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.

6 thoughts on “Introduction to Flash Dryers ( screen printing )

  1. When printing t shirts you would need something was very reliable and these custom made DIY dryers often break down.

  2. That can be a problem… I’ve seen some real horror story DIY units, but then again, commercial units are not exempt from bad design or manufacturing problems either. I’d say if someone is looking to DIY to be cheap, they are better off getting used gear. Cheapness always involves compromise, and until one has some experience, the compromises made could well be the wrong ones. You dont want to have an order for 200 shirts to go out in the afternoon, and have your DIY catch on fire in the morning.

  3. I want to dry monoprints more quickly than to put them between newsprint with heavy boards on top. I have seen plastic-bag-like dryers with heat blowers attached to one side and vents on the other. My studio is in my house so I need to have a low flow heater so the noise and air are not too fierce. I have thought to get a large vinyl bag and somehow attach a vacuum cleaner backwards to force hot air into the bag as it encloses the print which has to flattened while drying, usually under a heavy piece of hardboard. Do you have a suggestion for the outflow? Do you think my idea will work. Does someone make blowers that are not very expensive and more quiet than a vacuum cleaner? Thanks

  4. I’d be leery of some type of bagging approach as it would concentrate the evaporated solvents. If the solvents are flammable, this could result in an explosion. Another issue is likely the lack of uniformity in the bag… there might be cool areas where solvents could condense and then drip on places where you dont want them. The last issue is a matter of exhausting the air stream outdoors. In an air drying rack, solvents evaporate slowly, so over an 8 hour drying period, the probability of having a concentration of aqueous based solvents to the point of being hazardous without venting is pretty low. Ie, a houses natural air infiltration and leakage is probably enough to deal with relatively safe water based inks with an 8 hour drying period. If you add heat, and that changes to 1 hour, you might find some concentrations that are at or nearing hazardous levels.

    That being said, if you are comfortable with the solvent vapor concentration and/or can get it safely exhausted, and want to build something a source of really cheap and quiet surplus fans is Marlin Jones. Air movement alone will accelerate the drying process… not as much as heat will of course, but maybe that’s enough. If you want to add heat, likely the safest way to go is a hound heater. It wont add a great deal of heat, but you wont have to worry about wiring, fire, or shock hazards. If you need more power, and dont mind designing things, an Edison Cone Base Heater might work, but you need to be sure you know what you are doing.

  5. I read the post of custom flash cure. I agreed with that. However I am still searching whoever can custom made a flash cure which on/off control by a sensor.
    In that way I can insert into one of my auto printing machine arm for curing the ink before the last colour.
    If someone knows who can do that, please contact me.

  6. The challenge with doing so is the thermal inertia of the heating element. Most flash dryers I worked with over the years used either mineral insulated strip heater assembly (which are very robust) or a ceramic fiber heater (which responds quicker, but presents increased fragility).

    In high volume processes, thermal inertia issues are resolved via motion control. Ie, the flash dryer stays at a constant temperature, but when it is needed, it swings down close to the substrate, and once heating is complete, swings back up out of the way. I’ve even seen some units where in the set point gets bumped up in anticipation, and then is lowered to a more moderate temperature after heating is complete.

    A third possibility is to take advantage of the speed of quartz tube heaters. Alas, this can be a bit tricky to deal with, as the different spectral energy output of them vs the more broadband output of a MI or ceramic fiber heater can require different settings for different substrates.

    Back in the early 90’s, I remember designing a system which used quartz tubes, infrared temperature sensing, and a computer controlled temperature controller to do this. The biggest challenge in doing so were the safety aspects, in the event of conveyor jams or other failures. Ie, you don’t want the IR temperature sensor telling the controller things are cold if something gets jammed. Beyond thermal limit controls, we also had swing away arm on the quartz tubes to prevent substrates from bursting into flame if things were wrong

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