Barrel cooling

Specimen Location Coolant piping connections in extrusion barrel cooling jacket... 

Figure 9.16 depicts the arrangement of connections in the extrusion barrel cooling jacket. Embrittlement and cracking of these stainless steel connections (Figs. 9.17 and 9.18) were occurring as frequently as every 4 to 6 weeks. 

Kramer, W. A., Extruder Barrel Cooling, SPE ANTEC Tech. Papers, 33, 34 (1987)... 

On completion of the reaction, the bomb should be allowed to cool to room temperature and the barrel cooled further to about —15 °C in an ice-salt or acetone-Cardice bath. The valve F may then be opened to release any pressure (fume cupboard), and the bomb dismantled and the contents removed for workup. 

Until the 1960s TSs (thermosets) were primarily molded using compression or transfer presses (Chapter 14). At that time screw injection machines with modifications were developed to process TSs. These modifications included low to zero compression for screw depths, deeper channel depths, short length to diameter screws (L/Ds), tool steel construction, barrel cooling with heat transfer fluids, and spiral down discharge ends in place of non-return valves.3... 

Make a clear file in the forge, using charcoal if it can be obtained, heat the barrel very carefully until small pieces of solder will be melted when placed on the inside of the thimble. Have the soldering copper heated, and by using it and applying the acid an even amount of the solder can be applied to the joint outside the thimble where it joins the barrel. When all are soldered let the barrel cool, remove the binding wires and wash with warm water to re-... 

The working of the plastic the screw generates additional heat which can be of such magnitude, for example, to permit extrusion to continue even if the power to the heaters is switched off. Because of the heat produced by mechanical working, many extruders today are equipped with a barrel cooling system the barrel must be provided with means to both add and extract heat. 

Practically all extruders have a barrel cooling system, such as air fans or water circulators, to remove excess heat from the polymer. However, it is best that the unwanted heat not be added to the polymer in the first place. Therefore, when the temperature controller in an extruder cooling system (or zone) calls for cooling often, or all the time, it indicates inefficiency in the system. 

When high cylinder or barrel temperatures are used, then modify the above procedure slightly so as to prevent thermal decomposition of the resin. For example, turn off the cylinder heaters Oeave the nozzle heater on), put any barrel cooling on maximum and then, periodically pump resin through the machine while it is cooling. Close the slide at the base of the feed hopper and purge the cylinder clean by pumping the screw dry... 

The rate of temperature change upon cooling v. is much slower in this case because it is passive cooling. In other words, the barrel cooling has not been activated the temperature drops simply as a result of heat losses in the system. If AT = 1°C, t<) = 2 min, Vj, = 6°C/min, and v. = 2°C/min, the maximum temperature swing is ATmax 17°C. Thus, the amplitude of the temperature fluctuation is cut in half as compared to the first case. However, even a temperature fluctuation of 17°C would still be considered unacceptable in most extrusion operations. 

Figure 7.94 shows the temperature profiles at three different screw speeds with and without conduction the heat flux is -10,000 W/m. At a screw speed of 10 rev/sec (600 rpm), the temperature drops about 2°C after a distance of 10 D. At a screw speed of 3.34 rev/sec (200 rpm), the melt temperature drops about 6.5°C at 0.83 rev/sec (50 rpm) the melt temperature drops about 27°C. Clearly, at low screw speeds, the melt temperature can be affected significantly by conduction through the barrel. The reason for the large effect of barrel cooling at low screw speed is because the residence time of the polymer increases with reducing screw speed. As a result, more time is available to remove heat from the polymer melt at low screw speed. 

Further trials using screw jacking following rapid barrel cooling compared sections from a conventional screw with the one fitted with the Maddock element. The feed material was LDPE with carbon black masterbatch. Figure 7.12 shows photomicrographs of microtomed channel cross sections taken every two turns and Figure 7.13 shows cross sections from the die adaptor. 

Complete screw design must of necessity take into account the overall energy balance of the extruder and therefore cooling must also be considered. There are probably as many (if not more) different variations of barrel cooling systems as there are extruder manufacturers, but the majority of them are based on the use of either air or water or a combination of the two. To assess the relative merits of the different cooling systems available, it is necessary to consider two basic aspects, namely their heat transfer capacity and their controllability. The question of heat transfer capacity is more easily considered by reference to a basic equation governing heat transfer in a system such as an extruder barrel. This may be represented by the following relationship ... 

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