DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS

S. BINDHU MADHAVIPOST GRADUATE

Definition History Lost wax technique Sprue former Reservoir Forming the crucible & attaching the pattern Venting Preparation of the ring for casting Liner Preparing the wax pattern for investing Investment of the pattern Casting machines Casting techniques Cleaning the casting Compensation for shrinkage Casting defects

Casting – is the process by which a wax pattern of a restoration is converted to replicate in a dental alloy (Craig)

History – History of casting of objects in gold by the wax elimination process dates back to four or five thousand years ago by Chinese.

- In 1884 Aquilhon de Saran of Paris melted 24 Carat gold in an investment mould to form inlays.

- Matthaeus gottfried purmann (1700) first mentioned wax models in connection with prosthetic work.

- In 1897 Dr. D. Philbrook read a paper before “ IOWA STATE DENTAL SOCIETY”

- In 1903 Dr. John. A. Lentz of phoenix, Arizona - “disappearing wax method” for mold formation.

In 1906 Dr. William H. Taggart - First reported application of the lostwax technique in Dentistry

It is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out (“lost”) to create a space into which molten metal is placed or cast.

• Definition:Definition:Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated.

• Role of a Sprue:Role of a Sprue:1. Create a channel to allow the molten wax to escape

from the mold.2. Enable the molten alloy to flow into the mold which

was previously occupied by the wax pattern.

-Wax & Resin sprue formers have the advantage of being burnable and so do not need to be mechanically removed.

-Metal sprue formers can be solid or hollow.

-Hollow sprue formers are preferred since they hold less heat than a solid sprue former and will cause less heat transfer to wax pattern resulting in less distortion.

-Metal sprue formers must be mechanically removed prior to burnout.

-Wax & Resin sprue formers have the advantage of being burnable and so do not need to be mechanically removed.

-Metal sprue formers can be solid or hollow.

-Hollow sprue formers are preferred since they hold less heat than a solid sprue former and will cause less heat transfer to wax pattern resulting in less distortion.

-Metal sprue formers must be mechanically removed prior to burnout.

Often a matter of individual judgment, based on the shape & form of the wax pattern.

Some prefer placement at:- Occlusal surface Proximal wall Just below a nonfunctional cusp Ideal area- point of greatest bulk in pattern. .

1. To avoid distorting thin areas of wax during attachment to the pattern.

2. To permit smooth flow of the alloy.

The Sprue former should be directed away from any thin or delicate parts of pattern.

It should not be attached at a right angle to a broad flat surface. Sprueing should be done at a 45-degree angle to the proximal wall.

Maximum impedance to flow occurs when a Sprue former makes an angle of 90-degree to

the pattern

The molten metal may abrade or fracture investment in this area and may result in a casting failure.

The entering metal impinges the mold surface at this point (90-degree angle) and creates hot-spot, producing a localized lingering of molten metal after the casting has solidified. Creates shrinkage void, or suck-back porosity.

The length of the Sprue former depends on the length of the casting ring.

Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring.

The pattern should be placed as close to the centre of the ring as possible.

Short Sprue Length:The gases cannot be adequately vented to permit the molten alloy to fill the ring completely leading to Back Pressure Porosity.

Long Sprue Length:Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.

Sprue Former should be thicker than the pattern to which it is attached.

Since the thin sections solidify and contract first, they will draw molten metal from thicker sections.

Diameter 8(3mm),10(2.5mm),12(2mm) gauge Sprue act as a ideal reservoir for these sections.

The Crucible, Sprue and the Mould should be considered as being a wedge –shape: 1. correct 2.

incorrect

Failure to cater for this may result in a reduction in density of the casting by the presence of voids called Porosity

Reservoir portion of a Spruing system is a round ball or a bar located 1mm away from the wax pattern.

The diameter of the reservoir should be more than the average cross sectional area of the wax pattern.

Reservoir should be positioned in the heat centre of the ring.(Alleluia,1980; Ingersoll & Wandling,1986; McLean,1980; Naylor,1986)

This permits the reservoir to remain molten for longer and enables it to furnish alloy to the pattern until they complete solidification process

Reservoir is the largest mass of any part of the Sprue system & it is present in the heat centre of the ring, it is the last part to solidify.

These properties allow continuous feeding of the molten alloy to compensate for Solidification shrinkage & avoid Shrinkage porosity.

