Extrusion process, closed

In many cases drying operations are critical to the production of successful commercial catalysts. Close control of the drying process is necessary to achieve the proper distribution of the catalyst precursor within the pore structure of the support. Drying also influences the physical characteristics of the finished catalyst and the ease with which subsequent pelleting or extrusion processes may be carried out. 

Expanded Rubber This class of product has an essentially closed cell structure. These products are most usually formed by moulding or extrusion processes and have a solid outer skin around the cell structure. Organic blowing agents are used for this type of process. 

FIGURE 6.7 Extrusion process for fabricating tubular SOFC support layers, (a) Open-ended die with cathode slurry in it and (b) Mandril insertion into the die, extruding the cathode slurry into a closed-ended hollow tube. 

Dow Plastics is to unveil a new resilient polyolefin foam, Strandfoam, which offers packagers of lightweight electronic equipment and components reduced package size, and savings in materials and shipping costs. Strandfoam is made of a special blend of PE and is produced by a proprietary extrusion process that yields fused strands of closed-cell foam with a network of air channels parallel to the foam strands. Strandfoam provides the required levels of protection with substantially smaller volumes of foam compared to urethane. 

Blow extrusion, in which the initial lump of polymer is formed by an extrusion process, is the most common form of blow molding and is shown diagrammatically in Fig. 16.10. A short length of plastic tubing is extruded through a crossed die and the end is scaled by the closing of the mold. 

Polyethylene. This is essentially a closed-cell insulation manufactured at 448 =t 2 K by an extrusion process. A blowing agent and nucleating agent are employed to control the cell size, and primary use is in insulating pipelines for hot and chilled water lines, air conditioning, and processing... 

Considerable work has been done on mathematic models of the extrusion process, with particular emphasis on screw design. Good results are claimed for extrusion of styrene-based resins using these mathematical methods (229,232). With the advent of low cost computers, closed-loop control of... 

Starting with the governing equations of fluid flow we will collect the ingredients for the numerical simulation of fluid flow in ceramic extrusion. We start with the derivation of the governing equations to describe ceramic extrusion processes. Since most ceramic materials don t change in volume we will restrict our discussion to incompressible flow. A simple example closes the flrst chapter. 

The new mesoporous materials have extremely high surface-to-volume ratios. An example of these materials is MCM41, which was invented by DuPont. A simple structure that can be manufactured in the laboratory is illustrated in Eigure 15.14. The structure initially contained a periodic array of polymer spheres. When close packed, these spheres leave 26% of the volume empty. We can then infiltrate a liquid into these pores, burn out the spheres, and convert the liquid to a polycrystalline ceramic. Another synthesized porous ceramic is the cordierite honeycomb structure used to support the Pt catalyst in automobile catalytic converters. In this case the cylindrical pores are introduced mechanically in the extrusion process. 

In Figure 47.14, the gas supply to the column dryer is heated after dehumidification using a similar heat exchanger as in the fluid bed. This relatively small amount of gas mixes with the gas above the fluid bed and the same quantity is then vented off to atmosphere (air) or to a return line (nitrogen) after the supply fan, such that the whole system is kept in pressure balance. This small air loss is in fact the only energy loss in the system. Since the gas for the fluid bed is recycled, the heat input closely matches that required to heat the chips, which in any case are heated for the subsequent extrusion process. In many instances, the column dryer is positioned directly above the extruder. 

Gradient-index (GRIN) polymer fibers with a quadratic distribution of the refractive index are prepared by a novel closed extrusion process. The effects of the die geometry and the length of the diffusion zone on the refractive index distribution of the polymer fibers are studied. The results show that the non-quadratic refractive index portion of the GRIN polymer fiber can be removed by a coreshell separation die design. Also, the spatial refractive index... 

Several approaches have been developed to prepare GRIN polymer fibers in our laboratories, e.g., initiator diffusion technique (70), interfacial-gel copolymerization (77), and extrusion technique (12,13). Our previous study successfully used a closed extrusion process to prepare GRIN polymer fibers (72,75). This new process has also overcomed several drawbacks in conventional extrusion processes, e.g., poor reproducibility and low production rate. The refractive index profiles of the GRIN polymer fibers prepared by the new process are affected by the following two factors (1) the formulation of the reactant mixtures, e.g., different types of monomer combination and variation of host polymers and (2)... 

Figure 1. Experimental setup of the closed extrusion process for preparing GRIN polymer fibers. A and B, material supply tanks C and D, gear pumps E, a concentric die F, an enclosed zone G, (a) without core-shell separation die, (b) with core-shell separation die H, a hardening zone I, rolls.

The closed extrusion process is a liquid (core mixture)-liquid (shell mixture) diffusion control process. The key parameters for controlling the refractive index distribution in this process are the compositions of the reactant mixtures, pump speed, the temperature and the length of the diffusion zone. In our previous study, we have shown that the refractive index distribution of the GRIN polymer fibers was significantly affected by the monomer composition of the reactant mixtures and the temperature of the diffusion zone (13). The refractive index difference between the... 

In closing, significant development in the theoretical analysis of the extrusion process has contributed to a better understanding and design of new equipment, an increase of output in existing machinery, and development of concise control via computers. 

Steady state n. A condition of processes or parts of processes in which the state variables describing the process, e.g., temperature, pressure, compositions and velocities of streams, and amounts of materials residing in various process equipment, do not change with time. Most extrusion operations closely approximate the steady state except during startup and shutdown, whereas injection molding and sheet thermoforming are unsteady, intermittent processes. 

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