{"id":16109,"key":"Blown_film_extrusion","title":"Blown film extrusion","latest":{"id":1223800,"timestamp":"2026-01-08T23:25:56Z"},"content_model":"wikitext","license":{"url":"https://www.appropedia.org/Appropedia:Copyrights","title":"CC-BY-SA-4.0"},"source":"{{Topic notice}}\n\n[[File:Polyethylene.jpg|thumb|Fig 1: Model of polyethylene chain from polyethylene Wikipedia article.]]\n\n'''Blown film extrusion''' is a technology that is the most common method to make [[plastic]]{{W|plastic}} films, especially for the packaging industry<ref name=\"Plasticswiki\">Plastics Wiki. \"Blown Film Extrusion.\" Plastics Wiki. 2008. 6 November 2008. <http://web.archive.org/web/20170723214049/http://plastics.inwiki.org:80/Blown_film_extrusion>.</ref>. The process involves extruding a tube of molten polymer{{W|polymer}} 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{{W|polyethylene}} is used with this process, and other materials can be used as blends with these polymers.<ref name=\"Plasticswiki\" /> A diagram of a polyethylene chain is shown in Figure 1 to the right.\n\n== Background theory on polymers ==\n\nIn the cooling step of blown film extrusion, the amorphous{{W|amorphous}}, transparent melt crystallizes{{W|crystallizes}} to form a translucent, hazy, or opaque film. The point where opacity begins in the bubble is known as the frost line.\n\nThe frost line height is controlled by several parameters: the air flow, film speed, and temperature difference between the film and the surroundings.<ref name=\"Giles\">Giles, Harold F. Jr., John R. Wagner Jr., and Eldridge M. Mount III. Extrusion: The Definitive Processing Guide and Handbook. New York: William Andrew Publishing, 2005.</ref>\nProperties of the film, such as tensile strength, flexural strength, toughness, and optical properties, drastically change depending on the orientation of the molecules.<ref name=\"Giles\" /> As the transverse or hoop direction properties increase, the machine or longitudinal direction properties decrease. For instance, if all the molecules were aligned in the machine direction, it would be easy to tear the film in that direction, and very difficult in the transverse direction.\n\n== The film blowing process ==\n\n[[File:Blownfilm setup.jpg|thumb|Fig 2: Schematic of set-up from [[User:J.Chiang]].]]\n\nTypically, blown film extrusion is carried out vertically upwards, however horizontal and downward extrusion processes are now becoming more common<ref name=\"Chanda\">Chanda, Manas and Salil K. Roy. Plastics Technology Handbook, 4th ed. Florida: CRC Press, 2007.</ref><ref name=\"Giles\" /> Figure 2 shows a schematic of the set-up for blown film extrusion. This procedure consists of four main steps:\n\n# The polymer material starts in a pellet form, which are successively compacted and melted to form a continuous, viscous{{W|viscous}} liquid.<ref name=\"Callister\">Callister, William D. Jr. Materials Science and Engineering An Introduction. 6th ed. Massachusetts: John Wiley & Sons Inc, 2003.</ref> This molten plastic is then forced, or extruded{{W|extruded}}, through an annular die.\n# Air is injected through a hole in the center of the die{{W|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.<ref name=\"Chanda\" />\n# 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.<ref name=\"Giles\" />\n# After solidification{{W|solidification}} at 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.<ref name=\"Giles\" />\n\n== Advantages ==\n\nBlown 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.<ref name=\"Plasticswiki\" /><ref name=\"Rosato\">Rosato, Donald V. Concise Encyclopedia of Plastics. Massachusetts: Kluwer Academic Publishers, 2000.</ref>\n\nBlown 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,<ref name=\"Van\">Van, Der Molen Theodorus Jacobu. \"Process for the production of cast film from high-density polyethylene.\" European Patent EP0278569. August 1988.</ref> where as the temperature of blown film is about 135°C.<ref name=\"VanDer\">Van, Der Molen Theodorus Jacobu. \"Process for the preparation of blown film and oriented film.\" European Patent EP0156130. September 1991.</ref> Furthermore, the cost of the equipment is approximately 50% of a cast{{W|cast}} line.<ref name=\"Giles\" />\n\n== Disadvantages ==\n\nBlown film has a less effective cooling process than flat film. Flat film cooling is done by means of chill rolls or water,<ref name=\"Rosato\" /> which have significantly higher specific heat capacities than the air that is used in the blown film cooling process. The higher specific heat capacity{{W|specific heat capacity}} allows the substance to absorb more heat with less change in the substance temperature. Compared to cast film, blown film has a more complicated and less accurate method to control film thickness; cast film has a thickness variation of 1 to 2% versus the 3 to 4% for blown film.