Technical Field

The invention relates to a device for severing an endless tube made from a flexible material, the endless tube being conveyed in a transport direction in order to obtain tube bodies for packaging tubes. The device comprises a guide element for the endless tube, the guide element having a substantially cylindrical inner surface for guiding the outer surface of the endless tube, and a cutting element for cutting the endless tube along a substantially azimuthal cutting line, the cutting element being arranged adjacent to the guide element in the transport direction. Background Art

Apparatuses and processes for the manufacture of tube bodies for packaging tubes are known. As an example, DE 41 21 427 C2 (KMK Karl Magerle Lizenz AG) discloses an apparatus where an endless sheet or strip of a foil, in particular of a plastic laminate which may comprise metallic layers, is formed to a tube shape, employing a cylindrical mandrel extending along the transport direction of the sheet or strip. Edge portions of the sheet or strip overlap each other and are welded together employing heat and pressure such that a longitudinal welding line is formed. Downstream, in the transport direction of the formed endless tube, a cutting device is arranged in a certain distance after the mandrel in order to sever the endless tube into tube bodies of a predetermined length. In a further step, tube heads are joined to the tube bodies in order to form a packaging tube.

Cutting devices are known where the endless tube is guided by a substantially cylindrical inner surface of a guide element, the inner surface being made from a polished metallic material. A cutting blade is arranged adjacent to the downstream orifice of the guide element and allows for cutting the endless tube along a substantially azimuthal cutting line. In that context, reliably guiding the tube prior to the cutting step is crucial for obtaining a precise cut.

In the known devices, for guiding the endless tube, the outer surface of the tube is contacted by the inner surface of the guide element. Despite the polished surface of the guide, this leads to substantial friction and to striations in the outside of the endless tube and contamination of the inside of the endless tube with abrasion of the tube material. The striations of the outside of the tube may affect the visual appearance of the finished tube. The contamination of the inside of the tube is critical especially in case of packaging tubes for the pharmaceutical industry as for these applications a high degree of purity is required and usually tube materials having a high coefficient of friction are used. Furthermore, the guide has to be regularly cleaned. All these problems grow with increasing processing speed. Summary of the invention

It is therefore the object of the invention to create a device for severing an endless tube pertaining to the technical field initially mentioned, that allows for high throughput, the creation of precise cuts and a reduction of damages to the tube and of- contamination by abrasion.

The solution of the invention is specified by the features of claim 1 . According to the invention, at least in a region neighbouring the cutting element, the inner surface of the guide element comprises a plurality of gas delivery orifices.

The inner surface of the guide element defines a cylinder, the central axis of which (i. e. the longitudinal axis) being parallel to the transport direction of the endless tube. In the following, unless stated otherwise, the term "axial" relates to the orientation of the central axis of said cylinder.

The endless tube is pushed into the guide element by means arranged upstream of the inventive device, such as a device for forming and welding the endless tube from an endless sheet or strip of a foil or for forming the endless tube in an extrusion process. Using the gas delivery orifices a gas cushion may be created between the inner surface of the guide element and the outer surface of the endless tube, thereby avoiding physical contact between these two surfaces and therefore reducing or eliminating abrasion of the tube material. Therefore, a high throughput is achievable and the tube bodies have a smooth outer surface and are free from contamination. Accordingly, the device is suitable in particular for the manufacture of packaging tubes for pharmaceutical applications.

Preferably, the orifices lead directly into the inner surface of the guide element, i. e. they are not recessed with respect to the (geometrical) cylinder defined by the innermost areas of the inner surface. This allows for the buildup of an optimum gas cushion. Furthermore, the orifices are preferably formed in such a way that - at least in average - the gas flows radially from the outside to the inside (i. e. directed to the central axis of the inner surface, in a plane perpendicular to the central axis). This ensures that the section of the endless tube guided within the guide element does not experience rotational forces which could lead to slight twisting and therefore to an impaired cutting of the tube.