1. Direct Spruing: The flow of the molten metal is straight(direct) from

the casting crucible to pattern area in the ring. Even with the ball reservoir, the Spruing method is

still direct. A basic weakness of direct Spruing is the potential

for suck-back porosity at the junction of restoration and the Sprue.

2. Indirect Spruing:

Molten alloy does not flow directly from the casting crucible into the pattern area, instead the alloy takes a circuitous (indirect) route.

The connector (or runner) bar is often used to which the wax pattern Sprue formers area attached.

Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage

The Connector bar is often referred to as a “reservoir” bar

The crucible part of the investment assembly is cone shaped.

The sprue is attached to the crucible in the same way as the sprue is attached to the mold, i.e. it should be bulkiest in cross section, flared & smooth.

The deeper the crucible is & the more inclined its walls are, more velocity will be imparted to the melt on its way to the mold.

In order to prevent the trapping of gases a wax rod is added to the farthest or close to the farthest part of the pattern, which will stop short of the investment ring surface.

The metal used in the construction of a ring should be non-corrodible, hard & with a thermal expansion similar to the investment used.

Stainless steel has been found to produce the most acceptable rings with thermal expansion 12% at 700C which is compatible with the expansion of investments, provided a liner is used.

Average dimensions of the ring are app- 29mm (11/8 inch) in diameter & 38mm (1 ½ inches) in height.

A resilient liner is placed inside the ring to provide a buffer of pliable material against which the investment can expand to enlarge the mold.

If there is no liner present, the investment is in direct contact with the walls of the mold & will not be able to expand outward and so will expand in the direction providing less restriction, i.e. towards the center of the mold thus resulting in distortion of the casting.

1. Asbestos Ring Liner: Traditional material for lining casting rings until it

was learned that it posed a potential health risk to dental laboratory technicians (Davis,1987; palmer et al,1961; Priest & Horner, 1980).

Asbestos fiber bundles were found to produce hazardous-size respirable particles capable of causing lung disease.

2. Non-asbestos Ring Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose combination

To ensure uniform expansion , liner is cut to fit the inside diameter of the casting ring with no overlap..

Thickness of the liner should not be less than approximately 1mm..

Place the liner somewhat short of the ends of the ring, 3mm, tends to produce a more uniform expansion, therefore less chance for distortion of the wax pattern & mold.

The wax pattern should be cleaned of any debris, grease or oils before it is positioned in the ring.

This will decrease the surface tension & improve the wettability of the wax pattern.

Gentle washing with liquid soap using no. 2 paint brush is effective.

INVESTMENT:- a ceramic material that is suitable for forming a mould into which a metal or alloy can be cast

Procedure INVESTING

ADA # 2 ISO 7490

Phosphate bonded (High melting ; Metal ceramics)

Gypsum bonded (Low melting ; Gold alloys)

Silica bonded (High melting ; Base metal casting)

Inve

stm

ent

To meet ideal requirements - strong enough to bear pressure - withstand high temp

- expand enough - porous to vent gases

1. Binder: Magnesium Oxide (basic) & Mono-ammonium Phosphate (acid)--- phosphate

bonded investment Ca SO 4 Alfa hemi hydrate – gypsum

Ethyl Silicates- silicate bonded investment

2. Filler: Silica (quartz/Crystobalite or mixture of both)

Binder: It hold other ingredients together.Provide rigidity.

Filler: Regulate thermal expansion.

Silica is added for the compensation of Investment shrinkage during heating for wax elimination.

Silica exists in 4 allotropic forms :-I. QuartzII.TridymiteIII.CristobaliteIV.Fused Quartz

Two stable polymeric forms

Alfa – low temp.

Beta – high temp.

Transition temperature 573 ° C ( Alfa form to Beta form )

Expansion 1.4 % at 573 ° C

Two stable polymeric forms alpha and betaUniform till 200° C after which a sudden expansion from 0.5 –1.2 %Above 250° C becomes uniform – 1.6 %Transition temp. 220° C

Investment mixing

Hand mixing Vacuum mixing

Advantages Amount of porosity in the investment is reduced Texture of the surface casting is smoother with

better detail reproduction Compressive strength of investment is increased

1. Air pressure casting machinesAlloy is melted in situ in crucible hollow of the ring, followed by applied air pressure on the melt.

2. Centrifugal casting machineAlloy is melted in a crucible, and forced in to mold by centrifugal force.

3. Electrical Resistance/ Induction melting machine Alloy is melted electrically by a resistance or

induction furnace, then cast into mold centrifugally.

