Spruing and investing

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Introduction to Fixed Prosthodontics …

Introduction to Fixed Prosthodontics
Forth Year

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  • 1. SPRUING AND INVESTING Arriving at a completed casting after fabrication of wax pattern involves three steps: (1) investing-surrounding the wax pattern with a material that can accurately duplicate its shape and anatomic features (2) burnout-removal of the wax pattern so that a mold is created into which the molten alloy can be placed, and (3) casting-introducing the molten alloy into the previously prepared mold. Investment Materials: An investment must fulfill three important requirements: 1. It must reproduce precisely the detailed form of the wax pattern. 2. It must provide sufficient strength to withstand the heat of burnout and the actual casting of the molten metal. 3. It must expand sufficiently to compensate for the solidification shrinkage of the alloy Shrinkage Compensation: The molten alloy used for dental restorations shrink upon solidification: gold alloys by approximately 1.5% and nickel-chromium alloys by as much as 2.4%. If the mold is not made correspondingly larger than the original wax pattern, the resultant casting will be that much smaller. For crowns, it is necessary to compensate for the solidification shrinkage of the specific alloy used by expanding the mold enough to at least equal the shrinkage. There are four mechanisms that can play a role in producing an expanded mold: (1) setting expansion of the investment, (2) hygroscopic expansion, (3) wax pattern expansion, and (4) thermal expansion. Setting Expansion. Setting expansion of the investment occurs as a result of normal crystal growth. The expansion probably is enhanced by silica particles in the investment interfering with the forming crystalline structure of the gypsum, causing it to expand outward. This type of expansion, in air, normally is about 0.4%, but expansion is partially restricted by the metal investment. Hygroscopic Expansion: Hygroscopic expansion may be employed to augment normal expansion. The investment is allowed to set in the presence of water, producing additional expansion. The water in which the investment is immersed replaces the water used by the hydration process. Hygroscopic expansion ranges from 1.2% to 2.2%. More controlled amounts of hygroscopic expansion may be achieved by adding a measured amount of water to the setting investment. Hygroscopic expansion does occur in an unrestricted
  • 2. trough or an expandable investment ring. However, in a lined, rigid, metal ring, the expansion attributed to hygroscopic expansion is more likely due to expansion of the wax pattern caused by the elevated temperature of the water in which the pattern has been immersed. Wax Pattern Expansion. Expansion of the wax pattern while the investment is still fluid occurs when the wax is warmed above the temperature at which it was formed. The heat may come from the chemical reaction of the investment or from a warm water bath in which the ring is immersed. The low-temperature burnout technique employs a combination of wax pattern expansion and thermal expansion of the mold. After the investment-filled ring is removed from a 100°F (38°C) water bath, the ring is heated to only 900°F (480°C) before casting to produce the additional expansion needed. Thermal Expansion: Thermal expansion of the investment occurs when the investment is heated in the burnout oven. The high-temperature burnout technique relies primarily on thermal expansion of the mold. The investment around the wax pattern is allowed to harden in air at room temperature, and then is heated to approximately 1200°F (650°C). Regardless of which technique is employed, a precise routine for investing, burning out, and casting must be adhered in order to achieve consistent results. Two kinds of investments are in common usage for the fabrication of cast restorations: those bonded with gypsum are used for alloys that fuse below 1975°F (1080°C), and those bonded with phosphate are used for higher-fusing alloys. The manufacturer’s instructions for a particular brand should be followed. Gypsum-Bonded Investments: The gypsum-bonded investments are used with types I, II and III gold alloys. These investments are themselves classified as type I for use with the hightemperature technique, or type II for the low-temperature technique. After setting, both types of investment are composites containing a matrix of gypsum with silica as a refractory filter, and certain chemical modifiers. The gypsum matrix, or calcium sulfate hemihydrate, comprises 30% to 35% of the investment and acts as binder. The refractory material, either quartz or cristobalite, makes up 60% to 65% of the investment and provides the thermal expansion for the investment. The rigid metal ring in which setting takes places must be lined with a compressible material to allow setting expansion to occur in a radial direction. Expansion can be controlled to some extent by varying the thickness of the liner. Thermal expansion is achieved in the burnout oven through the normal expansion that occurs upon heating the silica as well as through phase changes that occur in the material. Investing Armamentarium:
