Dental casting investment material

Dental casting investment material

1. Dental Casting Investment Materials Dr. Deepak K. Gupta

2. Introduction A material which is suitable for forming a mould into which molten metal or alloy is cast. These materials can withstand high temperatures. Also known as refractory materials. In general, an investment is a mixture of three distinct types of materials R efractory material, B inder material Other chemicals facebook.com/notesdental

3. Components of Investment Refractory Material a form of silicon dioxide , such as quartz , tridymite , or cristobalite , or a mixture of these They serve two functions: Acts as a material that can withstand high temperatures. Regulates the thermal expansion . Binder Material refractory materials alone do not form a coherent solid mass, Some kind of binder is needed. Ex: Gypsum, phosphate, ethyl silicate Other Chemicals sodium chloride, boric acid , potassium sulfate, graphite, copper powder, or magnesium oxide , Small quantities - modify various physical properties . facebook.com/notesdental

4. Properties Required of an Investment Easily manipulated Sufficient strength at room temperature Stability at higher temperatures Sufficient expansion : compensate for shrinkage of the wax pattern and metal. Porosity : porous enough to permit the air or other gases in the mold cavity to escape. Smooth surface : Fine detail and margins on the casting. Ease of divestment Inexpensive . facebook.com/notesdental

5. Types of Investment Based on the nature of Binder, its classified on 3 types Gypsum bonded investments : used for casting gold alloys, withstand temperature up to 700°C. Phosphate bonded investments : For metal ceramic and cobalt-chromium alloys, withstand higher temperatures. Ethyl silica bonded investments alternative to the phosphate bonded investments , for high temperature casting. Principally used in the casting of base metal alloy partial dentures facebook.com/notesdental

6. Gypsum Bonded I nvestments ADA Sp. No. 2 for gold alloy casting investments again classify it 3 types. Constituents 65% to 75 %: quartz or cristobalite , or a blend of the two , 25% to 45%: α-calcium sulfate hemihydrate 2% to 3 % chemical modifiers Investment Use compensation for alloy casting shrinkage Type I casting of inlays or crowns Thermal expansion of the investment Type II casting inlays, onlays , or crowns hygroscopic expansion achieved by immersing the invested ring in a warm water bath Type III casting gold alloys Thermal facebook.com/notesdental

7. Gypsum: α- calcium sulfate hemihydrate Casting gold-containing alloys with melting ranges below 1000 °C Material is heated at temperatures sufficiently high 200-400 °C : shrink considerably 400 °C - 700 °C : slight expansion takes place between Above : decomposition and the release of sulfur dioxide causing contamination of alloy facebook.com/notesdental

8. S ilica It exists in 4 allotropic forms: quartz, tridymite , cristobalite and fused quartz. Quartz and cristobalite are of dental importance Investment should expand thermally - compensate partially or totally for the casting shrinkage of the solidifying alloy . Gypsum - shrinks considerably when it is heated, so it should be silica which should expand to cause overall expansion of investment facebook.com/notesdental

9. Silica When heated allotropes of silica changes from α (low room temperature form) to β (high temperature form) Phase transformation is called an inversion. Causes linear expansion – overall causing volume expansion facebook.com/notesdental

10. MODIFIERS Certain modifying agents Coloring matter , Reducing agents, such as carbon and powdered copper , Alkali-earth and transition-metal chlorides, boric acid, and sodium chloride regulate the setting expansion and the setting time facebook.com/notesdental

11. Properties Setting time: set initially in 9 to 18 mins It can be manipulated in the same way as of gypsum product NORMAL SETTING EXPANSION silica particles probably interfere with the intermeshing and interlocking of the crystals thrust of the crystals is outward during growth , Soft wax and thin pattern may get distorted, facebook.com/notesdental

12. Properties: HYGROSCOPIC SETTING EXPANSION Expansion when contact with heated water . G reater in magnitude than normal setting expansion Type II investments : 1.2 % - 2.2 % Directly proportional Silica content of the investment Water mixed during setting reaction Indirectly proportional : size of silica particles A, Normal setting expansion B , Hygroscopic setting expansion facebook.com/notesdental

13. Properties: THERMAL EXPANSION Thermal expansion of silica must be increased to counterbalance the contraction of the gypsum Contraction of the gypsum is entirely balanced when the quartz content is increased to 75% Type I : 1% - 1.6 %, Type II : - 0 % and 0.6% at 500 °C It depends on P article size of the quartz, T ype of gypsum binder: α or β Resultant W/P ratio Allotropes of quartz : Cristobalite > quartz facebook.com/notesdental

14. Properties THERMAL CONTRACTION Inversion of the β form to its stable α form at room temperature. Contracts to less than its original dimension EFFECT OF CHEMICAL MODIFIERS Increasing the silica content – reduces the strength of investment Small amounts of sodium, potassium, lithium chlorides or boric acid – eliminates the need for adding silica to cause expansion facebook.com/notesdental

