Maddock melting mechanism

Figure la Maddock melting mechanism observed from extruder freezing experiments... 

As mentioned earlier, the melting mechanism in screw extruders was first formulated by Tadmor (29) on the basis of the previously described visual observations pioneered by Bruce Maddock. The channel cross section and that of the solid bed are assumed to be rectangular, as in Fig. 9.26. The prediction of the solid bed width profile (SBP), that is the width of the solid bed X as a function of down-channel distance z, is the primary objective of the model, which can be experimentally verified by direct observation via the cooling experiment of the kind shown in Figs. 9.20-9.25. As shown by Zhu and Chen (40), the solid bed can also be measured dynamically during operation by equipping the extruder with a glass barrel. 

Results from computer simulations for a HDPE polymer show that the Maddock/ Tadmor melting mechanism occurs in the early stages of melting. At the beginning of the compression section, the solid bed tends to become totally encapsulated, while towards the end of the melting, the solid bed breaks apart into several pieces. At the time of publication of the paper by Viriyayuthakorn and Kassahun, no direct comparison was available between theoretical predictions and experimental results. Therefore, no statement can be made about the accuracy of the predictions. However, regardless of the accuracy of the predictions, this model provides new capabilities that no doubt will prove very useful in future work on the analysis of plasticating extrusion. 

Figure 12.3 (A) Melting mechanism of single-screw extruders as determined by Maddock. (B) Schematic diagram of barrier screw cross section showing separate solids and melt channels separated by the barrier flight as defined by Barr.

After the melting mechanism was described in 1959 by Maddock, variable width barrier section was invented by Maillefer [47] and the constant width barrier section screw design was invented by Barr [48]. Other barrier screw designs were patented by Geyer [49], Lacher [50], and Dray [51]. A good review of screw designs can be found in Chung [8],... 

This influence of starved feeding on melting and mixing was just one of many situations included in trials with colour pellet mixtures by Maddock in 1959 to investigate overall melting and mixing mechanisms [8]. 

The barrel will be heated in the melting region in the C-chamber. This represents a second mechanism of melting. Here heat eonduction prodnces a thin layer of melt adjacent to the barrel. This leads to a secondary mechanism similar to that described by Maddock [32] for single screw machines and Bawiskar, Jnng and White [33,34] for low serew speed co-rotating twin screw extmders. 

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