In the Injection Molding operation, a granular plastic material is softened by heat sothat it will flow under pressure and can bedelivered to a tightly closed mold, where it isheld for a specified time. The mold is maintained at a temperature that will permit theinjected material to become solid in a shorttime. After a prescribed  time interval, themold is opened and the injected material released as a finished product. 

           If we can clearly understand and picture the basic process, the more involved actual operations will be easy to understandand remember. The general description justgiven applies to thermoplastic as well as thermoset materials (see Chap. 6 for details onmaterials). 

           Now let us describe the operation of a machine in greater detail, starting with an injection screw machine for thermoplastics arranged for semiautomatic operation. Hard plastic granules are delivered  to a hopper, from which they  are fed through  a throat onto a rotating screw. The screw moves and compresses the material through  a heated chamber, where the granules soften to such a degree that they become fluid and can be delivered to a section of the heating chamber known as the measuring chamber. In addition to turning, the screw will on proper signal stop its rotation and move in a forward or reverse direction as a plunger. 

           When enough material for the mold cavity is supplied to the measuring chamber, as determined by the controlled distance of the backward-moving screw, an electrical command is given to it to act as a plunger and inject the fluid material into the tightly closed mold. The mold is maintained at a relatively low temperature that will cause the plastic to become rigid after a set length of curing time. Then the mold opens, and at the same time, the operator causes the gate to open, the parts are ejected from the mold (sometimes into the hands of the operator), the mold is checked to see that it is fully clear of plastic, the gate is closed again, and a new cycle is started. 

           The molded parts are briefly checked for quality and consistency in appearance and disposed of  either for storage or for other operations, such as gauging, auxiliary operations (hot stamping, etc.), or packaging. Normally, the work at the press is planned so that the attendant is kept occupied during the cycle; in this way, consistent results in part quality, cycle time, and safety of operation can be anticipated. The operation of  cycles becomes repetitive, and the attendant should exert every effort to have the motions organized and coordinated so that variables will not be introduced that could influence the consistency of quality and uniformity in the cycle. The best results are obtained when all elements in a cycle are repeated consistently from shot to shot. 

           In spite of  all precautions taken by the operator and setup person with respect to all machine parameters, there is occasionallysome need to interrupt the cycle. While anopen gate will prevent the starting of a cycle, there may be other reasons for stoppingthe motor and pumps, disconnecting electrical units on the chamber from the power, etc.Therefore, the operator should know how toactivate some of the switches of the controlpanel. Only those switches that should be activated in an emergency by the machine operator will be described here. 

           The “emergency” button, when activated and held in the proper position, will causethe clamp to return to the starting position.Opening the gate to correct the problem that necessitated the use of the emergency buttonshould reset the machine to the operating condition that existed before the interruption. Some machines may require the pushing of a “cycle reset” button before normaloperation can be restarted. If the correction, for example, requires removing an obstruction between mold halves or correction of a minor mold malfunction that can be accomplished in about 2 min, it should be possible to continue running the machine in a normal mannerby simply closing the gate and, if appropriate, pushing the cycle reset button.  

           A sudden oil leak in the hydraulic systemwould call for pushing the “motor stop” button in order to eliminate pressure in the hydraulic system and thus keep to a minimumthe loss of oil and its spread to the shop floorwhere it may cause safety hazards. The motostop may also have to be activated if unusuanoises develop on the injection end of  themachine, indicating some problems with therunning mechanisms. 

            A sizable leak of plastic material between machine nozzle and mold or anywhere on thefront portion of the injection cylinder wouldindicate an undesirable condition that couldlead to variation in the feed to the mold, thuscausing defective parts. The reasons for leakages of plastic material must be determinedand eliminated. The machine operator would turn the selector switch of  the extruder to“extruder-off,’’ stopping all action and leaving it  to the supervisor to take correctivemeasures. 

             Any change of  a button position on the control panel will bring about a modification in the sequence of operations in the electrical circuit, and consequently in the hydraulic circuit, that produce the orderly movement of components during a cycle. Definite information cannot be provided here for restoring machine operation after an interruption, because considerable variation exists among makes and models of  machines. This information is best obtained from supervising personnel, who have access to instructional manuals and wiring diagrams for each machine. When  given  an explanation  about restarting a machine after a specified interruption, the operator should make notes and save them for future reference. One should not rely on memory alone for such vital instructions. 

            We can better appreciate the above observations by considering a condensed version of sequences in the molding operation. Early in the chapter, the principle of molding was described with reference to the plastic material and its movement  in and out of  the mold. Here we will concern ourselves with switches and timers that accurately control the sequence of every action performed by the machine.