DENTAL CASTING DEFECTS
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.
An unsuccessful casting can results in considerable trouble and loss of time, in almost all instances.
DEFECTS IN CASTING:
Defects in castings can be classified under four headings:-
- Surface roughness and irregularities
- Incomplete or missing detail
Any marked distortion of the casting is most 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.
The setting and hygroscopic expansion of the investment may produce a non-uniform expansion of the proximal walls of the pattern.
The general margins are forced apart by the mold expansion, where as the solid occlusal bar of wax resists expansion.
Distortion happens due to-
- The configuration of the pattern
- Type of wax
- Thickness of pattern
For instance, Distortion increase as the thickness of the pattern decreases.
That is why some of the inaccuracies often occur in small castings.
Surface Roughness, Irregularities and Discoloration
Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.
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 inner cavity surface will prevent proper seating of the casting.
The surface roughness of the casting is invariably greater than that from its wax pattern.
The difference is probably due to particle size of the investment and its ability to reproduce the wax pattern in microscopic detail.
With proper manipulation technique, the normal enhanced 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.
Air bubbles create small nodules on the castings that become attached to the pattern during or subsequent to the investing procedure.
These nodules if present on the margins or on internal surface alter the fit of the casting during removal of these irregularities. 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 mixing is done manually:–
- Use of mechanical mixer with vibration both before and after mixing should be practiced.
- Wetting agent or surfactant may be used along. Wetting agent is applied in thin layer and air-dried because any excess liquid dilutes the investment and possibly produces surface irregularities.
Castings with phosphate bonded investments are more prone to such imperfections.
They are removed with ¼ or ½ round bin. A binocular microscope is extremely helpful in detecting and removing them.
If there is
- Large Nodule: Air is trapped during investing procedure.
- Multiple Nodules:
- Inadequate vacuum during investing
- Improper brush technique
- Lack of surfactant
- Nodules on occlusal surface: Due to excessive vibration
- Nodules on underside: Prolonged vibration after pouring
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:-
- If the pattern is slightly moved, paired or vibrated after investing
- If there is no intimate contact of the investment and the pattern
- Too high a liquid/powder ratio also produces these irregularities
Use of Surfactant helps to prevent such irregularities.
Fins are caused by the cracks in the investment that have been filled with molten metal.
Causes of Fins
- Weak mix of the investment i.e. high water/powder ratio
- Improper positioning of the pattern in investment that is when the pattern is placed to near the edge of the investment
- Too rapid heating
- Premature heating
- Excessive casting force
- Rough handling of the ring after investing
- Liner flushed with the end of the ring
Rapid Heating Rates
Results in Fins or Spines in the casting
Sometimes a characteristic surface roughness is evident due to flaking of the investment when the water/ steam are poured 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 evaporate.
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 slows it to be heated.
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 is unmanageably thick thus cannot be properly applied to the pattern in vacuum investing and air may not be sufficiently removed.
If the pattern is placed too near to the edge of the investment, it causes fins.
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”.
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 the 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.
When thermal expansion technique is employed, the mold is heated to casting temperature never higher than that and the casting should be made immediately.
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.
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.
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 doesn’t create surface roughness.
Presence of foreign bodies
- Surface roughness
- Incomplete castings
- Surface voids
- Surface discolouration
Any casting that shows sharp, well defined deficiencies indicates the presence of some foreign particles in the mold, such as;
- Pieces of investment
- Bits of carbon from flux
A rough crucible former with bits of investment clinging to it roughens the investment on its removal so that small particles of investment are carried into the mold with molten alloy.
Surface discoloration result from sulfur contamination either from investment breakdown or high sulfur content of flame torch.
Interaction of molten alloy with sulfur creates black/Grey layer on the surface of gold alloys that is brittle and does not clean readily during pickling.
Inclusion of flux shows bright concavities.
Impact of Molten Alloy: (Direction of the sprue)
Direction of the sprue former :
Directing the sprue 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 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.
- A crucible
- An improperly adjusted torch
- Carbon-containing investment
Carbon is absorbed by the alloy during casting.
It also leads to formation of carbides or even creates visible carbon inclusions
Certain surface discolorations/ roughness may not be evident when the casting is complicated but may appear during service.
In case of, 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).
- Porosities in noble metal alloy casting can be classified as:-
Localized Shrinkage Porosity
Aka: Shrink-Spot Porosity
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.
It usually occurs near the sprue-casting junction.
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 of 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.
We can avoid it by:-
- Using sprue of correct thickness
- Attaching the sprue at the thickest portion of the wax pattern
- Flaring the sprue at the point of attachment
- Placing the reservoir close to the wax-pattern
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.
Often occurs on occlusoaxial/ incisoaxial line angles that are not well rounded.
The entering metal impinges on to the mold surface at this point (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 shrinkage void or sucks back porosity.
These porosities can be eliminated:-
- By flaring the point of sprue attachment
- Reducing the mold melt temperature differential i.e. lowering the casting temperature by about 300.
Occurs from solidification shrinkage but is generally present in fine grain alloy castings when the solidification is too rapid for the micro voids to segregate to the liquid pool.
Microporosity voids are irregular in shape.
Such phenomenon 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 porosities are related to the entrapment of gas during solidification.
Both of them are characterized by spherical contour but size is varied [gas inclusion: larger in size compared to pinhole].
Many metals dissolve or occlude gases in their molten state e.g. both copper and silver dissolve oxygen in large amount in liquid state.
On solidification, absorbed gases are expelled resulting in pinhole porosities.
The larger voids of 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.
Oxygen is dissolved by some metals like silver in the alloy when they are in molten state.
During solidification: expelled gas – forms blebs and pores in metals.
Castings that are severely contaminated with gasses are usually black when removed and doesn’t go away easily on pickling.
The porosities that extend to the surface area are usually in the form of pin point holes.
Large spherical porosities: caused by gas occluded from poorly adjusted torch flame or by use of mixing or oxidizing zone of flame rather than reducing zone.
We can minimize them by pre-melting 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.
Cause: Simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls.
We can minimize it by controlling the rate at which the molten metal enters the mold.
Back Pressure Porosity
Aka: Entrapped Air Porosity
Can produce large concave depression
Cause: 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.
Frequently occurs 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 entrapment of air has increased by use of:-
- Dense modern investment
- Increase in mold density (vacuum investing)
- By tendency of mold to clog with residual carbon when low heat technique is used.
These factors tend 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 entrapped air porosity.
Thickness of investment between the tip of pattern and end of ring should be not more than 6mm.
Incomplete casting: when due to some reason molten alloy is prevented from completely filling the mold.
Insufficient venting of mold
It is 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.
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. These castings seen are generally shiny with rounded defects.
High Viscosity of Fused Metals
An incomplete casting resulting from too great 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 it does not solidify prematurity as it enters the mold.
Cool mold or melt
Wax pattern too thin
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.
Patience and effort pays off!
Good technique demands everyone involved give adequate attention to all details for safe and efficient operations. It is must for a dentist to have adequate knowledge of the procedures and their relative consequences to eliminate error with understanding.
A great force working against good experienced technique saves time.