CASTING PROCEDURES

Introduction

• Casting is the process by which a wax pattern of a restoration is converted to a replicate in a dental alloy.

• The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures.

Casting Procedure

The various steps in casting procedure are as follows : -

• Preparation of die

• Wax pattern

• Investing of the pattern

• Burnout of the wax

• Casting

• Cleaning of the casting

MASTER DIE Positive likeness of a single tooth made by

sectioning the cast which in turn is made out of an impression

Wax PatternThere are two fundamental ways to prepare a

wax pattern for a dental restoration : -

In the direct method the pattern is prepared on the tooth in the mouth. This method can be used for small inlay restorations.

In the indirect method a model ( die ) of the tooth is first made and the pattern is made on the die. The indirect method is used for all types of restorations

INVESTMENTSDEFINITIONS:

Invest: To surround or envelop in an investment material..Investing: The process of covering or enveloping, wholly or in parts; an object such as a denture, tooth, wax form, crown etc. with a suitable investment material before processing, soldering or casting.

Investment (K.J. Anusavice) refractory material used to form a mould casting for cast metals or hot pressed ceramics.

Classification according to Binders used:

GYPSUM BONDED INVESTMENT:- alloys that fuse below 1975F (1080c)- Casting of gold alloys inlays, onlays, crowns and fixed

partial dentures.

PHOSPHATE BONDED INVESTMENTS:- for higher fusing alloys- Designed primarily for alloys used to produce copings or

frame works for metal ceramic prosthesis and for some base metal alloys. It can also be used for press able ceramics.

ETHYL SILICATE- BONDED INVESTMENTS:- for casting of removable partial dentures with base metal

alloys (cobalt based or nickel based alloys)

Armamentarium

Vacuum mixer and bowl Vibrator Investment powder (gypsum or phosphate

bonded) Water or colloidal silica Spatula Brush Surfactant Casting ring and liner

In this technique, pattern is first painted with surface tension reducer; the surface must be wet completely. The procedures is as follows:

1. Add investment powder to the liquid in the mixing bowl and quickly incorporate it by hand.

2. Attach the vacuum hose to the bowl, evacuate the bowl, and mechanically spatulate.

The mixing should be carefully timed in accordance

with the manufacturer’s instructions and the type of mixing bowl used (high speed versus low speed).

3. Coat the entire pattern with investment, pushing the

material ahead of the brush from a single point. Gently vibrate through out the application of investment, being especially careful to coat the internal surface and the margin of the pattern.

After the pattern has been completely coated, the ring is immediately filled by vibrating the remaining investment out of the bowl.

4.When the investment reaches the level of the pattern, tilt the ring several times to cover and uncover the pattern, thereby minimizing the possible entrapment of air. Investing must be performed quickly within the working time of the investment

5.After the ring is filled to the rim, allow the investment to set.

Wax pattern removal • The sprue former should be attached to the wax

pattern with the pattern on the master die, provided the pattern can be removed directly in line with its path of withdrawal from the die.

• Any motion that might distort the wax pattern should be avoided during removal.

• The gauge selection and design for the sprue former are often empirical , but optimal performance during the casting process is based on the following five general principles.

Sprue Diameter • Select a sprue former with a diameter that is

approximately the same size as the thickest area of the wax pattern.

• If the the pattern is small , the sprue former must also be small, because attaching a large sprue former to a thin, delicate pattern could cause distortion.

• On the other hand, if the sprue former diameter is too small , this area will solidify before the casting itself and localized shrinkage porosity may develop.

• Reservoir sprues are used to help to overcome this problem

Sprue Position • Some clinicians prefer placement at the occlusal

surface, whereas others choose sites such as a proximal wall or just below the non functional cusp to minimize the subsequent grinding of occlusal anatomy and contact areas.

• The ideal area for the sprue former is the point of greatest bulk in the pattern to avoid distorting thin areas of wax during attachment to the pattern and to permit complete flow of the alloy.

Sprue Attachment • The Sprue former connection to the wax pattern is

generally flared for high density gold alloys, but it is often restricted for lower density alloys.

• The sprue former should be attached to the portion of the pattern with a largest cross sectional area.

• It is best for the molten alloy to flow from a thick section to surrounding thin areas ( eg. The margins) rather than the reverse.

• This design minimizes the risk for turbulence

• Patterns may be sprued either directly or indirectly.

• For direct spruing , the sprue former provides a direct connection between the pattern area and sprue base.

