2 Laboratory Procedures RPD
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Transcript of 2 Laboratory Procedures RPD
7/27/2019 2 Laboratory Procedures RPD
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Laboratory ProceduresFor RPD Framework
Construction
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Steps of Construction
1- Preparation of master cast
2- Duplication
3- Wax pattern & Sprueing4-Investing& burn-out
5- Casting6- Finishing & Polishing
7-Fitting the framework to the
master cast
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III- Wax Pattern & Spruing
* The investment cast is also called the
refractory cast or the working cast.
* It is used to make the wax pattern of
the denture on it.
* Following the wax pattern, a sprue must
be added.
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1- The Wax pattern
a- Design transfer to working cast
b- Wax pattern :
. Ready made wax patterns.
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a- Design transfer
* The master cast with the design drawn
on it should be available during making
the wax pattern.
* No surveyor marker or pencil drawn
are used.
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Sprueing the framework:
Sprueing is attaching wax or plastic form
to the wax pattern, to provide an entrance
to the mold space and to serve as a
reservoir of metal during castingprocedure
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Rules for Spruing
1- The sprues should be large enough.
2- The sprues should lead into the molten
cavity as directly as possible.
3- Sprues should be attached to the wax
pattern at its bulkier sections.
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Types of Sprues
* There are two basic types of sprues,
multiple & single.
* A single sprue is preferred for completedenture metal bases.
* The majority of partial denture castingsrequire multiple spruing .
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Types of multiple sprues
1- Top sprueing : central sprue from which
auxiliary sprues run to each corner of the
wax pattern
Used in maxillary cases
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2- inverted sprueing:
In which the base of the refractory cast
should have a hole in its center.
A cone- shaped plastic sprue is placed
into the hole.
Auxiliary sprues are attached between the
main sprue and the thick sections of the
wax pattern.
USED IN MANDIBULAR CASES
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3-Horizontal sprueing:
In which sprueing is from the posterior
edge of the casting
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Inverted sprueing
It is the best type of sprueing
because contraction of themetal during cooling tends to
pull the casting towards the
model rather than away it.
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Important points in multiple
spruing1- Use a few sprues of large diameter
rather than several smaller sprues.
2- Keep all sprues short & direct.
3- Avoid abrupt changes in direction.
4- Reinforce all junctions with additionalwax to avoid constriction in the sprue
channel.
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Investing
* Place the working casts on the ring .-
investment. mix is applied in two
layers: .
*1- Precision investment
layer(3to4mm).To insure that no air
bubbles are trapped
*2- Outer ( bulk ) investment.
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IV- Casting
* Prepare alloy & crucible
* Burn out
* Casting
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1- Prepare alloy & crucible
* The first step in casting is to load the
crucible with a suitable amount of alloy.
* The casting alloy is cobalt-chromium
* Its main properties:
. It is hard, rigid & corrosion resistant
. Its melting point is between 1250-1450 degree
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It expands the mold to compensate
for contraction of the metal on cooling.
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Casting: Molten metal is forced into the pre-heated
mold by the use of centrifugal force
casting machine
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VI- Finishing & Polishing
* Removal of investment
* Finishing
* Polishing
* Trial seating on the master cast
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