Monolayer Blown Film Line <br /> <br />Introduction <br />Blown film extrusion is a technology that is the most common method to make plastic films, especially for the packaging industry. The process involves extruding a tube of molten polumer through a die and inflating to several times its initial diameter to form a thin film bubble. This bubble is then collapsed and used as a lay-flat film or can be made into bags. Usually polyethylene is used with this process, and other materials can be used as blends with these polymers. A diagram of a blowing film process. <br /> <br /> <br />The Film Blowing Process <br /> <br /> <br />Typically, blown film extrusion is carried out vertically upwards, however horizontal and downward extrusion processes are now becoming more common . <br />This procedure consists of four main steps: <br />• The polymer material starts in a pellet form, which are successively compacted and melted to form a continuous, viscous liquid . This molten plastic is then forced, or extruded , through an annular die. <br />• Air is injected through a hole in the center of the die , and the pressure causes the extruded melt to expand into a bubble. The air entering the bubble replaces air leaving it, so that even and constant pressure is maintained to ensure uniform thickness of the film. <br />• The bubble is pulled continually upwards from the die and a cooling ring blows air onto the film. The film can also be cooled from the inside using internal bubble cooling. This reduces the temperature inside the bubble, while maintaining the bubble diameter . <br />• After solidification t the frost line, the film moves into a set of nip rollers which collapse the bubble and flatten it into two flat film layers. The puller rolls pull the film onto windup rollers. The film passes through idler rolls during this process to ensure that there is uniform tension in the film. Between the nip rollers and the windup rollers, the film may pass through a treatment centre, depending on the application. During this stage, the film may be slit to form one or two films, or surface treated. <br /> <br />Advantages <br />Blown film generally has a better balance of mechanical properties than cast or extruded films because it is drawn in both the transverse and machine directions. Mechanical properties of the thin film include tensile and flexural strength, and toughness. The nearly uniform properties in both directions allow for maximum toughness in the film . <br />Blown film extrusion can be used to make one large film, two smaller ones, or tubes that can be made into bags. Also, one die can make many different widths and sizes without significant trimming. This high level of flexibility in the process leads to less scrap material and higher productivity. Blown films also require lower melting temperatures than cast extrusion. Measured at the die opening, the temperature of cast film is about 220°C., where as the temperature of blown film is about 135°C. Furthermore, the cost of the equipment is approximately 50% of a line.