  • 3. 1. 200cc Vac-U-Spat bowl and lid. 2. Vacuum tubing. 3. Vac-U-Vestor (Whip Mix Corp. Louisville, KY). 4. Rubber crucible former. 5. Casting ring (32 mm diameter). 6. Plastic water measure. 7. Spatula. 8. PKT (Thomas) waxing instruments (no. 1 and no.4). 9. Cotton pliers. 10. Bunsen burner. 11. Matches. 12. Sticky wax. 13. Sprue formers (hollow plastic). 14. One package (50 g) of investment. 15. Four-inch (10 cm) strip of cellulose ring liner. Sprue Former Attachment: The sprue former is a small-diameter pin or tube made of wax, plastic, or metal. A 10-gauge (2.6 mm diameter) sprue form can be used on most patterns, while the 12 gauge (2.0 mm diameter) is used on small premolar patterns. One end of the sprue former is attached to the wax pattern and the other end to the crucible former (a conical rubber base). After the investment has hardened, the crucible former is removed from the ring, leaving a funnel-shaped entrance to the mold. The channel left by the sprue former following burnout is the sprue, an inlet for the gold that will be forced into the mold. A sprue former as large as possible should be used on each pattern. If the sprue is too thin or too long, the gold may solidify in the sprue before it does in the larger cavity formed by the wax pattern. The sprue former should be attached to the wax pattern as its point of greatest bulk, avoiding centric occlusal contacts if possible It is attached at an angle to allow the incoming gold to flow freely to all portions of the mold. If the sprue is directed at a right angle to a flat wall of the mold, a “hot spot” may be created at that point. Select a hollor plastic sprue former and place it inside the crucible former and casting ring for measurement. The sprue former should be just long enough so that the highest point on the wax pattern will be 6.0 mm from the end of the ring. Remove the sprue former from the crucible former and shorten it with a sharp knife if necessary. If the hold in the crucible former is too large to firmly grasp the sprue former, fill the hole with soft wax. With a PKT No.1 instrument. Place the sprue former into the molten bead of sticky wax. Melt in the sticky wax around the sprue former-wax pattern junction with the PKT No.1 instrument to provide a smooth conduit for the molten alloy. Do not expose the wax pattern to prolonged heat during this procedure. Avoid over
  • 4. bulking the sprue former attachment because this will increase the risk of shrink-spot porosity and also make removal of the sprue from the casting more difficult. The sprue former also should not be constricted at its attachment to the wax pattern. The best castability and least porosity is produced by a sprue former-wax pattern junction that is either straight or slightly flared. During the time period between removal of the wax pattern from the die and hardening of the investment, deformation will occur as stresses in the wax are released. To minimize this time, all the armamentarium must be at hand, the ring liner should be in place, and the water should be measured out before the pattern is removed. To produce niform expansion, surround the pattern on all sides with investment that is as uniform in thickness as possible. The closer to the centre of the ring that the pattern is placed, the greater will be the expansion. With pliers, push the sprue former down into the soft wax in the top of the crucible former until the top of the pattern is 6.0 mm below the end of the ring. To provide adequate bulk of gold during solidification, the sprue itself should be no longer than 6.0 mm (it can be shorter). To correct any discrepancy in length, add soft wax onto the sprue former, thus lengthening the cruicible former and shortening the exposed sprue former, Smooth the wax around the base of the sprue former. Investing Procedure: For a single crown or onlay, use metal casting ring with an outside diameter of 32 mm. Place a resilient liner on the inside of the ring to provide a buffer of pliable material against which the investment can expand to enlarger the mold. An alternative method uses a split plastic casting ring that offers no resistance to the setting expansion. The technique allows easier escape of gas from the mold during casting, but the mold is more vulnerable to cracking. Asbestos, ceramic papers are now used as substitutes for asbestos. Ceramic material does not readily absorb water except under vacuum. Place a dry strip of celluloseliner approximately 9.5 cm long into a 32-diameter casting ring, carefully adapting the strip to the walls of the ring with no overlap. The liner should be 3.0 mm short of both ends of the ring. Dip the ring into water to wet the liner, then gently shake off the excess. Do not compress the wet liner against the ring because its cushioning effect will be reduced. Rotate the ring firmly onto the crucible former, being careful to avoid snapping movements or contact of the wax pattern with the ring. The procedure for investing a pattern for a single-tooth restoration to be cast in type 11 gold with the vacuum mix, vacuum pour technique is as follows: Place the assembled ring and crucible former into the hole at the top of the Vac-U Spat investor. Hold the lid by the spindle with the paddle toward you and the inlay ring to the bottom. Look into the aperture through which the investment will flow into the ring, and make sure that the internal portion of the wax pattern is visible.
