Saturday, August 27, 2011

Less is more, part 5

Designing for small

In the last posting we stated that one of the challenges is linking product performance to material shape and properties. And then making the next link to environmental impacts/resource requirements.
An example of some helpful software that connects material properties to potential environmental impacts/burdens was given. By linking the potential burdens to material properties, and then to the design or production requirements, we can try to choose the least impactful material.

So, with respect to either a process/machine for manufacturing (manufacturing phase) or product (use phase), the challenge is to find the design/material/structure combination that:
i. gives the desired performance/meets specifications
ii. can be economically manufactured/operated at sufficient scale with required production rate, quality, and cost,
iii. while minimizing the environmental impact or, better, reducing the impact enough compared to the present performance to offer a "return on investment" that moves the operation of the process or product towards a more sustainable situation.

One of Ashby's techniques to start his analysis (see last posting for more info on Ashby!) is the "use matrix." This matrix arrays, vertically, energy intensive to material intensive products and, vertically, different product "load factors" from high impact to low impact. For example, the categories of energy to material intensity are from primary power consuming to non-power consuming. The primary power consuming products are energy intensive in their use phase and the non-power consuming are, then, material intensive. The figure below is reproduced from Ashby's CES Eco-selector white paper from February 2005.

In this matrix you can see examples of entries in the various categories with an automobile being a primary power consuming high load factor product (meaning the use phase impact is power related) while on the other end of the scale a tent or canoe is low load factor and material intensive since the tent requires no energy to operate so the material consumption in the manufacturing phase is the most significant.

Now, if we looked at manufacturing in the same manner, what could be a “use matrix of manufacturing classes”? Here is my attempt to fill in such a matrix for manufacturing.

You can follow the logic I think. An example of a high load factor energy intensive manufacturing process is something like a furnace for heat treating or a semiconductor manufacturing etch tool. A low load factor manufacturing process or element could be a warehouse or office for a factory which is midway between energy and material intensive depending on the exact activity in that warehouse or office.

Ashby's process uses such a matrix to help determine which phase of the "product" (here a consumer product but in our discussion a piece of manufacturing hardware for a process or factory component),  that is material production, product manufacture, product use or product disposal/end of lied, should be focussed on for the largest improvement.

If one is designing or producing high or modest load factor primary power consuming machines for production, such as rolling mills, forming presses or machine tools, etc. as in the manufacturing matrix above, then we would want to consider these four phases relative to those machines.

Let's consider the example of the design of a deep draw press. We'd like to come up with a press that meets the constraints posed at the beginning of this posting - gives the desired performance/meets specifications, can be economically manufactured/operated at sufficient scale with required production rate, quality, and cost, and minimizes the environmental impact.

If you are not sure what these are there is an excellent on-line video on the operation of one made in Taiwan and its manufacture. Note: this is a sales video but informative! The process performed on such a press is more objectively detailed on Wikipedia under deep drawing.

The elements to be considered in the design of a deep drawing press would include:

- Material production: steel mostly (several tons)
- Manufacture: welding (mostly), machining (some), electronics (not many)
- Use: electricity, hydraulic fluid, compressed air and other consumables
- Disposal: scrap (likely sold for re-use)

The design criteria would include:
- tonnage (pressure/power) which determines the size of the part to be made or thickness of the metal formed
- stiffness
- speed/strokes per minute
- ease of load/unload
- die changing/handling/setup

The press capacity is determined by the tonnage it provides for deep drawing while maintaining the necessary stiffness for the accuracy of the forming process. The speed is dependent on the efficiency of the energy to move the press given the weight of its components. A press that move rapidly (up/down strokes) either must be light (and hence low tonnage) or require a lot of energy to move.

Ashby data provides a measure of the relative "cost" in embedded energy of different materials per unit bending stiffness (affecting precision) and mass per unit of bending stiffness (for the speed vs precision tradeoff).

The curve below, from Ashby's software, shows the "trade-off surface" for this energy-mass for a stiffness limited design. The curve shows the range of reasonable candidate materials for achieving the required mass (for speed) and stiffness (for accuracy) normalized by embedded energy. Ideally, following along this curve gives the designer a set of material that will meet these constraints.

We see that one of the materials lying near the curve is cast iron and another is mild steel in the lower right part of the curve- both reasonable cost alternatives. Others on the curve, but with higher cost, are beryllium alloys in the upper left part of the curve- not likely to be used. Also not likely to be used is chipboard which is a bit below the curve. Another material not traditional used but worth considering is carbon fiber reinforced plastic - one the curve near the bend. These fiber-based materials offer very high strength/stiffness and very low mass so could be a new design for presses for high speed but high stiffness with similar embedded energy, for the amount needed, as steel.

These materials (the steel and cast iron at least) are also easily recovered at the end of life and, in fact, lend themselves to re-manufacturing (another good topic we'll delve into sometime) as well.

Next time we'll apply this to the manufacture of a precision machine tool.

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