The laboratory technicians will usually pour most
of the impressions, but there will be times when you
must perform this task. To fabricate a quality cast, you
must first start with a quality impression.
PREPARING TO POUR IMPRESSIONS
Once you have an accurate impression, producing
an accurate cast is simple if the impression and
materials are prepared correctly. A minor mistake in
any of these areas could cause a distorted cast.
Pour the impressions as soon as possible because
all impression materials are subject to distortion.
Cleanse impressions to remove mucous and saliva.
This helps to ensure accurate surface detail and
eliminates the chance of soft spots in the cast. To
cleanse the impression, rinse it under cool tap water, or
lightly sprinkle stone into the impression and
thoroughly rinse the stone away. If there is heavy
mucous and blood in the impression, you might need to
brush it lightly with a large soft sable paint brush.
Remove excess moisture by gently shaking the
impression. Never dry an impression or use an air blast
to remove moisture because you might distort or tear
the, impression material. Figure 8-29 illustrates
common problems with alginate impressions.
MATERIALS AND MIXING
To produce a good cast from the impression, you
need to use a properly mixed gypsum product. Most
often you will use hydrocal (dental stone). To assure a
good cast mix, perform the following steps.
Figure 8-29.Common problems with alginate impressions.