Investment/ dental implant courses by Indian dental academy

Investment Introduction The casting procedure by the lost wax technique which is now a common practice, was not popular until 1907. When W.H. Taggart introduced his technique and casting machine to the profession.

Modern dental practice involves a variety of casting operations, ranging from the simplest inlay to all forms of cast crown, bridge structure and removable partial dentures. Each of which makes use of the same fundamental practices in forming the cast restorations.

Once the investment was set for an appropriate period approximately one hour, for most gypsum and phosphate bonded investments it is ready for burnout. The procedure for the two types of investments are similar. The crucible former and any metal sprue former are carefully removed. Any debris from the in gate area (funneled opening at the end of the ring) is cleaned with a camel hair brush. If the invested ring is placed in a humidor at 100% humidity, if at all possible, the investment should not be permitted to dry out. Rehydration of set investment that has been stored or an extended period may not replenish all of the lost water.

Definition An investment can be described as a ceramic material which is suitable for forming a mold into which a metal or alloy is appropriately cast. The procedure for forming the mold is described as “investing” (wax pattern).

Depending on the melting range of the alloy and the preference of the clinician, generally two types of investment: 1. Gypsum – bonded and 2. Phosphate – bonded investment are employed.

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The gypsum based materials represent the type traditionally used for conventional gold alloys.

The phosphate based invest are designed purely for alloys used in the metal ceramic restoration.

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Casting Procedure Wax Elimination and Heating The invested rings are placed in a room temperature furnace and heated to the prescribed maximum temperature. For gypsum bonded investments the temperature is either 468째C for hygroscopic technique or 650째C for the thermal expansion technique.

With phosphate bonded investments, the maximum temperature setting may range from 700째C to 870째C depending on the type of alloy selected.

The temperature setting is more critical with gypsum bonded investments than with the phosphate type because the gypsum investments are more prone to investment decomposition.

During burnout, some of the melted wax is absorbed by the investment and residual carbon produced by ignition of the liquid wax becomes trapped in the porous investments, it is also advisable to begin the burnout procedures while the mold is still wet. Water trapped in the pores of the investment reduces the absorption of the wax and as water vaporizes. It flushes wax from the mold.

The formation of gases (CO2, H2O) depends however, on the presence of a sufficient supply of oxygen, the relatively high temperature of the oven and adequate time of heating of the ring.

If the amount of oxygen available to the wax in the mould cavity is not sufficient, the temperature of the oven is not high enough or the wax pattern is heated only for a short time, incomplete reaction between the wax and oxygen may result (Craig).

One of the most satisfactory way of eliminating wax pattern is to place the mold in the furnace with the hole down at first, so that the major portion of the wax drains out and is eliminated as a liquid. The ring is then inverted with sprue hole placed upward. In this way oxygen in the oven atmosphere can circulate into the mold morereadily.

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If more than one ring is placed in an oven a longer period of time is required for wax elimination. The general rule is to add 5 minutes to the wax elimination time for every ring placed in the oven at 500째C. According to Skinner This process is facilitated by placing the ring with the sprue hole down over a slot in a ceramic tray in the burnout furnace, when the high heat technique is used the mold temperature generates enough heat to convert carbon to either carbon monoxide or dioxide, (cause for the discolouration of alloy). These gases can then escape through the pores in the heated investment.

Hygroscopic (Low Heat) Technique This technique obtains its compensation expansions from three sources.

1. 37째C water bath expands the wax pattern.

2. The warm water entering the investment mold from the top adds some hygroscopic expansion.

3. The thermal expansion at 500째C provides the needed thermal expansions.

This low heat technique offers the advantages of less mold degradation, a cooler surface for smoother castings and the convenience of placing the mold directly in the 500째C furnace, the last benefit makes it possible to keep one or more furnaces at the burnout temperature, so that mold may be put in as they are ready at various times.

This is particularly useful in large laboratories where molds are ready at various times. Care must nevertheless be taken to allow sufficient burnout time because the wax is more slowly oxidized at the low temperature.

The mold should remain in the furnace at least 60 minutes, and they may be held up to 5 hours longer with little damage.

Molds placed in the furnace at intervals lower the temperature of the furnace.

Extra time should be given to ensure complete wax elimination. Even though the mold is held at this temperature for 60-90 minutes, sufficient residual fine carbon may be retained to reduce the venting of the mold.

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Because of this reduced venting back pressure porosity is a greater hazard in the low heat technique. Since the investments generally employed with low heat technique may be more dense.

Some times muffle furnaces may be so air tight that burnout takes place in a reducing atmosphere, preventing complete oxidation of the wax residues. Keeping the door open slightly permit air to enter and provides enough oxygen for elimination of the wax. This is particularly important for the hygroscopic expansion technique when a lower burnout temperature is used.

The standardized hygroscopic technique was developed for alloys with a high gold content. There may be a need for slightly more expansion for the newer noble alloys. This is obtained by making following changes.

1. Increasing the water bath temperature to 40°.

2. Using two layer of liner.

3. Increasing the burnout temperature to a range of 600°C to 650°C. High Heat (Thermal Expansion) Technique This approach depends almost entirely on high heat burnout to obtain the required expansion while at the same time eliminating the wax pattern.

Additional expansion results from the slow heating of gypsum investments or setting, thus expanding the wax pattern and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

Gypsum Investments These are relatively fragile and require the use of a metal ring for protection during heating. The molds are usually placed in a furnace of room temperature and slowly heated to 650°C to 700°C in 60 minutes and held for 15 to 30 minutes at the upper temperature.

The rate of heating has some influence on the smoothness and some instances overall dimensions. Initially, the rapid heating can generate steam that can cause flacking or spalling of the mold walls.

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Too many patterns in the same plane within the investment after cause separation of a whole section of investment because the expanding wax creates excessive pressure over a large area.

Too rapid a heating rate may also cause cracking of the investment. In such a case the outside layer becomes heated before the weaker sections, the outer layer starts to expand thermally, resulting in compressive stress in the outside layer that counteracts tensile stresses in the middle regions of the mold such a stress distribution causes the brittle investment to crack from the interior outwardly in the form of radial cracks. These cracks produce a casting with fine or spines.

This condition is especially likely to be present with a cristobalite investment.

Low inversion temperature of the cristobalite, rapid rate of expansion during the inversion makes it especially important to heat the investment slowly.

The reduction of calcium sulfate by carbon takes place rapidly whenever gypsum investments are heated above 700°C in the presence of carbon. Sulfur dioxide as a product contaminates gold castings and makes them extremely brittle. This emphasizes the need for complete elimination of wax and avoiding burnout temperatures, above 700°C particularly if the investment contains carbon.

After casting temperature has been reached, the casting should be made immediately, maintaining high temperature for longer time may result in a sulfur contamination of the casting also surface on the casting because of the disintegration of the investment.

Some manufacturers who advocate much more rapid burnout procedure suggest placing mold in a furnace at 15°C for 30 minutes and following with very rapid heating to the final burnout temperature. A few are offering investment that may be placed directly into a furnace at the final burnout temperature held for 30 minutes and cast.

