Agitating – Rubber or heavy plastic working – With specified feed means
Reexamination Certificate
2002-10-07
2003-11-11
Soohoo, Tony G. (Department: 1723)
Agitating
Rubber or heavy plastic working
With specified feed means
C366S076930, C366S176300, C425S382400
Reexamination Certificate
active
06644845
ABSTRACT:
The present invention relates to a buffer tank for supplying an extrusion device with a polymer melt in a substantially constant manner, the buffer tank comprising at least one polymer melt entry opening, at least one polymer melt exit opening, and at least one compensation chamber that is arranged between polymer melt entry opening and polymer melt exit opening and has the polymer melt flowing therethrough, and whose volume is variable in dependence upon a pressure prevailing in the polymer melt.
Such buffer tanks are generally used in extrusion devices which must be fed with a mostly high-viscosity polymer melt at a constant pressure, if possible. A special use of such buffer tanks for extrusion devices is encountered in the textile industry in the field of spinning systems and spinning machines, where the polymer melt is a spinning solution consisting of cellulose, amine oxides, such as N-methylmorpholine-N oxide (NMMO), and water. The spinning solution is spun by the spinning machines into yarns. Other applications of the buffer tank can be found in blow molding machines and in deep-drawing and injection-molding machines.
In spinning machines the quality of the yarns essentially depends on a constant supply of the polymer melt to the spinning machine. To ensure that the spinning machine is constantly fed with polymer melt, buffer tanks are used to compensate for variations in volume flow and pressure in the supply line of the polymer melt by varying the volume of a compensation chamber in the buffer tank in dependence upon a volume flow and/or pressure variation in the polymer melt.
However, the use of a buffer tank creates new problems. The mechanical and chemical characteristics of the polymer melts, e.g. a cellulose solution, are time-dependent and vary in response to the respective residence time of the polymer melt in the buffer tank and in the supply line to the spinning machine. To obtain a uniform spinning quality, it must therefore be ensured that the polymer melt flows at a uniformly rapid speed through all areas of the buffer tank, if possible.
Problems arise when the polymer solution is a high-viscosity fluid and flows at relatively slow flow speeds through the buffer tank. On account of the low flow speed and the high viscosity, the flow of the polymer melt through the buffer tank is predominantly laminar.
The laminar flow is above all critical in view of the time dependence of the characteristics of the materials: On the one hand, the flow speed considerably drops in a laminar flow near walls; on the other hand, a laminar flow is very likely to separate. When a flow separates in the buffer tank, dead water zones or recirculation zones are formed. These two groups of problems—irregular speed distribution and flow separation—effect different residence times of the polymer melt in the buffer tank. As a consequence, the chemical and mechanical characteristics of the “aged” polymer melt which flows through the areas of the buffer tank at a slower flow speed and thus remains in the buffer tank for a longer period of time are changed in comparison with a polymer melt flowing through the buffer tank at a fast speed.
The areas of separation of the polymer melt flow are flushed at least in part from time to time by the speed variations always observed during conveyance of the polymer melt, and because of the unsteady characteristics thereof. In the end, the polymer melt which has been trapped there for some time and has “aged” thus passes to the spinning machine and is spun in said machine, whereby the quality of the spun yarns varies considerably.
A measure for making the flow through the buffer tank uniform consists, according to WO 94/02408, in installing stirrers for actively transporting and mixing the flow through the buffer tank. Thanks to the mixing action the speed profile of the flow is made uniform and dead water and recirculation zones are avoided. However, the drive for driving the stirrers consumes a lot of energy, not least because of the high viscosity of the polymer melt. With an inaccurate design of the stirrer, there is the risk that the polymer melt heats up. Without expensive counter-measures, the chemical and mechanical properties of the polymer melt would change in such a case under the action of heat.
FR 2570323 relates to a device for supplying a low-viscosity elastomer at a predetermined constant throughput. The throughput is also to be achieved in cases where the supply with elastomer is briefly interrupted. The device is above all to be used for making connections for multiple glazings. In the device of FR 2570323, the elastomer is stored in a tank of a variable volume, a biased cylinder with a piston being arranged in the interior of the tank. In the stroke direction of the piston, a channel is passing therethrough for guiding the low-viscosity elastomer through the tank. A pump with two compression means is located at the outlet of the reservoir.
WO 96/05338 shows a buffer tank through which the polymer melt flows in axial direction. Behind the polymer melt entry opening, the compensation chamber is expanded in a diffusor-like manner. The compensation opening tapers towards the polymer melt exit opening in the manner of a nozzle. The volume prevailing between the diffusor section and the nozzle section can be increased or reduced in response to the necessary filling level in the polymer melt.
The polymer melt flow is very likely to separate because of the diffusor and the nozzle. Therefore, commercially available static mixer elements are installed in the compensation chamber of the buffer tank of WO 96/05338 for making the flow uniform and for preventing separation tendencies. The static mixer elements extend over the total cross-section of the compensation chamber.
The volume of the compensation chamber, however, must be increased considerably, so that the static mixer elements can be received therein. Therefore, the buffer tank with the static mixers may become too large for many applications. Moreover, on account of the long flow path and the large dimensions of the static mixer elements, the flow resistance of the buffer tank is increased many times, thereby increasing the amount of energy needed for conveying the polymer melt. This enhances the risk of a heating up of the polymer melt.
The buffer tank of WO 96/05338 has the drawback that the flow in the area of the diffusor and the nozzle tends to separate in cases where the angle of opening of diffusor and nozzle, respectively, becomes too large. This risk arises above all in the case of a slow flow of the high-viscosity polymer melt. In the separated flow, a separation whirl is formed, as well as dead water zones in which the high-viscosity polymer melt stagnates. Moreover, a new design and calculation of the opening angle are always needed for different flow conditions in different systems because the separating action depends on the speed of the flow.
In view of these drawbacks it is the object of the present invention to provide a buffer tank with improved fluid dynamics and to take measures for obtaining a uniform flow through the buffer tank, whereby the constructional size and the flow resistance of the buffer tank are not changed or only changed to an insignificant degree.
According to the invention this object is achieved for a buffer tank of the above-mentioned type in that the compensation chamber between the polymer melt entry opening and the polymer melt exit opening is divided into at least two partial chambers extending in the direction of flow.
Such a solution is simple and has the advantage that a diffusor-like enlargement and a nozzle-like narrowing of the flow cross-section can be dispensed with: Since the compensation chambers are divided into at least two partial chambers, the flow cross-section of the partial chambers can remain small so that a great discontinuous change in cross section is no longer required with respect to the polymer melt entry opening and the polymer melt exit opening. As a consequence, the tendency to form regions of separation is conside
Ecker Friedrich
Longin Michael
Zikeli Stefan
McDonnell & Boehnen Hulbert & Berghoff
Soohoo Tony G.
Zimmer A.G.
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