Method and apparatus for helical cutting of a flexible tubular film of polymeric material

(74) Agent

Ole-Bendt Rasmussen, Forchwal Strasse 23, CH-6318 Walchwil, CH Applicant

Knut B. Byklum, Bryns Patentkontor AS, Oslo (54) Designation Method and apparatus for helically cutting a flexible tubular film of polymer material (56) Published publications US 2644522 (57) Summary Flexible tubular film material (10) of polymer material is supplied in a flat shape to a first position (for example, between the rollers 11) and cut helically or splally with the knife (18) to form a cut strip which is pulled away by a unit (20). The flat strip is pulled from the first position and opened to a tubular shape and then pulled over a mandrel (14) where it is cut. Preferably, it is inflated with air from a fan (13).

<img file="NO178692B_D0001.tif" />

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method in which a flexible tubular film of polymeric material is delivered in a flat form to a first position and cut helically to form a cut strip which is pulled away from the tubular film, as will be more apparent from the preamble of the subsequent independent process claim.

The invention also relates to a device designed for carrying out this method.

¹⁰ Helical cutting of a flexible tubular film of polymeric material is disclosed in GB 816 607. The purpose of this patent was to produce strips with a strong molecular, oblique orientation.

The strips are mainly used for the production of high-strength cross-laminates of uniaxially oriented layers. To¹⁵ achieving the desired properties, the tubular film is strongly oriented in its original longitudinal direction before the helical cutting, which is usually performed over a mandrel. The tube is fed either with a screw movement before cutting with a fixed knife, or with a linear movement before cutting with a rotating knife²⁰ knife, in which case the winder for recording the helically cut film must rotate with the knife in a planetary motion.

Helical cutting of flexible tubular film is also used in 25 GB 1,526,722. The tube to be cut usually has a single-axis polymeric structure, which is produced during extrusion. However, prior to the helical cutting, the extruder need not provide any significant orientation of the film below its melting point. The object again is the production of high-strength 5 ° cross-laminates, but in this case the layers are biaxially oriented by stretching after or during the lamination, where the transverse and longitudinal stretching processes are mutually independent steps and each step is generally uniaxial.

These patents show no means for synchronizing the various motions which control the helical cutting, i.e. the advancement of the film, rotation of the film or cutter, and retraction of the cut film. However, some degree of such synchronization can be developed using conventional engineering techniques.

Nevertheless, certain problems remain.

One problem is that the cut of the cut occurs at an angle with respect to the original direction of travel of the film and is therefore apt to produce torque forces in the tubular film. This tends to cause distortion of the tube at the time of cutting, thus interfering with the cutting and the resulting cut strip. It is very difficult to balance the forces to avoid this distortion.

Another problem is that it can be difficult to obtain a regular and accurate feed of film to the cutting station, especially when the film is delivered in flat form wound on a very heavy drum.

As is well known, there are many instances where it is desirable to deliver films of film material oriented at an angle to the longitudinal axis and, as indicated above, the known helical cutting processes can be used for this purpose. However, it is desirable to provide a more technically feasible and reliable process, which is also suitable for heavy or wide drums and for high production speeds.

The objects of the invention are to overcome these various problems and to provide methods and devices for improved helical cutting of the films and to provide improved films.

In accordance with the present invention, there is provided a method and arrangement of the kind mentioned in the introduction, which are characterized by the features which are apparent from the characteristics of the following independent claims.

Thus, with the invention, air from the mandrel is pressed into the tubular film as it moves from its flat state to the mandrel, thereby inflating the film to a tube. The mandrel must have a diameter only very little smaller than the tube, to allow sufficient pressure to be inside the tube to make it sufficiently rigid. However, some air between the pipe and the mandrel inevitably escapes and this escape of air can provide lubrication between the pipe and the mandrel.

As a result of the tube being rigid as it slides across the mandrel toward the position where the helic is cut, the distortion of the tube by the cutting and pulling of the cut strip can be significantly reduced. The degree of stiffness required for a particular film will depend on the material used and the process conditions. However, it is easy to select the air pressure inside the pipe, and thus the rigidity of the pipe, to achieve the degree of resistance to deformation that is optimal for the particular process. Thus, with the invention, it is possible to apply much higher forces during the extraction of the helically cut strip from the mandrel and winding of this cut strip.

