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LABOTARORY PROCEDURES FOR FRAMEWORK CONSTRUCTION

LABOTARORY PROCEDURES FOR FRAMEWORK CONSTRUCTION . Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics. 1- Surveying of the study cast and designing of the RPD: After surveying of the study cast the design of the RPD is drawn on the study cast. 2- Surveying of the master cast:

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LABOTARORY PROCEDURES FOR FRAMEWORK CONSTRUCTION

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  1. LABOTARORY PROCEDURES FOR FRAMEWORK CONSTRUCTION Dr. Waseem Bahjat MushtahaSpecialized in prosthodontics

  2. 1- Surveying of the study cast and designing of the RPD: After surveying of the study cast the design of the RPD is drawn on the study cast. 2- Surveying of the master cast: a-The master cast is placed on the surveyor table In the previously established tilt. b- The survey line is drawn on the abutment teeth. Then the position of the clasp terminal at the required undercut depth is marked. c- Tripodizing the master cast. 3- Transfer the design from the study cast and drawing it on the master cast. Then beading of the maxillary cast is carried out by scrapping the master cast following the outline of the major connectors

  3. Block out and relief of the master cast : The elimination of undesirable undercuts on the master cast with wax is commonly referred to a blockout. The blockout wax must be trimmed parallel to the path of insertion. Areas of blockout: 1 - The soft tissue undercuts. 2- Mesial and distal undercuts facing the edentulous area. 3- Wax ledge: The blockout wax is shaped with hand instrument1.0 provide a slight ledge just apical to the clasp tip. This ledge guides the placement of the wax pattern to ensure that the clasp lies at the desired position in the undercut area. 4- Areas of severe undercuts not involved with the framework should also be recontoured with wax to minimize distortion during duplication.

  4. Relief areas: 1- The denture base area is relieved so that the retentive meshwork will be raised above the edentulous area to allow resin to completely surround it. The wax is trimmed to the internal finish line at an angle less than 9O• Also a small square (2 mm) of wax is removed at the tissue stop area. 2- Maxillary and mandibular tori. 3- The area under the mandibular major connectors.

  5. 5-Duplication of the master cast: The waxed up master cast is duplicated for the purpose of: 1- Making an investment east; The metal framework is cast against the investment cast: a-The investment cast can withstand the high casting temperature. b-It 'allows for thermal or hygroscopic expansion to compensate for casting shrinkage of the alloy. c- The investment porosity allows for gas escape during bum out of the wax pattern. 2- To preserve the master cast which is used to fit the framework.

  6. Procedure: The refractory (investment) cast is produced by making reversible hydrocolloid (agar/ agar) mold. Then the mold is poured into investment material. 1- The colloid is heated in a double boiler at 100°c to a fluid consistency. Then it is cooled to the working temperature (63c) to flow easily without melting the blackout wax. 2- The blacked out master cast is placed on the base of a duplicating flask. 3- Duplicating colloid at the proper temperature is poured in a steady stream into the flask.

  7. 4- The flask is placed in a cooling tank up to an hour to fully set the colloid. 5-The flask is then disassembled and the master cast is removed. The mold is the examined to detect any defect in the colloid. 6- The investment material is mixed according to the manufacturer's instruction and poured into the mold in small amounts to avoid air trap in the area of the teeth. The remainder of the refractory material is added to the mold with a minimum of vibration. 7-A after setting of the investment (60 minutes), the cast is removed from the mold.

  8. 5- Drying the investment cast and beeswax dip: The cast is dried in a hot air oven at 82-93°c for 1\2 to 1 hour, dipped into beeswax at 138-149c for 15 seconds. Beeswax dip ensures a smooth, dense surface to the cast.

