Method and embossing structure for maximizing pressure buildup at rotational embossing of foils

An embossing apparatus for embossing a foil material on both sides, the apparatus comprising:

• a pair of a first roll and a second roll configured to emboss the foil material which is intended to be fed into a roll nip formed by the first and the second roll, the foil material including a metal layer,
• the first roll and the second roll having a plurality of positive projections (P) and a plurality of negative projections (N) of identical shaped polyhedral structures, the positive projections are elevated above a mean cylindrical surface of their roll, and the negative projections are recesses reaching below the mean cylindrical surface of their roll, a first subset of the plurality of positive projections disposed on the first roll with a first periodicity on a first grid in an axial direction and in a second periodicity on the first grid in a circumferential direction of the first roll, and a first subset of the plurality of negative projection disposed on the first roll on the first grid with the first periodicity in the axial direction and with the second periodicity in the circumferential direction of the first roll on the first grid, the first subsets of the positive and negative projections interspersed between one another in axial and circumferential directions of the first roll, respectively, and a second subset of the plurality of positive projections and a second subset of the plurality of negative projections disposed on a second grid complementary to the first grid, on the second roll,
• each of the positive projections and the negative projections on the first roll being configured such that during intended operation of the rolls and in the roll nip, except for projections located on edges of the first grid, being surrounded on all sides by positive projections and negative projections on the second roll,
• the positive projections of the first roll together with alternating corresponding negative projections on the second roll forming during the intended operation of the rolls and in the roll nip, a first straight line (y-y) substantially parallel to the axial direction, and
• the negative projections of the first roll together with alternating corresponding positive projections on the second roll forming during the intended operation of the rolls and in the roll nip, a second straight line (x-x) substantially parallel to the axial direction,
• wherein on the first roll and on the second roll a disposition of the positive projections and the negative projections is configured such that in the axial direction on the first roll each positive projection shares a lateral base border with at least one negative projection adjacent to the positive projection,
• wherein the first straight line (y-y) and the second straight line (x-x) are coincident in a single third line (z-z), and
• wherein during an operation of the rolls and in the roll nip, lateral oblique surfaces of the positive and negative projections of the first roll are arranged to be substantially in parallel with lateral oblique surfaces of the respective negative and positive projections of the second roll that are facing each other, thereby enabling a homogeneous distribution of pressure to the foil material, wherein a common continuous surface is formed by a lateral oblique surface of a positive projection and a lateral oblique surface of a negative projection, the positive and negative projections neighboring each other and are located on the same roll, wherein no intervening flat portion is present between the corresponding lateral oblique surfaces of the positive and negative projections that are neighboring each other and are located on the same roll.