Working Casts & Dies Notes

Working Casts & Dies

Definitions

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

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

Working Casts

• Wax patterns are fabricated and margins finished on the die.

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

• Dies can be removable or an integral part of the working cast.

Requirements

• Accurate reproduction of prepared and unprepared tooth surfaces.

• Free of voids, especially in critical areas like prepared and adjacent teeth.

• Allows proper articulation with opposing casts.

• Properly recorded soft tissues, including residual ridges.

Dies

Requirements

• 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 proper cervical contour.

• Clearly visible and easily accessible margins.

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

Materials

General Requirements

• Accurate.

• Strong and abrasion-resistant.

• Easy to section and trim.

• Contrasting color to wax for easy visualization of margins.

• Compatible with impression material.

• Wettable by wax and compatible with separating agents to prevent the wax pattern from sticking.

Types of Materials

• Gypsum Products

• Resins

• Electroformed dies

• Divestment

Gypsum Products

• Type IV gypsum (die stone): 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.

Improving Abrasion Resistance

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

• Surface treatment with resins (epoxy, acrylic, or cyanoacrylate), but this can create an excessively thick surface layer.

Resins

• Epoxy resin and polyurethanes.

Advantages

• High strength and abrasion resistance.

• Good detail reproduction.

Disadvantages

• Expensive.

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

• Not compatible with polysulphides and hydrocolloids.

Electroformed Dies

• Process: Coating the impression by electroplating, depositing pure silver or copper, then pouring a cast with type IV gypsum or resin.

Advantages

• Good abrasion resistance and detail reproduction.

Disadvantages

• Distortion may occur if not handled properly.

• Time-consuming and requires special equipment.

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

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

Divestment

• A gypsum-bonded material used as both 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, eliminating the need to remove the wax pattern for investing and prevents distortion.

Limitations

• Cannot be used with high-fusing alloys (base metal and metal ceramic).

• Can be used only for conventional gold alloys.

Fabrication Methods

• Working cast with separate dies

• Working cast with removable dies

Working Cast with Separate Dies

• Impression is poured twice.

  • First, only the prepared teeth are poured with type IV gypsum for making separate die.

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

• Wax pattern is fabricated on the die and then transferred to the working cast to check contour, contact, and occlusion.

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

Advantages

• Simple method.

• The working cast requires 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 to working casts.

• Distortion is a concern.

Technique

• Remove moisture from elastomeric impressions using a surfactant and from hydrocolloid impressions by blowing with air.

• Mix die stone as per the manufacturer’s instructions and pour the prepared and adjacent sections of the cast.

• Build up the stone to a height of approximately 1.0 inch (2.5 cm) over the preparation to allow bulk for an adequate handle on the die.

• Allow to set for 1 hour and remove the sectional cast from the impression.

• 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.

• Use an acrylic bur 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 separate working cast is then poured of the entire arch with base and articulated.

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, reducing distortion.

Disadvantages

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

Methods for Making Removable Dies

1. Dowel pins

2. Di-Lok trays

Dowel Pins

• Metal pins used in stone casts to remove die sections and replace them accurately in the original position.

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

Types

• Various types are commercially available, differing in their mechanism to prevent rotation of the removable die.

Positioning Methods

1. Pre-pour technique: Pins are oriented before the impression is poured.

2. Post-pour technique: Pins are placed after the cast has been poured.

Pre-Pour Technique

• Any type of dowel pin can be used.

• 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.

• Steps:

  • 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 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.

Post-Pour 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 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 ball 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.

Pindex System

• A post-pour technique using a Pindex machine to place dowel pins accurately and parallel over the center of the prepared teeth.

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

• Steps:

  • 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.

Di-Lok Trays

• Use of a specially fabricated tray that allows precise reassembling of a sectioned working cast.

• Steps:

  • Impression is poured with die stone to a height of 1 inch or 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 Trimming

• Whichever procedure is used to fabricate removable dies, the apical portion below the margins should be trimmed similarly to that described for working cast with separate dies.

Die Spacer

• An agent applied to die to provide space for the luting agent in the finished casting.

• 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.

• 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.

Wax Pattern

• A wax pattern of the restoration is the next step in the fabrication of a fixed partial denture.

• For any restoration involving castings (lost wax process), a wax pattern is essential.

