Why does UV coating sometimes flake or scratch off?
Problems with adhesion of UV coating are primarily caused by one or a combination of the following factors. One is the use of inks that are not free of waxes or Teflon. Inks have waxes or Teflon in them to give them a greater rub resistance. The wax or Teflon acts much like wax on a car, causing the coating to bead up and not stick to the ink. Be sure to use inks without wax or Teflon to eliminate problems.
Another factor is slow drying of ink. The slow drying inks are usually not completely dry before they are coated. This is particularly true with multiple overprints and areas of heavy ink coverage. The slow drying inks release vapors as they dry. This interferes with the bonding of the water base and UV coating and can be aggravated by the heat of the curing unit. Eliminate the use of slow drying inks when coating to avoid potential problems.
The third factor is the VOC (Volatile Organic Content) of the inks. Inks that are to be coated, either with aqueous or UV, need to have a low VOC formulation. Low VOC inks (also called High Solid inks) have VOCs below 10%.
The fourth factor is the use of alcohol substitutes in fountain solutions. Many alcohol substitutes contain glycol that interferes with bonding of the UV coating to the substrate. You’ll need to use an alcohol substitute that does not contain glycol or replace the alcohol substitute with alcohol.
Why are metallic inks so difficult to laminate or UV coat over?
Metallic inks are normally manufactured from raw metal that has been milled into flat platelet type particles. To ensure an acceptable degree of luster the layers of metal particles must remain separate after the ink is laid down. This is achieved by adding a stearate type lubricant to the metal particles during the milling process.
Two primary factors influence the manner in which a metallic ink accepts a laminate or coating:
- The lubricant added in the manufacturing process of the inks can migrate to the surface as the ink dries. This layer of lubricant causes reticulation when the ink is UV coated since it will not allow the coating to wet out over the substrate. During the lamination process the adhesive is unable to bond to the lubricant on the surface and this results in poor adhesion.
- More prevalent is the poor cohesive binding of the metal particles in the ink. Consequently, when the ink is UV coated or laminated, adhesion only occurs on the surface or top layer of the metal particles in the ink.
Since the layers of metal are not bound tightly together, UV coating will “flake off” along with the top layers of ink. Laminated inks, especially where the metallic ink bleeds over the trim, will delaminate when subjected to stress. There are metallic inks that can be coated or laminated. Unfortunately, due to compromises in the manufacturing process they do not have the same degree of luster and are sometimes referred to as “fake” metallics.
Are there certain PMS colors that I should be careful about printing when the sheet is ultimately going to be laminated or UV coated?
Yes, some PMS colors react to the chemistry used in the UV coating process. Reflex blue, Rhodamine red and some purples have a tendency for fading after UV coating, so special care should be taken when preparing the project for post press UV coating. Also, please note that many PMS colors contain Reflex blue – your ink supplier may be able to offer an alternative mixing ink for this application. The film lamination process is not normally effected by these colors.
Why do I need to leave ½” trim on all 4 sides of the sheet?
An area around the perimeter of the sheet is needed to apply the film so that the laminate does not encroach on the printed copy, and, at no time does the film hang off the edges of the sheet. It is also necessary to maintain a clean Gripper and Side Guide so that subsequent processes have undisturbed registration points to work from.
Can I Aqueous coat my sheet in-line before laminating or UV coating?
An in-line aqueous “primer” has been proven to work successfully in many applications, although, a standard in-line aqueous can work, it is always recommended that testing be done to determine the compatibility between UV coating and press applied coating prior to running.
