Making Sense of a Heatset Offset Dryer

Even calling it a heatset offset dryer is perhaps one of the most misleading descriptions of all.

Take it from me, a heat-set dryer for a web offset press is a very misunderstood concept. Even for the first couple of years working on a Heidelberg M600 web press, I really didn't understand exactly what happened in the dryer.. and come to think of it, 15 years later, I don't think I will ever completely know what heatset printing ink is made of until I become a chemist.

Having said that, I would like to share, in lay mans terms, what really goes on in that long box after the last printing unit in a web offset heat-set press.

First of all, let's clarify what type of oven we're talking about here. There are a few types of dryers used on offset web presses.

• Direct Impingement Dryers
• Hot Air Dryers
  
• Combination dryers

All three of these refer to floating dryers, and you really don't need to understand in detail how they function. But suffice to say, hot air dryers are used on nearly all modern web presses that exist today. Hot air blows on the web through what are called blades and the solvent laden air is exhausted to be treated. That's as far as I'll take you on that one.

And here's where we clarify the most common misconception of heat-set printing. These do not actually 'dry' the ink per say. It is more proper to say that it sets. That is why we call it heatset printing and that is exactly what the oven and chill rolls perform, not drying, but setting. Let me explain.

Let's say you pour some gasoline on some paper. Would it be proper to say that the paper is wet? Well, maybe. If that's how you want to refer to it. A few hours later when the gasoline is gone, would it be better to say that the paper dried? You could say that too. But when it comes to using the terms wet and dry, water is the most widely understood medium.

Now let me return to the oven. When going through the oven, all the oils and solvents literally boil out of the ink into a layer called the 'boundary layer'. This is then exhausted. Now what is left? Pigment, of course. But in addition there are various resins and waxes that were added by the ink manufacturer which help you and I understand why we call this type of offset printing, heatset printing.

Upon hitting the chill rollers, all those waxes and resins solidify. I would liken it to dripping hot wax from a candle into cold water. Does the wax dry or does it set? The answer is obvious. So in reality, it is better to say that the ink sets as opposed to dries.

I will also add one more thing. Any good operator knows that the dryer and chill rollers are part of a single system. That's the way they have to be viewed to make proper sense of the 'drying' process.

Having said all that, I'll confess, I still call it a dryer. But for the record at least we can make sense of what it really does.