Prosthodontics Wax Patterns PDF

DIES AND WAX PATTERNS  INTRODUCTION The wax pattern is a precursor of the finished cast restoration that will be placed on the prepared tooth. Careful handling and manipulation...

DIES AND WAX PATTERNS  INTRODUCTION The wax pattern is a precursor of the finished cast restoration that will be placed on the prepared tooth. Careful handling and manipulation of the wax pattern is required to obtain an accurate casting DIE It is the positive reproduction of the form of a prepared tooth in any suitable substance DEFINITIVE CAST A replica of the tooth surfaces, residual ridge areas, and/or other parts of the dental arch and/or facial structures used to fabricate a dental restoration or prosthesis  DIE MATERIALS Type IV (high strength) dental stone. Type V (high strength and expansion) dental stone Resin strengthened gypsum products Resin dies – epoxy, polyurethane Electroplated dies Flexible die materials  DIE PREPARATIO N Thicknes s of die Cyanoacrylates: 1.0 to 2.5um hardener Acrylic lacquers: 4.0 to are: 10um DITCHING THE DIE Ditching or trimming the die defines the position of the margin and acts as a guide to gingival contour when the restoration is being waxed. Excessive trimming does not give the correct emergence profile and may lead to an over-contoured or bulky crown. DITCH BELOW MARGIN LINE THE MARGIN APPLY DIE HARDENER ABOVE AND BELOW MARGIN LINE FIRST RELIEF COAT BLOCK OUT WAX ADDITIONAL RELIEF COATS  Applied to die to increase cement space between axial walls of prepared tooth and restoration DIE Formulated to maintain constant thickness SPACER Should not coat entire preparation 1 mm space from the margin must be maintained Available as a paint on or pen type application  Die spacer is needed to provide space for the luting agent (cement) during cementation of the finished crown. When applying the die spacer over the DIE preparation leave the area 1mm above the SPACER margin line free of spacer. Close adaptation of the crown and cement (or luting agent) No disintegration and dissolution of the luting agent at the margin.  MARKING MARGINS Precise marking of preparation margin is crucial Color used for marking should contrast wax Ordinary lead pencil not recommended Marked margin can be coated with cyanoacrylate; blown dry Side of colored pencil used to keep line width minimal  A metal pin used in stone casts to remove die DOWEL sections and replace them accurately in the original PIN position TYPES OF DOWEL PINS Tapered, flat-sided brass dowel pin Flat-sided, stainless steel dowel pin Curved, single dowel pin Single dowel Double dowel Two separate dowels Horizontal contact tracks and vertical ribs  STRAIGHT DOWEL PIN – Commonly used for many years. – Brass dowel pin is used Advantages Resists horizontal displacement Removable die facilitates wax up and ceramic build up No special equipments required Disadvantages Technical skill is needed  CURVED DOWEL PIN Incorporated into the impression before or after the stone is poured.  SECTIONING REMOVABLE DIES Trim buccal and lingual sulcal areas adjacent to removable areas Mark intended saw cuts in pencil Saw cuts – parallel or converge Avoid undercuts Carefully position saw blade Not touch prepared tooth margin or proximal contact 0.007 to 0.01”  PINDEX SYSTEM Post-pour technique is used Reverse drill press is used to create a master cast The machine accurately drills parallel holes from the under side of the trimmed cast Dual pin, tri plus pin  DI LOK TECHNIQUE A snap-apart plastic segmented trays with internal orienting grooves and notches is used ACCUTRAC Used in laminate veneers Removable die system Modification of a plastic tray with internal orientation grooves and notches  WAX PATTERNS  WAX PATTERN A wax form that is the positive likeness of an object to be fabricated METHODS OF FABRICATING A WAX PATTERN DIRECT Pattern is waxed on the TECHNIQ prepared tooth in the UE patient’s mouth INDIRECT Pattern is waxed on a stone cast made from an accurate impression of the TECHNIQ prepared tooth Most popular method UE  ADVANTAGES OF INDIRECT TECHNIQUE Less chair-side time Better visualization of the restoration Ready access to waxing margins  INLAY WAX Inlay casting wax is used for all wax patterns. Inlay wax consists of:  Paraffin (40% to 60%).  Dammar resin to reduce flaking  Carnauba resin, ceresin, candelilla wax to raise the melting temperature.  Dyes to provide color contrasts  Formulated for making intraoral wax patterns TYPE Medium hardness wax I Resist flow at mouth temperature WAX Formulated for fabrication of wax patterns extra-orally Softer wax; Have a slightly lower TYPE melting point II Resist flow at room temperature WAX  REQUIREMENTS OF GOOD INLAY WAX 1. Flow readily when heated, without chipping , flaking or loosing its smoothness 2. When cooled, it must be rigid 3. It must be capable of being carved precisely without chipping, distorting or smearing. 