casting defects. final

Casting Defects

Evaluation of the casting

• The casting is never fitted on the die until the inner surface has been carefully evaluated under magnification

• Even tiny imperfections can cause damage to the stone die

• A die may be rendered useless if a casting is fitted prematurely.

Accuracy of the lost wax technique

• A casting should be as accurate as possible • A tolerance of +-0.05% for an inlay casting is acceptable• The tolerance limit are approximately 1/10 of the

thickness of human hair (40µm)• To obtain this rigid requirements must be placed on

– The investment material– Impression material– Waxes– Die materials– Technical procedures – Proper handling of materials

Defects in the casting

• An unsuccessful casting results in considerable trouble & loss of time. In almost all instances, defects an casting can be avoided by strict observance of procedures governed by certain fundamental rules and principles

• Defects in casting can be classified under four headings.

(1) Distortion

(2) Surface roughness and irregularities (3) Porosity

(4) Incomplete or missing detail

Distortion or Dimensional errors

CAUSES RESULT REMEDY

1. Distorted wax pattern

Delay in investing the wax pattern

Careless handling of the pattern

Improper fit Proper manipulation & handling of

pattern

- Invest wax pattern immediately

2. PowderlWater ratio too much results in increase in

setting expansion, hydroscopic expansion and

thermal expansion

Mold expansion too much &

casting too large

Correct proportion of powder /liquid

Correct technique

3. Powder / water ratio too less

Setting expansion, hydroscopic expansion & thermal expansion

decreased

Mold expansion is less & casting

is small

Correct proportion of powder / liquid, Correct

technique

• Other factors :

– Nonuniform expansion of the walls of the pattern

• Variable that influence amount of distortion:

– The configuration of the pattern

– Type of wax

– Thickness

• Distortion increases as the thickness of the pattern

decreases

Surface roughness and irregularitiesSurface roughness - Finely spaced surface imperfection whose height,

width & direction establish the predominant surface pattern.Surface irregularities - Isolated imperfections such as nodules that are

not characteristic of the entire surface area.

CAUSES RESULT PREVENTION

1. Air bubbles that attach to

the pattern during or subsequent to investing procedure

Surface projections or blebs/ small nodules

Vacuum investing technique

- preferred

Manual method

- Use mechanical mixer with vibration-Apply thin layer of investment over the wax pattern -Vibrate the investment ring to clear all air bubbles

- Application and air drying of wetting agent on the surface of wax pattern.


2. Water films

– if investment

becomes

separated from

the wax pattern

-Increased

water/powder

ratio

-Movement of

wax pattern after

investing

Seen as ridges/veins Wetting agent

helps prevent this

3. Rapid heating rates:

outside layer of

investment expands

more than centre,

resulting in

compressive stress in

outside layer & tensile

stress in middle

causing radial cracks

from interior outwardly.

- water or steam pours

into the mould – flaking

of investment &

carrying salts into mold

Fins/spines on the casting

Mold should be heated

gradually. At least 60min

should elapse during the

heating of the investment-

filled ring from room

temperature to 700°C.

Greater the bulk of the

investment, more slowly it

should be heated


4. Underheating/ insufficient air available in furnace-Incomplete wax elimination-Carbonized wax on the surface

Voids / porosity

Black casting

Maintain proper temperature

5. Prolonged heating

- disintegration of investment, walls of mold roughened

- deposition of contaminants (sulfur compounds)

- Rough surface

-Black casting

Maintain proper temperature



6.Increased temperature of the alloy

- Attacks investment surface

Surface roughness Avoid over heating the alloy

Color of the molten gold alloy should be no lighter than light orange

7. Water / powder ratio

Higher/lower

Rough casting Accurate measurement

8. High casting pressure

Rough surface Gauge pressure to

0.10 to 0.14 Mpa in air pressure casting machine

3-4 turns of spring in centrifugal casting machine


9. Composition of investment-Ratio of binder to quartz-Coarse silica

Surface roughness Investment should satisfy ANSI/ADA no.2

10. Foreign bodiesRough crucible former / carelessness on removal of sprue former- - Pieces of investment carried into the mold with the metal

-Bits of carbon from flux

-Sulfur contamination : investment breakdown at high temperature / high sulfur content of the torch flame

Sharp well defined deficiencies or surface voids

-Bright appearing concavities

-Surface discoloration & roughness-Produces black/gray area on the surface of gold alloy that is brittle & does not clear readily during pickling

11. Impact of molten alloy-

may fracture / abrade the mold surface

Surface roughness or irregularity

12. Pattern position –

- several patterns are invested in the same ring placed close together in the same plane.

- If the expansion of wax is much greater than that of the investment causing breakdown or cracking of the investment.

Spacing between patterns should be less than 3mm and should be placed in different planes.


