formation and liquid - filling of plastic bags is performed in a continuous high speed and low cost process . plastic film is extruded in tubular form and inflated with air or another gas . the tube is transported along a path of work stations at which the tube is successively : sealed to define a series of inflated sections ; filled with liquid , while inflated , by a large size hypodermic - type needle configured to puncture and displace entrapped gas with fill liquid , the displaced gas escaping around the needle ; seamed to seal off the needle - hole from the liquid - filled portion of the bag ; and cut to separate the individual bags , if desired .