Let us assume that we wish to maintain the 17.6 cu in./sec in a press that will
have a capacity of 30 cu inhec. The injection pump capacity is 75 gpm; the control-valve
capacity is also 75 gpm.

          We can set up a proportion as follows:

           17.6 cu in./30.0 cu in./sec = x / 75
or
           x=17.6 x 75/30.0 = 44.0 gpm (0.17 m³/m) 
           Thus, 44.0 gpm is needed to deliver 17.6 cu in./sec in the new press. Subtracting 44.0 from 75, we have to dispose of 31 gpm (0.12 m3/m). The control valves with setting increments of 7.5 gpm will call for 31.00/7.5 = 4.1 divisions, which will result in the desired 17.6-cu in./sec (289-cu cm/sec) rate of injected material.

           The information developed above is not only useful for setup purposes, but can also be instrumental in diagnosing potential problems in machine performance. For example, if the time of screw travel is well above the established value, that indicates a decrease in the volume of oil delivered to the injection cylinder and suggests possible pump wear.

           The various calculations may appear lengthy. However, all the needed information can be organized in chart form for each machine and thus be readily available for application to a specific job. In practice, the following will be needed

           1. Converting machine shot capacity into cubic inches 
           2. Finding the screw diameter and its area, to give the theoretical travel distance of the feed screw (melt decompress not included) 
           3. Speed of screw travel
           4. Flow rate in cubic inches per second
           5. Converting the weight of a shot for a job into cubic inches

           With the above information applied to the job at hand, we can determine the distance that screw travel is increased by the distance of melt decompress, the time needed to inject material, the timer setting for injection high pressure, and adjustments in pressure or speed of injection if necessary.

            For materials that are known to be shear-rate-insensitive and/or heat-sensitive, the setting of the back pressure is important. It is also significant for other materials, but to a lesser degree. For a better understanding of this problem, let us first explain how the injection pump pressure is reflected in the material pressure in front of the screw plunger and in the mold cavity.

            The force that causes the piston in the injection cylinder to move forward is the same force that moves the plasticating screw, since they are connected to each other. The force that moves the cylinder piston is the area of the piston in square inches multiplied by the pounds per square inch of  pump pressure. The above force is also equal to the area of the plasticating screw multiplied by the injection pounds per square inch on the material. Putting this information in equation form, we have

            area of piston x pump pressure = area of screw x pressure on material

            If we use the 250 ton, 32 oz. press as an example, where the screw  diameter is 2.75 in. (7 cm), the piston diameter 8; in. (22.2 cm), and the pump pressure 2,100 psi (14.5 MPa), we obtain, substituting the values in the above formula,

            60.132 x 2,100 = 5.9396 x pressure on material

             pressure on material=60.132 x 2,100/5.9396=21,260 psi (146.5 MPa)

             Since the force on the piston has to overcome its own friction and that of the screw, the actual pressure on the material will be reduced from 21,260 to about 21,000 psi. We can say that the multiplier of pump pressure against the cavity to obtain the material pressure is about 10 for a machine with the above specifications.

             The setting of the back pressure as read on the injection-pressure gauge is on the order of 50 to 100 psi (0.34 to 0.68 MPa). If we use the multiplier of 10, the pressure on the material in front of the screw plunger will be 500 to 1,000 psi. With the material in a highly fluid condition, these pressures are adequate for mixing the material thoroughly, driving out the gases, and measuring a reasonably accurate volume for a shot. The pressures on the material can climb as high as 5,000 psi (34 MPa) (500-psi gauge reading), but pressures higher than necessary can cause excessive drooling at the nozzle, overheating the material in the measuring chamber with resultant byproducts, and consequent molding problems. Such pressure settings should be used with care, especially when we consider that the readings are made on the dial portion of the gauge, which may not be very accurate.

             It was mentioned that the injection high timer setting should correspond to the maximum rate of injection of the machine. In the case of the 250-ton press, according to press specification, the time of injection would be equal to the volume of the injection chamber divided by the injection rate:

             59 cu in. / 22.5 cu inhec=2.62 sec 

             If the material is injected within this period, it will be quite fluid throughout  the cavity, and for practical purposes the solidification and cooling should occur in a uniform manner throughout the part. Pressure will also be applied uniformly over the molding surfaces. Both these conditions will result in good flow welds, minimal stresses in the part, and favorable appearance. On the other hand, when filling of the cavity takes 3 sec or more, the portion around the gate starts solidifying before the forward-moving material has filled the cavity, and this causes a decrease in the opening for material flow, as well as a differential rate of cooling of part surfaces.
In practical terms, higher injection pressures are needed, which cause stresses in the part and unfavorable conditions for self-welding of the flow, thereby creating poor and visible welds and a finished product whose appearance does not reflect the finish of the mold.       

             If the injection speed is such that the material is fluid throughout the cavity, even for a very short time, that may tend to cause mold opening and flashing. This indicates that the practical values of clamping pressure for the mold projected area do not hold-for example, the 2 tons/sq in. of cavity projected area for polyethylene. Since fast injection offers many advantages in product properties, we must beware of such undesirable side effects as flashing, poor dimensional control, and waste of materials. All these occur because the pressure generated in the cavity exceeds
that of the clamp.