Сraig. Dental Materials

wax is not completely eliminated, very fine particles of carbon cover the investment pores through which the gas in the mold cavity is supposed to escape. Depending on the amount of carbon remaining on the walls of the mold cavity when the molten metal enters, the casting may be complete but black in color (see Fig. 17-21, B), or it may be incomplete (see Fig. 17-21, 0.The black color in this instance cannot be cleaned by routine pickling action, which is described later in this chapter, because most pickling solutions are acidic and most acids are not effective in removing carbon from the surface of noble and high-noble alloys.

However, if the black color of the gold casting is removed by the normal pickling procedure, it was caused mainly by copper oxides formed during casting. Most casting investments contain some reducing agents to provide a reducing atmosphere when the molten alloys are entering the mold cavity. If the investment mold is left too long in an oven, all the deoxidizers will be decomposed and eliminated. Thus when the molten alloy enters the mold cavity,the oxidation of the copper is not prevented, and the casting will be black. Some investments routinely produce blackened castings because the manufactures do not add any deoxidizing agents. Also, if an oxidizing flame is used in melting the alloy, the casting will be black. Castings that are black because of oxidation of some of the elements in the alloy can easily be cleaned, and the original alloy color can be restored by normal pickling procedures.

Insufficient casting pressure or underheating of alloys usually results in castings with rounded margins (see Fig. 17-21, 0.The rounded margins are caused by freezing of the alloy before the gases are forced from the mold cavity. As mentioned previously,the slow escape of mold gases can be caused by the investment being too thick over the end of the ring or by a phosphatebonded investment that has a glassy surface coating that has not been removed. However, rounded margins can also be caused when the alloy cannot exert enough force while entering the mold cavity. This situation can be caused by an underheated alloy that is too viscous or does

not stay in the liquid state long enough to force the gases out and reach the marginal areas of the mold. Alternatively, the rounded margins can be caused when the casting machine is underwound and the force on the molten alloy is insufficient to drive the alloy into the mold before the alloy freezes.

Finally, investment errors can result in miscasts that are apparent only after the casting process is complete. Perhaps the most common problem is the trapping of air in the investment during the investing process (see Fig. 17-21, D, E). These air bubbles may occur on the outside or inside of the casting. If they occur on the outside (see Fig. 17-21, D), they are theoretically removable, but often at great expense of time and money (lost metal). However, the fit of the casting is not effected. If the voids occur on the margins or inside of the casting (see Fig. 17-21, E),successful removal is extremely difficult and often the restoration will have to be recast.

CLEANING AND PICKLING ALLOYS

The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy (see discussion on fluxes later in this chapter). Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.

Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution (Fig. 17-23).Today, most commercially available pickling solutions are not made of

Fig. 17-13 Pickling an alloy. After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition (see Fig. 17-21, B). The effect of the solution can be seen by comparing the submerged surfaces to those that have still not contacted the solution.

(Courtesy Dr. Carl W. Fairhurst, Medical College of Georgia School of Dentistty.)

the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.

There are several important notes about pickling. With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should be avoided. This practice may alter the phase structure of the alloy or warp thin

castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.

GENERAL SUGGESTIONS FOR SOLDERING

The details for dental soldering are adequately described in textbooks related to other fields of dentistry. Certain principles of the freehand and investment techniques are emphasized here. For successful soldering, both the technique and the selection of solder are important. Many of the materials and techniques used in casting are also useful in soldering.

Dental solders are supplied in a variety of shapes, such as strips, rods, wires, or cubes, each of which is convenient for certain operations. Thin strips are the conventional form for general applications, and small cubes, approximately 1-mm square, are convenient for soldering a contact area on an inlay or crown. Rods are often notched along two sides, which permits them to hold flux better than a smooth form. Also, when melted, the notched forms do not roll back into a ball as easily as smooth forms. Choosing a particular shape depends on the operation to be performed; each shape is available in a range of fineness.

