More and more, light metals as aluminium or magnesium take the place of polymers as polycarbonate with respect to the housing of smartphones, tablets or notebooks. Of these materials as such, polycarbonate is the lightest with a density of 1.2 g/cm3. This would argue in favour of this material; after all, the lighter such a mobile device, the more enjoyable to work with. Aluminium and magnesium are with a density of 2.7 and 1.7 g/cm3 much heavier than polycarbonate, when we compare the same material volume.
But density is not the only thing that counts here. Aluminium is twice as strong as polycarbonate, so an aluminium housing can be produced twice as thin as a polycarbonate housing. Hence, there is no longer a ‘density advantage’ for polycarbonate in practice. Aluminium as well as magnesium can be made quite easy into thin-walled – and light – pieces. For that matter, magnesium in its pure form is not strong enough. By making a solid solution of about 10 % aluminium and possibly some zinc or manganese, a magnesium alloy is obtained which is indeed strong.
For a user, a notebook has to be preferably lightweight and indestructible. You have to construct the housing in such a way that it is strong and stiff enough to protect the motherboard, harddisk, display and all other critical parts. Especially the stiffness is important here. After all, you do not want the glass display to break when you bend the notebook too much. Such a deformation may occur more and more in practice, due to the trend that notebooks are getting increasingly thinner. And this holds also to some extent for tablets and smartphones.
Besides low density and mechanical advantages, aluminium has another added value as a housing material. As the material conducts heat extremely well, the housing is able to remove the heat of the hard working microprocessors to the outside of the device. This way, the cooling fan inside a notebook has to ‘blow’ not so hard and loud. The heat conductivity of magnesium (and polycarbonate) is somewhat (and a lot) worse than of aluminium.
Metals as magnesium and to a lesser extent aluminium offer a good protection against electromagnetic radiation, where polymers allow this radiation to pass completely – which has its pros and cons. The antennas of the devices have to be able to receive a (mobile) phone signal or a WiFi signal as good as possible. At the same time, unwanted electromagnetic radiation – for example of other devices – has to be blocked. For optimum performance, you could interrupt a metal housing just beside the antenna(s) with a polymeric ‘window’ that allows the phone or WiFi signals to pass.
To put it briefly: there is more than one reason besides the density to choose for a certain light material as housing for a mobile device. So it’s no surprise that these different materials exist along each other for the same products. Anyway, magnesium is still being used in a device of long before the digital revolution, which is … a pencil sharpener!