Investing and Casting Techniques PDF

Investing And Casting Dr.Wayel Mohammed Huraib ( Part I ) ( Part II ) The lost wax casting technique has been used since ancient times to convert wax patterns to cast metal.  Prerequisites Sprue Diameter Location Attachment Venting Crucible Former Casting Ring Liner Ringle...

Investing And Casting Dr.Wayel Mohammed Huraib ( Part I ) ( Part II ) The lost wax casting technique has been used since ancient times to convert wax patterns to cast metal.  Prerequisites Sprue Diameter Location Attachment Venting Crucible Former Casting Ring Liner Ringless Investment Technique Spuing Technique  Sprue Prefabricated plastic and wax sprues are preferred over metal sprues because the material is eliminated during the heating cycle..  Sprue Requirements I- The Sprue must allow the molten wax to scape from the mold..  Sprue Requirements II- The Sprue must enable the molten metal to flow into the mold with as little turbulence as possible..  Sprue Requirements III- The metal within it must remain molten slightly longer than the alloy that has filled the mold..  Sprue Types I- Wax. II- Plastic. III- Metal  Diameter A 2.5 mm (12 gauge ) recommended for Molar metal-ceramic patterns. A 2.0 mm ( 10 gauge ) recommended for premolars casting and most partial coverage restorations.  Location The sprue should be attached to the bulkiest noncrotocal part of the pattern. Non functional cusp  Location On occlusal in accurate occlusion. In Proximal contact area (wide mesio- distal short occlosocervically).  Attachment The sprue’s point of attachment to the pattern should be carefully smoothed and should not be restricted or necking to avoid casting porosity.  Attachment Widening the attachment will lead to Shrink Spot Porosity.  Venting Vents have been recommended to -It helps gases to escape during casting. - Acting as heat sink. Crucible Former Made of rubber, which serves as a base for the casting ring during investing. Casting Ring and Liner It holds the investment in place during sitting and restrict the expansion of the mold. Casting Ring and Liner Wax pattern is sprayed with wetting agent (Debubblizer ) - Will decease surface tension. - Avoid air bubble formation. * Excessive wetting agent will give us rough surface. Ringless Investment Technique The ringless technique has become quite popular , this method entails the use of paper or plastic casting ring.  Ringless Investment Technique This method designed to 1 - allow unrestricted expansion. 2 - Useful with higher melting alloys that shrink more because of larger cooling trajectory. Spruing Technique 1 - Attach a wax sprue to the bulkiest nonfunctional cusp of the wax pattern.  Spruing Technique 2 - Add wax to the point of attachment and smooth it. Spruing Technique 3 - remove the pattern from the die. Spruing Technique 4 - Holding the sprue with forceps , insert it into the hole in the Crucible Former.  Spruing Technique 5 - Line the Casting Ring, keeping it flush with the open end, and moisten the liner. Spruing Technique 6 - Place the Ring over the pattern to ensure that is long enough to cover the pattern with about 6mm. Procedure for multiple castings When more than two units are being cast together, each is joined to a runner bar. A single sprue is used to feed the runner bar. Two units may be cast with a runner bar, or each unit may be fed from a separate sprue. Prerequisites Materials science. Selection of materials. Investing. Casting. Materials science Several Investment materials are available for fabricating dental cast mold. Consist Refractory material Silica and Binder Provides Strength Materials science Investment Classification by Binder : 1- Gypsum Bonded. 2- Phospahte Bonded. 3- Silica Bonded Materials science Gypsum bonded investment Phosphate bonded investment Not stable at high temp. (Should be not exceeding 650 C ) Stable at high temp. ( 1400 C ) Difficult to control the Sitting Expansion : Easy to control the Sitting Expansion : Less Water will increase the Sitting Expansion. Powder / Liquid ratio to be controlled. Additional Ring Liner will increase the sitting Expansion. Increasing of Colloidal Sillica will increase the Expansion Increase Mixing Time will increase the sitting Expansion. Used for Gold Alloys type : II , III and IV Used for Metal Ceramic Framework Long working time. Short working time. Materials science Gypsum bonded investment Phosphate bonded investment More porous. Less porous. Smooth casting. Rough casting. Casting easily to removed from investment. Casting difficult to remove from investment.  Materials science. Selection of materials. Investing. Casting. Selection Of Materials 1- Selecting a Casting Alloy 2- Selecting an Investment Material  Selection Of Materials Factors affecting in the selection casting alloy: intended use physical properties Color Composition. Cost. Clinical performance. Laboratory performance. Handling properties. Biocompatibility. Selection Of Materials According to their intended use: Type I: simple inlays Type II: complex inlays Type III: crowns and fixed dental prostheses Type IV: partial removable dental prostheses and Pin ledges, Porcelain: metal-ceramic alloys Selection Of Materials According to their physical properties: Type I: soft Type II: medium Type III: hard Type IV: extra hard  Selection Of Materials Color Color is not a good guide to gold content. 9 Carat jewelry alloy (37.5% gold) looks more yellow than a metal-ceramic dental alloy with 85% gold.  Selection Of Materials Composition To be accepted by the ADA as an alloy suitable for dental restorations. Alloys containing approximately 50% gold have been found to resist tarnish and corrosion.  Selection Of Materials Cost Base metal alloys (low cost). Alloys containing approximately 50% gold have been found to offer some economic advantage. Alloys containing primarily palladium and only a small percentage of gold are an alternative for use in the metal ceramic technique.  