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.
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.
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.Continue Reading