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.
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.
Challenge: Uneven pressure on repeat images (i.e., leading or trailing edges)
Solution: When ordering photopolymer plates for the printing of the adhesive, build in bearer bars. These bars help reduce bounce under high speed conditions, ensuring even pressure. Be sure foil is wide enough to cover the bars and running lines. Gear teeth must be clean of all debris, as this can add to bounce.