Machining PEI

PEI (polyetherimide) is an amorphous high-performance thermoplastic that is inherently flame retardant (UL 94 V0) without any flame-retardant additives, while producing very little smoke. Heat deflection temperature is 200 °C, and continuous use is possible up to about 170 °C, briefly up to 210 °C. In its base form PEI is amber-coloured and transparent. Because of its amorphous (non semi-crystalline) structure, the material shrinks and warps very evenly during machining – an advantage over semi-crystalline high-performance plastics such as PEEK when tight tolerances are required.

PEI machines similarly well to polycarbonate – turning, milling and drilling are unproblematic, and sharp tools with moderate feed rates avoid edge chipping. Because the material is amorphous, machining generates hardly any internal stresses of the kind that can arise in semi-crystalline materials through uneven cooling – this favours dimensionally accurate parts even without extensive annealing. Before bonding, or for very tight tolerances, we still recommend brief pre-drying, since PEI, like most polyetherimides, is somewhat hygroscopic.

PEI is a good fit when several of these requirements come together: continuous use at elevated temperatures up to around 170 °C, high requirements for flame retardancy and low smoke density (e.g. aircraft cabin interiors), very good electrical insulation for electronics and the electrical industry, use in semiconductor manufacturing (wafer handling, front-end and back-end processes, high- and low-temperature applications), and parts that need to stay dimensionally accurate and easy to machine despite high stiffness.

PEI is priced clearly below high-performance plastics such as PEEK, but above classic engineering plastics such as PC or POM – placing it in the mid-to-upper price segment of technical plastics. The part price is mainly determined by stock form and dimension, material utilisation (waste), machining effort and batch size. We are happy to quote binding prices on request based on your drawing.

PEI is not always the most economical or technically suitable solution: at moderate temperatures and loads, cheaper engineering plastics such as PC, PA or POM are often sufficient. Notched impact strength, at 4 kJ/m², is comparatively low – under impact loading or with sharp notches, PEI is more sensitive than PC, for example. For permanent outdoor weathering without UV protection, the amber base colour can darken. Where continuous use above 170 °C is required, semi-crystalline high-performance plastics such as PEEK or PPS are often the more resilient choice.

An honest assessment: Advantages: inherently flame retardant (UL 94 V0) without additives, very low smoke development in case of fire, high stiffness even at elevated temperatures, very good electrical insulation, good resistance against high-energy radiation, amorphous and therefore dimensionally accurate to machine. Disadvantages: comparatively low notched impact strength, more limited continuous service temperature than PEEK, amber base colour not UV-stable outdoors, higher material price than standard engineering plastics.