INVESTMENT MATERIALS FOR DENTAL CASTING PROCEDURES

PRESENTED BY DR. ALOK RANJAN SASMAL FIRST YEAR PG Dept. of Prosthodontics and Crown & Bridge

INTRODUCTION

CONTENTS-

DEFINITIONS

REQUIREMENTS OF INVESTMENT MATERIALS

MATERIAL SCIENCE

GYPSUM-BONDED INVESTMENT MATERIAL AND ITS PROPERTIES

PHOSPHATE-BONDED INVESTMENT MATERIALAND ITS PROPERTIES

SILICATE-BONDED INVESTMENTMATERIAL

IMPLICATION OF MATERIAL SCIENCE DURIING CASTING

OTHER INVESTMENT MATERIALS

SUMMARY

REFERENCES

DEFINITIONSInvesting: the process of covering or enveloping,

wholly or in part, an object such as denture, tooth, wax form, crown, etc. with a suitable investment material before processing, soldering or casting.

(GPT 8)Dental casting investment: A material consisting

primarily of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures).

(GPT 8)

REQUIREMENTS OF INVESTMENT MATERIALS

1. Must reproduce precisely the details form of the wax pattern2. Must provide sufficient strength to withstand the heat of burnout and the

impact and the pressure of cast molten alloy3. Must maintain the integrity at higher temperatures and should not

decompose to give off gases4. Must expand sufficiently to compensate for the solidification shrinkage of

the alloy5. Easily manipulated and setting time should be less6. Should be inexpensive7. Biological safety

Fundamentals of fixed prosthodontics,3rd edi.Herbert T. Shillingburg

MATERIAL SCIENCE

Investment materials are basically composed of

1. A refractory material

2. A binder material

3. Additives .

REFRACTORY MATERIAL

SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion. It exists in four allotropic forms such as Quartz Tridymite Cristobalite Fused quartz Each form of silica exists in two phases. Low temperature phase or alpha phase

High temperature phase or Beta phase

On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as inversion

IMPORTANCE OF INVERSION -

As form changes from Alpha to Beta Density

volume

Increase in liner expansion(quartz-0.45% ), which compensate shrinkage of gypsum and casting shrinkage

Quartz- 5730C Cristobalite- between 200 and 2700CTridymite- 1170C,1630cFused Qurtz- no inversion temperature

Inversion temperature

Quartz, cristobalite, or a combination of the twoforms may be used in a dental investment

BINDERIt binds the refractory materials together.According to the binder used investment materials are of

three groups.

1. GYPSUM-BONDED INVESTMENTS 2. PHOSPHATE BONDED INVESTMENTS 3. SILICA BONDED INVESTMENTS

ADDITIVESUsed are Reducing agents –provides a nonoxidizing

atomsphere in the mold cavity Modifying chemicals –regultes physical

properties like setting reaction, setting expansion Coloring matter

GYPSUM BONDED INVESTMENT MATERIAL

They are the mold materials used in the casting of dental gold alloys with temperature below 7000C.

Used for conventional casting of gold alloys inlays, onlays, crowns and Fpd.

ADA SPECIFICATION-2; divided them further into

TYPE I- for casting inlays/crowns. mode of expansion: Thermal

TYPE II-for casting inlays/crowns mode of expansion : Hygroscopic

TYPE III-for partial dentures with gold alloys

CompositionRefractoryCrystalline polymorphs

of silica (quartz or cristobalite)

55-75%Silica is added to provide

a refractory component during the heating of the investment and to regulate the thermal expansion.

BINDER – Alpha hemi hydrate form of gypsum(25-45%)

Strength of investment depends on amount of binder present.

ADDITIVES - (4-7%)

Used are Reducing agents

Modifying chemicals Coloring matter

Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold.

Ex– Copper

Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 200ºC .

