Melt stream size

In this section, we will answer this question Why do melt stream size and temperature affect the atomization results In addition, one might ask the following questions 1) Why do materials atomize differently and 2) What is the key material property responsible for the difference Furthermore, we will speculate on the broad appHcation of the GAP process to higher molecular weight polymers than the ones atomized thus far. 

Inputs were XI (melt temperature in°C),X2 (melt stream size in inches),X3 (type of material) outputs were weight fractions having micrometer ranges as specified by Y1 (0-53), Y2 (53-106), Y3 (106-150), Y4 (150-295), and Y5 (295-600)... 

Figure 10.35. Temperature distributions before and after a 1.77 inch inside diameter six element Kenics HEM mixer performing thermal homogenization of polyethylene melt. The apparent viscosity of polyethylene used in the test was 11,000 poises. A homogeneous melt stream was obtained using a Kenics Mixer of six elements. It was found that thermal homogenization in the Kenics Mixer is independent of the initial radial temperature profiles and the size of the unit. A radial thermal gradient reduction from 100°F to less than 1 °F was obtained in a PVC cast film production. In general, the unit delivers a polymer melt stream with less than a 3°F radial temperature gradient.

As it flows from section to section, changes in channel shape and size causes melt velocity to speed up or slow down. When velocity increases, flow is directed and guided when velocity decreases, flow wanders and loses direction. Typically at bends or turning comers, pockets of stagnant material fills the voids outside the main melt stream. 

The main causes of reduced output are increased flow restriction, commonly a result of clogged screens and screw wear. As screens perform their function properly and capture contaminants in the melt stream, they create an increased restriction to flow through the system. This increased restriction will result in higher head pressure. Additionally, there will be more recirculation of melt in the screw channel and less throughput. As discussed above in the section on high melt temperature, changing the screens should alleviate this problem. It is possible that other sources of flow restriction could exist, such as screens with an incorrect mesh size or a valve in the extruder head. 

The screw of the plastifier is designed to pick up pellets, not chopped and screened chips of your resin. In grinding, you do not achieve uniform ground resin. You get strings, fines, and various sizes of chips from your grinders. Thus, you are feeding the hopper throat of your machine with a different bulk density of resin constantly. The plastifier is nothing but a pump and with different bulk density, your melt has differential pressures within its melt stream. 

Another process, the Barton process, is based on molten lead. The core of such a device is the "Barton reactor", a heated pot that is partly filled with molten lead. It is continuously refilled by a fine stream of molten lead. Fine droplets of lead are produced by a fast rotating paddle that is partly immersed under the surface of the molten lead within the "Barton reactor". The surface of each droplet is transformed by oxidation into a shell of PbO by an airstream that simultaneously carries away the oxidized particles if they are small enough otherwise, they fall back into the melt and the process is repeated. Thus the airstream acts as a classifier for particle size. 

Process parameters of primary importance include roll speed, differential roll speed, roll gap, metal flow rate, metal stream velocity, and melt superheat. The mass median diameter of particles diminishes exponentially as the roll speed increases. It is possible to obtain a smaller mass median diameter when one of the rolls is kept stationary rather than rotating the two rolls at the same speed. Metal flow rate seems to have a negligible effect on the mass median diameter. However, the mass median diameter increases with increasing metal stream velocity, suggesting that the relative velocity of the metal stream to the periphery of the rolls may be a fundamental variable controlling the mass median diameter. The size distribution is approximately constant for the conditions studied. 

Some quantitative studies1498115011 on droplet size distribution in water atomization of melts showed that the mean droplet size increases with metal flow rate and reduces with water flow rate, water velocity, or water pressure. From detailed experimental studies on the water atomization of steel, Grandzol and Tallmadge15011 observed that water velocity is a fundamental variable influencing the mean droplet size, and further, it is the velocity component normal to the molten metal stream Uw sin , rather than parallel to the metal stream, that governs the mean droplet size. This may be attributed to the hypothesis that water atomization is an impact and shattering process, while gas atomization is predominantly an aerodynamic shear process. 

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