Which type of impression is often used for custom trays for dental appliances?

Preliminary impressions are commonly used to create custom trays for dental appliances. This type of impression is generally taken to capture the contours of the patient's mouth and provide a preliminary model that can be used for planning further procedures. The primary benefit of using preliminary impressions is that they allow for the creation of a custom tray that closely fits the patient's anatomy, which is essential for the proper fitting of dental appliances, such as dentures or orthodontic devices.

Unlike final impressions, which are more accurate and intended for definitive purposes, or bite registration impressions, which are used to record the occlusal relationship between the upper and lower teeth, preliminary impressions serve as a stepping stone. They enable the dental professional to establish a model to work from in order to produce more precise and functional appliances. Diagnostic impressions are typically used for initial assessments rather than the direct creation of applicatory devices. Hence, the preliminary impression is the preferred choice for custom tray creation.