When an application demands both structural strength and reliable performance at high temperatures, few plastics deliver like Ultem. Whether it’s an aircraft interior panel that has to meet strict flammability standards or a semiconductor component that needs to hold its shape under heat, Ultem machining consistently comes up as the answer for engineers who need more than a standard plastic can offer.
At Reading Advanced Machining, we’ve machined Ultem for aerospace, medical, defense, and industrial customers for years, and it remains one of the more misunderstood materials on the shop floor. It’s not just “a strong plastic” — its specific combination of properties is what makes it valuable, and understanding those properties is the first step toward specifying it correctly.
Ultem is the trade name for polyetherimide (PEI), an amber-to-transparent amorphous thermoplastic originally developed by GE Plastics (now part of SABIC) in the early 1980s. Unlike crystalline plastics, amorphous materials like Ultem don’t have a sharply defined melting point, which gives them more predictable dimensional behavior as they heat up — a useful trait in precision machined parts.
Ultem machining comes up frequently because the material checks a lot of boxes at once: high strength, high heat resistance, excellent electrical insulation, and flame resistance, all in a plastic that can be machined to tight tolerances. That combination is difficult to find elsewhere, which is why Ultem shows up in aircraft interiors, missile and defense components, electrical insulators, and semiconductor equipment.
Ultem’s strength comes from its rigid molecular backbone, which gives it high tensile strength and stiffness compared to many other thermoplastics. It also holds up well under constant mechanical stress, resisting the kind of long-term deformation (creep) that can gradually compromise a part’s dimensions over time.
On the heat side, Ultem can perform continuously at temperatures up to roughly 340°F, and it carries a strong flammability rating with low smoke generation, which is a significant reason it’s specified in aircraft interior components where fire safety standards are strict. It’s also resistant to hydrolysis, meaning it doesn’t break down easily when exposed to repeated steam sterilization, making certain Ultem grades a fit for reusable medical devices that need to withstand autoclave cycles.
Ultem also has some of the highest dielectric strength of any thermoplastic, outperforming materials like nylon and Delrin in electrical insulation applications. That’s part of why it’s a common choice for insulating components in high-voltage or high-frequency electronics.
Not all Ultem is the same. Ultem 1000 is the unfilled, “neat” grade of the resin. It’s well suited to aircraft interior components thanks to its flame resistance and low smoke output, and it’s also FDA, USDA, and NSF-compliant, making it usable in structural food processing equipment.
Ultem 2300 is a 30% glass-reinforced grade, which adds tensile strength, rigidity, and dimensional stability compared to the unfilled version. Because it has a coefficient of thermal expansion similar to 6061-T6 aluminum, Ultem 2300 is often specified as a lightweight, non-conductive substitute for aluminum in aerospace and defense structural components. It also holds up well to repeated autoclave sterilization, which makes it a common choice for reusable medical instrumentation.
Choosing between these grades comes down to the application: if flame resistance and FDA compliance matter most, Ultem 1000 is usually the better fit. If the part needs added stiffness or has to behave like a lightweight metal replacement, Ultem 2300 is typically the stronger choice.
Ultem machining isn’t quite as straightforward as machining Delrin or nylon, and a shop without direct experience can run into avoidable problems. As this detailed Ultem machining guide explains, Ultem should be properly annealed before machining to relieve internal stress from the manufacturing process, which reduces the risk of surface cracking or warping down the line.
Glass-reinforced grades like Ultem 2300 are more notch-sensitive than unfilled Ultem, so drilling operations in particular benefit from non-aromatic, water-soluble coolants rather than standard petroleum-based metalworking fluids, which can cause stress cracking in the material over time — sometimes not showing up until well after the part has shipped. Unfilled Ultem can typically be machined with high-speed steel tooling, while glass-filled grades generally require harder tooling to avoid excessive wear.
Ultem machining makes the most sense when an application needs a rare combination of heat resistance, strength, and electrical insulation in a single material — particularly in aerospace, defense, medical, and semiconductor applications where failure isn’t an option. If your part just needs general strength and machinability without the heat or electrical requirements, a more cost-effective material like Delrin may be the better fit.
At RAM, our CNC plastic machining capabilities include both unfilled and glass-reinforced Ultem, alongside PEEK, Vespel, and other advanced engineering plastics. We also stock a wide range of specialty plastics, so if you’re not sure which grade or material fits your project, our team can walk through the tradeoffs with you before you commit to a spec.
Specifying the wrong grade of Ultem, or machining it without the right process controls, can lead to costly rework down the line. If you’re evaluating Ultem for an upcoming project, request a quote and our team will help you determine whether it’s the right fit — and how to get it machined correctly the first time.
