Barrel control temperature

A first principle mathematical model of the extruder barrel and temperature control system was developed using time dependent partial differential equations in cylindrical coordinates in two spatial dimensions (r and z). There was no angular dependence in the temperature function (3T/30=O). The equation for this model is (from standard texts, i.e. 1-2) ... 

Reactive extruders and extrusion dies of different designs can be easily included in standard technological scheme of polymer production plants, such as those for polycaproamide synthesis, as shown in Fig. 4.39. In this case, a reactive material premixed in a tank 1 is fed into a static device 2 for prepolymerization, where part of the polymerization process takes place. Then the reactive mixture enters the extruder-reactor 3. The necessary temperature distribution is maintained along the extruder. Transfer of the reactive mass proceeds by a system of two coaxial screws mounted in series in a common barrel. Controlling the relative rotation speed of both screws provides the necessary residence time for the reactive mass in the extrader, so that the material reaching the outlet section of the die is a finished polymer. 

It is common for an extrusion line to be separated into several temperature control zones. The number of zones depends on the length of the barrel, the type of adapter or transfer line to the die, and the size and complexity of the die. An extruder may have as few as three or well over ten zones. Each zone, or circuit, contains up to four of the following components temperature controller, temperature sensor, heating unit, and cooling unit. Usually only barrel temperature zones utilize a cooling unit. 

Injection molding, to have higher output rates, corresponds to the heating zones on the barrel. For close tolerance in molding, the nozzle temperature must be precisely controlled with separate temperature controller to prevent freeze-off or drool. During processing, the hopper area should be with separate temperature controller, in order to not affect the ability of the sensor in the feed heating zone to control temperature. 

Plastic Temperature. Correct temperature and uniformity are crucial for a consistent process. Monitoring of plastic temperature is difficult and rarely utilized in the industry. Control of temperature is done via thermocouples partially embedded into the barrel wall, usually three to four along the length of the barrel. The actual molten plastic is not monitored for temperature and >90% of the temperature values provided as data are barrel wall temperatures that can be off by 25°C (45°F). Temperature control is done via proportional-integral-derivative (PID) controllers or PID algorithms on computer controlled presses. Calibration of thermocouples is seldom done. PID temperature control of the nozzle tips is also important, yet 40% of the industry uses variacs. 

Barrel (BBL) a barrel has traditionally been the standard liquid quantity of measurement in the petroleum industry for the production of oil. One barrel of oil equak 42 US gallons Basic Process Control System (BPCS) electronic, hydraulic, pneumatic, or programmable instruments and mechanisms that monitor and/or operate a facility or system to achieve a desired function, i.e., flow control, temperature regulation, etc., which are supervised by human observation... 

The odorless NF fluid is widely used in plastics and rubber machinery to precisely and uniformly control temperature in molds, barrels, dies, screws, platens, and calender rolls to 600F. It is highly efficient, cost effective and thermally stable. 

Figure 4 shows how the auto-tune PID controllers stabilized with a much different control interval for induction and band-heaters. With induction, temperature control was able to compensate promptly, each cycle, for the effect of the cold entering material. This was because the barreFs temperature at the measurement depth was affected more quickly by induction than by the introduction of cold material. By comparison, the band-heaters were unable to measurably affect the barrel s temperature within the short cycle interval, so PID control was forced to cycle at a much longer interval to produce a larger and slower, and therefore distinguishable, feedback temperature change. 

Loss of control of the product temperature can occur in cases of freezeout that occur over a large length of the barrel. Of course, as the layer becomes thicker, the loss of control becomes more severe. Product temperature may even increase as barrel zone temperature decreases. In extreme cases, the layer can be so thick that it is freed from the barrel walls by the flight and exits the extruder as solid particles in the melt stream. 

These consistent results occur in spite of the fact that only a single barrel metal temperature is used for the calculations. It is likely that the barrel had a temperature profile, especially near the ends. However, the barrel temperature was controlled by liquid medium, which helped to equalize it over the length of the extruder. 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

Immersion electrodes are the most common glass electrodes. These are roughly cylindrical and consist of a barrel or stem of inert glass that is sealed at the lower end to a tip, which is often hemispherical, of special pH-responsive glass. The tip is completely immersed in the solution during measurements. Miniature and microelectrodes are also used widely, particularly in physiological studies. Capillary electrodes permit the use of small samples and provide protection from exposure to air during the measurements, eg, for the determination of blood pH. This type of electrode may be provided with a water jacket for temperature control. 

CVD reactors can have one of several configurations. Each has particular advantages and disadvantages. Reactors that support wafers horizontally have difficulty controlling the deposition uniformity over all the exposed wafers. Reactors having vertical wafer support produce uniform deposition, but are mechanically complex. Barrel reactors are not suited for extended operation at temperatures greater than 1200°C. 

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