Today most industrial manufacturers use urea in the following reaction to produce melamine:
6 (NH2)2CO → C3H6N6 + 6 NH3 + 3 CO2
It can be understood as two steps.
First, urea decomposes into cyanic acid and ammonia in an endothermic reaction:
(NH2)2CO → HCNO + NH3
Then, cyanic acid polymerizes to form melamine and carbon dioxide:
6 HCNO → C3H6N6 + 3 CO2
The second reaction is exothermic but the overall process is endothermic.