brainiac wrote:
You are assuming that the 'plastic' is softer than the metal. Some 'plastics' are less plastic and harder than some metals. That is probably why engineering firms refer to them as polycarbonates. It is a shame that the word plastic has had its meaning perverted by an incorrect vernacular use. Plastic means inclined to deform permanently by applied force. The converse is elastic, which means inclined to return to original shape after application of deforming force. Most of the 'plastics' we depend on in our day to day lives are chosen more for their elasticity than their plasticity (at normal environmental termperatures).
These terms should not be confused with hardness, which is the ability to resist deformation by applied force, and brittleness which is the likelihood of shearing or shattering under applied force. Different solids and composite materials can have varying coefficients of hardness, brittleness, elasticity and plasticity in each of the four different dimensions as well as with temperature and the senses (e.g. tension, compression, shear).
The hood of my 200 f1.8 is made from a very plastic metal, and sometimes I have to hammer it back into a usable shape. A better design would have used polycarbonate or carbon fibre for the sake of their greater elasticity....Show more →
A lot of guys here seem to have a mixed idea of what constitutes an 'engineering plastic'.
Polycarbonate or sometimes known as Lexan is one of the many classes of plastic available for their specific properties and applications. There's also ABS, Nylon, Delrin, PEEK, HMPE, UHMPE, Polypropylene, PolyStyrene, PolyEthylene and many others. Then there are the fibre reinforced varieties of the above. There are the glass fibre filled, carbon fibre filled. Long fibre and short fibre varieties too. All of these and many, many more are classed as 'engineering plastics'.
Polycarbonate and it sub-variants are just a small group within this class of materials.
So.. what you read as engineering plastic, may not be even remotely close to polycarbonate as its pretty sensitive to certain oils. (Though all plastics are hydrocarbon based.) Whereas PEEK is one of those wonder materials that engineers love for its chemical resistance, and stability... but extremely expensive. Volume for volume, (not weight) costs more than most of the high grade steels out there!
So I hope everyone has some better awareness of what sort of materials constitute 'plastics' ..
