Putting the metals into RPDs Flashcards
what is casting
Casting is a process by which a detailed wax pattern of a dental restoration is converted into alloy or ceramic.
how is casting used
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.
what technique is used in waxing
lost-wax technique
what happens in the lost wax technique
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
why is the casting technique used for partial dentures different from other dental castings
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.
what is the refractory model
a model made of a material that is able to withstand high temperatures
what is the refractory model made in
phosphate-bonded investment material.
why must the refractory model be treated to harden it
they are subject to surface wear
what materials can be used to harden the refractory model
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
how is the model hardened
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.
what is the wax pattern
The wax pattern is a detailed model of the plate.
why is wax used to create the pattern
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.
what is the two stages of creating the maxillary plate wax pattern
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
what happens when adapting the palate
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.
why are two sheets used when adapting the palate
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.
how is the retention in the wax pattern made
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
describe how the clasps are made in the wax pattern
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.
why is it important that the mesh is filled to the clasp attachment to join 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.
what is a sprue
conducts the molten metal into the mould
where is the sprue attached
to the thickest area and closest to the ingress of metal
how should the casting cool
from the exterior to the centre of the metal inflow
what does allowing it to cool allow
prevent cooling shrinkage producing voids in the casting
why is the placement and number of sprue important
to prevent air turbulence or air being trapped which would cause voids or porosity within the casting.
where should the sprue attachment always be
higher than any part of the wax pattern
what happens in investing
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).
how is the furnace temperature increased
from 0 to around 1100 in approx 3-3.5 hours
why is the mould held at the precasting temperature to heat soak for 3-0 minutes
to ensure even the thinnest sections of the mould is hot to receive the alloy
what does failure to ensure the thinnest parts of the cast are hot result in
Failure to ensure this will mean the metal freezes on contact and the mould is not filled
what temperature does cobalt chromium allow melt at
1400 degrees celcius
what happens if the mould isn’t hot enough
The mould must be hot to receive the molten metal or the mould will expand and crack or explode
what do high frequency indication casting machines do
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.
what are the high frequency indication casting machines designed to do
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.
how do the casting machines cast the liquid
using centrifugal force
after casting is complete what happens
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
how is the mould cleaned
using a sandblasting cabinet with the aluminum oxide (300 microns particle size)
when inspecting what should be looked for
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.
what happens if there is abrasion on the cast when fitting
must be attended to by technician
how is the mould finished
It is smoothed using fine tungsten burs or fine stone
It is fitted to the cast and ensured that it fits
how is the mould prepared for electropolishing
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
