Investing Technique and Materials PDF

Inves&ng Technique and Materials • Lost Wax Technique: convert wax pa;erns to cast metal o Surrounding the wax pa;ern with a mold made of heat resistant investment material o Elimina0ng the wax by hea&ng o Then, introduce molten metal into the mold through a channel called the sprue, performed in a...

Inves&ng Technique and Materials • Lost Wax Technique: convert wax pa;erns to cast metal o Surrounding the wax pa;ern with a mold made of heat resistant investment material o Elimina0ng the wax by hea&ng o Then, introduce molten metal into the mold through a channel called the sprue, performed in a centrifuge • Shrinkage Compensa&on o Molten alloys used for dental restora&ons shrink upon solidifica&on. o Gold alloys shrink by 1.5% o Nickel-chromium alloys by 2.4% o Mold must be larger than original wax pa;ern (or the resultant cast will be too small) • Investment material- 3 important requirements o Must reproduce precisely the detailed form of the wax pa;ern o Must provide sufficient strength to withstand the heat of burnout and the actual cas&ng of the molten metal o It must expand sufficiently to compensate for the solidifica&on shrinkage of the alloy • Investment materials (Gypsum, Phosphate, Silica) o Gypsum bonded: sa&sfies most requirements but NOT for cas&ng metal ceramic alloys due to the high heat requirement § Gypsum = Binder = 30-35% § Cristobalite and Quartz = refractory material = 60-65% § Used for conven&onal type gold alloys (type II, III, IV): not stable above 650o o Phosphate bonded: for addi&onal expansion over Gypsum investment § Binder = phosphate ion that is water Soluble § Refractory Material = Colloidal Silica in Water § Used for high gold or palladium content alloys (stable above 650o C /1200o F) § Phosphate is: • Stable at high temperatures (excellent for metal ceramic alloys) • Rougher cas&ng surface • Difficult to remove investment • Common to have surface nodules on cas&ng o Silica Bonded: Limited applica&ons in fixed prosthodon&cs • Investment material ideal proper&es o Controllable expansion to compensate for shrinkage of the cast alloy during cooling. o Produce smooth cas&ngs with accurate surface reproduc&on and without nodules. o Chemical stability at high cas&ng temperatures. o Adequate strength to resist cas&ng forces. o Sufficient porosity to allow for gas escape. o Easy recovery of the cas&ng. • Investment Expansion (Se_ng, Wax pa;ern, Hygroscopic, Thermal) o Se_ng Expansion: as gypsum sets, it expands and enlarges the mold. § Influenced by: • The paQern • Metal cas&ng ring • Compressibility of ring liner • Water/powder ra&o: less water increases expansion = larger cas&ng o Wax Pa;ern Expansion § Expand while the investment is s0ll fluid, and the wax is warmed above the temperature at which it was formed. § The chemical reac&on of the investment § Immersion into a warm water bath o Hygroscopic Expansion § Ranges from 1.2% to 2.2% § Investment sets in water, producing addi&onal expansion (1000 F or 38oC) § In a lined, rigid, metal ring, the hygroscopic expansion is more likely due to expansion of the wax paQern § This wax pa;ern expansion is caused by the elevated temperature of the water o Thermal Expansion § Occurs as the mold is heated to eliminate the wax § The silica refractory material is responsible for thermal expansion • Cas&ng Alloy Selec&on o Type I (sod): simple inlays o Type II (medium): complex inlays o Type III (hard): crowns and fixed dental prostheses o Type IV (extra hard): par&al removable dental prostheses and pin ledges o Porcelain: metal-ceramic alloys o Other considera&ons: § Color § Composi&on § Cost § Clinical Performance § Laboratory Performance § Handling Proper&es § Biocompa&bility • Sprue: connects the wax pa;ern to the crucible former o Can be Wax, Plas0c, or Metal o Large diameter recommended (based upon cas&ng size) o Loca0on (a;ached to the bulkiest, non-cri&cal area) o AQachment (Smoothed to minimize turbulence) o Ven0ng (in small auxiliary sprue for thin cas&ngs) o Sprue requirements: § The sprue must allow the molten wax to escape from the mold. § The sprue must enable the molten metal to flow into the mold with as li;le turbulence as possible. § Metal within it must remain molten slightly longer than alloy that has filled the mold • • • • • Crucible former: a base for the cas&ng ring during inves&ng o made of rubber o Sprue aQaches here Cas&ng Ring and Liner o Container for the investment o A liner is placed inside the ring to allow for more expansion (Two liners can be used) Step by step o A;ach a 12-mm wax sprue to the bulkiest non-func&onal cusp of the wax pa;ern (at an obtuse angle to the adjacent axial walls and occlusal surface) o Add wax to the point of a;achment and smooth o Remove the pa;ern from the die o Hold sprue with forceps and insert into crucible former, smooth junc&on with wax o Line the cas&ng ring, keep it flush with open end of ring, and wet liner o Place ring over the pa;ern, adjust sprue if necessary Inves&ng Step by step o Paint pa;ern with surface tension reducer o Add investment powder to the liquid in the mixing bowl and quickly incorporate by hand o A;ach the vacuum hose to the bowl, evacuate the bowl and mechanically spatulate as manufactures instruc&ons o Coat the en0re pa;ern with investment o Place the lined cas&ng ring over the pa;ern and, with the aid of vibra&on, pour the investment down the side of the ring. o Fill the ring slowly o Allow investment to set or for hydroscopic technique place in a water bath for 1 hour Wax Elimina&on Step by step o Remove rubber crucible former, remove metal sprue if used, remove skin at top of the ring o Examine ring for any residual par&cles o Place in furnace at 200o C or 400o F for 30 minutes o Raise temperature based upon investment being used o Hold for generally 45 minutes o Cas&ng machines requires heat source and force o Ring is quenched into cold water o Recover your cas&ng