This turning knob is made of plastic because it is cheap and easy to mass produce. There are also multiple grooves on the side to allow good grip when twisting it.
This is a layer of plastic to separate the beads below from the rest of the etch a sketch. The plastic is their because the etch a sketch would not work because the beads below would stick to the glass, so the stylus could not scratch away anything.
The outer material of an Etch A Sketch is cheap because it allows mass production. The material also has low elasticity, plasticity, ductility and tensile strength. This makes the Etch a Sketch a good buy for families with young children as the material is unlikely to break or deform under high pressures. It also is a bright red to make the Etch A Sketch stand out on the shelf. The colour is also used because it makes the Etch A Sketch instantly recognisable. The outside of the plastic is very smooth to make it feel nice on touch.
This is a stylus which is mounted on a pair of orthogonal rails. This allows the lines to be drawn. The rails are controlled by the knobs at the top. the stylus itself is invisible under the beads. This allows lines to be drawn whilst keeping ugly rails out of the view of whatever drawn.
These beads are made up of a mixture of very fine aluminium powder and beads. These are used because without the beads the powder does not flow evenly at all. Also if the aluminium powder was not there then the reaction that creates the lines to be drawn would not happen. The lines happen because Aluminium powder sticks to everything. Therefore when you shake the Etch A Sketch, it coats the plastic. Then when you turn the dials the stylus scratches away parts of the aluminium powder, creating the lines.
At the top of the Etch A Sketch, there is a see through plastic that is light, hard wearing and easy to manufacture in large quantities.
Generally the materials used in the etch a sketch are all very cheap. This is because it allows the company to make a reasonable profit when selling the product itself cheaply. Also all the materials have an extremely low elasticity, allowing the Etch a Sketch to maintain its shape.