A device for the production of slabs of wax or other thermoplastic material is provided with two endless conveyor belts (1, 2) placed opposite each other and one above the other between which the wax is fed, with side guide (17) running along with and between the parts of the conveyor belts facing each other and with cooling means (9, 10) for cooling the wax. The device contains side guide belts (17) with two flat parallel surfaces, each of which sealingly engages with the opposite surface of the corresponding conveyor belt part. The device is suitable for processing wax with a sticky character due to the horizontal position of the belts and the relatively large sealing areas between the belts and the side guides.