STEPS IN MAKING DENTURE BASE:

IMPRESSION TAKINGo PRELIMINARY

Using modeling compound to make STUDY CAST (Plaster of Paris)

o SECONDARY By constructing an INDIVIDUAL TRAY

(working cast) CAST CONSTRUCTION

o Construction of working cast using dental stone WAX PATTERN CONSTRUCTION

o Use pink wax then heat it over open flameo Place the softened wax over the cast and press it hard

to get the exact shape of the cast then place another wax over the 1st wax to seal the periphery of melted wax

o Pour melted wax (using carver) into the periphery of wax pattern and allow it to flow to prevent Plaster of Paris from going inside wax pattern during investing

o Wax pattern should be stable to the working cast INVESTING OF WAX PATTERN

o Materials needed: Metal flask, plaster of paris/dental stone

(investing medium), separating medium (for easy separation of the upper half from the lower half)

The lower half of the metal flask is half-filled with Plaster of Paris

Place the cast with wax pattern (both coated with separating medium) into the plaster exposing only the wax pattern

Invest only up to the level of the base of w.c.

Wait for the plaster of Paris to set (initial setting time)

During initial setting time, coat the stone with a separating medium (COLORGARD) then wait for it to dry

Position the upper half of the flask over the lower portion

Pour a second mix of investing medium Remove excess so that there would be a

good metal to metal contact Allow the stone to set TWO METHODS OF POURING THE PLASTER

OF PARIS TO THE UPPER HALF OF THE METAL FLASK

SINGLE TECHNIQUEo Mixing the plaster

then pouring it to the upper half then place the cover

DOUBLE POUR TECHNIQUE/ TWO CAPPING TECHNIQUE

o Pouring is done twiceo First, pouring of the

investing medium is only to the level of

the wax pattern. This is good for the presence of PONTICS

ADVANTAGES OF DOUBLE POUR TECHNIQUE

o Easy location of teeth during deflasking

o Less danger of healing or moving teeth during deflasking

WAX ELIMINATION/BURNOUT PROCEDUREo Done to create a mold spaceo Immerse the flask in a water bath (60C, 5,mins)o Remove the flask; separate the upper and lower halveso Remove the softened wax patterno In case of overheating, flush out melted wax with

boiling water SEPARATION OF MOLD SPACE

o Make sure there are no melted wax on the cast so that all details will be copied

o Paint separating medium for easy deflasking and to prevent color change of the resin

PACKING OF RESIN DOUGHo Resin powder (polymer) liquid form/gel form

(monomer) CURING

o Heating process of resin to allow complete polymerization

o RAPID CURING 74C water bath for two hours then increase

to 100C for one houro SLOW CURING

74C water bath for 8 hours or overnight Better than rapid curing because

polymerization is more thorough DEFLASKING

o Removal of finished denture base from the metal flask and from the investing medium

o After curing, allow the metal flask to cool for 30mins (bench cooling)

o If metal flask will be put immediately to cold water, it will cause distortion

POLISHING THE DENTURE BASEo Trim excess resin, make surface shiny using wheel raq

and acrylustero It is important to remove the residual monomer (due

to low curing temperature and shorter time of polymerization) from the finished denture base because it can cause irritation to the oral tissues

STAGES OCCURING ONCE THE RESINS ARE MIXED DAMP/SANDY STAGE

o Mixture is cloudy and sandyo No reaction yet between polymer and monomer

STICKY/STRINGY STAGEo With threadlike projectionso Monomer attacks the surface of polymer

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o The mixture becomes viscous and sticks to the stirring rod

GEL/DOUGH STAGEo Mixture is easily molded into different forms or shapeso Ideal for compression moldingo Needed in packing of resin into the flasko No longer sticky

RUBBERY STAGEo Mixture is rubber-like in character and no longer flows

freelyo Cannot be molded anymore

STIFF STAGEo Mixture is dry and resistant to mechanical deformation

*Cover the mixture while waiting for the dough stage to prevent premature evaporation of monomers produces porous base

*When packing is done during the sticky stage, the denture base will be porous

*When mixture is in dough stage, collect the dough using moist cellophane for upper arch: roll the dough into a ball then place over the mold space then spread it for lower arch: form a horse shoe shaped dough

*Assemble again the upper and lower halves of the flask. Use the presser to remove excess resin(flash)

FLASH Excess resin

TRIAL CLOSURE Alternate opening and closing of the flask to remove excess resin Turn the handle slowly to pack the resin better

DIMENSIONAL CHANGES DURING CURING THERMAL EXPANSION

o Change from room temperature to temperature of water bath

CONTRACTIONo Polymerization shrinkage of resin

THERMAL CONTRACTIONo Change from temperature of water bath to room

temperature

DEFECTS OF DENTURE BASE: PRESENCE OF POROSITIES

o EXTERNAL POROSITIES Caused by improper manipulation of

investing medium Weaker, more retentive to food debris

(difficult to clean)o INTERNAL POROSITIES

Trapped within the finished denture base Caused by too rapid curing process and

improper manipulation of resin and packing too soon

Caused by premature evaporation of monomer

Not usually seen clinically

Weaker denture base CRAZING

o Small linear crackso Caused by too much pressure exerted during

deflaskingo Caused by accidental fall during deflasking

FRACTUREo Caused by too much pressure

PROPERTIES HEAT – CURED SELF - CURED

DIMENSIONAL STABILITY

Less dimensionally stable

more dimensionally stable

STRENGTH Higher lower

COLOR STABILITY More color stable Less color stable

HARDNESS VALUE 20KHN 16 – 18KHN

ACTIVATOR Heat Dimethyl paratoluene

RESIDUAL MONOMER

0.2 – 0.3% 3 – 5%

CAN WITHSTAND ABRASIONS

Can withstand

-Rosette Go 020109

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