What Is Ultem (PEI)? Properties, Processing, Applications, and Engineering Limits

When engineers ask “what is Ultem?” they mean Ultem® plastic, which is the trade name for polyetherimide (PEI). This is a high-performance amorphous thermoplastic made for tough thermal, mechanical, and regulatory situations. Ultem is a unique engineering plastic because it has high heat resistance, built-in flame resistance, great electrical properties, and long-term dimensional stability without needing fiber reinforcement.

Ultem is used in engineering when standard plastics like ABS, PC, or Nylon can’t handle the stress anymore, but metals would make things heavier, more complicated, or more expensive. So, PEI is often used in the interiors of airplanes, electrical parts, medical devices, and structural parts that need to stay strong and stiff even when they are under a lot of heat.

Ultem’s amorphous molecular structure makes it behave in a predictable way, with tight tolerances and great repeatability in injection molding and CNC machining. This is different from semi-crystalline super polymers. Its high glass transition temperature (about 217 °C) and natural ability to meet flame, smoke, and toxicity (FST) standards make it very appealing to regulated industries. This article talks about the material properties, processing issues, and engineering limits that help you decide when PEI is the best choice.

What is Ultem? What is polyetherimide?

Ultem vs. PEI: Name Brand vs. Generic Material

SABIC owns the registered trade name Ultem®, and PEI (Polyetherimide) is the name of the generic polymer family. This difference is very important for making sure that specifications are correct in engineering and procurement.

• Design and Drawings: To make sure that the material definition is technical and not commercial, include “PEI” and the appropriate ASTM standards or grades (for example, “PEI, Unfilled” or “PEI, 30% Glass Filled”).

• Procurement: When a specific SABIC formulation (like Ultem 1000) is needed for certified supply chains, you should ask for “Ultem®.” Please keep in mind that other suppliers may have PEI grades that work the same way.

• Engineering Communication: Use “PEI” to talk about how materials behave in general and “Ultem” to talk about specific commercial grades.

Basic Classification of Chemicals and Polymers

PEI is a thermoplastic that does not crystallize when it cools down. PEI’s amorphous structure makes it shrink in a predictable way, keeps its dimensions tight, and has isotropic properties. These are all important for precise CNC machining and injection molding. PEI is a type of high-Tg engineering plastic that has rigid aromatic backbones and imide linkages that make it more stable at high temperatures.

Where PEI fits in with other high-performance plastics:

• High Glass Transition Temperature (Tg ≈ 217 °C): Stays strong and stiff even when it’s hot.

• Inherent Flame Retardancy: Meets strict UL94 and FST standards without any extra chemicals.

• Great electrical properties: keeps its dielectric behavior stable over a wide range of temperatures.

A Quick Look at High-Performance Plastics (From an Engineering Point of View)

Engineering Takeaway: Ultem® is the brand name, and PEI is the material. PEI is an amorphous, high-Tg thermoplastic that is chosen for its accuracy, ability to resist heat, and ability to meet regulatory requirements, as long as its chemical resistance limits are appropriate for the job.

Molecular Structure and How Ultem Works at High Temperatures

Ultem’s ability to work at high temperatures comes from the way its molecules are arranged, not from fillers or post-processing. When you understand this structure-property relationship, you can see why PEI stays stiff while many other engineering plastics get softer or creep.

Aromatic Rings and Ether/Imide Groups in Backbone Chemistry

At the molecular level, PEI has a stiff aromatic backbone with ether linkages and imide groups:

• Aromatic (Benzene) Rings: Make the chain naturally stiff. Their flat, stiff shape limits how much they can rotate, which raises the modulus and makes them less likely to soften when heated.

• Imide Groups (–CO–NH–CO–): These are thermally stable, polar structures that make intermolecular interactions stronger, which makes it harder for chains to move around at higher temperatures.

• Ether Linkages: Allow for some flexibility in processing the polymer without losing thermal performance, which is what makes thermoplastics melt-processable.

Engineering Impact: The combined effect is a polymer that doesn’t move around when it’s hot, so it stays strong while it’s being used all the time.

Behavior of glass transition vs. melting

Ultem is an amorphous thermoplastic, which means it doesn’t have a real melting point. Instead, it is defined by its Glass Transition Temperature (Tg):

• Tg ≈ 217 °C: This is the temperature at which PEI changes from a hard, glassy state to a rubbery state.

• Below Tg: PEI stays stiff and keeps its shape.

• Above Tg: The modulus drops quickly, but the material doesn’t “melt” like semi-crystalline plastics do.

Engineering Comparison: Tg vs. Melting Point

Engineering Lesson: Tg, not a melting temperature, sets the upper and lower limits for Ultem service temperatures. Designs must make sure that operating loads and tolerances stay within acceptable limits as temperatures get close to, but do not go over, this transition.

