At the beginning of the process, the polymer comes in the form of a pellet. it is heated and melted into a viscous liquid between rotating screws and barrels of the extruder. This allows for the polymer to be fed through a die that shapes it in the form of a tube. This tube is then carefully inflated, so there is no risk of tearing, into a bubble by injecting it with air. The bubble is simultaneously being cooled in its interior, via a cooling system, and on the exterior surface, through the use of an air ring, to solidify the material. A set of collapsing frames or guides are then used to collapse the bubble into two, more defined, layers within closer proximity. Now that the layers are close, a series of nip rollers flatten the layers together to form a two layered plastic film that are then winded up into a cylindricalroll for packaging purposes. This process may vary depending upon the specifications and models of the machines.
Bubble Instabilities
In the case that the bubble formed from air injection is not handled with caution, the bubble may become unstable and deform in a number of different ways.
Draw resonance - Draw resonance exists when the film velocity at which solidification occurs is much higher than the velocity of the melted liquid as it exits the die. This causes the melt to stretch too quickly and the bubble diameter starts to vary along its surface. One way to fix this situation is to increase the speed of the melt through the die.
Helical Instability - Helical instability is noticeable when one side of the bubble is cooled more than the other due to the air ring. The bubble then starts to form a helical shape as it reaches the collapsing frames. This can be avoided by either lowering the melt temperature or increasing extruder output.
Freezeline Height Instability - Freezeline height Instability results in a variation of thickness of the bubble. This is caused by extruder motor amps and back pressure. To prevent this variation in thickness, improvements upon the feeding and melting of the material must be implemented.
Heavy-Bubble Instability - Heavy-Bubble instability refers to the bubble sagging towards the bottom. When this occurs it means that the it is not being cooled enough. Lowering the freezeline height or lowering the melt temperature will assist the bubble in its cooling face causing less sag.
Bubble Flutter - Bubble flutter appears below the freezeline when cool air impinges on the surface of the bubble. A higher freezline height, a lower melt temperature, and a narrower die gap can solve this problem.
Bubble Breathing - Bubble breathing occurs when the volume of the air in the bubble keeps changing periodically. This causes a variation in film thickness. Some solutions include controlling the cooling system and sensors, a reduction of melt temperatures, or decreasing extruder output.
Bubble Tear - Bubble tear appears as a tear in the bubble. this is happens when the force needed to draw up the bubble into the nip rollers is higher than the tensile strength of the melted film. A simple solution to this is to reduce extruder output or increase the die and melt temperature.
