Rubber-base impression materials possess a longer setting time than the irreversible hydrocolloid materials and lend themselves better to border molding in adequate supporting trays. The fact that a smoother surface results does not, however, preclude the possibility of a grossly inaccurate impression and stone cast resulting from other causes. Despite their accuracy, this has always been a disadvantage of all hydrocolloid impression materials. This is probably so because the rubber material does not have the ability to retard or etch the surface of the setting stone. Mercaptan rubber–base impression materials do have an advantage over hydrocolloid materials in that the surface of an artificial stone poured against them is of a smoother texture and therefore appears to be smoother and harder than one poured against a hydrocolloid material. Gypsum cannot be molded into different shapes. ![]() While Plaster of Paris (PoP) is a white powder. The accuracy of mercaptan rubber base is acceptable for making impressions for removable partial dentures however, as with hydrocolloid impression materials, certain precautions must be taken to avoid distortion of the impression. Plaster of Paris is a commonly used mineral obtained by Gypsum and has a chemical formula CaSO ♱/2H O. These materials are moderately inexpensive, have high tear strength and long working and setting times (8 to 10 minutes), and can be disinfected in liquid, cold-sterilizing solutions. Many of these materials have an unpleasant odor and can stain clothes. For complete instructions, visit our web page at. The material must be held still during the impression-making procedure because it does not have a snap set it should be allowed to rebound for 7 to 15 minutes after it is removed from the mouth and should then be poured immediately. This video demonstrates how to pour the plaster into the mold, remove air bubbles and scrape the mold flat. The long-term dimensional stability of these materials is poor because of water loss after setting. For example, when large numbers of teeth with natural tooth contours that display multiple undercuts remain, these materials will be subjected to clinically significant distortion upon withdrawal. Those materials that are highly cross-linked (medium and heavy body) do not recover well from deformation and should not be used when large or multiple undercuts are present. ![]() This necessitates the use of a carefully made individual impression tray of acrylic-resin or some other material possessing adequate rigidity and stability. To be accurate, the impression must have a uniform thickness that does not exceed 3 mm ( inch). The mercaptan rubber–base (Thiokol) impression materials can also be used for removable partial denture impressions and especially for secondary corrected or altered cast impressions. Mercaptan Rubber–Base Impression Materials
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