Proper spruing.- Avoid direct impact of the molten alloy at an angle of 90 to investment surface.- A glancing impact is preferred as it is less damaging & avoids undesirable turbulence.

13. Carbon inclusions

Carbon from crucible

2) Improperly adjusted torch

3)Carbon containing investment

Absorbed by the alloy during casting

Results in the formation of carbides or visible carbon inclusions

Other causes

1) Mixture of different casting alloys used together

2) Contamination of Gold alloy with Mercury

-Resulting mixture would not possess the proper physical properties & corrosion resistance may be low

-loss in ductility & greater susceptibility to corrosion

Never melt mixture of different casting alloys together


Porosity

- Presence of voids or pores within a structures

Porosities may be classified (According to the cause) :I. Solidification defects - A. Localized shrinkage porosity B. Microporosity

II. Trapped gases - A. Pinhole porosity B. Gas inclusions C. Subsurface porosity.

III. Residual air.

I. Porosity caused by solidification shrinkage 1. Localized shrinkage porosity

Cause Appearance Remedy

- Premature

termination of flow of

molten metal during

solidification

- If the sprue freezes

in its cross section

before this feeding is

completed to the

casting proper,

A localized shrinkage void will occur in the last portion of the casting that solidifies.

Usually occurs at sprue-casting junction

Occurs externally

Attaching one or more

small gauge sprues (18

inch) at a surface more

distant from the main

sprue, extending

laterally. These chill-set

sprues ensure that

solidification begins

within the sprues, and

they act as cooling pins

to carry heat away from

the pontic.Pontic area

Suck-back porosity/ shrink spot porosity/ pitted casting/ largest sized porosity


Hot metal impinging from the sprue channel on a point of the mold wall. Hot spot causes the local region to freeze last results in suck-back porosity

Usually on interior of crown, near area of sprue

Occurs at an occluso- axial line angle or inciso axial line angle that is not well rounded

- Flaring the point of sprue attachment

- Reducing the mold-metal temperature differential ie lowering casting temperature by 30°C.

Porosity caused by solidification shrinkage 2. Microporosity


Occurs when

solidification is too

rapid for the micro

voids to segregate to

liquid pool

Rapid solidification if

the mold / casting

temperature is tool low

Appears in the form of small,

irregular voids

Usually this defect is not

detectable unless the casting

is sectioned.

Not a serious defect

II. Porosity caused by Trapped gases1. Pinhole & 2. Gas inclusion porosity Entrapment of gas during solidification

Pinhole porosity

Metals dissolve/occlude gases when molten

On solidification, the absorbed gases are expelled, resulting in pin hole porosity

When severely contaminated, appears black in color

Spherical contour

-Smaller

-Black

Do not overheat the metal beyond the melting point

Gas inclusion porosity

Gas that is trapped mechanically by molten metal in the mold / Gas incorporated during casting procedure

-Poorly adjusted torch flame

-Use of oxidizing zone of the flame

Spherical contour

- Larger

Pre-heating the gold alloy on a graphite crucible (used alloy)

Correctly adjusting & positioning torch flame during melting


II. Porosity caused by Trapped gases

3. Subsurface porosity

Not completely

established

May be caused by the

simultaneous nucleation of

solid grains & gas bubbles

at the first moment that

the alloy freezes at the

mold walls.

Can be diminished by

controlling the rate at

which the molten

metal enters the mold.


III. Residual air.Entrapped Air Porosity / Back-pressure porosity

Inability of air in the mold to

escape through the pores in

the investment

Pressure gradient that

displaces air pocket towards

end of the investment

When casting / mold

temperature is so low that

solidification occurs before

entrapped air can escape

On inner surface of casting ; Large concave depressions

Found in a “pocket” at the cavity surface of a crown or mesio-occluso-distal casting

Found on outer surface

- Proper burnout

- Adequate mold

& casting

temperature

- Sufficient high

casting pressure-Proper L/P ratio

-Thickness of

investment

between the tip

of the pattern &

end of the ring

not greater than

6mm


Incomplete casting

1. Insufficient venting

& insufficient

casting pressure

2. Incomplete

elimination of wax

residues

3. Low L/P ratio

4. Viscosity of molten

metal & due to

insufficient heating

Rounded incomplete margins

Rounded margins & shiny appearance

Casting pressure atleast 4sec

Temperature of alloy should be raised higher than its liquidus temperature


Summary & Conclusion

• Investing and casting, a series of highly technique-sensitive steps, converts the wax pattern into a metal casting

• Accurate and smooth restorations can be obtained if the operator pays special attention to each step in the technique.

• When initial attempts at casting produce errors or defects, appropriate corrective measures must be taken so they do not recur.

Thank you

References

• TEXTBOOK OF OPERATIVE DENTISTRY -2ND EDITION-BAUM ,PHILLIPS, ZUND

casting defects. final

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