12. Casting and Finishing

Question 1

Aim of casting

Answer 1

To produce a complete base that can be used to mount teeth on

Question 2

Methods of performing casting

Answer 2

Lost-wax technique

Question 3

Process of lost-wax technique (3)

Answer 3

Precision model casting uses a method known as ‘lost-wax’ whereby a wax shape is made, a mould is made to surround the shape
The wax is removed from the mould by melting the wax and the shape is filled with molten metal
This type of casting is different for producing other dental castings since this method involves a refractory model, whereas other castings (crowns, bridges or implant bars) are waxed on a definitive cast and cast indirectly from the cast itself

Question 4

Features of refractory cast/model (2)

Answer 4

Made in phosphate-bonded investment material
Although phosphate-bonded investments are stronger than gypsum-bonded investments on heating, they are subject to surface wear and must be treated by hardening the surface of the model

Question 5

Methods of hardening the refractory models (3)

Answer 5

Beeswax - model is heated and dipped into hot wax
Resin - model is heated and dipped into cold resin
Aerosol - model is sprayed onto a cold cast. They give surface hardness

Question 6

Casting for retention (3)

Answer 6

A wax retention pattern is chosen and attached close to the major connector and joined using joining wax
An external finishing line is added using round profile wax smoothed against the connector
For anterior teeth or single teeth, it is advisable to use posts as retention as space in this area is limited for acrylic retention

Question 7

Casting for clasps (3)

Answer 7

The appropriate wax pattern is selected and positioned on the prepared ledge
The mesh has been filled to the clasp attachment to join with the major connector, this ensures the clasp is fed from the thickest part of the casting and not from this mesh which is thinner than the clasp which might result in a miscast
Wax pattern replaces relief wax

Question 8

Definition of sprue

Answer 8

A sprue conducts the molten metal into the mould

Question 9

Attaching sprues (4)

Answer 9

The sprue is attached to the thickest and closest to the ingress of the metal part of the casting
The casting should progressively cool from the exterior to the centre of the metal inflow
This should prevent cooling shrinkage producing voids in the casting
The placement and number of sprues is also important to prevent air turbulence or air being trapped which would caise voids or porosity within the casting

Question 10

Process of investing (5)

Answer 10

A mould is prepared around the refractory cast using the same investment as the refractory
A mould (muffle) former consists of a base and ring shape to hold the investment
Phosphate bonded investments do not require a supporting metal ring
One hour after setting the mould can be placed into a furnace
The height of the sprue former is higher than any part of the casting and the casting increases in thickness from pattern to sprue to button (cone)

Question 11

Casting involves induction melting and vacuum/pressure casting - the process (3)

Answer 11

When the casting is completed, 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 (to remove green oxide layer and residual material)

Question 12

Process of finishing (5)

Answer 12

A lost wax technique is used for processing the denture
A metal dental flask is used in the processing of the PMMA denture
A two-part closed mould is made enclosing the wax denture using dental plaster
The two-part mould is opened and the wax is evacuated from the mould
The space left by the removed wax is filled using PMMA

Question 13

Investing the casting - flashing (2)

Answer 13

The waxed denture, sealed to the duplicate cast is embedded in dental plaster in the deeper of the denture flask halves. The wax palate is left exposed. On setting, the plaster mould is coated with a separating medium (sodium silicate) and the mould is completed with a 50/50 plaster/stone mix
Following the setting of the completed mould the dental flask is immersed in hot water for 10 minutes to soften the wax inside. The flask is separated and the wax evacuated with boiling water

Question 14

Filling the mould - packing (4)

Answer 14

The mould prior to ‘packing’ the PMMA
The plaster surfaces and cast are coated with a ‘mould seal’ to prevent plaster adhering or penetrating the denture
When the mixed PMMA has reach a dough stage it is packed into the mould with special attention being paid to packing the mould around the teeth
The mould is closed under pressure to expel excess material which on re-opening is remove

Question 15

Procedure for packing (6)

Answer 15

On setting, the mould is placed in boiling water for ten minutes to melt the wax inside the flask and to assist the opening of the mould
On opening the flask, the wax is evacuated with boiling water and the mould cleaned with a detergent to remove any wax residue
The two halves of the mould are coated with a mould seal and PMMA is packed into the mould at the appropriate ‘dough’ stage
A trial of the packing stage is conducted to ensure the mould is completely filled
The flask is opened and excess removed
The two halves are closed and placed into a spring compress and processed

Question 16

Mixing PMMA procedure (4)

Answer 16

Powder (polymer) and liquid (monomer)
Accurate mixing (10ml:24g – granular porosity)
Volumetric contraction of PMMA on processing. Packed when a ‘snap dough/ stage is reached
Processed under pressure (contraction porosity), controlled heating (gaseous porosity) and bench cooling (internal stress)

Question 17

Checking the occlusion following processing (4)

Answer 17

Split cast system
The duplicate model (with denture) is secured to the original mounting with plaster bandage
The occlusion is checked and refined as required against the opposing dentition
The incisal post at rest on the incisal table indicates the occlusal vertical dimension

Question 18

Process following occlusal refinement (4)

Answer 18

The duplicate model is removed from the denture by sawing the model from the denture
The denture is finished and polished
Polishing the mucosa-bound/impression side of the cast will impair the fit
Following polishing the denture can be fitted to the definitive cast

Question 19

Summary of denture casting and finishing/design (7)

Answer 19

The denture is fitted to the definivite (master) cast
The anterior flange should have a thin edge to blend with the surrounding soft tissue
The denture is finished above survey lines
The post dam and palatal periphery is thinned to blend with the palate
The occlusal vertical dimension must be the same as the original restoration
There should be no sharp edges
The denture should be polished only on the non-impression surface