Ex– Boric acid Soluble salts of alkali or alkaline earth metals

SETTING REACTION-same as gypsum

Dissolution of calcium sulfate hemihydrate

Formation of saturated solution of calcium sulfate,

Aggregation of calcium sulfate dihydrate

Precipitation of dihydrate crystalsThe crystallization of calcium sulfate dihydrate occurs while most of the remaining hemihydrate particle dissolve

Theories to explain setting reaction

1.Colloidal theory2.Hydration theory3.Dissolution –precipitation theory

Among these dissolution-precipitation theory most accepted

Properties

SETTING TIMEAccording to ANSI/ADA Specification No. 2 for dental

inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min.

Should allow sufficient time for mixing and investing the pattern.

SETTING EXPANSION1. Normal setting expansion: the expansion takes place when

the material sets in air2. Hygroscopic setting expansion: when setting occurs under

water.3. Thermal expansion: when it is affected by heat

Purpose: to enlarge the mold to compensate for the casting shrinkage of the gold alloy

Normal Setting Expansion

Liner dimensional change as the investment sets

Silica particles interfere with the intermeshing and interlocking of crystals

resulting in outward thrust of crystals

Resulting in expansion

A mixture of silica and hemihydrate gypsum results setting expansion greater than that of gypsum product used alone

Normal Setting Expansion

ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0.6% setting expansion of modern investments is 0.4%.It can be regulated by accelerators and retarders .

Effective expansion in the mold depends on different factors like wax pattern in the mold may expand due to heat of reaction.

Soft wax- more NSE.

If wax softer than Type II inlay wax is used , it may cause distortion of the pattern.

Hygroscopic setting expansion

Greater in magnitude than NSE.The gypsum product is allowed to set in contact

with water for mold expansion.

Curve A-NSE; Curve B-Hygroscopic expansionWater is added 5 min after beginning of the mixing

The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment

The increased amount of expansion is because the water helps the outward growth of crystals

The investment should be immersed in water before the initial set is complete.

ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.

Factors affecting expansionComposition: more finer silica particles -more HSE. α hemihydrate greater HSE than β hemihydrate. W:P ratio-lower W:P RATIO-more HSE Spatulation: more mixing time- more HSE. Time of immersion: immerse in water before initial set more

expansion than immersion is delayed beyond the time of initial set.

Confinement: less opposing force from walls of casting ring (wet cellulose), immersion in water bath at 37.7°C- expansion of wax pattern-more HSE

Added Water : HSE directly related to amount of water added during the setting period until a maximum expansion occurs.

Shelf life: fresher investment-more HSE

Thermal setting expansion

The thermal expansion is directly related to the amount and type of silica present.

Contraction of gypsum is balanced when quartz contain increases to 75%.

Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%.

Type 1I investments should have thermal expansion of between 0% to 0.6% at 500°C.

Maximum thermal expansion should be attained at temperature not higher than 700 degree centigrade

Factors affecting thermal expansion

W/P ratio-more water less expansion as thermal expansion related to the amount of solids presents

When an investment is cooled from 700°C, it contracts but less than its original dimension. On reheating it expand thermally to the same maximum reached before.

PRECAUTION – Investment should not be heated a second time because internal cracks may develop.

SETTING CONTRACTION

COMPRESSIVE STRENGTH: According to ADA sp no 2 the compressive strength should

not be less than 2.4MPa when tested 2 hr after setting to withstand force of molten metal.

FINENESS: Surface roughness of the casting and setting time depend upon the fineness of the material. Fine silica- more hygroscopic expansion.

POROSITY: The material should be porous to allow escape of air from mold space while casting. The common method of venting the mold is through the pores of the investment.

CLINICAL CONSIDERATION- Sprue length should be adjusted so that the top of the wax pattern is within 6 mm of the open end to allow sufficient interconnectivity of the porous network, otherwise backpressure porosity will develop.

STORAGE

Storage room with high relative humidity , the setting time, setting expansion and hygroscopic expansion altered casting procedure adversely affected .

