Let us take as an example a part molded in a 250-ton press; the material used is polyethylene. The clamping pressure that is available for keeping the mold closed, in actual terms, is not 250 tons, but on the average 10% less, or about 225 tons. The reason for this is that molding conditions are never perfect; for example, the press platens are not perfectly parallel, the mold thicknesses from front to back are not exactly the same at all points, the guide pins and bushings may not be perfectly aligned. Such deviations from ideality use up a certain part of  the clamping force to get the mold tightly closed, so that both mold halves make intimate contact to prevent material leakage. Observations under actual operating conditions indicate that 10% of clamp capacity may be considered a reasonable estimate of the force used to straighten mold faces and bring them to the close condition. In the case of polyethylene, the usual requirement of clamp force is 2 tons/sq in. of projected mold area. In the selected example, the projected mold area should be 225/2 = 112.5 or, in round figures, 110 sq in. The force that can develop in the cavity should be around 220 tons maximum in order to prevent leakage from the cavity (flashing). This means that 220 tons or 440,000 lb = P x 110, or

           p= 440,000/110 = 4,000 psi (28 MPa) = pressure in cavity

            The parts we are molding will be 0.090 in. (0.23 cm) thick in the shape of a
box, and the material content will be 25 cu in. (410 cu cm). The recommended gate depth size is two-thirds the part thickness, and 2 gate widths is twice this depth. The gate area will be 0.060 x 0.120 sq in. (0.15 x 0.30 sq cm). What should the injection pressure gauge setting be? The pressure  that is indicated on the injection gauge is  that in  front of  the screw when the material is being injected from the measuring chamber into the mold. This pressure on the average molded product is about 50% higher than the average pressure in the cavity, because of  the pressure drop in the nozzle, sprue bushing, runner, and gate. This would make the injection pressure
gauge reading 6,000 psi (41 MPa). The injection time would be

            25 cu in. (size of our shot)/22.5 cu in./sec (from machine data) = 1.1 sec 

            Let us now assume that the prescribed pressure and time of filling did not produce complete parts. This would indicate that the gates could not accommodate so much material in 1.1 sec. 

            We shall apply the Newtonian  flow formula. which reads as follows:

            

            where Q = material flow, cu in./sec R = radius of cylinder (gate) through
which flow takes place, in.

            L = length of cylinder (gate), in.
            u=scosity, lb . secfin. 
            h = height of rectangular duct (gate), in.
            w= width of rectangular duct (gate) (usually 2h), in.
            p= pressure, psi

            The flow formula applies to viscoelastic materials such as thermoplastics when under one set of conditions (pressure and viscosity). In the molding conditions that we have set up, the pressure and viscosity will be the same as on the first trial run, and we shall change gate dimensions to improve the gate’s ability to accommodate twice the amount of material in the same time span. Since the volume per second increases as the fourth power of the gate depth, raising this dimension 19% will double the capacity of flow in the same time period. All other factors will remain the same. Thus, the gate will now be 0.071 x 0.143 sq in. (0.18 x 0.36 sq cm). This small change in size should have no effect on degating or any other aspect of the molding parameters.

            This modification should result in filled-out cavities; if  a small cushion is available and the hold pressure is set at about 1,000 psi higher than the injection high pressure, our parts should be of the desired quality.

            This example points out that an analysis of machine specifications and moldability features of the mold can lead to an arrangemen that will produce quality products, saving on power as well as wear and tear on machines by using lower injection pressure.

            Applying pressure transducers in strategic mold locations can lead to a more accurate determination of prevailing molding conditions. (See Chap. 7 on process control technology.)