Typically, Blown Film Plant Unit is carried out vertically upwards, however horizontal and downward extrusion processes are now becoming more common .
This procedure consists of four main steps:
• 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.
• 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.
• 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 .
• 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.
