The Authors wanted to establish which could be for every impression material tested, the most favourable moment for the development of plaster models in a steady environment conditions of temperature and relative humidity. From the analysis of the results obtained, we can clearly conclude that every material examined offers an ideal time interval in which we have to cast the model in plaster. This study aspires to be an invitation to all operator in the sector, to repeat this experience in their work environment to know and better manage the procedures and the materials used, from the taking the impression to the executing of the plaster model.