1. Closing the gates actuates a limit switch hat, in  turn, brings about rapid  forward movement on the clamp ram. 

         2. When the clamp reaches a position a couple of inches before closing, it activates another limit switch that causes clamp slow-down; finally, at a distance of about 1/16 in. before tight closing, a third limit switch is activated that signals the high pressure (2,000 to 3,000 psi) needed to squeeze the mold shut. 

         3. When pressure is fully built up behind the clamp, a pressure switch closes its contacts and initiates the following: The nozzle valve (if used) is opened, the injection high pressure  timer  started, and injection  high pressure movement of the extruder plunger action initiated. 

          4. When the injection high-pressure timer times out, it initiates the “injection overall timer,” which for several seconds maintainspressure on the material in the cavity. 

          5. When the injection overall timer times out, the melt decompression timer  starts. When melt decompression times out, the nozzle valve (if used) closes, the extruder startsturning, preparing the plastic for the following shot, and the clamp high pressure dropsto low hold. 

          6. While  turning and feeding the plastic into the shot chamber, the extruder moves backward (to provide space for the shot) until it contacts a limit switch that causes it to stop.  

          7. The overall timer or clamp timer times out, bringing  about  slow  opening  of  the clamp. 

          8. The opening clamp activates a limit switch that causes its rapid reverse movement until another limit switch is contacted thatslows down the clamp travel to the point atwhich the final limit switch contact provides the stop for the open position.  

          9. A  clamp-open timer is  provided  that either sets a time for removal of parts from the mold or, in the case of automatic (contin- uous) molding, can be energized by the reverse stop limit switch to perform the same electrical function as performed by manual gate closing and the activation of the limit switch by the gate.   

           All the limit switches and timers carryout their commands in an orderly manner,and any interference with this systematic arrangement by pushing a control button willthrow the plan out of order. There are certain steps required to restore the orderlyworking of the machine, but unfortunately,these steps vary from machine to machine.When we recognize that each timer alonecan have  three modes of operation, upontiming out, for resetting to zero for the fol-lowing restart, we realize that extreme caremust be exercised in restarting a machine after interruption. Close attention to the detailsof machine operation is very much in orderhere. 

           Repeating the cycle in a consistent manner is obviously the major responsibility of the machine operator. Also, certain observations must be made that will lead to a better understanding of the process and will aid the worker’s advancement in the field. Certain details require attention: 

            1. A machine in good working order should produce no unusual noises. It should close the mold by rapid movement of the ram, slow down as the mold faces come within in. of each other, and finally shut the mold by squeezing action under high pressure (no banging). During mold opening, about the first half inch should be done slowly, followed by  rapid  movement  up to the distance  at which ejection begins and then slowing down to a stop at the open position. 

            2. The tie rods of the machine and leader pins of the mold should be adequately lubricated to prevent excessive wear and associated problems. 

            3. The temperature of the hydraulic oil should be within the limits on the gauge mounted on the machine; overheated oil will bring about higher leakage in hydraulic pumps and valves, thereby making it difficult to maintain the required pressure for injection and clamping cylinders. Maintenance of  constant pressure on the components is an important factor in producing acceptable parts. 

            4. The extruder screw travel distance, forward and reverse, should be repeatable ensuring reproducible shot volume in mold filling. If screw travel is not in the normal forward position, not all the required volume of material is injected into the mold, with the result that parts are not dense, excessive shrinkage takes place, and the surface is not smooth. An increase in the backtravel of  the screw may cause an excess of material to be delivered and may overpack and flash the parts, causing enlarged dimensions, waste material, and possibly the need for a deflashing operation. Adequacy of the supply of material in the hopper should be checked when it is expected to reach a low level. 

           5. The injection high pressure, which can be read by depressing a button (at the hydraulic panel for injection pressure) during the injection time, should be checked for deviation from the required setup reading. Uniform pressure on the plastic in the moldis a very important determinant of productquality. 

           6. Temperature settings for the injectioncylinder at each zone should be recordedand checked at intervals of about 4 h tosee that unexpected variations are not introduced. A plastic material is not a pure chemical of certain description, but encompassesall kinds of additives (colorants, plasticizersfor flow, flame retardants, ultraviolet stabilizers, antioxidants, etc.), so the heating temperature must be confined within limits, notjust for the sake of the basic plastic, but alsoin the interests of protecting the additives.Excessive temperature and/or prolonged exposure to that of the normal melt heat cancause gassing, degradation of  the material,and change in flow properties, all of whichcan have a most undesirable effect on parts. 

           Automatic operation of  the thermoplastic injection screw machine is in every respect the same as that for the semiautomatic method, except that the stop limit switch for the clamp ram will initiate the clamp-open timer, which in turn will restart the cycle while the gate stays closed. 

           Molds that have been designed and tested for automatic operation require only intermittent observation to ensure that everything is working in an approved manner. The details requiring attention in the semiautomatic operation also apply to this mode, but the operator in this case will be concerned with checking product quality, ensuring an adequate supply of granular plastic material, and removing the molded parts to a designated station, in addition to these details. The duties of an operator can be to perform auxiliary operations, if necessary, at a single press or to attend to a number of cavities and required checking for quality. An operator can attend 4 to 16 presses. 

            A slight modification in the way a mold functions can enable automatic operationand thus improve productivity. Automatically operated molds usually result in better and more consistent quality and fewer rejects of parts. In most cases, mold life is also enhanced. 

            The thermoset injection machine is, from the operator’s point of view, very similar to a thermoplastic machine. There are, however, some additional points of concern: 

            1. The material content in the hopper should not fall below the half-point, so that there is always a sufficient weight of material to exert a pressure that will ensure good flow to the throat. 

            2. The temperature in the cylinder is critical. It must be observed that no increase inthe setting occurs that could cause hardeningof the plastic in the chamber, since this couldcause the operation to be interrupted. 

            3. The nozzle of the cylinder must be maintained at a low temperature to prevent hardening of the material in it. This is usually accomplished by retracting the nozzle from the sprue bushing of the mold. (The mold is usually at temperatures of 300°F and up, depending on the material.) The nozzle can also be maintained at a low temperature by incorporating a circulating coolant in it. Whatever the method employed, it must be seen that the material in the nozzle is maintained in soft condition to ensure free flow for each shot.