Ortho Impression Trays - Perforated LARGE Upper, Yellow 50/Pk. Trays provide extra clearance needed to fit over ortho brackets and wires without try contact. "Alginate locks" hold the impression to the tray without the use of messy adhesives. Strong plastic construction minimizes distortion. Improved impression material retention. Fabricates a precise bite registration. Built with a solid plastic construction. Excellent Lock Impression Trays are durable plastic impression devices that accurately capture the occlusion or the shape of the teeth. These disposable trays feature perforations that allow the dental impression to adhere well. The overall design includes a locking mechanism to ensure there are no gaps in the dental impression. Each pack comes with 50 trays.