Edge beads

The blown-film technique has several advantages the relative ease of changing film width and caliber by controlling the volume of air in the bubble and the speed of the screw the elimination of the end effects (e.g., edge bead trim and nonuniform temperature that result from flat film extrusion) and the capability of biaxial orientation (i.e., orientation both in the hoop direction and in the machine direction), which results in nearly equal physical properties in both directions, thereby giving a film of maximum toughness. 

R. Emami. Apparatus and method for edge bead removal. US Patent 6 786 996, assigned to Apphed Materials Inc. (Santa Clara, CA), September 7, 2004. 

Edge bead n. In some cast-film and sheet-extrusion operations, the narrow border at the edge of the sheet, usually somewhat thicker, that must be trimmed off prior to winding or stacking the product. 

Figure 5.47 Neck in and edge beading in flat film extrusion.

In contrast to the fiber-spinning process, the film-casting process has received less attention in the literature. Dobroth and Erwin [109] examined the causes of thick edges or the edge bead effect (also called the hone effect) and attributed it to the change from planar extension in the center of the film to uniaxial extension at the edges. 

Figure 4.43 Edge bead effect for the Newtonian and the Maxwell (UCM) models. At a given cross section, the Newtonian edge bead occupies a much larger area than the UCM edge bead, at the same flow rate [120, 122],...

Dobroth, T. and L. Erwin. 1985. Causes of Edge Beads in Cast Films. 43rd SPE Annual Technical Conference, Washington DC, 31, 89-92. 

Some of the studies in literature geared towards predieting the film formation process in the web have employed isothermal Newtonian models [2, 5], however, simulations that incorporate non-isothomal conditions and/or viscoelasticity [6-8] are physically more representative of the experimaital conditions observed. Smith and Stolle [7] studied factors responsible for neck-in reduction and improved thickness uniformity. Dobioth and Erwin [9] showed that the primary cause of edge-beads is an edge stress effect. 

Decreasing the polymer viscosity/molecular weight leads to an increase in neck-in at a constant draw ratio. Decreasing the polymer molecular weight decreases the polymer viscosity and this leads to a reduced restriction on neck-in by the edge beads. 

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