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1. CASTING DEFECTS
Contents:
 Introduction
 Defects in casting
1. Distortion
2. Surface roughness and irregularities.
3. Porosity
a) Solidification defects
 Localized shrinkage porosity
 Micro porosity.
b) Trapped gases
 Pinhole porosity
 Gas inclusions
 Subsurface porosity
c) Residual air.
4. Incomplete or missing detail.
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Introduction
An unsuccessful casting result in considerable trouble and loss of
time, in almost all instances, defects in castings can be avoided by strict
observance of procedures governed by certain fundamental rules and
principles. Seldom is a defect in a casting attributable to other factors
than the carelessness or ignorance of the operator. With present
techniques, casting failures should be the exception, not the rule.
Defects in castings can be classified under four headings:
1) Distortion
2) Surface roughness and irregularities
3) Porosity and
4) Incomplete or missing detail.
Some of these factors have been discussed in connection with
certain phases of the casting techniques. The subject is summarized and
analyzed in some detail in the following sections.
DISTORTION:-
Any marked distortion of the casting is probably related to a distortion
of the wax pattern. This type of distortion can be minimized or prevented
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by proper manipulation of the wax and handling of the pattern.
Unquestionably, some distortion of the wax pattern occurs as the
investment hardens around it. The setting and hygroscopic expansions of
the investment may produce an uneven movement of the walls of the
pattern. This type of distortion in part from the uneven outward
movement of the proximal walls. The gingival margins are forced apart
by the mold expansion, whereas the solid occlusal bar of wax resists
expansion during the early stages of stetting.
The configuration of the pattern, the type of the wax, and the thickness
influence the distortion that occurs, as has been discussed. For example,
distortion increases as the thickness of the pattern decreases. An would be
expected the less the setting expansion of the investment, the less is the
distortion. Generally, it is not a serious problem except that it accounts
for some of the unexplained inaccuracies that may occur in small
castings. There is probably not a great deal that can be done to control
this phenomenon.
SURFACE ROUGHNESS, IRREGULARITIES AND
DISCOLORATION: -
The surface of a dental casting should be an accurate reproduction of
the surface of the wax pattern from which it is made, excessive roughness
or irregularities on the outer surface of the casting necessitate additional
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finishing and polishing, whereas irregularities on the cavity surface
prevent a proper seating of an otherwise accurate casting. Surface
roughness should not be confused with surface irregularities. Surface
roughness is defined as relatively finely spaced surface imperfections
whose height, width, and direction establish the predominant surface
pattern. Surface irregularities refer to isolated imperfections, such as
nodules, that don’t characterize the total surface area.
Even under optimal conditions, the surface roughness of the dental
casting is invariably somewhat greater than that of the wax pattern from
which it is made. The difference is probably related to the particle size of
the investment and its ability to reproduce the wax pattern in microscopic
detail. With proper manipulative techniques, the normal increased
roughness in the casting should not be major factor in dimensional
accuracy. However, improper technique can lead to a marked increase in
surface roughness, as well as to the formation of surfaced irregularities.
Air bubble: - air bubbles that become attached to the pattern during or
subsequent to the investing procedure cause small nodule on a casting.
Such nodules can sometimes be removed if they are not in a critical area.
However, for nodules on margins or on internal surfaces, removal of
these irregularities might alter the fit of the casting. As previously noted,
the best method to avoid air bubbles is to use the vacuum investing
technique.
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If a manual method is used various precautions cab be observed to
eliminate air from the investment mix before the investing. As previously
outlined, the use of a mechanical mixer with vibration both before and
after mixing should be practiced routinely. A wetting agent may be
helpful in preventing the collection of air bubbles on the surface of the
pattern, but it is by no means a certain remedy. As previously discussed,
it is important that the wetting agent be applied in a thin layer. It is best to
air dry the wetting, a because any excess liquid dilutes the investment,
possibly producing surface irregularities on the casting.
Water films:- wax is repellent to water , and if the investment becomes
separated from the wax pattern in some manner, a water film may form
irregularly over the surface. Occasionally, this type of surface irregularity
appears as minute ridges or veins on the surface.
If the pattern is moved slightly, jarred or vibrated after investing, or if
the painting procedure does not result in an intimate contact of the
investment the pattern, such a condition may result. A wetting agent is of
aid in the prevention of such irregularities. Too high L: P ratio may also
produce these surface irregularities.
