Casting Defects

Seminar - 7 Casting Defects

CASTING DEFECTS

INTRODUCTION

Casting in dentistry is even more of an art today than

science. For this reason the skill of the casting technician is

critical in the fabrication of dental casting.

The transition of casting techniques in dentistry from those

based on empirical research to those based on sound scientific

principles can help eliminate uncertainties about the soundness

of the castings and ensure that individual skill is not an overriding

factor.

CAUSES OF DEFECTIVE CASTING:

Defects in castings can be classified under four headings:-

i) Distortion

ii) Surface roughness and irregularities

iii) Porosity

iv) Incomplete or missing detail

A) Distortion:-

- Any marked distortion of the casting is probably related to

the distortion of wax pattern.

- Distortion of the wax pattern occurs due to thermal changes

and the relaxation of stresses that are caused by

contraction on cooling, occluded air, molding, carving,

removal and the time and temperature of storage. (Waxes

DEPT. OF PROSTHODONTICS 131


like other thermoplastics tend to return partially to their

original shape after manipulation due to property known as

Elastic Memory).

- The setting and hygroscopic expansion of the investment

may produce a non-uniform expansion of the walls of the

pattern (proximal walls).

- The general margins are forced apart by the mold

expansion, where as the solid occlusal bar of wax resists

expansion.

- The configuration of the pattern

Type of wax Distortion

Thickness of pattern

For e.g. Distortion increase as the thickness of the pattern

decreases.

- This accounts for some of the inaccuracies that may occur

in small castings.

B) Surface Roughness, Irregularities and Discoloration:-

- 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 on the cavity surface will prevent

proper seating of the casting.

- The surface roughness of the casting is invariably greater

than that of the wax pattern from which it is made.

- The difference is probably related to particle size of the

investment and its ability to reproduce the wax pattern in

microscopic detail.

- With proper manipulation technique, the normal increased

roughness in the casting should not be a major factor in

dimensional accuracy.

- Generalized casting roughness may indicate a breakdown of

the investment from excessive burnout temperature

2. Air Bubbles: (Nodules)

Small nodules on the castings are caused by air bubbles

that become attached to the pattern during or subsequent to the

investing procedure.

These nodules if present on the margins or on internal

surface might alter the fit of the casting, if removal of these

irregularities is attempted. But if they are present in some non-

critical area they can be removed easily.

The best method to avoid air bubbles is to use the Vacuum

Investing Technique.

If manual method is used:-

i) Use of mechanical mixer with vibration both before and after

mixing should be practiced.



ii) Wetting agent may be used (surfactant). Wetting agent should

be applied in thin layer and air-dried because any excess

liquid dilutes the investment, possibly producing surface

irregularities.

- Castings with phosphate bonded investments are more

prone to such imperfections.

- They can be removed with ¼ or ½ round bin. A binocular

microscope is extremely helpful in detecting and removing

them.

- If there is

i. Large Nodule:- Air trapped during investing procedure.

ii. Multiple Nodules:-

Inadequate vacuum during investing

Improper brush technique

Lack of surfactant

iii. Nodules on occlusal surface:- due to excessive vibration

iv. Nodules on underside:- Prolonged vibration after pouring

2. Water Films:-

- Wax is a repellent to water, if the investment becomes

separated from the wax pattern in some manner, a water

film may form irregularly over the surface.

- This type of irregularity appears as minute ridges or veins

on the surface.

- This condition occurs:-

i) If the pattern is slightly moved, paired or vibrated after

investing



ii) If there is no intimate contact of the investment and the

pattern

iii) Too high a liquid/powder ratio may also produce these

irregularities

- Use of Surfactant help prevent such irregularities

3. Fins:-

- Fins are caused by the cracks in the investment that have

been filled with molten metal.

Causes of Fins:-

i. Weak mix of the investment i.e. high water/powder ratio

ii. Improper positioning of the pattern in investment i.e.

pattern placed to near the edge of the investment.

iii. Too rapid heating

iv. Premature heating

v. Excessive casting force

vi. Rough handling of the ring after investing

vii. Liner flushed with the end of the ring

4. Rapid Heating Rates:-

- May result in Fins or Spines in the casting

- Sometimes a characteristic surface roughness may also be

evident because of the flaking of the investment when the

water/ steam pours into the mold.

- Furthermore, this water/steam may carry some of the salts

used as modifiers into the mold and these salts are left as

deposits on the walls as the water evaporates.



- Ideally, 60 min should elapse during the heating of the

investment filled ring from room temperature to 7000C.

- The greater the bulk of the investment, the more slow it

should be heated.

5. Liquid-Powder Ratio:-

- The higher the liquid/powder ratio, the rougher the casting

(the investment becomes weak and develop cracks).

- If too little water is used the investment

unmanageably thick cannot be properly applied to the

pattern in vacuum investing air may not be sufficiently

removed.

6. Pattern Position:-

- If the pattern placed too near the edge of the investment

causes fins.

- Positioning of several patterns too close and in the same

plane in the mold should be avoided. Because, “The

expansion of the wax is much greater than that of the

investment, causing breakdown or cracking of the

investment if the spacing between patterns is less than

3mm”.