<ref name=\"Giles\" /> The resins used for casting typically have a lower melt flow index,<ref name=\"Giles\" /> which is the amount of polymer that can be forced through a standard die in 10 minutes according to a standard procedure.<ref name=\"Chui\">Chui, Q.S.H. et al. \"An interlaboratory comparison of the melt flow index: Relevant aspects for the participant laboratories.\" Polymer Testing. Vol. 26, Iss.5 (August 2007):576-586</ref> The melt flow index for cast film is about 5.0 g/10 min<ref name=\"Ebnesajjad\">Ebnesajjad, Sina. Melt Processible Fluoropolymers. New York: William Andrew Inc, 2003.</ref> where as for blown film it is about 1.0 g/10 min.<ref name=\"Chang\">Chang, A.C. et al. \"Mechanisms of ductile tear in blown film from blends of polyethylene and high melt strength polypropylene.\" Polymer. Vol. 43, Iss.24 (November 2002): 6515-6526</ref> Consequently, the production rates for cast film are higher: cast film lines can reach production rates of up to 300m/min where as blown film lines are usually less than half this value.<ref name=\"Dyson\">Dyson, R.W. Engineering Polymers. New York: Chapman and Hall, 1990.</ref> And finally, cast film has better optical properties, including transparency{{W|transparency}}, haze, and gloss.\n\n== Common problems ==\n\n* Air entrapment between film layers and rollers – this may cause film scratching or wrinkling, or processing problems when winding up the film due to reduced friction. Possible solutions to this is using a vacuum to remove entrapped air or by using winding rolls with a diamond shaped groove in the rubber cover to increase surface area and decrease amount of entrapped air in the film.<ref name=\"Giles\" />\n* Large output fluctuations from the die – this causes thickness variations, and can be prevented by keeping the extruder clean and by using more consistently shaped pellets in the extruder.<ref name=\"Cantor\">Cantor, Kirk. Blown Film Extrusion: An Introduction. Ohio: Hanser Publishers, 2006.</ref>\n* Melt fractures – these appear as roughness or wavy lines on the film surface, and can be eliminated by lowering the viscosity of the polymer melt. This can be done by increasing the melting temperature or by adding an internal lubricant to the material composition.<ref name=\"Cantor\" />\n* Thickness variations in the film – this can be avoided by centering the die in the extrusion line before every run, adjusting the air speed of the cooling system, or by using heated die lips.<ref name=\"Cantor\" />\n* Die lines on the surface of the film – this defect reduces the aesthetic appeal of the film, reduces optical properties, and weakens mechanical properties such as tear strength. This can usually be avoided by routinely cleaning the inner surfaces of the die and by refinishing scratched or roughened flow surfaces.<ref name=\"Cantor\" />\n* Gels – these defects are small, hard globules encapsulated in the film or stuck on the film surface and reduce the aesthetic appeal of the film and cause stress concentration points which may result in premature failure. These are caused by overheating to the point of polymer degradation in the die, and can therefore be avoided by cleaning the inner surfaces of the die on a regular basis.<ref name=\"Cantor\" />\n\n== Optimization of the process ==\n\n==== Coextrusion ====\n\nOne way to improve the line [[efficiency]] of blown film extrusion is to implement coextrusion. This is the process of extruding two or more materials simultaneously through a single die. The orifices in the die are arranged such that the layers merge together before cooling.<ref name=\"Giles\" /> This process saves time because it extrudes two or more layers at the same time, and it provides a method with fewer steps to produce multilayer films. The production rate for a coextruded multilayer film of three layers is about 65m/min,<ref name=\"Carneiro\">Carneiro, O.S., R. Reis, and J.A. Covas. \"Small-scale production of co-extruded biaxially oriented blown film.\" Polymer Testing. Vol. 27 Iss. 4 (June 2008): 527-537</ref> and the production rate for a single layer of blown film is about 130m/min.<ref name=\"Dyson\">Dyson, R.W. Engineering Polymers. New York: Chapman and Hall, 1990.</ref> Thus, in order to produce 10 000m of a three layer multilayer film, it would take almost 4 hours using a single layer blown film process, and only 2 and a half hours using the coextrusion process. Furthermore, the film produced from the single layer process would require an extra step to glue the layers together using some sort of adhesive. Coextrusion is the least expensive means of producing layered films and the coextrusion system is capable of quick changeovers to minimize production line down time.