Folding unit for tissue products

Abstract

The present invention refers to a folding unit of a high-speed folding machine, for folding sheet-like products (S) made of tissue paper or the like. Such a unit comprises a first folding cylinder ( 3 ) with pneumatic means ( 5, 6, 5′, 6′ ) adapted to promote an orderly and precise release of the material in the folding step even at high speed.

1 . A folding unit for folding sheet-like products (S) made of tissue paper or the like, comprising a first folding cylinder and a second folding cylinder mutually counter-rotating and tangentially cooperating in a release and pinch point to carry out the folding of said products (S), said first folding cylinder comprising a work surface, paper material cutting means extending along respective generatrices of the cylinder regularly spaced around the circumference, and vacuum means adapted to hold said products over said work surface, wherein said vacuum means comprise first vacuum means extensively distributed over said work surface adapted to transmit a high vacuum over a determined rotation angle of the first cylinder upstream of said release point, and to transmit a rough vacuum or no vacuum over a determined rotation angle downstream of said release point, and second vacuum means, pneumatically independent with respect to said first vacuum means, arranged immediately downstream of respective cutting means and adapted to transmit a high vacuum at least over a determined rotation angle downstream of said release point. 
     
     
         2 . The unit according to  claim 1 , wherein said determined rotation angle in which said second vacuum means transmit a high vacuum downstream of said release point is between 60° and 90°. 
     
     
         3 . The unit according to  claim 2 , wherein said rotation angle is between 60° and 75°. 
     
     
         4 . The unit according to  claim 1 , wherein the second vacuum means are also adapted to transmit a rough or medium vacuum over a determined rotation angle upstream of said release point. 
     
     
         5 . The unit according to  claim 1 , wherein said first and second vacuum means comprise axial channels that open at said work surface through radial holes, the pneumatic communication between said axial channels and external vacuum generation means being blocked by at least one selective operation valve comprising a slotted fixed disc arranged over at least one axial end face of the first folding cylinder. 
     
     
         6 . The unit according to  claim 5 , wherein said axial channels of said first and second vacuum means open at mutually opposed axial ends of the first folding cylinder, and are selectively shut by respective operation disc valves arranged at respective and opposed axial end faces of the cylinder. 
     
     
         7 . The unit according to  claim 5 , wherein said axial channels of said first and second vacuum means open at a same axial end of the first folding cylinder, in positions having mutually different distances from the center, and are selectively shut by a single operation disc valve with two ranks of arc-shaped slots having different radius. 
     
     
         8 . A method for folding sheet-like products made of tissue paper or the like by means of a first folding cylinder and a second folding cylinder mutually counter-rotating and tangentially cooperating in a release and pinch point to carry out the folding of said products (S), the method comprising: cutting the paper material in segments (S) on said first folding cylinder, said segments having a head edge which is arranged frontally with respect to the rotational advance direction; exerting a vacuum over a work surface of the first folding cylinder to hold said segments (S) over said work surface; and pinching said products by said second folding cylinder when a release portion of said work surface passes in said release and pinch point, wherein a high vacuum is extensively generated over said work surface for a determined rotation angle of the first cylinder upstream of said release point and a rough or no vacuum is extensively generated over said work surface for a determined rotation angle of the first cylinder downstream of said release point, and wherein said front edge of a segment (S) is also held locally by generating a high vacuum at least for a determined angle of rotation downstream of said release point, independently from the extensive generation of the rough or no vacuum in the same angle of rotation. 
     
     
         9 . The method according to  claim 8 , wherein said determined rotation angle in which said front edge is locally held downstream of said release point is between 60° and 90°. 
     
     
         10 . The method according to  claim 9 , wherein said rotation angle is between 60° and 75°.