12 Jan 2015 "Only an idiot would ..."

The world of coating and printing is full of people who think that these are "art" not science. This means that coating/printing is full of people making life very hard for themselves. I tend to get exasperated at the vast waste of time and effort caused by people who are fighting the laws of physics and, therefore, always losing. In exasperation I have ended up saying "Only an idiot would ..." Here are a few of my most common

"Only an idiot would choose a technique other than slot coating". I admit that there is some exaggeration here. If you want km of good-enough cheap coatings, a Mayer bar (metered rod) is more than adequate. But here I'm talking about 21st century coating where high quality and good control are essential. This instantly means that any technique which is not pre-metered should be banned, especially roll coating. By definition these "recycle" excess coating and this can only mean more waste or lower quality. And who in their right mind would use a technique where the levers you pull have a simultaneous action on both quantity (coatweight) and quality. Elementary quality theory tells you to separate variables. Control coatweight with one set of levers, control quality with another.

So any sensible coater will use slot coating unless their coatweight is high enough or their speed fast enough to use curtain coating. The problem with this rule is that most of us want thin coatings and the laws of physics make this inherently hard for slot coating. You can use "reasonable" slot setups to be thin at low speed, low viscosity, low width, but it needs incredible engineering to go wide width with modest speeds and viscosities but there's a limit to what engineering can deliver.

The saviour is tensioned-web slot which allows thin coatings over a surprisingly wide range of speeds and viscosities with merely "good" engineering. A very few Western companies worked this out for themselves, but Asia adopted the idea much more quickly. Thankfully the West is now catching up. So over a huge range of coating requirements, "only an idiot would choose anything else."

"Only an idiot would slot coat without a flow meter". In some corporations there would be astonishment that anyone would have to say such a thing. Of course you use a flow meter. Slot coating is litres/minute over metres/minute so to get your coatweight you simply pump what you need for the speed you've set. But countless slot coat facilities chase their tails by trying to control coatweight via the slot's lip-to-base gap. This is entirely stupid and doomed to fail, but they do it anyway. It sort of works because a smaller gap can give a larger backpressure which can reduce the flow. But the slightest change to the system and coatweight changes. And gap also controls quality so two variable are being controlled with one parameter. Sigh...

"Only an idiot would not control the temperature of their slot". Many years ago I was puzzled by the fact that the coatweight at startup was varying dramatically across the web. The operators would fiddle with the adjusment bolts but it would take a long time to settle down. It was some time before learning that they pre-cleaned the die by flushing with hot water. The hot water definitely did a good job of cleaning. The operators had no idea that any temperature change in the die changes the delicate balance of gaps within the slot, causing the coatweight to vary. The rule is clear. Keep your slot die at a constant temperature, for ever!

"Only an idiot would use an adjustable lip die". OK, I know that for polymer extrusion adjustable lips are probably necessary. But for normal coating they are positively bad. They add to cost and weight, they add to complexity, they compromise the design of the slot itself but above all they encourage operators to fiddle. It is impossible for an operator to resist the temptation to "improve" things by twiddling something and those bolts are a huge temptation.

"Only an idiot would open a die for cleaning". I learned this principle from one of the world's great die manufacturers and tried to stick with it. But in the end I gave in and announced the reverse: "Only an idiot would not open the die after each run to clean it". My heart is with the first principle. And if you are running the same stuff all day, every day and if you have good systems in place, it probably works well. If you have an ultra-precision die from a great supplier it is almost certainly a good idea never to open it. But for systems where formulations are changed regularly, my experience has been that a good clean between jobs makes life so much easier. There is a proviso. The die must be a nice, simple, light fixed-lip die. These are easy to remove, open, clean and close with a high degree of certainty that you won't scratch, bend or otherwise damage it. I once screamed when some excellent German operators carefully opened a die to clean it, had everything perfect and then placed the inner surface of the top of the die onto the benchtop. They were lucky that they didn't damage it then and they never made that mistake again.

When it comes to the borderline between printing and coating I am less confident of my pronouncements, though increasingly I'm at least thinking that "only an idiot would use gravure". This is a harsh judgement on a technique which has served many industries brilliantly. But it is a process that is not controllable in the scientific sense as there is little useful science behind it (not through lack of trying). And increasingly slot-based lane coating and patch (intermittent) coating are proving to be robust methods for simple shapes.

When it comes to my area of expertise in screen printing my list of "Only an idiot" is very long. Suffice it to say that they can mostly be encapsulated in "Only and idiot would not use a low EOM, low Rz stencil". But maybe that is for another blog.