<ref name=\"Cook\">Cook, Warren R. \"Co-extrusion apparatus.\" United States Patent 5324187. June 1994</ref>\n\n==== Minimizing the melt temperature ====\n\nThe efficiency of blown film extrusion can be improved by minimizing the temperature of the polymer melt. Reduction of the melt temperature causes the melt to require less heating in the extruder. Normal extrusion conditions have a melting temperature at about 190° C<ref name=\"Dowd\">Dowd, Laurence E. \"Blown film extrusion.\" United States Patent 4632801. December 1986</ref> despite the fact that the temperature of the melt only needs to be about 135°C.<ref name=\"VanDer\" /> However, it is not always practical to decrease the melting temperature by that much. By decreasing the melt temperature 2 to 20°C, the motor load can be decreased by about 1 to 10%.<ref name=\"Shannon\">Shannon, Porter C. \"Polyethylene films.\" United States Patent 7101629. September 2006.</ref> Furthermore, reduction of the melt temperature causes less need for cooling, so there is a reduced use of the cooling system. Moreover, removing heat from the bubble is usually the rate-limiting factor in this extrusion process, so by having less heat in the polymer to remove, the rate of the process can be increased, thus yielding higher productivity. A way to maintain the melt temperature at a minimum is to choose an extruder that is matched to the specific processing conditions, such as the material of the melt, pressure, and throughput.<ref name=\"Cantor\" />\n\n==== Heated extrusion die lips ====\n\nTypically, solutions to melt fractures involve decreasing the output or increasing the melt temperature to decrease the shear stress in the extruder. Both of these methods are not ideal because they both reduce the efficiency of the blown film line. Heated extrusion die lips can solve this problem. This targeted heating method allows for film extruders to be run at higher production rates with narrower die gaps while eliminating melt fractures.<ref name=\"Bentivoglio\">Bentivoglio, Alfredo. \"Heated die lips system.\" Canada Patent CA 2204548. June 2001.</ref> Direct heat is applied to the surface of the polymer melt as it exits the die so that viscosity is reduced. Therefore, melt fractures, which are caused when trying to extrude too much of the polymer at one time, will no longer act as a limiting factor to increasing the production rate.<ref name=\"Bentivoglio\" /> Furthermore, heated die lips use less energy than increasing the melting temperature because only the surface of the melt is heated and not the bulk of the liquid. Another benefit of using heated die lips is that thickness variations can be controlled by adding heat to certain areas along the die circumference to make the film at that position thinner. This would ensure that no excess material is used.<ref name=\"Moriarity\">Moriarity, Gregory J. \"Heated die lips for controlling extruded polymer film thicknesses.\" United States Patent 6273701. August 2001.</ref>\n\n== Applications ==\n\n[[File:Cling wrap.jpg|thumb|Fig 3: Consumer food wrap from [[Wikipedia:Plastic wrap]].]]\n\n* [[Agricultural]] film\n* Bags\n* Industry packaging, shrink film{{W|shrink film}}, stretch film\n* Consumer packaging, food wrap, transport packaging (shown in Figure 3)\n* Laminating{{W|Laminating}} film\n* Barrier film\n* Multilayer film{{W|Multilayer film}}\n* Research has been done to explore the incorporation of blown film extrusion into the large-scale manufacturing of carbon nanotube{{W|carbon nanotube}} and nanowire{{W|nanowire}} films<ref name=\"Dalton\">Dalton, Alan, and Izabela Jurewicz. \"Nanotechnology Meets Bubbleology.\" Nature Nanotechnology. Vol. 2 (June 2007): 339-340</ref><ref name=\"Yu\">Yu, Guihua, Anyuan Cao, and Charles M. Lieber. \"Large-Area Blown Bubble Films of Aligned Nanowires and Carbon Nanotubes.\" Nature Nanotechnology. Vol. 2 (May 2007): 372-377.</ref>\n\n== References ==\n\n<references />\n\n{{Page data\n| license = CC-BY-SA-3.0\n| part-of = MECH370\n| organizations = Queen's University\n| keywords = energy conservation, materials processing, extrusion, plastic, technology, energy efficiency, film blowing, blown film\n| description = Blown-film extrusion produces flexible packaging. Appropedia outlines methods to improve efficiency and reduce waste.\n}}\n\n[[Category:Materials processing]]\n[[Category:Plastic]]\n[[Category:Technology]]\n[[Category:Energy efficiency]]"}