Ultem’s Core Material Properties (Engineering View)

Ultem (PEI) is a material with a balanced set of properties that make it good for high-temperature, regulated, and electrically demanding uses. PEI has built-in thermal and flame resistance and predictable mechanical behavior.

Properties of Mechanics

Ultem is very stiff and strong, especially at high temperatures:

• Tensile Strength: ~100–110 MPa (grades that aren’t filled).

• Elastic Modulus: ~3.0–3.4 GPa, which means it is very stiff.

• Creep Resistance: Much better than regular engineering plastics when they are heated.

Engineering Meaning: PEI isn’t a material that can take a lot of stress, but it’s great for parts that need to hold their shape at temperatures between 150 and 170 °C. It is good for housings and structural parts where controlling deflection is more important than ductility.

Thermal Characteristics

Ultem’s thermal performance is one of the things that sets it apart:

• Temperature for Continuous Use: about 170 °C (depending on grade and stress).

• Tg: The temperature at which glass changes from a solid to a liquid is about 217 °C.

• HDT: The Heat Deflection Temperature is about 200 °C when the test loads are normal.

Engineering Meaning: Ultem is a type of plastic that falls between regular engineering plastics and ultra-high-performance polymers. Designers can count on long-term thermal stability without the processing problems that come with PEEK, as long as the temperature stays below Tg.

Performance in terms of electricity and fire

Ultem is a standard in the aerospace and electrical industries because:

• Dielectric Strength: Strong and stable over a wide range of temperatures.

• UL94 Rating: V-0 without any additives that stop flames from spreading.

• FST Performance: Low levels of flame spread, smoke production, and toxicity.

Engineering Meaning: The polymer backbone has flame performance built in, so it will stay compliant for a long time without any additives that could move or break down over time.

Resistance to chemicals and the environment

• Good resistance to oils, greases, hydrocarbons, and many cleaning products.

• Weak acids and bases at room temperature are okay.

• Limitations: Very sensitive to strong bases and solvents that are only partially halogenated.

• Low to moderate moisture absorption (about 0.25–0.5%), which makes it more stable in size than nylon.

Table of Typical Engineering Properties (Unfilled Ultem / PEI)

Ultem Grades and Variants: What Changes in Actual Use

Ultem (PEI) is not a single piece of material. Different grades change stiffness, how well they hold heat, and how easy they are to make.

Common Business Grades

• Ultem 1000 (Unfilled PEI): The base grade that has the best balance of toughness, heat resistance, and ease of machining. It works great for electrical housings and parts that have been machined to a high level of accuracy.

• Ultem 2300 (30% Glass-Filled PEI): This material is stronger and stiffer because it has been reinforced. It is less likely to bend, but it is also less strong and more brittle.

• Ultem 9085 / 1010 (Aerospace & Additive Manufacturing): Made just for FST compliance and FDM 3D printing. Ultem 9085 is strong and easy to work with, while 1010 is better at withstanding heat.

How Fillers Affect Performance

Adding glass fiber changes how PEI acts in a predictable way:

• Strength and stiffness go up a lot.

• Creep Resistance: Gets better, especially when temperatures are high.

• Impact Toughness: Goes down; parts are more likely to break at the edges.

• Machinability: Tools are worn down by reinforced grades.

Comparison of Ultem Grades (Engineering Trends)

Takeaway for engineers: grades that are not filled favor accuracy and toughness, while grades that are filled with glass favor stiffness and resistance to creep.

Ultem Plastic Processing and Manufacturing

Ultem is a thermoplastic that works very well, but it has a narrow processing window. Good thermal control and stress management are key to success.

Things to Think About When Injection Molding

• PEI needs high temperatures for melting (340–400 °C) and molding (170–200 °C).

• Risk: Low temperatures in the mold can cause stress to freeze in, brittleness, and stress cracking.

• Best Practice: Make sure the walls are all the same thickness and there is enough packing to keep them from sinking and warping.

Making Ultem Parts

Ultem machines well compared to other high-heat plastics, making it good for parts that need to be made in small quantities and with high precision.

• Tooling: To keep heat from building up, use sharp carbide tools.

• Tolerances: Unfilled PEI has very tight tolerances, while glass-filled grades are rougher.

• Risk: Clamping too tightly can cause distortion. It is best to anneal after removing a lot of material.

Extrusion and Thermoforming

• Extrusion: makes stock shapes (like rods and plates) that can be machined.

• Thermoforming: Needs even heating close to Tg. Rapid heating causes flaws on the surface, and uneven cooling makes the material spring back.

Ultem for 3D Printing (FDM)

• System: Needs FDM printers that can work at high temperatures and have heated chambers.

• Materials: Ultem 9085 and 1010 are the materials that are used to make functional aerospace prototypes in the industry.

• Risk: Anisotropy makes it hard to use in main load paths.

Processing Method × Risk Points

Common Uses of Ultem in Different Fields

Ultem is used when regular plastics don’t work and metals are too heavy or conductive.

