1. Extrusion Principles of the (Single) Screw Rubber Extruder and Gear pump The single screw rubber extruder is a constant pressure type pump that softens, pressurizes, and extrudes rubber compounds using frictional and shearing forces, between the rubber and barrel liner wall, with which the greater the extrusion pressure, the smaller the output becomes. The gear pump, on the other hand, is a constant output type extrusion pump, that outputs the rubber compound filling the space between the pump's revolving gear teeth by means of engaging the gear teeth, with which output remains constant even when extrusion pressure fluctuates (see Fig. 1). With gear pumps, pulsation proportional to the number of teeth on the gear is generated in the rubber flow, because the rate at which volume is compressed by the engagement of the gear pump's teeth is not constant, even when the gear is revolving at constant speed. On the other hand, although the screw type rubber extruder does not have any factor causing such mechanical pulsation, fluctuation in the coefficient of friction (due to the influence of temperature, characteristics of rubber material, and operating conditions, etc.) between the rubber, the liner, and screw wall makes it conducive for extrusion output to fluctuate.  2. Characteristics of the Single screw Extruder (1) Along with calender rolls, (single screw) rubber extruders are the most extensively used equipment in the field of continuous rubber molding. Rubber extrusion exhibits the following characteristics as it is subject to thrust pressure caused by friction between the rubber compound and barrel wall, and shearing resistance pressure at the outlet die part. (a) The greater the output pressure gets, the smaller the specific output (output/screw speed: kg/hr/rpm) becomes, and the higher the output rubber temperature rises as the amount of shearing induced heat goes up. (b) As the screw's rotation becomes faster, shearing generated heat increases and output temperature rises. The output capacity of a rubber extruder is often limited by the output rubber temperature. (c) When the viscosity level of rubber increases, the resistance pressure at the outlet die part increases, and specific output goes down. (2) Air drawn into compounds extruded by rubber extruders (a) Due to the degassing effect of its feed zone, the screw of a properly designed (single screw) rubber extruder attracts little air into its extruded rubber (see Fig. 2) (b) When an extensive level of degassing is required, a screw that has a vent capability is built in. (3) Self feed capability Today's single screw rubber extruder comes with a feed roller unit, and thus possesses the ability to self-feed itself with rubber. (4) Sealed part of high pressure rubber path structured without moving parts The high-pressure pathway, through which rubber flows between the screw exit and output die, does not contain any moving part, and has a sealed structure with a simple design. In addition, no rubber leaks out of this sealed structure. (5) Kneading effect While the screw itself exerts some kneading effect, a screw with an even higher kneading capability designed into it can also be built in to the machine. (6) Retention time As the term "L/D ratio" for the screw's length indicates, the pressure raising capability and output are proportional to the length of the screw, for which reason the screw of a high-capacity extruder tends to be longer and retention time for such extruder tends to be lengthier as well. (7) Self cleaning feature Since an extruder screw possesses a self-cleaning feature, albeit a weak one, there is no need to remove the screw and clean it every time the type of material is changed. (8) Rise time when commencing rotation Because time is needed for the temperatures of the screw, barrel, and head to stabilize, it takes 3 to 10 minutes after screw rotation is commenced to start an operation. 
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