What Is Converting? It Helps to Ask That Simple Question

Converting is unwinding a roll of relatively thin material, processing it in some fashion, and then rewinding it into a roll (unless it has been sheeted). Sounds very simple, but is it really all that easy?

The answer to this question obviously is no. Otherwise, you would not be reading this magazine devoted to the subject of converting each month. If converting were like falling off a log, the many products, processes, techniques, etc., described and advertised in this publication would not be necessary.

Thinking individually about the three steps in the converting process can be very revealing. Considering the above-mentioned steps as specific units or components can help you troubleshoot, prevent problems, and optimize performance, as the following discussion will show using some specific examples.

A very important step in the unwinding operation is making certain you are using the proper material. If the material is supposed to be 1-mil polyester, be certain that it is polyester and not PP or some other film. And, is it actually 1 mil and not 0.75 mil or 1.25 mil?

Know the specific details of both sides. Is one side treated and the other side not treated? Does one side have a coating and the other side have none? One side may have printing, and the other side may have no printing. In other words, know what you are using and be absolutely certain you are using it properly.

Pay attention to the physical characteristics of the material being unwound. Is the roll tight on one side and baggy on the other side? Does the roll contain wrinkles or any other visual defects? In the unwinding operation, make certain you apply the proper amount of tension. An inadequate level of tension may cause the web to track improperly during converting. Excessive tension sometimes can cause stretching of a web, with resultant difficulties.

The processing portion of the operation can be very complex. For example, it can consist of treating, coating, and drying. An obvious consideration during treating is the treatment level. It must be at the proper level — not too much and not too little.

A coating operation has more variables than this brief article can completely explore. A list of questions that you might compile should include the following:

  • Is the proper coating being used for the particular job? Is the coating head applying the proper amount of coating? As with treating, a certain range is necessary. Too much and too little are both unsatisfactory. If the coating has more than one component, make certain the proper ingredients were mixed in the correct levels.
  • Is the coating operation generating foam? Foam is usually detrimental to a coating operation. It can cause unacceptable variations in coating weight or contribute to appearance problems in the finished product.

A drying operation also has too many variables to cover completely in this article. One common concern is whether temperature and time of drying are coordinated, so that the proper amount of drying occurs in the oven. Usually, this is the level necessary to volatilize all the water or solvent in the coating. If the coating requires curing, excess heat may be needed to start or complete the curing reaction.

Physical characteristics also are important in the processing step, and tension still plays a crucial role. Avoiding wrinkles and having the same degree of tension all the way across a web are important factors.

As if all of this isn't enough to be concerned about, there are additional factors to consider in the rewinding operation. The first step obviously is to be certain that the correct side is wound inside. The roll also must be rewound with no wrinkles, baggy edges, uneven tension, or other physical problems. It should not be excessively loose or excessively tight.

Using this type of thinking to simplify your converting process will pay valuable dividends. You will have fewer problems and be able to solve those that do occur more quickly.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at e-mail: dbentley@unm.edu.


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