b. Two-Part Systems. All of the elastomeric impression materials consist of
two-part system. Generally, they consist of two-paste or putty components that are
manually mixed with a spatula and pad. Most are available in cartridge form for use in
an automix impression gun.
c. Usage. A thin layer of this material, uniform in thickness, is required to obtain
the most accurate impression with the material. For this reason, it is used in individually
designed (custom) acrylic resin trays. Synthetic elastomeric materials are not adhesive;
therefore, a tray adhesive is needed to prevent the impression from pulling away from
the tray. If the impression material pulls away from the tray, distortion will result when
the tray is removed from the patient's mouth.
d. Summary. The properties of elastomeric impression materials are
summarized in table 3-1.
Shrinkage on Setting
Poor to Good
Table 3-1. Mechanical and physical properties of elastomeric impression materials.
3-22. MODELING PLASTIC--CHARACTERISTICS
a. General. Modeling plastic (modeling composition) is an impression material
(thermoplastic type) which can be softened by heat into a soft plastic mass and then
hardened by cooling with either a stream of cold water or a blast of air. Modeling plastic
is used primarily to make impressions of the edentulous arches (the tooth ridges without
b. Properties and Materials. Modeling plastic is composed of shellac, talc, and
glycerides derived from certain tallow oils. The temperature range at which softening
occurs depends upon the proportions of the ingredients contained in the material.
Modeling plastic is supplied in cakes, wafers, or sticks and in various colors to aid in
distinguishing between products of different softening (fusing) temperature ranges.