To study the effect of recasting on the macrohardness of cobalt chromium (Co-Cr)alloy, and to evaluate the effect of finishing and polishing on it. Materials and methods: Two brand of Co-Cr alloys were used, divided into three groups, the specimens of the first group were casted from new materials, the second were casted from previously casted material without the addition of any new material (100% recast), the third were casted by combination 50% new material and 50% used material. Half of the specimens just finished and the other half were finished and polished , hardness for all of them were measured. Results: Showed that recasting, both 100% and 50%, significantly increase the macrohardness of Co-Cr alloys and no significant effect of finishing and polishing on it. Conclusion: Recasting may affect properties of Co-Cr alloy and new material should be used rather than recasted material.

Aims:To determine the dimensional accuracy of thermoformed dental base plates. Materials and methods: Six different types of thermoformed materials were used. The circumference of the triangle formed between the index marks on standard maxillary edentulous metal model and measurements of the surface area of the gap between the posterior margin of base plate and the metal model were captured with digital camera and measured with AutoCAD program. Measurements were made after 1 day after processing and then 2, 3 and 7 days. ANOVA and Duncan multiple range test were used to statistical analysis of the data. Results: The linear dimensional shrinkage and inaccuracy of fitness were significant for all of the thermoformed materials. The change after 7 days was not significantly different from that occurred after 1 day. Conclusions: All thermoformed materials exhibited dimensional instability, the Duran(hard material) and Durasoft(flexible material) were the most dimensionally stable and accurately fitted thermoformed materials.

Aims:The aims of this study are to compare the dimensional changes of record base from heat and cold cured acrylic resin, with the latter cured by two techniques, and to study the regions vulnerable to dimensional changes. Materials and Methods: The adaptation of the record base on edentulous maxillary casts were tested by sectioning the record base-stone casts sets transversely and measuring the gaps with travelling microscope. The gaps represent the crest of the ridge, midline, and the deepest points of buccal sulcus. Results: Record base made from cold cured acrylic resin showed significantly higher dimensional changes than that from heat curd acrylic resin. There were no significant difference between the bench-curd and thermopress curing for cold cured acrylic resin. No significant differences found between the ridge crest and midline , but both show significantly less dimensional changes than the buccal vestibules. Conclusions: Curing cold cured acrylic resin by thermopress machine would not improve dimensional stability. And to improve the dimensional stability, the denture base that is made from heat cured acrylic resin may be used as a record base.

Aims: To compare the compressive strength of refractory casts made from investments for cobaltchromium and investment for titanium and to study the effect of mixing fluid on compressive strength. Materials and Methods: Three types of investments were used, one for titanium and two for cobaltchromium, each one is mixed with three types of mixing fluids; tap water, distilled water and special liquid, to produce refractory casts. Compressive strength test was carried out using compression testing machine. Results: Showed significant differences in compressive strength between the types of investments, with titanium investment mixed with special liquid showed the highest value. There were significant differences between subgroups of the same type by changing mixing fluid type. Conclusions: Refractory casts made from titanium investment showed significantly higher compressive strength than refractory casts made from Co-Cr investments, and there was a difference between the two special liquids used, and tap water in which it produced casts with higher compressive strength than distilled water in two out of the three investment materials tested.

AIMS: To determine the effect of different denture cleansers on hue, value, and chroma of permanent silicone soft liner at different immersion periods. MATERIALS AND METHODS: Soft liner specimens were immersed in four denture cleansers ("citric acid + soda", salt, "apple vinegar + soda", and alum) for periods of 1, 3, 7, and 14 days. A digital spectrophotometer was used to access color differences of soft liner before and after each storage period in each solution. RESULTS: The citric acid and salt cleansers decrease the value, chroma, and hue. The apple vinegar cleanser does not affect significantly any color property. The alum cleanser decrease value and chroma, but does not affect hue of soft liner. CONCLUSIONS: The effects of different denture cleansers tested on different color properties of soft liner were different. The effect of the immersion period on the different color properties tested was different for each denture cleanser.