working cast and die part 1

what is the working cast?

also known as the master cast, it is a positive replica of the prepared teeth, ridge areas and other parts of the dental arch

what is the die?

the positive replica of the prepared tooth and consists of suitable hard substance of sufficient accuracy (usually an improved stone, resin or metal)

what are the ideal requirements for good casts?

1. must be bubble free, especially along the finish lines of the prepared teeth

2. all parts of the cast must be distortion free and dimensionally stable

3. the cast must be easily trimmed for easy wax pattern construction and carving

4. easy removal of the dies and can be easily repositioned

5. the cast should represent the entire dental arch so it can be mounted on an articulator for proper occlusal analysis and restoration fabrication.

what are the ideal requirements of the die?

1. must be able to reproduce all fine details of the prepared tooth with the exact size and shape

2. should be easy to remove and reposition in its original position in the cast every time

3. high abrasion resistance, so it should have a high surface hardness

4. should be compatible with all impression materials

5. easy to manipulate

6. excellent dimensional stability

7. good color contrast to easily detect margins

8. economic

what is the classification of die according to type of material construction?

1. metal, which can be amalgam, electroplating (copper or silver), bismuth-tin spray

2. stone, which can be plaster, stone and improved stone

3. ceramic (refractory die) which can be diamond or ceramco

4. cements, which can be silicophosphate or polyacrylic acid

5. resin, which can be epoxy or acrylic

6. flexible die material

when is amalgam die used?

only in copper band impression with modeling compound

why is amalgam die contraindicated with rubber base?

due to elastic property of the material as it will not withstand forces of amalgam condensation

what are the advantages of amalgam die?

hard and gives acceptable surface details

what are the disadvantages of amalgam die?

• longer setting time

• greater setting expansion

• mercury may effect the gold casting

what is electroplating?

a historical technique used to produce a metal die by depositing a thin layer of pure copper or silver onto the surface of an impression using an electric current. It is rarely used today.

what can electroplating be used to overcome?

poor abrasion resistance of gypsum

what is electroplating not compatible with?

not compatible with silicone rubber due to different surface energies that do not allow deposition of even thickness of the deposited layer

what are the components of electroplating apparatus?

• cathode, the impression to be coated

• anode, the metal to depositioned (copper or silver)

• anode and cathode holder

• electrolyte solution, either silver cyanide or copper sulphate

• ammeter register

• plating tank

• temperature

what are the advantages of electroplating?

1. strong

2. hard and high abrasion resistant

3. produces a smooth surface

4. better marginal definition

5. stable and can be stored for many years

6. dimensionally stable

what are the disadvantages of electroplating?

1. time of plating requires 8-12 hours and must remain undisturbed. during this time, some impression materials may undergo dimensional changes

2. time consuming

3. technique sensitive

4. liability to distortion in the metal coat if not performed slowly

5. not compatible with all impression materials

what is bismuth-tin spray?

it melts at 130°C and can be sprayed directly on the impression to form a metal shell, which can be filled with dental stone or cold cure acrylic. the metal can be readily sprayed into elastomeric impression materials

what is a disadvantage of bismuth-tin spray?

the alloy is soft and care is needed to prevent abrasion

stone and plaster are both products of what?

gypsum, which is the dehydrate of calcium sulphate

what are the particles of hemihydrates powder like in plaster?

porous and irregular in shape

what are the particles of hemihydrates powder like in stone?

dense and uniform

the more fine and small the stone particles, the better the what?

the reproduction of the die, greater strength and hardness and better heat resistance

what are types of dental gypsum products according to ADA specification?

1. type I (impression plaster)

2. type II (model plaster)

3. type III (dental stone)

4. type IV (high strength dental stone or die stone)

5. type V (high strength with high expansion or improved stone)

what are plaster and stone used for?

for the fabrication of the diagnostic casts, opposing arch and as the base portion of the cast containing removable dies

what is improved stone used for?

dies and working casts

what is the primary advantage of improved stone?

hardness and resistance of abrasion

what are the advantages of stone die?

• needs less requirements

• easy to manipulate

• can be used with all types of impression materials

• economic

• good strength

• has a smooth and hard surfaces

what do all gypsum products show?

a liner expansion during setting due to the outward thrust of the growing crystal during setting

when is setting expansion undesirable in dentistry?

impression plaster, dental plaster and stone as it results in inaccurate cast or change in occlusal relation if used for mounting

when is setting expansion desirable in dentistry?

in investment materials as it helps to compensate the shrinkage of the metal during casting

how can setting expansion be controlled?

• mechanical mixing is preferred to hand mixing

• increase in W/P ratio

• modifiers such as sulphate, sodium chloride and borax can decrease expansion

what is diamond die material?

a type of refractory die that is supplied as a powder and liquid that is mixed to form a putty like consistency. after one hour the material is removed from the impression and placed in the oven at 1000°C for 8 minutes. can be used with polysulfide and silicone impressions

what is ceramico?

a type of refractory die material that is packaged in a kit which contains vials of powder and liquid. can be used with elastomeric impression

what are the advantages of refractory die materials?

• accurate and fit is better

• easy technique

• used for porcelain restoration construction

what are the disadvantages of refractory die materials?

expensive

what is resin?

can be epoxy or acrylic, self-cure type is used as a die material to overcome the low strength of die stone

what are the advantages of resin as a die material?

• greater abrasion resistance than die stone

• compatible with silicone and polyether impression material

• recommended for fabrication of all ceramic restorations

what are the disadvantages of resin as a die material?

• polymerization shrinkage occurs during setting so it tends to produce an undersized die, so it is not suitable for routine die material

• expensive

• less dimensional stability

• viscous, does not flow readily

what is silico-phosphate cement used for?

to make dies from compound and elastomeric impression

what are the disadvantages of silico-phosphate cement?

• shrinks on setting

• has a tendency to crack due to dehydration

• air bubbles are easily entrapped and cause the die to become brittle

what is flexible die material?

similar to heavy bodied silicone or polyether impression material. have been found useful for the production of provisional restoration and indirect composite inlay or onlay made chairside

what are advantages of flexible die material?

• more rapid setting

• easy removal or provisional restoration

how are working cast and dies classified according to design and die system?

1. single die

2. working cast with separated dies

3. working cast with removable dies, which is broken down into

• dowel pin system

• straight dowel pin

• curved dowel pin

4. pindex system

5. di-lok tray

6. DVA model system

7. zeiser model system

what are single dies?

an old system that was used with a single copper band. the impression was made of either compound or rubber base impression materials