Melting drag melt removal

A common flow problem in polymer processing is a shear flow with a temperature gradient as depicted in Fig. 6.58. For example, this type of flow occurs within the melt film that develops during melting with drag flow removal, as will be discussed later in this chapter. 

Figure 6.64 Schematic diagram of the melting process with drag flow melt removal.

Melting with Drag Flow Melt Removal... 

Fig. 5.3 Schematic representation of drag-induced melt removal and pressure-induced melt removal mechanisms.

Removal of the melt, also discussed in Section 5.1, is made possible, in principle, by two mechanisms drag-induced flow and pressure-induced flow (Fig. 5.4). In both cases, the molten layer must be sheared, leading to viscous dissipation. The latter provides an additional, important source of thermal energy for melting, the rate of which can be controlled externally either by the velocity of the moving boundary in drag-induced melt removal or the external force applied to squeeze the solid onto the hot surface, in pressure-induced melt removal. 

Achieving a complete solution of the set of equations above is difficult, as pointed out earlier. In addition to the numerical solution (33), Pearson (35) proposed a heuristic approach. Insight into the nature of melting with drag-forced removal can be obtained, however, by considering some special cases that lead to analytical, closed-form solutions. These simplified cases per se represent very useful solutions to the modeling of processing methods. 

Thus the physical significance of U2 and U becomes evident. The former reflects the reduction (U2 < 1) of the rate of melt removal of the him by drag how as a result of temperature dependence and shear thinning of the viscosity, whereas U /2 is the rate of viscous dissipation (per unit width) in the melt him. The relative signihcance of conduction and dissipation for melting is obtained by comparing the two terms in the square Brackets in Eq. 5.7-55. 

Because of these limitations, and in particular because of the fact that, in such a mechanism, the temperature gradient at the wall that determines the heat flux to the solids drops exponentially with time, this melting mechanism is rather inefficient. However, the latter drawback can be alleviated if some mechanism continuously removes the molten layer. This, as shown in Fig. 5.3, can be accomplished either by applying a force normal to the heated surface, forcing out the melt by pressure flow, or by having the contact surface move parallel to its plane, dragging away the molten layer. These comprise the two... 

The drag-removal melting mechanism was discovered and mathematically modeled by Tadmor (27) in connection to melting in SSEs (see Section 9.3). It was further rehned, experimentally, verihed, and formulated as a self-contained computer package by Tadmor et al. (28-31). Later Vermeulen et al. (32), and Sundstrom and Lo (26) and Sundstrom and Young (33) analyzed the problem both experimentally and theoretically Mount (34) measured experimental rates of melting, and Pearson (35) analyzed the theoretical problem mathematically in detail, as shown in Fig. 5.12. In this section we follow Pearson s discussion. 

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