Die land

Fig. 4. Pipe or tubing die for in-line extmsion A, die body B, mandrel, pin, and male die part C, die, die bushing, and female part D, die-retaining ring E, die-retaining bolt F, die-centered bolt G, spider leg H, air hole I, seat for breaker plate J, ring for attachment to extmder and K, die land (15).

Example 5.12 Estimate the heat transfer from the die to the melt as it passes through the die land in Example 5.5. 

Thus the melt temperature after 2.08 x I0 seconds is the same as the initial melt temperature (Ti) so that as the melt passes through the die land it is relatively unaffected by the temperature of the die. 

A thrown die lands with the six side facing up. Use the many-worlds hypothesis to explain how the die actually landed on all values at once. 

According to the many worlds model each conceivable possibility corresponds to a different universe. We can understand how the die lands on all values at once by concluding that the die lands on different values in six different universes. You have 1/6 of a chance of being in the universe in which the die rolled ro a 6. Another you in another universe saw that it rolled to a 5. 

Flat Film Extrusion In flat film extrusion, the melt is extruded through a long slot in a T or coat hanger-type die, past the die lands. In this setup, the polymer melt is forced into the slot die at its center it reaches the slot opening by way of a manifold and over the lands. The principal advantages of film casting are substantial improvements in the film s transparency, freedom from haze, improved gloss, and other optical properties. 

You are to extrude a polystyrene tube at an average speed of 0.1 m/s. The relaxation time, A, of the polystyrene, at the processing temperature, is 1 second. The die land length is 0.02m. Will elasticity play a significant role in your process. 

The end-fed-sheeting die, as presented in Fig. 6.9, is a simple geometry that can be used to extrude films and sheets. To illustrate the complexities of die design, we will modify the die, as shown in the figure, in order to extrude a sheet or film with a uniform thickness. In order to achieve this we must determine the length of the approach zone or die land as a function of the manifold direction, as depicted in the model shown in Fig. 6.10. 

A manifold that generates a uniform sheet must deliver a constant throughput along the die land. Performing a flow balance within the differential element, presented in Fig. 6.11,... 

End-fed sheeting die. Design a 1000 mm wide end-fed sheeting die with a 1 mm die land gap for a polycarbonate film. For the solution of the problem assume a manifold diameter of 15 mm and the longest portion of the length should be 50 mm. Using the above information we can write,... 

Figure 6.14 Schematic diagram of die with two different die land lengths and thicknesses.

The die land thickness differences can be compensated by using different land lengths such that the speed of the emerging melt is constant, resulting in a uniform product. If we assume a power-law viscosity model, a uniform pressure in the manifold and an isothermal die and melt, the average speed of the melt emerging from the die is... 

Die design with two die land thicknesses. Determine the die land length ratios, L2/L1 for a die land thickness ratio, /i2/ i of 3, for various power-law indeces. Using eqn. (6.54), we can easily solve for the land length ratios for several power-law... 

Formulate the die design equation for an end-fed sheeting die with a constant manifold diameter and a variable die land gap. Assume a Newtonian viscosity. 

Using the geometry and notation given in Fig. 6.14, relate the die land length ratio, L1/L2 for a die land thickness h //i2 = 2 to the power law index of a shear thinning polymer. 

Reservoir (width X denth) Die land (width X depth) Aspect ratio (width / depth) Contraction ratio Ref... 

Figure 21. Discontinuities in the birefnngence pattern in the die land at high flow rate (LDPE FN 1010,145 °C, Va = 170 s l).

Fig. 23 illustrates another type of unstable behaviour for a polystyrene. In this case, an alternative discharge of the large entry vortices in the main flow leads to a periodic asymmetric pattern, both in reservoir and die land, resulting in an helical type defect at the die exit. Such observations were also reported by Oyanagi et al. [36],... 

A more precise comparison can be made by looking at the birefringence changes along the flow axis (Pig. 34). The birefringence increase in the reservoir is similar for both models and veiy close to experimental values, as already observed with results from other numerical computations [27,30,65]. The relaxation along the die land is very consistent for the mPTT model, where the stresses do not relax totally before the channel exit. Results with the GOB model indicate a less realistic relaxation, vdth a local overshoot near the exit, which is not observed experimentally. 

LDPE is a highly branched materied, whose flow behavioTir exhibits some peculiarities, which constitute a good test for a numerical simulation. As presented in Section lll-l, birefringence patterns are perturbed around the re-entrant comer, which leads to the appearance of W-shaped fringes at the entry of the die land. It can be seen in Fig. 39 that the mPTT model ilows on to obtain these characteristic shapes (the computation is related to experiments carried out at 175 °C and 21 s, for an estimated value of the stress optical coefficient of... 

The values of t] and x cannot be determined directly and, therefore, Benbow, Oxley, and Bridgwater, introduced the term jS, the die land viscosity factor, to replace r /x, as in Eq. (13) ... 

If the die land velocity factor p varies with the extrusion rate or the liquid layer at the die wall is non-Newtonian, Eq. (19) must be further modified fo Eq. (20) ... 

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