Transportation and Aerospace

• Common parts: structural brackets, interior panels, clips, ducts, and electrical housings.

• Why: It meets strict Flame, Smoke, and Toxicity (FST) rules without any extra chemicals. Dimensional stability makes sure that things fit near engines or avionics.

Electronics and semiconductors

• Typical parts: connectors, coil formers, sockets, and fixtures that can handle high temperatures.

• Why: The dielectric strength stays the same and there is little creep at high temperatures. Works with cleanroom standards.

Health and Medical

• Common Parts: Handles for surgical instruments, trays, and housings that can be sterilized.

• Why: It can be sterilized over and over again (with steam or an autoclave) without changing shape. Some grades are biocompatible (ISO 10993).

Energy and Industry

• Common parts: insulators, sensor housings, and chemical processing parts.

• Why: It can withstand heat and oils and fuels. Gets rid of the risk of metals corroding.

Choosing between Ultem and PEEK as an engineering material

Ultem (PEI) and PEEK are both high-end thermoplastics, but they are used for different engineering purposes because of how their polymers are made.

Comparing Performance: What Really Changes

• Ultem (Amorphous): Defined by Tg (about 217 °C). Keeps its stiffness up to Tg, but gets soft quickly after that. Better for tight tolerances and accuracy.

• PEEK (Semi-Crystalline): Tm (~343 °C) is what makes it what it is. Crystalline areas are better at resisting chemicals, lasting longer, and not wearing out, especially at temperatures above 200 °C.

Ultem vs. PEEK: An Engineering Decision Table

When Ultem Is the Best Option

When Ultem is the right engineering choice:

• Flame retardancy (FST) is required.

• It is very important to have good electrical insulation and precise dimensions.

• Cost control is necessary (less expensive than PEEK).

• There aren’t any extreme chemicals or dynamic fatigue in the environment.

Engineering Tip: Use PEEK in places where chemicals and fatigue are very strong. Ultem is a good choice for high heat, flame safety, and accuracy without the high cost of PEEK.

Ultem’s engineering limits and design factors

Ultem works very well, but it has clear limits. If you don’t pay attention to these, you’ll fail too soon.

Long-Term and Mechanical Limits

Ultem, like all thermoplastics, will creep when it is under a constant load at high temperatures.

• Limit: Not as good at resisting creep as PEEK or metals.

• Design: Don’t put too much stress on things near their thermal limits all the time. Use glass-filled grades for structural loads, but be aware that they will be more brittle.

Limits on heat and the environment

Tg (~217 °C) controls performance.

• Limit: Running above about 170 °C for a long time lowers safety margins for mechanical parts.

• Design: When parts are constrained, thermal cycling can cause stress cracking. Make room for thermal expansion.

• Chemical: Don’t use strong bases or solvents that are only partially halogenated.

Joining and Post-Processing

• Bonding: Moderate surface energy makes it hard to bond with adhesive; the surface needs to be prepared first.

• Fastening: Mechanical fastening is better, but it needs torque control to stop stress cracking.

When should you choose Ultem?

If you need something that can handle high heat, follow the rules, and be exact in size, choose Ultem (PEI). It has a high glass transition temperature (about 217 °C) and a built-in V-0 flame rating, which makes it great for aerospace, medical, and electronics.

Ultem is better than semi-crystalline plastics for making complex, lightweight parts that need tight tolerances. However, don’t use Ultem in places where there are a lot of strong bases or where long-term fatigue resistance is very important. In those cases, PEEK or PPS are better.

One-sentence engineering logic: Use Ultem when you need a plastic that can handle high temperatures, resist flames, and insulate electricity well. Only switch to PEEK when you need a plastic that can handle chemicals or dynamic loads.

FAQ

What do people use Ultem plastic for?
Ultem (PEI) is used in situations where high heat resistance, flame retardancy, and stability are needed. Some common uses are for the insides of airplanes, electrical connectors, medical device housings, and semiconductor fixtures.

What is the temperature at which Ultem melts?
Ultem is amorphous, which means it doesn’t really have a melting point. At its Glass Transition Temperature (Tg), which is about 217 °C, it starts to soften. Engineers should not use “melting point” to define thermal limits; they should use Tg.

Is Ultem stronger than PEEK?
No, usually not. PEEK is stronger and better at resisting creep, especially at very high temperatures. We chose Ultem because it has the right mix of flame resistance, electrical properties, and low cost.

Can you CNC machine Ultem?
Yes. Carbide tools work well with Ultem machines. It holds its shape and tolerances better than many semi-crystalline plastics, making it great for making precise prototypes.

Why does Ultem cost more than regular plastics?
The price is because of the complicated polymer chemistry and the need for high-temperature processing. The price is fair if the performance means that metal parts are not needed or if it meets strict safety standards that cheaper plastics can’t.