Download - 1 Laboratory Procedures RPD
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