Die exit regions

The tensile stresses acting in the direction of converging stream lines can ellipsoidally deform the big particles, but not so much as to form fine fibrils from small particles (region B). The matrix are also elongated in the converging section. As they pass the die exit (region C), recoil of the matrix occurs to release the stored energy... 

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. ... 

Pressure drops in die exit region Pressure drops in die ends Volumetric extrusion rate... 

Figure 7.117 Change in velocity profile in the die exit region...

The effect of die design and wall slip on the die drool phenomenon was investigated for metallocene based LLDPE. It has been found that die exit opening and wall slip can significantly reduce the die drool phenomenon. Moreover, theoretical research has revealed that die drool onset can be explained by the negative/non-monotonic pressure profile generated inside the die and/or at the die exit region due to melt elasticity. 

Secondly, the question is whether a more realistic constitutive equation than mWM model will also predict non-monotonic pressure profile at the die exit region. Due to the fact that level of non-monotonicity in the pressure profile depends on the streamline curvature and corresponding normal stress generation, as discussed above, the use of the more advanced constitutive equation will leads to the more precise determination of the normal stresses/non-monotonic pressure profile at the die exit region than in case of the mWM model which artificially predict Weissenberger number to be constant at very high flow rates [8]. On the other hand we can say that any constitutive equations capable to represent melt elasticity at least quantitatively should be able to predict nonmonotonic pressure profiles at the die exit region. 

It has been revealed theoretically that wall slip reduces the negative pressnre at the die exit region which is stabilizing effect from the die drool phenomenon point of view. 

The exit region of a die used to extrude a plastic section is 10 mm long and has the cross-sectional dimensions shown below. If the channel is being extruded at the rate of 3 m/min calculate the power absorbed in the die exit and the melt temperature rise in the die. Flow curves for the polymer melt are given in Fig. 5.3. The product pCp for the melt is 3.3 x 10. ... 

The exit region of a die used to blow plastic film is shown below. If the extruder output is 100 X 10 m /s of polythene at 170°C estimate the total pressure drop in the die between points A and C. Also calculate the dimensions of the plastic bubble produced. It may be assumed that there is no inflation or draw-down of the bubble. Flow data for polythene is given in Fig. 5.3. 

The site of the sharkskin distortion is again the die exit, and so is the screw thread pattern. The site of, and the mechanism for the gross extrudate distortion are problems that have no clear answers. The work of White and Ballenger, Oyanagi, den Otter, and Bergem clearly demonstrates that some instability in the entrance flow patterns is involved in HDPE melt fracture. Clear evidence for this can be found in Fig. 12.18. Slip at the capillary wall, to quote den Otter, does not appear to be essential for the instability region, although it may occasionally accompany it. ... 

To be exact, the origin of variable z is located not at the die exit, but just past the die-exit swell region (21). 

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... 

The flow channel of the die should be designed such that the plastic melt achieves a uniform velocity across the die exit. The shape of the land region of the die corresponds to the shape of the extruded product. An example of an in-line tube or pipe die is shown in Figure 17. The material flows into the die fi om the extruder then it flows around a torpedo. 

The orifice angle of the cone can signiflcantly influence the flow anomalies that are frequently found with high molecular polyethylene (PE). In such cases, changing the melt temperature has little effect. Computer programs are available to calculate the distribution of shear rate in the extrusion flow channel. It is important to ensure that the critical shear rate occurs near the die exit resp. is confined to as narrow a region as possible. With the accumulator head, the effects of flow anomahes can be influenced by the extrusion velocity. To achieve the shortest cycle times with continuous extrusion, the plastic usually exits from the orifice at close to critical shear rate. 

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