• It is used for making the metal portion of all metal and metal veneered with ceramic or resins.

• A wax pattern can be fabricated directly or indirectly.

Direct Technique

• The pattern is waxed on the prepared tooth in the patient’s mouth.

• It is made with type I inlay wax and can be adopted for small restorations like inlays and posts.

• It is practically impossible to make extracoronal restorations directly as it is cumbersome, technique sensitive, and will take away a lot of chairside time.

Indirect Technique

• The most commonly used method for fabricating fixed partial dentures.

• The pattern is fabricated on a die made from an impression of the prepared tooth.

Advantages

• Allows better visualization of the restoration.

• Margins can be accessed and finished better.

• Chairside time of the clinician is reduced.

• Better standardization in technique as it is fabricated in a laboratory.

Wax

• Type II inlay wax is used to fabricate indirect restorations.

Ideal Requirements

• It should soften uniformly.

• The wax color should be in contrast to that of the die to facilitate proper finishing of margins.

• It should not flake or chip when carved.

• It should burn out without any residue.

• It should be rigid and dimensionally stable.

• Must not flow at room temperatures.

Composition

1. Paraffin wax (40%–60%) – main ingredient

2. Carnauba wax, ceresin, candelilla wax – decreases the flow at mouth temperature.

3. Gum dammar – reduces flaking

4. Synthetic waxes – replace carnauba wax to improve the working characteristics.

5. Dyes – give color

Properties

1. Flow

   • At 45°C – type I and II should flow between 70% and 90%.

   • At 37°C – type I should not flow more than 1%.

   • At 30°C – type II should not flow more than 1%

2. Thermal properties

   • Low thermal conductivity – takes time for them to heat and cool uniformly.

   • High coefficient of thermal expansion – expand and contract when heated and cooled

3. Wax distortion

   • When distortion occurs, the wax is bent, and the inner molecules are under compression and the outer ones are in tension.

   • Once the stresses are gradually relieved, the wax tends to straighten and distort.

   • To minimize distortion:

     • Wax should be added in melted increments or drops, or by dipping in melted wax.

     • Margins must be remade just before investing.

     • The wax pattern should be invested as soon as it is completed

Waxing Instruments

1. PKT set: The waxing instruments designed by Dr. Peter K. Thomas (PKT) are the most commonly used. It consists of five instruments, each with a specific use:

   • No. 1 and No. 2 – wax adding

   • No. 3 – burnishing

   • No. 4 and No. 5 – carving

2. No. 7: Wax spatula is used for adding a large amount of wax.

3. Electric waxing instruments: These are also available for adding wax.

Advantages

• Allows precise temperature control.

• Carbon buildup in the instrument due to overheating will be minimal.

Wax Pattern Fabrication

Posterior crown/retainer

1. Coping fabrication: The coping can be made up of wax or resin. Wax is commonly used. It must reproduce the retentive features of the restoration. The die is first coated with a die lubricant (separating medium) to facilitate easy removal of pattern.

2. The coping is formed by adding wax a Number 7 wax spatula or dripping into molten wax.

   • Ensure that the previous layer of wax is melted before the addition of new layers of wax. This prevents the formation of voids and flow lines.

3. The proximal areas should be given extra bulk to help removal from die.

4. Excess wax is trimmed from the margins so that the coping can be removed and evaluated.

5. Axial contours: This involves fabrication of the proximal, buccal and lingual surfaces.

   • Proximal: Size and location of the contact is first established. If the contact area is too narrow, there may be wedging of food, and on the other hand, if the contact area is wide, it may injure the gingival tissues.

     • When viewed from the occlusal aspect, the contact of maxillary posteriors is located facially, except for the maxillary molars where it is located centrally

     • The contact of mandibular posteriors is located centrally.

     • When viewed buccally, the contact of maxillary and mandibular posterior teeth is located in the occlusal third, with the exception of the maxillary first and second molars, which are located in the middle third

   • Contour of the proximal axial surface below the contact point should be flat or slightly concave.

     • Overcontouring makes flossing and maintenance of periodontal health difficult.

   • Fabrication: The wax coping is replaced on the die. Wax is added in the contact areas ensuring that its size and location are in harmony with the anatomic form. The proximal surface gingival to the contact area is then correctly contoured.