Twin pressure head

Fig. 8 Diagrammatic view of a twin pressure head (Patent Officine Morando) for a two layer extrusion method...

Extrusion using pressure head with twin outlets. Extrusion with one... 

Finally, the effect of process variables, such as screw speed, differs with twin-screw extruders. Due to positive conveyance, the output of starve-fed twin-screw extruders (Fig. 5.36 b is independent of screw speed and, to some degree, head pressure. Head pressures and melt temperatures of intermeshing twin-screw extruders are also not as sensitive to screw speed as they are in single-screw extruders. In contrast, nonintermeshing twin-screw extruders exhibit output characteristics that are similar to those of single-screw extruders. A comparison of the characteristics of single and intermeshing twin-screw extruders is summarized in Table 5.9. 

The actual cylinders and pistons of a two-headed pump are constructed in a very similar manner to the single piston pump with a sapphire piston and a stainless steel cylinder. Each cylinder is fitted with nonreturn valves both at the inlet and outlet. The cams that drive the two pistons are carefully cut to provide an increase in flow from one pump while the other pump is being filled to compensate for the loss of delivery during the refill process and thus, a fall in pressure. A diagram of a twin-headed pump is shown in figure 10. 

LKB manufacture a reciprocating twin-pump system with pump heads made of KEL-F. Gradients are formed by a low volume mixer which is located on the low pressure side of the pump, thereby... 

Rectification takes place in a twin-column rectifier. The air fed to the pressure column is rectified to pure nitrogen which leaves the column over-head and is liquefied in the condenser. The major part of the liquid nitrogen is used as reflux to keep rectification going while a smaller percentage can be withdrawn as liquid product. The low-pressure column derives its reflux also from the pressure column. Liquid streams are withdrawn from the central and bottom part and flashed into the top column. 

There are two systems of spray-up in common use twin-pot and catalyst injection. The twin-pot machine uses one pressure pot for catalysed resin and another for accelerated resin. They are simultaneously fed to the spray gun and individually sprayed each through its own nozzle. The two streams are adjusted to meet at about 300 mm in front of the mould surface where they mbc. As the two streams only meet outside the spray head premature gelation cannot occur. The twin pot machine is relatively simple to operate but has the disadvantage that the pressure pots require frequent replenishment which is time consuming and particularly inconvenient for large mouldings. 

Figure 5.36 The effect of screw speed and head pressure on the output of starve-fed, fully intermeshing twin-screw extruders. ...

An essential feature of the I.D.T. apparatus is the measurement of pressure at each end of the capillary system. The pressure at the outlet end is held constant, as it is connected to a reservoir of effectively constant hydrostatic head. The pressure measurements are made by all stainless steel variable inductance transducers (EMI model SE 1150/WG) energized by oscillator/demodulator units. The pressure transducer outputs are fed to a twin channel flat bed potentiometric recorder. The pressure measurements have a sensitivity better than 1 Nm" (0.1 mm H2O). 

The low head pressure generated by twin-screw extruders initially limited their use to processing of shear-sensitive material, such as PVC, and to compounding. 

Knowing the head pressure prior to running material will aid extrusion personnel in system design and semp. For example, a high HPI value may indicate that the placement of a vent port on a modular twin screw line should be located further back from the die to prevent melt flow. Or, it may simply indicate that the assumed screw speed should be lowered for safety reasons. A low HPI value may point to the need for a tighter screen pack design in order to promote more mixing. 

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