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Manufacturing technology impact on haze
December 20, 2007
Manufacturing method in films definitely affects the optical properties as we have been discussing in the previous posts. In particular, the quenching conditions can have a profound impact on the optical properties. In blown-film applications, the formation of the solid film from the melt is oftentimes delineated at the “frost line” as shown in the diagram below of the blown-film process. Below the frost line, the film is clear because it is molten, and above the frost line, it is hazy because it is frozen and the crystals are scattering light as we have described. The limiting factor in blown-film production is usually the quenching rate because the film is cooled by moving air and the heat transfer coefficients are somewhere in the range of 15-20 btu/hr/ft2/ 0F (85.2 to 114 W/m2/0C)for high velocity air
In comparison, a cast line with its higher quenching rate of approximately 250 btu/hr/ft2/ 0F (1420 W/m2/0C) from the cooling drum will generally give better film clarity because the morphology of the crystals are different from those of the blown film, and this affects the internal haze of the film. A frost line can also be found on a cast line at the point where the film solidifies, but it may also be confused with the point where the film releases for the chill roll. It is difficult to choose between blown and cast lines at times because there are capital cost differences and throughput differences to be considered as well as film properties. But if you are looking for the best film optics, then the improved quenching obtainable from a cast line could be an important consideration.
Posted by Eldridge M. Mount on December 20, 2007 | Comments (0)