The back pressure is the resistance to backward movement of  the screw during preparation for a subsequent shot. This pressure is exerted by the material on the screw while it is being fed into the shot chamber. During rotation of the screw and the material under pressure, thorough mixing of the polymer is achieved, and some temperature increase also results. In dealing with heat-sensitive and shear-rate-insensitive materials, care must be taken to keep this value within prescribed limits. 

            There are two basic torque settings available on the machine. In practice, it has been found that the high torque setting is rarely adjusted and the low torque setting would be adjusted only if a highly liquid melt material is being molded, requiring high speeds of screw rotation. 

             This is related to the work input into a material; higher speeds are applied only when insufficient heat is absorbed from the cylinder for a particular shot. Heat-sensitive and shear-rate-insensitive materials do not tolerate the highest speeds. 

             There is a maximum vent depth beyond which the flow of material will not take place. However, this depth will be located away from the gate (by at least 90 to 180”). 

             Two  types  of  nozzles are available: the general-purpose  and ny- lon types. With the advent of screw-type injection machines and effective utilization of the melt  decompress  feature, the drooling present with a general-purpose nozzle while molding nylons can be effectively controlled. This is because the check-ring shutoff system fits the barrel properly to produce effective suction at the point of the nozzle outlet. 

             Materials  that  are moisture-sensitive and those that may pick up moisture for some other reason will have to be dried before molding. A drying temperature is used that will permit the removal of moisture without causing the granules to adhere to each other, behavior that could cause bridging  over the throat where  the screw picks up the material. It is also useful to set the water valve for cooling the throat so that its temperature will not be too low, causing condensation on the plastic, or too high, causing bridging. Attention to the correct setting of the water valve can yield savings in water and heat of plastication in the chamber. The preferred method of drying is the dehu- midifying process, whereby the humidity is removed and dry air supplied at the speci- fied conditions for each material. Also available are so called vented injection machines that are capable of removing moisture during the processing of the material. A simple test for moisture content has been developed by General Electric and is known as the T.V.I. test. 

              These data can be used in checking dimensions of parts, thus giving indirect verification that the setting of all parameters has been properly executed. 

              This value is used for such purposes as evaluating machine capacity in relation to polystyrene, screw travel, rate of injection, etc. 

              If a purging procedure or shutdown steps or any other precautionary  move is indicated  for a specific material, that should be suitably indicated under a similar heading. 

             Attaching of molds to platens should be done in a manner that will ensure retention of the mold in position without danger of shifting or loosening. Any change of position of a mold half will place an excessive burden on the leader pins and bushings that keep the halves aligned, thus causing wear on the pins and bushings and in time affecting the quality of the parts being molded. 

             The conventional method of holding mold halves in place is by employing mold clamps. The platens are tapped for bolts ranging from 1/2 to 1 in. in diameter, and the holes are laid out to an SPE standard design. 

              The forces holding  a mold  in the press have been analyzed, and the result is in Table 2-9. Only forged bolts with a yield strength of 120,000 psi (827 MPa) should be used. In order for each clamp to hold with equal force, a torque wrench is indicated. 

              When the calculations are made for an actual clamping system, the number of clamps should be divisible by four, since there are four clamping faces. For example, mounting a 300-lb (136-kg) mold with $-in. bolts would give 300/32 z, or 9.37 bolts. To be divisible by 4,12 clamps, or 3 on each side, are required. In all cases, the clamp surface should be parallel to the clamping slot and platen. The closer the holding bolt is placed to the flange of a mold, the higher is its holding power. 

              Moving a mold to the press and removing it to storage are normally done by means of  chains or wire-rope slings. These auxili- ary means for hoisting a weight are treated in technical handbooks under such headings as “Crane Chain and Hooks” and “Strength and Properties of Wire Rope.” Additionally, the Federal Government’s OSHA prescribes certain regulations for weight handling and makes the user liable to stiff penalties if they are not followed. 

              Under average conditions of a molding shop, the task of frequent inspection of the hoisting means, during as well as at the approaching end of their useful  life, should be assigned to one responsible person. This person should obtain literature from the suppliers of these devices and become familiar with such information and use it to instruct others in the safe handling of molds. Improperly lifted molds can be a hazard to workers and damage presses in the event they fall. They can also be damaged and rendered unusable. 

              Note: At this point in the instructions it is desirable to become familiar with the previously reviewed hydraulic system of the machine so that the following descriptions will be easier to comprehend.