The pressure inside the pipe, and the air supply through the mandrel, will be selected according to the materials used. Eg. relatively low pressures should be used when the film tends to split, e.g. when the film is of low aim, high degree of uniaxial orientation, and / or high stiffness. However, if the pressure is too low relative to the oblique tensile forces, then the tubular foil will lose its rigidity and will be twisted and the process will become less accurate or can cease. Provided that the film permits, it is usually desirable that the air pressure be sufficient to allow a tightening of at least 200 g / l. meters width in the foil upon extraction from the mandrel, and provided the foil permits, the tightening is preferably at least 2 kg per meter. m.

The advantages of the invention are mainly achieved when the flat tube has a width of at least 20 cm. The width can be up to 200 cm or even more.

The cutting angle is usually at least 20 '. Values ​​of up to 70 'can be obtained relatively easily with good accuracy, but it is also possible to obtain values ​​approaching 90'.

Preferably, the diameter of the mandrel is variable. Eg. it may be made of a tubular rubber plate held in a circular array of ribs carried by extendable devices (e.g., in the same way as the carrier ribs in an umbrella). Adjustment of these extensible devices will thus result in adjustment of the effective diameter of the mandrel and thus the same mandrel can be used for different pipe widths. The diameter can also be varied to control the air pressure. This is of particular value when cutting fragile films, such as thin, rigid and / or high-orientation sheets, when minimizing air pressure and air flow is important. In this situation, the variable diameter of the mandrel can be feedback controlled by recording the air flow or overpressure inside the tube.

The surface of the mandrel may be corrugated to reduce the friction between the foil and the mandrel. These corrugations may conveniently consist of substantially longitudinal ribs. The corrugations promote air flow over the entire circumference of the mandrel and therefore improve the lubrication of the annular air flow between the film and mandrel.

The helical cutting may be arranged in line with the first extrusion of the flexible tubular film, but the machinery for this combination will tend to be relatively complicated and will usually only operate at relatively low production rates or quantities. It is therefore preferred to first extrude the tubular film and then wrap it in flat form on a coil or drum. This coil is then mounted in a unwinding device and at the same time causes the tubular film to unwind in flat form and rotate about its center axis. This place of cutting could then be fixed relative to the room and the film moves linearly after cutting. This is more appropriate than making sure that the film moves linearly and for the knife to rotate around the mandrel, along with the winding machine.

In a simple form of apparatus for ensuring that the flat film is rotating in the first place, the axis of the drum is mounted perpendicular to the axis of rotation of the tube with the center of the drum on or near the latter axis. Such an arrangement is shown in FIG. 1 of the accompanying drawings. This method is very satisfactory for spiral cutting of drums which are not too heavy (e.g. up to about 400 kg) provided the drum is of the correct size and thickness and has its center of gravity correct on the axis of rotation and provided the rotation speed is not too high. However, the method is less satisfactory if the coil is very heavy, rotates very quickly, or has imperfect shape or tightness. These problems exist even when there is no air flow through the mandrel. A new flat film application system is provided which overcomes these problems.

Thus, in accordance with yet another aspect of the invention (which need not be combined with inflation of the tubular film with air through the mandrel), the site of cutting is fixed (i.e. is fixed relative to the space in which the process is performed), the foil is initially delivered from a coil and released from the coil into an exit zone and pulled to the mandrel by a screw motion, and the foil is pulled from the coil substantially along the axial direction of the coil while the exit zone spins around the coil to produce rotation of the tube (relative to the room). Preferably, the coil has substantially the same axis as the axis of rotation of the tube and the tubular foil is guided and maintained in flat form from the unwinding to the exit zone and conversion to tubular form begins at this zone, whereby the center line of the flat foil is led to be substantially coincident with coil axis.

This technique minimizes the problem of mechanical forces in the rotary unwinding and reduces the need for accurate adjustment of the coil position in the winding device. It also minimizes the risk of telescoping during unwinding of the film that is too loosely wound on the bobbin. Thus, it allows the use of much heavier coils and much higher speeds.

The bearing or bearings around which the unwinding unit rotates can be placed at only one end of the unit, namely at the end i5 opposite to the cutting as shown in FIG. 1. However, when heavy weights are included, this system is vulnerable to fatigue failure unless the shaft, bearings and support of the latter are of particularly heavy construction.

