Abstract:
An apparatus for the manufacture of a thermoplastic blown film is provided having a blower head for extruding a film tube of a thermoplastic material. An annular ring is provided for blowing a cooling gas on an outside surface of the film tube to solidify the film tube. This cooling gas is directed in a flow direction parallel to a wall of the film tube. The cooling gas is drawn off the tube by means of a suction ring positioned around the tube prior to flattening the tube to form a film tube.

Description:
FIELD OF INVENTION 
     The invention relates to a process of and a device for sucking off cooling gas for a film tube during the production of blown film, wherein the film tube, after having left the annular nozzle of a blowing head, is substantially annularly blasted with cooling gas from the outside either radially or in parallel with the wall. The cooling gas used is normally cooling air. However, for certain applications, it is possible to use inert gas. The cooling gas can emerge from a cooling gas ring in several planes and in different quantities, and it is possible to combine a fixed cooling gas flow and a controllable cooling gas flow. A process of and a device for said type are described in the applicant&#39;s DE 100 29 175 A1 for example. 
     BACKGROUND OF INVENTION 
     From U.S. Pat. No. 6,273,699 B1 there is known a device for conditioning a film tube during the production of blown film, which device comprises a plurality of closed annular sections along the film tube; the sections each comprise a ring of air ejection nozzles extending parallel to the longitudinal direction of the film tube, and at a distance therefrom they comprise a ring of air suction nozzles directed in the opposite direction, with the former extending in the direction of production and the latter in the direction opposed to the direction of production of the film tube. 
     DE 44 05 463 proposes a process and device for cooling a film tube extruded from a film blowing head, wherein cooling air is blown out of a cooling ring with an annular exit gap, which cooling ring encloses the tube and wherein cooling air is sucked out of a screened space surrounding the tube, at a distance from the cooling ring in the direction of production of the tube at an annular aperture. 
     SUMMARY OF THE INVENTION 
     During the production or extrusion of blown film, a thermoplastic material is pressed via an extruder into a blowing head out of whose annular nozzle or annular channel there emerges a film tube which is drawn upwards. The hot film tube is subjected to an internal excess pressure and immediately after having left the annular nozzle, it is substantially annularly blasted with cooling gas from the outside and optionally also from the inside, and thus cooled. From the moment of leaving the annular nozzle, after a film tube expansion phase, the thermoplastic material substantially solidifies, whereafter the film tube substantially retains its diameter. The place of solidification is referred to as the “freezing limit”. After the film tube has solidified, it is guided in the longitudinal direction via a calibrating basket and a flattening device and squeezed and pulled off in the form of a flat tube by an extraction device. The calibrating basket is positioned above the freezing limit. The diameter of the film tube and thus the subsequent film width is variable and is varied by the internal excess pressure in the film tube and via the setting of the adjustable calibrating basket. 
     During the production of blown film tube, after the hot tube material has left the annular nozzle, in the region of extraction of the film tube as far as the freezing limit, monomer-containing gases emerge from the thermoplastic material. Said gas emissions are carried along by the passing cooling gas in the direction of production and are deposited in the form of a wax-like coating on all plant elements following the blowing head in the direction of extraction such as the calibrating basket, the flattening device and the extraction tool as well on devices in the direct vicinity. Floating particles in the environment form additional dirt deposits on the sticky wax-like coating. Such wax and dirt deposits obstruct the production process of the film tube and adversely affect accurate functioning of the plant elements and thus the product quality. Particularly in the case of medical film and film for the food industry, the transfer of dirt deposits to the film tube leads to unacceptable quality defects. Therefore, it has so far been necessary for said plant elements to be cleaned regularly and extensively, wherein as a rule, access to said plant elements was complicated. Furthermore, it is suspected that breathing-in cooling gas contaminated with gas emissions in the gaseous phase is detrimental to a person&#39;s health. 