Air Pressure Casting Machine

crucible

Base

Cradle

Except for Air pressure casting machine, all other units require a crucible to hold the alloy before & during melting procedure.

1. Clay Crucibles2. Carbon Crucibles3. Quartz Crucibles (zircon-alumina)

Melting & Casting requires1. Heat source to melt the alloy2. Casting force, to drive the alloy into the mould

1. Zone in which air & gas are mixed before combustion:No heat is present in this zone.

2. Combustion zone:Green, gas & air are in partial combustion.This zone is oxidizing &

should always be kept away from metal during fusion.

3. Reducing zone: Dimly blue, hottest part of the flame. This area should be constantly on the metal during

fusion.4. Oxidizing zone: Outer most zone, here combustion occurs with

Oxygen in air. Under no circumstances should this portion be

employed to fuse the alloy. Low temperature, Causes oxidation of the metal.

When the reducing zone is in contact, the surface of the gold alloy is bright and mirror-like.

When the Oxidizing zone is in contact the metal there is a dull film of “dross” developed over the surface.

There are two casting techniques usually being used-

1. Thermal expansion technique ( high heat technique) 2. Hygroscopic expansion technique ( low heat

technique)

The investment is allowed to harden for a minimum of 45 min.

After the investment has thoroughly set, the crucible former is removed.

The metal sprue former is warmed slightly over the gas flame and carefully removed with pliers.

If the burn out or casting procedure are to be delayed for several hours or overnight, it is advisable to place the invested pattern in humid environment to prevent excess drying.

Burn out procedure:-

The casting ring should be placed in an oven preheated to approximately 900F (480C), held at that temperature for 20 min, & then the temperature is slowly raised to 1290F (700C) & held for 30 min.

Care should be taken to avoid heating gypsum bonded investment above 1290F.

This is because above 1290F ( 700C) calcium sulphate is reduced by carbon molecules releasing sulphur dioxide gases.

The sulphur dioxide gases, thus released may contaminate the gold alloy as it enters the mold.

It is advisable to burnout with the sprue hole facing downward, since this will allow the wax to run from the mold and carry investment inclusons out of the mold cavity.

This technique involves the immersion of the metal ring with investment into warm water bath set at 37C or 100F.

This technique compensates shrinkage by three mechanisms-

a. 37C water bath expands the wax patternb. Water entering the investment provides greater volume

into which gypsum crystals can grow & hence provides some hygroscopic expansion.

c. Thermal expansion at 468C ( burn out temperature for hygroscopic setting expansion)

After casting has been completed, ring is removed & quenched in water.Advantages:

1. Noble metal is left in an annealed condition for burnishing & Polishing.

2. When water contacts hot investment, violent reaction ensues. Investment becomes soft, granular & casting is more easily cleaned.

Chemical method- After casting the gold alloys become discolored or dark due to contamination & due to presence of sulphide in the investment material.

To remove these discolorations pickling procedure is done.

The solutions used for pickling are 50% HCL or 50% Sulphuric acid.

The best pickling solution for gypsum bonded investment is 50% HCL acid solution.

The disadvantage of HCL is that fumes from acids are likely to corrode laboratory equipments & can cause health hazards.

Mechanical method – The casting which is retrived from the investment possesses a surface that is too rough to be used in the mouth.

To remove the roughness of the casting surface, different abrasives are used- diamonds, silicon carbide, emery, aluminium oxide, garnet, sand, rouge etc.

Gold and palladium based metal ceramic alloys and base metals, these alloys are not generally pickled.

Molten alloys shrink on solidification. Mold must be made correspondingly larger than original wax pattern Mechanisms to produce expanded mold •Wax pattern expansion •Setting expansion •Hygroscopic expansion •Thermal expansion

Normal Setting Expansion:

occurs as investment hardens in air

Mechanism: silica particles interfere with the interlocking of crystals; the outward thrust of the crystals increases the expansion of investment.

Maximum expansion in air 0.6%

Hygroscopic setting expansion: occurs as investment hardens while immersed in

water.

Mechanism: A continuation of normal setting expansion b/c immersion in water encourages continued growth of crystals.

