03 Investment

8/3/2019 03 Investment

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Investment Materials


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Objectives

Students can select the type of

investments that is compatible with the

type of alloys used.

Students know why/how something has

gone wrong when they work with the

investment. (and hopefullyknow howto correct it!)


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Metal Casting

Lost wax technique (Taggart, 1907)

A crucible formerB sprue

C wax pattern

D investment material

E ring liner

G thickness of investment at

the top


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Metal Casting

Wax pattern shrinkageMetal shrinkage (~1.5-2%)

Wax pattern expansionInvestment expansion

Setting

HygroscopicThermal

Investment needs to expand 1.5 to 2 % before thecasting is made to compensate for metal expansionduring melting.


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Ideal Investment

Expansion: to compensate the shrinkage

of metal

Should not decompose at hightemperature

Strength at various temperatures

Porous (to allow the escape of gases)

Consistent casting temperatures


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Ideal Investment (cont.)

Easy to manipulate

Smooth castings

Easy to break out

Inexpensive


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Composition

Refractory Heat resistant silica based materials Thermal expansion

Binder Gypsum, Phosphate compound, Silica compound To hold refractory material particles

Modifiers

NaCl, boric acid, potassium sulfate, graphite,copper,..etc.

reducing agent, accel/retard, increase expansion


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Types of Investment(classified bybinder)

Calcium sulfate-bonded or Gypsum-

bonded

Phosphate-bonded

Silica-bonded


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Gypsum-bonded

investment


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SiO2

Composition Binder: E-hemihydrate form of

gypsum (25-45%) +H2O CaSO4.2H2O Shringkage when heated

Not to be heated above 700oC decomposition Refractory: Silica (SiO2)

Quartz, Cristobalite The density decreases as the E

form changes to the F form whenheated, with a resulting increase

in volumeincrease in the linearexpansion.

Modifiers: coloring agent, reduction agents

e.g. carbon, Cu

Em F

Em F

1.5%

1.2%


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ADA Specification No.2

For gypsum-bonded casting investments

(suitable for casting gold alloys)

Type I: For casting inlay-crown Type II: For casting complete and partial

denture


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Expansion

Setting Expansion

Normal setting expansion

Hygroscopic setting expansion Thermal Expansion


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Setting Expansion

(normal vs. hygroscopic)

Stage I: initial mix

Stage II: crystals of dihydrateare formed Lt: water around the particles is

reduced by the hydration andthe particles are drawn moreclosely together by the surfacetension action of the water

Rt: hydrated water is replaced Stage III: crystals grow

Lt: water is decreased, particlesare drawn together

Rt: water is replaced, crystalsgrow freely

Stage IV and V


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Normal Setting Expansion

(0.3-0.4%)

Occurs from CaSO4.1/2H2O CaSO4

.2H2O

A mixture of silica and gypsum hemihydrate

results in setting expansion greater than that ofthe gypsum alone. The silica particles probably interfere with the

intermeshing and interlocking of the crystals as theyform. The thrust of the crystals is outward during

growth, and they increase expansion. Can be regulated by retarders and

accelerators.


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The effectiveness of the setting expansion in enlargingthe mold containing the wax pattern may be related to thethermal expansion of the pattern caused by the heat of

reaction that occurs coincidentally with the setting of theinvestment.

The setting expansion is thus effective only to the extentthat the exothermic heat is transmitted to the pattern.

Manipulative conditions that increase the exothermic heatincrease the effective setting expansion.


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As the investment sets, it eventually gains

sufficient strength to produce a dimensional

change in the wax pattern as setting expansionoccurs.

The softer wax is more easily moved by the

expanding investment.

select appropriate material for a pattern


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Hygroscopic Setting Expansion Occurs when the gypsum

product is allowed to set

under or in contact with

water Can be obtained from

Water immersion technique Water added Wet ring liner

Greater magnitude than thenormal setting expansion (>

6 times)Normal setting expansion

Hygroscopic setting expansion


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Factors to Control Normal and

Hygroscopic Setting Expansion

Composition Proportional to the SiO2 content The finer the particle size of the SiO2, the greater the

hygroscopic expansion.

Water:PowderRatio Higher W:P less expansion

Spatulation

Insufficient spatulation decrease expansion Shelf life of the investment

Older investment lower expansion


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Time of Immersion Delayed immersion decrease expansion

Confinement

the walls of the investment container, or the walls of awax pattern

Water bath / Amount of Added Water Softens and expands wax pattern, allows maximal

hygroscopic expansion

The magnitude of the expansion is in direct proportionto the amount of water added during the setting perioduntil a maximum expansion occurs.


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Linear hygroscopic setting expansion

vs. Amount of water added


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Thermal Expansion

(1.0-1.5%) Directly related to the

amount of SiO2 presentand to the type of SiO2

employed (quartz vs.crystobalite).

The effect of SiO2 willbalance the contractionof the gypsum during

heating. The maximum thermal

expansion is attained ata temperature < 700oC.

SiO2


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Factors to Control Thermal

Expansion

Thinner mixed less thermal expansion

The sufficient amount of SiO2 to prevent any

contraction during heating can weaken theinvestment.

Chemical modifiers, e.g., sodium, potassium andlithium chlorides, are added to eliminates the

contraction caused by the gypsum and increases the

expansion without the presence of an excessive

amount of SiO2.


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Phosphate-bonded

investment


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Composition

Filler (refractory) = quartz or cristobalite or both

(80%)

Binder = magnesium oxide and acid phosphate Liquid = colloidal silica suspension


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Setting Reactions:

NH4H2PO4 + MgO NH4MgPO4 +H2O

The reaction is not simple and changed onheating.


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Setting and Thermal Expansion

Increased expansion andstrength is obtained by addinga combination of different silicaparticle size. (using a colloidalsilica solution instead of water)

Modified by altering the liquid:powder ratio or decreasing the concentration of

the special liquid.


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Working and Setting Time

Markedly affected by temperature

The warmer the mix, the faster it sets.

Increased mixing time and mixing efficiencyresult in a faster set and a greater rise in

temperature.

In general, the more efficient the mixing, the

better the casting in terms of smoothness andaccuracy.

Mechanical mixing under vacuum is preferred.


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OtherProperties

Increasing the special liquid:water ratio

used for the mix markedly enhances

casting surface smoothness but can leadto oversized extracoronal castings.


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ADA Specification No.42

Specifies two types of phosphate-

bonded investments for alloys having a

solidus temperature above 1080oC Type I: For inlays, crowns, and other fixed

restorations

Type II: For partial dentures and other cast,removable restorations

**Can also be used with alloys havingcasting temp. below 1080oC**


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Soldering Investments


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Are designed to have lower setting and thermal

expansions than casting investments

So the assembled parts do not shift in position during thesetting and heating of the investment

Do not have as fine a particle size as the casting

investment because the smoothness of the mass is

less important.


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Examples of Investment

Materials


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Summary

Ideal investment?

Composition of investment

Types of investment, properties and

material selection

Types of investment expansion & factors

to control Soldering investment


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End of InvestmentMaterials

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03 Investment