Because the design of the furnace, the proximity of the mold to the heating element. The availability of air in the shuffle may affect size and smoothness. It is advisable to examine these factors carefully before a casting is made in this manner.

Phosphate Investments Because the setting mechanism and reactions on heating are quite different there are several differences to gypsum – bonded investments.

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Phosphate investment obtain their expansion from : 1. The expansion of the wax pattern – this is considerable because the setting reaction raises the mold temperature considerably.

2. The setting expansion – this is usually higher than in gypsum – bonded investments especially because special liquid are used to enhance such expansion.

3. The thermal expansion is greater when taken to temperatures higher than these used for gypsum – bonded investments.

Phosphate-Bonded Phosphate investments are usually much harder and stronger than gypsum investments. They are quite brittle and are subject to the same unequal expansion of adjacent sections as phase changes occur during heating.

Phosphate Investments Require: 1. Higher burnout temperatures to ensure total elimination of wax.

2. The completion of chemical and physical changes.

3. Prevention of premature solidification of higher molding alloys, the usual burnout temperatures range from 750 to 900°C. The heating rate is usually slow to 315°C and is quite rapid thereafter reaching completion after a hold at the upper temperature for 20 minutes.

For time saving, there are now some investments that can be subjected to two stage heating more rapidly and placed directly in the furnace at the top temperature, held for 20-30 minutes and then cast. To save more time, the use of a ring and a liner is also eliminated the metal ring being replaced with a plastic ring that is tapered so that once the investment has set it can be washed out of the ring, held for a specified time to complete setting, and then placed directly into the hot furnace, obviously, the expansion on setting in different than when a lined ring is used, so that changes in overall fit must be considered. The required expansion may be adjusted by varying the liquid concentration.

Time Allowable for Casting The investment contracts thermally as it casts. When thermal expansion or high heat technique is used, the investment uses heat after the ring is removed from

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the furnace, and the mold contacts. Because of the liner and the low thermal ductility of the investment, a short period can elapse before the temperature of the mold is affected under average conditions of casting, approximately 1 minute can pass without a noticeable loss in dimension.

In the low heat technique, the temperature gradient between the investment mold and the root is not as great as with the high-heat technique.

Also the thermal expansion of the invest is not as important to the shrinkage compensation. However the burnout temperature lies on a fairly steep portion of the thermal expansion curve rather than on a plateau portion as in high-heat technique.

Therefore the alloy should also be cast soon after removal of the ring from the oven.

Casting Mechanisms Classified broadly into 2 types: 1. Centrifugal force type.

2. Air pressure to force the metal into the mold.

Other alloys are melted in one of the three ways: -

The alloy is melted in a separate crucible by a torch flame and the metal is cast into the mold by centrifugal force.

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The alloy is melted electrically by a resistance or induction furnace, then cast into the mold centrifugally by motor or spring action.

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The alloy is melted as in the first two ways but it is cast by air pressure, a vacuum or both.

Centrifugal Casting Machine The casting machine is first wound from two to five turns (depending on the particular machine and the speed of casting rotation desired).

The metal is melted by a torch flame in a glazed ceramic crucible attached to the “broken arm� of the casting machine. The broken arm feature accelerates the initial rotational speed of the crucible and casting ring, thus increasing the linear speed of the liquid casting alloy as it moves into and through the mold. Once the metal has reached the casting temperature and the heated casting ring is in position, the machine is released and the spring triggers the rotational motion.

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As the metal fills the mold there is a hydrostatic pressure gradient from the tip of the casting to the bottom surface is quite sharp and parabolic in form, reaching at the button surface. (0.21-0.28MPa).

Because of this pressure gradient, there is a gradient in the heat transfer rate, such that the greatest rate of heat transfer to the mold is at the high pressures and of the gradient (i.e., the tip of the casting).

Because this end is frequently the sharp edge of the margin of a crown, there is further assurance that the solidification progresses from the thin margin edge to the button surface.

A variety of centrifugal machine are available some designed to spin the mold in plane to the table top, on which the machine is mounted and other to rotate in a plane vertical to the table top.

Some are spring driven and others are operated by electric power.

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Attached to some machines is an electric heating unit to melt the alloy before the mold is started spinning to force the metal into mold.

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Others have a simple refractors tray mounted on the machine in which the alloy is placed to be melted by a blowtorch.

The advantage of the centrifugal machines and the simplicity of design and operation with opportunity to cast both large and small castings on the same machine.

When the air pressure type of machine to employ either compressed air or some other gas, such as carbon dioxide or nitrogen can be used to force the molten metal into the mold. The casting machine with an attached vacuum system designed to assist the molten mold falling into the mold are available.

In some casting, addition of the vacuum may advantageous but in general there is little evidence to indicate superiority in the quality of castings produced by this addition.

One should keep certain objectives in mind at the time of making the casting.

1. To heat the alloy no quickly as possible to a completely molten conditions.

2. Prevent oxidation by heating the metal quickly with a well-adjusted torch or other method and a small amount of flux on the metal surface.

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3. Produce a sharp details by having adequate pressure applied to the hot-melted metal to furnish it into the mold.

Electrical Resistance Heated Casting Machine In this there is an automatic melting of the metal in a graphite crucible within a furnace (rather than by use of a torch flame). This is an advantage for alloys used for metal ceramic restorations which are alloyed with base metals in true amount that had to oxidize on overheating.

Another advantages is that the: 3. Crucible in the furnace is located flush against the casting ring. Therefore the metal button remain molten slightly longer, again ensuring that solidification progresses completely from the tip of the casting to the button surface.

Induction Melting Machine With this unit, the metal is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing. Once the metal reaches the casting temperature. It is forced into the mold by air pressure, vacuum or both at the other end of the ring. Popular in the casting of jewellery more commonly used for melting base metal alloys.

There is little practical differences in the properties or accuracy of castings made with any of the three types of ceramic. The choice is a matter of access and personal preference.

Casting Crucibles Generally, three types of casting crucibles are available : Clay, Carbon and Quartz (Zircon alumina).

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Clay crucible are – many of crown bridge alloys such as high noble-alloy.

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Carbon crucible – not only for high – noble crown bridge but also for higher fusing gold based metal ceramic.

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Quartz crucibles are recommended for high fusing alloys of any type suited for alloys that have a high melting range and are sensitive to carbon contamination.

1. High palladium content.

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2. Palladium silicon from metal ceramic copings.

3. Nickel-cobalt based.

Methods of Melting Alloys The most common method of heating gold alloy for full cast metal restorations have been the use of a gas-air blowtorch. A properly adjusted blowtorch with develop a temperature that is adequate for melting distal gold alloys, whose melting average is between 870째-1000째C. Many descriptions of the proper flame for heating metals and alloys are found in the literature one practical method of checking and interpreting the flame condition is to apply the flame to a small piece of copper, placed on a soldering block. The blowtorch is adjusted and is then directed upon the copper. If the copper turns bright and clean as it is heated the flame and the blow torch manipulation are correct, if copper turns dark, dull red color oxydation is occurring and the heating is ineffective.