4. Wax should be of some colour that will contrast with and easily distinguishable from the stone die  Stresses – heating and manipulation Wax –thermoplastic material relaxes as these stresses are released – distortion Distortion – poor fit To minimize distortion patterns should never be left off the die, and they should be invested as soon after fabrication Investing and Casting  Lost-wax castings have been made since ancient times. In this technique, wax patterns are converted to cast metal patterns  The process consists of surrounding the wax pattern with a mold made of heat-resistant investment material, then eliminating the wax by heating, and introducing molten metal into the mold through a channel called the sprue.  In dentistry, the resulting casting must be a highly accurate reproduction of the wax pattern both in surface details and in overall dimensions  When the wax pattern has been completed and its margin has been reflowed The pattern is inspected under magnification, and any residual flash (wax that extends beyond the preparation margin) is removed.. A sprue is attached to the pattern. The pattern is then removed from the die and attached to a crucible former (Fig. 22-1). The wax pattern must be invested immediately because any delay leads to distortion of the pattern as a result of stress relief of the wax.3  There are three basic requirements, as follows: The sprue must allow the molten wax to escape from the mold. The sprue must enable the molten metal to flow into the mold with as little turbulence as possible. The metal within it must remain molten slightly longer than the alloy that has filled the mold. This provides a reservoir to compensate for the shrinkage that occurs during solidification of the casting alloy  Diameter In general, a sprue with a relatively large diameter is recommended because this improves the flow of molten metal into the mold and ensures a reservoir during solidification. A 2.5-mm (10-gauge) sprue is recommended for molar and metal- ceramic patterns. A smaller 2.0-mm (12- gauge) sprue is adequate for premolar castings and most partialcoverage restorations.  Location The sprue should be attached to the bulkiest noncritical part of the pattern, away from margins and occlusal contacts. Normally, the largest nonfunctional cusp is used  The sprue must also allow for proper positioning of the pattern in the ring. The objective is to center the pattern.  Attachment The sprue’s point of attachment to the pattern should be carefully smoothed to minimize turbulence  Venting Small auxiliary sprues or vents have been recommended to improve casting of thin patterns  Crucible Former The sprue is attached to a crucible former (sometimes referred to as a sprue former  which serves as a base for the casting ring during investing.  With most modern machines, the crucible former is tall, to allow use of a short sprue and also to enable the pattern to be positioned near the end of the casting ring  Casting Ring and Liner The casting ring serves as a container for the investment while it sets and restricts the setting expansion of the mold. Normally a liner is placed inside the ring to allow for more expansion because the liner is somewhat compressible.  Use of two liners allows for additional compression and enables increased setting expansion of the investment material.  At one time, asbestos was used as the liner; to avoid the health risks associated with asbestos fibers, cellulose paper liners or refractory ceramic fiber liners are now used  Wetting the liner increases the hygroscopic expansion of the mold and should be carefully controlled.  An absorbent dry liner removes water from the investment and causes the mix to become thicker, which leads to increase in the total expansion.  To prevent expansion restriction, care must be taken not to squeeze the liner against the ring. Expansion can be increased if the mold is placed in a water bath  The position of the pattern in the casting ring also affects expansion. For consistent results, a single crown should be centered in the ring, equidistant from its walls  Ringless Investment Technique With the use of higher strength, phosphate-bonded investments, the ringless technique has become quite popular  This method entails the use of a paper or plastic casting ring and is designed to allow unrestricted expansion.1  This can be useful with higher melting alloys that shrink more because of a longer cooling trajectory  Sprue Technique Armamentarium The following equipment is needed (Fig. 22-9): Sprue Sticky wax Rubber crucible former Casting ring Ring liner Bunsen burner Pattern cleaner Scalpel blade Forcep  MATERIALS SCIENCE Several investment materials are available for fabricating a dental casting mold. These typically consist of a refractory material (usually silica) and a binder material, which provides strength.  