Section III. IMPRESSION MATERIALS
An impression is a negative reproduction of a given area of the oral cavity. The
area reproduced may be composed of either hard or soft tissues or both. The material
must be inserted into the mouth while it is too soft to retain its shape. A rigid base is
needed to carry it to the mouth and hold it against the tissues until it hardens. For this
purpose, a variety of trays, called stock trays, are available. These are shaped to fit
over the average maxillary and mandibular arches. Some can be trimmed and bent to
the requirements of the individual patient. Trays may also be fabricated for each
3-13. REQUIREMENTS FOR IMPRESSION MATERIAL
a. General. An impression material must meet a wide range of requirements in
order to provide an accurate impression of the different tissues.
b. List of Requirements. The following are some of the more important
(1) The material should flow or be pliable at a temperature that will not injure
the oral tissue.
It should set quickly, preferably within 2 to 4 minutes, at body
(3) It should unite into a solid mass without adhering to the oral tissues or to
the material used for the cast.
(4) It should fall into all irregularities and fine lines in the area to reproduce
without displacing soft tissue.
(5) It must retain an accurate reproduction of surface detail when it solidifies
and is withdrawn from the mouth.
(6) It must have dimensional stability. It must not expand, contract, or
become deformed in any way because of temperature changes, atmospheric
conditions, or the pouring of the cast.
It must not be too unpleasant to the patient.
(8) It must not flake (after solidifying) when trimmed with a sharp knife at