• With indirect spruing a connector or reservoir bar is positioned between the pattern and crucible former.

• It is common to use indirect spruing for multiple single units and fixed partial dentures, although several single units can be sprued with multiple direct sprue formers.

• A reservoir should be added to sprue network to prevent localized shrinkage porosity.

• When the molten alloy fills the heated casting ring, the pattern area should solidify first and the reservoir last.

• Because of its large mass of alloy and position in the heat center of the ring , the reservoir remains molten to furnish liquid alloy into the mold as it solidifies.

• The resulting solidification shrinkage occurs in the reservoir bar and not in the prosthesis.

Sprue direction

• The sprue former should be directed away from any 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 .

• The sprue former should not be attached at a right angle to a broad flat surface.

• Such an orientation leads to turbulence within the mold cavity and severe porosity in this region.

Sprue length

• The length of the sprue former depends on the length of the casting ring.

• If the sprue is too short , the wax pattern may

be so far removed from the end of the casting ring that gases cannot be adequately vented to permit the molten alloy to fill the ring completely.

• When these gases are not completely eliminated, porosity may result

• Sprue length should be adjusted so that the top of the wax pattern is within 6mm of the open end of the ring for gypsum bonded investments.

• With the higher strength phosphate bonded investments it may be possible to position the wax pattern within 3 to 4 mm of the top of the investment for reproducibility of casting accuracy, the pattern should be placed as close as possible to the center of the ring.

Casting ring liner

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

• Traditionally, asbestos was the material of choice, but it can no longer be used because its carcinogenic potential makes it a biohazard.

• Two types of non asbestos ring liner materials have been produced ;

- An aluminosilicate ceramic liner and

- A cellulose (paper) liner

BURNOUT OF THE WAX PATTERN

• After the wax pattern has been invested, it should be set aside until the investment has hardened, usually 45 to 60 minutes.

• Investment must withstand the impact force exerted by the molten metal when it enters the hot mold during casting.

• During burnout, the mold is placed in an oven to completely eliminate the wax, thereby forming a cavity into which the molten metal is cast.

• During wax elimination, the investment expands thermally, which is necessary to compensate for the casting shrinkage.

• Although wax melts at a comparatively low temperature, its complete elimination requires much higher temperatures.

• When the molten metal enters the sprue hole, the resulting force causes the air in the mold cavity to be driven out through the pores of the investment, thus the mold cavity is filled completely.

• This process takes less than a second.

• When heated to higher temperatures, any organic material decomposes and forms carbon dioxide, water, or nitrogen oxides, all of which are gases and can be easily eliminated.

• However, formation of these gases depends on the presence of a sufficient supply of oxygen, the relatively high temperature of the oven, and adequate heating time of the ring

• A satisfactory way of eliminating the wax pattern is to set the mold in the furnace with the sprue hole placed downward at first, so most of the wax drains out and is eliminated as a liquid.

• The ring is then inverted with the sprue hole placed upward.

• In this position the oxygen in the oven atmosphere can circulate more readily into the cavity, react with the wax, and form gases rather than the fine carbon that interferes with the venting of the mold cavity.

Casting Machines: Alloys are melted in one of the four following

ways, depending on the available types of casting machines.

1) The Alloy is melted in a separate crucible by a torch flame and is cast into mold by centrifugal force .

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

1) The Alloy is melted in a

• 3) The Alloy is melted by induction heating, then cast into the mold centrifugally by motor or spring action.

• 4) The Alloy is vacuum arc melted and cast by pressure in an argon atmosphere. In addition to these 3 melting machine the molten metal may be caste by air pressure , by vacuum , or both.

Cleaning the casting

• Often the surface of the casting appears dark with oxides and tarnish.

• Such a surface film can be removed by a process known as pickling, which consists of heating the discolored casting in an acid.

• One of the best pickling solutions for gypsum

bonded investments is 50% hydrochloric acid solution.

• The hydrochloric acid aids in the removal of any residual investments, as well as the oxide coating.

• The disadvantage of hydrochloric acid is that the fumes from the acid are likely to corrode laboratory metal furnishings.

• In addition these fumes are a health hazard and should be vented via fume hood.

• A solution of sulfuric acid may also be more advantageous in this respect.

• The best method for pickling is to place the casting in a test tube or dish and to pour the 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 pickling, the acid is poured off and the casting is removed.

• The pickling solution should be renewed frequently, because it is likely to become contaminated after reusing the solutions several times

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