Preferably, the gas cushion is created by supplying pressurized air to the gas delivery orifices. For that purpose, the device preferably comprises a pressure generator. Advantageously, an excess pressure of the supplied air (or another gas) delivered by the gas delivery orifices is in the range of 0.5 - 10 bar, most preferably 3 - 6 bar. It is to be noted that the pressure required at the pressure generator will be higher due to pressure loss between the generator and the inner surface of the guide. The amount of pressure loss depends notably from the geometry of the gas feeds and the orifices. Preferably, at least two of the plurality of gas delivery orifices have a different axial position with respect to the guide element. This allows for reliably guiding the endless tube along an axially extended surface, thereby ensuring a consistent shape of the endless tube when leaving the downstream orifice of the guide.

Advantageously, a number of gas delivery orifices in the inner surface is at least 10, preferably at least 50, most preferably at least 1 Ό00. The larger the number of openings the smaller the openings may be. Furthermore, the distance between adjacent openings may be reduced. A high number of orifices therefore allows for an even distribution of gas over a comparably large surface area and thus creates a uniform gas cushion, ensuring optimum guidance of the endless tube. Preferably, the distribution of the orifices is symmetric with respect to rotations about the central axis of the cylindrical inner surface. In a given region of an embodiment the distribution may be uniform along the central axis, whereas in other embodiments or other regions the distribution changes gradually or stepwise along the central axis.

Preferably, in a region of the inner surface a number of gas delivery orifices per unit area is at least 5/cm ² , preferably at least 25/cm ² , most preferably at least 100/cm ² . This ensures even distribution of the gas and minimum affection of the movement of the endless tube by the gas introduced into the small clearance between the inner surface of the guide element and the outside of the endless tube. Preferably, the inner surface of the guide element is constituted at least in part from a microporous or nanoporous material. Such materials are known and preferably comprise a sintered material, a material made by thermal spraying, a metallic foam (in particular made from aluminium), a plastic foam, in particular from polyurethane, or a ceramic foam. Advantageously, the inner surface is polished. Microporous materials allow for having average pore sizes, corresponding to the size of the gas delivery orifices, of 0.05 - 200 μπΊ, preferably of 1 - 30 μηη, pore sizes of nanoporous materials are in the range of 0.5 - 500 nm. In this context, "average pore size" denotes the arithmetic average of the diameters of all pores. Due to the manufacturing processes of such materials there will usually be pores having a size less than the given lower limits as well as pores having a size larger than the given upper limits. However, the distribution of the diameters of the individual pores will be essentially centred about the given average.

Micro- or nanoporous materials allow for an optimum distribution of the delivered gas and therefore for an optimum guidance of the endless tube and minimum affection of the movement of the endless tube by the generated gas streams.

In regions where the guide element is provided with a microporous or nanoporous inner surface the wall of the guide element may be constituted of the microporous or nanoporous material. Alternatively, the wall of the guide element is constituted by a supporting body carrying a micro- or nanoporous layer or coating on the inside. In case air is to be delivered from the back side of the micro- or nanoporous layer or coating the supporting body is preferably made from a lattice-like structure such as an open-worked metal sheet. In further embodiments, the wall will consist of or include a number of micro- and/or nanoporous layers; in this case, the pore sizes preferably decrease in the direction of the inner surface in order to ensure uniform distribution of the gas (i. e. the innermost layer may e. g. be a nanoporous layer which is applied to a microporous layer). In any case the geometry and the specific material will be chosen such that the mechanical strength of the structure conforms with the requirements of the specific process. Furthermore, the inner structure of the microporous wall, layer or coating will be such that (pressurized) gas may be fed into the material and transported through the material to the orifices, corresponding to the pores that are open to the inner surface of the guide element. Preferably, the material, the geometry of the guide as well as the gas pressure are chosen in such a way that a gas flow per unit area amounts to about 0.02 - 0.6 cm ³ /s per cm ² of the micro- or nanoporous surface.

Instead of micro- or nanoporous surfaces orifices may be used that have, been produced by working a continuous material layer (such as a metal sheet), e. g. by drilling, cutting, laser drilling or punching.

