The die swell phenomenon

The optimization of the functional properties of a synthetic yarn (in particular from the point of view of its mechanical behaviour) requires maximum orientation of the macromolecules in the direction of flow. This objective can be difficult to achieve because of the generally pseudoplastic behaviour of such liquids. Tlie dies which the polymers cross typically have the shape of a funnel (see Rg. 2.6). When the macromolecules are brought into liquid state in the system, they will undergo stresses because of the irregularity of the die s diameter. Under the action of the pressures at the exit of the extm-sion device, the polymer will cross successively three zones in the die (see Fig. 2.6). The advance of the macromolecules from Zone I towards Zone II shows that the flow rate will increase because the section of the capillary 

The length L of the die in Zone II has also a very important influence on the swelling phenomenon at the exit of the die. Let us consider for example two extrusion experiments carried out with two dies of same diameters Do, but different lengths L and 2L. If the pressure exerted on each die is adjusted so that the same flow Q is obtained, we observe the production of rods with a diameter lower for the case of capillary length 2L. The Barus ratio thus decreases as the length of the die increases. The reduction of B with L (or —) can be explained by the material relaxation in the channel of the die where this is sufficiently long. In practice, extreme cases are chosen 

The flow rate in the capillary is another parameter which conditions swelling at the exit of the die. The Barus number in fact increases with the shear 

8 Evolution of the Barus number as a function of the shearing rate at the exit of a spinning die. 

Advances in filannent yarn spinning of textiles and polymers 

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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. 

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. 

Draw resonance, or surging, is defined as the nonuniformity in the diameter of the extrudate when a polymer melt is stretched at different take-up speeds as it comes out of an orifice. This phenomenon is shown schematically in Fig. 2.10. When the take up speed is small or when there is no stretching, only die swell is observed, as can be seen from Fig. 2.10a. When take-up speed is higher and the stretched extrudate is solidified by quenching, the contour appears as shown in Fig. 2.10b. Now the draw ratio is defined as the ratio of the linear velocity V of the extrudate settled in the quenching bath to the smallest linear velodly Vo in the die swell region. When the draw ratio (DR) )es beyond a oitical value DRc, then the resulting phenomenon is draw resonance as shown in Fig. 

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. 

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. 

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