Why must impression trays be rigid?

Impression trays must be rigid primarily to support the material in the patient's mouth effectively. This rigidity allows the tray to hold the impression material securely while it sets, capturing accurate details of the oral structures. A flexible or soft tray might distort under pressure or while the impression material is setting, leading to inaccurate impressions that do not reflect the true anatomy of the teeth and gums.

The ability to maintain structure during the impression-taking process is essential for ensuring the final dental restoration or appliance properly fits the patient, which ultimately contributes to their comfort and the success of the dental procedure.