Dental Casting Investment -D2 PDF

Dental Casting Investment Dr.Wayel Mohammed Huraib. Prosthodontics division  FUNCTIONS OF CONSTITUENTS The lost wax casting technique has been used since ancient times to convert wax patterns to cast metal.  SPRUE Prefabricated plastic and wax sprues are preferre...

Dental Casting Investment Dr.Wayel Mohammed Huraib. Prosthodontics division  FUNCTIONS OF CONSTITUENTS The lost wax casting technique has been used since ancient times to convert wax patterns to cast metal.  SPRUE Prefabricated plastic and wax sprues are preferred over metal sprues because the material is eliminated during the heating cycle..  Requirements For Sprue I- The Sprue must allow the molten wax to scape from the mold.  Requirements For Sprue II- The Sprue must enable the molten metal to flow into the mold with as little turbulence as possible.  SPRUE TYPES I- Wax. II- Plastic. III- Metal  SPRUE DIAMETER A 2.5 mm (12gauge ) : Recommended for Molar metal-ceramic patterns.  SPRUE DIAMETER A 2.0 mm ( 10 gauge ) : recommended for premolars casting and most partial coverage restorations.  SPRUE LOCATION The sprue should be attached to the bulkiest noncritical part of the pattern. Non Functional Cusp !  SPRUE LOCATION On occlusal Inaccurate occlusion. In Proximal contact area Wide mesio-distal and short occloso-cervically.  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.  CASTI NG RING AND LINER It holds the investment in place during sitting and restrict the expansion of the mold.  CASTI NG 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 : ๏ Allow unrestricted expansion. ๏ Useful with higher melting alloys that shrink more because of larger cooling trajectory.  SPRUE TECHNIQUE 1 - Attach a wax sprue to the bulkiest nonfunctional cusp of the wax pattern.  SPRUE TECHNIQUE 2 - Add wax to the point of attachment and smooth it.  SPRUE TECHNIQUE 3 - remove the pattern from the die.  SPRUE TECHNIQUE 4 - Holding the sprue with forceps , insert it into the hole in the Crucible Former.  SPRUE TECHNIQUE 5 - Line the Casting Ring, keeping it flush with the open end, and moisten the liner.  SPRUE TECHNIQUE 6 - Place the Ring over the pattern to ensure that is long enough to cover the pattern with about 6mm.  PROCEDURE FOR MULTIPLE CASTING 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.  INVESTMENT MATERIALS Several Investment materials are available for fabricating dental cast mold. Consist Refractory material Silica and Binder Provides Strength.  INVESTMENT MATERIALS Investment Classification by Binder : 1- Gypsum Bonded. 2- Phospahte Bonded. 3- Silica Bonded INVESTMENT MATERIALS 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. INVESTMENT MATERIALS 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.  SELECTION OF MATERIALS Factors that increase expansion of gypsum- bonded investments include the following: ๏ Use of a full-width ring liner. ๏ Prolonged spatulation. ๏ Storage at 100% humidity. ๏ Lower water/powder ratio. ๏ Use of a dry liner. ๏ Use of two ring liners.  SELECTION OF MATERIALS Factors that increase expansion of Phosphate- bonded investments include the following: ๏ Heat from the setting reaction softens the wax and allows free setting expansion. ๏ The increased strength of the material at high temperatures restricts shrinkage of the alloy as it cools. ๏ The powder mixed with colloidal silica reduces the surface roughness of the castings and also increases expansion.  INVESTING I- Investing technique. II- Wax elimination  II-WAX ELIMINATION Aims of wax elimination: ๏ Complete elimination of wax to obtain an empty mould. ๏ Elimination of carbon vapors from the mould. ๏ Obtain adequate thermal expansion. ๏ Elevates the temperature of the investment to prepare it for casting.  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.  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.  CASTING Casting technique: 4- The orifice of the crucible aligns with the sprue. 5- Heating continues for a few seconds so that the melting is complete and casting can proceed.  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. CAUSES OF CAST FALURE 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. CAUSES OF CAST FALURE Problem Appearance Rough casting Nodule Multiple nodules CAUSES OF CAST FALURE Problem Appearance Nodules on occlusal surface Fins Incomplete casting CAUSES OF CAST FALURE Problem Appearance Incomplete casting with shiny, rounded defect "Suck-back" porosity Inclusion porosity CAUSES OF CAST FALURE Problem Appearance Marginal discrepancy Inadequate or excessive expansion