Web Handling

What is web handling and why is proper web handling so important? A web is defined as a long thin flexible material (substrate) typically supplied in roll form. Web handling is the manipulation and movement of this flexible material through a machine to process the material. The goal is to control the flexible web through the machine rather than to modify it. Converting is the processing of these webs to permanently change them such as printing, embossing, coating, metallizing, or laminating. With these definitions behind us, we can now touch on the various aspects of web handling including tensions, web roller interactions, winding and some various defects that are most common.

There is a myriad of flexible webs used in the converting industry. These include paper, foil and plastic films such as polyethylene, polypropylene, polyester, nylon, cellophane, surlyn and a host of others. Each have their own set of unique physical properties and understanding these are key to proper web handling. Of this modulus, COF (Coefficient of Friction), heat resistance is some of the most important. It is important to note that flexible webs typically cannot withstand compressive forces and buckle easily. This is often the source of most web handling issues.

The first aspect for discussion is tension. Tension is the state of being stretched. Tension is needed to pull webs through the machine and provide web/roller interaction that allows control of the web through the converting process. This is known as good tension. However, if not controlled properly tension can cause curl, tunneling, wrinkles, fractures (in paper), web breaks, registration error and wound roll defects. This is referred to as bad tension. Tension of the web must be uniform. When combining two webs with different properties match the elongation of each substrate not the tension force on each. Correct tensions will result in equal amounts of elongation which will reduce or eliminate curl and tunneling. Run tension at the lowest possible value to maintain control of the webs. Matching elongation at higher tensions may cause other problems in the wound roll.  A laminate will curl in the direction of the web that has elongated the most if not matched. Curl can occur in both machine and cross(transverse) direction. Often it is wise to work with the film manufacturer to understand the modulus of each web. With modulus defined as the strength at a given strain. This will allow for proper matching of tension settings to achieve similar elongation.

Another aspect for review is web/roller interactions. Rollers provide the basis for moving/handling the web through the converting process. There are many types of rollers including those that transport the web and those that process the web. Transport rollers such as idler rollers move the web through traction between the roller and the web surface. The amount of traction is a function of applied tension, wrap angle and the surface of both the roller and web. There should be no relative movement between the web and roller that can lead to loss of tension, web marking and loss of edge control. Rollers must be perfectly aligned to prevent cross web tensions variations. These can create defects such as wrinkles, curl, tunneling and poor registration as well as others. Rollers must also be kept perfectly clean. Rollers can build up residue from adhesive, masking tapes, slip contamination from films and from scratching or abrasion.  Webs will not move as freely over these rollers creating friction and tension build leading to many of the defects mentioned above.

Another important aspect of web handling is winding. There are several types of winders found in the converting industry. Center winding is driven from the core while surface winding is driven from a drive roller that acts like a nip point. Center winding is by far the most common. Torque is transmitted through the roll layer by layer with each layer acting as the core for the next layer. Every wound roll has a torque capacity which if exceeded causes interlayer slippage and thus defects. Adjusting taper tension and lay on roll pressure can ensure that torque capacity is not exceeded. In addition to wrinkles and buckles other types of wound roll defects are loose or crushed core and telescoping. Telescoping is the abrupt change in roll edge during the winding procedure and is caused by interlayer slippage.

Remember that while converting is an art, web handling is a science. When trying to solve web handling problems do not commit to only looking at one way to solve the problem. Working in conjunction with film, machine and adhesive suppliers can often help pinpoint the problem. Key monitor points for proper web handling should be tension, roller traction, air entrapment, interlayer slippage and curl. Below are some tips that can often be used as general guidelines:

1. Tension is measured as total force divided by the width of the substrate and is given in PLI(Lbs. per linear inch). Some guidelines for films:

   1. Polyester 0.5-1.5 lbs/inch width

   2. Polypropylene .25-.3 lbs/inch width

   3. Polyethylene .25-.3 lbs/inch width

   4. Foil 0.5-1.5 lbs/inch width

   5. Nylon .1-.25 lbs/inch width

2. For 3 inch core 30” OD maximum and for 6 inch core 40 “ OD maximum.