Another aspect of the invention (which may again be in operation in combination with or without air inflation and with or without defined mounting of the coil) provides a solution to this problem. In particular, the site of cutting is fixed (relative to the room), the foil is delivered to a coil mounted in a winding unit and drawn to the mandrel from the coil by a screw movement provided by rotation of the winding unit, and the winding unit is mounted on a bearing including a bearing ring which surrounds the tubular plate.

The method of the invention is usually intended to produce high-strength cross-laminates, and for this purpose two or more helically cut films are subsequently laminated with each other with the original longitudinal directions of the tubular film intersecting. The tubular film is therefore preferably given a uniaxial melt orientation. Alternatively or in combination with this, the tubular film may be longitudinally oriented below the melting range of the polymeric material.

As geometric conditions or conditions show, the following equation relates to the width (h) of the flat tube, the cutting angle (v) measured between the longitudinal direction of the flat tube and the cutting direction, and the final width (w) namely:

w = 2h cosine v

By way of example, with a cutting angle of 60 ° C, the final width would be equal to the flat width, at 45 ° the flat width would be increased by Ί2 (1.41) and by a cutting angle of 30 ° 73 (1.73). With cutting angles higher than 60 °, the achievable width for a given helical cutting machine would rapidly decrease.

Another aspect of the invention has the object of increasing the achievable final width for a given final angle for the melting orientation and for a given spiral cutting machine, especially in the case of angles about 60 ° or closer to 90 °. Although this aspect is preferably used in combination with air inflation and possibly one or more of the other aspects of the invention, it can again be used independently of these.

In this aspect of the invention, the tubular sheet material was first prepared by extrusion through an extrusion nozzle with spiral melt orientation upon withdrawal in the molten or semi-molten state and at the same time relative rotation between the withdrawal device and at least the output of the extrusion nozzle, and the spiral cutting is performed in such a direction. in relation to the longitudinal direction of the cut strip.

In theory, it could be assumed that the helical cutting could be omitted (except when a 90 ° angle is desired) if the relative rotation was sufficiently fast. In practice, however, the difficulty of performing such extrusion increases with the increased orientation angle and it is found that an increased angle inherently leads to an increased component of biaxially melt oriented material, and this is usually not desirable, at least in the manufacture of cross-laminates. Eg. it is usually not possible to produce angles higher than about 30 ° at such relative rotations in conjunction with the extrusions from the extrusion die.

Thus, this aspect of the invention, in a simple practicable manner, allows a substantial increase in the angle with respect to the longitudinal direction of the cut strip. Eg. the 30 'angle from the extrusion can be made as a 60' angle by cutting below 30 ° or it can be made perpendicular by cutting below 60 °. In the former case, the flat width is increased by a ratio of 1.73: 1 and in the second case it is the same before and after the spiral cutting.

The strip obtained in all aspects of the invention may be relatively wide, typically 50 cm or more, and thus for the first time the invention provides for a strip of 50 cm or greater width and having an angle of melt orientation of 70 ° up to perpendicular . This material forms a further aspect of the invention. Preferably, but not necessarily, use is made of the air inflation technique described above. This material provides important advantages and yet its production, as a result of the invention, is relatively simple without the use of a tension frame or similar device. In particular, it is now relatively easy to produce substantially perpendicularly oriented film material e.g. 3 meters width even on multi-ton drums (especially when the second aspect of the invention is used, preferably in combination with the third aspect).

Extruded foil material will generally exhibit a structure showing the discharge direction from the extrusion die, and the new filler material according to the invention is characterized in that the width is at least 50 cm perpendicular.

This new filler material can e.g. is used to prepare a new type of biaxially oriented film in which at least the majority of the transverse orientation is produced at the outlet of an extrusion nozzle and the length orientation is produced at a much lower temperature. The longitudinal orientation and possibly extra cross-orientation at relatively low temperature can advantageously be carried out with the method described in WO 88/05378.

The various aspects of the invention are of particular value for producing a cross-laminate of layers having a substantially uniaxial material structure from the extrusion, and wherein the material structure of different layers is in a cross-laminated relationship but has subsequently become biaxially oriented. Such cross-laminates are described in GB 1,526,722. When such cross-laminates are manufactured, it is desirable to produce one or more layers with a material structure from the extrusion forming an angle of 70 to 90 'with the longitudinal axis, especially in combination with one or more layers having a material structure of 0' or close to it.

