The durability and quality of an excellent impression depends on water/powder ratio, spatulation, holding the impression in a stable position for optimum time and the correct selection of a tray.
Make a decision on the size of impression tray needed based on the dentition present, size of the arches and range of opening to help determine your tray size. The trays must extend past the most terminal tooth and clear the anterior teeth.
Mixing the alginate:
Use a measuring guide to fill bowl with distilled water. Slowly incorporate the powder with the water until all the powder is wet. Spatulate quickly pushing the alginate against the mixing bowl to remove air bubbles and to create a homogenous mixture.
Placing the alginate in the impression tray: Press a loaded spatula against the tray rim, evenly filling the tray. Avoid overloading tray as it could cause the patient to gag or displace tissue. Grasp the handle of the tray with the alginate facing up. Rotate the tray into the patient's mouth using the opposite hand to retract the opposite corner of the mouth.
Seating the tray: Center over the teeth and instruct the patient to extend their tongue to the palate and then relax on top of the tray. Apply uniform pressure over pre-molar area with your fingers and use your thumbs to stabilize the mandible, holding under the chin. Hold the tray securely until the alginate has set. Do not allow the patient to bite down on the impression tray.
Removal: Take the impression tray out by holding the tray handle with the thumb and finger. Retract the cheek and lip and release the edge of the impression by lifting up on the posterior buccal area, you will hear a suction release. Remove with a snap to prevent distortion.
Rinse and disinfect the impression and inspect for accuracy. Ensure all teeth are present, there are no voids, distortion of teeth, bubbles on the occlusal surfaces or teeth that have contacted the impression tray.