Extrudate swell region

The fiber-spinning process is a prime example of uniaxial extension. The process consists of two regions (i) the first is the extrudate swell region, where normal forces accumulated during extrusion suddenly relax to cause swelling (ii) the second is the draw-down region, where the fiber diameter decreases according to the velocity... 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

Continuum Depiction Local oscillation of the melt-wall boundary condition in the exit region causes peturbations on the exit stress and die swell Oscillation of the overall stress due to unstable boundary condition produces cycles of melt compression and decompression in the barrel and fluctuations in the extrude swell The extrudate distortion arises from formation of secondary flow (vortices) in the barrel due to the strong converging flow near the die entry... 

In the purpose of comparisons between theory emd experiments, this short review points out the need for significant experimental studies related to flow patterns, pressure drops and extrudate swell ratios for polymer melts characterized in shear and elongation. Streamlines, velocity components and stress fields related to flow birefringence studies should be used to validate numerical simulations of entry and exit flows. Progress in flow computations in domains involving the upstream and downstream channels together with the exit region are also necessary. 

With respect to Figure 7, it is clear that with short dies, the post-extrusion swelling includes the entrance effect contribution and the shear response in the viscometric region. Consequently, the extrudate swell not only depends upon the residence time within the die, but also upon the transit time within the entrance zone. Therefore, it is possible to define a correcte residence time, t, according to ... 

When plotting the extrudate swell versus the corrected residence time tj.j, an excellent fit is obtained for the four dies (x), as can be seen in Figure 10. This demonstrates that with the short dies of practice, the entrance converging flow region plays a very important role in post-extrusion swelling. 

The extrudate swell ratio, B, plotted as a function of composition, was found to go through a local maximum within the phase inversion region. As a rule, B in blends is large and shows PDB. The effect is not related to deformation of the macromolecular coil (as in homopol)nner melts), but rather to the form recovery of the dispersed phase. 

In practice, few extrusion dies are designed with square channels at the exit. However, many are designed with a rectangular channel in the lip region exiting the die. The effect of Reynolds number on the extrudate swell from such shapes appears to have been heretofore unreported. 

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. 

As to front coal body in large mining height face, it is extruded in the gob direction in the process of forming plastic area which makes coal body swell. In the situation that is out of swell deformation, the swelled coal area in gob side is equal to the reduced coal area in plastic region. Take the arbitrary point in plastic area as object, and the distance to coal wall is denoted by x. The horizontal displacement is denoted by S, and vertical displacement is Sy, then ... 

In this region, several important factors are taking place. First of all, the extruded melt experiences the phenomenon known as die swell. In this process, the extruded polymer melt expands its diameter beyond the limits of the spinning capillary. The diameter increase can be as much as 200% to 300% (see Fig. 9-12). 

Only for the lower MW rubber. A, at low shear rate, did the swell increase with increasing shear rate. This shear-rate region of sample A is in the flow state and therefore, the behaviour is similar to that of plastics melt. However, at the higher shear rate, and particularly after the melt fracture the swell decreased significantly. The extrudate going over maximum swell is not generally recognised. 

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