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Design Notes From the Field: Metal Cases
Jeremy Faludi, 2 Jan 07
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As I mentioned in an earlier post, I've been doing some green product design advice & research for a startup doing a consumer-electronics device. This post compares the impact of aluminum to two other metals, as well as plastic, and talks about the options for coating/finishing the metal.

Metal vs. Plastic

The company originally wanted the case to be injection-molded plastic, and wanted to know the greenest way to do that within their limitations. However, at some point they decided (again due to other limitations, not environmental reasons) to make the case be mostly extruded aluminum, with little bits of plastic to seal the ends and to be windows, somewhat like an iPod Nano.

Using aluminum instead of plastic does increase the device's environmental impact, in three ways: first, aluminum is far 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 (about the size of an iPod Nano), we need disassembly time to be extremely short, otherwise it won't be worth anyone's while to recycle it, because the value of metal and plastic you get for the amount of time spent is a financial loss. (I'll talk about another design for disassembly issue in a later post.) Basically the only advantage of using aluminum over plastic is that it makes for a more durable case. The electronics in the device may be working and useful long enough for this to make a difference, but they may not. (In most consumer electronics devices they're not, but this company is trying hard to be green.)

What Kind of Metal?

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" (80 kilowatt-hours). 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. However, because most smelting is such a large-scale operation, it appears difficult to source 100% recycled aluminum rather than getting what everyone else gets. (I didn't have any luck, though someone with a bigger operation might fare better.)

So, are there other metals that would be a better choice than aluminum? I only checked out two, steel and titanium, due to time constraints. It's possible that brass, bronze, or copper could have been alternatives too; feel free to investigate them or other metals and post your results as a comment.

Steel is also an extremely common resource (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 to produce as aluminum. It also requires less raw material to be mined per unit of pure metal. 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!) Steel is also very strong, and will make a device's case harder to break. Unfortunately, however, because of its high strength, steel cannot be extruded in the tiny dimensions of our device--I talked to about twenty companies throughout the US, 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.

Titanium was considered 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, titanium is a universally bad choice, unless you're doing a product that cannot be feasibly made with steel or aluminum. 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 (though stainless steel is good for this, too).


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--they require toxic solvents to set, and offgas volatile organic compounds while they're new. (In case you didn't already know, 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 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, the powder coat 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 (according to the three recyclers I talked to).

In the end we decided to go for anodization. It was unclear which of the two choices was best (an in-depth life-cycle analysis would need to be done to properly weigh the advantages and disadvantages), and anodization looks much nicer, so they chose that.

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I've no specific insight concerning aluminum or its impact, but wanted to thank Jeremy for these on-going posts. Although it is unlikely I'll be involved in consumer electronics anytime soon, reading about the project's development, the weighing of options, the guessing, etc., is extremely informative.

Please, keep them coming.

Posted by: Randy J. Hunt on 2 Jan 07

Thanks for another instructional article Jeremy.

Now, I would only like to add the importance of the material's density in the overall environmental equation.

Surely everyone knows the heavier an object the more energy you need to displace it. So, this is also an important factor to take into account when choosing a material. It's important to bear in mind the quantity of fuel that is used to transport the goods we consume, especially because products are transported more and more by plane, which have a very sensitive weight-fuel consumption relation.

Jeremy mentioned that less friendly materials were chosen because of their performance in certain aspects of functioning, and the same apllies here. But whenever possible, choosing the lighter material also contributes to a greener product.

Also bear in mind that density is not only equal to more or less weight, it is also related to the mechanical properties and volume. Volume can be decisive in logistics and an object made of steel will probably be less volumous than one made of aluminium with the same strength.

Anyway, that was a good and clear read, I hope you put more of your expertise into future articles.

Posted by: Sergio on 2 Jan 07

Just as a sidenote, aluminum does have one more advantage than listed, and it's in the hands of a consumer. Aluminum can be used to increase the perceived value of a product. When the user picks up two products with the exact same features; and can (even subconsciously) feel the physical difference between silver painted plastic and the cold sterile feel of aluminum, they are sure the aluminum product is worth the inflated additional cost. My two cents on the matter. Excellent writeup with many great facts and figures that sustainable designers can use for reference.

Posted by: Taylor on 3 Jan 07

There was a process called Metal Injection Molding a few years back that might meet your needs. IIRC, a metal/binder was injection molded to shape, then sintered to pure metal. Another process possibility might be superplastic forming. (I'm assuming you've already checked out the commoner processes.)

Posted by: Ron on 4 Jan 07

It should also be pointed out that there are huge diff. between the environmental impact of regular (partly or 100% recycled) steel and stainless steel. According to Okala, stailess steel is almost as bad as using virgin aluminum.
Anodized aluminum is a great choice as long as a high percentage of it is from recycled material and as mentioned above it can easiy dissasemble. There might be some bio-polimer that reacts to cold temperature and shrinks enough to snap out of the extruded part as a quick and cheap way to separate material for recycling

Posted by: Stiven (sustainableday) on 11 Jan 07



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