In a first preferred embodiment, the plurality of gas delivery orifices are arranged in a substantially ring-shaped region of the inner surface adjacent to the cutting element. This ensures controlled guidance of the endless tube immediately prior to cutting and therefore allows for reliably having a precise cut. Advantageously, the axial length of the ring-shaped region is at least 0.3 cm, in particular at least 0.6 cm. In the ring-shaped region the distribution of the orifices is essentially symmetric with respect to rotations about the central axis of the cylindrical inner surface, whereas it may be uniform or non-uniform along the central axis. The axial length of the guide element may be longer than the axial length of the ring- shaped region. Accordingly, the first section of the guide passed by the endless tube will not be provided by a gas cushion. However it may help centring the endless tube and improve the currents within the hollow cylinder of the guide. Furthermore, the first section may feature a tapered entry region further improving the introduction of the endless tube into the guide. In order to avoid direct contact between the inner surface of the guide and the outer surface of the tube, the inner diameter of the guide may be (slightly) larger in the section not having gas delivery orifices than in the ring-shaped region having the orifices. However, this is not compulsory as already the inner diameter of the ring-shaped region is larger than the outer diameter of the tube in order to create a clearance for accommodating the gas cushion.

In a second preferred embodiment, the plurality of gas delivery orifices are arranged in a first substantially ring-shaped region of the inner surface adjacent to the cutting element and in a second substantially ring-shaped region of the inner surface having an axial distance from the first substantially ring-shaped region. Again, in between the two ring- shaped regions preferably the inner diameter of the guide is slightly larger than in the ring- shaped regions itself in order to avoid direct contact between the guide and the endless tube. The second substantially ring-shaped region may be arranged adjacent to a tapered entry region of the guide element in order to ensure contactless introduction of the endless tube into the guide.

In a third preferred embodiment, the plurality of gas delivery orifices are distributed along the axial and azimuthal extensions of the inner surface. This provides for uniform guidance of the endless tube within the entire guide element. The substantially cylindrical inner surface of the guide element may be preceded by a tapered entry region of the guide element.

In preferred embodiments, in the region neighbouring the cutting element, the plurality of gas delivery orifices are divided into at least two sectors, whereas an excess pressure of the gas delivered by the gas delivery orifices of a first sector may be controlled to be different from an excess pressure of the gas delivered by the gas delivery orifices of a second sector. This allows for controlled deformation of the tube, in particular to force it to assume an oval cross-section. Compared to the case of symmetric feed of air, in regions of lower excess pressure the tube material will offer more resistance against pressure exerted by the knife, which in particular facilitates the entry of the knife into the tube material. Therefore, in connection with controlling the movement of the knife a clear cut may be ensured even at the position of the knife entering the tube.

As usually the guide element and the tube will be at rest with respect to each other during the cutting operation, the air cushion is not needed at this stage. Correspondingly, the pressure difference may be achieved by providing different values of positive excess pressure in all the sectors, but also by providing positive excess pressure in some of the sectors only, or even by sucking air in one or some of the sectors. Depending on the layout of the plurality of gas delivery orifices, the buildup of the corresponding surface of the guide element as well as the desired pressure difference, the different sectors need to be clearly separated with respect to the supply of air. In some cases a clear separation will not be required, e. g. if the surface is constituted by a layer of microporous material, a separation will not be required within that layer but only in a chamber arranged behind the layer.

Advantageously, the application of different pressures to the gas delivery orifices is synchronized with the operation of the inventive device, especially with the movement of the guide element and/or the operation of the knife. Even in cases where a deformation of the tube is not required, it may be advantageous to reduce or switch off the excess pressure during the cutting operation, i. e. when the guide element and the tube are at rest with respect to each other.

Advantageously, the device comprises at least one gas discharge opening in the inner surface of the guide element. This at least one gas discharge opening is not connected to the feed of (pressurized) gas but to a gas discharge and allows for discharging at least a part of the gas delivered into the clearance between the inner surface of the guide and the outer surface of the endless tube. The gas discharge opening may be large compared to the gas delivery orifices or it may be of a similar size (i. e. there may be a large number of gas discharge openings that may be micro- or nanoporous). In order to have a uniform discharge of excess gas it is preferred to have a large number of discharge openings and/or discharge openings that extend over a substantial area of the inner surface, such as a plurality of axial slits arranged along the circumference of the inner surface, extending essentially along the axial extension of the regions being provided with gas delivery orifices.

