Magnetic assemblies and processes for producing optical effect layers comprising oriented non-spherical magnetic or magnetizable pigment particles

A printing apparatus comprising a rotating magnetic cylinder or a flatbed printing unit, the rotating magnetic cylinder or the flatbed printing unit comprising at least one magnetic assembly configured to receiving a substrate in an orientation at least partially parallel to a first plane, comprising:

• a first magnetic-field generating device comprising at least four first dipole magnets having North poles pointing in a same direction and having magnetic axes oriented to be substantially parallel to the first plane, said first dipole magnets being spaced apart from each other,
• wherein each of the first dipole magnets is arranged on an intersection of at least two substantially parallel straight lines αi (i=1, 2, . . . ) and at least two substantially parallel straight lines βj (j=1, 2, . . . ), the straight lines αi and βj forming a grid, wherein at least two first dipole magnets are disposed on one of the straight lines ai and at least two other first dipole magnets are disposed on another one of the straight lines αi, wherein the magnetic axes of the first dipole magnets are oriented substantially parallel to the substantially parallel straight lines αi, and wherein the first dipole magnets of said first magnetic-field generating device are partially or fully embedded in a first supporting matrix;
• and
• a second magnetic-field generating device comprising one or more second dipole magnets having magnetic axes oriented to be substantially parallel to the first plane and wherein the one or more second dipole magnets are partially or fully embedded in a second supporting matrix;
• wherein the second magnetic-field generating device is disposed below the first magnetic-field generating device, and wherein each straight line αi and a vector sum H of the magnetic axes of the one or more second dipole magnets are substantially non-parallel and substantially non-perpendicular with respect to each other.

A process for producing an optical effect layer on a substrate comprising the steps of:

• i) applying on a substrate surface a radiation curable coating composition comprising non-spherical magnetic or magnetizable pigment particles, said radiation curable coating composition being in a first state so as to form a coating layer;
• ii) exposing the radiation curable coating composition to a magnetic field of a static magnetic assembly so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, said magnetic assembly being configured for receiving the substrate in an orientation at least partially parallel to a first plane, comprising:
• a first magnetic-field generating device comprising at least four first dipole magnets having North poles pointing in a same direction and having magnetic axes oriented to be substantially parallel to the first plane, said first dipole magnets being spaced apart from each other,
• wherein each of the first dipole magnets is arranged on an intersection of at least two substantially parallel straight lines αi (i=1, 2, . . . ) and at least two substantially parallel straight lines βj (j=1, 2, . . . ), the straight lines αi and βj forming a grid,
• wherein at least two first dipole magnets are disposed on one of the straight lines αi and at least two other first dipole magnets are disposed on another one of the straight lines αi,
• wherein the magnetic axes of the first dipole magnets are oriented substantially parallel to the substantially parallel straight lines αi, and
• wherein the first dipole magnets of said first magnetic-field generating device are partially or fully embedded in a first supporting matrix;
• and
• a second magnetic-field generating device comprising one or more second dipole magnets having magnetic axes oriented to be substantially parallel to the first plane and wherein the one or more second dipole magnets are partially or fully embedded in a second supporting matrix;
• wherein the second magnetic-field generating device is disposed below the first magnetic-field generating device, and
• wherein each straight line αi and a vector sum H of the magnetic axes of the one or more second dipole magnets are substantially non-parallel and substantially non-perpendicular with respect to each other;
• and
• iii) at least partially curing the radiation curable coating composition of step ii) to a second state so as to fix the non-spherical magnetic or magnetizable pigment particles in adopted positions and orientations.