Bubble draw ratio

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

Film blowing. A tubular 50 pm thick low density polyethylene film is blown with a draw ratio of 5 at a flow rate of 50 g/s. The annular die has a diameter of 15 mm and a die gap of 1 mm. Calculate the required pressure inside the bubble and draw force to pull the bubble. Assume a Newtonian viscosity of 800 Pa-s, a density of 920 kg/m3 and a freeze line at 300 mm. 

Figure 20 presents mesophase rods produced by extrusion alone and by a light draw after extrusion. As Jenkins and Jenkins observed (34), the strong preferred orientation induced by extrusion was easily disturbed by pyrolysis bubbles or even by small flow irregularities. However, modest draws (e.g., draw ratio = 2) after extrusion produced fibrous morphologies with good uniformity. At these draw levels, the nodes and crosses characteristic of wedge disclinations could be resolved on transverse sections. 

There are some characteristic parameters in the blown film process (see Fig. 24.1) the blow-up ratio (BUR), which is the ratio between the final radius (Of) and the radius at the die exit (Uq) the thickness ratio (TR) calculated as the ratio of thickness at the die exit (//q) and the final film thickness (//f) and the draw ratio (DR) defined as the ratio of take-up roller velocity (Vf) to the extrusion velocity (Vq). The stretching force (F ) is the force needed to take up the bubble by the roller system (Fig. 24.1). 

The blowup ratio, BUR, is the ratio of the final bubble radius to the initial radius and is an analog of the draw ratio in spinning. Primes denote differentiation with respect to the dimensionless axial length, which is normalized with respect to Ro. The differential equations are more complex for viscoelastic liquids, and the stress equations must be solved in parallel with the momentum equations, r = w = 1 at the exit from the die, and r = 0 at the freezeline. The last boundary condition assumes that the viscosity becomes infinite at the freezehne and that there is no further deformation. (This condition is approximate at best and need not be used when a solidification model in which the sohd phase evolves and locks in structure is employed.) Heat transfer is very important, although it has usually been handled with rather simplistic assumptions about the heat transfer coefficient. 

Tubular linear low-density polyethylene (LLDPE) film of thickness equal to 34.1 qm is produced (Kanai and White, 1984) by a blown film operation with draw ratio equal to 4. The annular die has an inner diameter 1.387 cm and outer diameter 1.496 cm. Calculate the pressure of the air to blow a bubble (i.e., the internal overpressure) of the given specifications and the axial tension to draw such a bubble. Consider that LLDPE is Newtonian with viscosity 720 Pa s, the process is isothermal at 180 °C, and LLDPE freezes at an axial distance of 15 cm. The mass fiow rate of the polymer is 0.21 g/s, and its density is 0.919 g/cm-. 

The blow and draw ratios of the film blowing process can be calculated from the known values of the radii of the die and final bubble, and from Eq. 9.176 as... 

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

PE films are usually produced by the blown film extmsion method. The alternative method is sheet extrusion followed by passing through chilled rollers. Blown film is more economic biaxial orientation is introduced by the draw-off and blow ratios. Bubble stability is critical to the blown film process. Bubble stabihty is provided by the melt strength and rheological characteristics of the PE. Long... 

Draw Resonance appears as a continuous variation in bubble diameter. As in other extrusion processes, such as profile, it occurs when the melt is stretched too quickly (i.e., a high take-up ratio). Solutions act to reduce the take-up ratio, for example increasing the melt (screw) speed. 

In biaxial tubular film extrusion, quenching on the mandrel causes metastability and paracrystallinity. This disorder changes on further heat treatment to the well-organized monodinic crystal form. During bubble forming, this change is increased further by draw in MD and by a force produced by pressure in TD. The b axes are randomly distributed about the machine direction at fixed blow-up ratio (BUR) and increasing take-... 

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