Apparatus for the manufacture of films

Abstract: The invention concerns an apparatus for the manufacture of films, in particular thin or very thin films, with a rotating roller (3) with which a plastically deformable film (2) preferably supplied by a slot extruder (1) is conveyed and stretched, wherein between the roller (3) and a metal belt (4) arranged parallel and at a short distance from the roller is applied a voltage by which the film (2) supplied is applied to the roller (3). In order to develop this apparatus in such a way that manufacture of films, in particular thin or very thin films, is possible with increased productivity and film quality, with the invention it is proposed that the metal belt (4) comprises a sharp longitudinal edge (4c) which is arranged closest to the roller and directed onto it.

Description: The invention concerns an apparatus for the manufacture of films, in particular thin or very thin films, with a rotating roller with which a plastically deformable film preferably supplied by a slot extruder is conveyed and stretched, wherein between the roller and a metal belt arranged parallel and at a short distance from the roller is applied a voltage by which the film supplied is applied to the roller. For the manufacture of films, in particular thin or very thin films, an apparatus is known in which the film supplied to the apparatus is applied to a roller and contacts the roller surface over a certain angle of wrap. Due to adhesion of the film to the roller surface, the moment produced by the roller is transmitted to the film, so that the film is stretched in the region located in front of the roller. By stretching, the film which is usually supplied by a slot extruder is reduced to a lower thickness. This thickness in the case of thin or very thin films is between 30 µm and <1 µm. Use of a slot extruder in the apparatus is preferred, as the heat supplied to the film in the slot extruder is utilised in plastic deformation of the film. Between the roller and a metal belt arranged at a distance of about 3-6 mm parallel to the roller, a relatively high d.c. voltage of 5 and 7 kV is applied, the belt being connected to the positive terminal and the roller to the negative terminal. The film is fed to the roller through the gap formed between-the metal belt and the roller, the film being applied to the roller on account of the electric field formed between the metal belt and the roller. This is intended to prevent the occurrence of trapped air between the film and the roller, which leads to reduced productivity and reduced film quality. It is known that metal belts of rectangular cross-section made of special steel can be arranged parallel to the roller by clamping. The known metal belts are cut from a film about 0.02 mm thick to the necessary width of 10 mm. These belts have the drawback that the edges produced by the cutting process are irregular, so that the electric field between the metal belt and the roller is distorted. These distortions lead on the one hand to electric arcing, by which the film is damaged, and on the other hand to a decrease in efficiency of the voltage, so that the effect of applying the film to the roller, which is produced by the voltage, is reduced, and breakdowns in production as well as trapped air between film and roller occur. Furthermore the metal belt cut from special steel film has the drawback that strong enough clamping forces to clamp the belt with the necessary precision at a uniform distance from the roller cannot be applied. This irregular distance also results in distortion of the electric field between the steel belt and the roller. Distortions of the electric field lead in general to reduced productivity and reduced film quality in the manufacture of films, in particular thin or very thin films, with this apparatus known from the state of the art. US 3,223,757 describes an Apparatus for the manufacture of films with a rotating roller with which a plastically deformable film is conveyed and stretched, wherein between the roller and a metal belt arranged parallel and at a distance from the roller is applied a voltage by which the film supplied is applied to the roller, wherein the metal belt comprises a longitudinal edge which is arranged at a distance to the roller and directed onto it. Disadvantages of the apparatus described in US 3,223,757 are deposits which are formed on the belt and/or electrocorrosion both of which lead to uneven belts so that the electric charge is no longer uniformly applied over the film surface along the belt. This may lead to electric arcing which damages or even destroys the film or it may result in air being trapped between the roller and the film leading to the above described disadvantages. It is the object of the invention to develop an apparatus of the kind described hereinbefore in such a way that manufacture of films, in particular thin and very thin films, is possible with increased productivity and film quality as well as the avoidance of electrocorrosion of the belt or formation of deposits on the belt. This object is achieved by an Apparatus for the manufacture of films with a rotating roller with which a plastically deformable film is conveyed and stretched, wherein between the roller and a metal belt arranged parallel and at a distance from the roller is applied a voltage by which the film supplied is applied to the roller, characterised in that the metal belt (4) comprises a longitudinal edge (4c) which is arranged at a distance to the roller (3) and directed onto it, and that the length of the metal belt (4) exceeds the width of the roller (3) by a multiple, and the metal belt (4) is movable past the roller (3) by the width of the roller (3). As the longitudinal edge of the metal belt is arranged closest to the roller and directed onto it, the shortest distance between the metal belt and the roller arises between the longitudinal edge and the roller, so that an electric field of high field strength spreads out, starting from this longitudinal edge. As the metal belt comprises only one longitudinal edge, the steel belt itself may have a thickness which allows application of the necessary clamping forces, so that the metal belt can be clamped with sufficient precision at a uniform distance from the roller. In general an electric field which is uniform over the length of the roller is produced, so that while avoiding electric arcing between the roller and the metal belt, the occurrence of trapped air between the film and the roller - is reliably avoided. As a result, with the apparatus according to the invention it is possible to manufacture films, in particular thin or very thin films, with increased productivity and film quality. According to the invention the apparatus is developed by a metal belt of which the sharp longitudinal edge is formed after the fashion of a cutting edge in a tapering region of the metal belt. The tapering region displays such stiffness and strength that the cutting edge retains its shape even under mechanical, thermal or other stress. By arranging the metal belt at a distance of less than 6 mm from the roller, an electric field is produced by which the film is reliably applied to the roller, so that with an ordinary d.c. voltage of 5 to 7 kV between the roller and the metal belt, trapped air between the roller and the film is reliably avoided. Preferably the apparatus is developed according to the invention by the fact that the centre line of the tapering region is aligned with the centre of the roller. This results in a symmetrical orientation of the electric field, as a result of which an increase in film quality is possible. In a development of the apparatus according to the invention, the length of the metal belt exceeds the width of the roller by a multiple, and the metal belt is moved past the roller in a time cycle by the width of the roller. As the metal belt which is clamped freely parallel to the roller is subject to electrocorrosion, and furthermore deposits and deformation can lead to a change of shape of the sharp longitudinal edge, due to such a development according to the invention after a predetermined time cycle a new, unused section of the metal belt can easily be arranged parallel to the roller. As a result it is ensured in general that even with continuous operation of the apparatus a film of constant quality is manufactured. Preferably the time cycle is six hours. Practical tests have shown that after an exposure time of the metal belt of six hours, the quality of the films manufactured declines. Preferably the metal belt is moved past the roller in a time cycle by unwinding from one reel and winding onto another reel. With this manner of operation of the apparatus according to the invention, at any given time an unused section of the metal belt can easily be arranged parallel to the roller, so that the film is always manufactured in the desired quality. Further characteristics and advantages of the invention are apparent from the claims and the description below, in which a practical example is described in more detail. The drawings show: Fig. 1 a schematic side view of an apparatus for the manufacture of thin films, Fig. 1 shows a schematic side view of an apparatus for the manufacture of films with an extruder 1, via the slot die 1a of which a film 2 is supplied to a roller 3. A metal belt 4 is clamped parallel to the roller 3, a voltage being applied between the metal belt 4 and the roller 3. The metal belt 4 comprises a tapering region 4a with a sharp longitudinal edge 4c, which is directed onto the roller 3. Between the sharp longitudinal edge 4c and the roller 3 remains a gap of about 2-5 mm, through which the film 2 is supplied to the roller 3. Starting from the position of the metal belt 4, the film 2 contacts the roller 3 with an angle of wrap of about 170°. The metal belt 4 comprises, adjoining the tapering region 4a, surfaces which are formed parallel to each other and at which the metal belt 4 is mounted at its respective ends. Mounting is such that the metal belt is clamped with high precision at a uniform distance from the roller. As the metal belt 4 is clamped freely over the whole width of the roller 3 of about 1.20 m, the metal belt 4 can easily be damaged, so that the mounting of the metal belt 4 allows mounting and dismounting of the metal belt without problems. In the manufacture of films, a high d.c. voltage of between 5 and 7 kV with a current of a few milliamperes is applied between the metal belt 4 and the roller 3. On account of the flow of ions including ions contained in the air to the negatively charged roller 3, the film 4 supplied to the roller 3 is applied to the roller 3 in the gap formed by the metal belt 4 and the roller 3. This prevents air from being trapped between the film 2 and the roller 3, which leads to a reduced quality of the film manufactured. As the shortest distance between the metal belt 4 and the roller 3 is between the longitudinal edge 4c and the roller 3, the electric field spreads out from the longitudinal edge 4c to the roller 3. Due to the secure mounting of the metal belt 4 and the exact shape of the sharp longitudinal edge 4c, this electric field forms with the same intensity across the full width of the roller 3. Distortions of the electric field are avoided as a result. Similarly electric arcing between the roller 3 and the metal belt 4 is prevented. As the film 2 is applied uniformly to the roller 3 in the gap formed by the metal belt 4 and the roller 3 and contacts the roller with an angle of wrap of about 170°, the film 2 can be taken off from the slot die 1a with a force which can be determined by the moment of the roller 3, and therefore stretched. Due to stretching the thickness of the film 2 is reduced, so that films of different thickness can be manufactured with the apparatus.