Electric melting units of various designs are used in some laboratories to melt the alloys. These units have the advantage that less skill may be required by the operator to control such devices.

This is necessary for the use of blow torch however many of these electric heating units have no limiting controls and as a result the operator is required to exercise regarding the proper conditioning of the alloy.

Electric units are heated either by induction or by resistance heating systems.

These heated by induction melt alloy much faster than those by torch melting, they can easily to overheated. An electronic monitor to induce the proper temperature is very useful. Units employing resistance heating require a longer time to complete the heating and casting operation when compared to torch melting.

Melting Noble Metal Alloy The alloy is best melted by placing it on the inner sidewall of the crucible. In this position the operator can better observe the progress of the melting opportunity for air gases in the flame to be reflected from the surface of the metal rather than to be absorbed.

The fuel employed is a mixture of natural or artificial gas and air. - Gold alloys cast restoration.

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Oxygen air acetylene can also be used for cobalt-chromium based alloys with high melting point.

The temperature of the gas-air flame is operately influenced by the nature of gas and the proportions of gas and air mixture. Care should be taken to obtain a nonluminous brush flame, with the different combination zones clearly differentiated.

If the air supply is excessive incomplete compaction and a lower temperature results, roaring sounds accompanies this type of flame.

The parts of the flame: -

The first long cone eminating directly from the nozzle is the zone in which air and gas are mixed before combustion. No heat is present in this zone.

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The next cone, which is green and immediately surrounding the inner cone and known as the combustion zone. Here the gas and air are partially burned. This zone is oxidizing and should be kept away from the molten metal during fusion.

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The next zone, deep blue, is the reducing zone it is the hottest part of the flame and is just beyond the tip of the green combustion zone. This area should be kept constantly on the metal during melting.

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The outer cone (oxidizing zone) is the area in which combustion occurs with oxygen in the air. This portion of the flame be added to melt the alloy, its temperature is lower than reducing zone also oxidizes the metal.

The proper zone in contact with the metal can be readily detected by the condition of the metal surface. With reducing zone – surface of gold alloys is bright and mirrorlike.

When oxidizing zone is in contact – there is a dull film of “dross” developed over the surface.

The alloy first appears to be spongy, then small globules of fused metal appears then the molten alloy soon assumes a spheroidal shape.

At the proper casting temperature, the molten alloy is slight orange and tends to spin or follow the flame which moved slightly, at this point the metal should be approximately 31°C to 66°C above its liquidous temperature. Casting should be made immediately when the proper temperature is reached.

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It is desirable to use the flux for gold alloys. The flux increases the fluidity of the metal and the film of flux formed on the surface of the molten alloy helps prevent oxidation.

Reducing fluxes containing charcoal are often used, small bits of carbon may be carried into the mold. They are excellent for cleaning old metal.

A better flux is made from equal parts of fused borax powder ground with boric acid powder. Boric acid aids in veterning borax on the surface. The flux is added when the alloy is completely melted.

Cleaning the Casting For gold crown and bridge alloys. After the casting has been completed, the ring is removed and quenched in water as soon as the button exhibits a dull red glow.

The disadvantages of quenching are the noble metal alloy is left in an annealed condition for burnishing, polishing and finishing procedures. When the water contacts the hot investments a violent reaction ensures. The investment becomes soft and granular and the casting is more easily cleaned.

Often the surface of the casting appears dark with oxide and tarnish such a surface film can be removed by a process known as “pickling� which consists of heating the discoloured casting in an acid. The best pickling solution for gypsumbonded investments is a 50% hydrochloric acid HCl acid aids in the removal of any residual investment as well as of the oxide coating. Disadvantage to the fumes from the air are likely to corrode laboratory metal finishings and these fumes are health hazard. A solution of sulfuric acid is more advantageous in this respect care should be taken not to over heat or margins of the casting get distorted.

Ultrasonic devices are also available for cleaning the casting, as are commercial pickling solutions made of acid salts.

The best method for pickling is to place the casting in a test tube or dish and to pour acid over it. It may be necessary to heat the acid, but boiling should be avoided because of the considerable amount of acid fumes involved. After heating, the acid is poured off and the casting is removed, in no case should casting be held with steel prongs so that both the casting and the tongs come into contact with the pickling solutions as this may contaminates the casting. The pickling solutions usually contains small amounts of copper dissolved from previous casting when the steel prongs contact this electrolyte, a small galvanic cell is created and copper is deposited on the

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casting at the point where the prongs grip it. This copper deposition extends into the metal and is a future success for discoloration in the area.

It is a common practice to heat the casting and then to drop it with the pickling solution. The disadvantages is that a delicate margin may be melted in flame or the casting may be distorted by the sudden thermal shock when placed in the acid.

Gold-based and palladium-based metal ceramic alloys and base metal alloys are bench cooled to room temperature before the casting is removed from the investment. Castings from these alloys are generally not pickled, and when it is recommended for certain metal ceramic alloys it is only to selectively remove specific surface oxides.

Neither the phosphate binder not the silica refractory is soluble in hydrochloric acid or sulphuric acid. Hydrofloric acid dissolves the silica refractory quite well without damage to a gold-palladium silver alloy but must be used carefully with other alloys.

Base metal alloys require a light sandblasting usually with fine aluminium oxide, chromium based partial dentures are usually sandblasted to remove the investment. Acid should never be used for cleaning base metal alloys.

Casting of Glass A castable glass ceramics inlay or crown is prepared in a manner similar to that of a metal casting, a wax pattern is made on a high strength stress die and all sections of the pattern should be more than 1 mm thick with the occlusal surfaces and marginal edges being 15 mm thick. Wax pattern is sprued with 8 or 10 gauge sprues.

The pattern is invested in a phosphate bonded investment and allowed to set for 1 hour.

The invested pattern is placed in a room temperature oven and heated to 280째C and held at that temperature for 20 minutes, after which the temperature is raised to 955째C for an additional 30 minutes.

A special centrifugal casting machine is used that has an electric furnace and is motor driven. The glass is heated to 1360째C (2480) and then cast and spun for a sufficient time to allow the casting to cool. The casting is allowed a set at room temperature for as minutes before diverting. At this point the casting is transparent.

After the sprue is cut off and the area finished it must be cerammed (recrystalized) to produce a transparent crown. The restoration is embedded in phosphate bonded

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investment prior to commonly for 6 hour at 1075°C. the cerammed restorations is one color and must be shaded with ceramic stains to yield an esthetic restorations.

A 3rd type is the ethyl silicate bonded invest used primarily in the casting of R.P.D. with base metal alloys.

According to ADA specification No.2 for casting investments for dental gold alloys are of three types of determined by whether the appliance is to be fabricated is fixed or removable and the method of obtaining the expansion required to compensate for the contraction of the molten gold alloy during solidification.

