3. Compression moulding technique :
( the most commonly used technique for acrylic resins )
Preparation of the trail denture
2. Disarticulation
3. Flasking produce
4. Dewaxing ( Boil out )
5. Placing retention grooves on the artificial teeth … ( Diatorics )
6. Application of the separation medium
7. Mixing of powder and liquid
8. Packing
9. Curing
10. Cooling
11. Deflasking
12. Finishing
13. Polishing
1.
4. Preparation of the trail denture :
wax contouring ( festooning ).
sealing.
9. Disarticulation
Articulator without any remounting plates.
The mounting plaster is carefully splitting from the cast.
The articulator is placed on cloth to avoid damage of the trail
denture.
Split the junction between the mounting plaster and the cast
with help of wax knife and plaster mallet.
After separation, the cast’s soaked in water.
13. Flasking
the festooned wax denture must be converted to resin to make a final denture.
Is the process of investing the cast with the waxed denture in a flask to make a
sectional mold that is used to form the acrylic resin denture base.
Dental flask : is a metal case used in investing the denture.
It consist of :
1. Base ( lower half ).
2. Body or counter.
3. Lid.
Body with lid called ( upper half ).
15. Flasking procedure:
The Inner surface of the flask is coated with Vaseline, while the
base of the cast is painted with separating medium, To prevent the
investment material (plaster of Paris) from attaching to the cast.
17. The first layer of gypsum investment is poured in the lower
half and the cast is placed on top of the investment.
18.
19. After the set of the first gypsum investment layer, a
separating medium is painted on it, to prevent the sticking of
the second layer of gypsum investment to the first layer.
20. A mix of plaster is placed over the surface of the teeth in
the invested trial denture.
21. The body of the flask is put in place, then the second mix
of gypsum investment is placed on the first layer and covers
the wax, denture base and teeth.
22. After setting of the second pour, apply layer of
separating medium.
Avoid applying separating
medium over the exposed
occlusal surface of the teeth.
23. The third pour of dental plaster is poured to fill the body.
Then, the lid is placed on the body.
24. The clamp is tightened to hold the flask in position
Leave the flask for 30-60 minutes to reach it’s final setting time.
25. The advantage of three pour technique is that it facilitate
easy removes of the denture after curing.
28. Dewaxing, Wax elimination or Boil out:
dewaxing is done to remove the wax in the wax pattern,
so that a mould space is created for acrylic to fill in.
put the flask in boiling
water (100 C ) for 5 min.
before placing the flask into
the water bath, the clamp
should be loosened.
dewaxing is done to just
soften the wax and not melt it.
29. the softened wax is flushed out using hot water or wax
solvent or soap solution.
even the finest traces of waxes should be removed.
34. Placing retentive groove on the artificial teeth:
use a round bur to create small retentive pot holes ( diatorics ).
this helps to increase the strength of attachment between the
denture base and the teeth .
39. Application of the separation medium
separating medium is applied to the dewaxed mould space
prior to packing the acrylic resin.
the plaster and stone is painted with e.g tinfoil medium
carefully and must prevent the penetration of the media to
the teeth.
40. Roles of separation medium
To prevent the passage of water from gypsum to resin
To minmize the passage of monomer to the plaster
To facilitate separation of the flasks
41. various types of separating media used are :-
- cellulose lacquers.
- soft soaps.
- sodium silicate.
- starches.
- evaporated milk.
- Tinfoil.
- sodium alginate.
43. Mixing of powder and liquid
Heat cure acrylic is used , polymer /monomer is mixed
according to manufacture instruction , usually 10cc of
monomer , and 30cc of polymer will be enough to pack an
average- sized denture. ( 1:3 ) M / P.
when the material reaches the dough stage, it is ready for
packing.
47. Packing
packing is introduction of denture base resin into the
mould cavity.
the powder liquid mixture should be packed into the flask
at the dough stage .
48. • Divide acrylic in half and place in tooth portion of flasks.
To trial pack, cover with plastic sheet before closing flasks so
flasks can be re-opened to add acrylic, remove flash or insert
names.
49. use cellophane or polyethylene flim as a separator for the
half of the flask which is lower half ( on the cast ).
51. the flask is opened and the flash is trimmed away .
52. before final closure the separating film is removed.
final closure of the flask or metal to metal contact of the flask
halves then completed in the process .
57. Curing (polymerization) :
after final closure the flasks are kept at room temperature for 30
to 60 min . this is known as bench curing.
Purpose of Bench curing:
Permit an equalization of pressures throughout the mould
space.
58. Curing cycle: It is polymerization cycle.
The curing cycle selection depends on the thickness of the resin.