Expansion range1.2 -2.2%

Factors that increase hygroscopic expansion: Composition: more silica, finer particles lead to more

outward growth of crystals W:P ratio: less water, more powder in mix Spatulation: more mixing time Time of immersion: immerse in water before initial

set Confinement : less opposing force from walls of

casting ring (wet cellulose) Water: more immersion water Shelf life: fresher investment

Thermal expansion: occurs as investment is heated. In quartz and cristobalite when α-form is converted in

to -form, there is ℬ decrease in Density and increase in Volume thus resulting in expansion.

Thermal expansion: 1 -1.6%

Other ways for compensation:- W:P ratio--- more powder, less water result in

increased thermal expansion. Using two Liners for greater setting, hygroscopic &

thermal expansion.

NOTE: Cristobalite gives highest thermal expansion

Casting Defects: Any impressions or irregularities that

result in unsuccessful casting which interfere with the fit of the final restoration or its esthetic and mechanical properties.

They are basically classified into 4 categories- Distortion Surface roughness and irregularities Porosity Incomplete or missing detail

Distortion of wax pattern may occur during its removal from the oral cavity, during direct technique owing to thermal changes, improper handling during removal, contraction on cooling, relaxation of stresses, distortion during storage etc.

Prevention:-• Soft wax is more susceptible to temperature changes than hard

wax; hence hard wax can be used for wax pattern fabrication.• Careful handling of the pattern during removal along the path

of insertion taking care not to disturb the margins. A separating medium must be applied on the die to ensure complete separation of the wax pattern with out distortion.

• Incorporation of residual stresses can be minimized by softening the wax at uniform temperature of 50C & adding wax in smaller increments at a time also reduces the stresses.

• To minimize warpage of the wax pattern, it should be invested immediately. If needed, the pattern must be stored in a refrigerator for no more than 30min.

◦Surface Roughness:- Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.

◦Surface irregularities:- These are isolated imperfections such as nodules that are not characteristic of the entire surface area.

Excessive roughness or irregularities on the outer surface of the casting necessitate additional finishing and polishing, whereas irregularities in the internal surface prevent a proper seating of an otherwise accurate casting.

particle size of the investment, proportion of the quartz & binder, influences the surface texture of the casting.

If the alloy is heated to very high temperatures before casting, the surface of the investment is likely to be attacked resulting in roughness of the surface.

The main causes of the surface roughness and irregularities could be due to-

1. Air bubbles 2. water films3. Too rapid heating 4. Under heating5. Water: Powder Ratio6. Prolonged heating7. Temperature of the alloy8. Casting Pressure9. Foreign Bodies10.Impact of Molten alloy 11.Pattern position 12.Carbon inclusions

Prevention:-

Proper burnout temperatures & adequate heating time prevent contamination with carbon. At the same time, excessive burnout temperatures should be avoided to prevent breakdown of the investment.

Small particle size of silica in the investment ensures smoother finish of the surface. For phosphate bonded investments, more effective the mixing, smoother is the casting.

• The mold should not be kept in furnace for too long, since this will eliminate all the deoxidizers added in the investments by the manufacturers.

• Casting pressure of 0.10 – 0.14 MPa in an air pressure casting machine or 3 to 4 turns of the spring in the centrifugal casting machine are sufficient for small castings.

Possible causes of incomplete casting are:-I. Diameter of the sprue- Using a sprue with a very small

diameter, attachment of the sprue other than at the bulky area of the pattern results in incomplete casting.

II. Discrepancy in temperatures between the casting ring/mold & the molten alloy causes premature solidification of the alloy.

III.Insufficient casting pressure during the casting process results in poor flow of the molten metal & incomplete filling of the mold, characterized by rounded incomplete margins.

• Incomplete elimination of wax & incomplete venting cause backpressure effects. The wax residue combines with O2 to form CO, a reducing agent, which causes sufficient backpressure to prevent rapid entry of molten metal.

• Higher viscosity of the molten alloy results in incomplete castings.

Prevention:-• Depending upon the size of the wax pattern, the diameter of

the sprue varies between 12 & 6 gauges. Larger sprue diameter ensures proper flow of the molten metal to the mold.

• Attachment of the sprue should always be at the bulkiest portion of the wax pattern, directed at 45 degree angulation.

• Proper balance between mold & molten alloy temperatures plays an important role in ensuring complete filling in to the mold.

• It is preferable that the casting should freeze by the wave of solidification traversing its mass, moving from the end of the mold towards the sprue.

