Exposure to UV light initiates the chemical reaction that causes the ingredients of a UV curable ink to bond together.  Exposure in the curing unit doesn’t a completely cure the ink though. In fact, only about 90% of the chemical reaction is complete.  This chemical reaction that the UV light initiates can continue and the chemical bonds continue to form.  The phenomenon of the ink continuing to cure after printing is called post cure or dark cure.

After some UV inks are initially cured by exposure to UV light, they continue to cure.  The general rule of thumb is that this post cure period for free radical UV inks lasts for 24 to 48 hours after printing.  Inks companies now have published in their tech bulletins that this curing period may last longer than originally thought. How long? Some think that this post cure process can last for weeks.  Factors affecting post cure include the printing environment, the intensity of the light and the substrate. Read more »