lab procedures in removable partial dentures

LABORATORY
PROCEDURES
in RPD
• www.asiandentalacademy.org

CONTENTS
• Duplicating a stone cast
• Waxing the rpd framework
• Spruing,
• investing,
• burnout,
• casting, &
• finishing of the rpd framework

DUPLICATING A STONE CAST
STONE CAST CAN BE DUPLICATED FOR
SEVERAL PURPOSES –
1) To preserve the original cast
2) Fitting of the RPD framework on the
duplicate cast
3) Processing of temporary prosthesis
4) To allow formation of investment cast for
framework fabrication• www.asiandentalacademy.org

• Blockout should be accomplished on the
master cast before making an investment
cast
• Wax or plastic pattern is formed on the
investment cast
• Metal framework is cast against its surface
• Finished casting is checked for fitting on the
duplicate cast

DUPLICATING MATERIALS & FLASK
• COLLOIDAL MATERIALS
• SILICONE MATERIALS
If wax or clay blockout is present, the
temperature of the duplicating material must
not be any higher than recommended to
prevent melting & distorting the blockout
material

Procedure for duplication

WAXING THE RPD FRAMEWORK
• On the investment cast, the wax or plastic
pattern is formed
• With free hand waxing, care must be taken
not to score or abrade the investment cast
• Use of preformed plastic patterns eliminates
the danger of damaging the cast to some
extent

PREFORMED PLASTIC PATTERNS

3 steps in making denture
framework using relief, blockout
ledges, & readymade pattern
forms

• 1) Master cast with relief, blockout of undercuts at
posterior right & anterior lingual regions, & shaped
blockout ledges for location of retentive &
nonretentive clasp arms

• 2) Completed pattern using lingual bar major
connector pattern, plastic clasp forms resting on
investment ledges, wrought wire, & open
retention mesh

• 3) Finished casting returned to master cast

Forming the wax pattern for a
mandibular class II RPD framework

• Occlusal view of mandibular kennedy class I,
modification 1 on a refractory cast
• The lingual bar major connector joins 3 clasp
assemblies – RPA, wrought wire & cast
circumferential

BUCCAL
VIEW
BUCCAL
VIEW rt. side
LINGUAL VIEW

Attaching wrought wire Retainer arms
by soldering
• May be attached to the framework after it has been
cast & finished
• May be accomplished by
# Electric soldering
# Direct heat method
In either method care must be taken to use
compatible alloys, appropriate solder, & flux in
conjunction with the careful application of controlled
heat

ELECTRIC SOLDERING
• An 18-gauge, round wrought-wire clasp is carefully
adapted & then secured to the framework &
duplicate stone cast using refractory investment

• The flux is placed at the proximal plate, followed
by placement of sufficient solder, & the electric
heat element tip is placed into contact with the
solder while the frame is grounded at another
location

Finished & polished wrought wire retainer

SPRUING, INVESTING, BURNOUT,
CASTING, & FINISHING OF THE RPD
FRAMEWORK

FACTORS THAT INFLUENCE THE EXCELLENCE OF A
DENTAL CASTING
According to BRUMFIELD-
1) Care & accuracy with which the cast is
reproduced
2) Intelligence with which the framework is
designed & proportioned
3) Care & cleanliness in waxing up the cast
4) Consideration of the expansion of the wax
caused by temperature

5) Size, length, configuration, points of
attachment, & manner of attachment of
the sprues
6) Choice of investment
7) Location of the pattern in the mold
8) Mixing water : amount, temperature &
impurities
9) Spatulation of the investment during
mixing

10) Restraint offered to the expansion of the
investment because of the investment ring
11) Setting time
12) Burn-out temperature & time
13) Method of casting
14) Gases; adhered, entrapped, & absorbed
15) Force used in throwing the metal into the
mold

16) Shrinkage on cooling
17) Removal from the investment after casting
18) Scrubbing, pickling
19) Polishing & finishing
20) Heat handling

SPRUING
• The sprue channel is the opening leading
from the crucible to the cavity in which the
framework is to be cast
• Sprues have the purpose of leading the
molten metal from the crucible into the
mold cavity
• Sprues should provide a reservoir of molten
metal from which the casting may draw
during solidification

• The sprues should be large enough that the
molten metal in them will not solidify until after
the metal in casting proper has frozen
• 8-12 gauge round wax is usually used for
multiple spruing of partial denture castings

• The sprues should lead into the mold cavity as
directly as possible and still permit a
configuration that will induce a minimal
amount of turbulence in the stream of molten
metal

• Sprues should leave the crucible from a common
point & be attached to the wax pattern at its
bulkier sections

• Sprue channels should not contain sharp
right angle turns, to avoid turbulence being
induced & leading to the entrapment of
gases which results in faulty castings

SPRUES ARE OF TWO TYPES-
SINGLE MULTIPLE

SINGLE SPRUE
• Used in maxillary RPD
because of presence of
palatal plate
• Single sprue must be
attached to the wax pattern
so that the direction of flow
of molten metal will be
parallel to the long axis of
the single sprue
• Disadvantage- extra large
investment ring should be
used for large castings

