Flow Control Reciprocating Machines

Cylinder loading changes the point in the cycle at which the inlet valve closes. If set at 75 % load, on reaching BDC the valve remains open. The piston begins the compression stroke but no actual compression takes places. Instead gas leaves via the inlet valve until it closes at point C75. At this point true compression begins but on a gas volume that is 75 % of maximum. The load can similarly be reduced to 50 % or 25 % of maximum. Cylinder loading is normally set manually local to the machine. However, there are examples of it being automated to provide true flow control. 

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Use is made of screw tip valves, popularly called non-return valve, ball check valve, or sliding ring valve. They are used in reciprocating injection and injection blow molding machines (IMM and IBMM) to control the melt flow in one direction (Chapter 4). There is also the smearhead for IMM, IBMM, and extruder. Back flow will not occur... 

Injection molding is a cyclic discontinuous process used to form three-dimensional plastic parts (25. 26). Both thermoplastics and thermosets are injection molded. The process is shown schematically in Figure 11 for a reciprocating screw injection molder. This type of a machine has a screw rotating in a barrel similar to an extruder. However, here the melt flow is discontinuous and controlled via a check valve at the tip of the screw. Material is melted in the extruder, and the melt is accumulated in front of the screw tip until sufficient melt is at hand to fill the mold cavity (cavities). 

A reciprocating screw machine is shown in Figure 7 in which materia flows in the channel of an helical screw the entire length prior to injection. The rotational flow and close contact with the heated barrel bring the material to a uniform temperature and metering, homogeneity, injection pressure control, reproducibility and cycle time are thereby improved ... 

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