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. 8-19