What Is Ultem? A Practical Guide to PEI Material and CNC Machining

Ultem is the best-known trade name for polyetherimide (PEI), a high-performance amorphous thermoplastic used when ordinary plastics do not provide enough heat resistance, stiffness, flame performance or dimensional stability. In its natural unfilled form, PEI is typically translucent amber. Glass-reinforced and specialty grades are also available for applications that need higher stiffness, lower thermal expansion, wear control, electrical performance, regulatory documentation or other targeted properties.

For engineers and buyers, the important distinction is that “Ultem” is not one single machining grade. ULTEM 1000, ULTEM 2300, healthcare grades, high-flow molding grades and other PEI formulations can behave differently. The drawing, material certificate and purchase specification should therefore identify the required resin or stock-shape grade rather than using “Ultem” as the only material callout. This is especially important when a part must meet flame, food-contact, medical, electrical or aerospace requirements.

SABIC describes the ULTEM resin family as amorphous PEI with high heat resistance, strength and stiffness, broad chemical resistance and a glass-transition temperature around 217°C for many standard grades. The material is widely selected for precision housings, electrical insulators, connectors, fixtures, structural supports and other components that need a combination of low weight, temperature capability and electrical isolation.

Ultem Material Properties That Matter in Real Parts

PEI is attractive because several properties arrive in one material rather than as separate trade-offs. It combines high-temperature capability, stiffness, electrical insulation and useful chemical resistance while remaining machinable as a thermoplastic. That combination is why it appears in aerospace interiors, electrical and electronic hardware, medical-device components, semiconductor equipment, laboratory instruments and industrial machinery.

Ultem 1000 vs. Ultem 2300

ULTEM 1000 is commonly referenced as an unreinforced PEI family, while ULTEM 2300 is a 30% glass-fiber-reinforced PEI grade family. The reinforced material provides substantially higher stiffness and improved dimensional control in many applications, but it also increases tool wear and can change edge quality, surface appearance and the machining strategy. A quotation should therefore separate “unfilled PEI” from “30% glass-filled PEI” rather than treating them as interchangeable.

How Machinable Is Ultem?

Ultem machines well when the process controls heat, cutting force and residual stress. It is not as forgiving as POM or ABS, but it is generally more straightforward to machine than highly wear-resistant imidized plastics. The main risks are localized heating, burr formation, edge chipping, stress movement after heavy stock removal and deformation from excessive clamping pressure.

Because plastics conduct heat poorly, a tool that would be acceptable in aluminum can still create too much localized heat in PEI if it is dull, rubbing, overloaded with chips or run at the wrong combination of spindle speed and feed. The goal is to cut cleanly and evacuate chips—not polish the material with the tool flank. Sharp tools, adequate clearance, controlled engagement and reliable chip evacuation are therefore more important than chasing a single “perfect” RPM.

Practical Machining Strategy

1. Verify the exact PEI grade, stock-shape producer, lot and material certificate before programming.
2. Saw or rough-machine with enough allowance for stress movement; avoid removing a deep pocket from only one face when flatness is critical.
3. Use sharp cutting edges with generous chip space and minimize tool rubbing.
4. Control temperature with chip evacuation, compressed air and—where technically compatible—an approved coolant strategy.
5. For close-tolerance parts, rough-machine, allow the part to stabilize, then finish with light, balanced cuts.
6. Inspect only after the part has returned to a stable metrology temperature and is supported without distortion.

 Tooling for CNC Machining Ultem

Milling

For unfilled PEI, sharp carbide end mills are a robust default, especially for production work. Low-flute-count cutters with generous chip space are useful because they reduce recutting and heat. Polished, non-ferrous-style geometries can improve chip release and surface finish. Climb milling is generally preferred where machine condition and workholding allow it because the chip forms more cleanly and the process tends to generate less rubbing heat.

Turning and Boring

Carbide inserts intended for aluminum or other non-ferrous materials are a practical starting point. A sharp edge, positive cutting action and a modest nose radius help avoid pushing the material away from the tool. Long stringy chips should be removed with air or suction so they cannot wrap around the workpiece or tool.