     OBJECT OF THE INVENTION 
     It is therefore the object of the present invention to provide a process and device of said type by means of which contamination of the plant elements by exhaust gas deposits can be reduced or avoided. The objective is achieved by a process of said type which is characterised in that the cooling gas contaminated with gas emissions out of the tube material is substantially annularly sucked off again on the outside of the film tube in the direction of extraction of the film tube downstream from the cooling gas supply, and by a device wherein at a distance from the cooling gas ring, above same (downstream), there is arranged a gas suction ring which encloses the film tube and comprises internal, substantially annularly arranged suction nozzles. Such means make it possible to suck off the cooling gas contaminated with gas emissions near their origin, thus avoiding or reducing the contamination of the subsequent plant components and reducing the health risk. 
     According to a first embodiment of the invention it is proposed that, downstream from the freezing limit, but already upstream from a calibrating phase, the contaminated cooling gas is sucked off. The gas sucking-off ring is arranged between the freezing limit and the calibrating device, without obstructing the film tube cooling operation up to the point of reaching the freezing limit. This is particularly advantageous with a view to protecting the calibrating device and the subsequent plant components from contamination. 
     According to a second embodiment it is proposed that the contaminated cooling gas is sucked off in the region of the calibrating phase of the film tube, i.e. that the gas sucking-off ring or several gas sucking-off beams are arranged in the region of the calibrating device for the film tube. This means that even in the region of the open structure of a calibrating basket, contaminated cooling gas can be sucked off in such a way that the components of the adjusting devices of the calibrating device can also be protected from contamination. 
     According to a third embodiment, it is proposed that the contaminated cooling gas is sucked off downstream from a calibrating phase of the film tube. This means that the gas sucking-off ring is arranged above (downstream from) a calibrating device for the film tube. 
     According to a preferred embodiment it is proposed that the film tube is sealed annularly directly downstream from the suction point relative to the suction means. The respective device comprises a diaphragm which is arranged directly above (downstream from) the gas sucking-off ring and which annularly seals the film tube relative to the gas sucking-off ring. On the one hand, this measure improves the effectiveness of the suction process and, on the other hand, it fully protects the subsequent plant components such as the flattening device, from being contaminated. The diaphragm can be provided in the form of an adjustable iris diaphragm. 
     More particularly, it is proposed that the iris diaphragm is provided with automatic control means for adapting the diaphragm diameter to the diameter of the film tube. The diameter of the film tube can be continuously recorded by contact-free ultrasound measurements. It is thus possible to compensate for slight diameter fluctuations of the film tube in the course of production and to avoid manual adaptation measures during production changes. 
     More particularly, it is proposed that the plurality of suction beams are connected to a segmented, adjustable calibrating device. 
     In a simple embodiment of the process, the sucked-off contaminated cooling gas can be blown into the environment outside the production hall. However, in view of environmental protection measures it is preferred to filter the cooling gas contaminated with gas emissions before it is blown out either inside or outside the production hall. For this purpose, more particularly, there is provided an electric filter in a suction line in front of the suction fan. 
     If the film tube production facilities include additional internal cooling, the contaminated, sucked-off cooling gas from the inner cooling operation can be combined with the contaminated cooling gas of the outer cooling operation and filtered together therewith. 
     According to a further process improvement, it is proposed that, downstream from the suction point of the contaminated cooling gas, the film tube, for being cooled subsequently, is again blasted in a substantially annular way with cooling gas, i.e. that directly above (downstream from) the gas sucking-off ring, there is arranged a further cooling gas ring. In this context, it is proposed that the renewed operation of blasting the film tube with cooling gas takes place downstream from the annular sealing region in that the further cooling gas ring is arranged directly above (downstream from) the diaphragm. Such means allow subsequent cooling of the still relatively warm film tube, such subsequent cooling allowing continued interference-free handling of the film tube, more particularly flattening same and winding same up on a coil. 
     As the already heated cooling air flowing along the film tube is sucked off completely from the film tube at the suction ring, the additionally blasted cooling gas reaches the film tube without any obstructions from a flow-technological point of view and has a high subsequent cooling effect because it is not mixed with the heated cooling gas. This ensures a high production output in high-performance production facilities and those with a low building height. 
     The quantity of sucked-off contaminated cooling gas can be automatically adapted to the quantity of cooling gas freshly supplied directly after the film tube emerges from the annular nozzle. For this purpose, it is necessary to provide a suitable control device. 