Rapid heating rates: - rapid heating results in fins or spines on the
casting or may result as a characteristic surface roughness may be evident
because of flasking of the investment when the water or steam pours into
the mold. Furthermore, such a surge of steam or water may carry some of
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the salts used as modifiers into the mold. Furthermore, such a mold,
which are left as deposits a on the walls after the water evaporates. As
previously mentioned, the mold should be heated gradually; at least 60
minutes should elapse during the heating of the investment- filling ring
from room temperature to 7000
c. The greater the bulk of the investment,
the more slowly it should be heated.
Under heating: - incomplete elimination of wax residues may occur if
the heating time is too short or if insufficient air is available in the
furnace. These factors are particularly important with the low-
temperature investment techniques. Voids or porosity may occur in the
casting from the gases formed when the hot alloy comes in contact with
the carbonaceous residues. Occasionally, the casting may be covered with
a tenacious carbon coating that is virtually impossible to remove by
pickling.
Liquid: powder ratio: -the amount of water and investment should be
measured accurately. The higher the L: P ratio, the rougher the casting.
However, if too little water is used, the investment may be unmanageably
thick and cannot be properly applied to the pattern. In vacuum investing,
the air may not be sufficiently removed. In either instance, a rough
surface on the casting may result.
Prolonged heating: - when the high –heat casting technique is used, a
prolonged heating of the mold at the casting temperature is likely to cause
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a disintegration of the investment, and the walls of the mold are
roughened as a result, furthermore, the products of decomposition are
sulfur compounds that may contaminate the ally to the extent that the
surface texture is affected. Such contamination may be the reason that the
surface of the casitng sometimes does not respond to pickling. When the
thermal expansion technique is employed, the mold should be heated to
the casting temperature- never higher than 7000
c – and the casting should
be made immediately.
Temperature of the alloy: - if an ally is heated to too high a temperature
before casting, the surface of the investment is likely to be attacked, and a
surface roughness of the type described in the previous section may
result. As previously noted, in all probability the ally will not be
overheated with a gas – air torch when used with the gas supplied in most
localities. If other fuel is used, special care should be observed that the
color emitted by the molten gold alloy, for example, is no lighter than a
light orange.
Casting pressure: - too high a pressure during casting can produce a
rough surface on the casting. A gauge pressure of 0.10 to 0.14 Mpa in an
air pressure casting machine or three to four turns of the spring in an
average type of centrifugal casting machine is sufficient for small
castings.
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Composition of the investment:-the ratio of the binder to the quartz
influences the surface texture of the casting. In addition, coarse silica
causes a surface roughness. If the investment meets ADA specification
no.2, the composition is probably not a factor in the surface roughness.
Foreign bodies:- when foreign substances get into the mold, a surface
roughness may be produced. For example, a rough crucible former with
investment clinging to it may roughen the investment on its removal so
that bits of investment are carried into the mold with the molten ally.
Carelessness in the removal of the sprue former may be a similar cause.
Usually, contamination results not only in surface roughness but also
in incomplete areas or surface voids. Any casting that shows sharp, well-
defined deficiencies indicates the presence of some foreign particles in
the mold, such as pieces of investment and bits of carbon form a flux.
Bright- appearing concavities may be the result of flux being carried into
the mold with the metal. Surface discoloration and roughness can result
from sulfur contamination, either from investment breakdown at elevated
temperatures or from a high sulfur content of the torch flame. The
interaction of the molten alloy with sulfur content of the torch flame. The
interaction of the molten alloy with sulfur produces black castings that
are brittle and do not clean readily during pickling.
Impact of molten alloy:- the direction of the sprue former should be
such that the molten gold ally does not strike a weak portion of the mold
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surface. Occasionally, the molten alloy may fracture or abrade the mold
surface on impact, regardless of its bulk, it is unfortunate that sometimes
the abraded area is smooth so that it cannot be detected on the surface of
the casting, such a depression in the mold is reflected as a raised area on
the casting, often too slight to be noticed yet sufficiently large to prevent
the seating of the casting. This type of surface roughness or irregularity
can be avoided by proper Spruing so as to prevent the direct impact of the
molten metal at an angle of 900
to the investment surface. A glancing
impact is likely to be less damaging and at the same time an undesirable
turbulence is avoided.
Pattern position:- if several pattern are invested in the same ring they
should not be placed too close together. Likewise, too many patterns
positioned in the same plane in the mold should be avoided, the
expansion of 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.
