Laboratory Procedures

For RPD Framework

Construction

Steps of Construction

1- Preparation of master cast

2- Duplication

3- Wax pattern & Sprueing

4-Investing& burn-out

5- Casting

6- Finishing & Polishing

7-Fitting the framework to the master cast

1- Preparation of Master Cast

A- Spraying

B- Beading of maxillary major connector.

C- Block out

C- Relief

Master Cast Preparation

A- SPRAYING

* Treating the master cast with a surface

sealer using a brush or an aerosol

spray to seal the cast and protect the

drawn design during the other steps of

cast preparation and during duplication.

* The sealer is a mixture of acetone,

diethyl phthalate, and cellulose acetate.

* The cast must be allowed to dry for

5 minutes.

Master Cast Preparation

B- Beading is the act of scraping the

outline of the major connector into

the master cast.

Master Cast Preparation

* The bead line is approximately

0.5-1 mm & becomes less distinct as

it approaches the gingival margin or

midline of hard palate.

Master Cast Preparation

* The bead line produces a raised edge

at the border of the major connector &

ensures positive contact of the major

connector with the palatal tissues to

prevent food particles from collecting

under the framework.

Master Cast Preparation

* Bead lines are not used with mandibular

major connectors because these

connectors rest on thin gingival tissues

that cannot tolerate the associated

pressures.

C- Block out

* Blocking out is the act of placing wax

and other materials into undesirable

undercuts on the master cast before

duplication.

Block out

* Both hard & soft tissue areas will

require block out so that the completed

casting will go completely to place

along the desired path of insertion.

Parallel blockout:

1-Proximal surfaces cervical to guiding

planes.

2-soft tissue undercuts that are crossed by

rigid connectors.

3-Soft tissue undercut crossed by bar

clasp.

Arbitrary Block out sites

* Gingival margins

* Spaces between teeth

* Tissue undercuts

* Gross undercuts not in the design but would

complicate the duplication procedure.

Ledges under clasp arms

Shaped Blockout

* Shaped block out or ledging is used on

the buccal or lingual surfaces.

* They guide the placement of the clasp

wax pattern on the investment cast.

D- Relief

* Any addition of wax intended to make

the framework stand away from the

master cast is termed “Relief”.

C- Relief

* In addition to serving as a spacer, one

margin of the relief wax forms the

“ Internal finish line” of the framework.

* It is essential that this finish line be

sharply defined.

Tissue stop.

A small square of 2mm of relief wax is

removed at the distal end of the saddle. It

provides stability of the framework during

acrylic resin processing

II- Duplication of the master cast

* Duplication is the procedure of

accurately reproducing a working

cast.

Steps of Duplication

* Production of the agar mold

* Making the investment ( refractory cast)

* Hardening the investment cast.

Duplication of master cast

* Prepared cast are duplicated to produce

an exact copy of the master cast in an

investment material “Refractory cast”.

* Refractory cast contains ceramic material that

permits it to withstand high burn out temperature

* Refractory cast has expansion to compensate

for metal shrinkage

Duplicating flaskDuplicating flask is a consists of:

1. Bottom..

2. Ring

3. Feeding top

Duplicating colloidsAgar-Agar

Re-used many times.

Cleaned and melted after each use.

Prepared and stored in automatic

duplicating machine.

The suitable working temperature is 63 °c

It prevent melting the blockout wax

Production of the agar mold

* Mount master cast

* Seat body of flask

* Prepare agar or silicone

* Filling the flask with agar

* after hardening remove the

master cast from the agar mold

Making the Investment Cast

* Make the investment mix

* Pouring

* Obtaining the investment cast.

Types of investment materials

1-Gypsum-bonded investment.

It can be burn out to 704 °c without

breakdown. It is used for low fusing alloys

as Gold

2- Phosphate-bonded investment

It can be burn out to 1037°c without

breakdown. It is used for high fusing alloys

As Cobalt chromium

The cast is removed from the colloid mold &

placed in drying oven at 180-200°F for ½- 1 hour.

(for hardening to avoid surface scratches)

Hardening the investment cast

while The cast is still warm it either:

• Dipped in Bees Wax at 280-300°F for

15 seconds or

•Sprayed

•This make the cast smooth and dense