INVESTMENT MATERIALS

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CASTING DEFECTS

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Introduction

The dental profession has used precision-type castings for the most part of century. Using the lost wax technique, Taggart in 1907 developed a process for making castings used in restoration of prepared teeth. Since the process did not take into account all the dimensional changes associated with the casting technique, the resultant product did exhibit some problems in terms of dimensional accuracy.

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Definition

• Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having a desired shape ; upon solidification, the metal assumes the shape of the mold.

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Classification Of Traditional Casting Alloys

TYPE HARDNESS USE

I Soft Single Surfaces

II Medium Inlays/Onlays

III Hard Crowns/Bridges

IV Extra Bridges & partial

dentures

JADA, Leinfelder ;1997;128;37-45

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Casting Shrinkage

Most metals & alloys, including gold & noble metal alloys, shrink when they change from the liquid to the solid state.

Shrinkage occurs in 3 stages:

• The thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus temperature.

• The contraction of metal inherent in its change from liquid to the solid state.

• The thermal contraction of the solid metal that occurs down the room temperature.

Phillips’; Science of dental material ; 10th edition

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COMPENSATION FOR SHRINKAGE

• Setting or Hygroscopic expansion of investment material: Hygroscopic low heat technique

• Thermal expansion of investment: High heat thermal expansion technique

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Hygroscopic low heat technique

Obtains its compensation expansion from 3 sources:

1. The 37˚C water bath expands the wax pattern 2. The warm water entering the investment mold

from the top adds some hygroscopic expansion 3. The thermal expansion at 500˚C provides the

needed thermal expansion.

Because of the potential for reduced venting, back pressure porosity is a greater hazard in this

technique than the high heat technique, since the investments generally employed with this

technique may be more dense.

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High heat thermal Expansion

• Obtain its compensation expansion almost entirely by burn-out.

• Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, & the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

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INVESTMENT MATERIALS

• Gypsum Bonded Investments

• Phosphate Bonded Investments

• Ethyl Silicate Bonded Investments

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Gypsum Based Investments

• The gypsum based investments have

traditionally been used for casting of gold alloy inlays, onlays, crowns and fixed partial dentures.

• Main Constituents:- a - hemihydrate of

gypsum and quartz which serves as a binder and gives strength to the investment.

• Gypsum shrinks at a temperature range between 200 and 400˚C, it slightly expands

between 400 and 700˚C and than it undergoes significant shrinkage beyond 700˚C. Because

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Phosphate Bonded Investments

• The spread of use of phosphate based

investment is caused by an increase in use of metal ceramic prosthesis, which require

higher melting temperatures than gold alloys.

• The investment consists also of binders and refractory filler, which are the same as for the gypsum based investments. However, the

binder in this case is magnesium oxide and a monoammonium phosphate. Carbon is also

often added to the investment in order to help

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Ethyl Silicate Bonded Investments

• Used in construction of high-fusing base metal partial denture alloys.

• Binder is Silica gel which reverts to silica on heating.

• This type of investment can be heated to

1090˚C -1180˚C & is compatible with higher fusing alloys.

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CASTING- LOST WAX PROCEDURE

The process involves producing a metal casting using a refractory mould made from a wax replica pattern.

The steps involved in the process or the lost wax casting are:

· Create a wax pattern of the missing tooth / rim

· Sprue the wax pattern

· Invest the wax pattern

· Eliminate the wax pattern by burning it (inside the furnace or in hot water) .This will create a mould.

· Force molten metal into the mould - casting.

· Clean the cast.

· Remove sprue from the cast

· Finish and polish the casting on the die.

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:SPRUE:

A sprue is the channel in a refractory investment mold through which molten metal flows.

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LINER:

Investment materials have a property of expanding on

setting.If the investment was confined on the outside this could cause shrinkage of the of the mould / wax pattern.

In order to prevent this effect a flexible split ring or a rubber ring could be used.

However, the most commonly used technique is to line the casting ring with either an aluminosilicate liner or a cellulose liner.

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CASTING MACHINES

1. Centrifugal Casting Machine

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2. Electrical Resistance-

Heated Casting Machine

3. Induction Melting Machine

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Melting Noble Metal Alloys:

There are several methods for melting of

alloys, which are combined with the casting methods and appropriate casting machines.:

· Torch melting,

· Electrical melting.

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Cleaning the cast:

• The dark surface of the casting can be removed by a process called

pickling – heating the discoloured casting in an acid until the discoloration vanishes.

• . Polishing of the casting is the final in its preparation. Rubber, rag or felt wheels impregnated with abrasives

• are used in the initial phase of this stage. Final polishing is achieved

using various oxides of tin and aluminium used in conjunction with a small rag or chamois buffing wheel, followed with an iron oxide rouge.

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CASTING DEFECTS

Defects in casting can be classified as:

1. DISTORTION

2. SURFACE ROUGHNESS & IRREGULARITIES 3. POROSITY

4. DISCOLOURATION

5. INCOMPLETE CASTING OR MISSING DETAIL

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Distortion:

Causes-

• Distortion of wax pattern

• Due to uneven movement of the walls of wax pattern when investment is setting. The gingival margins are forced apart by the mold expansion, whereas the

solid occlusal bar of wax resists expansion during the early stage of setting.

Solutions-

• Proper manipulation of wax & handling of the pattern.

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Poor Surface finish:

Causes-

• Air bubbles on the pattern

• Water films causing ridges & veins on the surface.

• Too rapid heating resulting in fins or spines

• Underheating causing incomplete elimination of wax

• Inappropriate water/powder ratio

• Prolonged heating

• Temperature of alloy too high

• Casting pressure too high

• Foreign bodies

• Impact of molten alloy

• Pattern position

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Solutions-

• Use of Vacuum Investing Technique

• Vibrate before & after mixing

• Use a wetting agent to reduce surface tension of wax pattern.