Claims: Apparatus for the manufacture of films with a rotating roller (3) with which a plastically deformable film (2) is conveyed and stretched, wherein between the roller (3) and a metal belt (4) arranged parallel and at a distance from the roller is applied a voltage by which the film supplied is applied to the roller (3), characterised in that the metal belt (4) comprises a longitudinal edge (4c) which is arranged at a distance to the roller (3) and directed onto it, and that the length of the metal belt (4) exceeds the width of the roller (3) by a multiple, and the metal belt (4) is movable past the roller (3) by the width of the roller (3). Apparatus according to claim 1, characterised in that the longitudinal edge (4c) is formed after the fashion of a cutting edge in a tapering region (4a) of the metal belt (4). Apparatus according to claim 1 or 2, characterised in that the metal belt (4) is arranged at a distance of less than 6 mm from the roller (3). Apparatus according to any of claims 2 to 3, characterised in that the centre line of the tapering region (4a) is aligned with the centre of the roller (3). Apparatus according to either of claims 1 to 4, characterised in that the metal belt (4) is movable past the roller in a time cycle by unwinding from one reel and winding onto another reel.

Inventors: APPELT HORST G, APPELT, HORST G.
Applicants: MITSUBISHI POLYESTER FILM GMBH
Application year: 1995
Main classification: B29C47
IPC codes: B29C47