Foil: Challenges
& Solutions
By Brian Hill
Foil stampers face challenges every day, but knowing
how to handle them can make the converter a valuable partner to the customer. Following are some
examples of foil application challenges and the techniques
to solve them.
Cold Foil
Challenge: Pinholes in images
Solutions: Pinholes generally occur in absorbent substrates like semi-gloss paper and are not usually an issue
with film stocks. Adhesive may dive into the stock, leaving
an insufficient deposit of adhesive atop the substrate, making it difficult to achieve a complete transfer.
Check the viscosity of the adhesive being used. There
may be a higher viscosity adhesive on the market. Prime
the substrate in the station immediately before the adhesive station to seal the sheet. By keeping the adhesive on
top of the substrate you form a deposit sufficient enough to
create a complete bond to the foil.
Try increasing the press speed. At higher speeds, there is
less time for the adhesive to dive into the substrate. Another
solution is to reduce the line count of the anilox in order to
print a heavier adhesive coating.
Challenge: Dirty images, flakes, dust
Solution: Check the area of the substrate that has the
excessive foil with a tape pull or even a firm wipe with your
fingertip. If excessive foil is bonded to the substrate, the
anilox roller may be too low (BCM too high).
If the tape pulls only the excessive foil while the desired
area has good adhesion (or you get a dirty fingertip), a
sharper stripping angle may improve the detail. Doing this
achieves a faster and more dramatic separation, especially
as compared with the substrate and foil running more parallel after the strip. An angle of 90° (or even more) is often
required. Typically, a narrower diameter stripping roller
does a better job at keeping sharp detail than a 3" idler
roller.
Another possibility is that the foil is stripped too early
after curing. Extend the foil path so that it is being stripped
farther from the lamp. Smooth and even rewind tension
assists a smooth strip. Increase rewind tension, especially
as the spent foil web increases in weight. Also, if it is possible, increase the rewind core from 3" to 6" as this will
increase the speed of the rewind and make the foil run with
the web.
Challenge: Air bubbles and/or fish eyes
Solution: Check the nip. If the nip roller is too soft ( 75-80
durometer), Shore A may cause the image area of the nip to
become concave, creating a fish eye or air bubble. A nip-roller durometer of 90 is usually recommended.
The stated durometer from the roller manufacturer may
be incorrect, and nip durometers can change with use and
age. For an accurate reading, test the nip roller durometer
with a durometer tester. Make sure both the nip roller and
anvil are clean and free of all dried inks, coatings, etc.
Challenge: Crisp edges with concave images
Solution: The plate printing the cold foil adhesive may be
too soft, so check the durometer settings. Use a photopolymer plate with 65-67 durometer for crisp edges. Sticky-back
(plate-mounting tape) should be a medium hard tape.
Challenge: Smears and cracks in images; broken images.
Solution: Check the UV lamp wattage. Without proper
wattage, under-curing can cause the foil to crack or smear.
In order to prevent smears and cracks, the lamp should be
400 watts per inch. Check the actual output, not just the
stated wattage, and change and maintain clean bulbs per
the manufacturer’s specification.
