Watch this short video of a HFFS machine.
Historically polyethylene has had limited application in horizontal form fill & seal HFFS. Because sealing jaws on HFFS machines are bare metal, the outer layer must have a higher melting point than the layer which seals to itself. This differential can be achieved either by coextrusion or lamination.
Simply coating the sealing jaws with Teflon obviates the need for this melting point differential so monolayer LDPE can be substituted.
LDPE film has been used as a sealant layer for laminations of polyethylene to polypropylene usually with buried reverse print to make a fin seal.
The most popular choice for non-laminated films has been polypropylene, referred to as either 2 side sealable polypropylene, or “coex prope”.
By far the largest application for single layer polyethylene film in horizontal form/fill/seal HFFS is USPS approved poly mailwrap.