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Melt-blown process is a one-step process in which high-velocity air blows a molten thermoplastic resin from an extruder die tip onto a conveyor or take-up screen to form a fine fibrous and self-bonding web. The fibres in the melt blown web are laid together by a combination of entanglement and cohesive sticking. The ability to form a web directly from a molten polymer without controlled stretching gives melt blown technology a distinct cost advantage over other systems. Melt blown webs offer a wide range of product characteristics such as random fibre orientation, low to moderate web strength. About 40% of melt blown material is used in the uncombined (monolithic) state. The remainder of melt blown materials are composites or laminates of melt blown webs with another material or nonwoven. The largest end-uses for monolithic melt blown materials are oilsorbents, air and liquid filtration media.
A typical melt blowing process consists of the following elements: extruder, metering pumps, die assembly, web formation, and winding.
The polymer pellets or granules are fed into the extruder hopper. Gravity feed supplies pellets to the screw, which rotates within the heated barrel. The pellets are conveyed forward along hot walls of the barrel between the flights of the screw, as shown in Figure 2. As the polymer moves along the barrel, it melts due to the heat and friction of viscous flow and the mechanical action between the screw and barrel. The screw is divided into feed, transition, and metering zones. The feed zone preheats the polymer pellets in a deep screw channel and conveys them to the transition zone. The transition zone has a decreasing depth channel in order to compress and homogenize the melting polymer. The molten polymer is discharged to the metering zone, which serves to generate maximum pressure for extrusion. The pressure of molten polymer is highest at this point and is controlled by the breaker plate with a screen pack placed near the screw discharge. The screen pack and breaker plate also filter out dirt and infused polymer lumps. The pressurized molten polymer is then conveyed to the metering pump.
The metering pump is a positive-displacement and constant-volume device for uniform melt delivery to the die assembly. It ensures consistent flow of clean polymer mix under process variations in viscosity, pressure, and temperature. The metering pump also provides polymer metering and the required process pressure. The metering pump typically has two intermeshing and counter-rotating toothed gears. The positive displacement is accomplished by filling each gear tooth with polymer on the suction side of the pump and carrying the polymer around to the pump discharge, as shown in Figure 2. The molten polymer from the gear pump goes to the feed distribution system to provide uniform flow to the die nosepiece in the die assembly (or fiber forming assembly).
From the feed distribution channel the polymer melt goes directly to the die nosepiece. The web uniformity hinges largely on the design and fabrication of the nosepiece. Therefore, the die nosepiece in the melt blowing process requires very tight tolerances, which have made their fabrication very costly. The die nosepiece is a wide, hollow, and tapered piece of metal having several hundred orifices or holes across the width. The polymer melt is extruded from these holes to form filament strands which are subsequently attenuated by hot air to form fine fibers. In a dies nosepiece, smaller orifices are usually employed compared to those generally used in either fiber spinning or spunbond processes. A typical die nosepiece has approximately 0.4-mm diameter orifices spaced at 1 to 4 per millimeters (25 to 100 per inch). There are two types of die nosepiece used: capillary type and drilled hole type. For the capillary type, the individual orifices are actually slots that are milled into a flat surface and then matched with identical slots milled into a mating surface. The two halves are then matched and carefully aligned to form a row of openings or holes as shown in Figure 3. By using the capillary type, the problems associated with precise drilling of very small holes are avoided. In addition, the capillary tubes can be precisely aligned so that the holes follow a straight line accurately. The drilled-hole type has very small holes drilled by mechanical drilling or electric discharge matching (EDM) in a single block of metal, as shown in Figure 3. During processing, the whole die assembly is heated section-wise using external heaters to attain desired processing temperatures. It is important to monitor the die temperatures closely in order to produce uniform webs. Typical die temperatures range from 2l5oC to 340OC.
The air manifolds supply the high velocity hot air (also called as primary air) through the slots on the top and bottom sides of the die nosepiece, as shown in Figure 4. The high velocity air is generated using an air compressor. The compressed air is passed through a heat exchange unit such as an electrical or gas heated furnace, to heat the air to desired processing temperatures. They exits from the top and bottom sides of the die through narrow air gaps, as shown in Figure 4. Typical air temperatures range from 230oC to 360oC at velocities of 0.5 to 0.8 the speed of sound.
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