Proper application of fluxes provides a protective coating that prevents oxidation of the solder and parts being soldered. The jlux dissolves surface oxides and allows the melted solder to wet and flow onto the adjoining alloy surfaces. The fluxes used for soldering gold alloys are combinations of borax and boric acid, with potassium fluoride added to some. Too much flux is as undesirable as not enough flux. See the discussion on fluxes at the end of this chapter.

The careful and skillful use of the soldering torch flame is important to a high quality soldered joint. A well-defined, not-too-large,

pointed flame is advisable for the final heating of solder in a localized area, but a larger, less welldefined flame of the "brush" type may be used for the initial heating. The flame should not be applied directly to the parts to be joined until the flux has melted and formed a uniform layer over the surface. Applying a hot flame to the flux too quickly causes the flux to form droplets rather than a film. Once the flame has been applied to the spot to be soldered, it should not be withdrawn until soldering is completed. A protective envelope should be created around the spot to be soldered with the reducing portion of the flame. The operation should be completed in the shortest time possible to avoid oxidation of the base metal ingredients of the alloys involved and to prevent damage to the microstructures of the alloys.

Overheating during the soldering operation will cause (1) pits in the solder, (2) penetration or burning through thin sections, and (3) a loss of strength because of diffusion of the solder into the other metals or loss of fibrous microstructure. Underheating will cause pitting from the retention of unmelted flux and failure of the solder to flow and adhere to all surfaces. Good wetting of the surfaces by the solder is imperative for satisfactory joining of the parts. Solder that is not well heated tends to "ball" and fails to spread properly. Both overheating and underheating will result in a weaker solder joint. For this reason, the proper and careful heating and fluxing of the solder are essential.

Sometimes the flow of the solder should be restricted from parts of the restoration, such as the margins or the occlusal grooves. Flow into these areas can be prevented with an antzJux material, which should be applied to the surface before the flux or solder is applied. Solder does not flow into an area contaminated with graphite; a soft lead pencil is therefore an effective antiflux. Other materials, such as rouge (iron oxide) or whiting (calcium carbonate) in an alcohol and water suspension, are effective antifluxesfor prolonged heating or high-temperature heating, which can burn off the graphite.

The distance between the parts to be joined can influence the accuracy of the final appliance. If the parts are in intimate contact, they tend to expand and push apart on heating, whereas if the distance is too great, the parts tend to draw together as the solder solidifies. A clearance of a few hundredths of a millimeter (0.13 mm) between the parts to be joined is optimal when using the investment-soldering technique. This thickness can be practically estimated by the thickness of a typical business card. When attempting free hand soldering of wires for orthodontic or other appliances, a closer adaptation of the parts is possible. The shape of the joint and the purpose of the appliance will ultimately determine the appropriate distance between the soldered parts.

Despite certain well-established principles, soldering is an art. The timing and type of heating, selection of the type and shape of the solder, contour of parts to be joined, and application of flux cannot always be dictated by specific rules. Ultimately, the experience of the operator plays a significant role in any successful soldering operation.

INFRARED SOLDERING

Instead of using a torch to provide heat, an infrared heating unit is available specifically for dental soldering. The unit uses the light from a 1000-watt,tungsten-filament, quartz-iodine bulb, which is mounted at the primary focal point of a gold-plated, elliptical reflector.The material to be soldered is placed at the reflector's secondary focal point, at which the reflected infrared energy of the tungsten light source is focused. This equipment, shown in Fig. 17-24,can be used for the high-temperature soldering of alloys for porcelain-fused-to-metal bridges at 1150' C. The main problem in the use of this unit is locating the focal center of the light on the spot to be soldered. The infrared energy must be focused on the crowns and not on the solder itself. Failure to focus on the right spot on the crown can result in cold joints that are porous.

Fig. 17-24 An infrared soldering unit that uses a focused light beam for melting the solder. The appliance is placed Into the lower half of the unit and the infrared beam is focused on the area to be soldered.

