Waxing Up, Spruing, Investing, Casting, and Porcelain Build-Up PDF

 Metal framework Porcelain Build-up  Laboratory steps Fixed prosthodontics 1 2 3 4 5 Waxing porcelain spruing investing casting up...

 Metal framework Porcelain Build-up  Laboratory steps Fixed prosthodontics 1 2 3 4 5 Waxing porcelain spruing investing casting up build-up Lost wax technique Metal  4 1 Lost wax technique 2 5 The process of surrounding the wax pattern with a mold made of heat resistant investment material, 3 eliminating wax by heating, then introducing molten metal into the mold through a channel 1 Waxing up  1 Waxing up The process of rebuilding of prepared tooth on a die by wax Why wax ?? Conveniently manipulated and precisely shaped by heating Can be completely eliminated from the mold after investing Non toxic & Environmentally neutral  Inlay casting wax wax that is used in forming the pattern for cast restorations General requirements Color Flow Carving Addition Residue Compensation Strength and Stress Burnishing Details of metal rigidity after relaxation &Polishing shrinkage cooling  Step by step wax pattern construction and evaluation Modification of the master cast Addition of the wax layers Evaluation Modification of the master cast Increase abrasion resistance 1 2 3 Die Preparation Define margins Die hardener 4 5 To facilitate To create removal of space for wax pattern luting cement Die spacer Separating medium Addition of wax layers Dipping of the first layer followed by anatomical building up of the wax pattern by addition  Evaluation Axial contours occlusal morphology Margins Removal of wax pattern Remove the pattern from the die, using extreme caution not to distort it by bending the fingers of the left hand 2 spruing to provide a Wax patterns spruing channel in the investment material through which molten wax can escape during the burnout process, and molten metal can reach the mold cavity. 2 spruing 1 2  Requirements for sprue Sprue Allow molten wax to escape from former 1 mold Enable molten alloy to flow into 2 mold Molten metal inside the sprue must 3 remain molten slightly longer than metal that had filled the mold.  Types of sprue formers wax plastic Metal Readymade Advantage : used in casting FPD in one piece as it adds Available in rigidity which minimize Solid or hollow different shapes distortion. and diameter Most preferred type as it melts at the same rate as the pattern Strong Disadvantage: soften at a higher and allows easy wax escape attachment temperature than wax which may block wax escape Should be removed from investment Properties of sprue 1- Diameter Sprue former must be thicker than thickest portion of the pattern this improves the flow of molten metal into the mold. And to permit solidification of metal in the casting prior to sprue Reservoir : Is a small amount of additional wax added to sprue former 1 mm below wax Reservoir pattern. Function : to compensate for the shrinkage occurs during solidification of the casting Properties of sprue 2- Location The sprue should be attached to the bulkiest noncritical part of the pattern. away from margins and occlusal contacts. Normally, the largest nonfunctional cusp is used Attaching the sprue to thin areas will result in porosities in thicker portions of the casting as thinner areas solidifies 1st so preventing compensation of shrinkage at thicker portions. (shrinkage porosity) Properties of sprue 3- Angulation Should be at angle (45°) (not perpendicular) to allow the incoming molten metal to flow freely to all portions of the mold and lessen the chance of excessive turbulence during casting and subsequent porosity formation. Properties of sprue 4- Length Provides adequate bulk of investment to According to investment used withstand force. Allows gases to escape from end of mold. Gypsum Phosphate bonded bonded investment investment 6-8 mm 3-4 mm investment investment bridge bridge  Avoid Very short < 2mm Very long > 6mm place the mold space As molten metal too far from the end of inside sprue may the ring, so the gases solidify 1st cannot escape easily preventing full resulting in back mold feeding by pressure porosity the molten alloy  5- Venting Small auxiliary sprues or vents improve casting of thin patterns and may helps in : gases escape during casting. compensate for the shrinkage during solidification auxiliary auxiliary sprue sprue  6- Number According to size of casting Single sprue Double sprue Multiple sprue Spruing technique Add wax to the point of Attach a wax sprue to attachment and smoothen the bulkiest it to prevent turbulence non-functional cusp of during casting the wax pattern, and angle it to facilitates filling of the mold Insert the sprue into the hole in the crucible former with forceps and lute with wax which should be smoothed Crucible former  Casting ring hold investment in place during setting and restricts expansion of mold Resilient liner is placed inside the ring Crucible former 1.Allow different types of investment expansion (act It forms the conical depression in the as cushion) end of investment that guides the 2.Facilitate removal of investment block after molten alloy into the channel left by casting the sprue former. 