Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Production of continuous or running length
Reexamination Certificate
1997-10-29
2001-04-10
Silbaugh, Jan H (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Production of continuous or running length
C264S560000, C264S561000, C264S563000, C264S237000, C264S348000, C425S326100, C425S393000, C425S404000
Reexamination Certificate
active
06214283
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the treatment of an extruded plastic section which emerges from an extrusion apparatus.
BACKGROUND OF THE INVENTION
In the Dutch patent application NL 7612518 an installation for manufacturing an extruded plastic section is disclosed wherein the extruded tubular section is cooled not only externally, which is the most common cooling method, but there is also internal cooling with cooling water which comes directly into contact with the inside of the plastic section to be cooled. As a result of said additional internal cooling, cool-down of the section can be improved and, at the same time, the length of the cooling section can be considerably shorter, theoretically by a factor of four, than if external cooling only were to be employed.
In the case of the known installation, for the purpose of internal cooling with cooling water, a sealed compartment is bounded by the insulated end face of the internal mandrel of the extruder on the one hand, and a closing means which is situated at a distance downstream therefrom in the extrusion direction on the other hand. The closing means, in the case of the known installation, is a cylindrical metal stopper which, via a hollow anchor rod, is fixed to the mandrel of the extruder. The external diameter of the stopper is such that the plastic section, which is cooling down and therefore shrinking, slides with friction along the rigid outer wall of the stopper. In the process, the contact pressure between the stopper and the plastic section brings about the fluid seal intended for the purpose of sealing the compartment, this being assisted by a plurality of circumferential grooves in the stopper, which provide the effect of a labyrinth seal. Through the hollow anchor rod cooling water can be introduced, upstream of the closing means, into the sealed compartment, said cooling water then flowing in a counter-current direction along the plastic profile to be cooled and then being discharged via the mandrel of the extruder.
The method and installation disclosed by NL 7612518 has not proved satisfactory in practice, particularly with respect to the closing means employed there. The seal obtained with said known closing means cannot be controlled in practice, as a result of which there will occur either undesirable leakage or precisely the problem of the plastic section lying with such force against the rigid circumferential wall of the stopper that as a result of the large frictional forces then produced damage is caused to the plastic section and the mounting of the stopper is subjected to undesirably large (tensile) loads.
For the abovementioned reasons, the purpose of effecting internal cooling has until now been served by preferably employing flexible closing means, for example as described in DE 25 06 517. In the case of the closing means shown in said publication, the outer wall of the closing means is made of a flexible material, and the pressure with which the flexible outer wall lies against the section to be cooled can be controlled. Since extrusion installations are in practice operated continuously, the inevitable result, inter alia under the influence of the prevailing temperatures and pressures and the frictional contact with the extruded section, is wear of the flexible wall of the closing means, which necessitates regular replacement thereof, for which purpose the extrusion installation has to be shut down.
OBJECT OF THE INVENTION
The object of the present invention is to provide a method and an extrusion installation for fabricating an extruded plastic section, which permit improved sealing of the compartment in the hollow space of the section, as a result of which it is possible to subject the plastic section at will, even during the production thereof, to one or more internal treatments.
SUMMARY OF THE INVENTION
This object is achieved by a method according to claim
1
and an extrusion installation according to claim
12
. Just as has been shown in NL 7612518, the rigid outer wall of the closing means preferably has such dimensions that in the absence of the fluid film according to the invention a frictional contact would exist at that location between the closing means and the plastic section. Since according to the present invention there is produced, at least over a portion of the outer wall of the closing means, a film of fluid, i.e. an incompressible or barely compressible medium, between the closing means and the section, the frictional forces are considerably reduced. The present invention further provides for the fluid in the film to flow in a counter-current direction, i.e. counter to the extrusion direction of the plastic section, over the surface of the outer wall of the closing means. This measure is based on the insight that the plastic section, which emerges from the extruder and is cooling down, will want to lie against the outer wall of the closing means with greater and greater force. By the fluid forming a film being fed in at the downstream end of the film, the supply of fluid at that location, by means of suitable feeding means, is utilized to prevent the at least one supply orifice from being closed by the plastic section. The plastic section, during normal operation of the installation, will lie directly, downstream of the at least one supply orifice, against the circumferential wall of the closing means and as a result form a fluid seal. Said lying against can be promoted by the plastic section being further cooled at the location of the closing means, this being effected, for example, by the closing means itself being cooled and/or by the fluid which forms the film being employed as a coolant.
As a result of the low friction obtained, according to the present invention, between the closing means and the plastic section, damage to the plastic section is avoided, the wear of the closing means is low, and the force exerted by the closing means on its mounting is also considerably smaller than in the case of the installation disclosed by NL 7612518. A further advantage of the inventive thought is that a very smooth surface of the wall of the hollow space in the plastic section is obtained.
The method and extrusion installation according to the present invention are also applicable, very advantageously, in cases where the closing means serves as an expansion mandrel and has such a shape, for example with a cross-section which conically increases in the extrusion direction, that the plastic section moving over the closing means fashioned as an expansion mandrel is expanded in the circumferential direction. Said purposeful expansion of the extruded plastic section is employed, in particular, in the fabrication of biaxially oriented tubing made of thermoplastic material. In this situation, too, by means of the closing means according to the invention, a considerable reduction of the frictional forces between plastic section, on the one hand, and closing means, the expansion mandrel, on the other hand, is obtained with respect to installations known hitherto, having an expansion mandrel with a rigid outer wall.
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International Search Report dated Jul. 20, 1995.
International Search Report dated Jul. 16, 1996.
Prenger Jan Hendrik
Schuurman Johan
Visscher Jan
Blank Rome Comisky & McCauley LLP
Silbaugh Jan H
Staicovici Stefan
Wavin B.V.
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