Honeycomb sandwich sheet or panel, based on polypropylene, with a central thermoformed film

A process for preparing a honeycomb sandwich sheet or panel comprising the following steps:

• a) contemporaneously extruding at least three multilayer films (10,11,12) starting from a polymer or from a polymer compound containing a mineral filler, said three multilayer films comprising a bottom outer film (10), an inner or central film (11), and a top outer film (12);
• b) thermoforming said inner or central film (11);
• and
• c) coupling said bottom and top outer films (10,12) to said inner or central thermoformed film (11) by thermowelding,
• wherein said thermoforming of the inner or central film (11) is performed contemporaneously and at a same location by applying a positive or a negative pressure both on an upper outer side and on a lower outer side of the inner or central film (11) to produce the inner or central film with a constant thickness, and
• wherein the coupling step begins contemporaneously for the bottom and top outer films and the inner or central film (10,11,12).