The discharge of the gas may be supported by the application of rough vacuum.

Preferably, the cutting element comprises a knife which is arranged to be rotated about a central longitudinal axis of the guide element as well as to be moved together with the guide element and the endless tube in the transport direction. This allows for cutting the endless tube during transport, along a strictly azimuthal cutting line using a knife of a simple construction. In order to clear the transport path of the endless tube during feeding the knife may be moved into a first position outside of the path of the tube as well as into a second (cutting) position where the knife interferes with the path of the tube. Alternatively, a ring-shaped cutting element is used which engages with the tube essentially along the entire azimuthal extension thereof.

Advantageously, the knife rests on an abutting face of the guide element. This allows for controlled guidance of the knife even when the blade thickness is small. Preferably, the knife rests on the abutting face even in the first (inactive) position. The knife and the abutting face of the guide element may be tensioned against each other with a predetermined force. The resilience of the blade and/or additional resilient elements (such as springs) bears that force. The force may be chosen by adjusting the axial distance of the abutting face and a knife holder or by further measures such as tensioning a spring acting on the knife holder.

Therefore, in this embodiment, the guide element is at the same time supporting ia'nd guiding the endless tube as well as supporting the knife, inter alia during the cutting operation. The minimum distance of the knife with respect to the guide (and possibly with respect to the orifices) ensures the best possible geometry of the endless tube at the cutting position.

Preferentially, a ratio between a length of the substantially cylindrical inner surface of, the guide element and a diameter encompassed by the substantially cylindrical inner surface is at least 1.5, preferably at least. 2.0. It has turned out that such geometries alloW for reliable guidance of the endless tube ensuring a precisely circular geometry of the tube at the exit of the guide, i. e. at the cutting position.

Alternatively, depending on the diameter of the endless tube and the rigidity of the tube material, other ratios are possible.

Advantageously, the guide element is surrounded by a chamber, whereas gas to be supplied to the gas delivery orifices is supplied to the chamber and further supplied to the gas delivery orifices. This allows for a simple construction and uniform gas pressure at the gas delivery orifices. In the case of a plurality of spaced regions being provided with gas delivery orifices, a single chamber may surround all or a plurality of these regions (allowing for differen gas pressures and/or temperatures) or all regions may be connected to the same chamber. Furthermore, a plurality of chambers may adjoin the outer surface of a single micro- or nanoporous body. Usually, at least if the pressure differences do not exceed a certain value, it is not required to seal neighbouring sections of the micro- or nanoporous body adjoined by different chambers from each other as transmission of the applied (pressurized) gas in the radial direction is usually strongly preferred against transmission in the axial direction. Therefore, the mixing effects in the boundary regions will be small and will not affect the guiding of the endless tube.

Alternatively, a tube shaped element providing the substantially cylindrical inner surface may be provided by one or a plurality of connectors for feeding (pressurized) gas and inner conduits distributing the gas to the orifices. Other advantageous embodiments and combinations of features come out from the detailed description below and the totality of the claims.

Brief description of the drawings

The drawings used to explain the embodiments show:

Fig. 1 A, 1 B schematic perspective and cross-sectional views of a first embodiment of a device according to the invention;

Fig. 2 a cross-sectional view of a guide element of a second embodiment of a device according to the invention;

Fig. 3 a cross-sectional view of a guide element of a third embodiment;

Fig. 4 a cross-sectional view of a guide element of a fourth embodiment; Fig. 5 a cross-sectional view of a guide element of a fifth embodiment; and

Fig. 6 a cross-sectional view of an asymmetrically deformed tube in a sixth embodiment of the invention.

In the figures, the same components are given the same reference symbols. Preferred embodiments

Figure 1 A shows a perspective schematic view of a first embodiment of a device according to the invention. Figure 1 B shows a cross-sectional schematic view of this embodiment along a plane including the central axis of the guide element of the device. The cutting device 100 comprises a guide element 1 10 having a substantially cylindrical inner guide surface 140. It further comprises a cutting element 150 which is supported on the guide element 1 10 in such a way that it is freely rotatable about a central longitudinal axis of the inner guide surface 140.

