strong, dimensionally stable, and highly resistant to heat and chemicals (see Figure 2). One familiar example is rubbery silicone bakeware. Cross-linking lets it easily withstand 400 degrees oven temperatures and makes it inherently non-stick—very desirable characteristics for bakeware, but thermosets have their liabilities as well. In harder forms, thermosetting plastics are not as impact resistant as thermoplastics and can tend to shatter.
At Protolabs, we also injection mold liquid silicone rubber (LSR), which is a thermoset material. Thermosets are injected in unheated liquid form into heated molds. The thermoplastics we use are molten under high pressure, injected into molds and allowed to cool before ejection. In some cases, thermoplastic materials can be used for prototyping and production in place of thermoset plastics. This is most often the case for elastic thermosets like rubber, vulcanized rubber, silicone (unless they are to be tested in high-heat applications), and some urethanes.
We stock many thermoplastic elastomers (TPEs), thermoplastic vulcanizates (TPVs), and thermoplastic polyurethanes (TPUs) that share elastic (resilience) and durometric (hardness) characteristics of elastic thermosets (except, as mentioned, the heat tolerance of silicone). We also stock multiple grades and durometers of LSR. Using these materials, Protolabs can make parts affordable parts for functional testing and end-use production.
While Protolabs cannot make resin choices for you, our applications engineers are available at 877-479-3680 to help you consider the characteristics of available thermoplastics and thermosets.