Model Cast and Die Materials: Types, Properties, and Applications

Model Cast And Die Materials

Casts and models are an important part of dental services. Plaster and stone are the usual materials used to prepare casts and models. However, it must be remembered that other materials can also be used for this purpose.

Models

Models are used primarily for observation, diagnosis and patient education, e.g. orthodontic study models, diagnostic casts, etc.

Read And Learn More: Basic Dental Materials Notes

Casts

A working model or master cast is the positive replica on which restorations or appliances are fabricated, e.g. complete denture, removable partial denture, orthodontic appliances. Casts should be made with a high level of accuracy. They should be handled with great care, taking care not to scratch or damage its surface.

Dies

A positive replica of a prepared tooth or teeth in a suitable hard substance on which inlays, crowns and other restorations are made. Similar care should be taken in ensuring its accuracy as well as handling.

Types Of Die Materials

Gypsum

• Type 4 dental stone
• Type 5 dental stone, high strength, high expansion
• Type 5 dental stone + lighosulfonates (this wetting agent reduces the water requirement of a stone and thus enables the production of a hard, stronger and more dense set gypsum)

Metal and metal-coated dies

• Electroformed
• Sprayed metals
• Amalgam

Polymers

• Metal or inorganic filled resins
• Polyurethane
• Epoxy

Cements Silicophosphate or polyacrylic acid bonded cement. These are no longer commonly used currently.
Refractory materials This includes investments and divestments. Investment casts are used to make patterns for RPD frames. Divestment dies are used in direct baking of porcelain crowns or preparation of wax patterns.

Ideal Requirements Of Die Materials

An ideal die material should

• Be dimensionally accurate.
• Have good abrasion resistance, strength and toughness to allow burnishing of foil and resist breakage.
• Have a smooth surface.
• Be able to reproduce all fine details in the impression.
• Be compatible with all impression materials.
• Have a color contrast with wax, porcelain and alloys.
• Be easy to manipulate and quick to fabricate.
• Be noninjurious to health by touch or inhalation.
• Be economical.

Alternate Die Materials

Improved Dental Stone Or Die Stone

The most commonly used die materials are still alpha hemihydrate type IV and type V gypsum products. Type 4 gypsum products have cuboidal-shaped particles and the reduced surface area produce the required properties of strength, hardness and minimal setting expansion.

The most recent gypsum product, having an even higher compressive strength than the type 4 is the high strength, high expansion type V stone. The setting expansion has been increased from 0.01 to 0.3%. This higher setting expansion is required in the stone used for the die to aid in compensation for the base metal alloy solidification shrinkage.

Advantages

• Good strength.
• Minimal shrinkage.
• Easy manipulation.
• Good working time.
• Sets quickly.
• Compatible with impression materials.
• Has smooth, hard surface.
• Can be easily trimmed.
• Has good color contrast.
• Is economical.

Disadvantages

• Brittle.
• Not as abrasion resistant as the epoxy and electroformed dies. Edges and occlusal surface may be rubbed off.

Electroformed/Electroplated Casts And Dies

Electrodeposition of copper or silver on the impression gives a hard metallic surface to the cast. Electroformed dies are not used currently; however, they will be described for historical reasons.

Advantages

• Dimensional accuracy.
• Hard and abrasion resistant.
• Imparts a smooth surface to the wax pattern in contact.
• Not very expensive.
• Better marginal definition.
• Does not absorb oil or water.
• Prevents cuspal wear due to repeated contact with opposing cast.

Disadvantages

• Difficult to trim.
• Silver bath is a potential health hazard.
• Not compatible with all impression materials.
• Color contrast not as good as die stone.
• Adaptation of wax not as good, pattern tends to lift from margins.

Electroforming

Electroforming (also known as electroplating or electrodeposition) is a process by which a thin coating of metal is deposited on the impression, after which a gypsum cast is poured. The cast thus obtained will have a metallic surface layer.

• Metals used for electroforming are
  • Copper
  • Silver
• Plating can be done for
  • Individual tooth impression
  • Full arch impression
• Plating is done on
  • Compound impression (usually copper plated)
  • Polysulfide impression (usually silver plated)
  • Silicone impression

Other impression materials show dimensional changes when plated.

Components Of An Electroplating Apparatus

A commercially available apparatus for electroplating is displayed.

• Cathode The impression to be coated is made the cathode.
• Anode is the metal to be deposited, i.e. copper or silver.
• Anode holder, cathode holder.
• Electrolyte is a solution through which the electric current is passed. Ions are deposited from anode to cathode, e.g. silver cyanide or copper sulfate.

• Ammeter registers the current in milliamperes (0–500 mA). The current passed is 10 mA per tooth area, for 12 hours.
• Plating tank is made of glass or hard rubber with a well-fitting cover to prevent evaporation.
• Temperature 77 to 80 °F (room temperature).

