More Causes, More Cures For Web Scratches

Last month I wrote that idler rolls (in high-speed processes) do not always turn at web speed even though they have sufficient wrap and free-turning bearings, and that this phenomenon may produce web scratches on clear, smooth films. Also, scratches may result in these types of webs if they are under high tension (with measurable elongation) by a driven roller, even though the roller is very accurately speed controlled.

There are other sources for scratches in the web path. One likely source is any set of nipping rolls consisting of one driven and one idler roll that are used to isolate web tension. Standard construction of these rolls dictates the driven roll should be made with a metal surface and the idler roll should be made with an elastomer surface. And, in order to achieve a uniform nipping pressure across the entire web width, the elastomer-covered roll surface must be profiled. The correct profile is a parabola and is defined by the desired nipping pressure and shell construction of both rolls.

Once the correct profile has been cut on the elastomer-covered roll, the actuating pressure on the trunnions of the idler roll must be kept at the design pressure to keep the nipping footprint uniform across the full width of the working faces.

These requirements result in a profiled surface velocity difference between the center and the ends of the idler roll. The idler roll angular velocity is determined by surface speed of the metal roll, though it is usually not equal to the metal roll's angular velocity. And, there can be a substantial difference between the metal roll surface velocity and the surface velocity at the ends of the elastomer roll on long nip rolls (more than 3 m).

There is also a small displacement of the elastomer in the nip, and thus, the driving velocity vector for the elastomer roll is slightly smaller than the surface velocity vector for that roll. The result is that there is a high potential to scratch a clear, smooth web surface as the higher elastomer surface velocity adjusts to the lower surface velocity of the metal roll in the high-pressure area of the nip.

Sometimes there are no substitute methods available to replace nip rolls for tension isolation and control because of the process demands. In such cases web scratches may be reduced by using three rolls to nip the web, where the two rolls that are touching the web are cylindrical in shape, and the roll that does the nip loading is profiled to create a uniform nipping profile (see figure).

A good design is one that uses a cylindrically shaped metal roll and a cylindrically shaped elastomer surface roll to nip the web, and a profiled metal roll to apply the loading pressure. Operation of three-roll systems is much the same as two-roll systems. As is the case of two-roll nips, there is only one loading pressure that is correct for uniform nipping when using nips with three rolls.

Also, when the rolls are opened, there should be separation between all rolls to prevent flat spots from developing in the elastomer when the rolls are not turning.

When scratch problems occur during web processing, materials such as paper or plastic film sometimes can be used to cover the offending rollers that are generating the scratches, and this procedure will allow production to continue. However, these types of fixes should be considered temporary, and a true fix should be found as soon as possible.

Cast film stretchers that use nip rolls and idler rolls to heat and elongate the web are likely to scratch clear webs. Very smooth surface rolls are required in these machines, because there is relative motion between the web and the stretching roll surface.

William E. Hawkins has 30-plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; fax: 740/474-3148; e-mail: fhswhawk@bright.net.


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