Irreversible Hydrocolloid

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Making an Impression with
Irreversible Hydrocolloid

Sharon Sparks Miss O’Keefe

January 19, 2010
Irreversible Hydrocolloid impression material also called “alginate impression material” is a powder that is mixed with water to obtain a paste in order to make an exact copy of the mouth. This creates a negative dental impression of the mouth into which a gypsum/plaster is poured. These impressions are taken in order to construct preliminary impressions for complete dentures, partial dentures with clasps and orthodontic models and study models. It is not used for crown and bridge impressions because of the lack of accuracy.

The advantages of using alginate are that it is economical, easy to use, quick setting, has a fair taste, hydrophilic (having a strong affinity for water, hydrophilic colloids swell in water and are relatively stable), can displace blood and saliva and you can use stock trays. Some disadvantages are that they have limited detail reproduction, low tear resistance, single pour only, and quick pouring required and low dimensional stability.

Alginate impression material is generally easy to use; the powder is supplied in a can and shaken up before use for aeration. The “Water to Powder” ratio should be carefully followed as specified by the manufacturer per directions on label. What we call “working time” is the time from beginning of the mix until the setting reaction is complete. The “setting time”, is the time from beginning of the mix until the setting reaction is over. There are two mixes that you may encounter in the office. First, is regular set alginate which takes 1 min, 2.5 min, 3.5 min to set and fast set alginate, which has a set time of 45 sec, 1.5 min, 2.5 min. Mixing time is the time it takes to complete the uniform blending of components. Adequate spatulation (mixing with spatula) produces a smooth, creamy mixture with a minimum of voids or air pockets. Lower water to powder ratio increases strength, tear resistance, and consistency; decreases working and setting times and flexibility. A Lower water temperature increases working and setting times. If not mixed properly it will produce a grainy mixture and will result in a poor recording of the details.

This set of instructions herein is provided to give you the ability to make a negative impression of any mouth you wish. Although the reasons for constructing an impression may vary, the materials are primarily the same. The instructions and materials below are for use in dentistry.

First prepare your work space by gathering the required materials, seat your patient and review patient history, then put on your personal protective equipment which includes but is not limited to the following: protective gown, goggles, mask, and gloves.

Nitrile gloves are considered thicker and stronger but any glove that completely protects your hands and wrists from splashes is preferred.

Materials Needed for 1 Impression:


·Measuring cups (for water-milliliter- and alginate-tablespoon)

·Flexible Mixing bowl (can hold at least 3 cups)


·Upper or lower dental tray

Making the negative impression first

1. Select the largest tray that will fit comfortably into the patient's mouth. Impression tray types: metal stock trays, disposable plastic trays. They must have retentive features: mechanical - rim lock, perforated or chemical - adhesive spray.

2. Prepare the patient’s mouth by rinsing and drying the patient's teeth. Note that if the teeth are too dry the alginate will stick.

3. Measure correct...
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