Melt flow model

D. Givoli, J. E. Flaherty, M. S. Shephard. Simulation of Czochralski melt flows using parallel adaptive finite element procedures. Model Simul Mater Sci Eng 4 623, 1996. 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

In the past three decades, industrial polymerization research and development aimed at controlling average polymer properties such as molecular weight averages, melt flow index and copolymer composition. These properties were modeled using either first principle models or empirical models represented by differential equations or statistical model equations. However, recent advances in polymerization chemistry, polymerization catalysis, polymer characterization techniques, and computational tools are making the molecular level design and control of polymer microstructure a reality. 

The physical process of melt ascent during two-phase flow models is typically based on the separation of melt and solid described by Darcy s Law modified for a buoyancy driving force. The melt velocity depends on the permeability and pressure gradients but the actual microscopic distribution of the melt (on grain boundaries or in veins) is left unspecified. The creation of disequilibria only requires movement of the fluid relative to the solid. 

Ribe NM Smooke MD (1987) A stagnation point flow model for melt extraction from a mantle plume. J Geophys Res 92 6437-6443... 

The flow properties of the polymer must be matched to the mold (or vice versa) in order to achieve optimum product quality. In particular, we must manage the pressure drop between the gate and the furthest extent of the flow path. Articles with thin walls, such as margarine tubs, require low viscosity resins. In other cases, the situation is more complex and the cavity may need to be designed so that the product is thicker near the gate than it is at its extremities. The development of complex products is aided by computer modeling of melt flow within the mold cavity. 

Modeling of Melt Flows and Heat Transfer in Near-... 

Equations (5.95), (5.96) and (5.97) are suitable for constant critical melting porosity. In a one dimensional steady state melting column as a result of decompression melting, the porosity may increase from the bottom to the top of the column. If melting porosities change as a function of the spatial position, the related differential equations need to be solved numerically. More details of various melt transport models by porous flow have been given by Spiegelman and Elliott (1993), Iwamori, (1994), and Lundstrom (2000). 

A single screw extruder was used to fabricate the filaments from the composite. The glass transition of the composite was found to be 126°C, which is higher than that of pure ABS material. In addition, the melt flow behavior was extensively studied by setting up a finite element model (79). 

N. Mostafa, H.M. Syed, S. Igor, and G. Andrew, A study of melt flow analysis of an ABS-iron composite in fused deposition modelling process, Tsinghua Science Technology, 14(Supplement l) 29-37, June 2009. 

The results of all the thermal-capillary models discussed so far have neglected the influence of convection in the melt in transporting heat to the solidification interface. The status of convection calculations that neglect the coupling to global heat transfer and capillary consideration is discussed later. The union of thermal-capillary analysis with detailed convection calculations is discussed in the subsection on melt flow. 

The correct barrel temperature to be used is the inner surface temperature. This is generally not known and a heat transfer problem in the barrel must be solved in conjunction with the flow model of the melt in the screw channel. Screw temperature is generally not controlled and it can be assumed to be roughly equal to the average melt temperature. 

The lumped-parameter model approach becomes particularly useful when dealing with the plasticating extrusion process discussed in the next subsection, where, in addition to melt flow, we are faced with the elementary steps of solids handling and melting. 

Fig. 10.48 Numerical simulation results of nonisothermal flow of HDPE, Melt Flow Index MFI = 0.1 melt obeying the Carreau-Yagoda model for a typical FCM model wedge of e/h — 3 and =15. (a) Velocity (b) shear rate and (c) temperature profiles [Reprinted by permission from E. L. Canedo and L. N. Valsamis, Non Newtonian and Non-isothermal Flow between Non-parallel Plate - Applications to Mixer Design, SPE ANTEC Tech. Papers, 36, 164 (1990).]...

Discharge parts are components located downstream from the screw tips through which the melt flows. These parts should, for example, exhibit as small a flow resistance as possible and an even product flow with zero dead space. In general, the flow channels in these parts are relatively simple in design. The polymer is completely melted, and the additives are homogenously distributed within it. The conditions for effective mathematical modeling would appear to be in place. 

However, these are not adequate for the strong non-Newtonian polymer melts under discussion here. The latest flow modeling tools are used for this task. They allow highly accurate calculation of pressure build-up and energy consumption. 

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