Basics Blown film
Basics Blown film extrusion by Michael Thielen Fig 3: Multilayer Blown film line (Photo: Reifenhäuser) Fig 1: schematic of a blown film line (here multilayer coextrusion) (Picture Reifenhäuser) 7 9 10 EVOLUTION W-P KIEFEL EXTRUSION 12 6 5 11 8 3 4 2 1a 1 Blown film extrusion is a technology that is one of the most common methods of manufacturing plastic films, especially for, but not limited to, packaging applications. The process basically consists of the extrusion of a molten polymer upwards through an annular slit die and the inflation of the tube to form a bubble. This bubble of thin film is then laid flat and can be used directly as a tube, converted into bags or sacks, or can be slit to form one or two flat films. It is not unusual to see this type of film blowing installation as a 10 metre high tower [1, 2, 3, 4]. Process In the first step of blown film extrusion plastic melt is extruded {1 in Fig. 1} through an annular slit die {2}, usually vertically upwards, to form a thin walled tube. Air is introduced via a hole in the die to inflate the tube to a multiple of its initial diameter {3}. By inflating the tube it is stretched and the molecules are oriented in the circumferential direction. The bubble is now pulled continually upwards from the die and a cooling ring {4} blows air onto the film. At a certain level (the so-called frost-line), the plastic is cooled such that the melt will solidify. The film moves upwards into a lay-flat device or collapsing frame {5}, pulled by a set of nip rollers {6} on top of the blown film tower. The lay-flat device (often also referred to as A-frame or V-boards) collapses the bubble and flattens it into two flat film layers {7}. A so-called calibration cage (or basket) {8}, which defines and stabilizes the film size (bubble diameter), is arranged between the die and the lay-flat device. The calibration cage thus has a direct influence on the film quality [5]. The haul-off speed of the puller rolls is usually higher than the extrusion speed so that the film is also stretched in the longitudinal (or machine) direction. Together with the inflation stretch this leads to a certain biaxial stretching of the film. The film passes through idler rolls {9} to ensure that there is uniform tension in the film. The puller rolls pull the film onto windup rollers {10}. There can be one windup roller if the film is wound up as a tube or slit only once to become a wide flat film. If the film is slit at both sides, two windup rollers are used for two flat films. 44 bioplastics MAGAZINE [06/12] Vol. 7
Basics Fig 2: oscillating turner bar haul-offs (Picture: General Extrusion Technology [6]) Some special devices Internal bubble cooling (IBC) Usually the air entering the bubble through the die replaces air leaving it, so that an even and constant pressure is maintained to ensure uniform thickness of the film. For better controlling the process in terms of cooling and thus wall thickness the film can also be cooled from the inside using internal bubble cooling (IBC). Here a higher flow of air is introduced into the film and exhausted through a separate bore in the die. This reduces the temperature inside the bubble, while maintaining the bubble diameter. Wall thickness control Circumferential stretching by inflating, longitudinal stretching by haul-off speed in combination with cooling air, lead to a certain wall thickness (gauge) of the film. In order to perfectly control the thickness of the film over its length and over its circumference, a contacting or contactfree oscillating wall thickness measuring sensor {11} can be installed. Turner bar haul-offs Even with the best wall thickness control devices, and sophisticated extrusion equipment, it cannot be avoided, that certain locations in the circumference of the film tube have slightly different wall thicknesses. If a small zone of a higher wall thickness remains at the same location it will inevitably lead to an accumulation on the final roll and create a problem. To avoid this, the ‘thick spot’ should somehow rotate in order to be evenly distributed on the reel. Solutions are rotating or oscillating extrusion dies or even complete extruder platforms. Other solutions are rotating or oscillating turner bar haul-offs {Fig 2 and 12 in Fig. 1}. Multilayer coextrusion By installing several extruders {Fig. 3 and 1a in Fig. 1} for different types of plastic, multi-layer film can be produced. The orifices in the die are arranged such that the layers merge together before cooling. Each plastic takes on a specific role, such as firmness, a barrier function, the ability to be welded etc. Co-extrusion lines with up to 9 layers are available today. Materials Polyethylene (HDPE, LDPE and LLDPE) are the most common resins in use, but a wide variety of other materials can be used as blends with these resins or as single layers in a multi-layer film structure, for example PP, PA, EVOH. Bioplastics that can be blown film extruded include PLA and PLA blends, TPS and TPS blends, PBAT, PBS and many more. Most of these can be processed on existing equipment, however, the process parameters such as temperature and extrusion speed have to be adjusted accordingly. The processing of biobased polymers compared with petroleum-based products requires special attention during the production. The raw materials are partly or mostly made of natural products and have a higher volatility in terms of melt index and in the range of the density. This has to be compensated by a modern extrusion technology [8]. Applications Products made from blown film are, for example, agricultural film, industry packaging, consumer packaging, rubbish sacks and bags for biological waste, hygienic foil for nappies, mailing pouches, disposable gloves and shopping bags, food wrap, transport packaging, shrink film, stretch film, bags, laminating film, and much more. References [1] Thielen, M.: Bioplastics: Basics. Applications. Markets., Polymedia Publisher, 2012 [2] N.N.: en.wikipedia.org/wiki/Plastics_extrusion#Blown_film_ extrusion, accessed 14 Nov. 2012 [3] N.N.: www.appropedia.org/Blown_film_extrusion, accessed 14 Nov. 2012 [3] N.N.: http://plastics.inwiki.org/Blown_film_extrusion, accessed 14 Nov. 2012 [5] N.N.: http://www.igus.it/wpck/default.aspx?Pagename=app_ blownfilmline&C=IT&L=it, accessed 14 Nov. 2012 [6] N.N. (General Extrusion Technology Ltd): http://www.getextrusion.com, accessed 14 Nov. 2012 [7] Wiechmann R.: personal information, Reifenhäuser GmbH & Co. KG Maschinenfabrik, Troisdorf, Germany, 2012 [8] Buth, K.: personal information, Wentus Kunststoff GmbH, Höxter, Germany, 2012 bioplastics MAGAZINE [06/12] Vol. 7 45