Curing (heating) cycle:
- Heat the flask in water at 60-70°C
for 9 hours.( long curing )
- Heat the flask
at 74°C for 90 minutes,
then at 100°C for 60 minutes.
( short curing)
64. the lid (cover) of the flask should be removed.
65. the flask is inverted and placed on the deflasker and
tightened with a thumbscrew.
pribars are inserted laterally to fit in to the slots of the flask.
66. these pribars are engaged and they are lifted up so that
separate the base and the body by lever action.
the body or connter will slide out of the investment.
68. three incisions are made one in the anterior mid-point and
two at the posterior corners.
use plaster knife to remove plaster around the denture
72. Finishing ( trimming & Sand papering )
Trimming :
Before trimming plaster at the necks of the teeth is removed
using a chisel .
Borders are trimmed using a laboratory size carbide bur.
Frenum relief is given using a straight fissure bur.
73. The thickness of palatal surface is reduced using a large
egg-shaped bur.
74.
75. Guide for trimming :
the denture should be held against the light and checked for
translucency.
Areas of opacity or darkness indicate excessive thickness of the
denture base. Uniform thickness of 2 to 2.5mm must be
maintained.
76. Sand paper finishing :
sand paper should be fixed on to a lathe mounted sand paper
mandrel.
Even the finest of all scratches should be removed during this
procedure.
Some technicians prefer the use of a wet sand paper to improve
the finish.
80. Polishing ( Pumice wash )
• Pumice should be loaded on a lathe-mounted rag wheel , A worn
out wheel should be conditioned prior to use.
• Pumice powder mixed with water is coated over the rag wheel.
• The finished denture should be polished by intermittently
pressing against the rotating rag wheel.
81. • a hand piece mounted rubber cup or a medium bristle brush can
be used to polish the inaccessible areas.
• After polishing, the denture should be thoroughly washed in
soap water.
• A tooth brush can be used to remove the remaining plaster and
pumice.
• After finishing , the denture are stored in water and stored till
the day of the insertion appointment.
87. Errors of sealing :
Improper sealing lead to change the relation between the trail
denture and cast.
88. Errors of disarticulation
Distortion of trail denture and the remounting plates may result
during disarticulation.
89. Errors of flasking:
Tooth movement:
causes:
-
the use of plaster instead of stone.
in complete closure of flask.
stone adhere to the surface of the denture:
causes :
-
the application of separating medium contimated with stone.
space between the teeth and resin base.
90.
91.
92.
93.
94. Errors of dewxing:
The soap solution is said to react with the gypsum forming a
membrane impervious to wax.
any traces.
95.
96. Errors of mixing the resin:
If too much monomer is used will lead to:
- there will be greater curing or polymerization shrinkage .
- more time is needed to reach the packing consistency.
- porosity can occur in the denture.
If too little monomer is used will lead to:
- not all the polymer beads will be wetted by monomer and the cured acrylic
will be granular.
- dough will be difficult to manage and it may not form a continuous mass
during processing.
Improper mixing & contamination of the mixture with dirty hand or instrument
may lead to color streaks.
97.
98. Errors of packing:
Insufficient separating medium will lead to:
- entering the water to the mould and may affect the rate of polymerization,
color and crazing.
- penetrating of the monomer into the mould material causing plaster to adhere
to the acrylic resin and producing a rough ( porosity ) surface .
the introduction of too much material termed over packing leads to :
- denture base has excessive thickness .
- movement the prosthetic teeth.
the introduction of too little material called under packing lead to :
- denture base porosity.
99. Adding the resin material in layers during trail packing may
lead to color streaks.
Packing the resin in sticky stage may lead to porosity.
Lack of pressure, lead to presence of fine pits.
100.
101. Errors of curing:
Immediate curing after closure of the flask without allowing the
packed flask to bench set for at least 30 minutes before curing, lead to
presence of fine pits in the resin.
Rapid rise of temperature during the initial curing, lead to gases
porosity, crazing and fractured in the acrylic denture base or teeth.
Incorrect time and temperature of curing, lead to incomplete cure of the
denture.
Too much delay in the curing after packing, lead to sandy appearance
in the resin material.
102. Errors of cooling:
Rapid cooling after curing, lead to cracked, warpage
or fractured of the denture base or the teeth.
103. Errors of deflasking:
Improper deflasking as used the hammer for knocking the flask,
may lead to cracked or fractured of the denture base or the teeth.
104. Errors of finishing and polishing :
Reduction should never be carried out on the tissue surface.
Care should be taken to avoid excessive pressure on the denture
to prevent warpage.
