What Is Post Cure of UV Curable Inks?

By Jim Hingst @hingst_jim        

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.

As the inks continue to post cure and more chemical bonds form, the ink becomes tougher and adhesion to the substrate becomes stronger.  There are a couple of problems that can occur as the inks post cure.  The first is that printed vinyl films can shrink.   Another issue is that as the inks continue to cure, they become brittle and can crack.  The same problems that digital printers are having today are some of the same problems that screen printers had when UV inks were first introduced. 

So how can you determine whether the inks have cured properly? Digital printers can get an idea of how well the ink has cured by performing a simple tape test to check ink adhesion.  In this test, burnish 3M’s #610 tape to the print and pull it from the substrate at a 90º angle. If the ink pulls from the substrate, you don’t have good adhesion and the inks most likely have not cured fully.

Another test that you can perform is a thumb nail scratch test. It’s not very scientific, but it’s a good way to tell if inks have cured. Just rub you fingernail against the printed surface. A fully cured ink will be hard as nails and will not scratch off.

Finally, your nose knows.  If the ink continues to give off an odor, it hasn’t completed the post cure process.  UV inks often smell when they are being processed because the  byproducts of curing  are partially volatile. Good ventilation is critical when you process UV inks, or the smell can get pretty objectionable.

When UV inks do not cure properly, the usual suspects that can cause the problems  include the type of ink used, the thickness of the ink deposit, the type of lamps, the age of the lamps and the type of reflector.   In the design of their equipment, manufacturers of UV curable inkjet systems take into consideration the ink chemistry, UV light wave length, intensity and amount of exposure to ensure that adequate curing takes place in all of the applicable print modes and with various substrates. Well-designed printer systems will prevent you from going outside the safety zone of the printer system. That way you cannot print so fast that there is not enough UV energy to properly cure the inks.

Visit Vinyl Sign Techniques Community

CLICK HERE FOR HINGST SIGN POST ARTICLES IN PDF FORMAT

Vinyl Application Videos

Squeegee Technique. Nothing is more basic in vinyl application than the squeegee. But some squeegees work better than others. And there are right ways and wrong ways to use this simple tool. This video clip reviews squeegee selection, squeegee care and squeegee technique. Click to Link

Wet Applications. Dry applications are typically recommended for most vinyl applications. For those exceptions to the rule, this instructional video explains the right way to perform a wet application. Click to Link

Application of Window Graphics. Installing vinyl graphics on glass can be challenging, because the adhesive aggressively grabs onto this high energy surface. Repositioning graphics on window is generally difficult, if not impossible. This instructional video explains how to dry apply window graphics right the first time. It also describes the necessary steps required for surface preparation. Click to Link

Transferring Frosted Window Graphics Films.  Because frosted window films are highly textured, transferring cut vinyl graphics can be problematic. This video provides direction in selection of the right application tape and how to apply these films quickly, easily and without problems. Click to Link

3-Step Surface Prep. This instructional video describes how to properly clean the surface of a vehicle before applying pressure sensitive vinyl graphics to a trailer or doing a full wrap of a car or van. This three step surface prep involves detergent washing, solvent cleaning and a final wipe down with IPA. Click to Link

Vinyl Application Over Rivets. Applying vinyl graphics on vehicle surfaces with rivets is challenging for most sign makers. Vinyl failures to these surfaces are all too common. This video demonstrates some tips and procedures, used by professional decal installers, which will make these demanding applications easier and more trouble-free.  Click to Link

Vinyl Application to Corrugations. Learn how to apply vinyl graphics to corrugations in a relaxed state. Forcing films into the valleys of corrugations puts too much mechanical stress on the film. Sooner or later, the vinyl will lift or tent in valleys. Click to Link

Applying Air Egress Vinyls. Vinyl films with air egress release liners have almost eliminated bubbles and wrinkles from applied graphics. Air egress release liners with their highly textured surface structure are problematic. Not much will stick to these liners other than the vinyl film. At RTape our best premask solution for air egress liners is a special Conform® tape called 4761RLA. Click to Link

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.  Vinyl Sign Techniques is available at sign supply distributors and at Amazon. 

Slides of Jim's Artwork

© 2016 Jim Hingst