Students do not realize that rubber base impression material is different than alginate. You must not fill up the tray with rubber base. If you do so then that will mean that the tray will not be seated completely (rubber base is more viscous than alginate) and the tray borders will not reach to the mucobuccal and mucolingual folds.

If your impression material is 5mm thick on the palate then your tray borders will miss the fold by 5mm. (A lot of excess material will also flow down the patient's throat.) With this one mistake you are undoing all your effort at border molding.

11. Rubber base too thick. A section of the palate has been cut out to measure the thickness.

11. Close-up of cut out. Rubber base is 5mm thick preventing the borders from reaching the fold.

©1999 by Julius Rosen, D.D.S.