Gypsum plaster is one of the most common auxiliary materials used in dental production.
Per GOST R51887-2002 and ISO 6873, all dental plasters are divided into five classes according to their purpose and hardness:
- Impression plaster - Soft and pliable low-hardness gypsum plaster. It is used to obtain partial and complete impressions, including the impressions of edentulous jaws. Such plaster hardens quickly and has the lowest degree of expansion.
- Medical-grade plaster of Paris - Alabaster plaster of standard hardness. This type of material is suitable for the manufacture of diagnostic anatomical models, as well as models for orthopedic planning of dental prostheses. Dental plaster of this class is considered an auxiliary material, since the models produced from it have an insufficient strength index. Therefore, impression plaster and medical-grade dental plaster are used only for technical purposes, and not for the production of working models.
- High-strength modeling plaster A class of hard gypsum plasters. It is used for the manufacture of removable dentures both for the entire dentition and for replacing missing teeth, for the manufacture of the base of fixed demountable dentures, and other similar products of this series. Unlike ordinary medical-grade plaster of Paris,materials of this class have a fairly high strength.
- Extra-hard modeling plaster (dental die stone) for low expansion models - Gypsum plaster with the highest strength characteristics, great for making collapsible master models and performing combined works.
- Extra-hard modelling plaster (dental die stone) for models with adjustable expansion rate - A fairly rare variety, designed for making models that require particularly high precision.
To successfully perform dental, orthopedic and dental technical work with the use of dental plasters, it is important to follow certain rules for their use:
- Dental plasters must be stored in a dry place.
Storage containers for plaster must be cleaned out before each new filling. - Tools and accessories used for working with dental plasters must be clean, and free of residues of previously used plaster.
- One batch of plaster should be enough to fill no more than two or three impression molds.
- The use of any curing accelerators is unacceptable. To achieve a similar purpose, the specialist may use quick-setting plaster or increase the mixing time by a few seconds.
- To achieve the necessary expansion rate of plaster, it is necessary to observe the ratio of gypsum plaster and water very precisely.
- Both water and gypsum powder should be at a temperature of 19-21°C.
- The powder should be slowly poured into the water to let it sink into it. Only after that stirring with a spatula may commence.
Mechanical stirring should not exceed 30 seconds; the time limit for manual stirring is one minute.
The mixture should be poured into the mold immediately after stirring. It is unacceptable to try to increase the filling time by vibration or adding water. - The plaster model may be removed from the impression mold only when its temperature drops.
If you following these instructions, you will be able to carry out any works with dental plaster comfortably, quickly, and economically.
The Department of Orthopedic Dentistry of the Voronezh State Medical Academy has performed a comparative analysis of dental plasters to evaluate the main characteristics of the most common brands of gypsum binders.
High-strength and extra-hard dental plasters were selected for analysis. The tests were carried out in accordance with GOST R 51887-2002.
The study allowed to establish the parameters that determine the quality of dental plaster, ensuring the manufacture of dentures with excellent functional and aesthetic properties.
Water consumption. In theory, the required amount of water to convert hemihydrate to dihydrate is 18.6% of the total mass of the binder. However, in practice, much more water is used to ensure the required mobility of the plaster paste; thus, the plaster paste has its own water requirement.
Water requirement is the smallest amount of water necessary to obtain the required consistency of the mortar. Excess water evaporates from the formed dental die stone creating pores that can significantly reduce the strength of the model. Therefore, the specialists should strive to measure the water accurately to obtain the ideal consistency.
During hardening, hydration of hemihydrate gypsum occurs. This reaction of adding water to the hemihydrate is accompanied by the release of 29 kJ of heat per kilogram of hemihydrate. The hardening process is gradual. When mixing with water, hemihydrate gypsum forms a supersaturated mortar, from which the dihydrate is released. Due to the formation of a large number of particles of dihydrate, the gypsum mixture thickens and becomes more compacted, which serves as the beginning of its setting.
The strength of the finished product depends on many factors: the purity of the raw material (gypsum powder), its structure, the processing methods, as well as the composition and amount of modifying additives. Ultimate strength is measured in megapascals: 1 MPa = 10 kgf/cm2.
Field tests at a dental laboratory have demonstrated that plasters with the highest quality demonstrate high stability on a spatula and a fluid consistency on a vibrating table, which allows to maximize the number of pore-free castings from one mixed batch.
Models produced using high-quality gypsum binders are resistant to chipping, reproduce the modeled surface perfectly. They are easy to polish, grind and saw up; the prosthetic margins are not damaged during the column preparation. The high quality of the gypsum raw material prevents edge breakage when removing the model from the impression mold, ensuring the best modeling result.
Production of cast dental models from plaster: