Keeping up Appearances
April 2002
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Learn prepress and on-press practices for ultimate aesthetic appeal using UV/EB and aqueous coatings and adhesives.
by Jessica Millward, Associate Editor
Energy-cured coatings/adhesives
When not matched with the right inks, coatings and adhesives carry the potential for poor visual performance, especially in the case of energy-cured products. Complementary coating/adhesive and ink formulation is therefore an essential first issue in identifying the source of print flaws.
SICPA Eastern Region Technical Manager Dave Steele lists the most common signs of improper coating formulation as deriving from poor wetting or poor adhesion characteristics. If the coating being employed is printed over an ink containing excessive silicone, the coating may exhibit reticulation, pinholing, etc., resulting in poor coverage and poor performance. High levels of silicone within inks also cause inefficient intercoat adhesion between the coating and the ink.
"For coating process printing, it is most effective to 'stage' the level of silicone in the inks," says Steele. This means applying less silicone in the first-down color, and gradually increasing the level in the second-, third-, and fourth-down colors if required. Other formulation possibilities for UV coatings include monomer and oligomer options, as well as specialty additives.
Perhaps the most common aesthetic challenge presented by UV coatings is a low-gloss factor. Rad-Cure's VP/Technical Director Jim Wittig identifies key gloss elements as stock type and coating weight. Porous stocks translate to a reduced degree of gloss, as do thin coat weights. A less obvious culprit, Wittig observes, may be the anilox. Ceramic aniloxes transfer more coating at a given cell count; therefore, a finer anilox (with a high cell count) will deposit a smoother coating, and, in turn, more gloss.
"Another factor could be that the anilox pattern is cured into the product, giving a rough, non-glossy look," he relates. An overly coarse roll, a high-viscosity varnish, or a varnish with poor flow-out might yield such dull results. Maximizing distance between the coating application point and the curing equipment will allow the coating to flow out before it's cured, minimizing the anilox pattern and increasing gloss.
Yellowing is commonly caused by either the coating formulation or over-cure. Wittig recommends keeping an eye on coating selection, because some yellow more than others. Also consider cure speed: "If you cure a high-speed formulation at a low speed, you could potentially over-cure the product, which will give a yellow appearance right off the press," he affirms. Lowering lamp power at slower speeds may reduce yellowing in such cases.
by Jessica Millward, Associate Editor
Energy-cured coatings/adhesives
When not matched with the right inks, coatings and adhesives carry the potential for poor visual performance, especially in the case of energy-cured products. Complementary coating/adhesive and ink formulation is therefore an essential first issue in identifying the source of print flaws.
SICPA Eastern Region Technical Manager Dave Steele lists the most common signs of improper coating formulation as deriving from poor wetting or poor adhesion characteristics. If the coating being employed is printed over an ink containing excessive silicone, the coating may exhibit reticulation, pinholing, etc., resulting in poor coverage and poor performance. High levels of silicone within inks also cause inefficient intercoat adhesion between the coating and the ink.
"For coating process printing, it is most effective to 'stage' the level of silicone in the inks," says Steele. This means applying less silicone in the first-down color, and gradually increasing the level in the second-, third-, and fourth-down colors if required. Other formulation possibilities for UV coatings include monomer and oligomer options, as well as specialty additives.
Perhaps the most common aesthetic challenge presented by UV coatings is a low-gloss factor. Rad-Cure's VP/Technical Director Jim Wittig identifies key gloss elements as stock type and coating weight. Porous stocks translate to a reduced degree of gloss, as do thin coat weights. A less obvious culprit, Wittig observes, may be the anilox. Ceramic aniloxes transfer more coating at a given cell count; therefore, a finer anilox (with a high cell count) will deposit a smoother coating, and, in turn, more gloss.
"Another factor could be that the anilox pattern is cured into the product, giving a rough, non-glossy look," he relates. An overly coarse roll, a high-viscosity varnish, or a varnish with poor flow-out might yield such dull results. Maximizing distance between the coating application point and the curing equipment will allow the coating to flow out before it's cured, minimizing the anilox pattern and increasing gloss.
Yellowing is commonly caused by either the coating formulation or over-cure. Wittig recommends keeping an eye on coating selection, because some yellow more than others. Also consider cure speed: "If you cure a high-speed formulation at a low speed, you could potentially over-cure the product, which will give a yellow appearance right off the press," he affirms. Lowering lamp power at slower speeds may reduce yellowing in such cases.
Companies Mentioned:
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Chemistry for the Graphic Arts
What the Printer Should Know About Paper