The guide element 1 10 includes a base body 1 1 1 which is made from stainless steel. Substantially, it has the form of a hollow cylinder, the polished inner surface of which forming a first part of the inner guide surface 140 of the guide element 1 10. In the vicinity of an intake opening 1 12, the base body 1 1 1 forms a conical surface 1 13 the diameter of which is increasing in the direction of the intake opening 1 12. In the vicinity of its other end, the base body 1 1 1 comprises a region 1 1 with increased inner diameter. This region accommodates an insert 120 which also has substantially the form of a hollow cylinder. The diameter and wall thickness of the insert 1 20 are chosen such that the back end of the insert 120 fits into the region 1 14 of the base body 1 1 1 with increased inner diameter. The inner surface of the insert 120 constitutes a second part of the inner guide surface 140 of the guide element 1 10. In the shown example, the inner diameter of the guide element 1 10 is 4.0 cm, the length of the cylindrical part is 10.2 cm, including the insert having a length of 3.0 cm.

The insert 120 is made from a micro-porous material, its inner surface having a large number of micro-openings 126 which are connected by the pores of the micro-porous material to a ring-shaped chamber 1 1 5 of the base body 1 1 1. The chamber 1 15 encloses the insert 120 along substantially its entire length apart from two supporting surfaces of the base body 1 1 1 in the end regions of the insert 120. The chamber 1 15 is connected to the outside of the base body 1 1 1 by a conduit 122. The mouth of the conduit 1 22 leaving the base body 1 1 1 is connected to a source of pressurized gas (not shown). Possible ways of distributing the air fed to the micro-openings 126 of the insert 120 are described below, in connection with Figures 2 - 4.

On the outside, adjacent to the front end of the base body 1 1 1 (i. e. in the direction to where the insert 120 is attached to the base body 1 1 1 ) a suitable roller bearing 135 is held on the base body 1 1 1 , as schematically schown in Fig. 1 B. The outer bearing surface of the roller bearing 135 is fixed to a ring 155 which forms part of the cutting element 150. A rocker 161 is connected to the ring 155 by means of a swivel bearing 162, the axis of which being oriented parallel to the longitudinal axis of the inner guide surface 140. One end of the rocker 161 accommodates a knife 163 comprising a sandwiched structure supporting a blade 164. The back side of a front portion of the blade 164 not being sandwiched by the sandwich structure is supported on the abutting surface 1 21 of the insert 120.

The rocker 161 may be swivelled about the swivel bearing 162 into a first position, where the blade 164 enters into the endless cylindrical space in front of the guide element 1 10, corresponding to a continuation of the interior space of the guide element 1 10, or into a second position where the blade 164 does not interfere with an endless tube 50 being fed through the guide element 1 10. Both the first and the second positions are defined by a corresponding stop element (not shown) for the rocker 161. Usually, at rest, the rocker 161 is held in the first position by means of a spring (not shown) accordingly acting on the rocker 161. As soon as the rocker 161 is rotated with a sufficient speed, the centrifugal forces acting onto the rocker 161 will move the rocker 161 into the second position. As soon as the rotational movement is decelerated such that the effect of the spring outbalances the effect of the centrifugal forces the rocker 161 will again be moved into the first position. The device shown in Figures 1 A, 1 B is operated as follows: The ring 1 55 of the cutting element 150 is set into rotational movement by means of a usual drive, e. g. a belt or pinion drive driven by a corresponding motor (not shown), whereas the rotation speed is chosen such that the rocker 161 is moved into its second position where the blade 164 does not interfere with the cylindrical space in front of the guide element 1 10. An endless tube 50 to be severed into base bodies for packaging tubes is supplied from the right hand side of the Figures 1 A, 1 B, i.e. fed into the guide element 1 10 through the discharge opening 1 12. The conical surface 1 13 adjacent to the discharge opening 1 12 facilitates the insertion of the tube 50. The inner diameter of the guide element 1 10 is slightly larger than the outer diameter of the tube 50 such that the tube may be guided essentially without direct contact between the guide element 1 10 and the tube 50.

