Plastic means pliable or impressionable. It is defined as the capability of being deformed continuously in any direction without breaking apart .Following this definition，plastic could include glass ，metal and wax .Thus ,it is evident that name plastic is misleading and not exactly correct. Today ,however, the term "plastic”is defined with the products which are derived from synthetic resins. The synthetic resins are made by various chemical processes .Plastics manufacturing is of comparatively recent date.
Today plastic are commonly found in homes ,automobiles ,numerous other products and machines . Some machines which utilize electricity require plastic parts with electrical insulating properties .
Plastics are usually shipped to manufacturing plants as pellets or powers ,and they are melted for just before the shaping process .Plastics are also available as sheet ,plate ,rod ,and tubing ,which may be formed into a variety of products .Liquid plastics are used especially in the making of reinforced plastic parts.
Plastics or polymers ,commonly called plastics ,fall into two broad classes :
Thermoplastic molding materials include polymers such as ABS and polycarbonate used for toys ,consumers electronic products and more flexible kitchenware products .The thermoplastic materials soften with heating and solidify and harden with cooling .The key features is that these materials can be remelted to a various fluid ,molded and cooled time aftertime without undergoing any appreciable chemical changes ,i.e. the change is reversible.
Thermosetting molding materials -these include the melamine-formaldehyde used in hard plastic tableware and the epoxy resins used for glues and reinforced cast products such as kayaks and tennis racket frames .Thermosetting products are heated until they become a viscous liquid , poured or injected into a mold ,and then allowed to solidify.
As thermosetting materials ,chemical cross-linking occurs to create an irreversible ,infusible mass. Upon the application of initial heat the thermosetting plastics soften and melt ,and under pressure they will fill every crevice of a mold cavity .Upon continued application of heat they polymerize or undergo a chemical change ,which hardens them into a permanently hard ,infusible ,and insoluble state .After this they cannot again be soften or melted by further heating.