Vacuum blanket

Abstract

A vacuum blanket for holding a sensitive sheet of material in precise registration with a film master against a rigid transparent plate includes a blanket of thin, flexible material having at least one vacuum port therein and an inside face opposing the rigid plate and adapted for biasing the sensitive sheet and film master toward the rigid plate in precise registration with one another. A flexible perimeter seal is provided around the edges of the blanket for sealing off a vacuum chamber established between the blanket and the plate when air is withdrawn through the vacuum port. A bladder of thin, flexible sheet material is secured to the outer surface of the blanket with a plurality of spaced apart seams forming a plurality of separate, inflatable air chambers on the outside surface of the blanket. An inflation port is provided in the bladder for directing air under pressure to inflate the air chambers and thereby create a plurality of induced bleed passages formed along the plurality of seams by a slight amount of movement of the blanket, sensitive sheet and film master away from the rigid plate. The bleed passages aid in rapidly evacuating the entire surface area of the vacuum chamber when air is initially withdrawn through the vacuum port and the bleed passages are then diminished when pressurized air is exhausted so that the sheets are pressed flat and held tightly in registration against the glass plate during an exposure cycle.