Self venting syringe plunger

Claim 1

In a syringe comprising a barrel having front and rear ends, and a through opening including a cylindrical portion defined by a cylindrical inner surface opening through the rear end of the barrel; a plunger adapted to be inserted in said cylindrical portion having front and rear surfaces, and a groove around its periphery defined by spaced front and rear generally radially extending walls of the plunger and a cylindrical bottom wall of the plunger extending between the front and rear walls; and a resiliently elastic O-ring within the groove around the plunger, said O-ring having a circular cross section with a predetermined diameter adapted to be compressed between the bottom wall of the plunger and the cylindrical inner surface of the syringe, with the front and rear walls defining the groove spaced at a distance exceeding said predetermined diameter to afford sliding relative movement between said O-ring and said bottom wall to move said front wall or said rear wall adjacent the O-ring, the improvement wherein: said plunger has a through passageway from said bottom wall to the rear surface of said plunger, said passageway being adjacent said rear wall, having a dimension from said rear wall to the edge of said passageway farthest from said rear wall measured axially along said bottom wall that is less than said predetermined diameter of said O-ring, and being so sized that (1) upon insertion of said plunger front surface first into said cylindrical portion, said O-ring will move to a position adjacent said rear wall and allow air to escape from said through opening through said passageway to afford seating said plunger against material in said through opening; (2) when the pressure that caused such seating of the plunger is released, the plunger will rebound slightly within the barrel, causing the plunger to move relative to said O-ring to position said front wall closely adjacent the O-ring with the passageway spaced from beneath the O-ring so that the O-ring makes an air tight seal between the plunger and the barrel; and (3) when pressure is again applied to the plunger to press the material out of the barrel, the plunger will again move relative to the O-ring to position said rear wall against the O-ring with the O-ring over the passageway, however, the O-ring will sufficiantly seal the passageway to restrict escape of a significant amount of the material through the passageway.