Screen Printing - December 2012/January 2013 - (Page 18)

SOLUTION SOURCE BOOK HOW TO CONTROL DYE MIGRATION ye migration is an unwanted reaction between plastisol ink and the dye used in the garment fabric—polyester fabrics and blends, in particular. This guide explains how dye migration happens and outlines steps you can take to prevent it from harming your garment-decorating jobs. Dye sublimation, migration, and bleeding These three terms are often used interchangeably, but each actually describes a different step in a process that results in a printed ink film taking on the color of the garment. The dye in the fabric sublimates (converts from a solid to a gas without the intermediate step of becoming a liquid) under high temperatures, migrates into the ink layer, and the result is bleed a discoloration of the cured print. Garment screen printers must deal with sublimation on a regular basis. Heat-set dyes are used in fabrics that contain polyester, and then these dyes are heated to their sublimation temperature, they convert to gas. If this sublimation process takes place in the presence of plastisol ink, the sublimated dye can migrate into the plastisol. The worst part is that the results of dye migration may take several days to show up after printing—perhaps after the garments have been packed and shipped to the customer. Any fabric that contains polyester is vulnerable to dye migration and bleeding, including polyester/cotton blends (Figures 1A-1D). Some colors are more prone to dye sublimation than others. Red shades are notorious for being the worst, but all colors are capable of sublimating to some degree. The best rule of thumb is that every dark shade of fabric containing polyester should be treated with respect and tested. The mechanics of dye migration On polyester fabrics, the gaseous, sublimated dye becomes trapped underneath the plastisol ink, and it solidifies as it cools. But now, the bond between the dye and the polyester fabric is broken, which means the dye molecule is free to attach itself to something else. Plastisol is a wonderful solvent for dye and allows the color to migrate throughout the printed ink film. The discolored print represents the diffusion of the dye particles throughout the ink layer. Polyester dyes sublimate at temperatures ranging from 360-420°F. Why is this a problem if you typically cure garments at 320°F? Well, when you set the dryer to 320°F, what you’re really doing is telling the dryer to maintain curing-tunnel temperature of 320°F. To compensate for heat loss and maintain the temperature you set, the dryer will reach temperatures beyond 320°F in certain locations. There are also opportunities on press to create dyemigration problems. Using a flash-cure unit to gel underbases, partially cure certain parts of a design, or partially cure thick 18 SCREENPRINTING D A B C D FIGURES 1A-1D In Figure 1A, portions of the red, 100% polyester garment printed with a white plastisol take on a pinkish cast, indicating that the fabric was exposed to excessive heat at some point during the printing process. Figure 1B shows how the same print on the same 100% polyester fabric should look. The potential for bleeding exists any time polyester in present in fabric. Figure 1C shows dye migration in a 50/50 polyester/cotton fabric. Figure 1D shows how the same print on the same 50/50 polyester/cotton fabric should look. ink deposits to prevent smearing when overprinted can create an environment that’s ideal for dye migration if proper flashcure parameters are overlooked. In addition, failure to control the amount of heat the press platens absorb from flash curing can create or worsen the problem. Low-bleed inks Low-bleed inks are formulated to prevent dye migration. They’re not, however, designed to prevent dye sublimation. No ink can stop dye from sublimating when it’s exposed to excessive temperatures. Low-bleed inks contain chemical-blocking agents that work on the dye to stop migration, but some dye migration may still occur when there’s more sublimated dye than the ink can handle. Controlling platen temperature Heat is the top consideration in preventing dye sublimation and migration. That means that you must control your flash temperature and time. Remember that more heat is needed to flash an ink layer when the press is cold. However, the ink film will gel much faster once the platens heat up. Warm platens pre-warm the fabric and ink before the platen even has a chance to swing under the flash-cure unit, thereby reducing the flash time required. Conversely, when the platen is still cool, it absorbs a large portion of the flash energy, necessitating longer dwell times.

Table of Contents for the Digital Edition of Screen Printing - December 2012/January 2013

Screen Printing - December 2012/January 2013
Prepress for Special-Effect Garment Printing
Tips for Optimum Screen Exposure
Maximizing Image Size and Position
Understanding Screen-Tension Loss
A Look at Screen Lift-Off
Staging Garment-Printing Jobs
How to Control Dye Migration
Fine Tuning for Fine Details
Application-Specific Considerations for Graphics Printers
Upping the Ante at SGIA 2012
Statement of Ownership
U.S. & Canadian Directory
Opportunity Exchange
Advertising Index

Screen Printing - December 2012/January 2013