When investments are classified by binder, three groups are recognized: gypsum-bonded, phosphatebonded, and silica-bonded investments  The gypsum-bonded investments are used for castings made from American Dental Association (ADA) type II, type III, and type IV gold alloys. The phosphate-bonded materials are recommended for metal-ceramic frameworks. The silica-bonded investments are for high-melting base metal alloys used in casting partial removable dental  Gypsum-Bonded Investments Gypsum is used as a binder, along with cristobalite or quartz as the refractory material, to form the mold. The cristobalite and quartz are responsible for the thermal expansion of the mold during wax elimination. Because gypsum is not chemically stable at temperatures exceeding 650 restricted to castings  Expansion Three types of expansion can be manipulated to obtain the desired size of casting: setting, hygroscopic, and thermal  Setting Expansion. As the gypsum investment sets after mixing, it expands and slightly enlarges the mold. The pattern, metal casting ring, and compressibility of the ring liner all influence this expansion  The water-to-powder ratio can be altered to reduce or increase the amount of setting expansion. The use of less water increases the setting expansion and results in a slightly larger casting.  Use of an additional ring liner increases the setting expansion, as does a slight increase in mixing time. If a smaller casting is desired, more water can be used or the liner can be eliminated, both of which curtail the amount of expansion  Hygroscopic Expansion. Hygroscopic expansion occurs when water is added to the setting gypsum investment immediately after the ring has been filled.  To accomplish this, the ring is usually submerged in a water bath at 37°C (100°F) for up to 1 hour immediately after investment. A significant amount of additional setting expansion results, enabling the use of a slightly lower wax elimination temperature.  A wet ring liner also contributes hygroscopic expansion to the portion of the mold with which it is in contact  Thermal Expansion. As the mold is heated to eliminate the wax, thermal expansion occurs The silica refractory material is principally responsible for this because of solid-state phase transformations. Cristobalite changes from the α (low-temperature) to the β (hightemperature) form between 200  ); quartz transforms at 575°C (1067°F). These transitions involve a change in crystal form, an accompanying change in bond angles and axis dimension, and a decrease in density, which produce a volume increase in the refractory components.  Phosphate-Bonded Investments Because most metal-ceramic alloys fuse at approximately 1400°C (≈2550°F) (as opposed to conventional gold alloys at 925°C [≈1700°F]), additional shrinkage occurs when the casting cools to room temperature. To compensate for this, a larger mold is necessary  The principal difference between gypsum-bonded and phosphate- bonded investments is the composition of the binder and the relatively high concentration of silica refractory material in the latter  The binder consists of magnesium oxide and an ammonium phosphate compound. In contrast to gypsum- bonded products, this material is stable at burnout temperatures above 650°C (1202°F) (Fig. 22-13), which allows for additional thermal expansion.  Investment strength increases with increasing temperature (Fig. 22-13, C). Most phosphate bonded investments are mixed with a suspension of colloidal silica in water. (Some, however, can be mixed with water alone.) Some phosphate- bonded investments contain carbon and therefore are gray in color. Carbon- containing materials should not be used for casting base metals because the carbon residue affects the final alloy composition. They may be used for casting alloys with high gold or palladium content.  Expansion In comparison with gypsum-bonded investments, phosphate-bonded investments offer greater flexibility in controlling the amount of expansion. The liquid-to powder ratio needs only slight modification to effect a significant change in setting expansion. Increasing the proportion of special liquid (colloidal silica) also increases expansion  Working Time Phosphate-bonded investments have a relatively short working time in comparison with gypsum materials. Their exothermic setting reaction accelerates as the temperature of the mix rises during manipulation.  The addition of water to the colloidal silica suspension increases the working time, with some loss of setting expansion.  