INVESTMENT MATERIALS - Harvard Universitymycourses.med.harvard.edu/ResUps/SDM404C.00/pdfs/HMS_2235.pdf · INVESTMENT MATERIALS Dental Materials Sang Park, DDS. Overview Investment

INVESTMENT MATERIALS

Dental MaterialsSang Park, DDS

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

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.

Steps

�� Spruing Spruing of wax patternof wax pattern

�� Investing Investing

�� Wax burnWax burn--outout

�� Casting with molten alloyCasting with molten alloy

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.

• (O’Brien, 1997)

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.

Casting:

�� Introducing molten alloy into the mold.Introducing molten alloy into the mold.

�� ShrinkageShrinkage

�� waxwax

�� goldgold

�� ExpansionExpansion

�� settingsetting

�� hygroscopichygroscopic

�� thermalthermal

�� waxwax

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

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

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.

Gypsum-Bonded Investments

�� UsesUses�� Casting gold alloys w/ metal < 700Casting gold alloys w/ metal < 700--1000 C1000 C

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

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.

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)

�� 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:

�� 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:

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%

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

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

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

Phosphate-Bonded Investments

�� UsesUses�� CastingCasting ceramometalceramometal alloys w/ metal < 1200 C (2192 F)alloys w/ metal < 1200 C (2192 F)

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)

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

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

Ethyl Silicate-Bonded Investments

�� Uses:Uses:

casting highcasting high--fusing metal partial denture alloysfusing metal partial denture alloys

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

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

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.