Folding or interfolding machine of sheets made of paper or similar material

A folding or interfolding machine for forming stacks of a predetermined number of folded or interfolded sheets according to a predetermined folding or interfolding configuration, said machine comprising:

• a first and a second folding or interfolding roll, arranged to rotate about respective rotation axes, said first and second folding or interfolding rolls being configured to fold or interfold said sheets according to said predetermined folding or interfolding configuration, obtaining a forming stack of folded or interfolded sheets adapted to grow in height along a predetermined forming direction;
• a separation group configured to separate a finished stack of folded or interfolded sheets, from a following forming stack of folded or interfolded sheets;
• a transfer group configured to transfer said finished stack of folded or interfolded sheets towards a discharge zone;
• and
• an image acquisition device configured to acquire at least one image of said forming stack, or of said finished stack, said or each image being processed by a control unit to define a quality parameter of formation of said forming stack, or of said finished stack.

A method for forming a stack of folded or interfolded sheets according to a predetermined folding or interfolding configuration, said method comprising the steps of:

• folding or interfolding a plurality of sheets in succession to each other according said predetermined folding or interfolding configuration by a first and a second folding or interfolding roll arranged to rotate about respective rotation axes, in such a way to form a forming stack of folded or interfolded sheets adapted to grow in height along a predetermined forming direction;
• separating by a separation group a finished stack of folded or interfolded sheets from a following forming stack of folded or interfolded sheets, once that a predetermined number of sheets is reached;
• transferring said finished stack towards a discharge zone by a transfer group;
• during said step of separating and said step of transferring, at least one between said forming stack and said finished stack being supported by at least a support surface;
• and
• detecting the force exerted by said forming stack or by said finished stack, on said or each support surface during said step of separating and/or said step of transferring.