Buy-to-fly ratio close to 1?
If you are involved in Additive Manufacturing (AM), you have probably heard of the ”Buy-to-Fly ratio” (BTF). The term originates from the aerospace industry and describes how much material is wasted in a manufacturing process. Per definition, the BTF ratio is the weight of the raw material divided by the weight of the final component. For conventional metal manufacturing like milling, the BTF ratio is often higher than 10, which means that less than 10% of the raw material remains in the final part. More than 90% is scrapped! We don’t like scrap, do we?
Many advocates of powder bed AM have embraced the BTF ratio. They claim that the BTF ratio is ”close to one”, based on the fact that AM powder can be used over and over again. No material is wasted! Wonderful!
But is this really true?
First of all, we know that some powder is indeed wasted in every AM build cycle: support structures must be scrapped, spatter particles and loose satellites must be removed by sifting, etc. etc.
Secondly, repeated recycling may cause chemical changes in the powder, for example oxygen pickup. If the powder degradation is fast, you may end up with a considerable powder quantity ”out-of-spec” that must be scrapped.
Finally, and even more important, you cannot assess your true BTF ratio without knowing how much material was wasted when your AM powder was atomized. Most AM processes make use of only a certain size fraction of a full atomization lot, for example 15-45um or 45-105um. Such a size fraction is typically only 30-50% of the full lot. What happens with the remaining powder? Can the powder manufacturer sell it to someone else? If not, then it will be scrapped by the powder manufacturer, sooner or later, and the cost of that scrap powder will be included in your powder invoice. Scrap is scrap, no matter who is responsible.
Let me give you a metaphor: Owners of electric cars often claim that they have ”zero emission vehicles”, without considering the origin of the electric energy. If the electricity comes from burning fossile fuel, then they have just moved the CO2 emission from their exhaust pipe to the chimney of the power plant. Emission is emission, no matter where it occurs.
Next time you hear AM people speak about Buy-to-Fly ratios close to unity, be skeptical and ask questions!
Yes, we all know that AM powders can often be recycled and reused several times, but this does not necessarily imply that BTF=1.
2020-02-13 05:39:30
Author: Ulf Ackelid, Principal Scientist, founder Freemelt AB
