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web positioning |  |
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Possible web configurations for coating 
Metering Bar un-supported except for the ends A metering bar supported only at its ends is the simplest system in general production use but it is only able to rotate with the web and has to have sufficient section to resist deflection. In this and other examples, single bar arrangements are shown for simplicity.
Metering Bar fully supported along its entire length A metering bar fully supported along its length illustrates the most common way of giving the bar support and wiping it along the entire length, enabling it to be rotated in either direction but commonly against the web.
End of machine arrangement The bar is commonly applied below a horizontal web run but the use of other configurations can be advantageous. In this example, using the end position on a machine gives the operator better access to the bar and bar bearing.
Multi bar coating Generally, bars are used singly. Multi-bar coating heads employing two or more bars are in general commercial use. Compound heads can give excellent coating quality and greater flexibility. Very rapid trial runs are achieved and flying changes save both time and material.
Use of a backing roll at point of bar contact A metering bar acting against a resilient covered backing roll ensures controlled contact between bar and web so the bar pressure can be adjusted without unduly disturbing the web tension or rap angle. This system has many advantages but is more costly to install and maintain. This also raises the question of compatibility between coating mix and roll covering.
Off-web size press coating In off-wen size press coating, both sides of the web are coated simultaneously by applying a coating mix to "rubber" applicator rolls and transferring the mix indirectly to the web. In this case, the behaviour is slightly different to that of conventional coating because the pressure and resilience of the roll coating are much more effective in controlling coating weight. [The following chart illustrates this, although it must be borne in mind that the coating weight represents double coating (both side of the web)]
Close-mounted backing roll This illustrates a system similar to the use of a backing roll but showing a more compact configuration giving a shorter contact time between initial application and metering station. |