It has been found that cross laminates of this type and with these angles (especially about 90 'combined with about 0') show maximum tear strength in the 45 'direction and it has been found that the need for high tear strength in a sewn seam is highest at the angle of 45 'in relation to the river direction. Therefore, such cross-laminates are e.g. very suitable for sewn sacks and for sewn raincoats and other sewn products.

A particularly preferred composition in such laminates, or for the main layer in an individual layer of the laminate (or in the main layer of the layer if the layer consists of several co-extruded sub-layers), is formed by a mixture of high molecular weight, high density polyethylene with considerably lower molecular weight, low density polyethylene. The latter is preferably selected from copolymers and / or branched polyethylenes having the same or higher break elongation (tested at room temperature under slow stretching) as the high molecular weight polyethylene capable of distinct segregation, while forming a distinct microphase, from the high molecular weight. the polyethylene by cooling a molten homogeneous mixture of the components. The polyethylene blend ratio is preferably 25:75 to 75:25. The incorporation of polypropylene with substantially lower molecular weight than high molecular weight polyethylene may also be advantageous in amounts from 0 to 70% based on the combined weight of polypropylene and both polyethylenes. The high molecular weight, high density polyethylene preferably has a melt flow index of 0.2 or lower at ASTM D1238 ratio E, and the low density polyethylene is preferably linear low density polyethylene.

The invention will now be explained in more detail with reference to the schematic drawings in which:

Fig. 1 is a horizontal top view of the helical cutting device with a rotary unwinding unit, in which the axis of the coil from which it is wound is perpendicular to the axis of the inflated tube. The rotary winding unit is carried in bearings at only one end.

FIG. 2 is a similar view showing another unwinding unit in which the axis of the coil from which it is unwound is substantially coincident with the axis of the inflated tube and the flat tube is passed over one end of this coil. The drawings also show a particularly suitable large roller bearing which surrounds the tubular foil. The extension system for removing the foil from the coil is not shown.

FIG. 3 is a perspective view of the guide system omitted from FIG. 2. Rollers shown only as dashed lines indicating their axes.

The device of FIG. 1 includes a unwinding unit 1 having a shaft 2 at one end and rotating about the axis of the shaft carried by a shaft 3 through large roller bearings 4. The shaft 3 is mounted on the floor of the room. For simplicity, the unwinding unit 1 is shown to have a housing consisting of an end plate 5 and two side plates 6. The drawings show the unit at the time of rotation when these side plates are in a horizontal position and only the upper plate, indicated by dashed lines, can be seen. In practice, a framework, mainly of profile steel, may be preferred over the panels, as it will be lighter and more manageable.

The bearing shaft 3 which carries a fixed sprocket 7 which engages another sprocket 8 mounted on the end plate 5 and therefore causes a planetary movement. The wheel 8 delivers operation to the various rollers and wheels 1 unit 1, but for the sake of simplicity the transmission of these movements is not shown.

A drum or coil 9 with flat tubular film 10 is mounted through bearings in the side plate and is provided with a brake, where the bearings and brake are not shown. The flat film 10 is removed from the coil 9 by means of a set of nip rollers 11 defining the first position in the process. The transmission between the gear 8 and the rollers 11 determines the cutting angle. Preferably, the gear ratio is arranged to be variable, e.g. by providing the possibility of changing gears.

There is a cylindrical cutter 14 and an air nozzle for inflating the film 10 and blown through a fan 13, such as a centrifugal fan, through the mandrel to the first position. The mandrel is mounted on the floor through a shaft 15 and makes a relatively tight fit with the tubular foil 10 sliding over it. The air will escape through the narrow space between the door and the foil and will produce some lubrication effect. The tip 16 of the mandrel can be rounded to avoid hanging the foil.

The lubrication effect can be improved by correcting the mandrel surface. The corrections are preferably a circular array of ribs. They can be arranged only on one side of the mandrel, namely the one opposite to the pull direction, or can be deepest on this side, since this is where the tensions are highest.

It can be seen that the axis of the inflated tube (23) coincides at least substantially with the axis of rotation of the unwinding unit 1.

