Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Patent
1998-03-27
2000-03-28
Soohoo, Tony G.
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
366100, B29C 4740, B29C 4576
Patent
active
060422602
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of carrying out continuous preparation processes on tightly meshing extruders rotating in the same sense such as twin-screw and multi-shaft screw-type extruders.
Tightly meshing twin-screw and multi-shaft screw-type extruders rotating in the same direction are used for continuous kneading processes with or without melting. Frequently, continuous degassing, mixing and expanding processes are integrated, too, and there are cases when the extruders are also used for reactions.
The products to be processed comprise plastics, resins, liquids, viscoplastic fluids, pulverulent and fibrous additives as well as food compounds. Discharging may take place for instance via filters and molding processes such as pelletization and section extruding.
Extruders of the type mentioned at the outset have been known, in which the screw diameter amounts to as much as 340 mm. The throughputs range from 5,000 to 35,000 kg/h at a ratio between the screw outer diameter and screw inner diameter (D.sub.a /D.sub.i) of 1.18 to 1.25 and 1.4 to 1.6, respectively. The ratio between torque and center distance.sup.3 of axes (m.sub.d /a.sup.3)--the so-called "torque density"--is in the range of 5 to 10. Depending on the size of the extruders, speeds of rotation are run, ranging from 200 to 500 rpm--in exceptional cases even up to 600 rpm.
Customarily, the design of the extruders is based on the principle of geometric and torque related similarity. Geometric similarity exists when the ratio D.sub.a /D.sub.i is constant; torque related similarity exists when the ratio M.sub.d /a.sup.3 is constant.
In addition to melt temperature and dwell time, the shear rate within the screw root filled with melt is another decisive factor for the dispersing, mixing and homogenization grade of the processed product.
In many processes the mixing, dispersing and homogenization grade will be the higher, the higher the shear rate. In today's state of extruder engineering, mean shear rates in the melting range of between 20 to 150 l/sec and mean product dwell times in the entire screw range of between 15 to 60 sec are customary in standard preparation processes.
In conventional extruders, the mean shear rates are restricted upwards by the screw speed of rotation and the so-called "volumetric ratio" represented by the ratio D.sub.a /D.sub.i. However, increasing shear rates are accompanied with higher specific values of the energy supplied, which may lead to inacceptably high melt temperatures. Together with high average dwell times of the product in the extruder, this may lead to quality reducing deterioration of the product in terms of thermal decomposition and cross-linking.
It is the object of the invention to put into practice quality increasing mean shear rates in the range of up to .gtoreq.1,000 l/sec accompanied with simultaneous reduction of the duration of action of temperature peaks in the product without possible occurrence of the difficulties described above.
This object is attained by the extruder being operated at a screw speed of rotation of at least 800 rpm while the so-called "torque density" (M.sub.d /a.sup.3) which can be induced is simultaneously increased by at least 11 Nm/cm.sup.3 and the volumetric ratio (D.sub.a /D.sub.i) is at least 1.5.
With the increased torque density (M.sub.d /a.sup.3) selected according to the invention of at least 11 Nm/cm.sup.3, the extruder can easily be operated at the high screw speeds of rotation without any inadmissibly high specific energy input resulting. Another advantage resides in a very high product throughput per time unit.
Suitably, the dwell time of the product in the extruder is less than 10 seconds.
In keeping with another embodiment of the invention, the extruder is operated at a screw speed of rotation of up to 3,000 rpm, while the so-called "torque density" (M.sub.d /a.sup.3) that can be induced is increased by up to 15 Nm/cm.sup.3 and the volume ratio is greater than or equal to 1.55 and the mean product dwell time is less than 2 seconds. This will give especially
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Burkhardt Ulrich
Haring Erwin
Heidemeyer Peter
Herter Rainer
Munz Rainer
Conte Robert F I
Krupp Werner & Pfleiderer GmbH
Soohoo Tony G.
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