• Silica-bonded & phosphate-bonded investments are more porous enough to allow venting of gases. Hence, a vent, 0.5mm in diameter should be provided to allow escape of gases toward the crucible end of the mold

Porosities in noble metal alloy castings may be classified as follows:

1. Solidification defects ◦ Localized shrinkage porosity◦ Suck-back porosity◦ Micro porosity

2. Trapped gases◦ Pinhole porosity◦ Gas inclusions 3. Subsurface porosity 4. Backpressure porosity

Localized shrinkage porosity:-Localized shrinkage porosity:-

• Solidification defects mainly occur due to improper flow of molten metal and premature solidification.

• As the molten metal in the mold starts to solidify, it shrinks linearly at a rate of at least 1.25%. Hence, as the metal solidifies, more molten metal has to be fed to compensate for this shrinkage.

• This is done by the still molten metal in the sprue & the reservoir.

• If the sprue is lesser in diameter without adequate reservoir, the molten metal in the sprue solidifies first before feeding molten metal to the shrinking alloy within the mold.

• This results in Localized shrinkage porosity in the last section of the casting that solidifies.

• Localized shrinkage porosity usually occur at the junction between the sprue and the casting.

When the sprue is attached at a right angle to the wax pattern, it creates a “Hot Spot”, where the molten metal impinges from the sprue channel onto a point on the mold wall, causing a higher localized mold temperature in this region.

This hot spot region retains heat longer; hence, the alloy here remain molten for a longer period of time after the other areas solidify.

When this alloy finally solidifies, it creates a shrinkage void known as “Suck-Back Porosity”.

It usually occur at the sharp occlusoaxial or incisoaxial line angle.

When solidification occur too rapidly (when the mold or casting temperature is too low), small, irregular microvoids known as “Micro-porosities” result.

This is especially seen in fine grain alloy castings. Although this type of defect cannot be seen unless the casting

is sectioned, it is not a serious defect, it does not amount to a casting failure.

Both pin hole & gaseous porosities are produced by gases that become dissolved in the molten alloy during solidification.

Copper, Gold, Silver dissolve Oxygen, while palladium & platinum have affinity to hydrogen and other gases in molten state.

On cooling, the alloys liberate the absorbed gases resulting in “Pin hole porosity”.

Both pin hole & gas inclusion porosities differ largely in their size; gas inclusion porosities are much larger.

The exact mechanism for formation of sub surface porosity has not been fully established; It may be caused by “simultaneous nucleation of solid grains & gas bubbles” at the first moment that the alloy freezes at the mold walls.

Backpressure porosity or entrapped-air porosity occurs on the inner surface of the casting as large concave depressions.

This is caused by the inability of the air to escape through the investment and accumulation of an air pocket that exerts pressure back towards the sprue & bottom.

Undersized & Oversized Castings:- The final fit of the casting depends on the balance of the

expansions & contractions, which occur during the investing and casting procedures.

The dimensional changes that occur due to casting shrinkage of the molten alloy should be compensated by the setting expansion, thermal expansion & inversion of the investment.

Undersized or Oversized castings can result due to improper L/P ratio, mixing time, & improper burnout temperature.

Sometimes, during the investing procedure, small air bubbles in the mix may get attached to the wax pattern; this manifests as small nodules of metal on the surface of the casting.

Such nodules can be removed easily if they are present in noncritical areas of the casting.

However, if a nodule is present on the margin or the internal surface of the casting, its removal may alter the proper fit of the restoration.

Can be prevented by:• Carrying out the mixing & investing procedure under vacuum.• When manual method is used, precautions to be taken are: Mechanical mixing with vibration both before & after mixing. Applying wetting agent on the wax pattern

Water films:• Since wax is hydrophobic, if the investment does not coat the

pattern evenly or if it gets separated from the wax pattern in some manner, a water film is formed over the surface of the pattern, which is reflected in the casting as minute ridge or vein on its surface.

Reasons include:- The pattern is slightly moved or vibrated after investing. The painting of the wax pattern with investment does not result

in intimate contact. Too high L/P ratio.

Fins or Flashes of metal:-Fins or Flashes of metal:-• These are thin metal extensions observed in casting.• The reason may be due to excess water, flask heated too

rapidly, invested ring allowed to dry or ring allowed to cool after burnout and before casting.

The selection of the investment material, the technique of expansion employed, & the method of investing and casting all have a bearing on the final outcome of the casting.

Fundamentals of Spruing, Investing & Casting

O’Brien, W., Dental Materials and Their Selection, 2nd ed., Quintessence Publishing Co. Inc., 1997, pp51-77; 237-247.

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