A film blowing machine involves one process used to make

plastic film Plastic film is a thin continuous polymeric material. Thicker plastic material is often called a "sheet". These thin plastic membranes are used to separate areas or volumes, to hold items, to act as barriers, or as printable surfaces. Plast ...

. Extruded tubular processing is most often used with

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

films but can be used with other polymers. The film may be laminating film, shrink film,

agricultural Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created f ...

covering film,

bags A bag, also known regionally as a sack, is a common tool in the form of a floppy container, typically made of cloth, leather, bamboo, paper, or plastic. The use of bags predates recorded history, with the earliest bags being lengths of animal s ...

or film for

textile Textile is an Hyponymy and hypernymy, umbrella term that includes various Fiber, fiber-based materials, including fibers, yarns, Staple (textiles)#Filament fiber, filaments, Thread (yarn), threads, and different types of #Fabric, fabric. ...

s and

clothing Clothing (also known as clothes, garments, dress, apparel, or attire) is any item worn on a human human body, body. Typically, clothing is made of fabrics or textiles, but over time it has included garments made from animal skin and other thin s ...

, and other packaging materials.

Technical information

Parts include: screw and barrel, motor, inverter, heaters, die head, winder, and tower. The main motor may have frequency control of motor speed to improve speed regulation and save electricity. The screw and material barrel may be made from a nitrogen-treated

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

Process

At the beginning of the process, the

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

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 is then wound onto a cylindrical roll for packaging purposes. This process may vary depending upon the

specifications A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard. There are different types of technical or engineering specificati ...

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 exists when the film

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

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 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 results in a variation of the 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 occurs when the bubble is not being cooled enough and sags towards the bottom as a result. Lowering either the freezeline height or the melt temperature will assist the bubble in its cooling phase, causing less sag. * 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 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, and decreasing extruder output. * Bubble tear appears as a tear in the bubble, caused by the force needed to draw the bubble up into the nip rollers exceeding the

tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...

of the melted film. A simple solution is to reduce extruder output or increase the die and melt temperature.

References

Books and general references

* Hawkins, William E, The Plastic Film and Foil Web Handling Guide CRC Press 2003 *Jenkins, W. A., and Osborn, K. R. Plastic Films: Technology and Packaging Applications, CRC Press 1992 * Yam, K. L., "Encyclopedia of Packaging Technology", John Wiley & Sons, 2009, {{Plastics Packaging machinery Industrial equipment