CASTING AND SOLDERING

A J u x is a substance applied to the surface of molten metal primarily to prevent oxygen from contacting the hot metal and thereby causing oxidation. In addition, flux dissolves oxides that may form while the metal is heated; the resulting solution of oxides or other extraneous matter in the flux constitutes a slag. A flux also facilitates the free flow of solder; it helps solder to wet and spread over the metal surface. For a flux to be effective, it must have a fusion temperature below that of the alloy being heated, but should not burn or volatilize readily.

NOBLE AND HIGH-NOBLE ALLOYS

Borax, or sodium tetraborate ((Na,B,,), . 10H20), can dissolve the metal oxides (mainly copper oxides) that occur on noble and high-noble alloys, and therefore is commonly used as a flux in dentistry. Dehydrated sodium tetraborate is known as borax glass and can be used in the dry powder form. When melted, borax glass is a clear, viscous liquid that does not volatilize readily when heated. It can be made more fluid by adding boric acid or other salts, which aid in spreading of the flux on the hot metal surface. Boric acid is not used alone for fluxing purposes, as is sometimes done with borax. Dehydrated borax glass is preferred to the ordinary hydrated borax, which liberates water vapor on being heated and, as a result, effervesces and bubbles up over the surface of the hot meal without forming an effective surface covering.

Fluxes are available in a variety of forms, which are usually designated for specific applications in soldering or casting operations. Liquid flux is principally a solution of borax and boric acid in water, and is applied in soldering operations of orthodontic appliances and bridge structures in which a minimum of flux is desired. A saturated solution, or lower concentration, of the ingredients may be used, and smaller quantities of other salts, such as potassium carbonate or ammonium chloride, may be added to some liquid fluxes. Also available are paste fluxes formed from mixtures of about one-third borax added to a mineral grease, such as petroleum jelly, with other chemicals added as desired. Pastes are convenient for soldering operations in which a large quantity of flux is desired and its application is directed to a specific area. Powder fluxes, which are normally used for metals during the melting for a casting operation, are sprinkled lightly on the metal. Powder fluxes may contain finely divided charcoal or other ingredients mixed with borax and boric acid; this combination gives added protection by producing what is described as a reducingJux. The charcoal not only helps prevent the formation of oxides on the metal surface, but also reduces oxides that have

already formed to free metal. Fluxes in the powder form also may contain a vely small percentage (1% to 2%) of finely divided silica flour, which holds the molten flux in position on the surface of the hot metal.

CHROMIUM-CONTAINING ALLOYS

When soldering stainless-steel or cast cohaltbased alloys that contain chromiuin, a special flux is required because borax and boric acid alone do not dissolve chromium oxides. Normally the fluxes for these soldering operations should contain about 50% to 60% potassium fluoride, or another fluoride, mixed with 25% to 35% boric acid (H3B0,), 6% to 8% borax glass, and 8%to 10% potassium or sodium carbonate. An equal mixture of boric acid and the fluoride salt may he formed into a paste for soldering when they are ground together with a few drops of water, which is considered effective in such soldering operations. Other similar compositions have been recommended. Pastes should not be

formulated with petroleum grease for these applications because the carbon formed in heating alters the properties of the alloy being soldered or cast.

Choosing the proper flux (borax or fluoride) for the soldering operation is important. The choice of flux is dictated by the type of alloy to be soldered or cast, not by the type of solder used. If the alloy contains chromium, such as stainless steel wires for orthodontics or cobalt-chromium alloys for partial dentures, the proper choice is fluoride flux, regardless of whether goldor silver-based solder is used. If noble or high-noble alloys are to be soldered, the proper choice is borax flux, regardless of the solder employed.

A principle governing the use of fluxes for any purpose is that neither too much nor too little should be applied to the metal during the heating operation. When too little flux is applied, it tends to burn off and be ineffective; when too much flux is applied, it may become entangled in the molten metal to produce a defect by inclusion.

1 SELECTED PROBLEMS

Problem 1

An invested casting ring was removed from the oven in preparation for melting and casting the alloy; however, there was a delay of several minutes before the alloy was cast. The casting did not fit. What was the probable cause of this problem, and how can it be corrected?

