CASTING DEFECTS
Contents:
Ø Introduction
Ø 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.
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 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 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.
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 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 7000c. 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 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 7000c – 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.
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 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 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
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 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 300c.
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 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 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. 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.
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.
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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