Polyetherimide is amorphous, so it shrinks little and evenly and holds a dimension well. It is chosen for parts that see heat, flame or repeated sterilisation. The risk in an Ultem part is rarely the tolerance — it is a sharp internal corner, a stress locked in by the process, or a solvent the part meets later.
With no crystallisation to vary, Ultem’s shrink is low and nearly the same in every direction, so a dimension moves less with gate position and packing than it would in PEEK or acetal. That makes it one of the easier resins to hold a tight molded dimension in — which is exactly why its failures tend to show up somewhere other than the drawing’s tolerance block.
Ultem is strong and stiff, and it concentrates stress at a sharp internal corner far more than its datasheet strength suggests. A crack that starts at a corner does not care how good the bulk properties are. Give every internal corner a radius the tool can cut, and treat a sharp corner on an Ultem drawing as a question to ask before the steel is cut, not a feature to reproduce.
Amorphous resins lock in the stress of filling and cooling, and it stays there until something releases it — a solvent, a cleaning agent, a thermal cycle. The part passes inspection and cracks later. A hot, well-controlled mold and a gate placed so the flow does not have to fight its way around the part are what keep that stress low, and for parts that will meet chemicals it is worth agreeing how residual stress will be checked.
Ultem absorbs moisture from the air, and wet resin at melt temperature leaves splay and bubbles and loses strength. Drying is not optional and not a guess — it is a time and a temperature, checked, before every run. At micro shot sizes the material also sits in the machine for many cycles before it is used, which is one more reason the resin has to go in right.
The shop’s molded-part figure is ±10 µm. On a 2 mm feature, this is how much of that band each grade’s own shrink range takes before the machine, the tool or the gauge have done anything — the same arithmetic the shrinkage calculator runs, on the Lab’s starting-point table for the family.
| Grade | Direction | Shrink range | Tolerance taken | Reading |
|---|---|---|---|---|
| Ultem (PEI) | Any direction | 0.5–0.7% | 20% | Room to work |
A family range is wider than a stable process window on one grade, so these are the worst-case budget, not a prediction. Replace the range with your grade’s datasheet figure in the calculator and the answer tightens.
Run your own dimension in the shrinkage calculator.
Flame-rated · autoclavable
Polyetherimide offers a rare combination of continuous-use temperature, inherent flame retardance without additives, and autoclave compatibility — making it the standard choice for reusable surgical instrument handles and housings.
These are standards the material grade is tested or supplied to by its manufacturer. They describe the resin or the bar stock, not a finished part — qualifying a part for patient contact is the device owner’s decision, made under their own quality system.
Have a part in Ultem (PEI)? Send the drawing with the grade and the dimensions that matter, or talk it through with Marve first.