Melt pressure control systems

FIGURE 1.52 Melt pressure control system ( Dynamic Feed ) using a conical needle with position control... 

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

For safety reasons, the pressure is set in such a way that the temperature is 5°C below the melting point of the product. On an hourly basis, the freeze-dryer automatically uses a drain valve to allow the pressure within the system to drop. A simultaneous fall in product temperature is indicative of the presence of frozen solvent that should thus be extracted during the sublimation or primary drying step. Detecting any residual solvent in this way avoids the risk of the product melting down. Before the desorption step is started, this pressure control cycle is repeated until no further drop in product temperature is observed. 

The operation of the injection and clamp units and other components of the injection molding machine (opening and closing of the mold and melting and injection of the polymer material) requires power, which is supplied by an electric motor. The orderly delivery of this power depends on auxiliary systems the hydraulic and control systems. The hydraulic system, the muscle for most maehines, transmits and controls the power from the electric motor to the various parts of the maehine. Maehine functions are regulated by a careful control of the flow, direction, and pressure of the hydraulic fluid. The elements of the hydraulic system for most injection molding machines are essentially the same fluid reservoir, pumps, valves, cylinders, hydraulic motors, and lines (Figure 11.8). 

Melt too cold] sensor error/control system error/lack temperature confirmation via hand-held pyrometer or laser sensor/cylinder too cold/screw rpm too slow/ back pressure too low/insufficient plasticizing capacity of machine/jnozzle too cold] /heating band fault/excessive flow length in mold. 

Ultrasonic measurements have also proven useful in the characterization of polymer melts (see Section 4.3.3.1). The ultrasonic transit time ( Laufzeit ) is dependent on the elastic properties of the material, pressure, temperature, chemical composition, and structure. It has been found [37] that the ultrasonic transit time is a sensitive measure of the condition of the polymer melt, in particular melt homogeneity. In a process control system, the ultrasonic transit time can provide a more useful feedback control signal than a single melt temperature measurement. 

The pressure relief system was not provided primarily to meet code requirements but rather Is an Integral part of the over-all protection system which includes the reactor power setback and scram systems Strict adherence to the code is impractical because the maximum thermal energy output of the reactor cannot be defined. Further at the hifpier conceivable powers fuel melting would probably occur in any event. Prevention of a fuel melting accident with consequent release of fission products depends primarily on the nuclear control and cool ant supply system and only Incidentally on the pressure relief systems. 3 Shutoff valves are not provided at the end of each instrument take-off connection, and the minimum size of instrument take-off connections is not necessarily l/2 or 3/if Inch as required by the ASA Code. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

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