Solution

When the casting ring is removed from the oven and placed into the casting machine, the investment rapidly cools and contracts. A delay of more than a minute generally results in sufficient shrinkage to produce an undersized

casting. The recommended procedure is to premelt the gold alloy in the crucible of the casting machine before removing the casting ring from the oven and to cast promptly. Reheating the ring is not recommended because the contraction of most investments is not fully reversible. Allowing the investment to cool too much also risks its fracture during the rapid contraction.

Problem 2

A casting that did not seat well had external and internal surface irregularities. How were these surface irregularities produced, and how might they be avoided?

Solution a

Air bubbles become attached to the wax pattern during investing, thereby producing nodules on the casting. Their removal can alter the fit of the casting. To avoid air bubbles, a wetting agent should be applied to the wax pattern, and the investment should be mixed and the pattern invested under vacuum.

Solution b

When too much water is used in mixing the investment, a rougher surface of the casting can result. The powder and liquid of the investment should be dispensed accurately.

Solution c

The position and direction of the sprue affect the turbulence of the molten alloy as it is introduced into the mold cavity. Localized roughness can result from abrasion of the mold. Attach the sprue to the bulkiest portion of the wax pattern. The point of contact should be flared, and the sprue should be directed toward the margins.

Solution d

Prolonged heating of a gypsum-bonded investment at a temperature above 650" C caused a breakdown of the investment, resulting in a rough surface on the casting. Casting should be completed promptly when wax burnout has been achieved.

Solution e

Phosphate-bonded investments produce water droplets as a byproduct of the reaction. These droplets can attach to the pattern and result in a positive defect in the casting. This problem can be prevented by accurately measuring the investment and liquid solution and following the mixing times and procedures exactly.

Problem 3

A full-crown casting was made by reheating an invested casting ring; the ring had been

heated to the burnout temperature the day before, then cooled when the procedure could not be completed. The resulting crown did not fit. What caused this problem, and how can it be corrected?

Solution

Irreversible chemical and physical changes in the binder and refractory of the investment occur when the investment is heated and then cooled. On reheating, the dimensional changes of the mold will be less than normal. Therefore, the casting was likely too small. If a casting procedure cannot be completed promptly with a heated investment, a new wax pattern should be invested and cast.

Problem 4

A soldered joint showed a large amount of porosity. What caused the porosity, and how can it be avoided?

Solution

Porosity in the soldered joint is usually associated with excessive heat applied during the fusion of the solder or with improper fluxing. Scrupulously clean and properly flux the surfaces to be soldered. The reducing part of the flame should be used, and the solder should be heated only as much as necessary to ensure adequate flow.

Problem 5

During a soldering operation, the fine margins of a casting were fused. What factor produced this problem, and how can it be avoided?

Solution a

If the layer of soldering investment protecting the margins of a casting is too thin or is porous, the refractory material will not be able to act effectively as a thermal barrier. The margins of a casting should be protected with a thick layer of investment carefully painted onto the occlusal areas and along the proximal walls.

Solution b

If the soldering investment is mixed with a high watedpowder ratio, the strength of the investment will be low. Portions of the investment may fracture during heating, thereby exposing margins to the soldering operation. The powder and liquid of the soldering investment should be dispensed accurately.

Solution c

Overheating the investment assembly for a long period can transfer heat to the protected margins, thus causing them to melt. Adequate temperature should be reached in the furnace, after which the reducing flame of the torch should be applied properly. The solder should be fused promptly.

Problem 6

During a soldering operation, the flow of the solder appears good, but the soldered crown no longer fits the die or the tooth after soldering. What could have happened?

Solution a

If the solder flowed onto the margins of the crown or into the interior of the crown, it will prevent the crown from seating properly on the die or tooth. This can be prevented by using antiflux appropriately to prevent the flow of solder into undesirable areas. The judicious use of soldering investment can also protect the margins somewhat.

