_Elastic._—A substance which exhibits rubberlike properties or “high elasticity” over a wide range of applied forces. _Elastic deformation._—When a substance reverts to its original dimensions on release of an applied stress. _Elastic limit._—The point at which a body begins to yield under a stress; that is, when the stress is equal to or greater than the internal friction. _Elasticity._—The property by virtue of which a body reverts to its normal bulk or shape after deformation by an applied force. _Extrusion molding._—A molding procedure for extended shapes of uniform cross section, whereby a heat-softened substance is forced through an orifice of form coinciding with the cross section of the article. _Flexibility._—Capability of bending without breaking. _Gums._—Viscous vegetable secretions which harden but, unlike resins, are water soluble. The name is often applied, particularly in the varnish industry, to natural resins such as copals. _Hardness._—Property of substances determined by their ability to abrade or indent one another. Often measured by the extent or depth of indentation produced by a standard substance under a predetermined load. _Impact strength._—The measure of toughness of a material. Generally determined by the energy required to break a specimen in one blow. _Injection molding._—A molding procedure whereby a heat-softened plastic material is forced from a receptacle into a cavity which gives the article of desired shape. Used particularly for thermoplastics since the scrap can be reused. As soon as the composition in the mold cools sufficiently to be rigid, the mold is opened and the molded article removed. An analogy of injection molding in another field is shown by the linotype machine. _Inserts._—Parts of a finished molded article which are of different material from the molding composition but are set in place or positioned by the molding operation. _Laminated products._—Sheets of material united by a binder. For example, sheets of paper or wood coated and/or impregnated with a resinous composition and subjected to pressure, generally with heat. _Monomer._—The simplest repeating structural unit of a polymer. For addition polymers this represents the originally unpolymerized compound. _Phenoplast._—A general term for phenol-aldehyde resins. Synonymous with popular term “phenolics.” _Plastics._—All substances that can be molded. In general a plastic is a substance which behaves as a solid at stresses less than a certain amount known as the yield value and as a viscous liquid at stresses greater than this. The name is also applied to substances which originally but not ultimately fulfill this condition. For example, it is applied to thermoset compositions or resinoids in the final stages.