Objectives: This study investigated the pressure characteristics of different impression materials for complete dentures using parallel disk models with vents with different diameter. Methods: Vinyl polysiloxane (EXADENTURE, GC, Japan) and polysulfide (SURFLEX F, GC, Japan) were selected as testing impression materials according to its viscosity. Four pressure sensors (PS-1KD, Kyowa, Japan) were set at from center to edge of 60 mm diameter model at 9 mm intervals (measuring point 0, A, B, and C). Seven trays were fabricated with same as model diameter. Six trays were with vents made in the center with 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mm of diameter and one without vent (N tray). The model and trays were connected to a rheometer (CR-200D, Sun, Japan) with loading force conditioned at 4,800 g and pressure speed at 120 mm/minutes. Measurements were carried out five times with each of seven trays using both materials. Impression pressure was gathered and differences between EXADENTURE and SURFLEX among the different diameter of vents were analyzed by Analysis of variance (ANOVA) followed by post hoc test (Tukey Kramer's). Results: By using N tray, the value 0, A, B and C points were 415.59, 345.30, 225.88 and 88.77 g/cm2 respectively with EXADENTURE and 342.52, 310.76, 205.23 and 97.36 g/cm2 with SURFLEX. There were significant difference between EXADENTURE and SURFLEX at four measuring points (<0.001). The impression pressure at 0 points decreased when the vent diameter was between 0.5 and 2.0 mm (EXADENTURE), and 0.5 and 2.5mm (SURFLEX) respectively. Conclusions: The effect of the vent differed according to the viscosity of the material.

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