The flexibility in adjusting plastic mechanical properties makes much of our technological advancement possible, from being able to produce fuel-efficient jumbo jets with high strength, to lightweight carbon fiber-reinforced plastic that can be made into impact-resistant silicone phone cases that save our precious phones from shattering into pieces.
This article provides an overview of the mechanical performance of the major families of thermoplastic and thermoset plastic. Note that because the mechanical performance of each plastic from different suppliers can vary, this overview serves as a guide, rather than a definitive catalogue of plastic types. It’s also important to note that different variants of the same plastic from the same supplier can have different properties, too!
Reversible or not: the basics of plastic chemistry
Thermoplastics soften when heated and slowly harden upon cooling, just like chocolate. This process is generally reversible, and thus, thermoplastics are ideal for recycling.
Unlike thermoplastics, thermoset plastic starts its life in a liquid state. Only upon contact with a special chemical (catalyst) or heat will it polymerize (set), giving us a hard and rigid material. This process is a one-way street; any attempts to heat it up will char or vaporize the material. You may have seen this destruction before, if you’ve ever accidentally placed your plastic tableware too close to the stove!>
This unique behavior is due to the way the polymer chains are bonded with each other. Here’s a quick chemistry review: Each polymer chain in thermoplastics is bonded via weak Van der Waals’ forces, which can be easily overcome with heat.
However, thermoset plastics polymer chains are bonded via the stronger covalent bonds, which unfortunately will burn the polymer before the covalent bonds are broken.
Rigid thermoplastic vs. thermoset plastic
Generally, thermoset plastics are preferred for applications in which softening of the material is risky. They’re widely used in tablewares, allowing hot content to be held in the container without any changes in shape or strength. Imagine you’re holding a saucepan, and the plastic handle gradually softens and bends under the weight of the hot soup!
The plot below shows how the flexural modulus of Delrin 100, a type of Acetate by DuPont, gradually decreases with temperature.
The table below shows several different types of plastics, classified according to their families.
Polycarbonate (PC) is chosen for a specific quality: optical transparency. Its superior optical property and dimensional stability ensures that your prescription glasses don’t deform under varying temperatures. It’s ideal for making riot shields, which require lightweight, high-impact resistance.
Like PC, ABS is impact resistant, but it’s cheaper and isn’t available in transparent form, which is a consideration when it comes to mass production.
POM has another unique property; it is self-lubricating and therefore an ideal choice for making super-smooth mechanisms without adding much grease.
Elastomer thermoplastic vs. thermoset plastic
Another class of plastic is foam-like. These plastics are processed a bit differently from rigid plastics and are typically used in cases requiring a high level of compliance. Keyboards have evolved with material advancement—the majority of keyboards used to consist of mechanical switches, but these complex parts have been replaced with a highly springy silicone membrane. Tech By Matt has a great video that compares these two technologies and includes a short teardown.
Because elastomers are very different from rigid plastic, the table below takes on a different set of mechanical properties. Porosity relates to a plastics’ barrier ability against air. Minimum service temperature refers to its lowest possible temperature before the elastomers becomes inflexible and brittle.
The selection of plastic is a complex process made challenging by the vast number of suppliers and variants. Keep in mind, though, that plastic is unlike metal—its non-linear mechanical properties require you to check constantly on the expected strain rate and working temperature.
Before selecting a plastic, focus on these two important parameters: service temperature and service strain rate. They’ll serve as a strong filter during your search for a plastic candidate. After all, candidates that don’t meet your requirements will impact both product quality and your warranty claim! To learn more about plastics, read our guide to plastic material for prototyping and production and our article about designing for stiffness using material properties.