CASTING and casting alloy M.Hosny 2023.pdf

CASTING and CASTING ALLOY Presented by Dr. Mohammed Hosny Associate Professor of Fixed Prosthodontics Collage of Dentistry Taibah University 2/12/2023 M.Hosny 2 Lab Steps 2/12/2023 M.Hosny 3 Metals used for casting Classification of dental alloys: 1. According to their noble metal conte...

CASTING and CASTING ALLOY Presented by Dr. Mohammed Hosny Associate Professor of Fixed Prosthodontics Collage of Dentistry Taibah University 2/12/2023 M.Hosny 2 Lab Steps 2/12/2023 M.Hosny 3 Metals used for casting Classification of dental alloys: 1. According to their noble metal content by the (ADA) 1-High noble metal alloy  Contain 60% noble metal and 40% gold e.g.: gold palladium platinum, gold palladium silver and gold palladium 2-Noble metal alloy  At least 25 % noble with no requirement of gold e.g.: silver Palladium, Palladium cupper gallium. 3-Base metal alloy  Less than 25% noble with no requirement for gold.  e.g.: Nickel chromium, Cobalt chromium and Titanium alloys. 2/12/2023 M.Hosny 4 2.The gold Alloy is classified according to there mechanical properties into: 1-Type 1(soft ) used in four wall simple cavities. 2- Type 2 (medium) used in complex inlays. 3- Type 3 (hard) used in crown and bridges. 4- Type 4 (extra hard) used in removal prosthesis. 2/12/2023 M.Hosny 5 requirements of casting alloy 1-High strength and hardness 2-Resist tarnish and corrosion 3-Have acceptable fluidity and castability 4-Ease in soldering and welding 5-Can be porcelanized 6-Biocompatiable with adjacent soft and hard tissue 7-Easly finished and polished 8-Can be burnished to fine margins 2/12/2023 M.Hosny 6 Comparison of gold alloys to base metal alloys Gold alloys Base metal alloys 1-Yellow in color 1-Silver white in color 2-High density 18.3gm/cm3 so require 2-Low density 7-9 gm/cm3 so require high less casting pressure casting pressure 3-Melting temperature 935-1150 (single 3-Melting temperature 1150-1400 (multi- orifice torch) orifice torch) 4-Casting shrinkage 1.6% 4-Casting shrinkage 2.4% 5-Lower modulus of elasticity so it must 5-Higher modulus of elasticity so can be be used in thick sections used in thin sections 6-High ductility so can be easily 6-Inductile so can not be burnished burnished 2/12/2023 M.Hosny 7 Casting Process Heat source for melting the alloy Casting force for driving the molten alloy inside the mold cavity 2/12/2023 M.Hosny 8 Heat Source Can be done by: a) Blowpipe flame (blow torch): I- Gas - air torch: Types II- Gas – oxygen torch: 1.Single orifice. 1. Multi-orifice torch. 2.Used to melt the precious, or semi-precious alloys 2. Used to melt the higher melting range alloys, base metal alloys 2/12/2023 M.Hosny 9 The blowpipe flame consists of: 1. Mixing zone: It is a colorless zone contains pure gas. 2. Combustion zone: mixture of gas and air. 3. Reducing zone: light blue in color, it is the hottest zone of the flame. This is the zone used to melt the alloy 4. Oxidizing zone: at the edge of the flame (reddish). 2/12/2023 M.Hosny 10 2/12/2023 M.Hosny 11 b)Electricity Convectional heating muffle Induction current 2/12/2023 M.Hosny 12 Fluxing  The flux is added when the gold starts to turn red  The flux is not recommended in non-precious alloys  Importance of flux: 1-To prevent oxidation of the alloy. 2- Increase fluidity of the alloy 3- Reduce the melting temperature of alloy  Examples of flux a- Borax powder with boric acid powder b- Charcoal powder 2/12/2023 M.Hosny 13 Casting Process Casting Force Air-pressure Centrifugal force Vacuum-pressure 2/12/2023 M.Hosny 14 Casting machines Molten alloy is forced inside the mold cavity using one of: 1- Air pressure casting technique:  The molten alloy is forced into the mold by air pressure which consists of compressed air or gases as CO2.  The alloy is placed in the crucible and melted by a torch, then pressure is applied.  Gives low casting pressure.  Used with precious and semi-precious alloys. 2/12/2023 M.Hosny 15 2-Vapor pressure technique It is used to cast the precious and semiprecious alloys, also gives low casting pressure. The machine consists of a main base and a cover. Wet asbestos disc adjusted on the cover. The cover is adapted properly over the heated ring. The produced vapor will press the molten alloy into the mold. 2/12/2023 M.Hosny 16 2/12/2023 M.Hosny 17 3-Centrifugal casting machine  It consists of a base joint with a mobile arm and a strong spring. One end of the arm the crucible with the molten alloy and the other a equal sized counter balance.  The spring is turned several times clockwise and fixed with a pin.  When all is ready the pin is released and the centrifugal force pushes the alloy into the mold.  A graphite crucible is used for gold alloys while a ceramic one is used in casting the base metal alloys, to prevent their contamination. 2/12/2023 M.Hosny 18 Centrifugal casting machine 2/12/2023 M.Hosny 19 2/12/2023 M.Hosny 20 4- Vacuum casting machine This machine evacuates the mold before it is filled with the molten alloy. Vacuum improves the mold cavity filling. The casting temperature of any alloy should be over its melting range with 30°F 2/12/2023 M.Hosny 21 2/12/2023 M.Hosny 22 Recovery of the casting For precious alloys:  The ring is quenched when the red glow of the button disappears. This helps disintegrate the investment.  Pickling in warm 50% HCl solution to remove black oxide layer. For base metal alloy:  Ring cooled to room temp., then broken manually.  NOT pickled--------contaminated, BUT Sandblasting with Al2O3 particles under pressure. 2/12/2023 M.Hosny 23 Recovery of the casting 2/12/2023 M.Hosny 24 2/12/2023 M.Hosny 25 2/12/2023 M.Hosny 26 References Contemporary of Fixed Prosthodontics. 5th edition. P. 614 2/12/2023 M.Hosny 27 2/12/2023 M.Hosny 28