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As the use of
Ultraviolet (UV) Cured Coatings increases in popularity within
the Printing and Packaging Industry, so does the need to
understand and monitor the potential for problems. As the
extremely high gloss of UV Coating exceeds all other coatings
(ie. varnish, aqueous coatings and even “in-line” uv
coating on press equipment), off-line UV Coating will enhance
the surface of whatever it is applied to. In addition, UV
Coating does not create environmental problems (as do solvent
based coatings) and when cured properly, the UV Coating will
polymerize to become a hard layer which in turn provides an
excellent protective surface.
In order for printers to
properly prepare the printed sheets for UV Coating, there are
certain guidelines that should be followed. The following
information will help the printer insure that the finished
product meets with their expectations.
LAYOUT
REQUIREMENTS
Please provide a layout sheet
indicating Guide & Gripper sides with final trim
marks and show areas to be left uncoated such as glue flaps,
etc. Allow a gripper of 5/8” and minimum ¼” from image to
edge of sheet on remaining three sides. top
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SPOT
COATING
Please supply a film positive (right
reading, emulsion down) with registration marks. Be sure to
tape film in position on a press sheet pulled from the center
of the load and strip to fit the whole of the sheet. Larger
sheets such as 28x40” and especially book weight stocks have
a tendency to grow when absorbing moisture on rainy or humid
days and shrink on warm dry days affecting film fit.
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INKS
UV Compatible Inks
must be used in conjunction with UV Coatings. In the past all
that was required was the inks to be wax free. Considering the
chemical make up of inks and the solvents used in them today,
UV Coating has three requirements. First, the inks must indeed
be free of paraffin waxes, silicones and teflon additives.
Although these additives are used in inks today to resist
scuffing, they greatly reduce the adhesion of the UV Coating
to the inks. Secondly, the pigments used must also be UV
Compatible. All UV Coatings today contain optical brightners
which in conjunction with the alkalines in pigments can cause
certain colors to bleach to a lighter shade. The colors most
prone to bleaching are Reflex Blue, Rubine Red and Rhodamine
Red and any PMS color containing these colors such as Purples.
The Third requirement is that the inks are hard drying so as
to resist the heat generated by the UV Curing Lamps. This is
to avoid the inks softening under heat and offsetting at the
delivery and when stacking in skids. Whatever inks are used,
ample drying time should be planned in the production process,
usually 24-48 hours depending on the coverage. top
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VARNISH AND AQUEOUS
COATING
Although varnishes and aqueous coatings can
be UV Coated, caution must be used as to which coatings are uv
compatible. Varnishes and aqueous coatings will seal in wet
inks slowing the drying process and when UV Coated will result
in unacceptable adhesion. Press Coatings (varnish / aqueous)
fall into two categories:
Solvent
Type: should be avoided as they
use extremely hard resins or wax additives which make adhesion
to the substrate virtually impossible.
Water
base topcoats: are usually of an acrylic type and many
of these will not allow UV Coating adhesion. While water base
primers are recommended and usually work well, they should be
tested for compatibility with the UV Coating used prior to
print production. top
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ADDITIVES
To reduce Volatile Organic Compounds (VOC”s)
and improve air quality, printers have had to eliminate the
use of Isopropyl Alcohol in dampening solutions. Alcohol was
used to improve the rheological properties of the ink and
promote faster ink setting and drying. With the introduction
of alcohol substitutes replacing Isopropyl Alcohol, specific
problems have resulted in the printing / UV Coating process.
After the sheets are
printed and aqueous coating applied, wet ink becomes sealed
between the sheet and the aqueous. The emulsified ink and
alcohol substitute (which have a high vapor pressure) are then
trapped which significantly slows the drying of the ink
requiring two to three times the normal drying time. The
glycols and glycol ethers used in alcohol substitutes act as
plasticizers softening the ink which results
in poor UV Coating
adhesion. To reduce adhesion problems, pressman should run the
lowest water setting possible and use fountain solutions which
contain the lowest levels of glycol and glycol ethers. The
correct aqueous primer coating will greatly affect adhesion
with UV Coating. top
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OFFSET SPRAY POWDER
Although WESTLAM has in line sheet
cleaning systems, these units are not a cure all system. Spray
powders should be kept to a minimum and all sheets should be
properly dusted on both sides prior to UV Coating. Spray
powder on printed sheets will be intensified when UV Coating
is applied resulting in a sand paper affect.
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SCORING,
FOLDING AND GLUING
Any scoring required on a job should be done
after the sheets have been UV Coated. Normal grain direction
will help insure that a coated sheet score and fold correctly.
When coated paper stock is to be printed with a heavy coverage
of ink, then UV Coated, then scored and folded and cracking
occurs at the fold, it is an indication that the stock may not
be suitable due to the clay coating on the stock. This can be
caused by stocks that have been stored for long periods of
time or by slight changes in chemistry of clay coating at the
mill. The clay coating should be one that has sealed the sheet
well so the UV Coating stays on top during the curing process
so as to achieve the deep mirror gloss or “wet” look.
Some stocks have a clay
coating that is porous and can absorb UV Coating before it is
cured causing a streaking affect. In addition, Cast Coated
sheets such as Kromekote & King James should not be used
in conjunction with UV Coating. Please be aware that when UV
Coating darker solid colors, fingerprints will show more than
on lighter solid colors or process colors. Uncoated stocks
should never be used for UV Coating. Remember that when
choosing a stock to be UV Coated, always be sure to use an
acid free stock. Acids in stock can cause UV Coating to
“yellow” and creates adhesion problems.
Glue tabs should always
be “spared-out” as UV Coating will not allow glues to
adhere during the converting process.
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FOIL
STAMPING AND EMBOSSING
UV Coating is not foil stampable, therefore the
foiling process should be done prior to UV Coating. UV
Coatable foils should always be used in conjunction with UV
Coating. Embossing should be done after the UV Coating
process. top
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METALLIC
INKS
Metallic Inks should be used with caution in
conjunction with UV Coating as these are actually tinted
varnishes with metallic particles. As the varnish dries, the
metallic particles rise to the surface and rub off easily
creating an unstable surface. This makes a very weak bond for
the UV Coating to adhere to. Most of the time the UV Coating
will flake off unless the metallic is sealed for protection.
If an Aqueous Coating is used it must be sealed after the
Metallic dries thoroughly or the wet Metallic will be sealed
under the AQ Coating and will not dry sufficiently to allow UV
Coating to bond. A water based primer AQ may be used if ample
time is allowed for drying, usually a minimum of 48 hours
prior to the UV Coating process. top
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| AVOID THE FOLLOWING:
For best adhesion results,
ink, varnish and aqueous coating must be free of waxes,
silicones and Teflon additives. Also, special attention should
be given to press chemistry as alcohol substitutes and one
step fountain solutions contain glycol, an oil base product.
UV Coating will not adhere to oil. Please consult your
supplier to be sure of product compatibility. top
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| PRIOR PLANNING
in print production is essential. Please
check with WESTLAM regarding equipment capabilities and overs
required for set-up, make ready and run waste before printing
your job. Please feel free to call WESTLAM with any questions
regarding your finishing needs.
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