By Scott Sabreen
Question: We UV inkjet print on polycarbonate using multiple-color printheads inline. We started printing a new product and have inconsistent quality. Buying multiple inks is too costly. Can you offer suggestions?
Answer: The problem you describe is common. There are several viable solutions, one of which originates from the pharmaceutical industry. As polymer substrates and surface textures differ, so do their inherent affinity for adhesion, wetting and print quality. One option is to preprint a clear universal basecoat that is chemically compatible for multiple substrates and the visco-elastic properties of your ink. This involves adding a printhead before printing the colors. Fiber optic “pinning” sets the basecoat, preventing smudge, smear and blur image issues prior to printing the colors. The final UV exposure cures the entire printed image.
A second option is to pretreat the substrate(s) prior to color printing using UV light. This technique is effective in which the ozone byproduct produced by the shortwave UV light functionalizes the substrate surface. Oftentimes, there is less effect on wetting (ink drops spreading) than flame treatment and electrical treatment, which can each create additional print quality issues.
Question: Our printed products don’t consistently pass our tape adhesion test. Shouldn’t the parts be fully cured after UV?
Answer: The printed image should be fully cured after UV exposure. Virtually all chemical crosslinking occurs during UV exposure. Use of a radiometer to measure the irradiator output energy, peak power and total dosage is a standard “best practice” protocol. Ensure that your radiometer is properly selected for the spectral range of the UV. Tape peel tests should be conducted in accordance with ASTM 3359. It’s important to remember that the tape peel test measures adhesion, not necessarily abrasion-wear. When robust adhesion is achieved, there is virtually no difference in results based upon the tack value of the tape. Several commonly used pressure-sensitive tapes are referenced in the chart below. The most widely used tape is Scotch 810 Magic Semi-transparent Tape, even though it is now stated in the ASTM.
For testing abrasion-wear, there are many test method options, including the Taber Linear Abraser Model 5700 and The Norman Tool Abrader (original manufacturer of the “RCA” Abrasion Wear Tester).