Melt curtain

Molten polymer is then extruded through a straight slot die, essentially similar to that used for polymer fihn casting and is drawn vertically down into the primary nip. The distance between the die and nip (air gap) is critical since it influences the degree of oxidation and, subsequently, adhesion of the coating. This distance is generally between 150 and 300 mm. Recent modifications of the technique concern the use of an ozone shower or specific active species introduced to the polymer melt curtain to improve adhesion. 

The die which can be center- or end-fed converts the melt from a circular cross section to a imiformly thick melt curtain of the required width. The thickness of the film is continuously measured across the web after the stenter process, giving a thickness or gauge profile. This profile data is used to make fine adjustments to flow profile at the die either through thermoviscous heating or by actuation of mechanical bolts (which physically modulate the die gap profile to achieve uniform film thickness profile). Combinations of thermoviscous and mechanical modulation are also employed in some cases. 

Therefore, an applicator position 40 mm from the melt curtain and low in the air gap will maximize delivery of ozone to the melt and away from the surrounding work area. In addition, by directing tube ports perpendicular to the melt curtain, or at a 45 degree angle directed toward the nip, the strongest extrusion coating bonds will be achieved. 

During melting, the air curtain should efficiently direct fume towards the exhaust off-take without allowing recirculation within the enclosure. The air curtain design should therefore also consider the fume trajectory when a lower updraft velocity from melting is experienced. 

The extruder is on a movable track or platform. On start-up the extruder is in the retracted position, that is the die is not over the web and its operating mechanism. Reason for this pull back is to permit the extruder s start up away from the web and its mechanism. Thus the web and its mechanism is not contaminated with the initial poor grade melt that has to exit the extruder. After the melt develops the desired melt the extruder is moved forward so the die is in the proper position to drop its curtain exactly where required. 

Mixtures of polar polymers, such as PVC, PC, PMMA, TPU, PA, PEST, PGI, SAN, or ABS could be compatibilized by incorporation of two copolymers, the first containing vinyl alcohol, the second an anhydride. For example, blends of TPU with Phenoxy, EVAl, COPO, modified cellulose, and/or polyalkylene oxide, had attractive physical, optical and barrier properties, and were melt-processable without degradation. They could be transformed into films, sheets, or bottles with good barrier properties. Blends containing PA were used for films, tubes, toys, gears, bearings, shafts, curtain sliders, door rollers, etc. The blends with elastomers were reported suitable for improved wiper blades [La Fleur et al., 1994]. 

C.S. = CS = cs = carbon steel, steel alloyed with a small amount of carbon, cullet = scrap pieces of glass recycled to a furnace for melting, cure (refractories) = to stabilize the chemical reaction in concrete and cement-based masonry and refractory materials by subjecting them to heat below 200 F (93 C). curtain wall = a baffle or wall to separate firing zones. 

Shirley, A.R., Nunelly, L.M. and Cartney, F.T. (1982) Melt granulation of urea by the falling-curtain process. Industrial and Engineering Chemistry Product Research and Development, 21, 617-620. 

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