1. Always use a clean mixing bowl and spatula. The best time to clean a bowl and spatula is immediately after pouring the impression while the material is still soft and easy to remove.
2. Measure the volume of water and weigh the powder before you mix any gypsum material. An accurate water-to-powder ratio is a must to preserve the properties of any gypsum product.
3. Always add the powder to the water, never the water to the powder.
4. Spatulate thoroughly by hand, incorporating al the powder evenly throughout the mix until creamy Avoid whipping the mix; doing so will cause the final product to have excessive air bubbles.
5. Vacuum mix using a power mixer-investor whenever possible. Vacuum mixing helps to eliminate incorporation of air into the mix. If you can't vacuum mix, tap the mixing bowl against the bench top or hold it on the vibrator for a few moments to express any air that might be trapped in the mix.
6. Never add water to a mix that is too thick; this interferes with the setting properties. It would be better to discard the mix and start over again.
The primary objective when you pour a cast is to capture all the surface detail of the impression, and as bubble-free as possible. This is done using a vibrator to make a thick gypsum mix, which flows into all the crevices of the impression. There are several ways to pour impressions, such as the upright, two-step, and boxed methods. We will only discuss the two-step method of pouring a cast, because it is the most successful method of pouring preliminary and final impressions.
Begin by holding the impression tray so that the handle rests against the vibrator. Start at one end of the arch and flow a small amount of stone into the impression, letting it slowly advance to the other side as shown in figure 8-30. Flow the stone slowly enough to watch the progress of the stone as you fill each tooth imprint. This should eliminate bubbles. If a bubble appears and does not go away with vibration, pop it with a small instrument. Use an acrylic mixing spatula to place small amounts of stone into minute preparations or teeth with wide incisal and narrow cervixes. Touch the impression to the vibrator to flow each addition of stone.
After covering all the critical surfaces of the impression, you may safely add progressively larger amounts of the mix. There is a rate of vibration that is best for each mix's ability to flow. The vibration intensity should be set high enough to make the material move across the surface of the impression. The vibrator is set too high if the impression "jumps" in your hand, moves so fast that it skips over surface detail, or if vibration wave patterns develop on the surface of the mix. Continue filling the impression stone to a level slightly above the height of the impression walls (about 2 mm thick). Do not flow stone over the outside of the tray because it must be removed before the impression can be separated from the cast.
Lastly, add retention nodules to this first pour as shown in figure 8-31(A). Stone retention nodules are used between the first and second pours so the two stone layers can be locked together mechanically. Place the handle of the tray in a holding device; do not lay the impression on the counter or the cast will be distorted. Now let this first pour set for about 45 minutes, or at least until the stone loses its glazed appearance before making the cast base.
It is now time for you to make the cast base. Use the cast trimmer to grind down the long retention nodules (if completely set), reduce the base of the cast's thickness, and make the top of the tray parallel to the cast's bottom. In general, the length of the retention nodules should equal the height of the trav flanges.
Figure 8-30. - Pouring the first pour of an impression.Continue Reading