In order to achieve a smooth screw movement of the inflated tubular film, it is very advantageous (and may even be necessary if the film is wide) to provide a driven support from the first position to a position at which the tube is inflated. Appropriate support for the inflation tube may be a pair of belts, and as shown in FIG. 1 there are two pairs of drive belts 17 which carry the tube against the backward force from the air exiting the mandrel 14 towards the first position, and promote a uniform change in the shape of the tube from the flat shape to the cylindrical shape. The straps are driven at substantially the same speed as that of the foil, or at a higher speed. Instead of using two sets of belts, a pair of single belts, or two sets of rollers of a relatively small diameter (only the smallest roller needs to be driven) could be used. For a relatively narrow, flat film, a pair of large diameter drum-like rollers may be sufficient.

The tubular foil is cut with a simple knife or blade on a support 18 which can conveniently be attached to the mandrel at an adjustable angle. The edge 19 shown in the drawing to the left of the knife, which becomes the right edge of the final foil, first moves downward and below the cutting door.

The cut sheet is drawn by a driven selection unit 20 shown schematically. The cutting angle, as mentioned, is determined by the ratio of the rotational speeds of the unit 1 to the rollers 11, but the direction of the knife or blade and the pull exerted by the unit 20 must also be adjusted to approximately fit the rotation-determined cutting angle. The winding unit 20 includes a dancing roller 21 and two additional frills 22 to control the speed of the winding device 20 and thus set up an appropriate tension.

By appropriately adjusting the air pressure inside the tube (relative to the ambient air pressure around the tube), high stresses can be applied by the unit 20 (except in the case of very fragile foils, when lower tension must be used). Thus, when a flat foil of about 100 micron dimensions and 1 meter wide is cut, the tightening from 5 to 20 kg / m of finished width is usually applicable.

It is to be understood that the support of the rotary unwinding unit, i.e. the shaft 2 of the bearing 4 of the shaft 3, is only one example of support systems. To carry heavier weights, this support may conveniently be supplemented or replaced by one or more large steel rings, which surround and form part of the rotary unwinding unit, which stand and roll on a plurality of support rollers or support wheels and serve as bearing rings. Alternatively, the carrier attached to the floor may form part of one or more bearing rings, on which support and roll one or more circular arrays of rollers or wheels, which surround and are mounted on the rotary unwinding unit.

As a further modification of the machinery shown, the sprocket 7, instead of being attached to the shaft 3, can be rotatable and driven by a separate motor which is electronically controlled relative to the motor for rotation of the entire winding unit so as to provide a simple adjustment of the cutting angles.

While the drawing shows the winding unit rotated and the knife as well as the winding unit being stationary, the opposite is also possible, namely a stationary winding unit, while the knife 5 together with the winding unit rotates around the axis of the inflated tubular film. Although this latter system is convenient for relatively narrow films, it is less satisfactory for wide films.

¹⁰ In FIG. 2 and 3, the coil 9 has its axis substantially coincident with the axis of the inflated tube and thus also with the axis of rotation of the unwinding unit 1. The shaft of the coil is carried in a bearing 24 fixed to the end plate 5 and the bearing 25 fixed to a beam 26, shown. in cross section. This beam connects the two side plates 6. Bearings 24 and 25 can be opened for changing coils with devices not shown. An adjustable brake 27 for unwinding or unwinding is shown schematically.

In order to facilitate the understanding of FIG. 3, the edges of the flat tube are marked a and b, respectively, at various stations along their path.

The coiled flat sheet is guided by the intermediate roll 28, the film 25 faces the rod 29 which may be a fixed roller, the intermediate roll 30 and the set of driven nip rolls 11 which are similar to the article 11 in FIG. 1. The aforementioned rollers and the rod of their lengths are attached directly or through beams (not shown) to the side or end plates 6 and 5 and the rewind unit 1.

The flat film is stretched from the coil at the output zone 31 and the entire film-conducting system and the output zone 31 will spin around the axis 23.

The rest of the coil cutting machine can be constructed like 55 FIG. 1, except that 1 fig. 2, a larger roller bearing 34 is shown. This is shown as the cross-section of a large ring attached to the side plates 6 (the connection is indicated by dashed lines) and two carrier coils or rollers 32 which are attached to a support (not shown) through 1'aggers. The two rollers, as shown, can only support the rotary rewind unit for lateral movements, and of course there must also be one or preferably several such wheels to hold the weight of the unit. Preferably, there is a circular array of such wheels or rollers.

The direction of rotation of the coil 9 1 relative to the housing or frame of the coil assembly 1 should preferably, but not necessarily, be opposite to the direction of rotation of the housing. This will make the absolute rotation of the coil 9 as small as possible.

