Impression Materials and Techniques Flashcards

Which impression material is classified as a reversible hydrocolloid?

Answer: Agar

Agar (colloid) is reversible because it can be repeatedly liquefied by heating and re-gelled by cooling, unlike alginate which sets irreversibly.

When taking a preliminary alginate impression, the dentist notices voids in the palate area. What is the most likely cause?

Answer: Air entrapment during spatulation or tray seating

Voids in the impression are commonly caused by air bubbles trapped during mixing or when the material fails to flow into all areas during tray placement.

The water-to-powder ratio for alginate primarily affects which property?

Answer: Tear strength and flow

Altering the water-to-powder ratio changes both the consistency (flow) and the tear strength of the set alginate.

A polysulfide impression has a distinctive smell. What causes this?

Answer: Sulfur-containing compounds in the base paste reacting with lead dioxide

Polysulfide rubber contains mercaptan (thiol) sulfur groups that react with lead dioxide accelerator, producing a strong sulfur odor.

What is the recommended action if an alginate impression cannot be poured immediately?

Answer: Store in an airtight bag with a moist paper towel for up to 1 hour

Alginate should be wrapped in a damp towel and sealed in a bag to prevent syneresis and imbibition; ideally poured within 1 hour.

Which type of impression tray is custom-made from the patient's diagnostic model?

Answer: Individual custom tray

Custom trays are fabricated directly over a patient's cast, providing a uniform spacer distance from tissues and improving final impression accuracy.

During impression removal, the tray should be removed with:

Answer: A quick, decisive snap in the direction of insertion

A swift, single-direction removal minimizes distortion by reducing the time elastic materials are stressed during withdrawal.