• Air dry the wetting agent as excess water will dilute investment, causing irregularities

• The mold should be heated gradually

• Heat the ring for sufficient period of time so that the carbonaceous residue is removed

• Water/powder ratio should be accurate

• Gypsum bonded investments should never be heated above 700˚C

• Alloys submitted to vacuum casting showed decreased surface roughness, compared to alloys submitted to acetylene- oxygen flame casting.

Journal of Prosthetic dentistry, 2004, vol 92, 274-277

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Porosity:

Porosity of the casting can be spread within the casting and on its surface. The surface porosity adds to surface roughness, but can also be a sing of the internal porosity. The internal porosity could weaken the casting, may cause discoloration if spread to the

surface and in extreme cases could lead to a leakage.

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Classification of porosities (According to Phillips’):

Porosities

Solidification

Defects Trapped gases Residual Air

Localized

Shrinkage Microporosity Pinhole Porosity

Gas Inclusions

Subsurface Porosity

Phillips’; Science of dental material ; 10th edition

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Localized Shrinkage Porosity Causes:

Generally caused by incomplete feeding of molten

metal during solidification.If the sprue is not properly designed and implemented then it may solidify

before the feeding is complete thus preventing a

continuous supply of molten alloy. This type of defect usually occurs close to the sprue-casting junction.

Solutions:

Ryge et al recommended the use of a sprue diameter larger than the thickest cross section of the casting.

Journal of dental rest.; Jan 1981; vol 60; no1; page 59-67 Phillips’; science of dental materials; 10th edition

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Macroporosity:

Causes-

• Primarily a shrinkage porosity

• The portion of a cylindrical casting which solidifies last is the low pressure side of the liquid metal close to the free surface of the button, therefore

macroporosity always appears in this portion of casting.

Solution-

• Reduce by providing a reservoir contiguous or close to the low pressure end of the liquid metal

• It may be affected by the closeness of the individual units in a multiple unit casting.

Phillips’; Science of dental material ; 10th edition Journal of dental restoration; Jan 1981;vol 60; no. 1;page 59-67

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HOT SPOT:-

Localized shrinkage porosity may result from the formation of HOT SPOT when metal

impinges on the mold surface so that here the metal remains molten while it solidifies

everywhere else. This hot spot causes the local region to freeze last and result in SUCK BACK

POROSITY.

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Solutions:

▪ Flare the point of sprue attachment

▪ Proper placement of sprue

▪ Do not use excessively long sprue

▪ Use a reservoir

▪ Reduce the mold-melt temperature differential (i.e. lower the casting temperature by about

30˚C).

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Microporosity:

Microporosity is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the microvoids to segregate. This in turn is caused the mould or casting temperature being too low.

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Trapped Gases 1. Pinhole Porosity

2. Gas Inclusion Porosity

• Both these porosities are related to the entrapment of gas during solidification.

• Both are characterized by a spherical contour.

• Gas inclusion porosities are much higher than pinhole porosities.

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SUBSURFACE POROSITY

Cause-

Simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mold walls

Solution-

• Diminished by controlling the rate at which the molten metal enters the mold

• Ryge et al reported that subsurface & microporosities are reduced by the use of a sufficiently long sprue & a melt temperature of 100˚C-150˚C above the melting point of alloy.

• According to Leinfelder et al reported that long sprues are

preferable for thick castings while short sprues are beneficial for thin castings i.e. the sprue design should be related to the surface area-to-volume ratio of castings.

Journal of dental rest.; Jan 1981; vol 60; no1; page 59-67 Phillips’; science of dental materials; 10^th edition

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ENTRAPPED AIR POROSITY

• Referred to as Back Pressure porosity.

• Can produce large concave depression.

Causes-

Inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket toward the end of the investment via the molten sprue & button.

Journal of dental rest.; Jan 1981; vol 60; no1; page 59-67 Phillips’; science of dental materials; 10^th edition

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Solution-

• Proper burnout

• An adequate mold & casting temperature.

• A sufficiently high casting pressure

• Proper liquid : powder ratio.

• The thickness of investment between the tip of the pattern and the end of the ring should not be greater than 6mm.

• Srickland & Sturdevant suggested that sprue design variables have no effect on these porosities; venting, high melt

temperatures, proper positioning of the pattern in the ring, choice of investment & the use of reservoir can assist in

reducing or eliminating the back pressure porosities.

Journal of dental rest.; Jan 1981; vol 60; no1; page 59-67

• Phillips’; science of dental materials; 10th edition

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INCOMPLETE CASTINGS

Causes-

• Inadequate spruing ( sprue former too small)

• Alloy not hot enough

• Incomplete elimination of wax residues from the mold

• Mold too cold

• Ingate obstructed

• Insufficient casting force

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Solutions-

• Use proper size of sprue former

• Casting temperature should be higher than the liquidus temperature of alloy

• Ensure that no debris blocks the ingate

• Use adequate amount of force for casting

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DISCOLOURATION

Causes-

• Sulphur contamination of casting causing black castings

• Contamination with copper during pickling

• Contamination with mercury Solution-

• Avoid overheating of investment

• Avoid use of torch flame as it contains sulphur

• Avoid use of steel tongs to hold casting during pickling

• Castings should never be placed with amalgam dies or kept on a table where amalgam scrap is present

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References

• Phillips’; Science of dental materials; 10^th edition.

• Journal of American Dental Association, Leinfelder

;1997

• Journal of American Dental Association: 1984

• Journal of Dental Restoration; Jan 1981; vol 60

• Journal of American Dental Association; 1993

• Journal of Prosthetic Dentistry; 1989

• Journal of Prosthetic Dentistry; 2004