Type I – are used for the casting of inlays or crowns, and the compensation of casting shrinkage is principally by thermal expansion of the investment.

Type II – are also used for the casting of inlays or crowns but major mode of compensation is by the hygroscopic expansion of the investment.

Type III – are used in the construction of partial dentures with gold-alloys.

Gypsum-Bonded Investment The essential ingredients are : 1. -hemihydrate of gypsum and 2. A form of silica. -hemihydrate gives greater strength to the material and acts as a binder to hold the other ingredients together and provide rigidity. Although depends on amount of binder – may contain 25% - 45% and is used for alloy with melting ranges below 1000°C (i.e., gold-containing).

When heated to the required temperatures it shrinks considerably and frequently fractures all form shrink considerably after dehydration between 200°C and 400°C. A slight expansion then occur between 400°C and approximately 700°C, and then a large contraction occur. This is most likely carried by decomposition and sulfur gases such as sulfur dioxide are emitted which contaminates the castings (with the sulfides of non habit allogest elements such as silver and copper). Thus not to be heated above 700°C. -hemihydrate requires less mixing water and shrinks less.

Silica

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Added to provide a refractory during the heating of the investment and to regulate the thermal expansion.

It exists in an allotrophic form.

1. Quartz 2. Tridymite 3. Cristobalite and 4. Fused quartz.

When heated a change in crystalline form occurs at a transition temperatures, characteristics of the particular form of silica.

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When heated quartz invasion from a ‘low’ form -quit to high form to quartz at 570°C.

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Cristobalite undergoes – between 200°C-270°C from  –cristobalite.

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Tridymite – 117°C to 163°C.

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-allotropic form are stable only above the transition temperature, inversion to the lower form occur on cooking in each case. The density decrease as the  -changes to  -form resulting increase in volume.

Fused quartz is amorphous and glucobites in character exhibits inversion at any temperature below its fusion points has an extremely low coefficient of thermal expansion and is of little use in dental expansion. Quartz, cristobalite, or a combination of the two forms may be used in a dental investment.

Modifiers Such as coloring matter reducing agents such as carbon powdered copper to provide a non-oxidizing atmosphere in the mold when the gold alloy is cast.

Some of the added modifiers such as toxic acid and sodium chloride not only regulate setting expansion and the setting time, but also prevent most of the shrinkage of gypsum when it is heated above 300°C. Setting Time

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According ADA specifications No.2 for dental alloy casting, S.T. should not be shorter than 5 minutes nor longer than 25 minutes the modern inlay investments set initially in 9-18 minutes.

Normal Setting Expansion A mixture of silica and gypsum hemihydrate results in setting expansion greater than that of the gypsum product when it is used alone. The silica particles probably interfere with the inert washing and interlocking of the crystals as they form.

Thus the thrust of the crystals is outward during growth and they increase expansion.

ADA specification No.2 for type I invest permits maximum setting expansion in air of only 0.6%, that of modern invest is approximately 0.4%.

The purpose of setting expansion is to aid in enlarging the mold to compensate partially for the casting shrinkage of the gold.

The effectiveness of the setting expansion in enlarging the mold containing the wax pattern may be related to the thermal expansion of the pattern caused by the heat of reaction that occurs coincidentally with the setting of the investment. It follows from such a theory that the setting expansion and effective only to the extent that the exothermic heat is transmitted to the pattern.

The amount of heat present depends on the gypsum content of the investment ; therefore the setting expansion of the invest with comparably high content of gypsum more effective in enlarging the mold than is a product with a lower gypsum content.

Likewise manipulative conditions that increase the exothermic heat increase the effective setting expansion, (eg, the lower the water powder ratio for the investment, the greater is the effective setting expansion).

Other variables are: As the investment sets, it eventually gains sufficient strength to produce a dimensional change in the wax pattern as setting expansion occurs.

The inner wall of the investment within a MOD wax pattern can actually force the proximal walls outward to a certain extent. If the pattern has a thin wall then the effective setting expansion, is somewhat greater than for a pattern with thicker walls because the investment can move the thinner wall more readily. Also the softer wax, the greater the effective setting expansion because the softer wax is more readily moved by the expanding investment.

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Hygroscopic Setting Expansion The hygroscopic setting expansion differs from the normal setting expansion in that it occurs when the gypsum product is allowed to set under or in contact with water and that it is greater in magnitude than the normal setting expansion.

This is related to the additional crystal growth permitted and not to any differences in chemical reaction.

In normal setting condition, the water around the particle is reduced by the hydration and the particle are brought more closely together by the surface tension action of the water.

In hygroscopic reaction the setting is taking place under water, the water of hydration is replaced and the distance between the particles remain same.

As the crystals of dehydrate grow they contact each other and the setting expansion begins in normal setting reaction the crystals being inhibited become intermershed and entangled much sooner than those on hygroscopic reaction which grow much more freely during the early stage before the intermeshing finally prevents with further expansion, the hygroscopic setting expansion is one of the methods for expanding the casting mold to compensate for the casting shrinkage of the gold alloys.

Commercial investments exhibit different amounts of wax expansion. ADA specification No.2 for such type II investments requires a minimum setting expansion in water of 1.2%, the wax expansion permitted is 2.2%. the factors controlling hygroscopic expansion.

Effect of Composition Proportional to the silica content of the investment the fine the particle size of silica the greater hygroscopic expansion ď Ą-hemihydrate produce more hygroscopic expansion with silica. Should have enough binder with silica, at least 15% of binder is necessary to prevent and drying shrinkage.

Effect of the Water/Powder Ratio (W:P) Higher the W:P ratio less the hygroscopic expansion.

Effect of Spatulation -

Mixing time is reduced hygroscopic expansion decreased.

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Older the investment lower is the setting expansion.

Effect of Time of Investment The greater amount of hygroscopic expansion is observed if the immersion take place before the initial set, the longer the immersion of the investment in the water both is delayed beyond the time of the initial set of the invest. The lower is the hygroscopic expansion.

The effect of confinement Both the normal and hygroscopic setting expansion are confined by exposing forces such as walls of the container in which the investment is placed on the walls of a wax pattern, the confining effect on the hygroscopic expansion is much more pronounced than the normal setting expansion.

The increase in the effective setting expansion when the investment is immersed in a 38째C water bath is caused mainly by the softening of the wax pattern at the water bath temperature permitting an increase in effective setting expansion, softened conditions of wax reduces its confining effect on the expansion of the setting expansion.

Effect of the Amount of Added Water The magnitude of the hygroscopic setting expansion can be controlled by the amount of water than is added to the investment.

Magnitude is in direct proportion to the amount of water added during the period until a maximum expansion occurs also further expansions to evident regardless of any amount of water added, the hygroscopic setting expansion is a continuation of the ordinary setting expansion because the immersion water replaces the water of hydration and thus prevents the confinement of the growing crystals by the surface tension of the excess water. Because of the diluent effect of the quartz particles the hygroscopic expansion in these invest is greater than that of the gypsum binder when used alone.

