Working Casts and Dies Notes

Working Casts and Dies

Introduction

• Fabrication of a fixed partial denture (FPD) is an indirect laboratory procedure.

• Begins with receiving the impression and instructions from the clinician.

• Procedures aim to fabricate a cast metal prosthesis veneered with resin or ceramic.

• Involves making a working (master) cast with removable dies where the wax pattern is fabricated.

• The pattern is invested and cast using the lost wax process.

• The metal restoration is then veneered with resin or ceramic.

Lab Procedures for Fabricating an FPD

• Pouring working casts and dies.

• Fabricating wax patterns.

• Spruing.

• Investing.

• Casting.

• Veneering.

• Soldering.

Working Cast and Dies Defined

• Working cast: A master cast that reproduces the entire arch, including prepared and unprepared teeth, soft tissues, and edentulous areas.

• Die: A positive reproduction of only the prepared teeth (abutments).

• Wax patterns are fabricated and critical margins are finished on a die.

• The die is placed on the articulated working cast to check occlusion, axial contours, and interproximal contacts.

• Dies may be removable or an integral part of the working cast, or separate from the cast.

Requirements

Working Cast

• Accurate reproduction of prepared and unprepared tooth surfaces.

• Free of voids in critical areas (prepared teeth and adjacent teeth).

• Proper articulation with opposing casts.

• Accurate recording of soft tissues, including residual ridges.

Dies

• Accurate reproduction of the prepared tooth in dimension and surface details, without voids.

• Accurate reproduction of unprepared tooth structure cervical to the finish line to aid in the development of the proper cervical contour.

• Clearly visible and easily accessible margins.

• Form should aid easy handling during waxing and other procedures.

Materials for Working Casts and Dies

Requirements

• Accuracy.

• Strength and abrasion resistance.

• Ease of sectioning and trimming.

• Contrasting color to wax for easy visualization of margins.

• Compatibility with impression material.

• Wettability by wax and compatibility with separating agents.

Gypsum

• Type IV gypsum (die stone) is the most commonly used die material.

• Type V gypsum can be used if higher expansion is required.

• Advantages:

  • Easy to use and cost-effective.

  • Good compatibility with all impression materials.

  • Adequate surface detail reproduction and dimensional accuracy.

• Disadvantages:

  • Poor abrasion resistance.

• Gypsum hardeners like aqueous colloidal silica or soluble resin solutions can be used instead of water.

• The surface of the die stone can be treated with a resin such as epoxy, acrylic, or cyanoacrylate.

Resins

• Epoxy resins and polyurethanes are used.

• Advantages:

  • High strength and abrasion resistance.

  • Good detail reproduction.

• Disadvantages:

  • Expensive.

  • Polymerization shrinkage (less of a problem with newer materials and polyurethane).

  • Not compatible with polysulphides and hydrocolloids.

Electroformed Dies

• The process of coating the impression by placing it in an electroplating bath.

• Pure silver or copper is deposited on the impression, and a cast is then poured with type IV gypsum or resin.

• Advantages:

  • Good abrasion resistance and detail reproduction.

• Disadvantages:

  • Distortion may occur if not handled properly.

  • Time-consuming procedure, and special equipment is required.

  • Incompatible with polyethers and sometimes even with some brands of silicones.

  • Silver electroplating uses cyanide solution, which is extremely toxic.

Divestment

• A gypsum-bonded material used both as a die and investment material.

• Mixed with colloidal silica to make the working cast and die.

• The wax pattern is fabricated on this, and the wax pattern along with the cast is invested in the same material.

• Eliminates the need to remove the wax pattern for investing and prevents distortion.

• Cannot be used with high-fusing alloys (base metal and metal ceramic); can be used only for conventional gold alloys.

• Type IV gypsum is the most popular and commonly used die material for fixed prosthodontics.

Fabrication Techniques

1. Working cast with separate dies.

2. Working cast with removable dies.

(1) Working Cast with Separate Dies

• The impression is poured twice.

