Agitating – Rubber or heavy plastic working – Stirrer is through-pass screw conveyor
Reexamination Certificate
2000-04-28
2001-10-23
Soohoo, Tony G. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stirrer is through-pass screw conveyor
C366S081000, C366S091000, C366S097000, C366S325200
Reexamination Certificate
active
06305831
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for mixing and advancing a polymer melt.
In the processing of a thermoplastic material, the material is initially melted, for example, by an extruder and supplied as a polymer melt to further processing operations. To this end, the polymer melt is advanced by means of a conveying device. For further processing the polymer melt, the homogeneity of the melt constitutes an important criterion besides the dwelling time of the melt. To attain an adequately homogenized melt, the melt is therefore mixed by means of a mixer. In particular, in the case of high quality requirements when additives or fillers are incorporated, it is necessary to adapt to each other the melt throughput and the intensity of the mixing.
EP 0 636 190 and corresponding U.S. Pat. No. 5,637,331 disclose an apparatus, wherein the mixer is combined with a conveying device in one unit. In this unit, the mixing shaft of the mixer is driven together with the conveying device. In this connection, the rotational speed of the mixer is defined by the discharge of the conveying device, so that the mixing result is directly dependent on the conveying device. In the known apparatus, a main flow of the polymer melt is mixed in a mixing chamber by means of a rotating mixing shaft, and subsequently divided by means of the conveying device into partial flows, and advanced to a spinneret. In this connection, it is especially important that the partial flows be uniformly homogenized, for purposes of attaining a uniform product quality in the subsequent processing, in this instance in a spinning line.
U.S. Pat. No. 4,128,342 discloses an apparatus wherein the mixer shaft of the mixer is arranged at the end of an extrusion screw and driven by same. In this arrangement, the rotational speed of the mixer shaft is determined by the rotational speed of the conveying device, in this instance an extrusion screw.
Thus, in the known apparatus, the mixing result is dependent on the speed of the mixer shaft and the length of the mixing chamber. To obtain an intensive mixing at relatively low rotational speeds, it is necessary to construct the mixer chamber and thus the mixer shaft very long in the case of large throughput quantities.
It is accordingly an object of the invention to design and construct an apparatus of the initially described type in such a manner as to ensure, despite low rotational speeds of the mixer shaft, an intensive mixing of the polymer melt and with the mixer being constructed as compactly as possible.
SUMMARY OF THE INVENTION
The above and other objects and advantages of the present invention are achieved by the provision of an apparatus for mixing a polymer melt which comprises a mixing chamber having an outer wall, an inner mixing shaft mounted within said mixing chamber so as to define a central axis, and a tubular outer mixing shaft mounted coaxially about said inner shaft. The outer mixing shaft is radially spaced from the inner mixing shaft so as to define an inner mixing chamber therebetween, and the outer mixing shaft is radially spaced from the outer wall so as to define an outer mixing chamber therebetween. A free end of said outer mixing shaft is spaced from the outer wall of the mixing chamber so as to define a passageway therebetween which interconnects the inner mixing chamber with the outer mixing chamber. A drive is provided for rotatably driving the outer mixing shaft about the central axis, and the mixing chamber further has a melt inlet and a melt outlet, with one of the melt inlet and the melt outlet communicating with the inner mixing chamber, and the other of the melt inlet and the melt outlet communicating with the outer mixing chamber.
The inner mixing shaft may be stationary, or it may be rotatably driven by a drive which is separate from the drive of the outer mixing shaft. In a further embodiment, a drive shaft is connected to both the inner mixing shaft and the outer mixing shaft, so that they rotate together in the same direction.
With the present invention, the mixing chamber includes both the inner mixing chamber (inner chamber) and the outer mixing chamber (outer chamber), through which the melt flows respectively in opposite directions of flow and undergoes in each a mixing by a mixing shaft. This makes it possible to realize an approximately doubled dwelling time of the polymer melt within the mixing chamber. Furthermore, the reversal of the melt flow prevents the formation of marginal zones with a less intensive mixing. In the outer marginal zone of the inner chamber, the polymer melt becomes an inner marginal zone of the outer chamber after reversing the flow. Thus, a very intensive mixing of the marginal zones occurs, whereby the polymer melt is uniformly homogenized. To this end, the polymer melt enters the inner chamber of the mixer through a melt inlet. In the inner chamber, the rotation of the inner mixing shaft (inner shaft) causes a thorough mixing of the polymer melt. At the end of the inner chamber opposite to the melt inlet, a passageway is formed at the end of the outer mixing shaft (outer shaft), which represents a boundary between the inner chamber and the outer chamber. The polymer melt flows from the inner chamber through the passageway into the outer chamber. In the outer chamber, the polymer melt undergoes a further mixing by the rotation of the outer shaft. The polymer melt leaves the outer chamber through a melt outlet that opens into the mixing chamber, preferably at the end of the mixing chamber opposite the inlet of the mixing chamber.
However, it is also possible to interchange the melt inlet and the melt outlet, so that the melt entering the mixing chamber is mixed first in the outer chamber and subsequently in the inner chamber.
This results in particular in the advantage over the state of the art, in that an intensive mixing is realized even with high throughput quantities and relatively low rotational speeds. The flow reversal of the polymer melt within the mixing chamber allows avoiding in particular radial temperature differences.
The mixer of the present invention permits a stationary arrangement of the inner shaft in the mixing chamber. With this arrangement, the rotating outer shaft that preferably mounts mixing elements, provides an adequate mixing.
Irrespective of the design of the mixing elements, which may be constructed, for example, as pins, spirals, or scoops, it is possible to drive the inner shaft and the outer shaft in the same direction of rotation or in opposite directions of rotation. With the use of mixing elements that have a conveying effect, it is recommended to operate the mixer shafts in the same direction.
As noted above, the inner mixing shaft and the outer mixing shaft may be mounted on a common drive shaft. It is thus possible to operate the mixer together with a conveying device.
According to an advantageous further development of the invention, the outer mixing shaft mounts a plurality of mixing elements, and the inner mixing shaft also mounts a plurality of mixing elements, which extend radially into the inner chamber. The inner chamber is thus especially suitable for uniformly mixing the polymer melt over the entire cross section of the mixing chamber. The special advantage lies in having the inner mixing chamber comprise a rotating chamber wall. Thus, it is possible to use for mixing the melt the rotation of the inner shaft and the rotation of the outer shaft, which each mount mixing elements extending into the mixing chamber.
The mixing elements of the inner shaft and the mixing elements of the outer shaft may intermesh in the inner mixing chamber, which is especially advantageous for realizing a very intensive mixing.
In a specific embodiment of the invention, a tube is slipped over the inner shaft in such a manner that the inner chamber forms between the inner shaft and the tube, and that an intermediate mixing chamber (intermediate chamber) forms between the outer shaft and the tube. At its free end, the tube defines a passageway for connecting
Dickmeiss Friedel
Gathmann Egon
Schafer Klaus
Stausberg Georg
Alston & Bird LLP
Barmag AG
Soohoo Tony G.
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