Extrusion processes solids conveying

However, the melting or plasticating process that follows the solids conveying zone is a relatively complex one, and unlike the melt extrusion and solids conveying processes, the detailed physical melting mechanisms cannot be easily visualized, predicated, and modeled from basic principles without experimental investigation. 

The fundamental processes and mechanisms that control single-screw extrusion are presented in Chapters 5 through 8. These processes include solids conveying, melting, polymer fluid flow, and mixing. The analyses presented in these chapters focus on easily utilized functions needed to assess the operation of the single-screw extruder. The derivation of these relationships will be presented in detail in the appendices for those who desire to explore the theory of extrusion in more detail. 

Like the original Darnell-Mol model, the Tadmor-Kleln model was developed with only very limited rate data because solids conveying measuring devices were not available at that time. Moreover, dynamic coefficient of friction data were also unavailable during their model developments. Complete plastlcatlng extrusion simulations, however, were developed with the Tadmor-Kleln model using engineering friction coefficients to estimate the characteristics in the process. 

Two solids conveying devices were built at the Clarkson Polymer Extrusion Consortium. The first device was used to visualize the flow in the section without a discharge pressure. The second device was built such that a relatively high discharge pressure could be applied to the process. 

Figure 12.28 Motor current and discharge pressure for a 114.5 mm diameter GPPS resin extrusion process with unstable solids conveying...

We now follow the modeling approach outlined for the melt extrusion process. We assume steady state conditions and a given mass flow rate then, starting from the hopper, where initial conditions are known, calculations are made in finite steps, ending up at the die, with extrudate pressure, mean temperature, and solids content. If the flow rate at these conditions does not match that of the die, or if the calculations break down for some reason (e.g., insufficient solids conveying), calculations are repeated at a new mass flow rate. 

Sections 9.2 and 9.3 discussed the elementary steps that make up the important and widely used screw extrusion process. The common and outstanding feature of all the elementary steps taking place in the screw channel is that they are induced by drag brought about by a single moving surface—that of the barrel. Solids are conveyed and compressed by the... 

If the actual solids conveying performance is as sensitive to the coefficient of friction as the theory indicates, small changes in the actual coefficient of friction can have a substantial effect on the entire extrusion process. This is particularly true for low values of the coefficient of friction against the barrel. The high sensitivity to the coefficient of friction will tend to result in unstable extruder performance. At high values of the barrel coefficient of friction the extruder performance will be less affected by changes in friction and the extruder will be more stable under these conditions. This explains why grooved feed extruders tend to be more stable than smooth bore extruders. 

Heat migration using an integral barrel-feed port barrel configuration will aid in solids conveying and increased output in the extrusion process, because of the increase in coefficient of friction between the pellet and the barrel wall in the feed section of the screw. 

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