Apparatus and method to adjust the thickness profile in the production of blown films

Abstract

Disclosed is an apparatus for the production of a blown tubular film (FT). The apparatus includes an independent device for the localized thickness adjustment at the strips adjacent to the folding edges resulting from the flattening of the tubular film (FT) by means of adjustment elements formed in angular sectors of limited extension symmetrically arranged 180° apart. The independent device for the localized thickness adjustment being a volumetric type device having a rotating ring that performs a rotating adjustment of the flow rate of a cooling air stream which takes into account the angular offset resulting from the operational parameters of the apparatus.

1 . A method for adjustment of a thickness profile of a blown tubular film (FT) at a cooling ring of an apparatus for the production of said blown tubular film (FT), wherein the apparatus comprises:
 an extrusion head,   a cooling ring containing a primary adjustment device for the adjustment of said thickness profile and means for localized thickness adjustment at strips adjacent to folding edges resulting from flattening of the tubular film (FT),   a haul-off that draws and flattens the tubular film (FT) into a flattened film (FA), said haul-off and the extrusion head being relatively rotatable to produce a rotating/oscillating relative motion therebetween,   means suitable to perform a stretch of the flattened film (FA),   a final thickness profile measuring device that measures a final thickness profile of the flattened film (FA), and   a control unit operatively connected to said means for the localized thickness adjustment, to said primary adjustment device, to said final thickness profile measuring device and to the extrusion head and the haul-off so as to coordinate the rotating/oscillating relative motion between the extrusion head and the haul-off and to carry out a feedback adjustment of the primary adjustment device on the basis of data of the final thickness profile measuring device,   
       wherein said means for the localized thickness adjustment comprise an independent localized adjustment device provided with adjustment elements formed in angular sectors of limited extension symmetrically arranged 180° apart, said independent localized adjustment device comprising a rotating ring that performs a rotating adjustment of a flow rate of a cooling air stream, which can be a stream coming from the primary adjustment device or a separate uniform stream, said control unit being operatively connected to the independent localized adjustment device so as to coordinate a motion of said rotating ring with the relative motion between the extrusion head and the haul-off and to carry out the feedback adjustment of the primary adjustment device on the basis of the data of the final thickness profile measuring device taking into account the effect of the independent localized adjustment device, 
       the method comprising the following steps: 
       a) measuring the final thickness profile of the flattened tubular film (FA); 
       b) calculating an angular offset caused by torsion of the tubular film (FT) around its longitudinal axis due to the relative motion between the extrusion head and the haul-off, taking into account a distance and a relative speed between the extrusion head and the haul-off; 
       c) calculating a starting time and a direction and amount of an angular speed of the rotating ring at each activation or inversion of the relative motion between the extrusion head and the haul-off; and 
       d) adjusting the independent localized adjustment device such that the flow rate of the cooling air stream, coming from the primary adjustment device or being a separate uniform air stream, is reduced through dispersion or limitation at angular sectors in which the strips adjacent to the folding edges resulting from the flattening of the tubular film (FT) will end up, so as to obtain a thickness profile suitably altered in the proximity of the folding edges with a shape complementary to non-uniformly distributed effects generated by the stretch of the flattened film (FA). 
     
     
         2 . The method of  claim 1 , further comprising the step of measuring the thickness profile of the tubular film (FT) between the extrusion head and the haul-off, wherein when calculating a correction of a base profile to be applied in the primary adjustment device said measuring is not taken into consideration in the angular sectors in the proximity of the folding edges. 
     
     
         3 . The method of  claim 2 , wherein said step of calculating a correction of the base profile to be applied in the primary adjustment device is carried out by discarding the present measuring of the angular sectors and using instead the latest measuring carried out on those sectors prior to the intervention of the independent localized adjustment device. 
     
     
         4 . The method of  claim 1 , wherein the independent localized adjustment device includes a static ring that is secured to the cooling ring, and shaped to mate with the rotating ring that is superposed thereon, as well as a driving mechanism that drives the rotating ring and is secured on top of the cooling ring; and wherein the rotating adjustment of the flow rate of the cooling air stream is performed through ducts and cavities formed in the static ring and in the rotating ring and provided with adjustment means located on the latter, so as to withdraw and disperse in a controlled manner a fraction of the cooling air, the dispersion of said air fraction taking place outside of the tubular film (FT) through release holes. 
     
     
         5 . The method of  claim 4 , wherein the static ring has a top channel with a U-shaped cross-section that on a bottom has one or more concentric circles of axial through holes in communication with a cooling air channel where the cooling air passes, while the rotating ring has a bottom shape matching said top channel so as to form with the static ring a sliding coupling whereby a bottom face of the rotating ring closes said axial through holes except at said two angular sectors of limited extension, symmetrically arranged 180° apart, in which said bottom face is provided with a plurality of cavities having a radial extension sufficient to cover all said circles of axial through holes and an angular extension sufficient to cover an axial through hole, an axial hole extending between each of said cavities and a top face of the rotating ring, and a radial hole extending between each of said axial holes and an internal face of the rotating ring, the portion of each axial hole above said radial hole and/or the portion of each radial hole to the inside of said axial hole being provided with a thread. 
     
     
         6 . The method of  claim 1 , wherein the independent localized adjustment device includes a static ring that is secured to the cooling ring, and shaped to mate with the rotating ring that is superposed thereon, as well as a driving mechanism that drives the rotating ring and is secured on top of the cooling ring; and wherein the rotating adjustment of the flow rate of a cooling air stream is performed through adjustable means capable of reducing the flow area in the cooling air channel, said adjustable means being integral with the rotating ring. 
     
     
         7 . The method of  claim 6 , wherein the static ring supports the rotating ring, which in turn carries an internal insert and a stabilizer mounted on the latter, through a sliding coupling achieved through a plurality of support wheels and centering wheels, said wheels being optionally equally spaced both with respect to the wheels of their own type and with respect to the wheels of the other type. 
     
     
         8 . The method according to  claim 7 , wherein the adjustable means capable of reducing the flow area in the cooling air channel are arranged below the internal insert and extend into said channel. 
     
     
         9 . The method according to  claim 8 , wherein the adjustable means capable of reducing the flow area in the cooling air channel are deformable segments secured to a ceiling of said channel and pressed from above through adjustment screws screwed into corresponding axial through holes formed on a bottom of the internal insert. 
     
     
         10 . The method according to  claim 8 , wherein the adjustable means capable of reducing the flow area in the cooling air channel are adjustment screws screwed into corresponding axial through holes formed on a bottom of the internal insert in correspondence with a ring that extends axially downwards under said bottom passing through said channel until it enters a corresponding groove so as to form a labyrinth seal, said bottom ring being crossed by one or more stacked circles of radial through holes whereby the cooling air stream must pass through said radial through holes in order to reach the tubular film (FT).