Choosing a Heat-resistant Plastic
Need to turn up the heat on plastic parts? Here are eight high-temperature polymers you should know about
If you’ve ever left a Rubbermaid food container in the microwave oven a bit too long, you’re well aware that some plastics don’t handle high temperatures too well. Depending on the vintage and type of container, you might be storing last night’s dinner in polypropylene (PP), polycarbonate (PC), or polyethylene (PE), none of which are heat-resistant superheroes. Polypropylene, for example, begins to lose strength at 180°F (82°C). Polyethylene does a better job at 266°F (130°C), but even so-called “high-heat” polycarbonate is only rated to 284°F (140°C).
Defining Hot: What is a Heat-resistant Plastic?
As evidenced by the tiny microwave oven symbol on the backs of these containers, each of the polymers just listed is clearly up to the task of heating leftovers. For high-temperature applications, however, something more robust is needed. But what does that mean, exactly? In other words, how hot is hot? The exact answer depends on the application requirements, but for the purposes of this design tip, let's define it as 350°F (177°C).
Let's also clarify that, for the most part, we're talking about working temperatures rather than those needed to melt or crystallize the polymer. That topic is covered elsewhere on our site. Nor are we discussing a polymer’s flame-retardant capabilities. As you'll see, this important attribute has little to do with a polymer's heat resistance.
Consider acrylonitrile butadiene styrene, a common plastic you might know as ABS. A favorite of plumbers and toymakers everywhere, ABS has a Vicat softening temperature—the temperature at which the material loses its “stability-form”—of roughly 219°F (104°C) and a heat deflection temperature of just 201°F (94°C). By adding an organic halogenated compound or other flame retarding compound, these values actually drop significantly, even though the material is much less likely to catch fire.
Teflon “Makes Things Better”
So what are some of these high-temperature polymers? Let’s start with polytetrafluoroethylene (PTFE), better known by its trade name, Teflon. Discovered by accident at DuPont in 1938, PTFE can withstand continuous service temperatures of 500°F (280°C). And despite what was said just now about flame retardancy, PTFE also boasts an admirable V-0 rating. It’s hydrophobic (repels water), has one of the lowest coefficients of friction possible (makes it extremely slippery). Plus, it’s highly resistant to most acids, solvents, and other corrosive chemicals.
PTFE is often used as a coating for carpets and clothing, but due to its great strength and impact resistance, is also an excellent choice for mechanical components such as bearing blocks and housings. Supporting this is the fact that PTFE is very easy to machine and dimensionally stable, too. Because it does not flow when heated above its 620°F (327°C) melting point, however, it cannot be plastic injection-molded, nor is it 3D-printable.
One heat-resistant thermoplastic that’s both machinable and injection moldable is polyetheretherketone, or PEEK. With a melting point close to that of PTFE, PEEK retains its mechanical properties—which are quite excellent, by the way—at temperatures of 482°F (250°C) or higher. It's also resistant to radiation, chemical attack, and hydrolysis. That last characteristic means PEEK can be sterilized in an autoclave, making it a favorite of the medical industry for use in spinal implants and fixation devices. These same properties make it suitable as a food-grade polymer.
PEEK is a dielectric as well, so it’s commonly used as an insulator in semiconductor applications. It is not "slippery" like PTFE but is very wear-resistant and sees extensive service in automotive seals, wear rings, and bearing surfaces. And thanks to its high strength-to-weight ratio and other physical attributes, PEEK often stands in as a replacement for metal alloys in various aircraft components (it’s 70% lighter than steel and roughly half the weight of aluminum). Like PTFE, PEEK is truly a wonder material…
The Rest of the High-temp Hit Parade
So is polyphenylene sulfide (PPS). Though not on par with PEEK and PTFE in terms of thermal capabilities, it still offers a respectable working temperature of 428°F (220°C). Known to automotive and electrical engineers as Ryton, this thermoplastic provides a good combination of corrosion resistance, mechanical strength, and dielectric properties. It also flows quite well in plastic injection molding operations and exhibits minimal shrinkage, both of which make it a good candidate for precision electrical connectors and similar components.
PPS is not a candidate for machined parts, but PPSU is. Polyphenylsulfone (aka Radel) has a working temperature quite close to PPS, enjoys similar mechanical and electrical characteristics, can be sterilized, and is quite machinable. It's used in aircraft window bezels, surgical instrument handles, hot water fittings, and because it’s FDA compliant (as are the other polymers listed thus far), is suitable for direct contact with food.
Similarly, there’s polyetherimide (PEI), known as Ultem. PEI is both machinable and injection-moldable, and is available in a range of glass fill (GF) levels. With a maximum continuous operating temperature of 340°F (171°C), Ultem is not quite cookie-baking compatible, but it is an excellent, all-around polymer for applications requiring strength, rigidity, solvent and flame resistance, and dielectric properties.
What About Heat-resistant 3D Printing Materials?
Other notable high-temp polymers include Vectra, a type of injection-moldable liquid crystalline polymer (LCP) commonly used in the SMT (surface mount technology) industry. It offers excellent flow characteristics, can produce parts with very thin walls, and has an operating range of up to 464°F (240°C). There’s also PC/PBT, a blend of polycarbonate and polybutylene terephthalate able to withstand temperatures up to 266°F (130°C)—nowhere near its counterparts described so far, but still offering a nice balance of toughness and weather resistance, especially where cold temperatures are a concern (as in -40°F, which is also -40°C).
You might be thinking, “But what about 3D-printed parts? What are the options for high-temp materials here?” You’re in luck. Chief among these is a ceramic-like advanced high-temp (PerFORM) stereolithography resin, able to handle temperatures up to 514°F (268°C) after an optional post-curing process. This gives designers the opportunity to prototype strong, stiff parts for use in applications such as wind tunnel testing, rapid tooling, electronic housings, and more. Similarly, PC-like advanced high-temp (Accura 5530) is a translucent material that combines optical clarity with good heat resistance. And like the polycarbonates used for machined and plastic injection-molded components, Accura 5530 withstands water, chemicals, fire, and electrical effects.
Because each of the engineering-grade polymers discussed here is both strong and stable, there’s little to worry about in terms of design for manufacturability. Some are more abrasive than others and require the machinist to use carbide drills and end mills, while those with very high melting temperatures might require some adjustments to the plastic injection molding process. Being that all are listed as standard Protolabs materials, though, any concerns will be addressed during the online quoting process.
We encourage you to check the extensive listing of material data sheets available on our website for additional details. With more than 140 polymers and 30 types of elastomer or liquid silicone rubber (LSR) available—some of which can withstand fairly high temperatures—there's sure to be the perfect material for your next project. If you have any questions, drop us a line. Our applications engineers are always available to help at 877-479-3680 or [email protected].