Casting is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures. Because castings must meet stringent dimensional requirements, the casting process is extremely demanding. In dentistry, virtually all casting is done using some form or adaptation of the lost-wax technique. The lost-wax technique has been used for centuries, but its use in dentistry was not common until 1907, when W.H. Taggart introduced his technique with the casting machine.
Casting can be defined as the act of forming an object in a mold .The object thus formed is also called as a casting .
Objectives of casting
1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with awell adjusted torch .
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.
STEPS IN MAKING A CAST RESTORATION
1 . TOOTH PREPARATION .
2 . IMPRESSION .
3 . DIE PREPARATION .
4 .WAX PATTERN FABRICATION .
- There are 4 methods for making wax patterns for a cast restoration .
5. SPRUING .
a) Sprue Former . (sprue pin ).
-provides channel for the molten metal .
-made of wax , plastic or metal .
-reservoir is attached to the sprue .
-ideally length of sprue is 3/8 th” to ½”
Lost Wax Process
The lost wax casting process is widely used as it offers asymmetrical casting withnvery fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:
1 .Create a wax pattern of the missing tooth / rim
2 .Sprue the wax pattern
3 .Invest the wax pattern
4. Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 .Clean the cast.
7 .Remove sprue from the cast
8 . Finish and polish the casting on the die .
The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out ("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process . A wax pattern is first formed on a die of the tooth to berestored or, occasionally, directly on the tooth. All aspects of the final restoration are incorporatedinto the wax pattern, including the occlusion, proximal contacts, and marginal fit. Once the wax pattern is completed, a sprue is attached, which serves as a channel for the molten metal to pass from the crucible into the restoration. Next, the pattern and sprue are invested in a ceramic material, and the invested pattern is heated until all remnants of the wax are burned away. After burnout, molten metal is cast into the void created by the wax pattern and sprue. Once the investment is broken away, the rough casting ispickled to removed oxides. Finally, the sprue is removed and the casting is polished and deliveredto the patient. If all steps have been done well, the final restoration will require minimal modification during cementation into the patient's mouth.
Dimensional Changes in the Lost-Wax Technique
If materials used during the casting process didn't shrink or expand, the size of the final cast restoration would be the same as the original wax pattern. However, dimensional changes occur in most of the steps and, in practice, the final restoration may not be exactly the same size as the pattern. The management of these dimensional changes is complex, but can be summarized by the equation:
wax shrinkage + metal shrinkage = wax expansion + setting expansion + hygroscopic expansion + thermal expansion .This equation balances the shrinkage (left sideof equation) against the expansion (right side ofequation) that occurs during the casting process. If the final restoration is to fit the die, the shrinkage and expansion during the casting process bmust be equal. Shrinkage forces in the casting process come from two sources: wax and metal. Although the die restricts the wax from shrinking to a large degree while the pattern is on the die, residual stresses may be incorporated into the pattern and released during investing, when the pattern isremoved from the die. Furthermore, if the investingis done at a temperature lower than that atwhich the wax pattern was formed, the wax willshrink significantly because of the high coefficientof thermal expansion of waxes. Metal shrinkage occurs when the moltenmetal solidifies, but this shrinkage is compensated by introducing more metal as the casting solidifies. However, once the entire casting has reached the solidus temperature of the alloy, shrinkage will occur as the casting cools to room temperature. As for wax, the metallic shrinkage that occurs below the solidus is caused by the coefficient of thermal expansion for the alloy. Cooling shrinkage may reach 2.5% for an alloy that cools from a high solidus temperature (1300" to 1400' C), depending on the coefficient of thermal expansion of the alloy. A typical shrinkage range for most alloys is 1.25% to 2.5%. Furthermore, because the casting is solid at this point, the only possible compensation mechanismis to start with a void space that is 1.25% to2. 5% too large. Thus, shrinkage of wax and metalmust be compensated with expansion in the investment if the casting is to have the appropriate dimensions.
