Process and device for cooling extruded hollow sections

Details Process and device for cooling extruded hollow sections Process and device for cooling extruded hollow sections

1998-02-05

2002-07-16

Capossela, Ronald (Department: 3744)

Refrigeration

Processes

Treating an article

C062S384000, C062S605000

Reexamination Certificate

active

06418732

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for cooling extruded hollow sections, especially pipes, that are made of plastic, whereby liquid or supercritical carbon dioxide is directed through the extruder head and depressurized and the interior of the plastic hollow section is cooled with the carbon dioxide in gaseous form that is produced. The invention also relates to a device for cooling extruded hollow sections that are made of plastic with at least one feeder for liquid or supercritical carbon dioxide, which runs through the extruder head into the interior of the hollow section and which has at least one depressurization nozzle.
BACKGROUND OF THE INVENTION
In the extrusion of plastic pipes, frequently only the exterior of the extruded pipes is cooled with water. In this case, cooling is done in a downstream calibrating and cooling segment. Especially in the case of thick-walled pipes, this type of cooling leads to only slow heat exchange. The ejection output of such plastic extrusion units is therefore limited basically by the cooling speed of the plastic, as well as of the calibrating form. Extending the cooling segment to increase ejection output is very costly and is possible only to a limited extent.
DE-OS 24 56 386, incorporated herein by reference, therefore proposes a process in which an extremely cold medium that evaporates or is sublimated there, especially liquid nitrogen, is sprayed into the interior of the plastic pipe in the area of the calibrating form. Also known from DE-OS 23 24 133, incorporated herein by reference, is a process for outer calibration of extruded hollow sections that are made of thermoplastic material in which, to provide cooling, a chemically inert liquid gas with a critical temperature of between −150 and +35° C., such as, e.g., nitrogen or carbon dioxide is evaporated in the hollow space of the hollow section.
When extremely cold nitrogen is used for internal cooling, it is necessary to be sure that nitrogen comes into contact with the plastic pipe only in gaseous form since liquid nitrogen can damage the inside wall of the pipe. In the case of extended operation or in the case of high nitrogen throughput, however, the danger exists that the nitrogen feeders will cool off greatly, which means that residue-free evaporation of the nitrogen can no longer be ensured.
DE-C-33 15 202, incorporated herein by reference, therefore proposes a device in which the exhaust nozzle for liquid nitrogen is encased by an evaporator element that is in thermal contact with the hot, extruded plastic pipe. The liquid nitrogen is sprayed from the nozzle to the evaporator element, is evaporated there, and is then fed via outlets in the evaporator element to the inner surface of the plastic pipe.
After extrusion, the plastic melt, which is still soft, flows downward under the action of gravity, in such a way that uneven wall thickness distribution develops over the periphery of the plastic pipe. This deformation of the pipe or the hollow section is effectively prevented by inner cooling. When liquid nitrogen is used, moreover, the extrusion rate can be increased over what is possible with water cooling.
When liquid nitrogen is run through the hot extruder head, a large temperature difference occurs between the liquid nitrogen, which has a temperature of −196° C., and the extruder head, which has a temperature of about 200° C. This requires a high insulation expense, but it cannot prevent nitrogen from exiting the depressurization nozzle in a pulsating manner, which results in the formation of droplets on the plastic pipe.
SUMMARY OF THE INVENTION
The object of this invention thus is to indicate a process of the above-mentioned type that avoids the drawbacks of the known processes. In particular, the invention is successful in dispersing the coolant in such a way that as uniform and good a heat transition as possible is achieved between the coolant and the inner wall of the hollow section. The risk of the hollow section being damaged by extremely cold, liquid coolant is eliminated, the extrusion speed is to be increased, and the technical expense, especially the insulation expense, is minimized in carrying out this process. A device is also provided for implementing this process.
On the process side, this object is achieved in that carbon dioxide that is in gaseous form is brought into heat exchange with the inner wall of the hollow section and in that solid carbon dioxide is brought into heat exchange with the inner wall of the hollow section only to the extent that the inner wall of the hollow section is not damaged.
According to the invention, carbon dioxide is used as a coolant, which generally is stored under pressure in liquid form in a supply tank. The liquid carbon dioxide is directed through the extruder head into the interior of the extruded hollow section and is depressurized there in one or more depressurization nozzles. When liquid or supercritical carbon dioxide is depressurized, a high-speed cooling jet that consists of carbon dioxide gas and carbon dioxide snow is produced. Sizeable carbon dioxide snow or dry ice particles can damage the surface of the extruded hollow section. The solid components that are contained in the cooling jet are therefore either kept away from the surface or evaporated, so that they do not come into contact with the surface of the plastic hollow section or are dispersed into the carbon dioxide gas so finely that damage to the surface of the hollow section cannot occur.
It has been found that damage to the section surface occurs only when a critical particle size that is dependent on, i.a., the temperature of the extruded section is exceeded. According to the invention, the solid carbon dioxide that is produced during expansion is therefore finely atomized, such that the resulting snow particles are sublimated as early as when they approach the section surface or such that the particles are so small that the section wall cannot be damaged.
Internal cooling with carbon dioxide instead of liquid nitrogen is significantly easier to handle under the conditions that exist in plastic extrusion, especially the high temperature of the extruder head, since the insulating wall can be minimized when the carbon dioxide feed is run through the extruder head.
The liquid or supercritical carbon dioxide is advantageously dispersed behind the extrusion tool in the hollow section and then depressurized. The liquid or supercritical carbon dioxide is dispersed onto several nozzles, especially preferably onto several nozzles that are arranged axisymmetrically with respect to the extruded hollow section or to an annular nozzle that is arranged coaxially with the hollow section. Owing to this dispersion of the carbon dioxide, a uniform cooling action is achieved over the entire periphery of the hollow section. Uneven wall thicknesses or deformation of the hollow section are thus avoided.
The depressurized carbon dioxide, i.e., carbon dioxide in gaseous form or carbon dioxide in gaseous form that is mixed with small carbon dioxide particles that do not damage the section wall is advantageously not only directed onto the inner wall of the hollow section, but is also guided along the inner wall of the hollow section. In this way, the heat transition between the inner wall of the hollow section and the carbon dioxide is intensified, and the cooling properties of the carbon dioxide are exploited as much as possible.
In a preferred variant, a twisting flow, i.e., a rotational movement component, is imposed upon the depressurized carbon dioxide. A coil-shaped or helical flow ensures even better dispersion of carbon dioxide over the periphery. Sinking of the carbon dioxide under the action of gravity and the associated uneven cooling of the hollow section are avoided.
It is also advantageous if the kinetic energy of the depressurized carbon dioxide that comes into contact with the inner wall basically corresponds to the kinetic energy of the carbon dioxide immediately after depressurization. The higher the kinetic ener

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