Selecting a Casting Alloy The choice of casting alloy largely determines the selection of investment and casting techniques and therefore is discussed first  Factors to Be Considered Intended Use. Alloys for casting were traditionally classified on the basis of their intended use, as follows: Type I: simple inlays Type II: complex inlays Type III: crowns and fixed dental prostheses Type IV: partial removable dental prostheses and pinledges Porcelain: metal-ceramic alloys  Physical Properties. In 1965, the ADA adopted the specifications of the Fédération Dentaire Internationale (FDI), which classified casting alloys according to their physical properties (specifically their hardness), as follows: Type I: soft Type II: medium Type III: hard Type IV: extra hard Porcelain-type alloys with a high noble metal content were found to have hardness similar to that of type III alloys, and base metal alloys were found to be harder than type IV alloys  Clinical Performance clinical performance (biologic and mechanical) is more important than cost. Biologic properties that can be evaluated include gingival irritation, recurrent caries, plaque retention, and allergies. Mechanical properties include wear resistance and strength, marginal fit, ceramic bond failure, connector failure, and resistance to tarnish and corrosion.  Handling Properties. The ease with which an alloy can be manipulated may influence its selection. An alloy that produces satisfactory clinical results, but only under extremely critical conditions or with expensive equipment, may be rejected in favor of one that produces acceptable results with less critical manipulation.  The ability to burnish an alloy to reduce marginal gap width and thus reduce the exposed thickness of the luting agent is important,24 although the areas where marginal adaptation is clinically most important (interproximally and subgingivally) are usually not very accessible for such manipulation.  Laboratory Performance. Sound laboratory data are essential in the selection of a casting alloy. Important areas of consideration are casting accuracy, surface roughness, strength, sag resistance, and metal-ceramic bond strength. Currently available data suggest that nickel-chromium alloys have lower casting accuracy21 and greater surface roughness22 than do gold alloys (Fig. 22-14) but higher strength and sag resistance because of their higher melting ranges.23  Biocompatibility. All materials for intraoral use should be biocompatible. In addition, it should be possible to handle them safely in the office or laboratory. Many hazardous materials—such as mercury, chloroform, silver cyanide, and hydrofluoric acid—are commonly used in dentistry.  The ADA26 requires nickel-containing alloys to carry a precautionary label stating that their use should be avoided in patients with a known nickel allerg  Selecting an Investment Material After the choice of casting alloy has been made, the investment material can be selected. Ideal Properties An ideal investment should incorporate the following features: Controllable expansion to compensate precisely for shrinkage of the cast alloy during cooling The ability to produce smooth castings with accurate surface reproduction and without nodules Chemical stability at high casting temperatures Adequate strength to resist casting forces Sufficient porosity to allow for gas escape Easy recovery of the casting  Gypsum-bonded Investments. Gypsum-bonded investments satisfy most of the requirements for an ideal material, although they are not suitable for casting metal- ceramic alloys because the gypsum is unstable at the high temperatures required and sulfide contamination of the alloy can occur  In addition, with some materials, obtaining adequate expansion may be difficult. This is critical in casting complete crowns. A casting that is slightly oversized (in a controlled manner) is advantageous for accurate seating  Factors that increase expansion27 of gypsum- bonded investments include the following: Use of a full-width ring liner Prolonged spatulation Storage at 100% humidity Lower water-to-powder ratio Use of a dry liner Use of two ring liners Hygroscopic technique with the pattern in the upper part of the ring  Phosphate-bonded Investments. Phosphate-bonded investment materials offer certain advantages over gypsum-bonded investments. They are more stable at high temperatures and thus are the material of choice for casting metal- ceramic alloys. They expand rapidly at the temperatures used for casting alloys, and their expansion can be conveniently and precisely controlled. The expansion is increased as a result of a combination of the following factors: Heat from the setting reaction softens the wax and allows freer setting expansion. The increased strength of the material at high temperatures restricts shrinkage of the alloy as it cools. The powder mixed with colloidal silica reduces the surface roughness of the castings and also increases expansion. Thus, expansion can be conveniently controlled by slightly diluting the colloidal silica with distilled water  However, castings made with phosphate- bonded investments are rougher than those made with gypsum bonded investments29 and are more difficult to remove from the investment.30 Because phosphate-bonded investments have lower porosity,31 complete mold filling is more difficult. Castings also are more likely to have surface nodules, which must be removed. (Vacuum mixing and a careful investing technique help reduce but do not eliminate the occurrence of nodules.  