Accuracy of the Lost-Wax Technique
A casting should be as accurate as possible, although a tolerance of rt0.05% for an inlay casting is acceptable. If the linear dimension of an average dental inlay casting is assumed to be 4 mm, +0.05% of this value is equal to only +2ym, which suggests that if two castings made for the same tooth have a variation of 4 ym, the difference may not be noticeable. To visualize this dimension, recall that the thickness of an average human hair is about 40 ym. Therefore the tolerance limits of a dental casting are approximately one-tenth the thickness of a human hair. To obtain castings with such small tolerancelimits, rigid requirements must be placed not only on the investment material but also on theimpression materials, waxes, and die materials. Naturally, technical procedures and the proper handling of these materials are equally important. The values for the setting, hygroscopic, and thermal expansions of investment materials may vary from one product to another, and slightly different techniques may be used with different investments. In each case, the values obtained for any one property should be reproducible from one batch to another and from one casting to another.
The Sprue :
Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Role of a Sprue: Create a channel to allow the molten wax to escape from the mold. Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.
FUNCTIONS OF SPRUE
1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage .
SELECTION OF SPRUE
1 . DIAMETER :
It should be approximately the same size of the thickest portion of the wax pattern .
Too small sprue diameter suck back porosity results .
2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax pattern .
It should be Flared for high density alloys & Restricted for low density alloys .
3 . SPRUE FORMER POSITION
Based on the
1 .Individual judgement .
2 .Shape & form of the wax pattern .
Patterns may be sprued directly or indirectly ..
Indirect method is commonly used
Reservoir prevents localised shrinkage porosity .
Reservoir And Its Location
Reservoir portion of a Spruing system is a round ball or a bar located 1mm away from the wax pattern. Reservoir should be positioned in the heat centre of the ring . This permits the reservoir to remain molten for longer and enables it to furnish alloy to the pattern until they complete solidification process . Round ball reservoir & a bar reservoir also called connector
Significance of Reservoirs:
Reservoir is the largest mass of any part of the Sprue system & it is present in the heat centre of the ring, it is the last part to solidify. These properties allow continuous feeding of the molten alloy to compensate for Solidification shrinkage & avoid Shrinkage porosity
The flow of the molten metal is straight(direct) from the casting crucible to pattern area in the ring. Even with the ball reservoir, the Spruing method is still direct. A basic weakness of direct Spruing is the potential for suck-back porosity at the junction of restoration and the Sprue.
Molten alloy does not flow directly from the casting crucible into the pattern area, instead the alloy takes a circuitous (indirect) route. The connector (or runner) bar is often used to which the wax pattern Sprue formers area attached. Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage .The Connector bar is often referred to as a “reservoir .
1. Sprue .
2 . Sticky wax .
3 . Rubber crucible former .
4 . Casting ring .
5 . Pattern cleaner .
6 . Scalpel blade & Forceps .
7 . Bunsen burner .
I . Procedure for single casting :
A 2.5 mm sprue former is recommended
for molar crowns 2.0 mm for premolars & partial coverage crowns .
II . Procedure for multiple casting :
Each unit is joined to a runner bar .
A single sprue feeds the runner bar
4 . SPRUE FORMER DIRECTION
Sprue Should be directed away from the delicate parts of the pattern
It should not be at right angles to a flat surface .(leads to turbulance porosity .)
Ideal angulation is 45 degrees .
5 . SPRUE FORMER LENGTH
Depends on the length of casting ring .. Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue former should be no longer than 2 cm. The pattern should be placed as close to the centre of the ring as possible.
Short Sprue Length:
The gases cannot be adequately vented to permit the molten alloy to fill the ring completelyleading to Back Pressure Porosity.
Long Sprue Length:
Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.
Top of wax should be adjusted for :
6 mm for gypsum bonded investments .
3 -4 mm for phosphate bonded investments .
TYPES OF SPRUES
I . - Wax . II . Solid
- Plastic . Hollow
- Metal .
Small auxilliary sprues or vents improve casting of thin patterns .
Acts as a HEAT SINK .
WAX PATTERN REMOVAL
Pattern should be removed in line with its path of removal
To minimise the irregularities on the investment & the casting a wetting agent can be used .
