Also, under ADA specification, the manufacturer
is required to include detailed instructions for use. The
dental assistant should read and follow these directions
Synthetic Rubber Materials
Rubber impression materials are supplied as
pastes in collapsible metal tubes that require mixing.
One tube contains the base, while the other contains an
accelerator or a catalyst. When mixed in appropriate
amounts, the mixture hardens to a synthetic rubber.
Other types of materials come in the form of
double-barreled injector cartridges that do not require
CONSISTENCY TYPES.Rubber impression
materials can be used for almost any impression. They
come in three consistencies and are discussed in the
paragraphs that follow.
Light Bodied. Light bodied impression
materials are injected with a syringe onto preparations
for inlays, crowns, and FPDs. It is also used as a
"wash" impression for full dentures, relinings, and
RPDs. Its high degree of flow registers the fine detail.
Regular Bodied.Regular bodied impression
materials are used in an impression tray for inlays,
crowns, and FPDs.
Heavy Bodied. Heavy bodied impression
materials are used in a tray to force light bodied
impression material onto the cavity preparation or with
a copper band for impressions of single teeth.
materials can be grouped into three types depending on
their composition: polysulfides, silicones, and
Polysulficles. The polysulfides (rubber base)
can be identified by the usually dark color of one of the
two pastes and their resulting opaque mix and sulfur
smell. If the materials are improperly mixed, the
impression will have streaks in it, thereby affecting
dimensional stability. Mixing time is between 45 and
60 seconds with a 5-minute working time. The
impression must not begin setting before placement in
the mouth. If the 5-minute working time is exceeded,
the resulting impression will have inadequate
expansion, producing a smaller cast. The impression
must set completely before removal from the mouth
and poured no later than 1 hour after removal.
Silicones. Silicone (vinyl polysiloxanes)
materials are generally lighter in color, translucent
when set, and have a slight odor. Silicone types come
in the form of a heavy putty, light, regular, and heavy
bodied viscosities. The silicone material is used with a
stock tray to make up the bulk of the impression and
minimize distortion. Manufacturers have been able to
control shrinkage resulting in impressions with greater
accuracy when compared to all other rubber products.
Impressions made from silicone do not have to be
poured immediately. The material will remain
accurate for several days so they can be repoured as
Polyethers.PoIyetliers have lighter colors than
polysulfides, but are darker than silicones. The
working and setting times are much shorter than the
other two rubber impression materials. Polyether is
just as good to use as polysulfides to control shrinkage.
Unlike polysulfide, polyether will absorb water. This
type of impression material is very stiff, making it
difficult to remove from the mouth and a cast. The
dentist must take care when removing the tray with the
material from the mouth, because the polyether tears
easily in thin areas like the subgingival sulcus. For best
results, use this material with a custom tray.
Gypsum products are supplied in powder form.
When mixed with water in the correct proportions, a
paste forms that will eventually harden. This setting
process takes place over several minutes, during which
time the mixture is soft and pliable, and can be formed
into the desired shape. During the setting process,
gypsum gives off heat, which is characteristic of all its
products. Each material in the gypsum group is
carefully compounded to give it the particular
combination of physical properties needed for a
particular work order. Dental plaster, stone, and die
stone are the most frequently used gypsum products.
that the plaster must set within a definite time limit.
Plasters made for dental use are specially
processed to provide high purity and suitable working
properties. One of the most important requirements is
Plaster has many uses. It can be used to form casts,
construct matrices, and attach mount casts to an
articulator. The initial setting time for most dental
plaster is from 7 to 13 minutes. The final set is
completed within approximately 45 minutes.