Moulding machine with an adjustment device assigned to allow the use of dies of different thicknesses, said machine comprising at least a conveyor for a plurality of supports; each support is equipped with plier means provided at least with a first jaw means and with a second jaws means, both flat or defining respective planes, facing each other and parallel to each other, and assigned to match with, and clamp, corresponding main faces (17, 19) of at least one die means having a predetermined thickness (a, b); the adjustment device comprising, for each support, at least: - a slide seat means fixed to the support itself and assigned to the axial sliding of a respective column means which is rectilinear and perpendicular to the plane of a corresponding second jaw means to which its first end is fixed; - a reversible block means fixed to the slide seat means and engaged by a portion of the column means; said machine further comprising an unlocking drive of the block means to lock or unlock the sliding of such column means along the slide seat means, wherein the first jaw means is fixed to the support and the second end of the column means, opposed to its first end, protrudes outside from the slide seat means and the block means; such that in an adjustment condition, in which the support is devoid of the die means and the unlocking drive is actuated to allow the sliding of the column means along the block means and the slide seat means, the column means can be axially translated to move the second jaw means thus setting the distance between the latter and the first jaw means until a distance between them (13, 15) is reached, which corresponds to the thickness (b, a) of the new size of the die means and until said distance is fixed by actuating the unlocking drive to lock the column means by means of the block means. wherein downstream of an area of the conveyor for the unloading of the die means and upstream of an area for the introduction of the die means, within the pliers means of each support, the device comprises an activation means for the column means, and a block means of each support, wherein such activation means comprises at least one first cam means equipped with a mouth, connected with a linear adjustment segment located downstream of the mouth, parallel to the path of the conveyor and movable while approaching and leaving such path; and it comprises a second linear cam means parallel to said path and whose operative front and rear ends precede the corresponding ends of the first cam means with respect to the movement of the supports along the conveyor; wherein in an adjustment condition, the second cam means is assigned to match with first rolling means of the unlocking drive of the block means of a running support to unlock the respective column means; the mouth and the adjustment segment of the first cam means are assigned to match with second rolling means fixed to the second end of the column means of the running support to translate the second jaw means at a distance from the first one, which corresponds to the thickness (a, b) of a die means of a desired size; the detachment of the first rolling means from the second cam means locks the column means by fixing it to the desired thickness. - a slide seat means fixed to the support itself and assigned to the axial sliding of a respective column means which is rectilinear and perpendicular to the plane of a corresponding second jaw means to which its first end is fixed; - a reversible block means fixed to the slide seat means and engaged by a portion of the column means;