  • 5. Connect once end of the clear plastic vacuum tubing to the vacuum outlet on the Vac-UI vestor. Insert the metal connector on the other end of the tubing into the hole in the lid of the Vac-U-Spat. Turn on the Vac-U-Vestor briefly. Pour the recommended amount of room-temperature water into the bowl. This must be carefully measured, since the water-powder ratio has a critical effect on expansion. Add a package of investment ot the water and mix it with a hand-held spatula until all of the investment has become wet. Place the lid on the bowl and make sure it is firmly sealed. Turn on the Vac-U-Vestor and insert the spindle of the lid of the Vac-U-Spat into the smaller of the two drive chucks on the bottom of the unit. The gauge should register a vacuum. Powder-spatulate for 15 seconds. Since the length of spatulation can affect expansion of the investment measure the time of spatulation precisely. Overspatulation will increase thermal expansion. Remove the spindle from the drive chuck. Do not turn off the Vac-U-Vestor, and do not disconnect the vacuum at this point. Place the drive nut of the Vac-U-Spat spindle on the vibrator knob. More sure that the shaft is horizontal and the casting ring is in the lowest position on the circumference of the lid. Hold the Vac-U-Spat in this position for a few seconds until the investment has run to the lower side of the bowl. Slowly invert the Vac-U-Spat until the shaft points straight down, keeping the drive nut in contact with the vibrator. It should take slightly less than 30 seconds to traverse the 90 degree arc from the horizontal to the vertical position. Remove the drive nut from the vibrator knob, keeping the Vac-U-Spat inverted. While it is still in this position, turn off the vacuum pump and disconnect the vacuum hose. Then remove the casting ring and crucible former from the Vac-USpat lid. Place the crucible former on the vibrator knob for a few seconds to settle any investment that might have spilled during separation of the ring from the lid. Do not overvibrate; this may cause air to slip around the seal between the ring and the crucible former, rising up and lodging on the underside of the pattern. If a high-temperature (1200°F, 650C°) burnout technique will be used, place the casting ring and crucible former into a humidor (a covered plastic container or sealed plastic bag with wet paper towels in the bottom) and let set at room temperature. If a low-temperatur (900°F, 480°C) burnout technique is to be used, immediately immers the ring in a 100°F (38°C) water bath to produce expansion of the wax pattern. Allow the investment to set for a minimum of 30 minutes. Leave the ring in the humidor until you are ready for burnout and casting. To prevent clogging the drains with accumulated investment, empty the unused portion of investment remaining in the Vac-U-Spat bowl into the investment
  • 6. remaining in the Vac-U-Spat bowl into the investment envelope. Fold over the top of the package so the waste can be disposed off neatly. Use a brush and running water to clean the bowl, lid, and paddle before the investment hardens on them. Investment of Inlay and Dowel-Core Patterns: Less mold expansion is required for dowel cores and inlays than for crown. Omitting the ring or increasing the investment water-powder ratio by 1.0 mL will result in a slightly undersized casting that will fit more easily into the cavity prepared in the tooth. The following technique is recommended for investing and casting a dowel-core pattern. Invest the pattern in Beautycast using the standard water-powder ratio, without a ring liner. Burn out at 1200°F (650°C). Because a gas-oxygen torch or electric induction casting machine must be used to melt the alloy. It is possible for an experienced operator to cast and cement gold inlays and dowel cores on the same day that the teeth are prepared by using the following accelerated technique for investment and burnout. 1. Invest the pattern in a phosphate-bonded investment using a ring liner and standard special liquid dilution of 50/50. 2. Allow the investment to harden for 12 to 15 minutes. It should feel firm and warm. 3. Place the invested pattern directly into a 1300°F (705°C) oven and allow 12 to 15 minutes for burnout. 4. Cast in gold alloy (type II or III for inlays, type III or IV for dowel cores). In this way, investment, burnout, and casting can be completed in 1 hour, saving the patient an additional appointment.
