The biggest time delay tends to occur between the film being extruded and when it is used for printing. This can be a matter of a few days, but more often stretches into weeks and occasionally months. So, storage of the film is critical. The problem is that substrate manufacturers do not give clear guidance on this subject in the data sheets relating to the different films they produce.
For a start, some customers are better equipped to handle storage than others, and dyne decay is not necessarily consistent. Each case is different. It is essential the film was correctly surface treated at extrusion. This will have an impact on its shelf life and ability to be boost treated after storage.
If you are storing film, keep the temperature in the 60 degrees Fahrenheit to 68 degrees Fahrenheit range, and the humidity between 50 percent and 60 percent. Some transport trucks are air conditioned, which is good. Further, make sure you keep the rolls wrapped to keep out dust and dirt. Finally, to understand the converting parameters, you need ink/adhesive surface tension data from your suppliers. If you then measure the film surface tension, you can use the curve profile to calculate the boost dosage needed to reach the correct dyne level at each process stage.
The choice you must make is either spend more money up front on top-coated material, or have the cost and inconvenience of boost treating the cheaper stock after a period of storage.
Decay Profile
There are four factors involved in dyne decay during storage: time, humidity, additives and contamination. These make up what we call the “decay profile.” In general terms, the more you have of each of the four factors, the greater the dyne decay will be.
Using a dyne pen is a simple, reliable and inexpensive method for converters to test substrates, especially when compared with the cost of attempting the next process with mis-matched surface energy levels between liquid and substrate.
Let me give you an example of a customer that supplied us with different materials for testing in our lab. These were two types of polyethylene (PE): one with 600 ppm of additive; the other a top-coated stock. Both decayed initially at the same rate, but the top-coated material leveled out after one month at 42 dyne, while the other continued to decay over the three months of testing, by which time it had decayed to 36 dyne.
Again, the choice the customer must make is either spend more money up front on top-coated material or have the cost and inconvenience of boost treating the cheaper stock after a period of storage.
Our lab test also shows that while corona treatment will achieve the desired dyne level for printing or adhesive application, the decay is relatively quick compared with ordinary plasma treatment, where the dyne level attained at treatment is higher and the decay is slower and flattens out at a higher dyne level.
Plasma is the best form of surface treatment if you need to obtain a high dyne level that can be retained over a long period, typically on films used in agriculture, but for other applications too. Using high-power plasma treatment achieves the highest dyne level, and that level is retained for a much longer period. But again, it’s a question of cost—Plasma is a far more complex and expensive process. Corona is better suited in most cases.
Knowing your film parameters is essential. How pure is it? How many ppm of additive does it have? What is its granulation? These are all key elements, so you really need to master the decay profile or go for just-in-time (JIT) production, where storage is not an issue.
There are no short cuts. Be responsible. Make best use of your time and money! “As they say, Knowledge is power!” In today’s complex and competitive marketplace, the better informed you are, the greater your chance of success!
About the Author
The company is headquartered in Kolding, Denmark, where it manufactures both corona and plasma surface treatment equipment for the printing, converting, laminating and extruding community, with control, consistency and efficiency ever in mind. Printers can learn more about Vetaphone and how to enlist its lab in performing the tests described in this article by visiting vetaphone.com.
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