an apparatus for heat sealing composite lids , made of a laminate of a light ductile alloy coated with a layer of thermoweldable plastic material , on glass containers . the apparatus applies , under pressure , a flexible membrane onto said lid while disposed in position on the mouth of a glass container . the membrane is a simple sheet stretchingly supported by its outer periphery on a movably and hingedly supporting frame . the side of said membrane opposite said lid is subjected to a controlled heat flow , thus sealing the lid to the container .