Due to different ingredients of investment possesses different specific gravity ,they have a tendency to separate as they settle influence properties of material

Should be stored in airtight and moisture-proof containers.

For an infrequent basis of use, it is advisable to purchase prepackaged investment in relatively small quantity.(also cost effective)

Quality of investment products depends onHomogeneity of particulate componentsVariations in weight of powder in packets

So products with weight variations of 2% or less between packets in a box represents excellent quality control &variations of 5% implies serious consequences in its properties.

PRECAUTION

Available products in market-

Gloriaa Special

SPOFA DENTAL

PHOSPHATE BONDED INVESTMENT MATERIAL

Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast at a mold temperature higher than 700 ºc.

To withstand these high temperatures ,molds require different types of binders such as phosphate compounds.

They are used in construction of high melting temperature

dental alloys .Soldering and porcelain veneering

Types

Type 1 For casting of inlays crowns and other restorations

especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium

Type 2 For casting of removable partial dentures

COMPOSITION

Refractory materials – (concentration of approximately 80%)silica in quartz , cristobalite or a mixture of two .Purpose To provide high temperature thermal shock resistance To provide high thermal expansion.

Binder (<20%)Magnesium oxide (acid) and a phosphate (base)

• Originally phosphoric acid was used but mono ammonium phosphate has replaced it as it can be

incorporated in powder form

Modifiers :CARBONCarbon is often added -clean casting. Facilitates easy divesting of casting and mold. Generally added when casting alloy is gold.Not used with Ag-palladium alloys or base metal alloys as; -palladium reacts with carbon at temperatures above 15040C -carbon embrittles the alloys.CLINICAL CONSIDERATION- So for base metal alloys and palladium containing alloys carbon free phosphate bonded investment should use. Also, a carbon crucible should not be employed for melting the alloy.

It is available as two component systems 1- It is a Powder which contains refractory materials

and binders and modifiers2- Aqueous solution stabilized with colloidal silica Colloidal silica suspension facilitate greater

expansion of the investment which can compensate the greater casting shrinkage of alloys used in Metal Ceramic & newer gold alloys.

.

SETTING REACTION

The chemical reaction that causes the investment to set and harden is

NH4H2PO4 +MgO+5H2O→NH4MgPO4 +6H2O The product formed is the predominantly colloidal

multimolecular (NH4MgPO4·6H2O)n aggregate around excess MgO and fillers.

Properties

1 Compressive strength Type 1- 2.5 MPa Type 2- 3 Mpa Thermal expansion0.8% when 50:50 mixture of liquid and water

working & setting time

Temperature Warmer the mix faster it setsThe setting reaction also liberate the heat andaccelerates rate of settingMixing timeincreased mixing time and mixing efficiency result in faster set.

Surface Quality of cast metal

In the past, detail reproduction and surface smoothness of ametal-ceramic gold alloy restoration cast in a phosphate bondedinvestment were considered inferior to those characteristicof a conventional gold alloy that had been cast in agypsum-bonded investment.

Due to improvement in technique and in composition of material Phosphate bonded investments now approach the surface quality or fineness of the gypsum investments,

Advantages1.They have high fired strength. This make them

handle without breaking before they are placed in a furnace for the wax burn out process and strong enough to withstand the impact and the pressure of centrifugally cast molten alloy

2.They also provide high setting and thermal expansion enough to compensate cast metal prosthesis or porcelain veneers during cooling

3. They can withstand temp more than 700°C

Disadvantage

When used with alloys having casting temperature greater than 10300C results in mold breakdown & rougher surfaces on casting.

Although high strength of these investments is an advantage during casting, but can make divesting a difficult and tedious task.

