In the discussion of clamp size vs. counteracting pressure generated in the cavity during Injection Molding, it was remarked that the average force used to straighten out mold faces amounts to about 10% of clamp capacity. The question arises, how do we determine the actual force for full contact of mold faces if the suspicion exists that the case under investigation wastes a higher percentage of clamp force than the
10% cited? The following steps will provide a reasonably close answer.

          In order to maintain the integrity of the land area outside the cavity, a pressure of 3½;  tonsisq in, is allowed for steels, Bhn 300 and 5 tons/sq in. for H13 heat-treated steel (or similar tool steels). These values not only lead to long tool life, but also provide enough concentrated pressure to give the mold effective closing force. To test the size of the force needed to obtain good contact between faces  of the mold halves, we first see that the land area is so dimensioned as to give approximately 3;  or 5 tons/sq in. (depending on the steel).

           Having verified this, we take a piece of paper whose area is the same as the mold base, of 0.003- to 0.005-in. thickness, cut out the shape of the cavity, and place it between the mold halves. Applying a force of f ton/sq in. by reducing the clamp pressure, we close the press, and upon opening it, we check to see if the impression is uniform all over the contact area of the paper. If  contact is lacking in any part of  the land circumference, the test should be repeated at increased pressure.
The increase should be made in increments of 5 tons of clamp size until complete contact is established. The tonnage read when the impression on the paper covers the full circum-ference of the cavity is the tonnage wasted straightening the mold. The difference  between it and rated capacity is the amount left to keep the mold from opening during injection of the fluid plastic.

            Let us continue with the example in which we decided that the clamp would keep a mold closed with 110 sq in. (710 sq cm) of projected area. The rectangular 110-sq in. part will have dimensions of 10 in. x 11 in. and a perimeter of 2x 11 in. +2x loin., or42in. (107cm). We are working with a mold of 300-Bhn hardness. The square inches are calculated as follows:

            tonnage = area x 3.5
or
            250 = A x 3.5

and 
            A=250/3.5=71.4 sq in. (461 sq cm)

            A is expressed as perimeter times width of land, from which the width is calculated:

            71.4 = 42 x W

            W = 71.4 /42=1.7 in. (4.3 cm)

            When contact of the 1.7 in. x 42 in. land area is uniform after being compressed with 1/3 tonhqin. on 71.4  sqin., or 23.8 tons of clamping force, we have obtained the tonnage needed to straighten out the mold halves. Otherwise, the clamp size must be increased in steps of 5 tons until good contact is observed, and a reading taken. If, for example, this reading were 33.8 tons, then about 216 tons would be available to prevent the mold from opening and the 110-sq-in. (710-
sq-cm) cavity from flashing-a  pressure that under normal conditions would be expected to keep the mold closed.

            A time element that deserves more consideration than it normally
receives is residence time in the heating chamber to which a material is exposed during molding.  

            The average chamber with an L/ D (length-to-diameter) ratio of 20/1 has a volume twice its rated capacity. Thus, a32-oz (0.9-kg) nominal machine with about 59-cu in. (968-cu cm) actual  chamber volume  would  have about 118-cu in. capacity with the screw in the full forward position. If the full shot (32 oz) had a cycle time of 60 sec (1 min), the material on the screw would be exposed to the full heat for 2 min. If the shot were only 16 oz and the cycle half a minute, the exposure would still be only 2 min because of the reduced cycle time. With a shot of 8 oz (0.2 kg) and the cycle again half a minute, the exposure would be double the 2 min, or a total of 4 min. This length of time may be excessive for some materials and can cause degradation of properties. Whenever the residence time is on the high side and the danger of polymer damage exists, corrective measures must be taken.

            The most important corrective step is to keep the heat derived from the work of plastication to a minimum. This means that the screw rotation speed should be at the low end, the back pressure should be as low as practical, and pressure drops (from such sources as small nozzle diameter, small sprue, small runners, small gates, rough finish in runners, sharp corners at bends, and rough surfaces of cavity and core) should be minimized, so that the mechanical energy converted into heat
will be at the lowest possible level. In addition, cylinder temperatures should be arranged to be as low as possible in the lead section area, with a gradual increase toward the metering portion to the level required for adequate melt temperature.

             If all these measures do not remedy the  problem, then the only relief can come from a machine with a cylinder of lower shot capacity.