CNC Machining Ultem Services: How to Pick a Supplier
This guide is for engineers and buyers sourcing machined PEI parts. It lists the checks that separate a supplier who has run Ultem before from one who will learn on your order, and it shows the material and process limits you should expect in writing.
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What matters most when buying Ultem parts
Which Ultem grade fits which job
Ranges below are typical for machined stock shapes. Confirm the exact grade and wall section with your supplier before release.
| Grade | Filler | Pick it when | Avoid it when |
|---|---|---|---|
| PEI 1000 | Unfilled | Transparent parts, electrical insulators, tight chemical exposure | You need stiffness above 120 °C continuous |
| PEI 1010 | Unfilled | Machined housings and manifolds needing good impact strength | Dimensional creep under sustained load is a concern |
| PEI 2100 | 10% glass | Connectors, brackets, moderate stiffness at temperature | Optical clarity or a smooth cosmetic surface is required |
| PEI 2300 | 30% glass | Structural parts, load-bearing fixtures, thermal cycling | Thin walls under 1.5 mm or fine internal threads |
| PEI 2400 | 30% carbon | Stiff, low-creep parts, ESD paths in fixtures | Electrical isolation is the design intent |
| PEI AT* | Unfilled | Higher heat resistance near 180 °C continuous | Budget is tight and 170 °C service is enough |
What Ultem does to a cutting tool
Ultem is an amorphous thermoplastic with a glass transition near 217 °C and a continuous service temperature around 170 °C. It machines more like a hard, brittle plastic than like PEEK or POM. Chips come off as powder or as sharp shards, and the edge of the tool does the work. A dull cutter rubs, heats the surface, and leaves a smear that shows up after annealing.
The glass-filled grades are abrasive. PEI 2300 wears carbide quickly, so tool life on a 30% glass job may be a third of what you get on unfilled stock. Suppliers who quote glass-filled PEI with the same tooling plan as unfilled PEI are guessing. Ask what coating and what change interval they use.
Heat is the main enemy. PEI has low thermal conductivity, so heat stays at the cut instead of leaving through the chip. Without air blast or chilled air, the surface can reach its glass transition locally. That is where crazing and small cracks start. Once a crack is in the part, no amount of polishing removes it.
Dimensional stability depends on stress. A block machined from extruded stock carries residual stress from the extrusion process. Remove material from one side and the part bows. Rough machine, anneal, then finish is the standard sequence for a tight-tolerance PEI part. Any supplier skipping the middle step is trading your yield for their cycle time.
- 1Air blast beats flood coolantUnfilled PEI absorbs moisture and can stress-craze near coolant. Use dry cutting with high-pressure air.
- 2High rake, sharp, polished flutesTwo or three flutes on aluminum-style geometry, not a heavy roughing cutter.
- 3Moderate speeds, healthy feedRubbing is worse than cutting. Keep the chip load up so the tool cuts rather than polishes.
What tolerance to ask for on PEI
A ±0.005 mm tolerance on a PEI part is a machining claim, not a material claim. The machine can hold it. The material may not. PEI moves with moisture uptake and with temperature, so a measurement taken at 20 °C and 50% RH is the only one that counts. Put the measuring environment in the drawing notes.
For most Ultem work, ±0.05 mm on metal-to-metal fits and ±0.1 mm on non-critical features is realistic and cheaper. Tighten only the features that touch something else. A drawing with a global ±0.01 mm block tolerance forces the shop to slow every cut and inspect every dimension, and the price reflects that.
Thin walls are the real limit. Below about 1.5 mm, glass-filled PEI becomes hard to hold without chatter and hard to anneal without distortion. If the design needs a 1 mm wall in PEI 2300, expect the supplier to propose a fixture, a slower finishing pass, or a different grade.
Surface finish on PEI is largely a function of tool condition. A fresh cutter with a polished edge gets Ra 0.8–1.6 μm without extra work. Ra 0.2–0.8 μm needs a deliberate finishing pass and a clean setup. A scratched part cannot be buffed back to a fine finish, because buffing smears the surface rather than cutting it.
Drying, annealing and why they are on your quote
PEI absorbs moisture from the air. Machining wet stock traps that moisture, and the heat of cutting turns it into steam inside the material. The result is a bubble or a hairline crack that appears hours after machining. Drying the stock before the first cut costs oven time, and that time is a real line item.
Annealing is the second oven step. After rough machining, the part goes through a controlled heat cycle to relax the stress the cutter introduced. For a part with a tight flatness callout, this is what keeps the face flat. For a part with a thin web, this is what stops the warp.
The schedule depends on section thickness. A 10 mm section and a 50 mm section do not share the same ramp. A supplier who quotes a single fixed anneal cycle for every PEI job is either over-charging the thin parts or under-treating the thick ones. Ask for the ramp rate and the hold time.
You can check whether a shop actually did this. Look at the part after a week. A properly dried and annealed PEI part holds its dimensions. One that skipped the steps drifts, and the drift shows up as a fit problem at assembly, not at incoming inspection.
Five checks before you release a PO
First, ask how the shop will hold the part. A PEI part with a tight flatness callout needs a fixture plan, and the answer should mention soft jaws or a vacuum plate, not just a vise. Second, ask for the tool list. A shop that names a specific cutter geometry and coating for glass-filled PEI has done this before.
