Abstract: The specification and drawings disclose a method and apparatus of forming foam plastic envelopes by simultaneously engaging two juxtaposed sheets of foam plastic with at least three pairs of opposed jaw members positioned to fully define three marginal edges of an envelope. As shown, the jaw members are positioned in a generally U-shaped pattern so that each pair of jaws completely defines a separate marginal edge. The preferred apparatus of the invention includes first and second opposed platens mounted for movement between a first position in which they are spaced a substantial distance apart and a second position wherein they are relatively closely spaced. Each platen carries at least three jaws including two elongated, spaced, side-by-side jaws and one intermediate jaw which extends transversely between the side-by-side jaws.