Process of using thermoplastic paste for the production of foundry mold cores

A process for the production of foundry mold cores comprising the steps of:

• providing a thermoplastic paste comprising a mineral filler composed of, by weight, from 60% to 85% fused silica, from 15% to 35% zircon, and from 1% to 5% cristobalite, said paste also comprising, per 100 parts by weight of said mineral filler, from 0.2 to 0.5 parts by weight of a mold release agent, from at least 15 to 20 parts by weight of an organic binder formed by a polyethylene glycol having a molecular weight between 1400 and 1600, and, optionally from 1 to 5 parts by weight of cetyl alcohol as a plasticizer;
• subjecting said paste to a shaping operation to form a mold core;
• and
• subjecting said shaped mold core to a single firing cycle comprising the following four steps,
• (a) raising the temperature to 300.degree. C. at a rate of between 30.degree. C. and 50.degree. C. per hour,
• (b) raising the temperature from 300.degree. C. to a predetermined maximum temperature at a rate of between 100.degree. C. and 200.degree. C. per hour,
• (c) maintaining the temperature at said maximum temperature for a period of between 4 and 5 hours, and
• (d) cooling rapidly using pulsed air,
• so as to ensure, in the one firing cycle, elimination of the binder, consolidation by sintering of the material of the mold core, and stabilization of the core structure by conversion of fused silica into cristobalite, the total duration of the firing cycle being between 24 and 36 hours.