Dealing with Heat & Humidity
Many people consider adjusting ink for printing under conditions of extreme heat and relative humidity to be akin to “voodoo.” While it is true that high temperature and humidity can pose problems, overcoming them isn’t really that difficult.
The best way to minimize the negative effects of temperature and humidity variation is to control your printing environment. By keeping the temperature between 68 and 74 degrees Fahrenheit, the relative humidity between 40 and 60 percent, and acclimating your substrate to the controlled environment before printing, you can eliminate most problems. If you don’t have a controlled environment, you’ll probably have to adjust your ink under warmer and/or more humid conditions.
When the mercury rises, solvents evaporate more rapidly. As a result, you need to add either a higher percentage of thinner by weight, or add a “slower” evaporating thinner. Most ink manufacturers have slow, medium and fast evaporating thinners available for each ink type. When experimenting with thinners of different speeds or blends of compatible thinners, start by first setting the machine speed to the desired cycle time (if that is important). Then begin testing by adding a medium speed thinner at a percentage in the middle of the manufacturer’s recommended range. For example, if the range is 10 to 20% by weight, try 15%. After allowing the machine to cycle a few dozen times on a piece of scrap, stop the machine and clean the pad, then print a clean part at normal cycle speed.
If you see a problem with the image, perform the following simple test:
- Pick up the image at normal speed, stopping the machine before it prints. Look at the image on the pad. If the whole image is there, clean the pad and proceed to Step 2. If the image is only partially there, and particularly if areas of finer detail are missing whereas areas of higher coverage are not, it is a good indication that the ink is not thin enough. To see if a little more thinner will solve the problem, clean the pad, then place a few drops of thinner on your finger and rub it on the image area of the pad. Cycle the machine through pick-up and stop, once again observing the pad to see if the entire image is there. If it is, then you know the addition of a little thinner should solve the problem.
- Once you have an acceptable image on the pad, print a part. If the print is acceptable and there is no ink remaining on the pad, you’re ready to begin production. If the image looks smeared or transparent, or there is still ink on the pad after transfer, it is a good indication that your ink is too thin. In that case, add some more ink (don’t forget to add hardener, too, if you’re using a two-component ink) and try again.
You may run into a situation where one thinner alone may not work. For example, you may find that by the time you get enough medium speed thinner in the ink to make it transfer under warm conditions, it is so thin and the print quality has deteriorated. In these cases, you can usually mix two compatible thinners together to achieve the desired speed. For example, if an application requires too much medium speed thinner, try mixing medium and slow together.
In any experiment you try using varying percentages of thinner and/or blends of thinner, make sure you keep good notes so that you can go back and recreate the desired results later on.
Humidity is a bigger problem than high temperatures. When the relative humidity is high, there is a thin, invisible layer of water on everything. The cliché, the pad – even the substrate – become saturated. Since oil and water don’t mix, this water causes all kinds of problems. If you are printing in an enclosed area, try using a simple dehumidifier. If it can efficiently remove the excess humidity, your problem will be solved. If not, you might try an additive such as TOSH PR2 Paste that increases the cohesiveness of the ink.
Most ink manufacturers have an additive similar to PR2 for use in maintaining dots sizes when printing four-color process. Another common problem in high humidity are “spider webs,” those pesky little stingers of ink that stick out from the image area. This is caused by the water forming a barrier that prevents the efficient transfer of the ink near the images’ outside edges. If you have a TOSH machine, increase the delay for “on substrate,” allowing the pad to compress and hold in its compressed position on the substrate momentarily before lifting away, thus giving the image more time to leave the surface of the pad in favor of the substrate. Dealing with wild fluctuations in temperature and humidity can be frustrating, but the situation is controllable.