INVESTMENT MATERIALS - Harvard...
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INVESTMENT MATERIALS
Dental MaterialsSang Park, DDS
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Overview
�� Investment MaterialsInvestment Materials�� ADA Specification 2ADA Specification 2�� Chemistry: Chemistry: Composition, Setting reactionComposition, Setting reaction�� Physical Properties: Physical Properties: Setting, Expansion, StrengthSetting, Expansion, Strength�� Types of Investments Types of Investments
�� GypsumGypsum--bonded Investmentsbonded Investments�� PhosphatePhosphate--bonded Investmentsbonded Investments�� Ethyl silicateEthyl silicate--bonded Investmentsbonded Investments
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Casting Procedures
�� The lost wax casting process: a metal casting is made The lost wax casting process: a metal casting is made using a refractory mold made from a wax replica or using a refractory mold made from a wax replica or pattern.pattern.
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Steps
�� Spruing Spruing of wax patternof wax pattern
�� Investing Investing
�� Wax burnWax burn--outout
�� Casting with molten alloyCasting with molten alloy
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Spruing & Investing:
�� Surrounding wax pattern with a material that can Surrounding wax pattern with a material that can accurately duplicate its shape and anatomic accurately duplicate its shape and anatomic features while leaving a channel for entry of features while leaving a channel for entry of molten alloy.molten alloy.
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• (O’Brien, 1997)
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Burnout:
�� Removal of wax pattern to create mold into which Removal of wax pattern to create mold into which molten alloy can be placed.molten alloy can be placed.
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Casting:
�� Introducing molten alloy into the mold.Introducing molten alloy into the mold.
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�� ShrinkageShrinkage
�� waxwax
�� goldgold
�� ExpansionExpansion
�� settingsetting
�� hygroscopichygroscopic
�� thermalthermal
�� waxwax
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Shrinkage Compensation:
�� Molten alloys shrink on solidificationMolten alloys shrink on solidification
�� Mold must be made correspondingly larger than original Mold must be made correspondingly larger than original wax patternwax pattern
�� Mechanisms to produce expanded moldMechanisms to produce expanded mold
�� Wax pattern expansion Wax pattern expansion
�� Setting expansionSetting expansion
�� Hygroscopic expansionHygroscopic expansion
�� Thermal expansionThermal expansion
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Types of Investments
�� GypsumGypsum--bonded investmentsbonded investments�� Used for casting gold alloysUsed for casting gold alloys
�� PhosphatePhosphate--bonded investmentsbonded investments�� Used for alloys used in Used for alloys used in ceramometal ceramometal
restorationsrestorations�� Ethyl silicaEthyl silica--bonded investmentsbonded investments
�� Used for casting base metal alloy partial Used for casting base metal alloy partial denturesdentures
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Gypsum-bonded investments
�� The mold materials most commonly used in the casting The mold materials most commonly used in the casting of dental gold alloys.of dental gold alloys.
�� Decomposes at high temperatures; not suitable for Decomposes at high temperatures; not suitable for casting highcasting high--melting gold alloys, or base metal alloys.melting gold alloys, or base metal alloys.
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Gypsum-Bonded Investments
�� UsesUses�� Casting gold alloys w/ metal < 700Casting gold alloys w/ metal < 700--1000 C1000 C
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Gypsum-bonded investments:ADA Specification No. 2
�� Type IType I�� THERMAL expansion THERMAL expansion
�� Inlays or crownsInlays or crowns
�� Type IIType II
�� HYGROSCOPIC expansionHYGROSCOPIC expansion
�� Inlays or crownsInlays or crowns
�� Type IIIType III�� Partial dentures with gold alloyPartial dentures with gold alloy
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Gypsum-Bonded Investments�� CompositionComposition
�� RefractoryRefractory�� Crystalline polymorphs of silica (quartz orCrystalline polymorphs of silica (quartz or cristobalitecristobalite))�� 5555--75%75%
�� BinderBinder�� Calcium sulfate Calcium sulfate hemihydrate hemihydrate (plaster or stone)(plaster or stone)�� 2525--45%45%�� In setting, In setting, hemihydrate hemihydrate binder combines with water to form binder combines with water to form
dihydratedihydrate(gypsum)(gypsum)
�� ModifiersModifiers�� Accelerators,Accelerators, retardersretarders, reducing agents or additives that reduce the , reducing agents or additives that reduce the
thermal contraction of the binder. Coloring agents. thermal contraction of the binder. Coloring agents.
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Microstructure of a set gypsum-bonded investment. The large particles are cristobalite: the small acicular crystals are gypsum formed during setting. (SEM at x 3,000) (O’Brien, 1997)
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�� Purpose: to enlarge the mold to compensate for the Purpose: to enlarge the mold to compensate for the casting shrinkage of the gold alloycasting shrinkage of the gold alloy
(1) (1) Normal setting expansionNormal setting expansion: occurs as investment : occurs as investment hardens in airhardens in air
(2) (2) Hygroscopic setting expansionHygroscopic setting expansion: occurs as investment : occurs as investment hardens while immersed in waterhardens while immersed in water
(3) (3) Thermal expansionThermal expansion: occurs as investment is heated: occurs as investment is heated
Types of Expansion:
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�� Mechanism: silica particles interfere with the Mechanism: silica particles interfere with the interlocking of crystals; the outward thrust of the interlocking of crystals; the outward thrust of the crystals increases the expansion of investment.crystals increases the expansion of investment.