3.Prevent distortion of the pattern by permitting Made of rubber, metal or plastic the outward expansion of the mold 3 Investing  Ideal properties of investment materials Accurate surface Controllable reproduction expansion Components Refractory material Sufficient porosity Chemical stability to allow gas at high casting escape temperatures Binder material Chemical modifiers Adequate strength Easy recovery of to resist casting the casting forces Produce smooth castings  Types of investment materials According to the binder magnesium oxide and ammonium phosphate Gypsum binder compound binder Silica binder Gypsum-bonded Phosphate-bonded Silicate-bonded with cristobalite or quartz Silica refractory material alternative to the phosphate refractory material Mixed with colloidal silica bonded investments, for high temperature casting. Not chemically stable at Higher strength Principally used in the casting of temperatures above 650° Higher expansion base metal alloy partial dentures Used for casting of Lower porosity Its use has declined -more complex conventional type II,III, and IV Rougher surface and time-consuming procedures gold alloys Stable at high temperatures; NO porosity suitable for metal ceramic alloys  Types of expansions Setting expansion Hygroscopic expansion Thermal expansion Extension of setting Solid-state phase Occurs during setting of transformations of expansion by adding the material as a result silica. water to the setting of crystal growth investment More controllable How to increase expansion ?? Mixing of investment Hand mixing Vacuum mixing Reduce the amount of porosity in the investment. Texture of the surface casting is smoother with better detail reproduction. Compressive strength of the investment is increased Wax elimination (burnout) 4 Casting Heat source to melt the alloy. Casting force to force molten alloy into mold high enough to overcome the high surface tension of the molten alloy, as well as the resistance of the gas within the mold Heating Source Hottest part of the flame Kept constantly on the alloy during melting  Casting force Centrifugal casting machine Air pressure casting machine The alloy is melted in a separate crucible by a torch The alloy is melted in the crucible followed flame and cast into the mold by centrifugal force by applied air pressure  Casting Defects Casting defects are attributed to poor casting technique and failure to adhere to casting laws Rough casting Large nodules Small nodules Fins Incomplete casting Incomplete casting Marginal discrepancy Cleaning the casting After the casting has solidified the ring is removed and quenched in water. Divesting “It refers to removal of casting from the investment mold Often the surface of casting appears dark with oxides and tarnish, such a film can be removed by process called pickling removal of casting from the Heating a discolored casting in an acid investment by sandblasting (50%) dil HCL + (50%) dil Sulphuric acid To sum up : Preparing wax pattern Spruing Attaching to Casting Ring Investing crucible former Wax burnout Casting Divesting Finishing of metal Lost wax technique Remember this ?  Laboratory steps Fixed prosthodontics 1 2 3 4 5 Waxing porcelain spruing investing casting up build-up Lost wax technique Metal 5 Porcelain build up Ceramic layers Metal coping 1- opaque porcelain 2- body porcelain 3- incisal porcelain Types of porcelain for metal ceramic restoration Opaque porcelain they wet the metal surface and establish a metal-porcelain bond; they mask the color of the metal substructure; an they initiate development of the selected shade. Body porcelain Term used to collectively describe four principal types of porcelain powders used to create a restoration: dentin porcelains, enamel porcelains, translucent porcelains, and body modifiers Stains and glazes Glazes are generally colorless low-fusing porcelains that possess considerable fluidity at high temperature. opaque porcelain Dentin porcelain Enamel porcelain Reading material: Fundamentals of fixed prosthodontics, Herbert T. Shillingburg, 2012 (chapters 19, 20, 23) Fixed prosthodontics, principles and clinics, H.W. Anselm Wiskott, 2011 (chapter 18) Contemporary fixed prosthodontics, Rosenstiel, Land, Fujimoto, 4th edition, 2006 (chapters 18, 19, 22, 24, 25, 26, 27, 29)