Depending on the cutting angle v and the flat width of the tube h, there will be an instantaneous radius of the coil (r = - ', „) for which the absolute rotation tangent v ^u the speed of the coil 9 is zero although the housing of the rewind unit (and with this output zone 31) can spin around the axis 23 at high speed. The weight of the rotating machine parts is therefore a significant limiting factor for the capacity of the spiral cutter and even more in this embodiment than it is in the embodiment according to FIG. 1. Therefore, instead of the unwinding unit based on the end wall 5 and the side wall 6, it is particularly desirable to use a low weight framework, in which the ring 31 and optionally a corresponding ring at the other end are integral parts.

In order to guide and flip the flat tube in the desired manner, the rod or fixed rollers 29 must form an angle close to 45 ° to the axis 23. This angle is preferably made automatically adjustable over a small angle range and adjusted upon insertion into a sensor that senses the position of the one of the edges. In this way, the center of the plate 10 is controlled to reach the axis of rotation 23, even when the coil 9 is not wound clean or has been placed in some incorrect position on the shaft.

When the rotation of the rewind unit is very fast, as is usually intended, the guide of the foil shown in FIG. 3, brought out by the arrangement of air turbulence. To overcome this, a wind shield or baffle (preferably transparent) may be provided around the entire rotary rewind unit 1, attached to and rotating with the latter.

When the knife is at a constant location and the tubular film is carried away from the coil along the axial direction of the latter, while the zone 31 spins around the coil, a guiding system is similar to FIG. 3 advantageous, but not essential. Especially when the cutting angle is relatively small, e.g. about 30 ', the foil can be allowed to curl and even compact to form, since subsequent inflation with air can return it to tubular form. However, carrier belts 17 will generally not suffice in such cases and should preferably be replaced by a circular array of many narrow belts or belts. At the exit from the support system, the diameter of this arrangement should be close to that of the tubular film. The optimum diameter of the alignment at the inlet to the support zone, and the required length of the zone, can be established by simple experiments.

Assuming that the alignment of support belts rotates around the axis of the alignment at exactly the same rotational speed as the speed at which the output zone 31 spins around the coil, it is not absolutely necessary for the nip rollers 11 to follow this rotation. If they do not, the tubular foil may be compacted and twisted in front of the rollers 11, but will twist to open as it moves away from the rollers.

As shown, the coil 9 is held from the inside and the tube is pulled from the outside, but the coil 9 can alternatively be kept from the outside where the coil takes place from the inside.

Although the device shown in FIG. 2 and 3 do not have an air supply through the mandrel 14, the features of these systems are also valuable even when the air supply is omitted. Thus, the rotary rewind system is shown in FIG. 2 and 3, and also the rewind systems in which the tubular film is removed over one end of the coil while the exit zone is spinning, are of significant value for spiral cutting of materials from heavy coils and can be exercised independently of the use of the mandrel and air supply shown in FIG. 1. Similarly, obtaining helical orientation by relative rotations between the extrusion nozzle and the withdrawal device, combined with helical cutting which makes the orientation angle larger, can also be used regardless of air inflation from the cutting door.

The device constructed for use in the defined process is novel and forms a further aspect of the invention.

Example

Helical cutting below about 30 ° of a 100 micron tubular, low density polyethylene film with flat width of 2080 mm and taken from a 200 kg coil.

In principle, the device is constructed as shown in FIG. 1. However, the housing of the rewinding unit is replaced by a more lightweight frame construction, and the main bearing to support this unit is a large steel ring surrounding the unit forming part of the frame and the bearing is housed in a circular array of rollers.

The control of the cutting angle is electronic as explained at the end of the description in FIG. 1. The outside diameter of the mandrel, including an arrangement of thin ribs on the surface, is 1 300 mm, while the flat width 2080 mm corresponds to a diameter of 1,324 mm, i.e. the difference between the radius of the tubular plate and the mandrel is 12 mm. The final width of the foil is 3 600 mm, and the recording speed is 60 m. minute.

The air jet is established with a centrifugal fan, the full performance of which is 2,000 m³ pr. hour, which is estimated to have worked at about half the full capacity, ie about 5 1,000 m³ pr. hour. According to the fan data, this corresponds to 600 mm H₂0 overpressure. (This is a reliable, though rough indication, since the pressure does not depend on airflow very significantly.) Io The machinery works very reliably and the helical cut is right.