Extrusion 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 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. 

In many processes based on extrusion the material is subjected to further manipulation after leaving the die—as examples, by stretching or casting on chill rolls in the manufacture of film. In all such cases it is essential that an extrudate withstand the forces applied to it and not tear—in other words, while there should be some strength and elasticity the main requirement is that the molecules of which it is comprised can flow relative to each other (in this sense its viscous behaviour is the most important feature). 

Forcing of Cl into the head die is a multifunctional operation leading to reduced friction of the melt against the die walls, in addition to improved orientation stretching and reinforcement of the film. The distinguishing feature of the employed extrusion head [100] (Fig. 2.24) is the presence of an annular chamber 6 in mandrel 4. The chamber communicates with Cl channel 8 and is shut off by a porous wall 5 from the side of channel 7 over which the polymer melt is fed from the extruder into the head. The outside surface (a) of the porous wall 5 and inner surface (b) of matrix 1 are made in the form of a cone whose apex faces the side opposite to the inlet to channel 7. 

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. 

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