FUNCTIONS OF A WETTING AGENT
1 . Reduce contact angle between liquid & wax surface .
2 .Remove any oily film left on wax pattern .
DISTORTION OF THE PATTERN
Distortion is dependant on temperature &time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .
It serves as a base for the casting ring during investing .Usually convex in shape.
May be metal , plastic or rubber .
Shape depends on casting machine used .
Modern machines use tall crucible to enable the pattern to be positioned near the end of the casting machine .
CASTING RING LINERS
Most common way to provide investment expansion is by using a liner in the casting ring .Traditionally asbestose was used .
Non asbestose ring liner used are :
1) Aluminosilicate ceramic liner .
2) Cellulose paper liner .
The aim of using a resilient liner is to
-. allow different types of investmentbexpansion (act as a cushion)
_. facilitate venting during casting procedure.
_. facilitate the removal of the investment block after casting.&. prevent the distortion by permitting the outward expansion of the mold.
The casting ring holds the investment in place during setting and restricts the expansion of the mold. Normally a resilient liner is placed inside the ring leaving about 2-3 mm from both ends to allow for supporting contact of the investment with the casting ring.
Purpose of Casting Ring Liner
Ringer liner is he most commonly used technique to provide investment expansion. To ensure uniform expansion , liner is cut to fit the inside diameter of the casting ring with no overlap. Thickness of the liner should not be less than approximately 1mm. Place the liner somewhat short of the ends of the ring, 3mm, tends to produce a more uniform expansion, therefore less chance for distortion of the wax pattern & mold .
Traditional material for lining casting rings until it was learned that it posed a potential health risk to dental laboratory technicians . Asbestos fiber bundles were found to produce hazardous-size respirable particles capable of causing lung disease.
Non-asbestos Ring Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose combination Safety of the ceramic ring liners remains uncertain, because aluminum silicate also appears capable of producing hazardous-size respirable particles
RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .
Investing is the process by which the sprued wax pattern is embedded in a material called an investment. The investment must be able to withstand the heat and forces of casting, yet must conform to the pattern in a way such that the size and surface detail are exactly reproduced. In dentistry, gypsum- and phosphate-bonded investment materials are the two types of materials used for this purpose . After spruing, the pattern a casting ring is added to contain the investment while the investment material is poured carefully around the pattern. For the setting and hygroscopic expansion of an investment to take place more uniformly, some allowance must be made for the lateral expansion of the investment. Solid rings do notpermit the investment to expand laterally duringthe setting and hygroscopic expansions of themold.
To overcome this lateral restriction, a ceramic paper liner is placed inside the ring.The ceramic paper liner is cut to fit the inside ofthe metal ring and is held in place with the finger.The ring containing the liner is then dipped intowater until the liner is completely wet and wateris dripping from it. The ring is shaken gently toremove the excess water. After the liner has beensoaked, it should not be touched or adaptedfurther with a finger because this reduces itscushioning effect, which is needed for the lateral expansion of the investment. A liner that is about3 mm short at each end of the ring is preferred.When the liner is equally short at each end of thering, the investment is locked into the ring, and uniform expansion of the cavity form occurs.
During investing, the water-based gypsummaterial must flow around the pattern and captureevery surface detail. However, the wax sur-faces generally are not easily wetted by water.The surface of a wax pattern that is not completelywetted with investment results in surface irregularities in the casting that destroy its accuracy.These irregularities can be minimized byapplying a surface-active wetting agent on thewax. The function of the wetting agent is toreduce the contact angle of a liquid with the waxsurface. Wetting agents also remove any oily filmthat is left on the wax pattern from the separatingmedium. Thecontact angles are 98' for the plain wax surfaceand 61" for the treated wax surface. The lowercontact angle indicates that the treated wax surfacehas an affinity for water, which results in theinvestment being able to spread more easily overthe wax. Because the surface-active agents arequite soluble, rinsing the wax pattern with waterafter the application defeats the purpose of theiruse.