�� Maximum expansion in air 0.6%Maximum expansion in air 0.6%
Normal Setting Expansion:
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Hygroscopic setting expansion:
�� Mechanism: a continuation of normal setting Mechanism: a continuation of normal setting expansion b/c immersion in water encourages expansion b/c immersion in water encourages continued growth of crystals.continued growth of crystals.
�� Expansion range 1.2Expansion range 1.2--2.2%2.2%
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Factors that increaseincrease hygroscopic expansion:
�� CompositionComposition: more silica, finer particles lead to more outward : more silica, finer particles lead to more outward growth of crystalsgrowth of crystals
�� W:P ratioW:P ratio: less water, more powder in mix: less water, more powder in mix
�� SpatulationSpatulation: more mixing time: more mixing time
�� Time of immersion:Time of immersion: immerse in water before initial setimmerse in water before initial set
�� ConfinementConfinement: less opposing force from walls of casting ring (wet : less opposing force from walls of casting ring (wet cellulose)cellulose)
�� WaterWater: more immersion water: more immersion water
�� Shelf lifeShelf life: fresher investment: fresher investment
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Thermal expansion:
�� In quartz and In quartz and cristobalitecristobalite, the high, the high--temp phase is less temp phase is less dense than lowdense than low--temp phase; result in expansion.temp phase; result in expansion.
�� Thermal expansion: 1Thermal expansion: 1--1.6%1.6%
�� At high temp, sulfur dioxide gas is released causing At high temp, sulfur dioxide gas is released causing discoloration and discoloration and embrittlement embrittlement of alloy.of alloy.
�� W:P ratioW:P ratio------ more powder, less water result in increased more powder, less water result in increased thermal expansionthermal expansion
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Compressive strengthCompressive strength: : minimum strength is necessary to prevent fracture of investment from the impact of metal enteringthe mold---more powder, less water increase investment strength.
FinenessFineness: : a finer particle size leads to higher hygroscopic expansion and smoother surface on casting.
PorosityPorosity: : venting the mold, air must be eliminated through the pores of investment to prevent buildup of pressure so that molden metal can flow into the mold during casting.
StorageStorage: : preweighed packages
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Phosphate-Bonded Investments
�� UsesUses�� CastingCasting ceramometalceramometal alloys w/ metal < 1200 C (2192 F)alloys w/ metal < 1200 C (2192 F)
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Phosphate-Bonded Investments�� CompositionComposition
�� BinderBinder�� MagnesiumMagnesium--oxide (basic) and phosphate (acid, monooxide (basic) and phosphate (acid, mono--ammonium)ammonium)
�� RefractoryRefractory�� Colloidal silica liquid. Increases expansion and enhances castiColloidal silica liquid. Increases expansion and enhances casting ng
surface smoothnesssurface smoothness
�� ModifiersModifiers�� Carbon: to produce clean castings and facilitate the Carbon: to produce clean castings and facilitate the devestingdevesting. Don’t use . Don’t use
with palladiumwith palladium--containing alloys because carbon containing alloys because carbon embrittles embrittles alloy)alloy)
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Phosphate-Bonded Investments
�� PropertiesProperties�� Complex reaction. Complex reaction.
�� Setting and thermal expansionSetting and thermal expansion
�� Water gives less thermal expansion than silica solWater gives less thermal expansion than silica sol
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Phosphate-Bonded Investments
�� Working and Setting timeWorking and Setting time
�� Markedly affected by temperatureMarkedly affected by temperature
�� The warmer the mix, the faster the setThe warmer the mix, the faster the set
�� Increased Increased spatulation spatulation increases set and gets increases set and gets greater rise in temperaturegreater rise in temperature
�� Increased Liquid/Powder ratio increases Increased Liquid/Powder ratio increases working timeworking time
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Ethyl Silicate-Bonded Investments
�� Uses:Uses:
casting highcasting high--fusing metal partial denture alloysfusing metal partial denture alloys
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Ethyl Silicate-Bonded Investments
�� CompositionComposition
�� BinderBinder
�� Silica gel that reverts to silica (Silica gel that reverts to silica (cristobalitecristobalite) ) on heatingon heating
�� Refractory:Refractory:
�� Quartz or Quartz or cristobalitecristobalite
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Ethyl Silicate-Bonded Investments
�� PropertiesProperties
�� Setting: drying/Setting: drying/gelation gelation causes contractioncauses contraction
�� Complicated and timeComplicated and time--consuming procedures consuming procedures involvedinvolved
�� Heating temp from 1090 Heating temp from 1090 –– 1180 C (20001180 C (2000--2150 F)2150 F)
�� Fragile castFragile cast
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Reference:
�� O’Brien, W., Dental Materials and Their O’Brien, W., Dental Materials and Their Selection, 2nd ed., Quintessence Publishing Co. Selection, 2nd ed., Quintessence Publishing Co. Inc., 1997, pp51Inc., 1997, pp51--77; 23777; 237--247.247.