![]() One approach is to apply additives used in industrial applications (e.g., concrete manufacture) to dental gypsum products. 6 Experts continue their efforts to improve the properties of die stone. Care is needed when the resin is selected and applied so that the resin film will have no significant thickness. As mentioned earlier, abrasion resistance is the physical property most improved by this technique. ![]() An alternative approach 5 is to impregnate the surface of the die with a low-viscosity resin such as cyanoacrylate. 4 Their use is accompanied by a slight increase in setting expansion, but such is probably not clinically significant. Attempts to overcome this have included the use of so-called “gypsum hardeners.” Although these materials (e.g., colloidal silica) have relatively little effect on the hardness of the stone, they improve abrasion resistance (some by as much as 100%). ![]() Gypsum’s greatest disadvantage is its relatively poor resistance to abrasion. If soaking is required, it should be done in water saturated with plaster slurry and only long enough to achieve the desired degree of wetting. However, although it appears to be insoluble, the gypsum slowly dissolves, which ruins the surface detail of the cast. With some techniques (e.g., when a cast is prepared for duplication), it is necessary to soak the set gypsum in water. 1 However, not all brands of die stone are compatible with all brands of impression material, 2, 3 and if poor surface detail reproduction is experienced, an alternative product should be selected. The materials are capable of reproducing a 20-μm-wide line as prescribed by ADA specification No. Surface detail reproduction is acceptable with type IV and type V gypsum products. Newly poured casts should be left undisturbed for at least 30 minutes superior results are achieved at 1 hour, although these times may vary among brands. Porosity is reduced, with a concomitant increase in strength, after only 15 seconds of mechanical mixing. Hand mixing of gypsum products is easy, but results are better when the mixing is done mechanically in a vacuum. The physical properties of die stone are improved over those of dental stone and plaster because less water is needed to obtain a sufficiently fluid mix. The differences between the various types of dental gypsum are attributable to calcination. ![]() The hemihydrate is manufactured by heating the dihydrate under controlled conditions to drive off some of the water of crystallization (a process called calcination). The setting reaction results from the hydration of calcium sulfate hemihydrate: The gypsum components are identical chemically. Manufactured in enormous quantities for industrial use, it can easily be modified for dental use.ĭental gypsum products are available in five forms (American Dental Association types I to V), defined as impression plaster model plaster dental stone high-strength dental stone and high-strength, high-expansion stone. This material is inexpensive, is easy to use, and produces consistent results. The two crucial characteristics of cast-and-die materials, dimensional accuracy and resistance to abrasion while the wax pattern is being formed, are adequately achieved with gypsum. ![]()
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