Best Lab Equipment

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twin screw extruder

A twin screw extruder type batch mixer that is comprised of two screws parallel to each other with the distance between the two screw center lines being larger or equal than the half of the sum of two screw diameters and the two screws conveying material in the opposite direction during mixing and reacting stage and the same direction during charging and discharging stages; and a barrel casing completely enclosing said screws which can run clockwise and counter-clockwise independently.
he most widely used batch mixer for bulk polymer materials mixing is rotor type mixer called internal mixer which was originally designed by Banbury (Banbury, F. H., U.S. Pat. No. 1,200,070, 1916). Over the years since its inception, numerous modifications in design have been introduced, but the basic principle remains the same. They are all comprised of two parallel kneading rotors, each rotating within the corresponding region of a double trough or W-shaped chamber, but with the walls of the chamber continuing upwards over the rotors to constitute a closed mixing chamber. Although widely used in rubber and plastics compounding, these types of mixers have some shortcomings. One of them is material discharge. In these machines, at the completion of the mixing cycle, the discharge door is opened and the mass was discharged by gravity as shapeless lump or lumps which are difficult to handle for the next operation. This discharge mechanism makes it necessary to interpose at least one more intermediate processing stage before final shaping or next operation. It is also difficult for some materials to discharge and discharge completely. For small laboratory internal mixer machines as produced by C. W. Brabender Inc. and Haake Buchler Instruments Inc., there is no discharge door. The discharge has to be performed by disassembling the mixer and then removing the material manually. Very often, time spent on recovering the final material and cleaning the machine is more than testing time.

Another shortcoming with a internal mixer is in tts use in evaluating materials for an extrusion process. One of the major uses of a laboratory internal mixer is to test the processibility of thermoplastics, thermosets and elastomers, and transfer the information to a continuous production process such as extrusion. As both the geometry of a internal mixer and mixing mechanism are different from those of an extruder, it is often difficult to predict extrusion results from information learnt through a batch internal mixer study. Scott and Macosko (Scott, C. E. and Macosko, C. W., Polymer Engineering and Science, 33, 1065, 1993) developed a batch mixer named the recirculating screw mixer(RSM) based on single screw extrusion technology. The mixing mechanism in RSM is more representative of that in a continuous single screw extruder and thus results obtained are more meaningful in scale up for continuous single screw extrusion operation. However, a single screw extruder and RSM do not have very good mixing capabilities. More polymer mixing and reaction processes are performed in twin screw extruders which have much better mixing capabilities. Thus a twin screw type batch mixer should provide not only a good batch mixer but also a tool with which useful information which can be easily transformed to continuous twin screw extrusion can be learnt.

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