Background and Aim: Dimensional changes of Alginate due to late pouring of the impression material is not only a concern but a significant indication for manufacturing long-extended Alginate impression materials. The aim of this research was to assess the effect of time on the dimensional changes of Hydrogam-5 Alginate.

  Materials and Methods: In this experimental study, a model was prepared from which, 15 impressions were taken using Hydrogam-5 Alginate. The samples were divided into three groups which were stored in sealed plastic bags for a period of 30min, 24hrs, and 120hrs. ( Iso 1563: 1990 E ) Afterwards, the impressions were poured with improved stones. The large and small diameters as well as the distance and height between the dies were measured by profilometer microscope and data was analyzed by Kruskal Wallis statistical method.

  Results: The percent of dimensional changes in large and small diameters, the height of the large die, as well as the distance between the dies in sample groups after 30min, 24hrs, and 120hrs were: (0.7, 0.5,0.2,0.6), (0.6, 1.3, 1.1, 0.4) and (1.2, 0.3, 0.3, 0.3) percent respectively. The difference was not significant (p<0/1).

  Conclusion: The time elapsed did not cause dimensional changes in the Hydrogam-5 Alginate, however the extent of changes were more than the claim of its manufactures.