Foil: Challenges & Solutions

March 14, 2008

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.

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.

Hot Foil

Challenge: Foil will not stick over heavy ink coverage, especially multiple stacked inks, stair-step effect.

Solution: If the ink is not formulated correctly and has a concentration that is too high, it gives the substrate a rough surface and will not allow the foil to stick. Check the ink concentration and separate the colors into two stations to ensure that ink is smooth in texture.

To reduce stair-stepping, be sure that the anvil is sufficiently hard – 90 to 95 durometer.

Consider plate design to reduce multiple inks ending at the same line and increase or reduce trap size to have foil overlap ink layers individually while still trapping all the inks.

Challenge: Slow running speed

Solution: Check machine wattage; it may be too low. Dies need to recover the temperature transferred to the foil, and the lower the wattage, the slower the dies recover.

The true fix is to use a high wattage unit for electric heat, or a hot oil unit which speeds recovery time. Consider larger diameter dies if they will fit in the machine.

Challenge: Uneven pressure on repeat images (i.e., leading trailing edges)

Solution: The die should have running lines outside the final trim, and foil width should be sufficient to cover the running lines. Gear teeth must be clean of all debris, as this can add to bounce. Be sure to communicate to your die maker the typical temperature at which you run so he can make the correct heat expansion calculation to prevent the die from expanding to an oblong shape.

Challenge: Dirty images, flakes, dust

Solution: With a tape pull or a firm wipe with your fingertips, check the area of the substrate that has excessive foil. If the excessive foil is bonded to the substrate, check the hardness of the backup roller to ensure it is correct for the application. If the hardness is less than 70 durometer, the backup roller would be like a sponge absorbing the impact of the stamp. This could cause the material to concave to the image giving false indentation of pressure, appling foil to the stock.

If the tape (or your fingertip) pulls only the excessive foil while the desired area has good adhesion, check the entry angle of the foil to be sure that it is not preheating.

Another solution is to extend the stripping angle after the stamp. You can also decrease application temperature or increase press speeds.

Brian Hill is a technical graphic specialist for KURZ Transfer Products, Charlotte, NC, USA. Hill has been with KURZ for 11 years and in the printing industry for 25 years. KURZ is an international supplier of hot stamping technology. The company has more than 3,000 employees as well as nine production facilities located throughout Europe, the US, and the Pacific region. It also has 19 international branch offices and 70 exclusive distributors.
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