The phenomenon is purely physical, the hemihydrate binder is not necessary for the hygroscopic expansion. Investment with other binder exhibit similar expansion when allowed to set under water. Expansion can be detected when water is poured into a vessel containing only small smooth quartz particles, the water is drawn between the particles by capillary action and thus causes the particles to separate, creating an expansion. Any water insoluble powder that is wettable can be mixed with

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hemihydrate and hygroscopic expansion results. The greater the amount of silica or inert filler the more easily the added water can diffuse thus the setting material and the greater is the expansion.

The term hygroscopic is a misnomer, although the added water may be drawn into the setting material by capillary action, the effect is not related to hygroscopy.

On the basis of theory the hygroscopic expansion is a normal phenomenon as that which occurs during normal set expansions the terms have gained general acceptance by usage.

Thermal Expansion The thermal expansions of a gypsum bonded investment is directly related to the amount of silica present and to the type of silica employed, the contraction of the gypsum is entirely balanced when the quartz content is immersed to 75%. The thermal expansion curves of the quartz is influenced by particle size of the quartz, the type of the gypsum binder and the resultant water powder ratio necessary to provide a workable mix.

Much greater expansion occurs during the inversion of cristobalite, the normal contraction of the gypsum during heating is easily eliminated. The expansions occurs at a lower temperature because of the lower inversion temperature.

Investments containing cristobalite expand earlier and to a greater extent than those containing quartz.

ADA specifications no.2 requires that the thermal expansion must be not (066%) less than 1% nor greater than 1.6%.

Maximum thermal expansion is obtained at a temperature not higher than 700째C. W:P Ratio More water that is used in mixing the investment the less is the thermal expansion that is achieved during subsequent heating.

Effect of Chemical Modifiers The addition of small amounts of sodium, potassium or libuim chlorides to the investments eliminates the contraction caused by the gypsum and increase the expansion without the presence of excessive silica.

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Strength The strength of the investment must be adequate to prevent fracture or chipping of the mold during heating and casting gold alloy. When the alloy is still quite hot and weak the investment and resist alloy shrinkage by strong and constant dimension. After burnout of the pattern (mold), the strength need be no greater than that required to resist the impact of the metals containing the mold.

ADA specifications no.2 the compressive strength for the inlay investments should not be less than 2.4 Mpa for gypsum.

Other Gypsum Considerations Investments fineness affect the setting time, the surface roughness of the casting, a fine silica results in higher hygroscopic expansion.

Porosity As the molten metal enters the mold, the air must be forced out ahead of it. If not a back pressure builds up to prevent the gold alloy from completely filling the mold, the common method for venting the mold is though pores of investment, the more gypsum crystals, the less is its porosity lower the hemihydrates content and the greater the amount of gauging water used to mix, the more porous it becomes.

More uniform the particles size, the greater the porosity.

Storage Phosphate Bonded Investment The rapid growth of use of metal ceramic restorations and the increased use of higher melting alloys have resulted in an increased use of phosphate or silica bonded investment.

Composition Consists of refractory fillers and binder, the filler is silica, in the form of cristobalite, quartz or a mixture of two – 80% concentration approximately. The purpose of silica is to provide high temperature thermal shock resistance and a high thermal expansion.

The binder consists of magnesium oxide (basic) and a phosphate that is acid in nature.

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Originally phosphoric acid was used, but mono ammonium phosphate has replaced it, because it can be incorporated into the powdered investment.

Newer gold-containing alloys and other alloys used for metal ceramic have higher melting temperature ranges and then contraction during solidification is also greater. This necessitate greater expansion, can be achieved by using colloidal silica suspensions with the phosphate investments, in place of water colloidal silica liquid suspension freeze, should be assessed before winter, freeze solid at low temperature.

Some are made to be mixed with water, for predominantly base metal alloys, a 23% dilution of the colloidal silica is required.

Carbon is often added to the powder to produce clear castings and facilitates the divesting of the casting from the mold, appropriate when the casting alloys is gold not with silver containing and base metal alloys. It is believed carbon embrittles the alloys.

Latest evidence palladium reacts with carbon if heated above 1504째C in this case investment without carbon should be used.

Setting and Thermal Expansion There is a slight expansion during the reaction compared to gypsum products, and this can be increased considerably by using a colloidal silica solution instead of water.

When phosphate investments are mixed with water this exhibit a shrinkage within essentially the same temperature range as gypsum inert (200째C-400째C). this contraction is practically eliminated when a colloidal silica solution replaces.

Some users of phosphate bonded - expansion can be decreased by the increasing the liquid : powder ratio also by decreasing the concentration of the special liquid or by they may use a combination of these methods.

Working and Setting Time Phosphate investments are markedly affected by temperature. The normal the mix, the faster it sets the setting reaction itself gives off heat (this itself gives heat) and this further accelerates the rate of setting.

Increased mixing time and mixing efficiency results in a faster set and a greater rise in temperature. The ideal technique is to mix as long as possible yet have just enough time for investing. Mechanical mixing under vacuum is preferred.

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Ethyl-Silicate – Bonded Investments Involves more complicated and time consuming procedures involved. Used in the construction of the high fusing base metal palladium alloys.

The binder is a silica gel, that reverts to silica cristobalite on heating.

Several method may be used to produce the silica or silicic acid gel binder.

When the pH of sodium silicate is lowered by the addition of an acid salt, a bonding silicic acid gel forms. The condition of magnesium oxide strengthen the gel. An aqueous suspension of colloidal silica can be converted to a gel by the addition of an accelerator, such as ammonium chloride.

Another system for binder formation is based on ethyl silicate. A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water. The solution is then mixed with the quartz or cristobalite to which is added a small amount of finely powdered magnesium oxide to render the mixture alkaline. A coherent gel of polysilicic acid then forms accompanied by a shrinkage. The soft gel is dried at a temperature below 168°C. During the drying process, the gel losses alcohol and water to form a concentrated hard gel, a volumetric contraction accompanies the drying which reduces the size of the mold. This contraction is known as “green shrinkage”, and it occurs in addition to the setting shrinkage.

The gelation process is slow and time consuming certain types of amines can be added to the solution of ethyl silicate so that hydrolysis and gelation occurs simultaneously.

The Sprue Former The purpose of a sprue former or sprue pin is to provide a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated.

With large restorations or prosthesis, such as removable partial denture frame works and fixed partial dentures, the sprue former are made of wax. For smaller casting metal pins can be used, plastic sprue forms are also available.

The diameter and length of the sprue former depends to a larger extent on the 1. Type and size of the pattern, 2. The type of casting machine to be used, 3. And the dimensions of the flask ring in which the casting is to be made.