7. Underheating:- (Low temperature investment

technique)

- Incomplete elimination of wax residues (too short heating

time/ insufficient air available in furnace)

- Voids/ Porosities due to gasses formed when hot alloy

comes in contact with carbon residues.

- Occasionally, casting may be covered with tenacious carbon

coating that is virtually impossible to remove by picking.

8. Prolonged Heating:-

When high heat casting technique is used

Prolonged heating of the mold

Cause disintegration of gypsum bonded investment

As a result, walls of the mold are roughened

Furthermore, products of decomposition are sulphur compounds

that contaminate the alloy to the extent that surface texture is

affected

Such contamination do not respond to pickling

- When thermal expansion technique is employed, the

mold should be heated to casting temperature – never

higher – and the casting should be made immediately.

9. Temperature of the alloy:-

- If alloy heating to too high temperature before casting



Surface of the investment will get affected causing

surface roughness

- If alloy heated with GAS-AIR torch will not be overheated

- Other fuels – colour emitted by gold alloy No lighter than

light orange.

10. Casting Pressure:-

- Too high pressure during casting -> causes fins.

- A gauge pressure of 0.10 to 0.14 Mpa in an air pressure

casting machine. Or

3 to 4 turns of the spring in an average type of centrifugal

casting machine is sufficient.

11. Composition of the Investment:-

- Ratio of binder/ quartz

Influences surface texture (A coarse silica causes surface

roughness)

- If the investment meets ANSI/ADA specification No.2, the

composition not a factor in the surface roughness.

12. Foreign Bodies:-

- Presence of foreign bodies

i. Surface roughness

ii. Incomplete castings

iii. Surface voids

iv. Surface discolouration



- Any casting that shows sharp, well defined deficiencies

indicate the presence of some foreign particles in the mold,

such as;

i. Pieces of investment

ii. Bits of carbon from flux

- A rough crucible former with bits of investment clinging to it

may roughen the investment on its removal so that small

particles of investment are carried into the mold with molten

alloy.

- Surface discolouration result from sulphur contamination

either from investment breakdown or high sulphur content of

flame torch.

Interaction of molten alloy with sulphur black/Grey layer on

the surface of gold alloys that is brittle that is brittle and does

not clean readily during pickling.

- Inclusion of flux shows bright concavities.

13. Impact of Molten Alloy:- (Direction of the sprue)

- Direction of the sprue former

Such that molten alloy does not strike a weak portion of the

mold surface.

- Such an abraded area in the mold reflects as a raised area

on the casting often too slight to be noticed but at the same

time large enough to prevent complete seating of

investment.



- This type of irregularities can be avoided by proper spruing

so as to prevent direct impact of molten alloy at an angle of

900 to the investment surface.

- A glancing impact is likely to be less damaging at the same

time an undesirable turbulence is avoided.

14. Carbon Inclusions:-

- Carbon as from A crucible

An improperly adjusted torch

Carbon-containing investment

Can be absorbed by the alloy during casting.

May lead to formation of carbides or even create

visible carbon inclusions

15. Other Causes:-

- Certain surface discolourations/ roughness may not be

evident when the casting is complicated but may appear

during service.

- For e.g. If a new amalgam restoration is placed adjacent to

high noble alloy restoration there may be chances of

contamination of the alloy by mercury.

- Mercury penetrates rapidly into alloy and causes loss in

ductility and greater susceptibility to corrosion.

- Dissimilar metals form Galvanic cell that can lead to

breakdown of anode (Amalgam) relative to that of cathode

(noble alloy).

C) Porosities:-

- Porosities in noble metal alloy casting can be classified as:-

Solidification Trapped Gases Residual Air



* Localized shrinkage * Pinhole porosity (Back pressure Porosity porosity)

* Micro-porosity * Gas inclusions * Subsurfac porosities

* Localized Shrinkage Porosity: (Shrink-Spot Porostiy)

- Cause -> premature termination of the flow of metal during

solidification.

- Linear contraction of noble metal alloys in changing from

liquid to solid – 1.25% therefore continual feeding of molten

metal through the sprue must occur to compensate for

casting shrinkage i.e. shrinkage during solidification.

- If the sprue solidifies before the casting it usually results in

localized shrinkage porosity.

- Occurs usually near the sprue-casting junction.

- If can be avoided by:-

i. Using sprue of correct thickness

ii. Attaching the sprue at the thickest portion of the

wax pattern

iii. Flaring the sprue at the point of attachment

iv. Placing the reservoir close to the wax-pattern

Suck-Back Porosity:-

- Localized shrinkage may also occur in the interior of crown

near the area of the sprue, if a hot spot has been created by



the hot metal impinging from the sprue channel one point

of the mold wall.

- The hot spot causes the local region to freeze last and

results in suck back porosity.

- Occurs often on occlusoaxial/ incisoaxial line angles that are

not well rounded.

