Using Metal

November 8th, 2006 by Team Member

by Jeremy Faludi

This week’s episode of green design for Exbiblio is about metal. They recently decided to change plans about how the first release of the oPen will be made–instead of the whole body being recycled injection-molded plastic, most of the body will be an aluminum extrusion with holes machined into it for the screen and buttons, and there’ll just be plastic bits on the ends, much like an iPod Nano. The reasons for this had to do with schedule and design flexibility–we have a very tight schedule to make, and need to get to production as soon as possible, but still have not nailed down all of the design considerations. Using an extrusion with machined holes gives us a great deal of flexibility, as machining can be reprogrammed at any time to cut different holes, and extrusions are fast and easy to get into production–easier than injection-molding.

Using aluminum instead of plastic does increase the device’s environmental impact, in three ways: first, aluminum is more energy-intensive to produce than plastic; second, it’s more energy-intensive to manufacture with and requires harder tooling; third, having the case be made out of multiple different materials makes it harder to recycle because it needs to be more carefully disassembled and sorted. With a device this small, we need disassembly time to be extremely short, otherwise it won’t be worth anyone’s while to recycle it, because the amount of plastic and metal you get for the amount of time spent is small. I’ll talk more about design for disassembly in a later post.

Almost all consumer electronic devices which use extrusions are made out of aluminum. The ecological advantages of aluminum are that it’s a very common material (the most common metal in the Earth’s crust), it’s not toxic, and it’s very recyclable–according to the US Geological Survey, 44% of aluminum production in the US is recycled material (the industry calls it “secondary” production as opposed to “primary” production.) The disadvantage of aluminum is that extracting and refining it is enormously energy-intensive–so much so that it’s often called “solidified electricity”. The US EPA estimates that 2-3% of all electricity use in the US is for making aluminum, and New American Dream estimates that 16% of all electricity used in Oregon & Washington goes to aluminum smelting. (Also from that site: “every three months, Americans alone throw away enough aluminum [cans] to rebuild the entire US commercial air fleet.” So remember to recycle your cans.)

Exactly how energy-intensive is aluminum? An MIT study found “the production of 1 kg of aluminum requires on the order of 12 kg of input materials and 290 MJ of energy”. However, recycled aluminum only requires 5% as much energy to produce. This is why so much aluminum is recycled–it’s advantageous purely from a money point of view, even if you don’t care about the environment. Estimates vary on how much of the US’s total aluminum production is recycled, and it depends on the grade of aluminum you buy, but on average it’s at least 20%. Smelting tends to be a very large-scale operation, so everything gets mixed together in large batches–that unfortunately means it’s nearly impossible to source 100% recycled aluminum. (The upside is that everyone who buys aluminum of most grades gets some recycled content whether they care or not.)

Steel, on the other hand, is also extremely common (iron is the fourth most common material in the ground), and doesn’t require aluminum’s enormous energy budget–it uses less than 1/8 (almost 1/9) as much energy as aluminum to produce. And stainless steel has a great aesthetic, very classy without being pretentious. Steel is also recycled even more than aluminum: the USGS says that 71% of steel in America is recycled. (Surprisingly, they also say that the auto industry’s steel recycling rate is 102%, meaning that they’re recycling more steel from old cars than the steel they’re putting in new cars.) Unfortunately, however, steel cannot be extruded in the tiny dimensions of our device–I talked to about twenty companies throughout the country, and the thinnest wall they could extrude was usually the thickness of our entire device! Some places can do what’s called cold-rolling for thinner walls, but even then I couldn’t find a single company who felt they could make what we needed.

We also considered titanium briefly, but it has none of the advantages of aluminum and far worse disadvantages. It’s a rare metal; it’s not toxic itself, but its refinement requires toxic chemicals like chlorine and hydrochloric acid; and it uses over three times the amount of energy to produce as aluminum (twenty-six times that of steel). (See that same MIT study for details.) Titanium is also expensive, due to the difficulty and energy-intensity of processing it; it’s also very hard to work with: a European Commission study says that “80% of material from forged parts for the aerospace industry becom[e] turnings or other scrap. Titanium machines at between 10-20 times slower than aluminum and can account for 70-80% of the cost of the component.” So, no titanium for us. Best to leave it for high-performance engineering applications that require its impressive physical properties, or to the medical industry that likes its non-reactivity in the body.


Bare aluminum gets smudgy and ugly when handled, and can get your hands a little smudgy too, so for aesthetics you should coat it with something. Paint is a poor choice, because all but a few special paints cause a great deal of pollution, as they require toxic solvents to set, and offgas volatile organic compounds while they’re new. (I hate to break it to you, but that “new car smell” isn’t very good for you.) Lacquering, plating, and buffing are also not so great. The two good choices are powder-coating and anodization.

Powder-coating is like paint, except there are no solvents. Instead, the pigment is a fine, usually non-toxic, dust that gets sprayed onto a product with a compressed-air sprayer (no CFC’s required). The powder sticks to the product is by static cling: the powder shooting out of the sprayer gets positively charged, and the product is hanging on a rack that’s negatively charged (meaning that the product has to be electrically conductive to powder coat it.) Then the powdered product is put in an oven to bake the powder into a hard coating; this does use some energy, but is still a much smaller impact than painting. The only problem with powder coating is that, like paint or lacquer, when you recycle the aluminum it’s coating, it burns off into toxic fumes, so a recycling plant needs to have special emissions-control equipment to do it cleanly. This means that powder-coated aluminum is less recyclable than bare aluminum (and painted or powder coated aluminum scrap is worth less money than bare aluminum.)

Anodization is a microns-thin layer of oxidation on the surface of the aluminum. It involves some nasty chemicals (depending on the color, these can include sulphuric acid, nitric acid, phosphoric acid, nickel acetate, and others; some use hexavalent chromium, but we will definitely avoid that), but small amounts of them (because the oxidation layer is so thin). The coating is non-toxic to the user, the concern is the waste and worker safety in manufacturing; any decent modern plant has emissions/effluent controls, but it would be better not to use toxins in the first place. The main advantage of anodization is that it does not hurt recyclability of the aluminum–it is such a thin coating (and even that coating is mostly aluminum itself) that anodized parts can be thrown right in with bare parts in recycling plants, and their value as scrap is just as high as bare aluminum.

In the end we decided to go for anodization. It was unclear which of the two choices was best (they both have advantages and disadvantages), and anodization looks much nicer.

2 Responses to “Using Metal”

  1. James Says:

    What causes powder coating to peel?

    I have a patio set and the powder coating is peeling off. Can the water from a salt water pool cause this to happen?

  2. ari salomon Says:

    I use different materials when i design signs for my clients and I have been trying ot find greener materials. I’m glad google brought me here – your explaination of these different processes is really helpful.

    i’ll stick with powder-coated aluminum. Some people like Dibond but I bet you can’t recycle it.