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Prefabricated sprue formers are available in a wide range of gauges or diameters sprue former gauge selection is often empirical, yet it is based on the following five general principles (Skinner’s).  Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern.  If the pattern is small the sprue former must also be small because a large S.F. attached to a thin, delicate pattern could cause distortion.  However, if the sprue former diameter is too small, this area will solidify before the casting itself and localized shrinkage porosity (“suck back” porosity) may develop. Reservoir sprues are used to help overcome this problem.  If possible, the sprue former should be attached to the portion of the pattern with the largest cross-sectional areas, it is best for the molten alloys to flow from a thick section to surrounding thin areas not the reverse. This minimizes the risk for turbulence.

Porosity : Also, the sprue former orientation should minimize the risk of metal flow on to flat areas of the investment or small areas such as line angles.

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The length of the sprue former should be long enough to be within 6 mm of the trailing end and yet short enough so the molten alloy doesn’t solidify before it fills the mold.

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The type of sprue former selected influences the burnout technique used it is advisable to use a two-stage burnout technique, whenever plastic sprue former or pattern are involved, to ensure complete carbon elimination because plastic sprues soften at temperature above the mounting point of inlay wax.

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Patterns may be sprued either directly or indirectly.

For direct spruing the sprue former provides a direct connection between the pattern area and the sprue base or crucible former area.

In indirect spruing, a connector or reservoir bar is positioned between the pattern and the crucible former – commonly used for multiple single units and fixed partial dentures.

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Also several single limits can be sprued with multiple direct sprue. Reservoir should be added to a spruing network to prevent localized shrinkage porosity.

When the molten alloy fills the heated casting ring, the pattern area should solidify first and reservoir lost. Because of its large mass of alloy and position in the heat centre of the ring, the reservoir remains molten to furnish liquid alloy into the mold as it solidifies. Resulting solidification shrinkage occurs in the reservoir bar and not in the restorations.

Sprue Former Attachment The sprue former connection in the wax pattern is generally flared (telescopic) for higher density gold alloy, but is often restricted for lower density alloys.

Flaring act much in the same way as a reservoir, facilitating the entry of the fluid alloy into the pattern area.

Sprue Former Position sprue former attachment is often a matter of individual judgement, based on the shape and form of the wax pattern.

Some prefer at the occlusal surface, others choose sites such as a proximal wall or just below non functional cusp to minimize subsequent grinding of occlusal anatomy and contact areas, as indicated earlier the ideal area for the sprue former is the point of greatest bulk in the pattern to avoid distorting this areas of wax during attachment, and to permit a smooth flow of the alloy.

Sprue Former Direction The sprue former should be directed away from thin or delicate parts of the pattern, because the molten metal may abrade or fracture investment in this area and result in a casting failure.

It should not be attached at a right angle to a broad flat surface, this will lead to turbulence within the mold cavity and serve porosity in this region, if it is sprued at a 45째 angle to the proximal area a satisfactory casting can be obtained.

Sprue Former Length Length depends on the length of the casting ring, if sprue is short, the resulting mould space may be far from the end of the casting ring that gases can not be

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adequately vented to permit the molten alloy to fill the ring completely (mould space), thus may result in the porosity.

For gypsum bonded should be within 6mm of the open end of the ring, with the higher strong phosphate bonded investments position may be within 3-4 mm of the top of the investment.

For reproducibility of casting accuracy, the pattern should be placed as close to the center of the ring as possible.

Wax Pattern Removal Sprue former should be attached with the pattern on the master die, provided the pattern can be removed directly in line with its path of withdrawal from the die.

Preparation of the Master Die The most commonly used die materials are type-IV (dental stone, high strength) and type-V (dental stone, high strength, high expansion).

Relatively, inexpensive, easy to use and generally compatible with all impression materials. Type-IV stones have a setting expansion of 0.1% or less whereas the harder type-V stones expand as 0.3% this greater expansion is useful for compensation of the relatively large solidification shrinkage of base metal alloys.

To increase the abrasion resistance several means including silver plating, coating the surface with cyanoacrylate and adding a die hardner to the gypsum.

However each may also increase the die dimensions, thus reducing accuracy.

Methods of Altering Die Dimensions To reduce the setting expansion of the type-IV die stone to less than 0.1% there by reducing diameter additional accelerator (potassium sulfate) and retarder (borax) can be added to the gauging water.

To produce relief space for cement, die spacer can be used with a stone die, the most common die spacers are resins. Although proprietary point on liquids are sold for this purpose, model paint, colored nail polish or thermoplastic polymers dissolved in volatile solvents enjoy wide spread popularity.

These spacers are applied in several coats to within 0.5mm of the preparation finish line to provide relief for the cement luting agent and to ensure complete seating of an otherwise precisely fitting casting.

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Die Stone – Investment Combination In this technique (that has been developed) the die material and the investing medium have a comparable composition. A commercial gypsum bonded material, called divestments (whip mix corporation, Louisville KY) is mixed with a colloidal silica liquid. The die is made from this mix and the wax pattern constructed on it.

Then the entire assembly (die and pattern) is inserted in a mixture of divestment and water thereby eliminating the possibility of distortion.

Casting Ring Liners With the use of solid metal rings or casting flasks, the mold may actually become smaller rather than larger because of the reverse pressure resulting from the confinement of the setting expansion.

This effect can be overcome by using a split ring on flexible rubber ring that permits the setting expansion of the investment.

The most commonly used technique to provide investment expansion is to line the walls of the ring with ring liner.

Traditionally, (earlier) asbestos was the material of choice, no longer be used because of its carcinogenic potential.

Two types of non-asbestos ring liner used are aluminium silicate ceramic liner and a cellulose (paper) liner.

To ensure uniform expansion, the liner is cut to fit the inside diameter of the casting ring with no overlap. The cut liner is added in position with sticky wax and then is used with a dry or wet, with a wet liner technique the liner ring is immersed in water for a time and the excess water is shaken away.

Squeezing the liner should be avoided because this leads to variable amounts of water removal and uneven expansion.

Ceramic liner doesn’t absorb water like a cellulose liner, its network of fibres can retain water on the surface.

In the liner the absorbed water causes a semihygroscopic expansion as it is drawn into the investment during setting. A thicker liner material or two layers of liner provide even greater semihygroscopic expansion and also affect a more unrestricted normal setting expansion of the investment in any case, the thickness of the liner should not be less than approximately 1mm.

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The length of the liner remains a matter of controversy. If the liner is shorter than ring, the investment is confined at one or both end of the ring, the longitudinal setting and hygroscopic expansion are thereby restricted as compared with the end where the liner is flush with the ends of the ring.

The expansion of the investment is always greater in the unrestricted longitudinal direction than in the lateral direction that is toward the ring itself.

Therefore it is desirable to reduce the expansion in the longitudinal direction.

Placing the liner somewhat shorter of the end of the ring tends to provide a more uniform expansion; thus there is less chance for distortion of the wax pattern and the mold.

Investing Procedure The wax pattern should be cleaned of any debris, grease or oils. A commercial wax pattern cleaner or a diluted synthetic detergent is used. Any excess liquid is shaken off and the pattern is left to air dry while the investment is being prepared. The thin film of cleaner left on the pattern reduces the surface tension of the wax and permits better “wetting� of the investment to ensure complete coverage of the intricate portions of the pattern.