The distortion of the wax pattern after itsremoval from the die is a function of the temperatureand time interval before investing. Thenearer the room temperature approaches the softening point of the wax, the more readilyinternal stresses are released. Also, the longer apattern is allowed to stand before investing, thegreater the deformation that may occur, even atroom temperature. A pattern should therefore beinvested as soon as possible after it is removedfrom the die, and it should not be subjected to awarm environment during this interval. In anycase, a pattern should not stand for more than20 to 30 minutes before being invested. Once itis properly invested and the investment has set,there is no danger of further pattern distortion,even if it remains for some hours before the finalstages of wax elimination (burnout) and casting
During investingof the pattern, the correct water powder ratioof the investment mix, a required number ofspatulation turns, and a proper investing techniqueare essential to obtain acceptable castingresults. There are two methods of investing thewax pattern: hand investing and vacuum investing.In both cases, the proper amount of investmentpowder and water should be used, followingthe manufacturer's instructions exactly. Thewater is added first, followed by the slow additionof the powder to encourage the removal ofair from the powder. The powder and liquid aremixed briefly with a plaster spatula until all thepowder is wetted.
In hand investing, the cover of the bowl containingthe investment mix is placed over thebowl . The cover contains a mechanicalmixer, and the mixing is done by hand,usually for 100 turns of the spatulator. The settingrate of an investment depends on the number ofspatulation turns, which also affects the hygroscopicexpansion. The investment, after beingspatulated, is placed on the vibrator to eliminatesome of the air bubbles from the mix and tocollect all of the mix from the sides of the rubber bowl into the center. Thefilled ring is then set aside for the investment toset completely, which usually requires 45 to60 minutes. When a phosphate-bonded investmentis used, the ring is slightly overfilled, the topof the ring is not leveled off, and the investmentis allowed to set. After the investment has set, the excess investment is ground off using a modeltrimmer. This procedure is necessary because anonporous, glassy surface results, which must beground off to improve the permeability of the
investment and allow for gases to readily escape from the mold during casting.
In vacuum investing, special equipment is used to facilitate the investing operation. With this equipment, the powder and water (or special liquid) are mixed under vacuum and the mixed investment is permitted to flow into the ring and around the wax pattern with the vacuum present. Although vacuum investing does not remove all the air from the investment and the ring, the amount of air is usually reduced enough to obtain a smooth adaptation of the investment to the pattern. Vacuum investing often yields castings with improved surfaces when compared with castings produced from hand-invested patterns. The degree of difference between the two procedures depends largely on the care used in hand investing. Whether hand- or vacuum-investing procedures are used in filling the casting ring, the investment should be allowed to harden in air before burnout of the wax.
Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.
a). Brush technique:
The accurate water-powder ratio is mixedunder vacuum. A brush is then used to paintthe wax pattern with mix then the casting ringis applied over the crucible fromer and thering is filled under vibration until it iscompletely filled.
b). vacuum technique:
• The mix in first hand spatulated, and then withthe crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to theassembly. The bowel is inverted and the ring isfilled under vacuum and vibration
Two-step investing technigue:
The investing procedure is carried out in twosteps:
• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.
• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former isremoved. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the waxpattern. Any loose particles of investment should beblown off with compressed air should be placed in ahumidor if stored overnight.
Wax elimination (burnout):
Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.
• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .
Melting & Casting Technique Melting & Casting requires Heat source to melt the alloy Casting force, to drive the alloy into the mould
Casting Torch Selection Two type of torch tips: Multi-orifice Single-orifice Multi-orifice tip is widely used for metal ceramic alloys. Main advantage is distribution of heat over wide area for uniform heating of the alloy. Single-orifice tip concentrate more heat in one area.Three fuel sources are used for Casting Torch; Acetylene ,Natural Gas ,Propane
Four types are available ;
1) Clay .
2) Carbon .
3) Quartz .
4) Zirconia –Alumina .
It is a device which uses heat source to melt the alloy casting force .
Heat sources can be :
1) Reducing flame of a torch .
