Putting the metal into RPD's

Question 1

Precision model casting uses a method known as the ‘lost-wax’ technique. What is this?

Answer 1

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.

Question 2

Why is the lost wax technique different than the techniques used for producing other dental castings?

Answer 2

This method involves a refractory model whereas other castings (crowns, bridges or implant bars) are waxed on a different cast and cast indirectly from the cast itself

Question 3

What is the refractory model made of?

Answer 3

Made in phosphate-bonded investment material

• (it is vitally important that the investment is mixed according to the manufacturers instructions regarding powder/liquid ratio and mixing time)

Question 4

Although phosphate-bonded investments are stronger than gypsum-bonded investments on heating they hare subject to surface wear. How can we overcome this?

Answer 4

• They must be treated by hardening the surface of the model

Question 5

The refractory model can be hardened by using a variety of materials. What are the 3 options?

Answer 5

• Beeswax (the model is heated and dipped into hot wax)
• Resin (the model is heated and dipped into cold resin)
• Aerosol (the cold model is sprayed)

Question 6

What is the process of hardening the model? (4 points)

Answer 6

• The model is heated to 200*C for 45 mins and then dipped for 10 secs in the model hardener
• The hardener is absorbed into the model - no dimensional change takes place
• The model is allowed to cool (15-20mins)
• The wax pattern can be placed directly onto the model surface

Question 7

When hardening the refractory model, will any dimensional change take place?

Answer 7

• No

Question 8

What is stage 1 of preparing the maxillary plate with wax?

Answer 8

• Fill the space made by the preparation wax for the internal finishing line
• Start where there could potentially be weak points - add a little extra wax to this area

Question 9

What is stage 2 of preparing the maxillary plate with wax?

Answer 9

Although the plate will be 0.5mm a sheet of 0.2mm is adapted over the centre of the plate to provide strength and resistance to flexure

• Middle of the plate is quite a deep vault so have a little extra wax for support and strength

Question 10

What is the process of adapting the palate when playing the wax?

Answer 10

• A wax sheet of 0.5mm is used and adapted in 2 stages
• The second sheet is placed overlapping the first sheet and using a fine blade the knife is drawn down the junction and the two pieces fitted together
• The two halves are adapted and lightly burnished to remove the visible junction

Question 11

How is retention added to the wax design?

Answer 11

• A wax retentive 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 (sometimes) as retention as space in this area is limited for acrylic retention

Question 12

What does a sprue do?

Answer 12

A sprue conducts the molten metal into the mould

Question 13

Where is a sprue attached to?

Answer 13

• It is attached to the thickest area and closest to the ingress of metal

Question 14

The casting should be progressively cooled from the exterior to the centre of the metal inflow. It is recommended to let it bench cool once you take it out of the casting machine. What advantage does this have?

Answer 14

• This should prevent cooling shrinkage producing voids in th e casting

Question 15

The is the placement and number of sprues important?

Answer 15

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

Question 16

Does the cone and sprue have to be higher than any part of the wax pattern?

Answer 16

Yes

Question 17

What does a mould (muffle) former consist of?

Answer 17

A base and ring shape to hold the investment

Question 18

Do phosphate bonded investments require a supporting metal ring?

Answer 18

• No

Question 19

How long after setting can an investment mould be places into a furnace?

Answer 19

One hour after setting

Question 20

What is the max temp of the furnace and how long does it take to achieve this?

Answer 20

• Goes from 0-1100

- In approx. 3-3.5 hours

Question 21

The mould is held at the pre-casting temp to ‘heat-soak’ for 30 mins. What does this ensure and why is that important?

Answer 21

• Ensures that even the thinnest section of the mould is hot

- Failure to ensure this will mean the metal freezes in contact and the mould is not filled

Question 22

At what temperature does Co/Cr alloy melt at?

Answer 22

Around 1400*C

Question 23

Why must the mould be hot to receive the molten metal?

Answer 23

• As if it is not the mould will expand and crack or explode

Question 24

When casting is completed the mould is allowed to cool. The cooled mould is then removed from the metal. How is this done?

Answer 24

By tapping the residue of the casting button

Question 25

Once the mould is removed from the casting it is cleaned. What is it cleaned with?

Answer 25

Cleaned using a sandblasting cabinet with aluminium oxide (300 microns particle size)

Question 26

Sometimes there can be air bubbles on the surface of the cast. What can this be cased by?

Answer 26

• Caused by possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast
• These should be removed by the technician

Question 27

What do abrasions on the cast indicate?

Answer 27

• Indicates that a bit of the metal has to be taken away to make the denture fit better

Question 28

What is electro-polishing and how should you prepare the cast for this?

Answer 28

• Passes a current through and strips the surface away form the alloy
• Makes it shiny
• This can reduce the thickness of the Co/Cr so:
• Wax should be placed on occlusal rests to stop this happening at these points