While the wax pattern cleaner is air drying, the approximate amount of distilled water (gypsum investment) or colloidal silica special liquid (phosphate investment) is measured. The liquid is added to a clean dry mixing bowl, and the powder is gradually added to the liquid care should be taken to minimize air entrapment, mixing be started gently until all the powder has been wet, or the unmixed powder may inadvertantly be ejected from the bowl. Hand mixing is an option.

It is far more common place to mechanically mix all casting investments under vacuum.

Vacuum Mixing Mechanical mixing under vacuum removes air bubbles created during mixing and eliminates potentially harmful gases produced during chemical reaction of the high heat investment.

Once the mixing is completed, the pattern may be hand invested or vacuum invested. For investing by hand, the entire pattern is painted (inside and out) with a thin layer of investment. The casting ring is positioned on the crucible former, and the

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remainder of the investment is vibrated slowly into the ring, with vacuum investing, the same equipment used to mix the investment is employed to invest the pattern under vacuum.

Amount of porosity in vacuum investment is reduced the texture of the cast surface is smoother with better detail reproduction and tensile strength also increases.

In one study it has found 95% of vacuum invested castings were free of nodules where as 17% castings made in hand investment molds were entirely free of defects.

Air bubbles that are remain in the mix, can be entraped on flat or concave surfaces that are not orientated suitably for air evacuation tilting the ring slightly aids in releasing these bubbles so they can rise to the surface.

Excessive vibration is to be avoided it can cause solids in investments to settle and may lead to free water illumination adjacent the wax pattern. Resulting surface roughness. Excessive vibration may also dislodge small pattern from the sprue former with miscast.

If the hygroscopic technique is employed, the filled casting ring is immediately placed as 37째C water bath with crucible former side down. For high heat expansion, the invested ring is allowed to bench cool undisturbed for the time recommended by the manufacturers.

Compensation for Shrinkage A number of factors influence the mold size: 1. Two liners allows a greater setting and thermal expansion than does a single liner.

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2. Setting thermal and hygroscopic expansion can be controlled to a certain extent by varying the liquid : powder ratio of the investment.

3. Lower the L:P ratio greater the potential for expansion, thinner mixes reduces the expansion.

With some investment minor adjustments with L:P ratio is insignificant.

There is a limit to which L:P can be altered if it is too thick, it can’t be applied to the pattern without distorting the pattern and producing air voids. If the mixture is too thin, a rough surface on the casting may result.

In controlling hygroscopic expansion along with L:P ratio can also be regulated either by reducing the time of immersion of the setting investment or by controlling the amount of water to be added during the setting process.

The longer the delay before immersion in the water bath, the less the hygroscopic expansion that occurs.

Increasing the burnout temperature and the water bath temperature increases the expansion and vice versa.

Controlled Water – Added Technique Another technique, in which the shrinkage compensation is controlled by the addition of water during the setting of the investment.

Here the linear hygroscopic expansion increases directly with the amount of water added until a maximal expansion is attained.

The compositions of investments in this technique ensure maximal expansion during immersion in water. The amount of hygroscopic expansion needed is then obtained by adding enough water to provide the desired expansion. A soft, flexible rubber ring is employed instead of the usual asbestos lined metal ring.

A specified amount of water is then added on the top of the investment in the rubber ring and the investment is allowed to set, usually at room temperature. This technique is rarely used, since the hygroscopic expansion method described earlier provides adequate expansion in most cases.

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Causes of Defective Casting Introduction In almost all instance, defects in casting 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.

Defect in casting can be classified under four headings: 1. Distortions 2. Surface roughness and irregularities 3. Porosity 4. Incomplete or missing detail Distortion: Any marked distortion 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.

Some distortion of the wax pattern occurs as the investment hardens around it, the setting and hygroscopic expansion of the investment may produce uneven movement of the walls of the pattern. Eames W.B. O`Neal et al (1978) established that die spacing was one of the most suitable methods to compensate for casting variables and it ensured improved marginal adaptation yet increasing retention by 25 percent.

This type of distortion occur in part from the uneven outward movement of the proximal walls. The gingival margins are forced apart by the mold

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expansion. Whereas the solid occlusal bar of wax resist expansion during the early stages of setting.

The configuration of the pattern, the type of wax, thickness all influence the distortion that occur. Distortion increase as the thickness of pattern decreases, and the less the setting expansion of investment, the less is distortion. There is probably not a great deal that can be done to control this phenomena. However, Grajower R., Lewinstein (1985) found that shrinkage of wax pattern on dies created marginal gap at shoulders and bevels which was attributed to elastic stress in wax. Remodeling of pattern margins by heating marginal wax with spatula was found to improve the adaptation of die. Surface roughness, irregularities and discoloration.

Influence 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 necessitates additional finishing and polishing where as irregularities on the cavity surface prevent a proper setting of an otherwise accurate casting.

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 nodule, that do not characterize the total surface area.

The difference in the surface roughness of the casting and the wax pattern from which it is made is probably related to the particle size of investment and its ability to reproduce the wax pattern in microscopic detail.

Improper technique can lead to a marked increase in surface roughness as well as to the formation of surface irregularities.

Air Bubbles Small nodules on a casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure. Such nodule can sometimes be removed if they are not in a critical area. The best method to avoid air bubbles is to use the vacuum technique.

If manual method is used, various precautions can be observed. The use of a mechanical mixture with vibration both before and after mixing should be practiced

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routinely. A wetting agent may be useful in preventing the collection of air bubbles on the surface of the pattern, but it is by no menas a certain remedy. It is important that the wetting agent be applied in a thin layer. It is best to air dry the wetting agent because any excess will dilute the investment , producing surface irregularities on the casting.

Water Films Wax is repellant to water, and if the investment becomes separated from the wax pattern in some manner a water film may form irregularly over the surface. 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 with pattern, such a condition may result. A wetting agent is of aid in the prevention of such irregularities. Too high a W : P ratio may also produce these irregularities.

Too Rapid Heating It results in fins or spicules on the casting. The mold should be heated gradually ; at least 60 minutes should elapse during the heating from room temperature to 700°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. It is particularly important with the low-heat technique. 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 casting that is virtually impossible to remove by pickling.

Liquid Powder Ratio: 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 high heating casting technique is used, prolonged heating is likely to cause disintegration of the investment and the walls of the mold are roughned as a result. Further more the products of decomposition are sulfer compounds that may contaminate the alloy to the extent that the surface texture is affected. Such contamination sometimes doesn’t respond to pickling. When thermal

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expansion technique is employed the mold should be heated to the casting temperature, never higher than 700째C and the casting should be made immediately.

Temperature of the Alloy : If an alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked and a surface roughness result. 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 produces a rough surface on the casting. A gauge pressure of 0.10 to 0.14 Mpa [15 to 20 psi] 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 influence, the surface texture of the casting. In addition, a 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 carried into the mold with the molten alloy. Carelessness in the removal of the sprue former may be a similar cause.