Carbon inclusions: -carbon, as form a crucible , an improperly adjusted
torch or a carbon-containing investment, can be absorbed by the alloy
during casting. These particles may lead to the formation of carbides or
even created visible carbon inclusions.
Other causes: - there are certain surface discolorations and roughness
that may not be evident when the casting is completed but that may
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appear during service. For example, various gold alloys, such as solders,
bits of wire, and mixtures of different casting alloys should never be
melted together and reused. The resulting mixture would not posses the
proper physical properties and might form eutectic or similar alloys with
low corrosion resistance. Discoloration and corrosion may also occur.
POROSITY
Porosity may occur both within the interior region of a casting and on
the external surface. The latter is a factor in surface roughness, but also it
is generally a manifestation of internal porosity. Not only does the
internal porosity weaken the casting but also if it also extends to the
surface, it may be a cause for discoloration. If severe, it can produce
leakage at the tooth-restoration interface, and secondary caries may
result. Although the porosity in a casting cannot be prevented entirely, it
can be minimized by use of proper techniques.
Porosities in noble metal castings may be classified as follows: -
I. Solidification defects
a. Localized shrinkage porosity
b. Microporosity.
II. Trapped gases
a. Pinhole porosity
b. Gas inclusions
c. Subsurface porosity
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11. CASTING DEFECTS
III. Residual air.
Localized shrinkage: - is generally caused by incomplete feeding of
molten metal during solidification. The linear contraction of noble metal
alloys in changing form a liquid to a solid is at least 1.25%. Therefore,
there must be continual feeding of molten metal through the sprue to
make up for the shrinkage of metal volume 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. The porosity in the pontic area is cause by the
ability of the pontic to retain heat because of its bulk and because it was
located in the heat center of the ring. This problem can be solved in the
future simply be attaching one or more small-gauge sprues (e.g. 18
gauge) at the surface most distant from the main sprue attachment and
extending the sprue(s) surface most distant from the main sprue
attachment and extending the sprue laterally within 5mm of the edge f the
ring. These small chill-set sprues ensure that solidification begins within
these sprues and they act as cooling pins to carry heat away from the
pontic.
Localized shrinkage generally occurs near the sprue –casting junction,
but it may occur anywhere between dendrites, where the last part of the
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casting that solidified was in the low melting metal that remained as the
dendrite braches develop.
This type of void may also occur externally, usually in the interior of a
crown near the area of the sprue, if a hot spot has been created by the hot
metal impinging form the sprue channel on a point of the mold wall. This
hot spot causes the local region to freezed last and result in what is called
suck-back porosity. This often occurs at an occlusoaxial line angle or
incisoaxial line angle that is not well rounded. The entering metal
impinges onto the mold surface at this point and creates a higher localized
mold temperature in this region that is called a ht spot. A hot spot may
retain a localized pool of molten metal after other areas of the casting
have solidified, this in turn created a shrinkage void, or suck-back
porosity, suck –back porosity can be eliminated by flaring the point of
sprue attachment and reducing the mold –melt temperature differential,
that is, lowering the casting temperature by about 300
c.
Microporosity: also occurs from the rapid solidification but is generally
present in fine grain alloy castings when the solidification is too rapid for
the micro void to segregate to the liquid pool. This premature
solidification causes the porosity.
Such phenomenon can occur from the rapid solidification if the mold or
casting temperature is too low. It is unfortunate that this type of defect is
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13. CASTING DEFECTS
not detectable unless the casting is sectioned. In any cast, it is generally
not a serious defect.
Pinhole and gas inclusion porosities: - are related to the entrapment of
gas during solidification. Both are characterized by a spherical contour,
but they are decidedly different in size. The gas inclusion porosities are
usually much larger than pinhole porosity. Many metals dissolve or
occlude gases while they are molten. For example, both copper and silver
dissolve oxygen in large amounts in the liquid state; molten platinum and
palladium have strong affinity for hydrogen as well as oxygen. On
solidification, the absorbed gases are expelled and the pinhole porosity a
results. The larger voids may also result from the same cause, but it
seems more logical to assume that such voids may be caused by gas that
is mechanically trapped by the molten metal in the mold or that is
incorporated during the casing procedure. All casings probably contain a
certain amount of porosity. However, the porosity should be kept to a
minimum because it may adversely affect the physical properties of the
casting.
