Introduction to Flash Dryers ( screen printing )

A young fellow was asking about building flash dryers a few weeks back. He wanted to build his own flash dryer, and a lot of the screen printing guys were coming down a bit hard on him. Having designed a number of these over the years *, I thought I’d share a few bits of information about them such that others could benefit as well.

The thing is, flash dryers are the type of thing that are very easy and pretty inexpensive to build, but not necessarily so easy to engineer because of the numerous trade offs involved. I remember telling the young fellow, if you build your own, you can have a much better unit than you can buy for not a great deal of extra money… but, you need to balance out how much time and money it will take to develop a design. That is where the rubber hits the road so to speak.

For those unaware, a flash dryer is used in screen printing in order to bring freshly screen ink to a partially cured state (gelled) in a short time frame such that other inks can be overlaid on top. In dark colored t-shirt printing, its common practice to put down a layer of white, flash dry it, and then proceed to screen print the rest of the colors.

For those somewhat aware, a flash dryer is no substitute for a conveyor oven for purposes of final curing. It would be similar to using a pipe wrench as a hammer… it can work under some conditions, but its far from the optimum, and unintended consequences can abound.

The reverse is also true. One can use a conveyor oven to flash cure ink, but its slow, could affect durability, and it opens the door to major registration hurdles in removing and replacing items on the platen.

The driving off of solvents is really the key part of the flash drying process. The ink must not to be fully cured, or its likely the additional inks will not adhere properly, and as such the durability of the final article would be compromised. By the same token, if one tries to cure too quickly, the solvent may bubble off, and produce pinholes, or in other cases, the shirt/ink ends up scorched. And of course, if the article enters the next stage before being in the gel state, expect a mess.

Proper ink selection is also key. Not all inks are equally suitable for flash drying, some are subject to after flash tack, which requires an additional cooling station. Such an extra process step could impact production speed, but also if the ink is misplaced, impact durability. Ultimately, one needs to review the manufacturers ink specifications for flash drying, both as for suitability, but also the proper temperature.

Now that we’ve looked at the need for flash drying, and some of the issues involved, lets take a look at design. Earlier, I mentioned that the manufacturing of flash dryers is pretty easy, but the design part is where things get a lot more complex. There are a multitude of things to consider, and the fact that each one impacts one or more other factors makes for a challenge. As such, the old school printers are probably correct that DIY flash dryers are probably not the best way to go. Yet, if one is willing to trade design time for final performance, most certainly a DIY dryer can exceed the capabilities of pretty much any commercial product on the market. As I go through the design issues involved, I’ll throw out a few blue sky ideas, that are not commercially viable for a product manufacturer, yet for the dedicated experimenter may prove useful.

Being each design issue can end up being fairly long, I’ll break them out into individual entries. As I do so, I will hot link them here for ease of navigation.

Some of the design issues and tradeoffs:

• safety in use
• safety in manufacturing processes chosen
• speed of cure vs energy used
• speed of cure vs production bottle necks
• static uniformity
• dynamic uniformity
• manufacturing labor costs
• raw material costs and availability
• usability
• user maintenance
• product costs and features
• time to market, and engineering costs

* disclaimer… I’m a hardware designer, not a screen printer. While I’ve been in a ton of shops ranging from 1 man opt with a single press to 300 employees with a multitude of octopus presses and others, my hands on time is exceedingly limited. Keep such in mind, some of the concerns and concepts I have may not necessarily be the optimum for your operation, or any operation for that matter.