Drilling and Threads

Drilling creates a concentrated heat load. Sharp drills, peck cycles and frequent chip evacuation are important, particularly when hole depth exceeds roughly two diameters. Pilot-hole size must also be validated because plastic holes can recover slightly after drilling. For internal threads, thread milling or well-controlled tapping cycles can reduce the risk of tearing and inconsistent thread form. Metal inserts may be appropriate where repeated assembly, high clamp load or vibration would otherwise damage a plastic thread.

Glass-Filled PEI

Glass-filled PEI is more abrasive. Carbide is preferred, and PCD/CVD-diamond tooling may be justified for longer production runs, fine finishes or high tool-life requirements. Do not copy unfilled-PEI tool life or cutting conditions directly into a 30% glass-filled program.

Ultem Speeds and Feeds — Use Reference Windows, Not Universal Recipes

Published machining data are useful for building a safe starting process, but they are not a substitute for a shop trial. Mitsubishi Chemical Group publishes machining data for its Duratron U1000 PEI stock shape. Those values are shown below as an engineering reference for unfilled PEI stock—not as a guaranteed setting for every ULTEM resin grade, cutter diameter, machine or part geometry.

To convert cutting speed to spindle speed, use RPM = (1000 × cutting speed in m/min) ÷ (π × tool diameter in mm). That formula is only the conversion step; the acceptable cutting speed still depends on the grade, tool geometry, radial/axial engagement, machine stability and heat-control strategy. A production program should be qualified by actual part measurements, surface condition and tool-wear observations.

Moisture, Drying, and “Post-Curing” — What Actually Applies to Ultem?

Moisture Control

PEI is not as moisture-sensitive as nylon, but it is not completely moisture-free. The source material supplied for this article cites a representative 24-hour water-absorption value around 0.25% for PEI; the exact number varies by grade, test method and conditioning. For precision machining, the practical control is to keep stock identified and protected from unnecessary humidity changes, then allow finished parts to stabilize before final inspection when tight dimensions are involved.

Do not automatically apply an injection-molding pellet-drying schedule to machined plate or rod. Pellets are dried to protect melt processing; a machined stock shape has a different thermal history, section size and residual-stress condition. If stock has been stored in high humidity, must undergo a high-temperature secondary process, or carries very tight dimensional requirements, use the stock-shape manufacturer’s drying or conditioning recommendation.

Post-Curing vs. Stress-Relief Annealing

Ultem/PEI is a thermoplastic, so a machined PEI component does not normally require a chemical “post-cure” in the way a thermoset or some imidized resin systems might. The relevant secondary heat treatment is stress-relief annealing. It is optional and geometry-driven, not a mandatory step for every PEI part.

Mitsubishi Chemical Group lists a reference PEI annealing procedure of a 200°C hold temperature, a heating rate of 10–20°C per hour, a hold time of about 10 minutes per millimeter of wall thickness, and a controlled cooling rate of 5–10°C per hour. The same guide recommends rough machining, annealing and then a light finish cut when extremely close tolerance, flatness or non-symmetrical geometry requires it. This should be treated as a supplier-specific stock-shape procedure. The exact ULTEM grade, finished geometry, oven uniformity, fixturing and stock supplier instructions must be reviewed before using any annealing cycle.

Inspection of CNC-Machined Ultem Parts

A good PEI machining process is not complete until the measurement plan accounts for the behavior of plastic. Parts should be measured after cutting heat has dissipated and after any required stabilization or annealing. Heavy CMM clamping, aggressive fixture pressure or excessive probe force can distort thin walls and create a false pass or false fail.

Recommended Inspection Approach

• Verify material identity, grade, color, reinforcement and lot/heat-equivalent traceability from incoming stock documentation.
• Use first-article inspection for critical dimensions, datums, hole locations, thread requirements and functional interfaces.
• Use CMM, optical measurement, height gauges, pin gauges, thread gauges or custom fixtures according to geometry—not one inspection method for every feature.
• Let finished parts reach the drawing/customer-specified metrology temperature before final dimensional acceptance.
• Support large plates and thin walls in a neutral state; avoid forcing a warped part flat during measurement unless the drawing defines that restraint condition.
• For very tight dimensions, confirm repeatability after a defined stabilization period and document the inspection condition on the report.

