Welex Inc. has introduced what officials of the Blue Bell, Pa., company call an easy system for making high-barrier plastic sheet.
Welex has perfected a simple laminating system that allows the conversion of a single-layer sheet line to produce barrier sheet, by adding a separately produced, high-barrier film to one side of the sheet.
The barrier film is produced by the blown film process, where such films are made on lines that use three to five extruders.
According to Welex, film lamination on sheet lines has long been done by introducing the film at the first nip near the die. But that can destroy the film's barrier properties if a bank arises at this point, although it is not readily visible, the company said.
The patent-pending Welex innovation applies the film using a cooled pressure roll, after the sheet has passed through the first nip. At that point, the sheet is formed and is flat, so there is no danger of a bank formation. The sheet remains hot enough for film adhesion, because it has only had brief contact with the upper roll of the three-roll stack, according to Welex.
The package can be retrofitted to existing sheet lines. It includes an overhead, tension-controlled, two-roll film payoff, followed by some guiding rolls and an expander to eliminate wrinkles before the film is applied to the sheet on the middle roll of the stack.
Usually, the barrier film includes a core of ethylene vinyl alcohol, surrounded by two adhesive layers, which are in turn surrounded by two polyolefin layers that can be heat-set. Since the sheet usually is thermoformed into the package, the residual scrap will be recycled back into the base sheet layer.
The adhesive content can cause severe die buildup, but Welex said that is easily corrected by using a small, two-layer virgin material coextruder on the line. This assures that the die is always in contact with virgin material.
Tel. 215-542-8000, fax 215-542-9841, e-mail [email protected]