  • 7. Phosphate-Bonded Investments: Phosphate-bonded investment are much stronger and withstand much higher temperatures than do gypsum-bonded investments used for investing and casting alloys with higher melting temperatures eg. Silver-palladium, gold platinum, and nickel-chromium. To obtain sufficient expansion for crowns of these alloys, the mold must be heated to 1400°F (760°C) or higher, temperatures. In general , any alloy with a casting temperature in excess of 2100°F (1150°C) should be cast into an investment with q binder other than gypsum. The powder contains phosphates of magnesium and ammonium, graphite (carbon), and large silica particles, while the special liquid provided with these investments contains an aqueous suspension of colloidal silica. Carbon-free phosphate investments. Magnesium phosphate reacts with primary ammonium phosphate to produce magnesium ammonium phosphate, which gives the investment is strength at room temperature. At higher temperatures, silcophosphates are formed; they give investment its great strength. Expansion can be varied by the proportions of silica sol and water: 1. More silica sol and less water = more expansion. 2. Less silica sol and more water = less expansion. The usual proportion is three parts silica sol liquid to one part distilled water. Overall liquid powder ration for Ceramigold investment should remain constant: 9.5 cc liquid to 60 g of powder. Investing Armamentarium for Phosphate-Bonded Investments: 1. 200-cc Vac-U Spat bowl and lid. 2. Vacuum tubing. 3. Vac-U-Vestor. 4. Rubber cruicible former. 5. Casting ring. 6. Plastic water measure. 7. Spatula. 8. PKT (Thomas) waxing instruments (no.1 and no.4). 9. Cotton pliers. 10. Bunsen burner and matches. 11. Sticky wax. 12. Sprue formers (hollow plastic or wax). 13. One package (60 g) of Ceramigold investment. 14. Special liquid.
  • 8. 15. Strip of liner 9.5 cm long. 16. Small camel’s hair brush. Investing with Phosphate-Bonded Materials: Attach a 10 gauge (2.6 mm) plastic sprue former to the tip of the incisal portion of a single crown wax pattern with sticky was, using a PKT no. 1 instrument to melt and blend the junction. Carefully remove the pattern from the die and grasp the sprue former with cotton pliers. Seat the sprue former into the soft wax in the centre of crucible former. The sprue former’s length should be adjusted so that the pattern will be 6.0mm from the end of the ring when it is in place. Build up the crucible former with wax, if necessary, so that more than 6.0 mm of the sprue former will be exposed. Posterior patterns are used on the tip of the cusp with the greatest bulk. An 18gauge wax sprue former (0.8 mm diameter) should connect the other cusp tip (in the veneering area) with the base of the crucible former (attach to the pattern while it is still on the die). The tip of this cusp should be lower than the point of every of the main sprue. Adapt a layer of dry cellulose liner to the inside of the ring, immerse the ring briefly in a bowl of water to moisten the liner. Assemble the ring, crucible former, and Vac-U-Spat lid. Place 9.5 cc of the liquid in the Vac-U-Spat bowl and add the contents of a 60-g package of Ceramigold investment. Connect the vacuum tubing and mechanically spatulate under a vacuum for approximate 15 seconds. Disconnect the vacuum and remove the ring from the lid. This type of investment possesses poor surface-wetting characteristics. Either vacuum or open investing can be used. Allowing the investment to set a pressure pot will further reduce the size and number of bubbles. If there are small, restricted areas in the interior of a wax pattern, gently brush the investment into the pattern with s small brush. Then place the ring over the crucible former and slowly pour the investment down one side of the ring with vibration. You should see a small stream of investment flow over the margin on one side of the pattern, down into the deepest recess, and gradually fill the pattern from the bottom up. Once the pattern is covered, the ring can be filled the rest of the way with a minimum of vibration. There