There should be no blanket tolerance promise for “Ultem machining.” Achievable tolerance depends on feature size, wall thickness, stock orientation, amount of material removal, reinforcement, thermal condition and inspection method. A supplier should review the drawing and then quote a part-specific capability rather than applying a single ±0.005 mm, ±0.02 mm or ±0.05 mm claim to every PEI component.

Where Ultem Is Commonly Used

Ultem/PEI is chosen when the part needs more than simple shape and dimensional accuracy. Typical applications include electrical connectors and insulators, sensor and instrument housings, aerospace interior hardware, high-temperature brackets, sterilizable equipment components, semiconductor and electronics fixtures, laboratory hardware and lightweight structural components. Its combination of stiffness, electrical insulation, flame performance and temperature capability can reduce the need for metal when electrical isolation or weight reduction is important.

Application approval must still be grade-specific. For example, healthcare-oriented ULTEM grades may carry documentation for biocompatibility or sterilization processes, while aerospace-focused grades may have specific flame/smoke/toxicity evidence. The fact that one ULTEM grade has a certification does not automatically transfer that certification to all PEI stock shapes or colors.

DFM Checklist for Ultem CNC Machining

• Avoid sharp internal corners where stress concentration is unnecessary; use practical radii.
• Avoid very thin unsupported walls beside deep pockets unless function requires them.
• Balance stock removal from opposing faces when flatness matters.
• Use realistic thread engagement and consider inserts for repeated assembly or high clamp load.
• Call out the exact PEI grade, reinforcement and required certification on the drawing or PO.
• Define which dimensions are truly critical and the inspection condition required for acceptance.
• If annealing is required, plan machining allowance and the heat-treatment sequence before the first roughing operation.

FAQ — What Buyers and Engineers Ask About Ultem

Is Ultem the same as PEI?

Ultem is SABIC’s trademarked PEI resin family. PEI is the polymer category; other manufacturers also sell PEI materials under different product names.

Is Ultem easy to machine?

It is generally machinable with conventional CNC equipment, but heat control, sharp tooling, chip evacuation and stress management are more important than with common plastics such as POM or ABS.

What tool material is best for Ultem?

Sharp carbide is a strong default for unfilled PEI. Glass-filled PEI is abrasive and may justify PCD/CVD-diamond tooling for production or demanding surface requirements.

Does Ultem need to be dried before CNC machining?

Not automatically. Machined rod and plate should follow the stock-shape supplier’s conditioning guidance. Do not copy a pellet-drying schedule into a CNC work instruction without confirming it applies to the stock form.

Does Ultem require post-curing after machining?

Normally no. PEI is a thermoplastic. Some close-tolerance parts benefit from stress-relief annealing, but that is a controlled optional process, not a universal post-cure requirement.

What tolerances can be held in Ultem?

There is no responsible single tolerance for every PEI part. Feature size, wall thickness, grade, residual stress, machining sequence and inspection condition determine achievable capability. The drawing should be reviewed before a tolerance is promised.

Can Ultem replace PEEK?

Sometimes. PEI can be a cost-effective choice where its temperature, chemical and mechanical performance are sufficient. PEEK generally offers higher extreme-temperature and chemical capability, so substitution should be based on the actual environment rather than price alone.

Can Ultem be used in medical or aerospace parts?

Yes, selected grades are used in those industries. However, certification, biocompatibility, sterilization or FST claims must be verified for the exact grade and application.

RFQ CTA — Get an Ultem / PEI Machining Review Before You Cut Material

If your PEI part includes tight flatness, thin walls, deep pockets, precision threads, press-fit features, sterilization exposure or high-temperature service, the machining route should be reviewed before material is cut. Send Rollyu Precision your 2D drawing, 3D model, required PEI/ULTEM grade, quantity, critical tolerances, surface requirements and any regulatory documentation requirements. Our engineering team can review stock form, workholding, rough/finish sequence, tooling, annealing risk and inspection planning, then return an RFQ with DFM feedback and a part-specific manufacturing plan.

RFQ information to include: material grade and reinforcement; drawing revision; annual or batch quantity; critical-to-function dimensions; thread and insert requirements; operating temperature; sterilization or chemical exposure; required certificates; inspection-report requirements; packaging and cleanliness expectations.