( conventional alloys & metal ceramic alloys )
2) Electricity .(Base metal alloys )
Advantages of electric heating :
-heating is evenly controlled .
-minimal undesirable changes in the alloy composition .
- Appropriate for large labs .
Casting machines use :
1) Air pressure .
2) Centrifugal force .
3) Evacuation technique .
Alloys can be melted by :
1) Alloy is melted in a separate crucible by a torch flame & is cast into the mold by centrifugal force .(centrifugal C M )
2) Alloy is melted by resistance heating or by induction furnace & then cast centrifugally by motor or spring action (springwound CM electrical resistance )
3) Alloy is melted by induction heating cast into mold centrifugally by motor or spring action .(Induction CM )
4) Alloy is vacum melted by an argon atmosphere
Torch melting / Centrifugal casting machine
Electrical resistance /Heated casting machine
Melting of the alloy should be done in a graphite or ceramic crucible .
-Oxidation of metal ceramic restorations on
overheating is prevented .
-Help in solidification from tip of the casting to the button surface .
Induction casting machine
Commonly used for melting base metal alloys.
- Highly efficient .
- Compact machine withlow power consumption
-No pre heating needed ,
- safe & reliable.
Direct current arc melting machine
A direct current arc is produced between two electrodes :
The alloy & the water cooled tungsten electrode .Temp used is 4000 degrees .
High risk of overheating the alloy .
Vacuum or pressure assisted casting machine
Molten alloy is drawn into the evacuated mold by gravity or vacuum & subjected to aditional pressure
For Titanium & its alloys vacuum heated argon pressure casting machines are used .
Accelerated casting method
This method reduces the time of both bench set of the investment & burnout .
Uses phosphate bonded investments which uses 15 mnts for bench set & 15mnts for burnout by placing in a pre – heated furnace to 815 degrees .
Effect of burnout on gypsum bonded investments
Rate of heating has influence on smoothness & on overall dimensions of the investment
Rapid heating causes cracking & flaking which can cause fins or spines .
Avoid heating gypsum bonded investment above 700 degrees .Complete the wax elimination below that temp .
Effect of burnout on phosphate bonded investments
Usual burnout temp is 750 -1030 degrees.
Although they are strong they are brittle too .
Since the entire process takes a long time two stage burnout & plastic ring can be used .CLEANING AND PICKLING ALLOYS
The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy . Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.
Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution . . After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition The effect of the solution can be seen by
comparing the submerged surfaces to those that have still not contacted the solution. the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.
Precautions to be taken while pickling
With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should beavoided. This practice may alter the phase structure of the alloy or warp thin castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove
castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.
To prevent oxidation of gold alloys during melting always use a reducing flux .
Boric acid & borax are used .
Casting of glass or ceramic
A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .
Classification (combe ):
2) Surface roughness .
3) Porosity .
4)Incomplete casting .
5) Oxidation .
6) Sulfur contamination .
It is usually due to the distortion of wax pattern.
To avoid this :
Manipulation of the wax at its softening temp
Invest the pattern at the earliest .
If storage is necessary store it in a refrigerator .
May be due to :
Air bubbles on the wax pattern .
Cracks due to rapid heating of the investment .
High W/P ratio .
Prolonged heating of the mold cavity .
Overheating of the gold alloy .
Too high or too low casting pressure .
Composition of the investment .
Foreign body inclusion.
May be internal or external .
External porosity causes discolouration .
Internal porosity weakens the restoration .
Classification of porosity .
I .Those caused by solidification shrinkage :
a) Localised shrinkage porosity .
b) Suck back porosity .
c) Microporosity .
They are usually irregular in shape .
II ) Those caused by gas :
a) Pin hole porosity .
b) Gas inclusions .
c) Subsurface porosity .
Usually they are spherical in shape .
III ) Those caused by air trapped in the mold :
Back pressure porosity .
Localised shrinkage porosity
Large irregular voids found near sprue casting junction.
Occurs when cooling sequence is incorrect .
