Some time ago (July last year to be specific) we discussed the concept of “buy to fly ratio” used to track the ratio of the amount of material that an aircraft manufacturer starts with to the amount that actually ends up on the airplane. It was part of a discussion on degrees of perfection and was a led in to a series of discussions about how to actually measure the impact of what we are doing in terms of “greening” manufacturing. That is, if we keep track of everything – are we ahead at the end of the day or not?
The buy-to-fly ratio came out of the aerospace industry but has applicability to manufacturing broadly. Unfortunately, numbers for this ratio are not too impressive and I cited some published from aircraft manufacturing that are in the 30’s – meaning 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.
And here is the issue. Even with recycling of “wasted” materials in manufacturing we use energy and resources. So, recycled content is not free!
Recall Henry Ford’s comments cited in one of the first postings for this blog on “why green manufacturing” two years ago: “… we will not so lightly waste material simply because we can reclaim it — for salvage involves labour. The ideal is to have nothing to salvage.” This was published in his book “Today and Tomorrow” (1926).
At that time Henry was probably generating his own electricity from “waste” steam from steel or coke making or wood chips from his wooden frame production so he wasn’t even thinking about the cost of energy. And, I don’t think the concept of global warming/CO2 was a topic of discussion then.
(Note: Today’s Ford Motor Company is fully engaged in energy and resource efficiency in both product and manufacturing. You can find their CSR report online)
So, back to the chips (or the “hole”).
The International Society of Industrial Ecology just held their annual meeting in Berkeley. One of the attendees was Professor Julian Allwood from Cambridge University and we had a chance to meet up and talk a bit about his work under the banner of “WellMet 2050.” I introduced this project in a blog earlier this year on ‘resource dieting.’ He is a creative thinker about green manufacturing challenges and firmly grounded in processes and analysis.
One of the big 4 themes of their research is “less metal, same service” and John was discussing, basically, the “buy to fly ratio” problem. He focusses specially on metals in his research.
The details are documented in the “Going on a metal diet” report from the study and you can download it from their website.
One focus of the study as part of the “less metal, same service” is on reducing the scrap in manufacturing. There is a common misconception (or, at least, benign neglect) that recycling hits the reset button on inefficient use of material. This is a big mistake!
Inefficient use of materials is usually referred to as yield loss. That is, in the course of normal manufacturing (whether you are making airplanes, automobiles, semiconductors or polo shirts) material gets ‘left on the foor.’ Shapes are cut out of sheets and the bits around the shape that need to be held in the press, or due to standard size sheets larger than the part being produced, etc. are left over.
At best these leftover pieces are large enough to be used for other pieces (a concept called “nesting” in manufacturing). At some point there is not enough material left to be used productively in the operation and it is discarded and, hopefully, recycled.
Recall that recycled can mean anything from remelting and added to virgin material for making new sheets of material (as used here); mixed with other similar materials to produce lower quality metal; or collected and dumped somewhere (recycled to you – waste to the collection organization).
Take a peak at the Ricoh Comet circle from prior blogs to see the various paths of “down cycling” of materials.
The metal diet report states boldly “Going on a metal diet has much greater potential for CO2 emissions abatement than the pursuit of further efficiency measures in an already efficient liquid metals production process.” So Professor Allwood’s research team is focussed on the data to prove that statement.
First, let’s define what we mean by yield. The figure below, from Allwood’s “Going on an energy diet” shows how yield is determined based on the ratio of
metal going on to a downstream process over the sum of all process inputs. That which is not part of yield along the process chain is lost.
Now, how about the connection between the yield losses and the embodied energy of the material? Allwood has used a very novel way to display this that pretty clearly points out the challenge.
In the graph below, also from Allwood’s “Going on an energy diet” report, the horizontal, x, axis shows the “yield path” of a material amount during processing through several steps. That is, starting with 1 ton of liquid metal, it plots the mass remaining after each step of the process. This, essentially tracks the buy to fly ratio across several process steps. The vertical, y, axis shows the cumulative increase in embedded energy with each process step. Constant embodied energy contours are shown.
Reading this figure, for a specific product, tracks the consumption of energy and the loss of mass of the product (relative to the original raw material input at the start). You’d start with the liquid metal, cast it into billets or other shapes, rolled/formed into finished raw material stock (like sheet or bar) and then further processed by stamping or cutting, then finishing, etc. to yield the final product. These process steps are the “process AB” and “process BC” shown in the individual lines of product manufacture on the chart. You’d use as many process steps as needed to complete the product.
One interesting thing to note is that if you want to maintain constant embodied energy in the manufacture of the product you need to follow the constant energy contours.
We will see that this is the real manufacturing engineering challenge for green manufacturing!
In the next posting we’ll show some examples of real products from this study (like a beverage can or a car door panel) to illustrate the use of this energy yield vs material yield chart.
Once we are comfortable with the metrics for measuring our success (or documenting our failure!) we’ll talk about engineering tools to overcome this scenario in product design and manufacture.
This is part two in a series. Read part one here.
David Dornfeld is the Will C. Hall Family Chair in Engineering in Mechanical Engineering at University of California Berkeley. He leads the Laboratory for Manufacturing and Sustainability (LMAS), and he writes the Green Manufacturing blog.“