Degrees of Perfection

Green Manufacturing (Dornfeld) - I know I said in the last posting that this time we'll look at how some industries are doing and guidelines/strategies they are using to move up on the "sustain-o-meter." Well, let's start this process by another type of self examination - dealing with the "degree of perfection" for manufacturing.

The term "degree of perfection" is a ratio of useful products to inputs. The most recent discussion I read that referred to this was a paper by a clever person I've referred to before, Tim Gutowski at MIT, and others, in Env. Sci. Technology on "Thermodynamic Analysis of Resources used in Manufacturing Processes."

One of the more novel uses is in the aerospace industry where it is called "buy to fly ratio." Boeing, for example,  has a long history of tracking this value. Due to the peculiar requirements of aircraft components (demanding precision, unique shapes, incredible strength and fatigue requirements, etc.) many structural components (from wing spars to ribs) and many other parts, like landing gear, are machined out of large blocks of material. This results in most of the material going to waste. Buy to fly ratios in the 30's are common. This means, only a bit over 3% of the material purchased actually ends up on the plane. This waste for machined components is usually in the form of chips - which are recycled of course but discarded never-the-less.


Switching to other production methods (beside machining away most of the material) such as laser welding of complex rib components can make huge savings. Using laser welding to produce a rib component that had previously been machined resulted in a reduction in the buy-to-fly ration from 30:1 to 3:1 (see article). Ditto for use of carbon fibers. But in this case, the concern is how to better reuse the fibers or replace processes that generate so much scrap. A recent article in Plastics Today discusses Boeing's recent efforts to find secondary outlets for carbon fibers reclaimed from aircraft production.

...But, the original concept of degree of perfection does not speak specifically to material use ratios but useful output in terms of energy compared to input energy. The term used is exergy - a term you should have heard if you went to engineering school and took a thermodynamics course and may remember or - if you had a good physics course in high school.

Next time we will dive deeper into exergy and the concept of available energy and useful work. This forms an interesting basis for measuring the performance of manufacturing processes and material conversion/transformation and could allow us to look at the potential for greening and process improvement in a new way. This could be a better way to evaluate alternate technology.