Figure 8-31.Two-step method of cast fabrication.
With a second mix of stone, form a patty for the base.
Add stone around the retention nodules and invert the
first pour into the patty. Now use you spatula to shape
the sides of the cast base as shown in figure 8-31(B).
Take care not to bring stone up onto the tray
embedding the tray into the base. With a mandibular
cast, you must take time to smooth and contour the
tongue space while the second mix is still soft.
Casts should present as neat and attractive as
possilbe. After the heat generated by the final setting
reaction dissipates completely (about 45 minutes),
trim any stone from the outside of the tray with a
plaster knife and separate the cast from the impression
as shown in figure (8-31(C). Do not hurry and force the
tray or you may break off some teeth, especially if the
tray is embedded in the base. If a cast is not separated
from an alginate impression before the alginate shows
signs of dehydration, the cast will probably show
unacceptable surface damage.
Before trimming the cast, you must soak it in a
saturated dihydrate solution (SDS). Never trim a dry
cast because the slushy debris coming off the trimming
wheels will fall on the dry surface and become
permanently attached to the cast surface. Mark the cast
with trimming lines. These lines will help you
determine the base thickness; align the base plane to
the occlusal plane of the teeth; and trim the outer
boundary alongside the posterior teeth, the anterior
teeth, and the posterior border of the cast.
Figure 8-32 shows the desired cast dimensions of a
trimmed cast. Start trimming the cast by grinding the
cast bottom parallel to the occlusal plane of the teeth.
The cast should be about 15 mm (5/8 inch) thich at its
thinnest place (usually the palatal vault of the upper
and the tongue space region of the lower). Make sure
the cast includes all the denture support areas and all of
the features that define denture borders. Keep the cast
free of nodules or voids. When trimming a maxillary
cast, make it as much like the general shape shown in
fighre 8-32. Trim a mandibular cast to correspond with
the shape shown in the same figure. Fully represent the
sulci area in the cst, but not more than 3 mm deep. The
sulci are routinely protected by a peripheral "land"
area or ledge extending 4 mm outward. Make sure the
cast extends 5 mm beyond the hamular notch area of
the maxillary arch and 5 mm beyond the retromolar
pads of the mandibular notch.