Curing

Post cure is simply the time it takes a UV coating to completely polymerize or harden. Generally a UV coating is considered to be 'post cured' when:

• It has very low residual odor.
• It cannot be scratched or removed with tape.
• If it can be removed, no trace of wet ink, odor, or a shadow remains.

Post Cure Identification
Some UV inks have 'post cure' characteristics. Depending on the processor, the belt speed, and the ink film thickness, these post cure inks will be easy to scratch off and will fail the tape test as they exit the UV processor. However, after the first pass through the processor, these inks should, at a minimum, pass the thumb twist test. Failure to do so indicates the ink film is too thick or the printing speed is too rapid. Generally, post cure inks require up to 48 hours to reach an acceptable level of adhesion. This time, however, may be longer if the ink film is too thick or cured improperly.

A 90% post cure can be approximated by passing the printed sheet through the processor three times and allowing it to cool completely before testing.

NOTE: Due to the instant and complete cure of EB processing, only very minimal post curing occurs.

Determining Proper Print Speed
The speed of the UV processors being used is determined by the substrate, the ink, the ink film thickness, number of lamps, type of lamps, age of lamps, and type of reflector. The best way to determine the best speed is to run tests at increasing speeds until the ink fails to cure and adhere properly. Once the maximum belt speed is determined for a particular ink/substrate combination, it can be reduced to a reasonable production level.

Ink Film Thickness
The thicker the UV ink film, the longer it takes to cure the ink up to a point. Beyond that point, only the surface of the UV ink will cure. That is why it is extremely important to carefully control the amount of ink printed on a substrate.

Ink film thickness is not an issue with an EB curing system.

Adhesion and Performance Tests
Adhesion is the ability of a coating to resist 'reasonable' attempts to remove it from the material to which it was applied. Reasonable refers to the requirements of the 'finished' product based on treatment it will receive in the hands of the end user. Avoid over-engineering a product to meet unnecessary specifications.

Although a second surface coating needs only to look good with minimal adhesion, a notebook cover must undergo repeated flexing and handling. A bumper sticker needs varying degrees of weatherability and flex, but a poster must only be scratch resistant. The point is to understand the end use of the product.

Once the end use is known, the following tests can be used more effectively:

Film Tack Test
There should be little or no difference in surface texture between the uncoated substrate and the ink when rubbed with the fingertips.

Fingernail Scratch Test
100% cured ink should not be able to be scratched off a substrate with a fingernail.

Tape Test

• Peel Test
  • Apply transparent tape to the ink film with part of the tape on the uncoated substrate. Rub the tape down well and allow it to remain on the surface for a minimum of one minute.
    (consult with your technical service representative for tape suggestions)
  • Rapidly remove the tape by peeling it back on itself.
  • The ink should not be affected by the tape. Undercured ink will come off and will be slightly tacky and smell wet on the bottom surface which was pulled off the substrate.
  • If the ink comes off but appears thoroughly cured, the problem is likely one of adhesion.
• Cross-Hatch Adhesion Test is a more severe form of the tape test.
• Score the film through to the substrate in a one-inch '#' sign
• Apply transparent tape to the '#' area and rub down well
  (consult with your technical service representative for tape suggestions)
• Wait one minute and snap the tape off with a quick upward jerk
• Any removed ink film is evidence of incomplete 'adhesion'.
• Other Adhesion and Performance Tests may be specified by customers for end use requirements:
  • Sutherland Rub Test - The hardness of the cured ink film is evaluated using a Sutherland Rub Tester.
  • MEK Double Rub Test - Ink film is rubbed with a cotton swab soaked in MEK
  • 5 Minute Potassium Permangenate (Stain) Test - Used to check cure density and degree of cure for cure rate.
  • Accelerated Weather Test - A preprogrammed weathering device can be used to approximate the type of performance which might be expected in the field.

Polyethylene and Polypropylene Flame Treatment
All polyolefins must be flame or corona treated prior to printing with any ink. The purpose of the treatment is to increase the surface tension, which gives the ink something to "grab", and thereby improves adhesion.