This rate is measured in cubic inches per second. Many machine suppliers show this information as part of their specification sheet. In the case of 250 tons and 32 oz, the rate is shown as 22.5 cu in./sec, so that the time required to fill the complete shot of 59 cu in. is 

             59/ 22.5  = 2.62 sec

              The number of cubic inches of a plastic material injected per second, if not given in the machine specification, can be established by determining how many gallons per minute (gpm) are fed into the injection cylinder by the pump or pumps, and the diameter of the shooting piston. These quantities are, as a rule, shown on the hydraulic diagram of the machine. Since the injecting piston and screw are connected to each other, for each inch of piston travel there will be an inch of screw travel and a corresponding displacement of plastic volume. 

              The speed of piston travel therefore is the number of gallons of oil per minute delivered by the pump, divided by the area of the piston, Converting gpm to cubic inches per second and the piston area to square inches, we obtain 

              speed =231 cu in./gal x flow rate / 60 sec/min x area of piston 

             Using the 32-02 (0.91 kg), 250-ton machine as an example, in which the injection pump capacity is 60 gpm and the piston has a diameter of 8.75 in. (22.2 cm) or an area of 60.132 sq in., we have 

              speed = 231 x 60 / 60 x 60.132 = 3.84 inhec (9.8 cmisec) 

              so the full stroke will take 

              10 in./3.84 in./sec = 2.6 sec 

              and will displace in this machine 

              59/2.6 sec=22.7 cu in./sec (372 cu cmisec) 

              (The proportion was set up from the figures obtained above, under “Screw travel.”) Since the screw speed is equal to the piston speed, or 3.84 inhec, the number of cubic inches per second will be found using the ratio: 

              3.84/10 x 59 cu in. = 22.7 cu in./sec

              For the 14-oz shot of  polypropylene, wehave established that screw travel includingmelt decompress will be 5.5 in. The time forthis travel will be 

              total distance / speed=5.5 in./3.84 in./sec=1.43 sec 

              This is the total injection time needed for theabove shot and is the guide for the injectionhigh timer setting. 

              In some cases, the shots are not filled with the screw traveling at full speed, because if the injection pressure is set high enough to do this, it may cause flashing at the parting line. In such cases, the pressure is set to have the screw travel 90 to 95% of the distance at full speed, with the remainder of the stroke slowed down to allow more time for filling, causing the material to be less fluid and thus have less tendency to flash. This action will increase the time shown above (1.43 sec) by some amount that can be determined by the use of a stopwatch for the complete injection time. 

             Another way to accomplish the filling ofthe last 5 to 10% of the shot is to add a limitswitch at the desired distance from the endof the screw travel, and electrically signal thehold timer to take over the job of completing the shot, as well as maintaining a pressure on the material in the cavity until thegate is frozen shut. Some machines have thistype of limit switch for the injection stroke,so that a low volume pressure hold pump canbe signaled to replace the high-volume pumpand thereby slow down the screw travel at theend of the filling action of the cavities. In effect, the limit switch cuts on the injection lowtimer, instead of the injection high timer. Thisis a desirable feature and can easily be addedif not provided on an existing machine. 

             This hold pump is of particular value when the machine clamping capacity in relation to the projected area of the molded part does not provide a reasonable margin for viscosity variation in the plastic, and thus can allow flashing at the parting line. The pressure on the injection hold pump under its normal usage is considerably lower than the injection high-pressure pump. However, for the application of completing cavity filling, the pressure setting may have to be equal to or even higher than the injection high pressure. The use of the limit-switch system permits the calculation of the time required to fill the cavities with the low-volume pump.  

             The 250-ton press selected as an example has a low-volume pump of 17-gpm (0.06-cu mlmin) capacity. Therefore, the speed of screw travel with this pump will be lower in the proportion of pump capacities: speed =17/60x3.84 = 1.09 in.isec (2.8 cmisec) Seventeen gpm is the capacity of the hold pump, 60 gpm (0.23 cu m/min) is that of the high-pressure pump, and 3.84 in. (9.8 cm) is the distance traveled per second when the 60-gmp pump is active. 

              In this type of application, the hold pressure pump should be activated over a distance of 0.25 in. (0.6 cm) or less. If we use in our example a 0.25-in. distance, the time involved will be 0.25/1.09 = 0.23 sec for the hold pump during the screw travel distance of 0.25 in. The travel at high-pressure pump ca- pacity will now be 0.25 in. less; therefore, the time will be 5.25/5.5 x 1.43 = 1.36 sec, in proportion to the distance, so the corrected time is 1.36 sec plus 0.23 sec for the hold pump, giving 1.59 sec for the total injection time. 

              In this case, the injection high timer is ineffect bypassed, and the injection hold timerinitiates the subsequent machine functions. 

             There are molds with part configurations inwhich the rate of injection must be reduced to a value that will permit trouble-free filling of cavities. For this purpose, the injectionmachines are equipped with a flow controlvalve that is rated in gallons per minute andnormally has 10 settings; each increment represents one-tenth of the valve capacity. When the valve is set at a number other than zero, it indicates the number of tenths of the pump oilthat will be bypassed to the tank. In the machine chosen as an example, the control valve has a capacity of 45 gpm (0.17 cu m/min), and each division represents 4.5 gpm. Let us as-sume a setting of 3; then the bypassed oil will be 3 x 4.5 = 13.5 gpm (0.05 cu m/min). Thehigh-pressure pump will now deliver an effective volume of 60 gpm -13.5 gpm = 46.5 gpm(0.18 cu m/min). 

             The rate of injection with the control valvesetting at 3 will be 

             46.5/60 x 3.84 = 2.98 in./sec (7.57 cm/sec)

             and the time required for the screw to travel the 5.5 in. in the example will be 

             5.5/ 2.98 = 1.85 sec

             The number of cubic inches per second will be 

             59/10 x 2.98 = 17.58

             or 17.6 cu in./sec (289 cu cmhec).  

             Occasionally, there are jobs that require a certain number of cubic inches per second to be injected into a mold to ensure a good-quality product. If the mold is run in the same press, the recorded settings can be repeated. Frequently, it becomes necessary to transfer a mold to a press with different specifica- tions, in which the requirement of a specified number of cubic inches per second must be repeated. The following example points out how this can be accomplished.