Less porous than gypsum bonded investment .CLINICAL CONSIDERATION-For proper venting of investment, wax pattern should be within 3 to 4 mm from the top of the investment . The permeability low casting pressure should be

greater than that of gypsum investment

Bellavest SH

Properties:

Processing time approx. 3.30 minThermal expansion linear 0.8 % with 70 % BegoSol K Compressive strength: approx. 11 N/mm²Mixing ratio 100 g : 20 ml

Properties:

Processing time 4-5 minThermal expansion linear 0.8-0.9 %Total expansion linear 1.7-2.2 %Compressive strength 4.2-5.1 N/mm²Mixing ratio 100 g : 25 ml.

Properties:

Mixing liquid: BegoSol (Anti-freeze optimization up to -10 °C)Processing time at 20 °C: approx. 3 min.Total expansion: approx. 2.3 %Shelf life in unopened bag: 2 years

indications in the partial denture

Universally applicable for precious metal alloys, non precious metal alloys, pressed all-ceramic and press-over ceramic.

(SH-Shock heat)

Not for metal casting. Used for pressed ceramic

phosphate-bonded investment for the partial denture technique.

The adequate expansion can be adjusted for every type of alloy by changing the concentration of the apropriate Adentatec Liquid.

suitability for use with non-precious dental alloys.

GC Stellavest

Adenta-vest

SILIKAN( P) &SILISAN (L) SPOFASANIVEST INVESTMENT MATERIAL

for precious and non-precious metal applications

WiroFast( Phosphate bonded investment material)

Partial denture investment material suitable for shock heating

Mixing liquid distilled water Powder : liquid 100 g powder : 26 – 30 ml distilled water Recommended liquid factor 100 g powder : 28 ml distilled water Mixing under vacuum 30 sec. Working time approx. 5 min. Initial setting time approx. 14 min. Final setting time 30 - 45 min. Max. preheating temperature 700 °C Setting expansion ca. 0,6 % Linear thermal expansion approx. 1.0 % Hinrivest® G

(ERNST HINRICHS)

SOME IMPLICATION OF MATERIAL SCIENCE DURIING CASTING

Investing procedure-(common for both gypsum and phosphate investment) Mixing by-1.Hand mixing 2.vacuum mixing.In vacuum mixing-removes air bubbles,texture somewhat smoother and better details reproduction.• Excessive vibration should be avoided as- . causes solids in the investment to settle and may lead to free water accumulation adjacent to wax pattern

Surface roughness

.May dislodge small pattern Produce a miscast

Wax elimination and Heating-

After investment set, approximately-1hr,it is ready for burnout

BURNOUT TEMPERATURE- Gypsum investment-5000C for hygroscopic technique 7000C for thermal expansion technique Phosphate bonded investment- range from 7500C to 10300C depending of the type of alloy

The temperature setting is more critical withgypsum-bonded investments than for the phosphate typebecause the gypsum investments are more prone to investment decomposition. As temperature rises above 7000C ,calcium sulfate reduce as following reaction-

CaSO4 + 4C CaS + 4CO3CaSO4 + CaS 4CaO+ 4SO2

Carbon produce from the wax burnout. The sulfur dioxide as a product of this reaction contaminates gold castings.

Heating in burnout furnaceFor gypsum bonded investment- The molds are usually placed in a furnace at room temperature, slowly heated to 650 °C to 700 °C in 60 minutes, and held for15 to 30 minutes at the upper temperature.

For phosphate bonded investment- The heating rate is usually slow to 315 °C and is quite rapid thereafter, reaching completion after a hold at the upper temperature for 30minutes.

.

RAPID HEATING IN BURNOUT FURNACE-

Generate steam Causes flaking or spalling of the mold walls

Outside layer of the investment expands more than central

Tensile stress develop in the middle

Crack from interior outwards in the form of radial cracks

Casting fins or spines

ETHYL SILICATE BONDED INVESTMENTS

Though losing popularity because of the more complicated and time consuming procedures, still used in the construction of high fusing base metal partial denture alloy.

Composition

Refractory material – Silica

Binder –Silica gel that reverts to silica (cristobalite) on heating.

Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator

Binder

Lowering the pH of sodium silicate by addition of an acid or an acid salt.

Converting colloidal silica to gel by adding ammonium chloride

Ethyl silicate: colloidal silicic acid is formed by hydrolyzing ethyl silicate in presence of Hcl, ethyl alcohol & water.

Si(OC2H5)+4H2O Si(OH)4+4C2H5OH

Methods used to produce the Binder

It is then mixed with quartz or cristobalite and magnesium oxide ( alkaline). Coherent gel of polysilicic acid formed(accompanied by a setting shrinkage) ,dried at temp 168°C, loses alcohol & water to form concentrated hard gel . Volumeric contraction during drying is known as green shrinkage.

As the gelation process is slow and time consuming , amines are added to ethyl silicate- hydrolysis and gelation occurs simultaneously.

It is supplied as a powder and liquid

Powder consists of refractory particles of silica and glasses along with the magnesium oxide and some other refractory oxides in minor amounts

Liquid contains stabilized alcohol solution of silica gel

Manipulation The powder is added to hydrolyzed ethyl silicate liquid,

mixed quickly and vibrated into a mold , that has an extra collar to increase the height.

The mold is placed on a vibrator that has a tamping action

This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top .

In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation).

The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1%

Can be used for higher temp castings ( 1090°C-1180°C) and compactible with higher fusing alloys.

Care should be taken while handling and burnout as inflammable alcohol is given off.

Disadvantage

Non porous material.

Complicated manipulation

Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily.

Other investment materials

Soldering investment

Divestment materials

Investment material for titanium alloys

Investments for all ceramic restoration

Soldering investment

ANSI/ADA Specification No. 93 (IS0 11244) for dental brazing investments defines two types of investment:

Type 1: Gypsum-bonded dental brazing investments Type 2: Phosphate-bonded dental brazing

investments

Soldering investment differ from casting investment by lower setting and thermal expansion most often ingredients do not have as fine particle as

casting investment USES TYPE 1- for soldering of low melting alloys TYPE 2 - for soldering of high melting alloys

DIVESTMENT

Die stone and investment combination.These mixed with colloidal silica liquidThe die is made from this mix and the wax pattern is

then constructed on it.The whole complex is then invested in a mixture of

divestment and waterThis combination used to compensate distortion of wax

pattern of long span bridges or RPD frameworks during removal from die.

Special GBI or PBI investment materials are used.

Newer investments for casting titanium based alloys

Problems with conventional investment materials-0 Molten titanium is highly reactive with the oxygen and

is capable of reducing some of the oxides commonly found in the investment.

0 Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investments.

0 These elements can also embrittle titanium in the solid state.

Objective for a titanium investment

0 To reduce breakdown of the investment

0 To reduce contamination of the titanium – refractory materials that are less easily reduced by titanium should be used

INVESTMENTS USED FOR CASTING TITANIUM ALLOY

Silica based investmentMagnesia based investmentZirconia based investmentAlumina based investmentResin based calcia investment

SUMMARY

Due to increase in the use of the higher melting alloys ,the use of phosphate bonded investment materials increases.Gypsum bonded investment cannot withstand temperature higher than 7000C and it can be used only with conventional gold alloys.The complex procedures limited the use of silicate bonded investments.Due to highly reactive of titanium with the oxygen in the mold cavity , newer investment materials are developed.

• Gypsum bonded

investment(700OC)

• Mold for gold casting alloys

Phosphate bonded investment

(700-1030oC)

Silica bonded investments

(1090-11800C)

Mold for base metal and gold casting alloys, cast ceramic and glasses

Mold for base metal casting alloys

References-

PHILLIPS”Science of Dental material, 11th edition

Restorative Dental materials, Robert G. Craig.11th edition

Fundamentals of fixed prosthodontics,3rd edi.Herbert T. Shillingburg