     The inventive devices are particularly advantageous in that they can easily be adapted to different film tube diameters, more particularly in the region where the suction ring mentioned first is sealed relative to film tube by means of an adjustable diaphragm and in the region of the calibrating device by combining individual suction beams or suction funnels with the segments of an adjustable calibrating device. 
     According to a further advantageous embodiment, it is possible to structurally connect the suction ring mentioned first, the diaphragm and the additional cooling ring in such a way that the assembly requires only one single common suspension facility in the plant frame. 
     Preferred embodiments of the invention are illustrated in the drawings and will be described below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a film blowing plant in accordance with the invention in a first embodiment illustrating the entire system. 
         FIG. 2  shows parts of the plant according to  FIG. 1  in an enlarged illustration. 
         FIG. 3  shows a film blowing plant in accordance with the invention in a second embodiment illustrating the entire system. 
         FIG. 4  shows parts of the plant according to  FIG. 3  in an enlarged illustration. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a side view and partially in a longitudinal section through the axis of a blown film tube,  FIG. 1  shows a plant for carrying out the inventive process, including the characteristics in accordance with the invention. On a machine foundation  11 , there is erected an extruder  12  comprising two charging hoppers  13 ,  14  for thermoplastic material. Thermoplastic material introduced in granular form through the hoppers  13 ,  14  is plasticized and homogenised by the pressure means and additional heating means in a worm of the extruder  12  and pressed into a blowing head  15  with a vertical axis, which follows the extruder. At its upper end, the blowing head  15  comprises an annular nozzle (not identifiable in detail) from which there emerges a expanding axis-symmetrical film tube  16  initially consisting of a still plasticized film material. After the material has solidified, the film tube  16 ′ substantially retains its diameter. The film tube  16 ′ is flattened in a flattening device  17  and extracted upwards by an extraction device  18 . Subsequently, the flattened film tube  16 ″ is wound up on coils. In the direction from the blowing head  15  to the extraction device  18 , the functional terms “upstream from” and “downstream from” are used to mean in a spatial relationship “underneath” and “above”. 
     Directly above the blowing head  15 , there is shown a cooling gas ring  19  with several gas supply lines  20 . The cooling gas ring  19  comprises internal exit nozzles  21  out of which there flows the cooling gas and, substantially annularly and parallel to the wall, flows against the film tube  16  which is subjected to an increased internal pressure. The cooling gas flow is symbolised by arrows  22 . Under said internal excess pressure, the diameter of the film tube  16  plasticized in this region, until the film tube  16  hardens under the effect of the cooling gas and assumes a constant diameter. The place of transition from the plasticized material to the hardened material is referred to as the “freezing limit” and has been given the reference number  23 . Above, i.e. in the direction of extraction downstream from the freezing limit  23 , there is arranged a calibrating device  24  which contains roller assemblies which are positioned one above the other and which are positioned substantially annularly around the film tube  16 ′. To permit adaptation to film tubes with different diameters, the roller assemblies, as a rule, are located on individual pivotable segments forming circumference parts, by means of which segments the diameter of the calibrating device can be changed. In a cross-sectional view, said segments, in a simplified way, can form a polygonal calibrating device. The still relatively warm film tube  16 ′ is stabilised in its cross-section by the calibrating device. 
