Die swell measurements

The shear viscosity can be used for relating the polymer flow properties to the processing behavior, extruder design, and many other high shear rate applications. Elongational viscosity, die swell measurements as well as residence time effects can be estimated. Typical data are shown in Figure 6. 

A die swell measurement of a strand extruded from a capillary die is used to measure the elastic properties of a material. As a polymer flows down a capillary tube, its molecules become stretched out and aligned in the direction of the flow. Once the material leaves the confines of the die, the molecules recoil and draw back up to themselves, causing the extrudate to swell beyond the size of the opening in the die. The amount of swell displayed by a polymer is a characteristic of its elastic nature. 

Die swell is measured by determining the diameter of the extruded strand after it has exited the capillary die. Usually a noncontact laser measuring system is used to get accurate strand diameter measurements. Die swell is a strong function of shear rate, so most systems are configured to get a die swell measurement each time a viscosity point is taken by the rheometer. Die swell data are usually presented as a ratio of the cross-sectional area of the die to the cross-sectional area of the strand ... 

Aside from viscosity curves, the capillary rheometer can be used to determine other material properties. The effects of time and temperature on processability and chemical stability can be studied, and other properties such as the melt density can be measured. Elastic data can be collected with accessories such as a die swell measurement system, and extensibility measurements can be performed with a melt tensile tester. The capillary rheometer is the instrument of choice for any practically oriented polymer laboratory. 

Die swell measured as percentage increase of external diameter of tubing over that of die. 

Automatic running die-swell measurements can be made by using laser scanning. This can be carried out in a temperature-controlled chamber at the die exit for isothermal die-swell. Together with stress-relaxation experiments (by stopping the piston descent), this can provide information on the viscoelastic nature of the material. High die-swell and long relaxation times would indicate a more elastic material. Many rheometers also include automatic MFR calculations on thermoplastics. 

The phenomenon of die swell is complex, and the method for incorporating it into die design calculations is unclear. In most cases the processing die geometry is considerably different from that used to make die swell measurements. For example, in the extrusion of a parison used in blow molding there is swell of both the thickness and outer diameter of the parison. How to translate die swell from a capillary to that of a parison is certainly not straightforward. 

To illustrate one possible way of translating capillary die swell measurements to some other die geometry we consider the swell of extrudate leaving an annular die. In the swell of polymer extruded from an annular die as shown back in Figure 3.1 (this figure is associated with Design Problem II), there is swell of the diameter as well as the thickness of the extrudate. The two most eommon swell parameters are the diameter swell, Bi, and the thiekness swell, B2, defined, respectively, as... 

The results presented here illustrate the complexity in trying to extend die swell measurements from a capillary to other die geometries. As an initial approximation one can use the relations between Bi, B2, and B given in Eqs. 7.13, 7.14, and 7.15. However, one must be aware of the fact that when significant strain hardening arises in the extensional behavior, the data will deviate more dramatically from these... 

Die swell measurements can also be made using the capillary rheometer and Fig. 2, from the Mills and Giurco paper,shows the dependence of die swell on the molecular mass distribution for emulsion polymerised SBRs and solution polymerised SBRs. 

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

This composition contains abont 90 to 60 pbw of a PP block copolymer and abont 10 to 40 pbw of a PE. The block copolymer contains abont 99 to 90 wt.% of a crystalline PP and abont 1 to 10 wt.% of an amorphons ethylene/alpha-olefm copolymer and has a melt flow rate of about 2 to 15 g/10 min and a die swell ratio, measured by a capillary rheometer, of at least 1.7. Foamed materials containing a fine and uniform foam are obtained from this composition. 

Two types of flows were investigated. At CEMEF, experiments on a plane geometry were carried out. Birefringence measurements were performed, the results of which are presented in Section III-l. Comparison with computed results will be presented in paragraph 6.2. At LRMP, flows in axisymmetric contractions were studied. We present in the next pages the experimental results obtained for presstire drops, entrance pressure losses and die swell. 

Jerman and Baird recently conducted rheological studies on the same copolymers using an Instron capillary rheometer. They also measured die swell and entrance pressures. They observed that the viscosity of the 60 mole % HBA/PET copolymer was two orders of magnitude lower than that of PET when compared at the same temperature of 285 °C, which is similar to the results reported earlier by Jackson and Kuhfuss Die swell of the copolymers was highly temperature dependent. In general,... 

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