Selection Of Materials Clinical performance Biologic properties that can be evaluated include gingival irritation, recurrent caries, plaque retentions, and allergies. Mechanical properties include wear resistance and strength, marginal fit, ceramic bond failure, connector failure, and tarnish and corrosion. Selection Of Materials Laboratory performance Important areas of consideration are: Casting accuracy. Surface roughness Strength Sag resistance Metal-ceramic bond strength. Selection Of Materials Handling properties Alloy selection may influenced by the following: Easy manipulation. Critical for dental laboratory. Need an expensive equipment. Selection Of Materials Biocompatibility All materials for intraoral use should be biocompatible. In addition, it should be possible to handle them safely in the office or laboratory. Selection Of Materials The ideal investment should incorporate the following features: 1. Controllable expansion to compensate precisely for shrinkage of the cast alloy during cooling. 2. The ability to produce smooth castings with accurate surface reproduction and without nodules. 3. Chemical stability at high casting temperatures. 4. Adequate strength to resist casting forces. 5. Sufficient porosity to allow for gas escape. 6. Easy recovery of the casting. Selection Of Materials Factors that increase expansion of gypsum-bonded investments include the following: 1. Use of a full-width ring liner. 2. Prolonged spatulation. 3. Storage at 100% humidity. 4. Lower water/powder ratio. 5. Use of a dry liner. 6. Use of two ring liners. Selection Of Materials Selecting an Investment Material Factors that increase expansion of Phosphate-bonded investments include the following: 1. Heat from the setting reaction softens the wax and allows free setting expansion. 2. The increased strength of the material at high temperatures restricts shrinkage of the alloy as it cools. 3. The powder mixed with colloidal silica reduces the surface roughness of the castings and also increases expansion.  Materials science. Selection of materials. Investing. Casting. Investing I- Investing technique. II- Wax elimination Investing I- Investing technique. A. Brush technique. B. Vacuum technique.  Investing I- Investing technique. A. Brush technique. 1. The mix is hand-spatulated to incorporate powder quickly. 2. The investment is vacuum mixed according to manufacturer recommendations.  Investing I- Investing technique. A. Brush technique. 3- The mix is vibrated for a few seconds. 4- The wax pattern is carefully coated the investment by the brush. 5- Be sure that all surfaces of the pattern are covered.  Investing I- Investing technique. A. Brush technique. Investing procedure brush technique. 6- Note the finger that is interposed between the crucible former and the top of the vibrator. 7- The ring is slowly filled, with the bowl held on the vibrator to facilitate investment flow.  Investing I- Investing technique. B. Vacuum technique. Investing procedure vacuum technique. 1- The mix is hand-spatulated. 2- With the crucible former in place, the ring is attached to the mixing bowl. 3- The vacuum hose is attached.  Investing I- Investing technique. B. Vacuum technique. Investing procedure vacuum technique. 4- Mixing is performed according to manufacturer. 5- The bowl is inverted, and the ring is filled under vibration. 6- The vacuum hose is removed before the mixer is shut off.  Investing II - Wax elimination. Aims of wax elimination: 1- Complete elimination of wax to obtain an empty mould. 2- Elimination of carbon vapors from the mould. 3- Obtain adequate thermal expansion. 4- Elevates the temperature of the investment to prepare it for casting.  Investing II - Wax elimination. Burnout ovens are available with : Manual. Semiautomatic. Fully programmable controls.  Investing II - Wax elimination. Burn out temperature: - Gypsum-bonded: 650-687°c (for gold alloy) - Phosphate-bonded: up to 834°c (for base metal alloy) Gradual rise in temperature is required to prevent investments cracking. the temperature is held for half an hour.  Materials science. Selection of materials. Investing. Casting. Casting Casting machine. Casting technique. Causes of cast failure. Casting Casting Machines Required: Heat source to melt the alloy using reducing flame of a torch or electricity. Conventional alloys melted by gas air torch. Metal ceramic alloys (higher melting range) melted by gas oxygen torch. Casting force to overcome the high surface tension of the molten alloy. Casting Casting Machines A: Gas-air casting torch. B: Gas-air tip. C: Gas-oxygen casting torch. D: Multi orifice tip. Casting Casting technique. 1- Preheating the crucible. 2- Making the melt. When the alloy is molten, the casting ring is removed from the furnace and placed in the cradle. Casting technique. 3- Tongs are used to slide the crucible platform into contact with the casting ring. 4- The orifice of the crucible aligns with the sprue. Casting Casting technique. 5- Heating continues for a few seconds so that the melting is complete and casting can proceed. 6- The casting arm is pulled forward until the pin drops. Casting Casting technique. 6- The casting arm is pulled forward until the pin drops. 7- The melt, seconds before casting. 8- Centrifugal force carries the melt into the mold cavity. Casting Causes of cast failure. Evaluation: The casting is never fitted on the die until the inner surface has been carefully evaluated under magnification. Defects in the casting: Investing and casting requires careful attention to obtain a successful, properly fitting casting. Casting Problem Possible causes Appearance Excess surfactant Rough casting Improper water/powder ratio Excessive burnout temperature Air trapped during investing Nodule Procedure Inadequate vacuum during investing. Multiple nodules Improper brush technique. Lack of surfactant. Casting Problem Possible causes Appearance Nodules on occlusal Excessive vibration surface Increased water/powder ratio Pattern too near edge of Fins investment. Too-rapid heating Dropped mold Wax pattern too thin Incomplete casting Cool mold or melt Inadequate metal Casting Problem Possible causes Appearance Incomplete casting with Incomplete wax elimination shiny, rounded defect if the metal in the sprue solidifies before the metal in the mold. "Suck-back" porosity Improper pattern position Narrow, long sprue Particle of investment Inclusion porosity dislodged during casting Casting Problem Possible causes Appearance Wax pattern distortion Uneven (increase) expansion Marginal discrepancy Improper water/powder ratio Inadequate or excessive Improper mixing time expansion Improper burnout temperature