• First, only the prepared teeth are poured with type IV gypsum for making a die, and the partial cast is removed from the impression.

• Then a full arch cast is poured for the working cast, which is then articulated.

• The wax pattern is first fabricated on the die and then transferred to the working cast for checking contour, contact, and occlusion.

• It is finally returned to the die to refine the margins.

• Advantages:

  • Simple method.

  • The working cast requires only minimal trimming.

  • Intact gingival tissues around the prepared teeth in the working cast guide proper contouring of the cervical aspect.

• Disadvantages:

  • May be difficult to transfer and seat delicate patterns from dies and working casts.

  • Distortion is a concern.

Techniques

1. Moisture is removed from elastomeric impressions by using a surfactant and from hydrocolloid impressions by blowing with air.

2. The required amount of die stone is mixed as per the manufacturer's instructions, and prepared and adjacent sections of the cast are poured.

3. The stone is built up to a height of approximately 1.0 inch (2.5 cm) over the preparation. This allows bulk for an adequate handle on the die.

4. It is allowed to set for 1 hour, and the sectional cast is removed from the impression.

5. The handle of the die should be octagonal in cross-section, slightly larger in diameter than the preparation, and parallel to the long axis of the tooth. The ideal length of the handle should be about 1 inch.

6. An acrylic bur is used to trim the die below the margin, and the contour near the margin can be finished using a scalpel blade. The contour of the die apical to the finish line should be similar to a natural tooth root to allow good axial contours of the finished restoration.

   • A. Contour below margin similar to natural tooth.

   • B. Die trimming influences axial contours.

(2) Working Cast with Removable Dies

• The die is an integral part of the working cast and can be removed to facilitate fabrication and finishing of the restoration.

• Advantages:

  • The wax pattern need not be removed from the die while transferring it to the working cast. This reduces the chances of distortion of the wax pattern.

• Disadvantages:

  • The gingival portion of the abutment is destroyed while making the die, and hence it becomes difficult to verify the emergence profile of the restoration.

Removable Dies Can Be Made Using

1. Dowel pins.

2. Di-lok trays.

(1) Dowel Pins

• A metal pin is used in stone casts to remove die sections and replace them accurately in the original position.

• Available with a metal or plastic sleeve to facilitate accurate repositioning on the cast.

• Types:

  • Flat-sided single dowel.

  • Curved dowel.

  • Straight double dowel with common head.

  • Two separate parallel dowels.

  • Dowel with plastic sleeve.

Methods Employed to Position the Dowel Pin

1. Prepour technique – pins are oriented before the impression is poured.

2. Postpour technique – pins are placed after the cast has been poured.

Prepour Technique

• A bobby pin is most commonly used to hold and position the dowel pin. Other materials that can be used for this purpose are anesthetic needles, paper clips, and paper matches.

• A dowel pin is placed between the arms of a bobby pin. The bobby pin is then positioned buccolingually across the impression such that the dowel pin will be centered directly over the preparation, without touching the impression, and along its long axis.

• The bobby pin is attached to the impression with sticky wax.

• Die stone is mixed and poured into the impression covering the knurled end of the dowel pin and at least 3–4 mm beyond the gingival margins of the teeth.

• Paper clips are placed in the stone before it sets in other areas of the cast to aid retention to the base.

• After the stone sets, the bobby pins are removed from the impression, and a small ball of soft utility wax 5 mm in diameter is placed on the tip of each dowel to aid in removing the dowel pin after setting.

• A ‘V’-shaped groove is carved on the cast with a Bard–Parker (BP) blade running buccolingually from each dowel pin to aid in reseating the die completely and accurately during use.

• The open lingual/palatal space of the impression is covered with baseplate wax or a paper towel to enable pouring of a complete base.

• The cast is lubricated around each dowel with Vaseline or separating medium to permit the easy separation of the dies from the working cast.

• The base of the cast is poured with dental stone.

• After the stone has set, the cast is removed from the impression, and the excess is trimmed till the wax balls are exposed.