INVESTING Vacuum mixing of investment materials (Fig. 22- 16) is highly recommended for consistent results in casting with minimal surface defects, especially when phosphate bonded investments are used. Good results are possible with brush application of vacuum-mixed investment or when the investment is poured into the ring under vacuum pressure. Vacuum mixing with brush application of the investment is the suggested mode. To expedite the procedure and minimize distortion, all necessary items and materials should be prepared before the wax pattern is reflowed and removed from the die  Armamentarium The following equipment is needed (Fig. 22-17): Vacuum mixer and bowl Vibrator Investment powder (gypsum or phosphate bonded) Water or colloidal silica Spatula Brush Surfactant graduated cylinder Crucible former Casting ring and liner  Step-by-Step Procedure Brush Technique In this technique, the pattern is first painted with surface tension reducer; the surface must be wet completely. The procedure is as follows: 1. Select the correct program on the mixing unit in accordance with the manufacturer’s instructions (Fig. 22- 18, A). The mixing bowl can be either wiped completely dry or shaken dry.  remember that the residual water adds about mL to the mix. Add investment powder to the liquid in the mixing bowl (see Fig. 22-18, B). 2. Attach the bowl to the mixer, and mechanically spatulate (see Fig. 22- 18, C and D).  3. Coat the entire pattern with investment, pushing the material ahead of the brush from a single point (see Fig. 22-18, E). Gently vibrate throughout the application of investment, being especially careful to coat the internal surface and the margin of the pattern (see Fig. 22-18, F).  A finger positioned under the crucible former on the table of the vibrator minimizes the risk of excessive vibration and possible breaking of the pattern from the sprue. After the pattern has been completely coated, attach the ring and immediately fill by causing the remaining investment to vibrate out of the bowl  4. Place the lined casting ring over the pattern (see Fig. 22-18, G) and, with the aid of vibration, pour the investment down the side of the ring (see Fig. 22-18, H). Fill the ring slowly, starting from the bottom and moving up (see Fig. 22-18, I).  5. 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. If the investment begins to set too soon, rinse it off quickly with cold water. The wax pattern can then be replaced on the die, and material can reflow into its margins again.  6. After the ring is filled to the rim, allow the investment to set. 7. If the hygroscopic technique is used, place the ring in a 37°C (100°F) water bath for 1 hour  Wax Elimination Wax elimination, or wax burnout, consists of heating the investment in a thermostatically controlled furnace (Fig. 22-19) until all traces of the wax are vaporized. The temperature reached by the investment determines its thermal expansion  All water in the investment must be driven off during wax elimination. The temperature to which the ring is heated during wax elimination must be sufficiently high. It should be maintained long enough (“heat soak”) to minimize a sudden drop in temperature upon removal from the furnace. Such a drop may result in an incomplete casting because of excessively rapid solidification of the alloy as it enters the mold.  Once the investment is heated during the wax-elimination procedure, heating must be continued, and casting must be completed. Cooling and reheating of the investment can cause casting inaccuracy because the refractory mold and binder do not revert to their original forms (hysteresis). Inadequate expansion and cracking  Step-by-Step Procedure 1. Allow the investment to set for the recommended time (usually 1 hour), and then remove the rubber crucible former (  If a metal sprue is used, remove it as well. The ring should be placed in a humidor if stored overnight. The smooth “skin” that forms on the ring with phosphate bonded investments should be removed with a plaster knife, and any loose particles of investment should be blown off with compressed air  2. Reexamine the ring for any residual particles, and then place it with the sprue facing down in the furnace on a ribbed tray. The tray allows the molten wax to flow out freely  3. Heat the furnace to 200°C (392°F), and hold this temperature for 30 minutes. Most of the wax is by then eliminated. 4. Increase the heat to the final burnout temperature (generally 650°C [1202°F] or 480°C [896°F] if a hygroscopic technique is used; follow the manufacturer’s instructions), and hold that temperature for 45 minutes. Because the heating rate affects expansion,32 it also should be standardized as part of the investing and casting protocol in order to routinely obtain accurately fitting castings.  