“Mix Thick; Don’t Flip.”
L.T. Armstrong, DMD: Pouring of the final impression is the second most critical step in constructing any successful restoration. Producing a bubble-free,accurate model from your impression requires strict attention to three areas, mix. position and time.
The first area is the consistency of the model stone mix. Properly mixed die or model stone is very thick and uniform (smooth). It should never be runny or lumpy. A test for proper consistency is, after mixing, invert and lightly shake the plaster bowl. Correctly mixed stone will not run out.
Using a vibrator, slowly feed the stone into the impression. Properly mixed stone will smoothly flow under vibration, giving the operator more control when pouring an impression. Thin mixes run under vibration, making it more difficult to control flow without trapping air (bubbles). A thick mix also provides a denser, harder model.
The second critical area is the position of the newly poured cast. Never invert your partially poured impression into a stone patty. Gravity will cause the stone to sag away from the surface of the impression. Unfortunately, the sag is slight and not noticeable to your eye. However slight, it is enough to cause inaccuracy and result in a poor fit particularly in rest areas, and palatal/horseshoe bars on upper removables.
A solution is to mix the stone thickly, pour the impression, turn off the vibrator, set the impression on the table top and mound the thick stone on top to sufficient model thickness. Place PlayDoh in the tongue area of the lowers to prevent the stone from locking around the lingual flange of the tray. A model grinder will easily get rid of the excess stone.
The third area is time. Do not try to separate the model from the impression for at least one hour after pouring, if possible, overnight is better. Gypsum products reach their maximum compressive strength after 24 hours.
These procedures will save you time by avoiding remakes.
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