Monday, August 8, 2011

Less is more, part 4

How much less is less?

The last several postings have been discussing the elements of reducing consumption in manufacturing. Not just cutting but making better use of the resources available. This stretched from reviewing the "buy-to-fly" ratio concept to yield issues in metal production and use. We discovered that there is a lot more potential in the material that is left on the floor in production than we might think. In fact, improving material processing yield may actually offer more potential for impact reduction than many other strategies.

But these are technically complex issues. Manufacturers don't waste material on purpose. The swarf from machining is due to material removed to achieve the desired shape. The farther the input workpiece is from the final shape the more material must be removed and shows up as chips on the floor. These chips are routinely recycled of course. But that is a far cry from not using it in the first place.

The term "net-shape processing" (defined as making things to a final or near final shape without removing material - such as forging) is one approach to reducing the amount of material that needs to be removed to achieve the final shape. This cannot address the requirements of surface conditions (like very low roughness) or some form requirements but it goes a long way. This does not work for all materials. But, for example, plastic injection molding is a classic example of net shape forming (except for the runners, sprue, etc. unless done with hot runner systems as in high production.)

The challenge is linking performance to shape and properties. And then making the next link to environmental impacts/resource requirements.

Engineers like to use "tools" for assisting in making these links. By tools we mean software or other analysis methodologies that assist in presenting data or alternatives to the designer, or manufacturing engineer, to be used in decision making. These tools often help the engineer answer questions like

- what is the function of the device or piece of hardware or component that is being designed?
- what are the objectives that need to be optimized?
- what constraints must be satisfied?

These questions are common to all engineering design problems but are part of the concept behind a wonderful tool from Granta Design called the CES (for Cambridge Engineering Selector) methodology developed by Professor Mike Ashby and his colleagues at Cambridge University in England. This was first developed to help engineers and designers to select materials for use in products and components.

They give the example of a design relative to these questions as "For instance, a car body panel (function) needs to be as light as possible (objective) for a specified stiffness and cost (constraint). Other constraints on the design might be acceptable resistance to mechanical impact and to contact with various environments." This is described in great detail on the Granta website.

I need to mention here that I am in no way associated with Granta Design or Mike Ashby and am not being paid to pitch his product or company! We are, in my lab, using this software (and we paid for a license) and I am only a big supporter because it is one of those products that is very useful and enables us to do things we otherwise would not be able to do. I also use Granta software in my class on sustainable manufacturing.

Ashby developed the concept of selection charts that show one type of material property as a function of another - for example elasticity as a function of thermal distortion. So if you are designing a component, say of a machine tool, and you need a material that has a certain stiffness but is less sensitive to temperature variations (and the accompanying distortion, growth with increase in temperature and shrinkage with reduced temperature) then you could see where different material groups fall and choose a material that is in the range desired.

The data is the same data you'd get from a handbook, or tests, or another expert but the method of presenting it offers additional insight to the designer.

For example, the figure below, from the CES EduPack Manual from Granta (this is available on the Granta's website for teaching tools) shows a typical "Ashby chart" plotting Young's modulus (this is the "elastic limit" for materials in load per unit area which serves as a measure of the stiffness of an elastic material) as a function of density (mass/volume).

This would be the kind of information that an auto designer would use to pick a material that has the required stiffness but with the least possible weight (since lighter vehicles require less fuel to move and lighter frames to support them).

Ashby fits the use of this data in assessing material selection impacts over the life cycle of a product. The figure below, also from Ashby and available at the link above, shows the stages of material usage in a product lifecycle. Ashby data makes it easier for an engineer to see

the magnitude of these impacts. And, of course, it gets back to our "make vs use" impact discussion some time ago (for example as part of the material diet discussion). The take away from this figure is that there are materials issues at all stages of product life.

The one limiting element of Ashby is that he looks at product design through the lens of materials and there are, of course, other concerns. But, this is a small issue compared to the benefit of his approach.

So, where does the "green-stuff" come in?

One of the axes of information that Ashby provides is embodied energy (and its equivalent in CO2 emissions). The figure below, also from Ashby, shows embodied energy (GJ/volume) for a wide array of materials. This data, specially when plotted as a function of material parameters, opens up

the possibility of connecting design parameters to environmental impact. The embodied energy data Ashby relies on is generally reliable. In a number of cases the data may reflect a specific region of the world or particular means for processing the material but it is an excellent base to work from. The red dotted line is only for comparison of materials and embodied energy.

As you can see in the figure above, the potential for looking at energy (and thus CO2) impact for materials is wide open. Within a materials group, like polymers, there is a factor of 10 range of embodied energy. Within the ceramics group this range grows to a factor of 1000 (three orders of magnitude.)

Clearly, all these materials in each group do not have the same properties are, hence, are not interchangeable. But, linking them to material properties, then to the design or production requirements, lets chose the best, least impactful, material.

That's where we start next time and we'll include an example.

1 comment:

  1. The discussion around using the best material vs the easiest to manufacture is a very legitimate and common discussion amoungst Engineers, especially in the Automotive realm. I entered the Auto Manufacturing era when HSLA steels were being pushed hard. I know for a fact that when a material is lighter, although maybe more expensive, it is considered a plus. We worked on seating and was required to reduce ounces out of assemblies by less welding and adding holes in stampings to lighten them.

    All in all it turns out that lighter cars save on gas. And lighter cars mean less raw materials. We saw a down trend in material costs once a few bold Engineers demanded HSLA as a standard and consumption went up. Then the good old free market started to kick in and it got less expensive to make.

    ps-I love the article but the "Professor" side of you showed up in this one. I had to re-read a few times to gain the gist of the charts. Whew!

    Keep them coming!


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