Melting mechanisms

As mixing cannot occur until the polymer has melted, delays in start of melting and the late completion of melting, are of concern with regard to thorough incorporation of additives. 

An extruder screw will contain unmelted pellets at the feed end and molten polymer at the discharge end prior to die entry, assuming the machine is functioning correctly. Between these two points there will he a transition from solid pellets to molten polymer over a number of screw turns, mayhe occupying anywhere from one to two thirds 

Melting has been studied in depth over the years and the technique usually used is that pioneered by Maddock [1] and by Street [2]. This technique had previously been used by Grant and Walker to study mixing [3]. 

The usual procedure is to run an extruder under steady conditions, stop the machine and apply rapid cooling to freeze the polymer in the screw channel. During this period the die and adaptor are removed. The screw is then ejected, usually with a hydraulic ram and sometimes with a short reheat to release the polymer from the barrel surface. Thompson and co-workers [4] used a clear tube to contain loose feed section material. Edwards and co-workers [5] used a barrel split axially to avoid risks of disturbing the surface during removal, whilst Edmondson and Eenner [6] used both methods. 

In order to visually inspect the transition from pellets to melt, mixtures of coloured pellets were used right from earliest investigations so that some appreciations of screw mixing behaviour was established concurrently with the melting studies. 

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Melting Mechanisms of Crystals (Ubbelohde). Metal-Ammonia Systems, Structure and Properties of... 

Figure 6.28 Photograph of a PVDC powder resin that is melting via the one-dimensional melting mechanism. Even at this early location in the melting process, the thickness of the melt film adjacent to the barrel wall was thicker than normal...

This is consistent with the observations of Tang [31] when evaluating PP melting. In his experiments, the melting mechanism could be switched from the conventional mechanism as shown by the photographs in Fig. 6.30 to a one-dimensional... 

The potential cause of these differences in melting mechanisms may be found in the pressure development in the delay zone as described by Kacir and Tadmor [59] and the source of the energy needed for melting. For the delay zone, it is generally... 

Mount, E.M., III, A Review of Melting Mechanisms in Single-Screw Extrusion, SPE ANTEC Tech. Papers, 51, 304 (2005)... 

Foley SF (1992) Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas. Lithos 28 435-453... 

In polymer processing, compaction is an important and necessary step in order to reduce the interparticle, unoccupied spaces and thus eliminate air. It is essential for melting in both single-screw extruders as well as for twin-rotor processors, as we shall see in Chapters 5 and 10. In twin-rotor devices, such as Co-TSEs, for example, the large and repeated deformation of compacted particulates by the kneading elements, which induces large plastic deformation of particulates, is the dominant melting mechanism. 

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... 

Although feasible, as shown by Erwin and Suh (4), using dielectric heating as an energy source is rather limited in polymer processing practice as a primary melting mechanism. 

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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