RPD LAB - PUTTING THE METALS INTO RPD

Question 1

what is casting

Answer 1

Casting is a process by which a detailed wax pattern of a dental restoration is converted into alloy or ceramic.

Question 2

how is casting used

Answer 2

Casting is used to fabricate inlays, onlays, crowns, ceramic-allow crowns, some all ceramic crowns, partial dentures, implant restorations and frameworks and occasionally a complete denture.

Question 3

what technique is used in waxing

Answer 3

lost-wax technique

Question 4

what happens in the lost wax technique

Answer 4

wax shape 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 5

why is the casting technique used for partial dentures different from other dental castings

Answer 5

this method involves a refractory model whereas other castings (crowns, bridges or implants) are waxed on a definitive cast and cast indirectly from the cast itself.

Question 6

what is the refractory model

Answer 6

a model made of a material that is able to withstand high temperatures

Question 7

what is the refractory model made in

Answer 7

phosphate-bonded investment material.

Question 8

why must the refractory model be treated to harden it

Answer 8

they are subject to surface wear

Question 9

what materials can be used to harden the refractory model

Answer 9

 Bees wax
o The model is heated and dipped into hot wax
 Resin
o The model is heated and dipped into cold resin (most common)
 Aerosol
o The cold model is sprayed

Question 10

how is the model hardened

Answer 10

The model is heated to 200 degrees Celsius for 45 minutes and then dipped for 10 seconds in the model hardener. The hardener is absorbed into the model – no dimensional change takes place. The model is allowed to cool (15-20 minutes) and the wax pattern can be placed directly onto the model surface. It is important that the model is cooled fully otherwise the wax will melt on the surface.

Question 11

what is the wax pattern

Answer 11

The wax pattern is a detailed model of the plate.

Question 12

why is wax used to create the pattern

Answer 12

Wax is used because it is easy to manipulate, inexpensive and well suited to make custom restorations. It is also easy to completely eliminate (via burnout) after investing.

Question 13

what is the two stages of creating the maxillary plate wax pattern

Answer 13

 Stage 1: fill the space made by the preparation wax for the internal finishing line
 Stage 2: 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

Question 14

what happens when adapting the palate

Answer 14

A wax sheet of 0.5mm is used and adapted in two 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 are fitted together.
The two halves are adapted and lightly burnished to remove any visible junction.

Question 15

why are two sheets used when adapting the palate

Answer 15

because there is a deep vault in the palate and you can end up adapting it to much so using two halves of wax sheets results in better adaption.

Question 16

how is the retention in the wax pattern made

Answer 16

A wax retention pattern is chosen and attached closed 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 17

describe how the clasps are made in the wax pattern

Answer 17

The appropriate wax pattern is selected and positioned on the prepared ledge
The mesh is filled to the clasp attachment to join with the major connector, this ensures that 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.

Question 18

why is it important that the mesh is filled to the clasp attachment to join the major connector

Answer 18

this ensures that 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.

Question 19

what is a sprue

Answer 19

conducts the molten metal into the mould

Question 20

where is the sprue attached

Answer 20

to the thickest area and closest to the ingress of metal

Question 21

how should the casting cool

Answer 21

from the exterior to the centre of the metal inflow

Question 22

what does allowing it to cool allow

Answer 22

prevent cooling shrinkage producing voids in the casting

Question 23

why is the placement and number of sprue important

Answer 23

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

Question 24

where should the sprue attachment always be

Answer 24

higher than any part of the wax pattern

Question 25

what happens in investing

Answer 25

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 26

how is the furnace temperature increased

Answer 26

from 0 to around 1100 in approx 3-3.5 hours

Question 27

why is the mould held at the precasting temperature to heat soak for 3-0 minutes

Answer 27

to ensure even the thinnest sections of the mould is hot to receive the alloy

Question 28

what does failure to ensure the thinnest parts of the cast are hot result in

Answer 28

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

Question 29

what temperature does cobalt chromium alloy melt at

Answer 29

1400 degrees celcius

Question 30

what happens if the mould isn’t hot enough

Answer 30

The mould must be hot to receive the molten metal or the mould will expand and crack or explode

Question 31

what do high frequency indication casting machines do

Answer 31

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 cobalt chrome partial denture.

Question 32

what are the high frequency indication casting machines designed to do

Answer 32

These machines are designed to make casting precise parts for dental restorations simple and repeatable as possible, and some automate much of the process including melting the metal.

Question 33

how do the casting machines cast the liquid

Answer 33

using centrifugal force

Question 34

after casting is complete what happens

Answer 34

the mould is allowed to cool (manufacturers guidelines differ for different investments)
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

Question 35

how is the mould cleaned

Answer 35

using a sandblasting cabinet with the aluminum oxide (300 microns particle size)

Question 36

when inspecting what should be looked for

Answer 36

Air bubbles on the surface of the cast, caused by possible non-appropriate mixing time or vacuum mixing being used when making the refractory cast.

Question 37

what happens if there is abrasion on the cast when fitting

Answer 37

must be attended to by technician

Question 38

how is the mould finished

Answer 38

It is smoothed using fine tungsten burs or fine stone

It is fitted to the cast and ensured that it fits

Question 39

how is the mould prepared for electropolishing

Answer 39

This passes electric current through the cobalt chrome making it shiny. It can also reduce the thickness slightly so wax is being placed on rests to stop this.
It is also maybe not a good idea to polish retention as it may shine through