Case Study - 13A plug

Plug body: selecting the material

Also, out of all the polymers available, why are only 2 commonly used?

To hold the pins securely and to protect the conducting parts, the plug body must be sufficiently strong and stiff. Plugs are also likely to suffer impacts, so toughness may also be a factor in material choice. A materials selection chart showing strength against toughness for various materials allows us to compare various polymers and we might be able to see why other polymers are not suitable.

The strength of polymers is relatively low compared to other materials. However, good design of the moulding shape is able to provide sufficient strength to support the pins. Correct shaping is also important for providing sufficient rigidity for the pins as the stiffness of polymers is relatively low (not shown here).
In practice, ABS and urea formaldehyde are both used for plugs. The toughness of ABS is a lot greater than that of urea formaldehyde (remember these are logarithmic axes) - this means it can withstand a greater impact before any damage is caused.
Other materials: Nylon appears to combine the strength of urea-formaldehyde with the toughness of ABS, so why isn't it used for plug bodies? Looking back at the resistivity-cost chart, we can see that it is much more expensive than ABS or urea-formaldeyde, although this may not be the only reason.
So why are 2 different polymers used?

Plug body: two different solutions

Although both ABS and urea-formaldehyde are used for plugs, they are in fact used for slightly different applications. In addition, ABS is used for one piece moulded plugs which prevent access by the user whereas urea-formaldehyde is used for two piece plugs that can be fitted by users. From the selection chart and processing information we can see why:

ABS is used for one piece plugs because thermoplastics can be easily joined after moulding. ABS is also much tougher and as a result is ideal for pre-fitted plugs which might suffer impact during service (e.g. vacuum cleaners).
As a thermoset, urea-formaldehyde is stronger (and stiffer) than ABS so it is ideal for a two piece construction where each half must be individually stronger. The lower toughness means that urea-formaldehyde plugs should only be used for fixed appliances where the plug is unlikely to suffer impact (e.g. computers or freezers)