However, the investment should not be overheated or kept at the chosen temperature too long Gypsum- bonded investments are not stable above 650°C. Also, some carbon in carbon containing investments burns off, which causes increased surface roughness of the casting  CASTING Casting Machines A casting machine (Fig. 22-21) requires a heat source, to melt the alloy, and a casting force. For a complete casting, the casting force must be high enough to overcome the high surface tension of the molten alloy,39 as well as the resistance of the gas within the mold  The heat source can be either the reducing flame of a torch or electricity. Conventional alloys can be melted with a gas-air torch (Fig. 22- 22, A and B), but for the metal- ceramic alloys in a higher melting range, a gas oxygen torch (see Fig. 22-22, C) is needed. For base metal alloys, a multiorifice gas-oxygen torch (see Fig. 22-22, D) or an  Electric heating can occur by convection from a heating muffle or by generation of an induction current in the alloy (Fig. 22-23). Advocates of the latter maintain that heating can be more evenly controlled, which prevents undesirable changes in alloy composition caused by volatilization of the elements with lower melting points.  In general, the electric machines are expensive and more appropriate for larger dental laboratories, whereas a torch may be the equipment of choice for smaller laboratories and dental offices. The combination of alloy and casting technique influences the marginal fit of the restorations.  In present-day casting machines, either air pressure or centrifugal force is still used to fill the mold; both were first proposed in the early days of lost-wax castings. Some machines evacuate the mold before it is filled with metal, and vacuum has been shown to improve mold filling, although it is not clear whether the difference is clinically  Casting Technique The ring is not removed from the burnout furnace until the alloy has been melted and is ready to cast. Cleaning a previously cast alloy is necessary to remove investment debris and oxides before its reuse  remove investment debris and oxides before its reuse. Noble metal alloys can be melted on a charcoal block with a gas-air torch, which provides a reducing atmosphere  Remaining impurities are removed through pickling and ultrasonic or steam cleaning  Alloys from different manufacturers should not be mixed, even if they are similar. According to one report, recasting nickel-containing alloys with 65% surplus metal addition significantly increased the cytotoxic activity  Armamentarium The following equipment is needed (Fig. 22-24): Broken-arm (Kerr) centrifugal casting machine Crucible Blowtorch Protective colored goggles Tongs Casting alloy Flux  Procedure The casting machine is given three clockwise turns (four if metal-ceramic alloys are used) and locked in position with the pin. The cradle and counterbalance weights are checked for the appropriate size of the casting ring  A crucible for the alloy being cast is placed in the machine. The torch (gas-air for regular alloys, gas- oxygen for metal-ceramic) is lit and adjusted.  For metal-ceramic alloys, the clinician should wear a pair of colored goggles to protect the eyes and also to enable direct viewing of the melt.  The crucible is preheated (Fig. 22-25, A), particularly over the trajectory that will be in contact with the alloy, and the alloy is added. Preheating avoids excessive slag formation during casting.  when metal-ceramic alloys are cast, a crucible that is too cool can “freeze” the alloy, which results in an incomplete casting.  The mass of the alloy must be sufficient to sustain adequate casting pressure. With a high-density noble metal alloy, 6 g (4 dwt*) is typically adequate for premolar and anterior castings, 9 g (6 dwt) is adequate for molar castings, and 12 g (8 dwt) is adequate for pontics  The alloy is heated in the reducing part of the flame until it is ready to cast. A little flux can be added to conventional gold alloys (not to metal-ceramic alloys  Gold alloys ball up and have a mirror- like shiny surface that appears to be spinning. Nickel-chromium and cobalt alloys are ready to cast when the sharp edges of the ingot round over  The mold is placed in the cradle of the casting machine (see Fig. 22-25, B) and kept on the alloy with the reducing flame until the crucible is moved into position (see Fig. 22-25, C to G).  The casting machine arm is then released to make the casting (see Fig. 22-25, H). The machine is allowed to spin until it has slowed enough that it can be stopped by hand, and the ring is removed with casting tongs  Recovery of the Casting. After the red glow has disappeared from the button, the casting ring is plunged under running cold water into a large rubber mixing bowl (Fig. 22-26).  