To quickly explain the difference between thermoplastic and thermosetting polymers, let’s make an omelette (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.
Figure 1: In eggs and thermosets the change is permanent; for cheese and thermoplastics it is reversible.
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 bake ware. Cross-linking lets it easily withstand 250°C oven temperatures and makes it inherently non-stick—very desirable characteristics for bake ware, but thermosets have their liabilities as well. In harder forms, thermosetting plastics are not as impact resistant as thermoplastics and can tend to shatter. The materials cannot be reused, and in some cases surface finishing can be difficult.
Figure 2: Thermoplastic vs. thermosetting polymers
At Protomold we are occasionally asked if we can mould thermoset resins. Unfortunately, we cannot. Thermoset materials are injected in unheated liquid form into heated moulds. The thermoplastics we use are molten under high pressure, injected into moulds and allowed to cool before ejection. In some cases, however, we can suggest thermoplastic materials that can be used for prototyping (and occasionally for production) in place of thermoset plastics. This is most often the case for elastic thermosets like rubber, vulcanised rubber, silicone (unless they are to be tested in high-heat applications), and some urethanes.
Proto Labs stocks a wide variety of thermoplastic elastomers (TPEs), thermoplastic vulcanisates (TPVs), and thermoplastic polyurethanes (TPUs) that share elastic (resilience) and durometric (hardness) characteristics of elastic thermosets (except, as mentioned, the heat tolerance of silicone). Using these resins, Protomold can make prototype parts suitable for functional testing faster and at significantly lower cost than they could be made using thermosets. While Protomold cannot make resin choices for you, our customer service engineers are available at +44 (0)1952 683047 to help you consider the characteristics of available thermoplastic resins. You can determine whether any might make suitable prototypes for parts that will ultimately be made of thermoset plastic, or perhaps you’ll find that thermoplastic resin is the way your project will go.