Polyetherimide (PEI; branded as Ultem^[1]) is an amorphous, amber-to-transparent thermoplastic with characteristics similar to the related plastic PEEK. When comparing PEI to PEEK, the former is cheaper but has lower impact strength and a tighter temperature range.^[2]

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Quick Facts Names, Identifiers ...

Polyetherimide

Names
Other names PEI, Ultem
Identifiers
CAS Number	61128-46-9 Y
ChemSpider	none
ECHA InfoCard	100.126.800
CompTox Dashboard (EPA)	DTXSID90976644
Properties
Chemical formula	(C37H24O6N2)n
Molar mass	Variable
Appearance	Amber-to-transparent solid
Density	1.27 g/cm3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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Due to its adhesive properties and chemical stability it became a popular bed material for FFF 3D printers.

The molecular formula of the PEI repeating unit is C₃₇H₂₄O₆N₂ and the molecular weight is 592.61 g/mol.^[3] It contains phthalimide and bisphenol A sub-units.

The glass transition temperature of PEI is 217 °C (422 °F). Its amorphous density at 25 °C is 1.27 g/cm³(.046 lb/in³). It is prone to stress cracking in chlorinated solvents. Polyetherimide is able to resist high temperatures while maintaining stable electrical properties over a wide range of frequencies. This high strength material offers excellent chemical resistance and ductile properties suitable for various applications, even those involving steam exposure.^[4]