15. Properties STRENGTH Adequate to prevent cracking, bulk fracture, or chipping of the mold affected by the W/P ratio same way as gypsum. It decreases after heating to 700 C – microcracks FINENESS Fine silica results in a higher hygroscopic expansion Also it will give finer details accurately and minimal surface roughness. facebook.com/notesdental

16. PHOSPHATE-BONDED INVESTMENT Increased use of metal-ceramic, hot-pressed ceramic prostheses and base metal alloy – requires investment which can bear higher temperature Phosphate-bonded or silicate-bonded investments can be used in such condition But they are difficult to disinvest – however these problem has been sorted out recently. facebook.com/notesdental

17. COMPOSITION Refractory filler : silica (80% by weight) in the form of cristobalite , quartz, or a mixture of the two forms. particle size varies from a submicron level to that of a fine sand Binder : magnesium oxide (basic) and a phosphate (acid) Originally phosphoric acid was used, but monoammonium phosphate has replaced Other: Carbon Produce clean castings and facilitate the divesting of the casting from the investment mold Appropriate when the casting alloy is gold Others: palladium reacts with carbon at temperatures above 1504 °C to make it brittle Colloidal silica suspensions in place of water Greater expansion : newer gold-containing alloys and other alloys - higher melting temperature ranges than traditional gold alloys, facebook.com/notesdental

18. SETTING REACTION Ammonium diacid phosphate reacts with magnesium oxide - green strength, or room temperature stregth . ammonium diacid phosphate is used in a greater amount. additional amount can react with silica at an elevated temperature P 2 O 5 and SiO 2 forms silicophosphate increases the strength of investment facebook.com/notesdental

19. SETTING AND THERMAL EXPANSION slight expansion occurs compared to gypsum bonded increased considerably by using a colloidal silica solution in place of water early thermal shrinkage - decomposition of the binder, accompanied by evolution of ammonia. Gypsum investments the shrinkage is caused by the transformation of calcium sulfate from the hexagonal to the rhombic form. facebook.com/notesdental

20. WORKING AND SETTING TIME markedly affected by temperature warmer the mix, the faster it sets setting reaction itself gives off heat, and this further accelerates the rate of setting Increased mixing time and mixing efficiency , result in a faster set Better smoothness and accuracy of the casting. Mechanical mixing under vacuum is preferred L/P ratio increase in the L/P ratio increases the working time facebook.com/notesdental

21. SURFACE QUALITY OF CAST METALS In the past, detail reproduction and surface smoothness were inferior to gypsum bonded increasing the ratio of special liquid to water used for the mix – gave a better details lead to oversized extracoronal castings Improvement in the technique and also in the investment composition - few surface imperfections facebook.com/notesdental

22. ETHYL SILICATE–BONDED INVESTMENT Its use has declined - more complex and time-consuming procedures But still used in the construction of high-fusing base metal partial denture alloys . Here the binder is silica gel . Its made either by 2 method from Sodium Silicate Ethyl silicate facebook.com/notesdental

23. Types Sodium silicate pH lowered by addition of an acid or an acid salt, a bonding silicic acid gel forms. Accelerator: ammonium chloride Ethyl silicate It is hydrolyzed in the presence of hydrochloric acid, ethyl alcohol, and water. Sol is then mixed with quartz or cristobalite , Finely powdered magnesium oxide is added - keep the mixture alkaline. A coherent gel of polysilicic acid then forms, accompanied by a setting shrinkage facebook.com/notesdental

24. Ethyl silicate When this soft gel is dried at a temperature below 168 °C loses alcohol and water to form a concentrated, hard gel Green shrinkage , which is additive to the setting shrinkage As well it takes longer time to hydrolyze and gelation – amines can be added to faster the reaction Thus in this type of investment, mold enlargement should compensate not only casting shrinkage, but green shrinkage and setting shrinkage facebook.com/notesdental

25. Manipulation The powder is added to the hydrolyzed ethyl silicate liquid, mixed quickly, and vibrated into a mold that has an extra collar to increase the height The mold is placed on the platform of a special type of vibrator that provides a so-called tamping action. This allows the heavier particles to settle quickly while the excess liquid and some of the fine particles rise to the top. In about 30 minutes, the accelerator in the powder hardens the settled part and the excess at the top is poured off. Thus, the L/P ratio in the settled part is greatly reduced and the setting shrinkage is reduced to 0.1%. The remaining cast is somewhat fragile because the amount of binder is quite low and it is essentially composed of silica. facebook.com/notesdental

26. Various Investment Material facebook.com/notesdental

27. References Phillips' Science of Dental Materials- Phillip Anusavice_12 th Basic Dental Materials -2nd.ed Mannapalli Clinical Aspects of Dental Materials Theory, Practice, and Cases, 4th Edition Craig's Restorative Dental Material 13th edition facebook.com/notesdental

28. THANKS…… Like, share and comment on https://www.facebook.com/notesdental http://www.slideshare.net/DeepakKumarGupta2 facebook.com/notesdental