Blown films draw ratio

Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

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. 

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... 

Wang and Cakmak (52) studied the development of structure hierarchy in tubular film blown dynamically vulcanized PP/EPDM blend. The blown films were found to exhibit an unusual asymmetric structure. The PP phase was found to fibriUate at all the outside surface, while the inner surface remained relatively featureless. This was attributed to disproportionally rapid cooling of the outside surface by the air steam blown externally onto the film being extruded. This, in turn, resulted in sohdification of very thin PP surface layers that caused their fibrillation under the heavy stress they had to endure. Increase in the blow-up ratio was found to expand this web-like surface texture. As a result of this fibrillation mechanism, the increase of both the blow-up ratio and draw-down ratios was found to reduce the mechanical properties. 

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). 

Blown film samples, draw ratio between 1 0.32-0.74 ... 

Draw resistance instability] for blown film, fiber spinning, blow molding draw ratio too high. 

Draw resonance occurs in processes where the extrudate is exposed to a free surface stretching flow, such as blown film extrusion, fiber spinning, and blow molding. It manifests itself in a regular cyclic variation of the dimensions of the extrudate. An extensive review [169] and an analysis [170] of draw resonance were done by Petrie and Denn. Draw resonance occurs above a certain critical draw ratio while the polymer is still in the molten state when it is taken up and rapidly quenched after take-up. 

With a given die cross-sectional area, there is only 1 ratio of puller speed to extruder throughput rate that produces a product with the correct cross-sectional dimensions. If the extruder throughput is increased, the puller speed must be increased proportionally to maintain the same finished product dimensions. Likewise if the throughput is decreased the puller speed must be decreased proportionally to maintain the same finished product cross-sectional area. The draw ratio and molecular orientation can only be increased or decreased by changing the die cross-sectional area relative to the puller speed assuming the final product dimensions are kept constant. This is easily done with sheet dies, cast film dies, or blown film dies that have adjustable die lips. Profile and cross head dies... 

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-. 

Figure 10.39. Effect of nucleating agent on sptierulite size in tubular blown films made out of polypropylene at different draw down ratios. [Data from Bheda, J. H. Spruiell, J. E., Polym. Eng. Sci., 26,11,736-45,1986.]...

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