FOUR-BUBBLE EXTRUSION PROCESS FOR BLOWN POLYMER FILM

摘要

A multi-layer tubular film is co-extruded and cooled to form a first bubble, then collapsed, preheated and blown into a second bubble. The second bubble is further heated from outside to produce a decreasing temperature gradient from the outside surface of the film to the inside surface of the film. The outside surface of the film is hotter than the inside surface of the film and the inside surface of the f film is hot enough for re-blowing but sufficiently cool to avoid blocking. The film is then collapsed, blown into a third bubble and cooled to produce biaxially-oriented film, which is then collapsed, blown into a fourth bubble, heated and then cooled film to relieve stress and impart desired shrink properties to the film to produce ship-stable biaxially-oriented film.