- The entering metal impinges on to the mold surface at this

point (i.e. occlusoaxial/ incisoaxial line angle) and creates a

higher localized mold temperature at this region known as

“Hot Spot”.

- A hot spot may retain a localized pool of molten metal after

other areas of the casting have solidified. This in turn

creates a shrinkage void or suck back porosity.

- These porosities can be eliminated:-

i. By flaring the point of sprue attachment

ii. Reducing the mold melt temperature differential i.e.

lowering the casting temperature by about 300.

Microporosity:-

- Occurs from solidification shrinkage but is generally present in

Fine Grain Alloy Castings when the solidification is too rapid for

the microvoids to segregate to the liquid pool.

- Microporosity voids are irregular in shape

- Such phenomena occurs from rapid solidification if mold or

casting temperature is too low.

- This defect is not detectable unless casting is sectioned.



Pinhole and Gas Inclusion Porosity:-

- Both these porosities are related to the entrapment of gas

during solidification.

- Both these are characterized by spherical contour but size is

varied i.e. gas inclusion larger in size compared to perihole.

- Many metal dissolve or occlude gases in their molten state e.g.

Both copper and silver dissolve O2 in large amount in liquid

state.

- On solidification, absorbed gases are expelled resulting in

pinhole porosities.

- The larger voids i.e. gas inclusion porosity are caused by gas

that is mechanically trapped by the molten metal in the mold

or by gas that is incorporated during the casting procedure.

- O2 is dissolved by some metals like silver in the alloy when

they are in molten state.

During solidification gas expelled forms blebs and pores

in metals.

- Castings that are severely contaminated with gasses are

usually black when removed and do not clean easily on

pickling.

- The porosities that extend to the surface area usually in the

form of pin point holes.

- Large spherical porosities also caused by gas occluded from

poorly adjusted torch flame or by use of mixing or oxidizing

zone of flame rather than reducing zone.

- These can be minimized by premelting the gold alloy on

graphite crucible or a graphite block, if the alloy has been used



before and by correctly adjusting the torch flame during

melting.

Subsurface Porosity:-

- Caused by simultaneous nucleation of solid grains and gas

bubbles at the first moment that the alloy freezes at the mold

walls.

- Can be minimized by controlling the rate at which the molten

metal enters the mold.

Black Pressure Porosity:-

- Also known as entrapped air porosity

- Can produce large concave depression

- Caused inability of air to escape through pores of

investment or by pressure gradient that displaces air towards

end of the investment via molten sprue and button.

- Found frequently on cavity surface of crown or mesio-

occlusodistal casting.

- Occasionally found on the outer surface of casting when the

casting or mold temperature is so low that the solidification

occurs before the entrapped air can escape.

- The incidence of entrapped air has increased by use of:-

i. Dense modern investment

ii. Increase in mold density (vacuum investing)

iii. By tendency of mold to clog with residual carbon

when low heat technique is used.

- These factors tend to slow the venting of gases from the mold

during casting.



- Proper burnout, an adequate mold and casting temperature, a

sufficiently high casting pressure and proper L/P ratio can help

to eliminate entrapped air porosity.

- Thickness of investment between the tip of pattern and end of

the ring should be not more than 6mm.

Incomplete Casting:-

- Incomplete casting when due to some reason molten alloy is

prevented from completely filling the mold.

- Causes:-

i. Insufficient venting of mold:-

Directly related to back pressure exerted by the air in mold.

If air not vented molten alloy does not fill the mold before

solidifying. Now, if insufficient casting pressure is applied

the back pressure cannot be overcome, therefore pressure

should be applied for 4 seconds.

(The mold is filled and the alloy solidifies in 1sec, yet it is

quite soft during early stages therefore pressure should be

maintained for few seconds beyond this point).

ii. Incomplete elimination of wax residue:-

Too many products of combustion remain in the mold,

the pores of the investment become full and air cannot

be vented properly.

Contact of molten metal with wax or moisture produces

an explosion that may produce sufficient back pressure

to prevent the mold from being filled. Castings seen are

generally shiny with rounded defects.

iii. High Viscosity of Fused Metals:-



An incomplete casting resulting from too great a

viscosity is attributed to insufficient heating.

Temperature of the alloy should be raised higher than its

liquid temperature so that its viscosity and surface

tension are lowered and so that it does not solidify

prematurity as it enters the mold.

iv. Inadequate metal

v. Cool mold or melt

vi. Wax pattern too thin

vii. If there is marginal discrepancy due to wax pattern

distortion and uneven expansion.

If short rounded margins with lumpy/ rounded button alloy

not hot enough/ inadequate casting force.

If short rounded margins with sharp button – Pattern too far

from the end of ring. If casting is shiny Incomplete Burnout.

CONCLUSION:

Good technique pays off !

These are the words of Wisdom. Good technique demands

everyone involved give adequate attention to all details for safe

and efficient operations.

A great force working against good technique is the habit of

taking short cuts which are risky attempts to save time by

modifying proven procedures.


Date post:	27-Nov-2014
Category:	Documents
Upload:	vishal-malusare
View:	7 times
Download:	4 times

Casting Defects

Documents