A powder bed fusion apparatus including a build chamber, a build platform in the build chamber for supporting a powder bed, a layer formation device for forming layers of powder to form the powder bed, a scanner for scanning an energy beam across the powder bed to fuse the powder and a gas circuit for forming a gas flow across the powder bed. The gas circuit includes a separator for separating particles from gas in the gas circuit, a nozzle for propelling gas into the build chamber and an exhaust for extracting gas from the build chamber and delivering the gas to the separator. The exhaust includes an exhaust channel or opening located for receiving powder wiped from the powder bed and/or build platform.