Also, under ADA specification, the manufacturer is required to include detailed instructions for use. The dental assistant should read and follow these directions carefully.
Synthetic Rubber Materials Rubber impression materials are supplied as pastes in collapsible metal tubes that require mixing. One tube contains the base, while the other contains an accelerator or a catalyst. When mixed in appropriate amounts, the mixture hardens to a synthetic rubber. Other types of materials come in the form of double-barreled injector cartridges that do not require mixing.
CONSISTENCY TYPES. - Rubber impression materials can be used for almost any impression. They come in three consistencies and are discussed in the paragraphs that follow.
Light Bodied. - Light bodied impression materials are injected with a syringe onto preparations for inlays, crowns, and FPDs. It is also used as a "wash" impression for full dentures, relinings, and RPDs. Its high degree of flow registers the fine detail.
Regular Bodied. - Regular bodied impression materials are used in an impression tray for inlays, crowns, and FPDs.
Heavy Bodied. - Heavy bodied impression materials are used in a tray to force light bodied impression material onto the cavity preparation or with a copper band for impressions of single teeth.
MATERIAL TYPES. - Rubber impression materials can be grouped into three types depending on their composition: polysulfides, silicones, and polyetliers.
Polysulficles. - The polysulfides (rubber base) can be identified by the usually dark color of one of the two pastes and their resulting opaque mix and sulfur smell. If the materials are improperly mixed, the impression will have streaks in it, thereby affecting dimensional stability. Mixing time is between 45 and 60 seconds with a 5-minute working time. The impression must not begin setting before placement in the mouth. If the 5-minute working time is exceeded, the resulting impression will have inadequate expansion, producing a smaller cast. The impression must set completely before removal from the mouth and poured no later than 1 hour after removal.
Silicones. - Silicone (vinyl polysiloxanes) materials are generally lighter in color, translucent when set, and have a slight odor. Silicone types come in the form of a heavy putty, light, regular, and heavy bodied viscosities. The silicone material is used with a stock tray to make up the bulk of the impression and' minimize distortion. Manufacturers have been able to control shrinkage resulting in impressions with greater accuracy when compared to all other rubber products. Impressions made from silicone do not have to be poured immediately. The material will remain accurate for several days so they can be repoured as necessary.
Polyethers. - PoIyetliers have lighter colors than polysulfides, but are darker than silicones. The working and setting times are much shorter than the other two rubber impression materials. Polyether is just as good to use as polysulfides to control shrinkage. Unlike polysulfide, polyether will absorb water. This type of impression material is very stiff, making it difficult to remove from the mouth and a cast. The dentist must take care when removing the tray with the material from the mouth, because the polyether tears easily in thin areas like the subgingival sulcus. For best results, use this material with a custom tray.
Gypsum products are supplied in powder form. When mixed with water in the correct proportions, a paste forms that will eventually harden. This setting process takes place over several minutes, during which time the mixture is soft and pliable, and can be formed into the desired shape. During the setting process, gypsum gives off heat, which is characteristic of all its products. Each material in the gypsum group is carefully compounded to give it the particular combination of physical properties needed for a particular work order. Dental plaster, stone, and die stone are the most frequently used gypsum products.
Plasters made for dental use are specially processed to provide high purity and suitable working properties. One of the most important requirements is that the plaster must set within a definite time limit.
Plaster has many uses. It can be used to form casts, construct matrices, and attach mount casts to an articulator. The initial setting time for most dental plaster is from 7 to 13 minutes. The final set is completed within approximately 45 minutes.Continue Reading