The tube 50 leaves the guide element 1 10 at the front end of the insert 1 20. As soon as the tube 50 is forwarded into a position, in which the plane defined by the rotational movement of the blade 164 of the knife 163 (i. e. essentially the plane defined by the abutting surface 121 of the insert 120) corresponds to a desired cut, the section of the tube 50 that has already passed the guide element 1 10 is severed from the rest of the endless tube 50. For that purpose, the ring 1 55 is decelerated (e. g. by the motor itself and/or using brake means known as such). As soon as the speed falls below a certain critical speed the rocker 161 is swivelled about the swivel bearing 162 in such a way that the blade 164 engages with the endless tube 50. The swivelling angle of the rocker 16 1 is caused by the spring mentioned above and defined by one of the stop elements of the ring 155. The ring 1 55 and thus the knife 163 are rotated such that the blade 164 forms a circumferential cut in the endless tube 50.

The continuous movement of the endless tube 50 is accounted for in a way that is known as such, by moving the entire cutting device 100 together with the endless tube 50 such that the relative axial position of the tube 50 and the cutting element 1 50 remain fixed during the cutting step. As soon as the tube body has been severed from the endless tube 50, the rotational movement of the ring 155 is accelerated until the centrifugal forces acting onto the rocker rotating about the central axis of the guide element 1 10 lead to a swivelling motion by which the blade 164 is moved out of engagement with the endless tube 50. Subsequently, the entire cutting device 100 may be moved in a direction opposite to the feed direction of the endless tube 50 in order to engage the tube 50 with the cutting element 150 at the next cutting position. During all the phases of the process, and especially while cutting, the endless tube 50 is reliably guided within the guide element 1 10 by the cushion of pressurized gas supplied through the micro-openings of the insert 120. Figure 2 shows a schematic cross-sectional view of a guide element of a second embodiment of a device according to the invention. In contrast to the embodiment shown in Figure 1 , the guide element 210 is unitary, i. e. the functions of the base body 1 1 1 and the insert 1 20 are assumed by a single element, namely the guide body 21 1. It is entirely manufactured from a micro-porous material such as a polished metal foam (e. g. made from aluminium). With the exception of the conical surface 213 in the vicinity of the intake opening 21 2, the inner surface 240 of the guide body 21 1 is cylindrical. Essentially the entire outer surface of the guide body 21 1 is enclosed by a chamber 21 5. In Figure 2, this chamber is shown in a schematic way, there are a number of ways of implementing this chamber. In particular, the chamber may be formed by a jacket closely surrounding the base body 21 1 of the guide element 210, leaving only a passage for circulation of gas in between. Alternatively, essentially the whole length of the cylindrical inner surface is surrounded by a more spacious chamber.

A source 270 of pressurized air leads into the chamber 2 15. Suitable sealing elements (not shown) are provided against the escape of substantial amounts of pressurized air out of the chamber 2 15. The pressurized air fed to the chamber 2 15 traverses the wall of the guide body 21 1 and provides a cushion of pressurized air on the inner surface 240. The second embodiment therefore allows for guiding the endless tube along substantially the entire length of the guide body 21 1.

Figure 3 shows a schematic cross-sectional view of a guide element of a third embodiment. Again, the guide body 31 1 of the guide element 310 is unitary, however it is not made from a single material, but the guide body 31 1 is made from stainless steel with the exception of the inner surface adjacent to the exit end of the guide element 310. In this region the wall thickness of the guide body 31 1 is reduced to form a recess on the inside and a ring-shaped layer 316 of a microporous polished metal foam is accommodated within this recess. The thickness of the layer is between 1 and 5 mm. Behind the ring-shaped layer 316 a number of uniformly distributed air conduits 317 are connected to air feeds 318 formed in the wall of the guide body 31 1. The air feeds 318 lead to a chamber 31 5 which is fed from a source 370 of pressurized air. In the context of the third embodiment, the pressurized air fed to the chamber 31 5 is further fed to the conduits 317 by the air feed 318 and traverses the ring-shaped layer 316 to form a cushion of pressurized air on the inner surface 340. The inner diameter of the back part of the guide body 31 1 made from stainless steel is polished, due to the fact that it is slightly larger than the diameter of the endless tube to be fed there will usually be no direct contact between the inner surface of the guide element 310 and the tube.