Third, ask about the oven. Drying and annealing are the two steps most often left off a quote, and they are the two that decide whether the part survives. Fourth, ask what they inspect and when. A plan that covers incoming stock, in-process checks, and a final dimensional report is the baseline for any regulated industry.
Fifth, ask what happens if a dimension misses. A shop that re-machines a PEI part is usually making it worse, because a second setup adds stress. The right answer is a new part from a corrected process, and the supplier should say so without prompting.
GreatLight runs Ultem on 5-axis and mill-turn centers with ±0.005 mm capability, and the process plan includes drying, roughing, annealing, and finishing as separate steps. The quote covers all four. If your current supplier's quote does not mention the ovens, that is the gap.
- 1Fixture planSoft jaws, vacuum plate, or a dedicated nest. A vise alone will not hold a thin PEI wall.
- 2Tool listSpecific geometry and coating for the grade you named, not a generic aluminum cutter.
- 3Oven scheduleDrying before machining, annealing after roughing, with ramp rates tied to section thickness.
- 4Inspection planIncoming stock, in-process, final report. Ask to see a sample report format.
Lead time, MOQ and what a fair quote looks like
A fair Ultem quote separates material, machining, heat treatment, and inspection. When those four are bundled into one line, you cannot tell whether the price is driven by the PEI stock or by the oven time. Ask for the breakdown. A supplier who has run PEI will have the numbers on hand.
Lead time on a machined PEI part is longer than the equivalent aluminum part, and the reason is the ovens. Drying and annealing add calendar time that cannot be compressed by running the spindle faster. A shop that quotes PEI at aluminum lead time is either skipping the heat steps or planning to ship late.
MOQ should not be a barrier. Ultem stock is available in plate and rod, and a single prototype is a normal order. If a supplier pushes a 50-piece minimum on a PEI bracket, they are protecting their setup cost, not yours. A shop set up for high-mix work will take the one-off.
Confidentiality matters more here than on commodity parts, because Ultem usually shows up in defense, medical, and semiconductor programs. Ask for an NDA before you send drawings. A supplier with a standard NDA process will not treat the request as unusual.
Step by step: from drawing to shipped PEI part
- 1Name the grade and the environmentPut PEI 1000, 1010, 2100, 2300 or 2400 on the drawing, plus the service temperature and the chemical exposure. A generic 'Ultem' callout invites substitution.
- 2Mark only the critical tolerancesKeep the global block tolerance loose (±0.1 mm) and tighten only the features that mate. A drawing full of ±0.01 mm calls raises cost without adding function.
- 3Ask for the process plan with the quoteThe reply should list drying, roughing, annealing, finishing, and inspection as separate steps. If it lists only 'CNC machining', push back.
- 4Confirm the oven schedule by section thicknessRamp rate and hold time should change with the thickest section. A single fixed cycle for a 5 mm and a 40 mm part is a warning sign.
- 5Approve the first article before the run continuesCheck dimensions at 20 °C and controlled humidity, then re-check after 24 hours. Late movement means the anneal was incomplete.
- 6Request the final inspection reportDimensional results plus material certification. For medical and aerospace work, keep the report with the lot.
Questions buyers ask about Ultem machining
Can Ultem be machined to ±0.005 mm?
The machine can hold ±0.005 mm on PEI, and GreatLight quotes that capability. The material is the limiting factor. PEI moves with moisture and temperature, so the tolerance only means something if the measuring environment is fixed.
Write the temperature and humidity into the drawing notes, and expect the supplier to measure after the part has stabilized rather than straight off the machine.
Is coolant allowed when machining PEI?
For unfilled PEI, dry cutting with high-pressure air is safer. Flood coolant can be absorbed at the cut and lead to stress crazing later.
Glass-filled grades generate more heat and some shops use a light mist. Ask what the plan is and why. There is no single answer that fits every geometry.
How long does an anneal take?
It depends on section thickness. Thin sections cycle faster than thick ones, and the ramp rate matters as much as the hold time.
A supplier should give you the ramp, the hold, and the cool-down rate for your part, not a generic number.
What is the minimum order for Ultem parts?
One piece is a normal order. Ultem stock comes in plate and rod, and a prototype bracket or manifold does not need a production run.
If a shop insists on a high minimum, they are set up for long runs, not for the high-mix work PEI usually shows up in.
Why did my PEI part crack after machining?
Most cracks trace back to two causes: moisture in the stock turning to steam at the cut, or residual stress from a heavy roughing pass without a follow-up anneal.
Both are process problems, not material problems. The fix is in the oven schedule, not in a different grade.
Which certifications should the supplier hold?
For general industrial work, ISO 9001:2015 covers the quality system. For medical devices, ISO 13485:2016. For automotive, IATF 16949:2016. For programs handling sensitive drawings, ISO 27001:2022.
GreatLight holds all four. Ask for the certificate scope, not just the certificate.
Send your PEI drawing and get a process plan back
Upload the model and drawing. We reply with a quotation and a DFM review within 12 hours, and the process plan names the drying and annealing steps before you commit.
12-hour quote±0.005 mmNDA on requestNo MOQ
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