Problems Curing White Ink?

November 11, 2009 at 12:03 am 2 comments

Kent Hudson
Kent Hudson, National Sales Manager

We asked our National Sales Manager, Kent Hudson, to go into depth on a topic we continuously receive questions on, namely that of curing Plastisol inks.  Specifically, we receive many inquiries as to how to correctly cure white inks.  Here is Kent’s explanation:

“One frequent technical question I receive is “Why am I having problems curing white ink”. To help solve this question, I first need to have a few questions answered that will help me to pinpoint the solution:

 

 

  • Number one: what type of dryer are you using and how long is the heat chamber? Also, how long is the printed garment in the heat chamber?
  • Next, what mesh count do you normally use when printing white and do you double print or use a print flash print technique to improve coverage?
  • Are there other inks printed on the same garment that seem to be fully cured and what mesh counts do you normally use for these inks?
  • What type of fabric are you printing on?
  • And lastly, which white ink do you use?

Some of you may already see where I am going with these questions, but if you do not let me explain:

The Dryer: First and foremost, to cure any plastisol ink you need to introduce heat. Normally the cure temperatures of plastisol are around 300º F to 320º F (149º C to 160º C) depending on the series of ink being used. Your dryer will be either electric or gas. Electric dryers are available in smaller sizes and are usually more economical to purchase.

The question on curing white ink, 99 percent of the time, comes from a printer that has one of these electric dryers. Electric dryers use a type of heat energy called infrared or IR. IR heat is very commonly used for many different applications. Electric ovens in your home use IR heat to cook your food. The sun heats the earth with IR energy. So let me explain some of the nuances of IR energy.

Colors: First off, IR “sees” colors. What I mean by this is, it heats darker colors faster than lighter colors. This is due to the reflective nature of the color. You have likely heard it said that a black car is too hot in the summer. That’s due to the fact that black color absorbs IR energy faster. White or metallic silver colors reflect IR and absorbs heat slower. This being said, white ink will absorb heat slower and will need more time to cure.

Heating Elements: IR heating elements like the one in your dryer, flash cure or oven at home, usually only heat to one temperature. Around 800º F to 1200º F (427º C – 649º C) is common. The way these heating units control the temperature is by turning the heating elements on and off. The heat penetrates much faster when the elements are on and not when they are off.

A good example is when you toast bread in your oven at home. If the temperature is set too low, let’s say 250º F (121º C) on broil, you will start to see the toast turning brown and then the oven reaches 250º F, so the burner cycles off. The bread will stop browning until the element cycles back on again. Even if you left the toast in the oven for hours at 250º the bread will not brown but only dry out.  It takes the extreme heat of the IR element being “on” to brown the toast.

Cycle Time: To help control the lack of heat during an off cycle, screen printing dryers have the elements cycle on and off at a much faster rate. However, most printers will set their dryers to a higher temperature and increase the belt speed to compensate.   You can be adversely affected by this on-off cycle time if you try to set the belt speed too fast for the size of the dryer. The ink still needs enough time to absorb the heat.

The Mass: If you have ever watched mom cook a turkey for Thanksgiving, you know that she determined the time needed for the turkey to cook by the size of the bird. A larger bird needs more time to cook than a smaller bird. This is the same with plastisol. A thicker coating of ink needs a longer time to cure.

So let’s put this all together for you: White ink is normally printed through coarser mesh counts like 60 to 110. For additional opacity, most printers will flash the first coat and add a second coat. This amount of ink will take more time to cure than a color printed only once or through a finer mesh count.

 The white will also reflect some of the IR heat. This means it will need more time to cure as well. The dryer settings should be set high enough to cure the thickest and the most reflective ink printed on the shirt. My suggestion is to slow down the belt speed and allow plenty of time for the ink mass and color to absorb enough energy to complete the curing process. Most manufactures suggest at least 1 to 2 minutes in the heat chamber.

 So next time you see a little cracking on your white ink, try slowing the belt down and giving it just a little more time to complete the process. Next time I will discuss the differences between gas and electric dryers and why you would choose one over the other.”

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