Gypsum-bonded investments disintegrate quickly, and residue is eliminated easily with a toothbrush. Final traces can be removed ultrasonically. Oxides are removed by pickling in 50% hydrochloric acid (or, preferably, a nonfuming substitute; Fig. 22-27)  Phosphate-bonded investments do not disintegrate equally well, and some must be removed forcibly from the casting ring. They can be handled as soon as they have been sufficiently cooled under running water  A knife is used to trim the investment at the button end of the ring (Fig. 22- 28, A). The other end is not trimmed because of the risk of damaging the margin.  When the ring liner is exposed, the investment can be pushed out of the ring (see Fig. 22-28, B). It is then broken apart under running water (because it is still hot; see Fig. 22-28, C).  The remaining investment is carefully removed with a small blunt instrument (see Fig. 22-28, D), and any traces are dissolved in hydrofluoric acid or a less caustic substitute.  Evaluation. The casting is never fitted on the die until the inner surface has been carefully evaluated under magnification; even tiny imperfections can cause damage to the stone die. A die may be rendered useless in a matter of seconds if a casting is fitted prematurely  Nodules. Bubbles of gas trapped between the wax pattern and the investment produce nodules on the casting surface. Even minute nodules can limit the seating of the casting to a considerable degree.  Keys to avoiding nodules include a careful investing technique, use of a surfactant, vacuum spatulation, and careful coating of the wax pattern with investment. Castings made with phosphate-bonded investment are especially prone to imperfections, and experience and care are necessary to routinely produce castings that are free of nodule  Fins. Fins are caused by cracks in the investment that have been filled with molten metal. These cracks can result from a weak mix of investment (high ratio of water to powder), excessive casting force, steam generated from too-rapid heating, reheating an invested pattern, an improperly situated pattern (too close to the periphery of the casting ring), or even premature or rough handling of the ring after investing  Incompleteness. If an area of wax is too thin (less than 0.3 mm), which occurs occasionally on the veneering surface of a metal-ceramic restoration, an incomplete casting may result. Thickening of the wax in these areas is recommended. Incomplete casting of normal-thickness wax patterns may result from inadequate heating of the metal, incomplete wax elimination, excessive cooling (“freezing”) of the mold, insufficient casting force, not enough metal, or metal spillage.  Voids or Porosity. Voids in the casting (in particular in the margin area) may be caused by debris trapped in the mold (usually a particle of the investment undetected before wax elimination). A well-waxed smooth sprue helps prevent this.. Porosity resulting from solidification shrinkage (“suck-back”) occurs if the metal in the sprue solidifies before the metal in the mold, as may happen when a sprue is too narrow, too long, or incorrectly located or when a large casting is made in the absence of a chill vent. Gases may dissolve in the molten alloy during melting and leave porosity  Back-pressure porosity47 may be caused by air pressure in the mold as the molten metal enters. Its occurrence is reduced through the use of a more porous investment, location of the pattern near the end of the ring (6 to 8 mm), and casting with a vacuum technique  Marginal Discrepancies. Inaccuracies of fit at the margin can be caused by distortion during removal of the wax pattern from the die. They may also result from increased setting expansion (hygroscopic technique) after uneven expansion of the mold.  Dimensional Inaccuracies. The casting can be either too small or too large. Attention to detail is essential for an accurately expanded mold. A standardized procedure is needed in regard to liquid-to-powder ratio, spatulation, the ring liner, the amount of liquid added, and mold heating.  REVIEW OF TECHNIQUE The following list summarizes the steps involved in investing and casting (Fig. 22-30) and should prove helpful in reviewing the material covered in this chapter: 1. A sprue 2 or 2.5 mm in diameter (10- or 12- gauge) is attached to the bulkiest nonfunctional cusp (the larger size for molar and metal-ceramic patterns, the smaller size for premolar and partial coverage). Sprues can be attached to multiple units with a runner bar (see Fig. 22-30, A)  2. The pattern is carefully removed from the die and attached to a crucible former (sprue length should be 6 mm or less; see Fig. 22-30, B). 3. The pattern is painted with surface tension reducer (see Fig. 22-30, C) and then carefully coated with vacuum-mixed investment (see Fig. 22-30, D).  4. The ring is filled, and the investment is allowed to bench set for a minimum of 1 hour. 5. After wax elimination, the casting machine is prepared, and the crucible is preheated. The alloy is melted, the ring is transferred, and the casting is made promptly (see Fig. 22-30, E).  6. The casting is recovered from the investment (see Fig. 22-30, F). 7. Defects are identified and corrected if possible (see Fig. 22-30, G).