This in-vitro study investigated the dimensional accuracy and stability of polyether and polyvinylsiloxane impression materials. The study included the effect of mixing methods where conventional mix versus auto-mixing technique, the effect of special tray on the accuracy of impression and pouring the impressions at different times of storage (1hour, 48 hours and 7 days). Impressions were made for a master model cast of mandibular arch with two prepared abutments; right second premolar and right second molar with a shoulder finish line made as to give a definite margin that was easily
defined where there was an occlusal shoulder and gingival shoulder used for occlusogingival (O-G) measurements. There was an occlusal holes made on occlusal surface of both teeth to be used as a reference points for inter-abutment distance record of the prepared abutments. All dimensions were measured on casts retrieved from impressions made on master model. The impressions were then poured with extra-hard stone into stone casts, according to times under investigations. The results revealed that the polyether elastomeric impression material presented a higher degree of accuracy than
vinylpolysiloxane. The automixing technique showed better results than conventional mixing, while the light- cured special tray presented slightly better dimensions. Also, the one- hour pouring time presented the best dimensions, while two- days and sevendays pouring times had almost the same dimensions.