Investment materials are basically consist of 1. A refractory material 2. A binder material 3. Additives used to improve the handling character. It is available in four allotropic forms such as Quartz Tridymite Cristobalite Fused quartz Quartz and Cristobalite are used extensively in dental investments.
Cristobalite occurs naturally as a rare mineral but is normally manufactured by prolonged heating of the quartz at high temperatures to induce the appropriate slow inversion. Each form of silica exists in two phases. Low temperature phase or alpha phase 2. On heating the change between the two phases is rapid and readily reversible on cooling.
It binds the refractory materials together. According to the binder used investment materials are of three groups. Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties. They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than c. Used for conventional casting of gold alloys inlays, onlays, crowns and Fpd.
Ex Copper. Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above c. They act by reducing the two large contractions of gypsum binder on heating to temperatures above c.
Ex Boric acid Soluble salts of alkali or alkaline earth metals. Usually, the modern inlay investments set initially in 9 to 18 min. Should allow sufficient time for mixing and investing the pattern. ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0. It is enlarging the mould containing the wax pattern and it depends on the thermal expansion of pattern caused by heat of reaction.
High gypsum content- high expansion. Thin wall pattern- more expansion than thick walls. Soft wax- more NSE. Greater in magnitude than NSE. The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment The increased amount of expansion is because the water helps the outward growth of crystals The investment should be immersed in water before the initial set is complete.
ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1. Composition: more finer silica particles -more HSE.
W:P ratio: less water, more powder in mix. Spatulation: more mixing time- more HSE. Time of immersion: immerse in water before initial set. Confinement: less opposing force from walls of casting ring wet cellulose , immersion in water bath at Water: more immersion water. Shelf life: fresher investment. The thermal expansion is directly related to the amount and type of silica present. On reheating it expand again but not recommended as investment may crack on heating.
According to ADA sp no 2 the compressive strength should not be less than 2. Lower content of hemihydrate- more water gaugingmore porous. It is a gypsum bonded material mixed with colloidal silica Setting expansion is 0.
Divestment phosphate is a phosphate bonded investment used as a divestment for high fusing alloys. Easier to manipulate. Adequate strength. Adequate porosity. Less expensive. Amount of dimensional change easier to control. Thermally unstable at high temp. Powder is hygroscopic. Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast at a mold temperature higher than c.
To withstand these high temperatures ,molds require different types of binders such as phosphate compounds. Type 1 For casting of inlays crowns and other restorations especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium Type 2 For casting of removable partial dentures. Purpose To provide high temperature thermal shock resistance To provide high thermal expansion. Modifiers : Carbon is often added -clean casting. Facilitates easy divesting of casting and mold.
Generally added when casting alloy is gold. When silver palladium or base metal alloys are invested with the investment containing carbon, it embrittles the alloys even though the investment is heated to the temperature that burn out the carbon.
Palladium reacts with carbon even at above temp C, so carbon free phosphate bonded invst used for higher temp. Sign Up. Upcoming SlideShare. Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Share Email. Top clipped slide. Download Now Download Download to read offline. Casting procedures in dentistry Oct. Abhilasha Dass Follow. Attended Sharad pawar dental college. Casting in dentistry. Dental Waxes. Waxes in dentistry. Endodontic mishaps - Dr.
Jagadeesh kodityala. Complications of local anasthesia in dentistry. Related Books Free with a 30 day trial from Scribd. Jen Gunter.
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Find Yourself First. John Kim. Casting procedures in dentistry 1. Good Morning 1 2. Pradnya Nikhade Dr. U Manwar Prof. Anant Heda Dr. Abhilasha dass 4. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures. Philbrook demonstrated the 1st cast gold inlay in the Dental Society. H Taggart. Goldberg recommended the use of the wax with 0.
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