Die swell phenomenon

The die-swell (extrudate swell) effect describes the significant expansion of the diameter of the fluid column after exiting from a small pipe (Figure 4.3.8(b)). Some polymer fluids can have a swelling of up to two or three times the exit diameter. A simple proposition for the mechanism of the die-swell phenomenon is that while the fluid is inside the exit pipe, it is subject to a velocity shear, similar to the pipe flow with a maximum shear stress at the wall [18]. This velocity shear stretches... 

Figure 9.4. Results of melt-spinning a simple bicomponent fiber. Light and dark portions represent different polymer materials. Note the ballooning effect (the die-swell phenomenon) as the blend leaves the common capillary. Since the pressure drop in the common capillary must be the same for each component, careful regulation of the homopolymer capillary diameters is necessary to obtain the desired result.

The complex changes taking place in the die swell phenomenon could have sizable effects on the development of melt-spun fiber properties. 

As a starting point, consider the behavior of the fluid after extrusion. Immediately after extrusion, the fluid experiences the die swell phenomenon, where the velocity profile flattens. Ultimately, when the fiber solidifies, the velocity profile will be flat (i.e., plug flow). In between, a velocity profile will possibly be first formed and then distorted by solidification at the fiber exterior. Even if the profile becomes fully developed, however, it will not have a parabolic shape but rather will have a blunted form because of the polymer s non-Newtonian fluid behavior. In essence, then, the fiber in the post-extrusion-solidification region will have a velocity profile that can be closely approximated by assuming plug flow (i.e., a constant V across the fiber cross section). 

Pereira, C.C., Nobrega, R., Borges, C.P., 2000. Spinning process variables and polymer solution effects in the die-swell phenomenon during hollow fiber membranes formation. Braz. J. Chem. Eng. 17 (4-7), 599-606. http //dx.doi.Org/10.1590/S0104-66322000000400024. 

Present an explanation of the observations relating to the die-swell phenomenon shown in Fig. 6.46. 

Most thermoplastic polymers have a strong tendency to crystallize during cooling. This phenomenon generally results in an isotropic arrangement of the macromolecules and must be controlled to ensure orientation in the fibre direction. The relaxation of the macromolecular chains at the exit of the dies causes a die swell phenomenon, which must also be treated to optimize the final properties of the yarns. 

Mayer H J, Stiehl C and Roeder E (1997), Applying the finite-element method to determine the die swell phenomenon during the extrusion of glass rods with noncircular cross-sections ,/oMrnn/ of Materials Processing Technology, 70,145-150. 

The polymer jet exiting from the spirmeret is subject to relaxation, which is indicated by the die swell phenomenon. As a resrrlt, the molectrlar orientation developed in the spinneret is largely relaxed. 

Such elastic effects are of great importance in polymer processing. They are dominant in determining die swell and calender swell via the phenomenon often... 

When a polymer is extruded through an orifice such as a capillary die, a phenomenon called die swell is often observed. In this case, as the polymer exits the cylindrical die, the diameter of the extrudate increases to a diameter larger than the diameter of the capillary die, as shown in Fig. 3.9. That is, it increases in diameter as a function of the time after the polymer exits the die. Newtonian materials or pure power law materials would not exhibit this strong of a time-dependent response. Instead they may exhibit an instantaneous small increase in diameter, but no substantial time-dependent effect will be observed. The time-dependent die swell is an example of the polymer s viscoelastic response. From a simplified viewpoint the undisturbed polymer molecules are forced to change shape as they move from the large area of the upstream piston cylinder into the capillary. For short times in the capillary, the molecules remember their previous molecular shape and structure and try to return to that structure after they exit the die. If the time is substantially longer than the relaxation time of the polymer, then the molecules assume a new configuration in the capillary and there will be less die swell. 

Die swell is dependent upon the L/D ratio of the die. The phenomenon is a limiting factor in the drive to reduce moulding cycles, since the conditions which lead to excess swelling lead also to quality deficiencies in appearance, form and properties of the extrudate. In order to control the swelling the temperature of the melt can be increased, which causes a decrease in relaxation time. A long tapered die has also been found to reduce post-swelling. 

We have already discussed one aspect of non-linear behavior in polymer melts, namely shear thinning. A second aspect manifests itself when we examine the flow of polymer melts through small diameter tubes or capillaries. This is the phenomenon of jet or die swelling, where a polymer forced into a narrow tube, diameter d0, swells when... 

The flow computations in a short die (here T= 145 °C) obviously imply free surface determination corresponding to the swell phenomenon. It may be seen in Fig. 37 that the general birefringence patterns are similar, even if the results of the mPTT model seem more realistic in the downstream region than those of the GOB model, for which discontinuities appear along the centreline, apparently due to an incorrect determination of the relaxation time at very low shear rate. 

---