Oxygen is dissolved by some 0f the metals, such as silver, in the alloy
while they are in the molten state. During solidification, the gas is
expelled to form blebs and pores in the metal. As was pointed out earlier,
this type of porosity may be attributed to abuse of the metal. Castings that
are severely contaminated with gases are usually black when they are
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removed from the investment and do not clean easily on pickling. The
porosity that extends to the surface is usually in the form of small
pinholes appearing.
Larger spherical porosities can be caused by gas-occluded form a
poorly adjusted torch flame, or the use of the mixing or oxidizing zones
of the flame rather than the reducing zone. Premelting the gold alloys on
a graphite crucible can minimize these types of porosities or a graphite
block if the alloy has been used before and by correctly adjusting and
positioning the torch flame during melting.
Subsurface porosity:- the reasons for such voids have not been
completely established. They may be caused by the simultaneous
nucleation of solid grains and gas bubbles at the first moment that the
metal freezes at the mold walls. As has been explained, controlling the
rate at which the molten metal enters the mold can diminish this type of
porosity.
Entrapped air porosity: - On the inner surface of the casting, sometimes
referred to as backpressure porosity, can be produced large concave
depressions. This is caused by the inability of the air in the mold to
escape through the pores in the investment or by the pressure gradient
that displace4s the air pocket toward the end of the investment via the
molten sprue and button. The entrapment is frequently found in a
“pocket” at the cavity surface of a crown or mesio-occlusal-distal casting.
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Occasionally it is found even on the outside surface of the casting when
the casting temperature or mold temperature is so low that solidification
occurs before the entrapped air can escaped. The incidence of entrapped
air can be increased by the dense modern investments, an increase in
mold density produced by vacuum investing, and the tendency for the
mold to clog with residual carbon when the low-heat technique is used.
Each of these factors tends to slow down 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 this phenomenon. It is good practice to make sure that the
thickness of investment between the tip of the pattern and the end of the
ring not be greater than 6mm.
INCOMPLETE CASTINGS: -
Occasionally, only a partially complete casting or perhaps no casting
at all, is found. The obvious cause is that the molten alloy has been
prevented, in some manner, from completely filling the mold. At least
two factors that might inhibit the ingress of the liquefied metal are
insufficient venting of the mold ant the mold and high viscosity of the
fused metal.
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The first consideration, insufficient venting, is directly related to the
back pressure exerted by the air in the mold. If the air cannot be vented
quickly, the molten alloy does not fill the mold before if solidifies. In
such a case, the magnitude of the casting pressure should be suspected. If
insufficient casting pressure is employed, the back cannot be overcome.
Furthermore, the pressure should be applied for at least 4 seconds. The
mold is filled and the metal is solidified in 1 second or less, yet it is quite
soft during the early stages point. These are usually exemplified in
rounded incomplete margins.
A second common cause for an incomplete casting is incomplete
elimination of wax residues from the mold. If too many products of
combustion remain in the mold, the pores in the investment may become
filled so that the air cannot be vented completely. If moisture or particles
of wax remain, the contact of the molten alloy with these foreign
substances produces an explosion that may produce sufficient
backpressure to prevent the mold from being filled. It can be seen as
rounded margins with quite shiny rather than dull appearance. The strong
reducing atmosphere created by carbon monoxide left by the residual wax
causes this shiny condition of the metal.
The possible influence of the L: P ratio of the investment has been
discussed. A lower L: P is associated with less porosity of the investment.
An increase in casting pressure during casting solves this problem.
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Different alloy composition and temperature probably exhibit varying
viscosities in the molten state, depending on composition and
temperature, however, both the surface tension and the viscosity of a
molten alloy are decreased with an increase in temperature. An
incomplete casting resulting from too great a viscosity of the casting
metal can be attributed to insufficient heating. The temperature of the
alloy should be attributed to insufficient heating. The temperature of the
alloy should be raised higher than its liquidus temperature so that its
viscosity and surface tension are lowered and it does not solidify
prematurely as it enters the mold. Such premature solidification may
account for the greater susceptibility of the whit gold alloys to porosity
because their liquidus temperatures are higher, thus, they are more
difficult to melt with a gas-air torch flame.
REFERENCES
1. Fundamentals of fixed prosthodontics- shillinburg
2. Contemporary fixed prosthodontics- roesensteil
3. Dental laboratory procedure- rudd and marrow
4. Dental materials and their selection-willian .j.o’ brien
5. Restorative dental materials-craig
6. Phillips sciences of dental materials- anusavice.
7. Removable prosthodintics- stewart
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