• The wax is removed to expose the tips of the dowel pins.

• A die cutting saw with thin blades is used to section the dies.

• Cuts are made vertically and interproximally on either side of the prepared teeth, such that it converges towards the base.

• The exposed end of the dowel is gently tapped with an instrument handle to loosen and remove the die.

• Advantages:

  • Simple method without any elaborate equipment.

• Disadvantages:

  • Pins can be displaced while the cast is poured.

Postpour Technique

• A full arch cast is first poured to cover the entire impression.

• After setting, the cast is removed from the impression, and the base is trimmed such that at least 5 mm of base is available beyond the gingival margins of the teeth.

• Holes are drilled with a fissure bur on the base of the cast corresponding to the center of the prepared teeth and other areas of the cast so that all the holes are parallel.

• Dowel pins are cemented with cyanoacrylate or luting cement such as zinc phosphate on the prepared holes.

• Wax balls and grooves are placed as described previously. Separating media is applied around the dowels on the prepared teeth. The cast is boxed, and the base is poured with dental stone.

• The die is sectioned and removed as described with the previous technique.

• Advantages:

  • The final impression can be poured immediately without any obstructing pins.

  • The removable section is planned with a master cast rather than impressions.

• Disadvantages:

  • Fracture of the first pour of the cast if made too thin.

Pindex System

• Used to place the dowel pins accurately parallel over the center of the prepared teeth.

• After the cast is retrieved and trimmed to the appropriate dimensions as described previously, the pins are placed with the help of the Pindex system.

• The desired location of the pins is marked on the occlusal surfaces of the teeth and preparations. There should be two pins for each die, each for an edentulous area, and the segment containing unprepared teeth.

• The machine is turned on, and the cast is placed on the worktable.

• The red illuminated dot from the light beam director is aligned with the pencil dot marked on the cast.

• A hole is drilled by the machine directly below the red light on the base of the cast as the cast is pressed down. All the marked areas are drilled, and dowel pins are cemented with cyanoacrylate.

• When the cement has dried, the sleeves are placed over the pins such that the flat sides of their bases face each other.

• The base of the cast can be added by either boxing the cast conventionally or by using specially fabricated base formers.

• After sectioning the dies, die hardener and die spacer are added, and the casts are mounted.

• Advantages:

  • Pins are placed accurately and parallel to each other.

• Disadvantages:

  • Special equipment is required.

(2) Di-Lok Trays

• Involves the use of a specially fabricated tray that allows precise reassembling of a sectioned working cast.

• The impression is poured with die stone to a height of 1 inch (2.5 cm).

• The ‘U’-shaped cast must have an open lingual area and must be trimmed so that it fits into a Di-Lok tray.

• Stone is mixed and vibrated into the tray until it is three-quarters full.

• The cast is seated such that the cervical lines of the teeth are about 4 mm above the edge of the tray.

• The stone is allowed to set.

• The cast can be removed by tapping on the front of the base of the tray with the handle of a laboratory knife.

• A saw blade is used to cut between the prepared and adjacent teeth.

• The die and the other parts of the cast can be removed and reassembled as required.

• The Di-Lok trays can be attached to an articulator by means of undercut rails on it.

• Advantages:

  • It is a simple procedure.

• Disadvantages:

  • The large size of the tray makes articulation and manipulation cumbersome.

Die Spacer

• Applied to a die to provide space for the luting agent in the finished casting.

• It is applied prior to commencing the fabrication of wax patterns.

• The die spacer is a material that is applied to the surface of a die to create space between the tooth and the restoration. This space is required for the cementing material.

• The margins should not be coated with the spacer. A band of about 1 mm adjacent to the finish line is not coated.

• This allows for adequate marginal adaptation.

• The thickness of the die spacer depends on the number of coats applied. A thickness of about 20–40 μm is adequate.

• The solvent of the die spacer has a tendency to evaporate; hence, a thinner is supplied along with the spacer.