Any casting that shows sharp well defined deficiences indicates the presence of some foreign particles in the mold such as pieces of investment or bits of carbon from a flux. Bright appearing concavities may be the result of flux being carried into the mold with the metal.

Impact of Molten Alloy The molten alloy, should 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. Sometimes the abraded area is smooth so that it can not 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 can be avoided by proper spruing so as to prevent impact at an angle of 90째 to surface.

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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 too close together. Likewise too many patterns positioned in the same place in the mold should be avoided, the extension of the wax is much greater than that of the investment, causing breakdown or cracking of the investment if the spacing between pattern is less than 3mm.

Carbon Inclusions : From a crucible, improperly adjusted torch or a carbon containing investment can be absorbed by the alloy during casting.

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May lead to the formation of carbide or even a 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 various gold alloys, solders, bits of wire and mixture of different casting alloys should never be melted together and reused. The resulting mixture would not possess the proper physical properties form eutectic or similar alloys with low corrosion resistance.

Discoloration and corrosion may also occur.

A source of discoloration often overlooked is the surface contamination of a gold alloy restoration with mercury. Mercury penetrates rapidly into the alloys and causes a marked loss in ductility and a greater susceptibility to corrosion. Thus it is not a good practice to place a new amalgam restoration adjacent to high noble alloy restoration, it also forms a galvanic circuit leading to the breakdown of the anode i.e., amalgam.

Porosity May occur within the interior region of a casting and on the external surface.

The later is a factor in surface roughness but also it is generally a manifestation of internal porosity.

Internal porosity weaken the casting and extends on the surface it may because for discoloration. If severe it may produce leakage at the tooth restoration interface and 54secondary caries may result. Although the porosity in a casting cant not be prevented entirely, it can be minimized by use of proper techniques.

Porosities in noble metal alloy castings may be classified as follows : I. Solidification defects.

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a. Localized shrinkage porosity. b. Microporosity.

II. Trapped gases. a. Pinhole porosity. b. Gas inclusion porosity. c. Subsurface porosity.

III. Residual air : Localised shrinkage porosity is generally caused by incomplete feeding of molten metal during solidification. The linear contraction of noble metal alloys in changing from 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 feeding of molten metal through the sprue to make up for shrinkage of metal volume during solidification. If the spure 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 caused by the ability of the pontic to retain heat because of its bulk and because of it is located in the heat centre of the ring.

This problem can be solved by attaching one or more small gauge sprues at the surface most distant from the main sprue attachment and extending the sprue laterally within sprue of the edge of ring. These small (auxiliary) sprues ensures that solidification begins within sprues and they act as cooling pins to carry heat away from the pontic.

Localized shrinkage generally occur near sprue casting junction but it may occur any where between dendrites where the last part of the casting that solidified was in the low melting metal that remains as the dendrite branches 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 from the sprue channel on a point of the mold wall. This hot spot causes the local regions to freeze last and result in what is called suckback porosity.

This often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well roudned. The entering metal impinge onto the mold surface at this point and

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creates a higher localized mold temperature in this region that is called a 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 suck back porosity. Suck back porosity can be eliminated by flaring the point of the sprue attachment and by reducing the mold melt temperature differential, that is lowering the casting temperature by about 30째C. Microporositiy also occur from solidification shrinkage but is generally present in fine grain alloy castings when the solidification is too rapid for the microvoids to segregate to the liquid pool. This premature solidification causes the porosity in the form of small irregular voids.

Such phenomenon can occur from the rapid solidification of 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 many case it is generally not a serious defect.

Both the pinhole and the gas inclusion porosities are related 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 in molten state. For e.g. both copper and silver dissolve oxygen in large amounts in the liquid state, molten platinum and palladium have a strong affinity for H2 as well as oxygen.

On solidification the absorbed gases are expelled and the pinhole porosity results.

The larger void 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 on that is incorporated during the casting procedure.

All castings 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.

The porosity that extends to the surface is usually in the form of small pinpoint holes, when the surface is polished other pinholes appear.

Larger spherical porosities can be caused by gas occluded from a poorly adjusted torch flame, or the use of the mixing or oxidizing zones of the flame rather

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than the reducing zone. These types of porosity can be minimized by premelting the gold alloy on a charcoal or a graphite block if the alloy has been used before and by correctly adjusting and positioning the torch flame during melting.

Subsurface Porosity : Occurs due to simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mold walls. This type of porosity can be eliminated by controlling the rate at which the molten metal enter mold.

Entrapped Air Porosity On the inner surface of the casting. Sometimes referred to as back pressure porosity. Can produce 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 displaces 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 escape.

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. 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 Casting Occassionally a partially complete or perhaps no casting at all is found because that the molten alloy has been prevented in some manner, from completely filling the mold.

The two factors responsible are: -

Insufficient venting of the mold and

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-

High viscosity of the fused metal.

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 doesn’t fill the mold before it solidifies. In such a case the magnitude of the pressure should be suspected.

The pressure should be applied at least 4 seconds. The mold is filled and solidified in 1 second or less yet it is quite soft during the early stages.

These failures have castings with 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 combusion remains in the mold the pores in the investment may become filled so that the air cant be vented completely. If mixture or particles of air remain, the contact of the molten alloy with these foreign substances produce an explosion that may produce sufficient back pressure to prevent the mold from being filled.

The rounded margins are quite shiny in some cases because of the strong reducing atmoshpere created by carbon monoxide left by the residual wax.

A lower L:P ratio of the investment has been associated with less porosity. An increase in casting pressure during casting solves this problem.

Different alloy compositions probably exhibit varying viscosities in the molten state, depending on composition and temperatuere. 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. Temperature of the alloy should be raised higher than its liquidus temperature so that its viscosity and surface tension are lowered and its doesn’t solidify prematurely as it enters the mold. Such premature solidification may amount for the greater susceptibility of the white gold alloys to porosity because their liquidus temperature are higher thus they are more difficult to melt with a gas air flame.

To gain an understanding of dental materials we need a basic knowledge of matter and its behaviour during handling.

Assuming that the wax pattern is satisfactory, the procedure techniques become a matter of enlarging the mold uniformly and sufficiently to compensate for the casting shrinkage of the gold alloy. Theoretically, if the shrinkage of the wax and the gold alloy are known, the mold can be expanded an amount equal to such shrinkages and the problem is solved. There are variables in the behaviour of the materials involved, especially the wax that cannot be rigidly controlled.

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Summary and Conclusion The overall dimensional accuracy possible with current technique has never been clearly defined.

Neither the allowable tolerance of accuracy in the fit of the casting nor that obtainable during the procedure is known. (In the last analysis the procedure is partially expired and a matter of routine procedure. The latter should be rigidly followed.

There are however many steps in the procedure for which a considerable number of facts are known and there are also certain variations in technique described have produced equally satisfactory results. However, any technique involves strict adherence to certain fundamental principles that are common to all.

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