Figure 4 shows a schematic cross-sectional view of a guide element of a fourth embodiment.

Again, the guide body 41 1 of the guide element 410 is unitary, made from stainless steel with the exception of the inner surface adjacent to the exit end as well as the inner surface adjacent to the intake end of the guide element 410. In these regions the wall thickness of the guide body 41 1 is reduced to form ring-shaped recesses on the inside, ring-shaped layers 416a, 416b of a microporous polished metal foam are accommodated within this recess. Behind the ring-shaped layers 416a, 416b a number of uniformly distributed air conduits 417a, 417b are connected to air feeds 418a, 418b formed in the wall of the guide body 41 1. The air feeds 418a, 418b are fed by a source 470 of pressurized air. In the context of the fourth embodiment, the pressurized air is fed to the conduits 417a, 417b by the air feeds 418a, 418b and traverses the ring-shaped layers 416a, 416b to form two cushions of pressurized air on the inner surface 440. The inner diameter of the intermediate part of the guide body 41 1 made from stainless steel is polished, due to the fact that it is slightly larger than the diameter of the endless tube to be fed there will usually be no direct contact between the inner surface of the guide element 410 and the tube.

Figure 5 shows a schematic cross-sectional view of a guide element of a fifth embodiment. The guide element 510 of the fifth embodiment essentially corresponds to the third embodiment shown in Figure 3. In addition, the guide body 51 1 is provided with a number of axial grooves 519 extending from the front region being provided with the ring-shaped layer 516 of the micro-porous polished metal to the end of the cylindrical inner surface 540. These grooves 519 allow for discharging excess air from the air cushion. Figure 6 shows a cross-sectional view of an asymmetrically deformed tube in a sixth embodiment of the invention. The guide element of this embodiment comprises a section neighbouring its exit end which has a microporous inner surface 626. About its circumference this section is divided into four equal-sized sectors which may be independently charged with pressurized air, i. e. it is possible to independently vary the amount (or pressure) of supplied air to each of the sectors. As shown schematically in Figure 6, this allows for asymmetrically deforming the tube 50: The air pressure in two opposing sectors 626.1 , 626.3 is higher than that in the other two opposing sectors 626.2, 626.4. Accordingly, the tube 50 is slightly deformed to assume an oval cross-section, the longer semi-axis roughly extending between the centers of the sectors 626.2, 626.4 of lower pressure. It is to be noted that the representation of the asymmetry in Figure 6 is greatly exaggerated for presentation purposes.

When starting the cut of the tube 50, the blade 664 is controlled to enter the tube 50 at one of the positions where the tube 50 closely contacts the inner surface 626 of the guide element. Due to the asymmetric feed of air, the tube material will resist the forces of the entering blade 664, thus ensuring a clean cut in the entry region of the blade 664.

It is to be noted that the tube 50 does not move with respect to the guide element during the cutting process. Therefore, the feed of air in the sectors 626.2, 626.4 of lower pressure may be stopped within this phase of the process, it is even possible to suck air through the microporous inner surface 626. Alternatively, air is fed during the entire processing, whereas a pressure difference is created during cutting, or at least at the time of the blade 664 entering the tube. Furthermore, it is to be noted that the number of sectors is not restricted to four, but it may be different, in particular higher than four.

The invention is not restricted to the embodiments described above. First of all, individual elements of these embodiments may be combined differently. A surrounding chamber and/or specific feeds may be provided for supplying pressurized gas to the orifices in the inner surface of the guide. Any structures featuring a microporous inner surface may be formed entirely from the microporous material (or a combination of different microporous materials) or formed by a supporting structure featuring a microporous inner layer. Advantageously, this inner layer has a thickness of 0.5 - 10 mm, preferably of 1.0 - 5.0 mm. The supporting structure may comprise a geometry that allows for uniformly distributing air to the microporous inner layer.

In summary, it is to be noted that the invention creates a device for severing an endless tube that allows for high throughput, the creation of precise cuts and a reduction of damages to the tube and of contamination by abrasion.