The flame retardancy of plastics is in itself a tricky matter; after all, they are organic materials. The requirements vary from application to application, but it is most difficult for railway components. The relevant fire protection standard EN 45545 describes different risk classes. The highest class must be observed for trains that travel underground. Their passengers cannot simply get out and move away from the danger zone. In an emergency, the fire must not spread quickly. Passengers must have time to get past the train to safety in the tunnel. In addition to flames and heat, flue gas plays an important role here. The smoke must not have an immediate toxic effect in these cramped conditions in the train and tunnel. The availability of such safe plastics is already low.
If these components are then to be manufactured using 3D printing, the only option so far has been to use expensive high-performance polymers in filament printing. Materials such as PEI, PEEK or PEKK are inherently flame-retardant due to their chemical structure and also comply with the limit values for smoke gas toxicity. This was even more difficult to implement if laser sintering is the preferred 3D printing technology. Very expensive and rare high-temperature machines had to be used. The high temperatures of up to 390°C in the printers put a strain on the powder, which resulted in high refresh rates and a lot of powder consumption. It should also be borne in mind that high-performance materials from the top of the plastic pyramid were made to resist - neither chemical nor mechanical load should break them down to dust. This makes the production of fine powders for laser sintering very difficult. “It is therefore not surprising that the price of such powders suggests a life-extending drug rather than a material that offers the user a competitive advantage in difficult markets,” says Dr. Marcus Rechberger, plastic powder expert at LEHVOSS. Testing laboratories commissioned by individual players in railway construction certified that these materials fully met the EN 45545 criteria, but these results never reached the supply chain as certificates. Their use made economically no sense.
What this industry therefore needs are inexpensive but extremely flame-retardant plastic powders with low smoke toxicity that can be processed safely and in high quality on available laser sintering machines. SIEMENS Mobility has developed the basics for this, which were transferred to a new laser sintering material in close cooperation with the LEHVOSS Group: LUVOSINT PPS 9268 BK.
LUVOSINT PPS 9268 BK is based on a Polyphenylene sulfide (PPS). Although PPS is inherently flame-resistant, it does not inherently meet the requirements of the highest hazard classes of EN 45545. This was achieved by additionally modification of the polymer. In laser sintering, the PPS powder offers a comfortable process window at part bed temperatures of around 250°C. The inherently high flowability of PPS supports the pressure-free sintering of the powder into dense components during laser sintering. These show typical PPS mechanics with high rigidity and strength. Components do not absorb moisture and are therefore dimensionally stable. PPS is also chemically and thermally very stable, and as a consequence used part bed powders can be completely recycled. The material is colored black and can be processed with CO2 lasers as well as fiber lasers. The latter bring speed without loss of precision and thus high productivity for industrial production. All factors together enable component costs that were previously unthinkable for EN 45545-compliant components using laser sintering. A declaration of conformity from SIEMENS Mobility shows that printed components from LUVOSINT PPS 9268 BK meet the requirement sets R1HL3, R7HL3 and R17HL3 according to DIN EN 45545-2. Nevertheless, the price level of the material corresponds to that of conventional polyamide powder for laser sintering.
The use of the LUVOSINT PPS 9268 BK is not just limited to railway applications. The material also meets the flame retardancy requirements placed on components for use in aircraft and buses. Aside from flame retardant applications, an HDT A of 196°C enables use in high temperatures. If chemical resistance is part of the requirement profile, PPS is particularly suitable. Up to 200°C there is no known solvent that could attack PPS. Printed components made from LUVOSINT PPS 9268 BK even exceed the chemical resistance of those made from PEEK and PEKK.