Solution b

If the crown was heated to too high a temperature, then sag or other distortion of the crown can occur, which then prevents the crown from fitting. For this reason, a solder should be selected that has a liquidus at least 50" C less than that of the crown. Furthermore, the solder and crown should be heated only to the extent necessary to melt the solder.

Problem 7

A full-crown wax pattern was sprued to the bulkiest portion with a single sprue. After casting, porosity was observed on the pulpal floor of the gold crown. What could have caused the problem, and how can the problem be avoided on the next casting?

Solution

The porosity, commonly known as suckback porosity, resulted from improper freezing of the alloy so that molten alloy could not compensate for shrinkage of the solidifying alloy. Several conditions could have caused the problem: (1) The diameter of the sprue may have been too small or the length of the sprue too long, resulting in the alloy's freezing in the sprue before freezing in the crown. A shorter, larger-diameter sprue should be used. (2) The single sprue directed the molten alloy to an area of the pulpal floor, and the investment in this area became much hotter than the surrounding investment. This condition, coupled with the poor thermal conductivity of the investment, may have caused the alloy in this area to stay molten after it had frozen in the sprue. The direction of the sprue should be toward the margins of the restoration, and a Y-shaped sprue should be used to distribute the flow of the alloy more uniformly. Increasing the temperature of the high-heat investment before casting reduces the differences in the temperature of the investment from the impingement of the molten alloy. (3) The amount of alloy forming the sprue button may have been too small in relationship to the size of the pattern; again, the sprue and button froze before the alloy in the crown, causing the porosity.

Problem 8

A removable, cobalt-chromium partialdenture alloy was melted with an oxygen-

acetylene flame. There were problems adjusting the torch, however, and the alloy was heated three times longer than normal. What, if any, problems might be expected with the cast framework?

Solution

Extended heating of the alloy with such a flame will probably increase the carbon content of the cast alloy. The alloy contains several elements that form carbides under such conditions, and their precipitation increases the brittleness and decreases the elongation of the alloy. The clasps of the cast partial denture will therefore be susceptible to fracture during minor adjustments.

Problem 9

A low-fusing point (Be-containing) nickelchromium alloy was to be cast to embedded platinum-gold-palladium (PGP) wire in the preparation of a partial-denture framework. A gypsum-bonded investment was used, but after the investment had been heated to the casting temperature, casting of the alloy was delayed for four hours. Later, when the PGP wire was bent to form a clasp, it was brittle and broke. What caused this embrittlement and could it have been prevented?

Solution

Low-fusing point nickel-chromium alloys melt at substantially higher temperatures than gold-based alloys, just under 1300" C (compared to 900' to 1100' C). Therefore special gypsum-bonded investments are often used that contain oxalic acid, oxalates, carbonates, and carbon, which release CO, at various temperatures during heating and keep the mold cavity flushed with CO, to remove any sulfur oxides from the calcium-sulfate binder. On extended heating these CO, sources are used up and the sulfur oxides react with the PGP wire, causing embrittle-

ment. Embrittlement will also occur if a gypsum-bonded casting investment (designed for gold alloys) that does not contain these protective chemicals is used, even if there is no delay in the casting procedure. Thus casting of the nickel-chromium alloy to the embedded PGP wire should be done promptly after the investment has reached proper temperature.

Problem 10

Why is it necessary to prepare a duplicate investment cast from the master stone model when preparing a wax pattern for partialdenture framework?

Solution

The large wax pattern is too fragile to be handled and sprued as a free-standing wax pattern as is done for crowns and bridges. Waxing must therefore be done on a cast that becomes part of the investment mold. If the stone model (master cast) were used, it would not survive the temperatures necessary during casting and burnout, and its decomposition during heating would embrittle the alloy. Furthermore, the model would be destroyed in the process of casting and there would then be no model on which to add the acrylic portions of the partial denture. Thus a duplicate model, called the refractory cast, is made and the wax pattern is constructed on this model and invested in place. After casting, the framework (now in metal) is transferred to the master cast for subsequent steps in the fabrication process.

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