The process comprises impregnation of first sheets of paper with a first aqueous resin having a solids content of 30-60 % of urea-melamine-formaldehyde resin; from 4-10 % of a plasticizer; a surfactant wetting agent; a catalyst, and a mould stripping agent; impregnation of second sheets of paper with a second aqueous resin having a solids content of from 30-60 % of urea-melamine-formaldehyde resin; 0-8 % of a plasticizer; a surfactant wetting agent; a catalyst and a mould stripping agent, this impregnation until the sheets have a resin solids content of 30-60 % and a volatiles content of 5-7 %; forming an ensemble with a first sheet of paper, a second sheet of paper, a board, a second sheet of paper and a first sheet of paper; and pressing the ensemble, the press surface being at a temperature ranging from 130 to 180 °C