If the sprue solidifies before the rest of the casting , no more molten metal is supplied from the sprue which can cause voids or pits
(shrink pot porosity )
This can be avoided by -
- using asprue of correct thickness .
- Attach the sprue to the thickest portion of the pattern .
-Flaring of the sprue at the point of atttachment .
-Placing a reservoir close to the pattern .
Suck back porosity
It is an external void seen in the inside of a crown opposite the sprue .
Hot spot is created which freezes last .
It is avoided by :
Reducing the temp difference between the mold & molten alloy .
Fine irregular voids within the casting .
Occurs when casting freezes rapidly .
Also when mold or casting temp is too low .
Pin hole porosity :
Upon solidification the dissolved gases are expelled from the metal causing tiny voids .
Pt & Pd absorb Hydrogen .
Cu & Ag absorb oxygen .
Gas inclusion porosities
Larger than pin hole porosities .
May be due to dissolved gases or due to gases Carried in or trapped by molten metal .
Apoorly adjusted blow torech can also occlude gases .
Back pressure porosity
This is caused by inadequate venting of the mold .The sprue pattern length should be adjusted so that there is not more than ¼” thickness of the investmentbetween the bottom of the casting .
This can be prevented by :
- using adequate casting force .
-use investment of adequate porosity .
-place the pattern not more than 6-8 mm away from tne end of the casting .
Casting with gas blow holes
This is due to any wax residue in the mold .
To eliminate this the burnout should be done with the sprue hol facing downwards for the wax pattern to run down.
This is due to :
- insufficient alloy .
-Alloy not able to enter thin parts of the mold .
-When the mold is not heated to the casting temp .
-Premature solidification of the alloy .
-sprues blocked with foreign bodies .
-Back pressure of gases .
-low casting pressure .
-Alloy not sufficiently molten .
Too bright & shiny casting with short & rounded margins :
occurs when wax is eliminated completely ,it combines with oxygen or air to form carbon monoxide .
Small casting :
occurs when proper expansion is not obtained & due to the shrinkage of the impression .
Contamination of the casting
1) Due to overheating there is oxidation of metal .
2) Use of oxidising zone of the flame .
3) Failure to use a flux .
4) Due to formation sulfur compounds .
It is due to :
1) Overheating of the investment .
2) Incomplete elimination of the wax .
Investing and casting , a series of highly technique sensitive steps , converts the wax pattern into metal casting . Accurate and smooth restorations can be obtained if the operator pays special attention to each step in the technique .
When initial attempts in the casting procedure produce errors or defects , appropriate corrective measures must be taken so that they do not recur .
• Fundamentals of fixed prosthodontics: Shillingburg
• Dental laboratory procedures: Rudd and Morrow.
• Philip’s science of dental ceramics;Anusavice.
• Dental materials: Craig.
• Tylman’s theory of fixed prosthodontics
• * Notes on Dental Materials , E .C . Combe .
• Applied Dental Materials , Mc Cabe .
• Contemporary fixed Prosthodontics; Rosensteil.
1 . The effect of sprue attachment design on castability and porosity .J Prosthet Dent , 61 :418 -24 , 1989 .Flared & straight sprue attachment optimised castability and minimised porosity
2 . Setting & thermal reactions of Phosphate bonded investments . J of Dentistry rest :1478 -1485 , 1980 .
3) Delayed hygroscopic expansion of phosphate bonded investments . Dental Mater 3 : 165 -7 ,1987 .
Delayed hygroscopic expansion occurs when the investment is immersed in water after setting .
4) Sprue design in RPD casting : J of dentistry ,Nos 1-2 .vol 24 ,99-103 ,1996 .
Correct sprue designs is a major factor in reduction of casting defects .
5)Creating buttonless casting by using preformed wax sprues ; JOP Sept 1996 ; 327 -329 .
This method conserves metal by allowing a minimum of metal for each casting .Smaller button size allows more new metal to be added with subsequent castings .
6) Effect of burnout temp in strength of phosphate bonded investments ,J of Dentistry ,vol 25 ; No :2 , 153 -160 ,1997 .