Blown films stretching force

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 majority of the studies presented in Table 24.1 focus on the final dimensions of the blown films. However, the calculated stretching forces are not in close agreement with the reported experimental values. 

The stretching force is a direct consequence of the distance of polymer solidification (DPS) in both processes (i.e., FLH in film blowing processes or cooling length in ribbon extrusion). In this sense, controlling DPS offers the possibility to control the final properties of films and ribbons. Table 24.3 lists typical values of stretching force as function of FLH and DR for blown films of LDPE. 

Films made from LCP and LCP-thermoplastic blends have been made using the blown film process [6,10,11,15-17], combined with a counter-rotating die. This process provides for multiaxial orientation of the LCP through controlled shear forces in the die prior to extrusion from the lips. Then, after extrusion from the die, the semi-molten LCP film tube is simultaneously stretched in the longitudinal (machine) direction and the circumferential (transverse) direction. The die shear and stretching were discussed previously in section 3. 

Depending on the application, blown and cast stretch films ideally provide high stretchability for increased product yields, strong holding force to secure pallets during transportation, and high puncture resistance to prevent failure on corners and irregularly shaped loads. 

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