Taking a good impression is one of the most hands-on skills a dental assistant develops — and one of the easiest to get visibly wrong in a way the dentist immediately notices. This guide covers the materials, the technique, and the most common mistakes that lead to a redo.
The golden rule of impressions: a slightly slower, more careful impression taken once beats a fast one that has to be retaken — patients remember the gagging and discomfort of a second attempt far more than the extra thirty seconds the first one took.
Common impression materials
Alginate is the most common material for study models and quick impressions — it's inexpensive, sets quickly, and is comfortable for patients, but it's also the least dimensionally stable, meaning it should be poured into a model within about 15 minutes or it can distort.
Polyvinyl siloxane (PVS / silicone) is used when more precision matters — crown and bridge work, implant cases, anything going to a lab for fabrication. It's more expensive and sets slightly slower, but it's far more dimensionally stable and can be poured hours later if needed.
Choosing the right tray
Tray selection matters more than most new assistants expect. A tray that's too small won't capture the full arch, including the crucial retromolar pad and vestibule areas needed for a denture or partial. A tray that's too large can cause gagging or simply not seat properly against the teeth, leading to a thin, weak impression that tears on removal.
When in doubt between two sizes, most experienced assistants size up slightly rather than down — a little extra material trimmed later is preferable to a short impression that has to be retaken.
Mixing technique
For alginate, the water-to-powder ratio directly affects set time and strength — too much water creates a weak, tear-prone impression; too little makes it set too fast to seat properly. Most assistants develop a feel for the right consistency: it should look like thick pancake batter and pull into a smooth peak off the spatula, with no dry powder pockets.
For PVS dispensed from an automix gun, consistency is less of a variable, but purging a small amount before loading the tray is essential — the first bit out of the tip can have an uneven base-to-catalyst ratio from sitting in the nozzle.
Seating the tray
Seat from the back of the mouth forward, allowing material to flow naturally rather than forcing the tray straight up, which traps air bubbles against the occlusal surfaces. Hold steady without rocking until the material begins to gel — movement during the early set phase is one of the most common causes of a distorted, unusable impression.
Removing the impression
Wait for a full set before removing — test by pressing a fingernail into excess material at the tray's edge; it should resist without denting. Removing too early causes permanent distortion that won't show up until the lab tries to use it, often days later.
Remove with a quick, firm snap rather than a slow pull, which is more comfortable for the patient and reduces tearing, especially with alginate.
Common mistakes that cause redos
- Voids or bubbles — usually from air trapped during seating or material that wasn't mixed thoroughly
- Pulled or torn margins — often from removing too early or using a tray that was too shallow at the margins
- Patient movement — talking or swallowing during the set phase can shift the impression; clear pre-instructions ("breathe through your nose, try not to swallow") help
- Delayed pouring (alginate specifically) — left too long before pouring, the impression can shrink or distort from water loss
The bottom line
A good impression comes down to the right material for the job, correct tray selection, careful mixing, and a steady, patient seating and removal technique. Most redos trace back to one of those steps being rushed — slowing down on a first attempt almost always saves more time than it costs.
Read the full article
Create a free SmileSmart account to unlock every article instantly.