MULTIPLE SPRUES
• Use few sprues of larger diameter
rather than several smaller sprues
• Keep all sprues as short & direct as
possible
• Avoid abrupt changes in direction;
avoid T-shaped junctions as much as
possible
• Reinforce all junctions with
additional wax to prevent
constrictions in the sprue channel &
to prevent V-shaped sections of
investment that might break away &
be carried into the casting

Investing the sprued pattern
• Investment of a RPD
casting consists of –
Investment cast on
which the pattern is
formed &
Outer investment
surrounding the cast &
the pattern

• The outer investment is confined within a metal
ring, which may or may not be removed after the
investment has set
• If the metal ring is not to be removed, it must be
lined with a layer of cellulose, asbestos
substitute, or ceramic fiber paper to allow for
both setting & thermal expansion of the mold in
all directions
• The investment must conform accurately to the
shape of the pattern & preserve the
configuration of the pattern as a cavity after
pattern itself has been eliminated through
vaporization & oxidation• www.asiandentalacademy.org

PURPOSES OF THE INVESTMENT
• Provides strength necessary to hold the forces
exerted by the entering stream of molten metal
until this metal has solidified into the form of
pattern
• Provides a smooth surface for the mold cavity
• Provides an avenue of escape for most of the
gases entrapped in the mold cavity
• Provides (together with other factors), necessary
compensation for the dimensional changes of the
alloy from the molten to the solid, cold state

INVESTMENT MATERIALS
• PLASTER BOUND INVESTMENT MATERIALS
– used for alloys having low melting
temperature
• SILICA BOUND INVESTMENT MATERIALS –
used for alloys having high melting
temperature

Procedure of investing the pattern
• Line the casting ring with a ring liner. Liner
should be 6 mm shorter at the crucible end.
Liner permits hot gases to escape
Wet the liner after it is in place
No need of liner if a split type forming flask is
used that will be opened and removed as
soon as the investment has set

• Investment cast should be soaked in room
temperature water before the application of
investment material
• Mix the investment according to the
manufacturers directions
• Paint the pattern with a wetting agent
(debubblizer) just before applying the
investment to reduce the surface tension of
the wax so that the outer investment readily
covers & adheres to the pattern

• Apply the investment with a soft moistened brush to
assure complete adaptation of the investment. This
will help in reducing the number of bubbles and make
for a smoother cast surface
• Entire pattern should be covered with about 6 mm of
investment

• After the painting investment has reached its intial
set, it may be invested in the casting ring or flask
former

BURNOUT
The burnout operation serves three
purposes-
1) It drives of moisture in the mold
2) It vaporizes & thus eliminates the pattern,
leaving a cavity in the mold
3) It expands the mold to compensate for
contraction of the metal on cooling

• For the investment to heat uniformly, it should be
moist at the start of the burnout cycle. Steam will then
carry the heat into the investment during the early
stages of burnout
• Mold should be placed in the oven with the sprue hole
down
• Burnout should be started with a cold oven.
Temperature should be increased slowly to a
temperature recommended & maintained for the
period recommended by the manufacturer
• More time must be allowed for the plastic patterns

• Invested pattern is removed from from the
burnout oven following complete elimination of
the wax pattern

CASTING
• Method of casting will vary widely with the alloy &
equipment being used
• All methods use force to quickly inject the molten
metal into the mold cavity. This force may be either
centrifugal or air pressure
• If too little force is used- mold is not completely filled
before the metal begins to freeze
• If too much force is used- excessive turbulence may
result in the entrapment of gases in the casting
• With centrifugal casting machines, this can be
regulated by the number of turns put on the actuating
spring

• Metal may be melted with a gas-oxygen blowtorch or
by an electric muffle surrounding the metal
• Induction method may be used which provides a rapid
& accurate method of melting the metal

• Currently available casting machines
include those that are electrically
controlled to heat alloys to a specified
temperature and to release the molten
metal at precisely the correct casting
temperature
• These machines are expensive

CENTRIFUGAL CASTING MACHINE

ELECTRICALLY CONTROLLED CASTING MACHINES

VACCUM PRESSURE CASTING MACHINES

REMOVING THE CASTING FROM THE
INVESTMENT
• Chromium cobalt alloys are usually allowed to cool in the
mold, are divested & are not cleaned by pickling

• After removal of the investment from the casting by
brushing under water with a stiff-bristle brush, the
casting should be further cleaned by pickling
PICKLING
The casting is placed in a porcelain dish & acid pickling
solution such as dilute sulfuric acid is poured over it to
a depth sufficient to cover it.
The dish is heated over a flame until the surface of the
casting brightens
Then the pickling solution is poured off & the casting
washed in abundance of water

FINISHING & POLISHING
• Important rules in finishing the casting-
1) High speeds are preferable to low speeds
2) The wheels or points and the speed of their
rotation should do the cutting. Excessive pressure
heats the work, crushes the abrasive particles,
causes the wheels to clog & glaze, & slows the
cutting
3) Definite sequence for finishing should be adopted
& followed for every framework
4) Clean polishing wheels should be used
5) Be sure each finishing operation completely
removes all scratches left by the preceding one• www.asiandentalacademy.org

Electropolishing

lab procedures in removable partial dentures

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