a press - formed plastic coated paperboard tray is hermetically sealed by applying a hot melt or wax to indentations formed in a flange of the tray at the corners of the tray . a film cover is then adhered to the tray over the flange and the hot melt or wax filling the indentations in the corner of the flange of the tray provides a hermetic seal for the interior of the tray and its contents , such as sterilized medical instruments , and precludes leakage of liquids housed within the tray .