A machine at- tendant may be involved in occasionally supplying plastic material to the hopper. However, in most cases, he or she will deal with defective parts, runners, and sprues to be reground for future use. It must be recognized that plastic materials can be easily contaminated, unless proper precautions are taken to assure chemical cleanliness. The following is an explanation of how to keep plastic materials protected from contamination. 

           In addition to machine variables, there is one major source of problems in controlling quality Plastic Parts namely, the cleanliness and conditioning of the material as it is placed in the hopper. If we keep the material free of contamination that  is, free of foreign matter as well as other plasticour  chances of making good products are enhanced. It takes only a few parts per million of contamination to affect the properties of some materials. The way contamination will influence properties is not known without extensive research.  Even when  materials  are intentionally combined, the component ingredients lose some of their original characteristics while gaining some new ones. Take, for example, ABS, an alloy of acrylonitrile, butadiene, and styrene. Although ABS itself has desirable properties, the styrene part of it has lost its rigidity and clarity, the butadiene has lost chemical resistance, and the acrylonitrile has lost resistance to ultraviolet rays and weathering. The combination, however, has toughness, impact resistance, and good moldability, entitling it to a vital place in the plastic family. 

           It must be remembered that the ABS combination is achieved under predetermined favorable conditions. Accidentally contaminated materials may not look objectionable, but properties may be adversely affected. Think for a minute of one cubic foot of material as containing about two million cubes of the material; it only takes 10 to 20 similar cubes of another material to cause contamination. To make matters still worse, these small cubes in many instances cannot be distinguished from each other, nor can they be seen in the molded part if  it happens to be opaque. 

           A greater variety of materials will be used in the future, and the products that they will be applied to will be more intricate and functionally more important. Thus, it behooves us to seek immediately a foolproof manner for handling the materials so that all dangers of contamination are eliminated, and the chances of weakened parts are avoided. Above all, care, and more care, will be needed. (See Chap. 10 on material handling and size reductionigranulating.) 

           Principles of machine operation Duringthe process of converting a plastic raw material into a finished molded product, threebasic elements in modeling time, temperature, and pressure must be correlated in away that will produce a part with anticipated properties. Most deviations in product quality can be traced to variations from established values in time, temperature, or pressure. Changes in any of these individually or in combination spell problems in product properties and performance characteristics. 

           Time involves these elements: time beginning with material entering the heating cylinder until injected into the mold (also called residence time in the cylinder); time of injection into the mold; time of maintaining pressure in the mold cavity; time of solidification, or cure time; press open time; press opening and closing time; time of part ejection in relation to mold opening time. 

            Temperature is affected by the temperature of material entering the hopper; throat temperature; heat contributed by screw compression and rotation; heat absorbed from the cylinder and the setting arrangement of pyrometers in the heat zones; averaging of heat by continuous mixing and homogenizing up to injection time; mold temperatures; flow control of coolant in mold passages for desired temperatures; and temperature of the environment. 

            Pressures that require  consideration are the injection high pressure (the pressure needed to fill cavities to proper part density); the hold pressure (the pressure that is maintained on material during solidification and prevents backflow into the nozzle area); the back pressure, which influences mixing and feeding of material into the measuring chamber; and the clamp pressure, which achieves mold closing.