     Downstream from the calibrating device  24 , there is provided an inventive gas sucking-off ring  25  with internal annular suction apertures  26 , which ring  25  can suck off the cooling gas contaminated with gas emissions from the film tube. Circumferentially distributed suction lines  27  are connected to the gas sucking-off ring  25 . The cooling gas contaminated with gas emissions is indicated by arrows  28 . Directly downstream from the gas sucking-off ring  25 , there is provided an adjustable diaphragm  29  which is mechanically connected to said ring  25  and which sealingly encloses the film tube  16 ′. The diaphragm  29  is adjustable by an adjusting device  30  in the aperture cross-section and can therefore be adapted to different diameters of the film tube  16 ′. By sealing the film tube above the gas sucking-off ring  25 , pure air is prevented from being sucked in from a region downstream from the gas sucking-off ring, so that the process of sucking off the contaminated cooling gas is optimised while keeping the energy consumption low. In the region of the calibrating device  24 , radially outside the calibrating device, there is provided a further gas sucking-off ring or a plurality of individual gas sucking-off funnels or gas sucking-off beams  31  which are connected to a plurality of circumferentially distributed suction lines  33 . One gas sucking-off beam each can be associated with one of the above-mentioned segments, so that an adaptation of the calibrating device to different diameters of the film tube  16 ′ can be accompanied by an adaptation of the additional suction device to the different diameters. The plurality of suction lines  27  is first joined to form a line  27 ′. Equally, the plurality of suction lines  33  is then combined to form one single line  33 ′. The lines  27 ′ and  33 ′ are then combined to form one single gas sucking-off line  43 , with the control flap  34  being arranged in the region of a Y-pipe. The suction pipe  43  is followed by a suction fan  44 . In the suction line  43 , in front of the suction fan  44 , there can be integrated a filter, more particularly an electric filter, if, for environmental reasons, it is impossible to blow the gas emissions into the environment outside a production hall. A control device  36  with a processor controls via a control line  37  the suction fan  44  and, via a control line  38 , a further control device  39  for controlling the adjusting device  30  for the diaphragm  29 . Via a measuring line  40 , the signals of a cooling gas quantity sensor  41  are transmitted in the cooling air supply lines  20  to the control device  36 . Via the measuring line  42 , the signals on the pressure and flow quantity conditions at the electric filter  45  are transmitted to the control device  36 . Via a further control line (not illustrated), the control device  36  can regulate the cooling gas fan (not illustrated either) in a processor-controlled way. The entire region from the first cooling gas ring  10  to the gas sucking-off ring  25  is enclosed by a casing  53 , so that the cooling gas contaminated with gas emissions is kept clear of the working area of the operatives. The lines  27 ,  33  are sealed by the casing. 
     In  FIG. 2 , the details identical to those shown in  FIG. 1  have been given the same reference numbers. To that extent, reference is made to the preceding description. In this Figure it is indicated that the cooling gas ring  19  comprises two exit nozzles  21 ′,  21 ″. It can also be seen that in the suction ring  25 , there are provided labyrinth elements  25 ′ in order to level out the pressure level at the suction aperture  26  across the circumference and to suppress the influence of the individual gas sucking-off lines  27 . It can also be seen that in the gas sucking-off beams  31  there are provided facilities  31 ′ in order to calm the pressure level in the gas inlet aperture  32 . 
     In  FIG. 3 , the details identical to those shown in  FIG. 1  have been given the same reference numbers and have the same function as described above. In addition to the details described in  FIG. 1 , there can be identified a further cooling gas ring  46  which, via inner exit nozzles  47 , blows cooling gas for subsequent cooling purposes substantially radially against the film tube  16 ′. The additional cooling gas streams are indicated by arrows  48 . Gas supply lines  49  which, in total, are supplied via a supply line  50  by a further cooling gas pressure fan  51  are connected to an additional cooling gas ring  46 . The cooling gas pressure fan  51  is controlled via a control line  52  by the control device  36 . The still relatively warm film tube  16 ′ can thus be re-cooled, so that the flattening assembly  17  and the extraction device  18  can be made to operate without the risk of the film layers sticking during the flattening operation. In this embodiment, too, the region from the first cooling gas ring  19  to the gas sucking-off ring  25  is enclosed by a casing  33 , which sealingly adjoins said parts and which prevents the working environment from being subjected to cooling gas contaminated with gas emissions. 
     In  FIG. 4 , the details identical to those shown in  FIG. 3  have been given the same reference numbers. To that extent, reference is made to the above description. It is indicated that the cooling gas ring  19  comprises two exit nozzles  21 ′,  21 ″. Furthermore, it can be identified that labyrinth elements  25 ′ are provided in the suction ring  25  in order to unify the pressure level at the suction aperture  26  around the circumference and to suppress the influence of the individual gas sucking-off lines  27 . It can also be seen that in the gas sucking-off beams there are provided elements  31 ′ in order to calm the pressure level in the gas inlet apertures  32 . Finally, the additional cooling gas ring  46  is provided with labyrinth elements  46 ′ to unify the additional cooling gas stream at the exit nozzle  47 . The assemblies of suction ring  25 , diaphragm  29  and additional cooling gas ring  46  are structurally connected to one another.