2 laboratory procedures rpd

7/27/2019 2 Laboratory Procedures RPD

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Laboratory ProceduresFor RPD Framework

Construction



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



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.



1- The Wax pattern

a- Design transfer to working cast

b- Wax pattern :

. Ready made wax patterns.



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.



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



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.



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 .



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



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



3-Horizontal sprueing:

In which sprueing is from the posterior

edge of the casting



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.



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.



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.



IV- Casting

* Prepare alloy & crucible

* Burn out

* Casting



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



It expands the mold to compensate

for contraction of the metal on cooling.



Casting: Molten metal is forced into the pre-heated

mold by the use of centrifugal force

casting machine



VI- Finishing & Polishing

* Removal of investment

* Finishing

* Polishing

* Trial seating on the master cast

2 laboratory procedures rpd

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