Powder Consumed by a Build

Mass equals density times volume, and everything interesting about this page is in which density and which volume.

The density is the apparent, or bulk, density — powder as it pours, with all the air between the particles still in it — and it is roughly half the density of the solid metal. A 316L powder that pours at 4.2 g/cm³ becomes a part at 7.9. Using the solid density to budget powder nearly doubles the answer; using the tapped density overstates it too, because a recoater spreading a fresh layer does not tap it. Measuring apparent density yourself takes fifteen minutes and is worth doing on every lot: pour freely through a funnel into a cup of known volume, strike the top level without pressing, and weigh. That is the Hall or Carney flowmeter procedure in essence.

The volume is the whole envelope the build sweeps — plate area times build height — and not the volume of the parts. This is the point people find hardest to accept about a powder-bed machine, and it is not negotiable: the machine fills the box. If the parts occupy five per cent of the envelope, ninety-five per cent of the powder in the machine is there to hold up the recoater and support the geometry, and it has to be present, bought and in the room.

None of that surplus is destroyed. It is sieved and returned, and a well-run shop recovers the great majority of it. But recovered powder is not the same powder. Every pass through the machine changes it: fines are lost to the filter, spatter and satellited particles are picked up, oxygen and moisture creep in, and the particle size distribution drifts coarser. The flow changes, the packing changes, and eventually the qualified parameters stop applying. Reactive alloys — titanium above all, aluminium close behind — are the strictest, and titanium powder handling carries genuine fire and health hazards on top. Every serious shop runs a defined blend ratio of virgin to recovered powder and tests oxygen content on the blend, and the real consumption figure for a job is the virgin top-up, not the envelope mass this page returns. The envelope mass is what has to be in the machine; the top-up is what leaves the store.

One more practical margin. The dose factor is above 1 on every machine, typically 1.5 to 2.5, because the dispenser must deliver more powder than the layer needs so the recoater always runs against a full wave rather than running dry halfway across the plate. The surplus goes to the overflow bin and is recovered — but it passes through the machine first, and a build that empties the dispenser at layer nine hundred is a scrapped build and a scrapped plate of parts.