  9. 6-Waxing the framework: The design is again transferred from the master cast to the refractory case. -Ready made plastic patterns are used for waxing up the framework The plastic pattern, which will be used for clasp arms, major connectors and retentive meshwork, is selected. The plastic patterns are glued to the refractory cast with a mixture of acetone and plastic pattern scraps mixed to a watery consistency. The tacky liquid is painted on the design outline with a fine brush and allowed to dry for few seconds. The pattern is then adapted to the cast with a soft rubber pencil eraser shaped into a wedge. -Once the plastic patterns are in place on the refractory cast, they must be joined together with blue inlay wax. This wax is also to seal down the margins of the major connector and freehand wax the minor connectors and rests. -Final contouring of the pattern is carried out with carvers and small round blades. Finally smoothness of the pattern is performed using alcohol torch

  10. 7- spruing the framework: A sprue is a channel, which allows molten metal to flow to the mold cavity during casting. The sprue should be as thick as the pattern and should be attached to a bulk area. All sprue connections should be round to not leave sharp edges of investment, which is liable to fracture during casting. There are two main types of sprues: 1-single sprue placed horizontally and all parts of the pattern are connected to it. 2-Multiple Spruing: a- Top sprues: Gold castings and chrome cobalt alloys sprue the casting from above with multiple sprues. b- Inverted sprue: approach through the refractory cast.

  11. lnvesting the refractory cast: Purpose of investment: 1- It provides strength necessary for casting. 2- Provides a smooth surface mold. 3-It provides an escapement for the gas in the mould during casting. 4- Compensate for metal shrinkage when it changes from the molten to solid state. Plaster bound to silicate investment is used for cast gold alloy (with expansion of 1-1.7%). While cobalt chromium alloys quartz bound by ethyl or sodium silicate is used (with expansion of 2.3%). Venting is usually used with chromium-cobalt alloy to avoid entrapment of gases in the molten metal.

  12. Procedures: -The refractory cast is dipped into slurry water to moisten its surface, to prevent the dry refractory cast from taking up water from the investment. Then paint on layer of investment material is applied to the surface of the cast. -The cast with the paint on layer is placed in an investment ring. The cast should be placed in proper relation to the bottom of the ring so that an adequate thickness of investment is available to strengthen the mold and still allow for the escape of gas during casting. Then the investment material is measured, mixed and placed in the ring. -After setting of the investment, the sprue lead is removed and the entrance of the mold is inspected to eliminate any debris or sharp edges. The investment ring is removed before burnout in case of cobalt chromium casting.

  13. 9-Burnout Purposes: 1-Vaporizes the pattern leaving a cavity in the mould. 2-Drives off moisture from the mold to avoid porosity. 3-Provides thermal or hygroscopic expansion for the mold . The mold is placed in a burnout furnace for the required time and temperature. The time and temperature required to eliminate the wax are factors of the refractory- alloy system.

  14. 10- Casting: The metal is melted and forced to the mold by centrifugal force. The metal is melted by one of two ways: a-The torch method: Gas and oxygen are used for alloys that melt close to 1093c. Oxyacetylene mixture is used for casting require higher temperature. b-Induction casting based on electric current in a metal core: It is a controlled melting procedure and eliminates the variables in torch method

  15. Removing the casting from the mold: For cobalt chromium alloys the casting is allowed to cool in the mold. The outer layer of investment is broken by a wooden mallet- The first layer is then removed by sandblasting. For gold alloy castings, the ring is quenched in water and the investment is removed from the casting by brushing. Then the casting is placed in diluted sulfuric or hydrochloric acid (pickling), for final cleaning.

  16. 12-Finishishing and polishing: -With high speed lathes and large abrasive disks the sprue leads are cut from the casting. -The casting is finished to the proper form and to remove any blebs. Coarse disks and stones at high speed and with minimal pressure, is used first, and then going into finer stones. -The framework is then fitted on the master cast. Special powdered spray and liquid disclosing media are used to detect areas that prevent seating of the casting. The seating and spot grinding is continued until the rests are completely contacting the cast. -The cobalt chromium alloys are electro polished to give it the high shine appearance. -Rubber, rag and felt cones are used on the high speed to apply the final polish. An ultrasonic cleaner is commonly used to remove all traces of the polishing material.

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