should be an excess of investment above the end of the ring so the hardened glaze can be easily ground away on a model trimmer. If it is needed, an additional 0.7% expansion can be obtained by placing the investment-filled ring into a 100°F (38°C) water bath before it has hardened. If this is done, the surface of the investment should be protected from the softening effect of water by a thin sheet of rubber or plastic wrap held in place by a rubber band. Wax patterns for metal-ceramic fixed partial dentures are invested and cast as one unit whenever possible. The wax pattern for a fixed partial denture should
  • 9. be invested in a large ring (found or oval, with a diameter of approximately 6.3 cm) to produce the most accurate casting. For lower-fusing gold alloy castings, sprue formers run directly from crucible former to wax pattern to provide rapid, turbulence-free access of the metal to the mold during casting. Patterns for metal-ceramic fixed partial dentures, however, must sprued by an indirect method because the alloys used fuse and solidify at much higher temperatures. Bulky horizontal runner bar is placed between crucible former and pattern. Place a piece of 8-gauge (3.4 mm diameter) hollow plastic sprue former material horizontally into the sprue former network to form a manfold between the crucible former and the wax pattern. Be sure to plug both ends of the hollow sprue former with a wax to avoid he formation of thin projections of investment that might break off in the mold. The metal farthest from the manifold-the margins, and the surface of the button exposed to ambient room temperaturewill cool first while the feeder bar is still fluid and can serve as a reservoir for solidification contraction in the fixed partial denture. The runner bar also helps to stabilize the pattern against distortion during investing, and it equalizes the flow of metal so that all parts of the mold will be filled evenly and simultaneously during casting. To facilitate proper orientation, a wax dot can be place on the crucible former. This will leave an imprint on the surface of the investment which can be seen when the ring is placed in the casting machine. As an alternative, two dots can be scribed on the outside of the ring, one directly opposite the other. These dots should be aligned with the axis of the pattern before investing.
  • 10. be invested in a large ring (found or oval, with a diameter of approximately 6.3 cm) to produce the most accurate casting. For lower-fusing gold alloy castings, sprue formers run directly from crucible former to wax pattern to provide rapid, turbulence-free access of the metal to the mold during casting. Patterns for metal-ceramic fixed partial dentures, however, must sprued by an indirect method because the alloys used fuse and solidify at much higher temperatures. Bulky horizontal runner bar is placed between crucible former and pattern. Place a piece of 8-gauge (3.4 mm diameter) hollow plastic sprue former material horizontally into the sprue former network to form a manfold between the crucible former and the wax pattern. Be sure to plug both ends of the hollow sprue former with a wax to avoid he formation of thin projections of investment that might break off in the mold. The metal farthest from the manifold-the margins, and the surface of the button exposed to ambient room temperaturewill cool first while the feeder bar is still fluid and can serve as a reservoir for solidification contraction in the fixed partial denture. The runner bar also helps to stabilize the pattern against distortion during investing, and it equalizes the flow of metal so that all parts of the mold will be filled evenly and simultaneously during casting. To facilitate proper orientation, a wax dot can be place on the crucible former. This will leave an imprint on the surface of the investment which can be seen when the ring is placed in the casting machine. As an alternative, two dots can be scribed on the outside of the ring, one directly opposite the other. These dots should be aligned with the axis of the pattern before investing.