reserved areas 27 each accommodate a plate 28 a made of a flexible material for example silicone the plate 28 a has two parallel oblong openings at a slant a first short opening 30 corresponds to a conduit nozzle 28 the opening 30 is equipped with two lips 31 having a low height a second oblong opening 32 corresponds to the conduit 29 this opening 32 has a lip for doctoring 33 the maintenance cycle of the heads and nozzles is shown in fig8 in a continuous line the different stages of die cycle and the displacement of the head or the heads being shown by the letters a b c d e f and g at a the projection head is guided at a speed v 1 above the lips 31 of the opening 30 at b the head is lowered at a speed v 2 onto the lips 31 at c the partial vacuum at the head is interrupted the suction is implemented by the action of the device using the venturi effect 16 over a time t 1 at d the head is brought back up by a value of 2 millimeters above the lips 31 the suction is stopped the partial vacuum is started again at e the head is moved at a speed.