Putting metal into dentures

Question 1

What is the method called for doing precision model casting

Answer 1

‘lost-wax’ technique

Question 2

describe the lost wax technique

Answer 2

• a wax shape is made
• a mould is made to surround the shape
• the wax is removed from the mould by melting the wax
• the shape is fulled with molten metal

Question 3

how is the method used for model casting different from producing other dental castings such as crowns, bridges or implant bars

Answer 3

the lost wax technique involves a refractory model

other castings are waxed on a definitive cast and cast indirectly from the cast itself

Question 4

what material is the refractory model made in

Answer 4

phosphate-bonded investment material

Question 5

how must phosphate-bonded investments be treated and why?

Answer 5

• subject to surface wear

- so must be treated by hardening the surface of the model

Question 6

how do you harden the refractory model

Answer 6

The refractory model can be hardened by using a variety of materials including:

• beeswax
• resin
• aerosol

Model is heated to 200oC for 45mins and then dipped for 10s in the model hardener.
Hardener absorbed into the model (no dimensional change)
Model allowed to cool
The wax pattern can be placed directly onto the model surface

Question 7

Describe how the wax patterns are put onto the casts

Answer 7

1. fill the space made by the preparation wax for the internal finishing line
2. a sheet of 0.2mm is adapted over the centre of the plate (maxillary) to provide strength and resistance to flexure
3. A wax sheet of 0.5mm is used and adapted in 2 stages
4. the second sheet is placed overlapping the first sheet and using a fine blade the knife is drawn down the junction and the 2 pieces fitted together
5. the 2 halves are adapted and lightly burnished to remove any visible junction
6. a wax retention pattern is chosen and attached to the major connector
7. external finishing line added
8. anterior posts added for anterior teeth/ single teeth
9. wax pattern for clasps added

Question 8

What conducts the molten metal into the mould

Answer 8

a sprue

Question 9

how do you attach a sprue

Answer 9

it’s attached to the THICKEST area and CLOSEST to the ingress of metal

Question 10

why should the cast progressively cool from the exterior to the centre of the metal inflow

Answer 10

to prevent cooling shrinkage producing voids in the casting

Question 11

why is the placement and number of sprues important

Answer 11

to prevent air turbulence or air being trapped which would cause voids or porosity within the casting

Question 12

what does a mould former consist of in the investing stage

Answer 12

a base and ring shape

Question 13

what is the procedure for putting the mould into the furnace

Answer 13

• temp of furnace 0-1100oC in 3 hours

- mould is held at pre-casting temp to ‘heat-soak’ for 30mins

Question 14

what happens if the mould isn’t held at the pre-casting temp to ‘heat soak’ for 30mins

Answer 14

the metal will freeze on contact and the mould won’t be filled

Question 15

what temp does cobalt chromium alloy melt at

Answer 15

1400oC

Question 16

why must the mould be hot to receive the molten metal

Answer 16

if it isn’t the mould will expand and crack or explode

Question 17

what do high frequency induction casting machines do

Answer 17

• use pressure and high temperatures to cast precious, semi-precious and base metals into specific shapes, such as a frame for a crown or bridge, or for a co/cr RPD.
• use centrifugal force to cast the liquid metal

Question 18

what happens when the casting is complete

Answer 18

• the mould is allowed to cool
• the cooled mould is removed from the metal by tapping the residue of the casting button
• once the mould is removed the casting is cleaned using a sandblasting cabinet with aluminium oxide (300µm particle size)

Question 19

on inspection of the cast why might there be air bubbles

Answer 19

caused by possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast

Question 20

when refining the fit of the base using a gypsum cast what do you need to remember

Answer 20

gypsum can be abraded much easier than tooth enamel

Question 21

what happens after you are happy with the fit of the denture to the cast

Answer 21

finishing and electro-polishing