The processing data for a material family and specifically for a grade within a family as supplied by theproducer of the raw material are of utmostimportance to the setup person. These dataprovide guidelines for setting parameters thatwill safeguard the properties described in the data sheet for the particular grade. 

               Plastics are man-made materials known bythe general name polymers. Each of  themhas a different prescription for processing orconverting it into a finished product. (SeeTable 2-8.) 

               Polymers are created from atoms that are joined to form a molecule. The atom is the fundamental component in a chemical compound. The molecules resulting from joined atoms are caused to combine with each other to create a long chain (the molecular chain), in  a process called polymerization. These chains fold, intertwine with each other, and are held together by forces between them. The molecular chain mixture becomes a plastic raw material called a polymer. 

              This oversimplified definition is not intended to mislead anyone into believing that the science of polymers can be learned easily. It is one of the most complex branches of chemistry, and a great deal of skill is required to master it. 

              Fundamentally, these materials are chemicals, but the molder hardly ever works with the pure polymer because modifications are needed to make conversion of these materials into useful products practical. A variety of additives are compounded into the materials before they are marketed. 

              Specific additives are essential for reduction in heat sensitivity during molding, stability during exposure to ultraviolet light, color; antioxidization during exposure to the air; lubrication during molding, reduction of flammability, acting as reinforcement to improve properties, serving as extenders to reduce the cost of  material, and many other uses, as requirements may demand. In working with materials of such complex composition, it is imperative to follow the exact  specifications for processing outlined by the manufacturer for each one of the grades. 

              Variations in processing requirements exist not only from one type of plastic material to another, but also from one grade to anotherwithin a family type. A good example is thevarious grades of ABS, in which the pre-scription for processing changes with mostof  the grades. Thus, it should not be takenfor granted that setup conditions for similarmaterials apply to any grade; careful investigation is required if quality problems are tobe avoided. 

              It should be noted that at melt temperature, all plastics are amorphous. After molding, some of the properties of both crystalline and amorphous structures are observed (Chap. 6). 

             Examples of crystalline materials arepolyethylene, nylon, and polypropylene.Amorphous plastics are polystyrene andpolycarbonate. 

             Crystalline materials have high shrinkage,with the component in the direction of flowusually greater than that perpendicular to it.When a symmetrical part such as a round cupis fed at its center, the shrinkage is uniformand usually the average of the two components. Amorphous materials have low shrink-age, which is the same in all directions. 

             Crystalline plastics require more heat than amorphous ones to bring them to the desired flow, because of the heat of fusion. (Heat of fusion is the heat necessary to bring about a change of state for example, the heat necessary to melt ice at 32°F to water at the same temperature.) After the material is brought to melt temperature, additional heat is needed so that it will flow properly. 

              When plastics flow through gates and runners, their molecules tend to be oriented inthe direction of flow. A smaller gate area willcause greater orientation, except that there isa lower limit on the gate area for amorphous,heat-sensitive, and long-molecular-chain materials, such as polycarbonate. 

              Oriented plastics gain in strength in the direction of flow. One application of this feature is the living hinge in polypropylene, where the gate opening is 0.020 in. thick, andthe direction of material flow is perpendicular to the hinge action. Under these production conditions, the living hinge will notcrack.