A question that is often asked about the Web Coating Process is “ What are the one or two most critical areas of the process that I should concentrate on to obtain a coated product with high quality and low cost?”. A possible basis for this question can be found in an interchange in the 1991 movie “City Slickers”, between Billy Crystal and Jack Palance
Curly: You know what the secret of life is?
Mitch: No, what?
Curly: This.
Mitch: Your finger?
Curly: One thing. Just one thing. You stick to that and everything else don't mean anything.
Mitch: That's great, but what's the one thing?
Curly: That's what you've got to figure out
This exchange suggests that there is only one answer to any decision, dilemma or course of action. This can lead to the possibility that there are one or two key process elements that should be worked on to achieve desired goals and the rest can be ignored. Unfortunately, this is not true and there are many process elements which must be optimized and controlled to in insure a successful process.
When I first thought about applying this concept, I thought that it does not apply to the coating process, I had a hard time trying to figure out the one thing that applies to the web coating process. I got bogged in trying to prioritize the many different process elements coating, drying, web transport, solution preparation, rheology etc. and decided the premise of the exchange was wrong.
Then it occurred to me that there was indeed Just One Thing. It was that you have to do everything the correct way and no one element can be ignored. The coating process is complex and a failure in any one component will lead to failure in the process. An important reason for this is that once a defect is introduced. There is no way to correct in subsequent process steps. An out of specification viscosity can lead to poor coating quality and out of limits coating weight. There is no redundancy in the process to correct errors.
Figure 1 is a representation of a Pilot Coater. I like this diagram because it shows all of the major components of the process.
Figure 1
There are 14 specific process elements in this coater. These transport he web through the coater, apply the liquid coating coating, dry it, cure it of needed, laminate it if needed and wind the final roll. If any one of these is out of standard operating specifications the coating will be defect. For example, an out of specifications edge guide can lead to a poorly wound roll and an uneven coating. An out of specification dryer can lead to a wet coating, which when wound will stick or an over dried coating with poor performance properties.
In addition to the above process elements , there are several support technologies which are essential to the successful operation of the coater. These are shown in the following table:
Table 1
Coater Support Equipment
Each of these support functions must function as specified. Again, the failure of one component can lead to poor quality. Lost product and high cost. Surface treatment is often needed to insure adequate adhesion and help clan the web. If it is running out of limits then product will have poor adhesion. The instrumentation system must insure all process control variables are on target and have alarms to show when they are out of limits. Procedures to indicate how to use equipment and set variables for all process hardware is essential. Incorrect settings will lead to defects. In addition, operation personnel need to be trained to use equipment and how
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