Blown Film
“Blown film extrusion is the process by which most commodity and specialized plastic films are made for the packaging industry. The film blowing process basically consists of a extruding a tube of molten thermoplastic and continuously inflating it to several times initial diameter, to form a thin tubular product that can be used directly, or slit to form a flat film.”
"Plastic melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ' lay-flat' tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling).” (Plasticstech, 8/5/2010)
http://www.plasticstech.info/processes/extrusion/blown-film-extrusion/
Recent US Patents
9/20/2011
8,022,124
Process for preparation of ethylene polymers for blown films
Rohde et al of Basell Polyolefine, Germany, developed ethylene homopolymers or copolymers by ethylene or 1-olefins polymerization in the presence of a chromium catalyst at temperatures of from 20 to 200.degree. C. and at pressures of from 0.1 to 20 MPa, where the chromium catalyst can be prepared via application of one or more chromium compounds to a silicate support and subsequent thermal activation under oxidative conditions at temperatures of from 620 to 900.degree. C. and mixing the polyethylene in the melt in the presence of from 1000 to 2000 ppm of an antioxidant comprising at least one sterically hindered phenol and comprising at least one phosphite in a mixing ratio of from 1:5 to 5:1, with specific energy input of at least 0.17 kWh/kg. (RDC 9/25/2011)
10/12/2010
7,811,073
Blow head for producing blown tubular film
Mahler et al has developed a blow head which avoids the so-called spiral strips. Based on annular gap-shaped melt channels arranged concentrically around the center axis of the blow head. Spiral strips are dull strips or wavy areas in the blown film. (RDC 1/6/2011)
10/5/2010
7,806,674
Air ring for controlling blown film thickness provided with individual heat conducting segments
Joppe et al of Windmoeller & Hoelscher, Germany developed an air ring with increased-control over the gauge uniformity of blown plastic film in a film extrusion device. The air flow path is segmented along a circumference of the annular plenum, the segments including a material with high heat conductivity and guiding the air from the annular plenum to the orifice. Heating elements are provided for individually and selectively heating the segments of the air flow path such that the temperature of the cooling air discharge against the extruded plastic tube can be locally varied. (RDC 12/19/2010)
Recent Journal Articles
A mathematical model to study the effect of the air jet in the film blowing process
(1301–1315) Polymer Engineering & Science 51 #7 (2011)
Housiadas of the University of the Aegean, Greece, studied the aerodynamic effect of the air jet that supports the polymeric films produced in the film blowing process. The nonlinear coupling between the air and the polymer is examined by assuming steady, axisymmetric, and isothermal flow for both phases. The governing equations for the polymer are simplified by following the thin-film approximation and the corresponding ones for the air are derived by applying a boundary-layer type analysis. The latter are solved analytically by first applying the Mangler transformation that reduces the boundary layer equations for the axisymmetric case to those for a plane boundary layer. Then a similarity solution is obtained, allowing the shear stress and the pressure on the outer polymer/air interface to be evaluated in terms of the film shape only. The final set of ordinary differential equations for the film is solved numerically using finite differences. The results show that the force caused by the airflow has a significant effect on the film shape and the characteristics of the final product. (RDC 6/24/2011)