Composition Of The Electroplating Bath

Procedure

• Wash and dry the impression.
• Metallizing Most impression materials do not conduct electricity. They are made conductive by applying a metallizing solution or powder with a brush.
  The metallizing agents are
• • Bronzing powder suspended in almond oil.
  • Aqueous suspension of silver powder.
  • Powdered graphite.
• The surface of the impression tray is covered with wax 2 mm beyond the margin of the impression. This protects the tray and prevents its plating.
• With a dropper, the impression is filled with electrolyte, avoiding air-bubbles.
• The impression is attached to the cathode holder with an insulated wire.
• The electrode is attached to the cathode and the impression is immersed in the electrolyte bath. Distance between the cathode (impression) and anode (metal) should be at least 4 inches.
• Initially, current should not exceed 5 mA. Later the current is increased to 10 mA per tooth for 12–15 hours, to get a deposit of 0.5 mm (If a high current is used the surface will be granular, uneven and weak. With low currents the deposit is smooth and hard).
• The current is disconnected. The impression is washed. The die is completed by pouring resin or dental stone to form the cast and base.

Polyurethane

Resin die materials were developed for applications where increased hardness and abrasion resistance is desired. One such material developed for this purpose is a polyurethane resin.

Mode Of Supply

Supplied in glass bottles containing

• Base material (200 mL)
• Hardener (100 mL)
• Filler (400 g)

Indications

Indicated for use with elastomers. Die separator must be applied when casting polyether impression.

Contraindication

Not indicated for use with alginates and hydrocolloid impression materials.

Properties

It is flowable, accurate in detail and dimensionally stable. It has high edge strength and abrasion resistance and is easy to trim and saw.

Manipulation

Briefly shake the bottles containing both the base material and hardener prior to mixing. Close the glass bottles carefully immediately after use. Do not allow the material to come into contact with water (foam).

Fill the required amount of base material in the dispensing and mixing container supplied. Then add the correct quantity of filler and thoroughly spatulate the mixture.

Add the correct quantity of hardener and spatulate the mixture again thoroughly.

Mixing ratio Base : hardener = 2:1. (10 g:5 g).

Approximately 15 grams filler is required for a full dental arch.

Mixing time Approximately 30 seconds.

Pouring After mixing the resin is poured in a thin stream into the cleaned and dried impression. The material remains flowable for approximately 2 minutes at 20 °C.

Curing In order to prevent air voids the die may be hardened for 15 minutes, after pouring, in a dry pressure vessel at 2–4 bars. The die is sufficiently hard after 1 hour to permit trimming and grinding.

Epoxy Resin Die Materials

Epoxy is another resin material that has been developed for die construction. They are most effective with rubber impression materials.

Advantages

• Tougher and more abrasion resistant than die stone.

Disadvantages

• Slight shrinkage (0.1%).
• Viscous, does not flow readily.
• Setting may take up to 24 hours.

Available As

Two components—resin paste and hardener.

Refractory Cast For Wax Patterns

A refractory cast is a special cast made from a heat resistant (investment) material. Such casts are used in the fabrication of certain large metal structures, e.g. cast removable partial dentures. Small wax structures like inlays, crowns and small FPDs can be constructed on a regular die as it can be removed from the die without significant distortion and invested separately. However, larger wax structures like that for the cast RPD, would distort if removed from the cast. RPD patterns are best constructed on a refractory cast. The pattern is invested together with the refractory cast.

Why not invest an ordinary gypsum cast?

• The conventional (nonrefractory) gypsum cast cannot withstand the high temperatures involved in the casting of metal and would disintegrate under these conditions.

Refractory Cast For Ceramics

• Refractory dies are also available for ceramic restorations [e.g. polyvest and VHT—Whipmix]. The all-porcelain restoration is directly built up on these refractory dies and fired (further detail in chapter on investments).

Die Stone – Investment Combination (Divestment)

This is a combination of die material and investing medium. A gypsum-bonded material called divestment is mixed with a colloidal silica liquid. A die is prepared from the mix and a wax pattern is constructed on it. Then the wax pattern together with die is invested in divestment.

The setting expansion of divestment is 0.9% and thermal expansion 0.6%, when heated to 677 °C.

As it is a gypsum-bonded material it is not recommended for high fusing alloys, e.g. metalceramic alloys.

Advantage

• It is a highly accurate technique for conventional gold alloys, especially for extracoronal preparations. In this technique, removal of the wax pattern from the die is not required. Thus, possibility of distortion of wax pattern during removal from the die or during setting of the investment is minimized.

Divestment Phosphate Or DVP

This is a phosphate-bonded investment that is similar to the divestment and is suitable for use with high fusing alloys.

Difference Between Divestment Cast And Refractory Investment Cast

Though both are quite similar, there are some fundamental differences. The investment casts are not as strong and abrasion resistant as the divestment cast. In fact, they are quite fragile and can disintegrate easily. Manufacturers have provided certain hardening solutions to compensate for this. Divestment is generally used for smaller castings, whereas investment refractory casts are used during the fabrication of larger structures, such as partial dentures frames and complete denture bases.