The application relates to a twin lane tray sealer (100) comprising a pair of conveyors aligned with respective lanes for conveying trays to a heat sealing tool (6) and a pair of transfer mechanisms (66) each for transferring a known plurality of trays together and at a known separation from the respective conveyors to the heat sealing tool wherein the heat sealing tool (6) comprises a plurality of individually operable heaters (61a, 61b), each aligned with a respective tray in the tool in use. Control means is utilized to determine if a tray is missing from a group of trays transferred by the transfer mechanism. If a tray is determined as missing the control means is configured to not activate the heater aligned with the position of the determined missing tray in the tool.