Gypsum is one of the most common auxiliary materials used in dental production.
According to GOST R51887-2002, ISO 6873, all dental gypsum are divided into five classes, in accordance with their purpose and hardness:
- Plaster for prints - Soft and pliable low hard plaster. It is used to obtain partial and complete impressions, including those with jaws without teeth. Such gypsum hardens quickly and has the smallest expansion.
- Medical plaster - Alabaster gypsum of normal hardness. This type of material is suitable for the manufacture of diagnostic anatomical models, as well as models used for planning an orthopedic design. Gypsum of this class is attributed to auxiliary materials, since the model from it has an insufficient strength indicator. Thus, impression gypsum and medical dental gypsum are used only for technical purposes, and not for the manufacture of working models.
- High-strength gypsum for models - Class of hard gypsum. It is used for the manufacture of removable dentures as the entire dentition, and replacing the missing part of the teeth, for the manufacture of fixed non-removable prostheses and other products of this series. Unlike conventional medical gypsum, material of this class has fairly high strength indicators.
- Heavy-duty gypsum for models with a low expansion rate - The gypsum with the highest strength indicators is excellent for making collapsible master models and performing combined work.
- Heavy-duty gypsum for models with an adjustable rate of expansion - A fairly rare variety intended for the manufacture of models requiring particularly high precision.
- Dental gypsum must be stored in a dry place. Plaster storage containers must be cleaned before each new filling.
- Devices and accessories used when working with dental gypsum must be clean and free from residues of previously used gypsum.
- One portion of gypsum should be the amount needed to fill no more than two to three prints.
- The use of any solidification accelerators is unacceptable. If necessary, use quick-setting gypsum or increase the kneading time by a few seconds.
- To obtain a given expansion of gypsum, it is necessary to very accurately observe the ratio of gypsum to water.
- Water and gypsum powder should have a temperature of 19-21 ° C.
- The powder must be slowly poured into water, then allowed to immerse in it, and only after that proceed with mixing with a spatula.
- Machine kneading should not exceed 30 seconds, manual - one minute.
- The mixture should pour into the mold immediately after mixing. It is unacceptable to try to increase the pouring time by vibration or adding water.
- It is possible to remove the gypsum model from the print only when the temperature of the model drops.
On the basis of the Department of Orthopedic Dentistry of the Voronezh State Medical Academy, a comparative analysis of dental gypsum was performed, the task of which is to assess the main characteristics of the most common brands of gypsum binders.
For analysis, high-strength and heavy-duty dental gypsum were selected. The tests were carried out in accordance with GOST R51887-2002.
As a result of the study, parameters were established that determine the quality of dental gypsum, ensuring the manufacture of prostheses with high functional and aesthetic properties.
Water consumption. In theory, the required amount of water for the conversion of hemihydrate to hemihydrate is 18.6% of the total binder mass. But in practice, much more is spent to ensure the required mobility of the gypsum dough: thus, the gypsum dough has its own water demand.
Water demand is the smallest amount of water required to obtain a given solution consistency. Excess water evaporates from the formed gypsum stone, leaving pores in it that can significantly reduce the strength of the model. Therefore, it is necessary to strive to accurately measure the water to obtain the perfect consistency.
During hardening, hydration of semi-aquatic gypsum occurs (the reaction of the addition of water to hemihydrate), at which 29 kJ of heat per kilogram of hemihydrate is released. The hardening process is gradual. Semi-aquatic gypsum forms a supersaturated solution with water, from which dihydrate is released. The formation of a large number of particles of hemihydrate leads to the fact that the gypsum mixture is compacted and thickens, 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, its processing methods, composition and amount of modifying additives. The tensile strength is measured in megapascals: 1 MPa = 10 kgf / cm2.
Direct tests in the dental laboratory have shown that the most high-quality types of gypsum show high stability on a spatula and a fluid texture on a vibrating table, which allows you to maximize the number of non-porous fillings with one kneading.
Models obtained from high-quality gypsum binders are resistant to chips, they perfectly repeat the modeled surface, are well polished, sanded and sawn, and when processing a column, the preparation boundaries are not damaged. The high quality of gypsum raw materials eliminates breaking edges when removing the model from the impression, providing the best simulation result.
Production of models of the dentition from plaster: