Dye migration is an unwanted reaction between plastisol ink and the dye used in the garment fabric—polyester fabrics and blends, in particular.

Dye migration is an unwanted reaction between plastisol ink and the dye used in the garment fabric—polyester fabrics and blends, in particular.

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 dye-migration problems. Using a flash-cure unit to gel underbases, partially cure certain parts of a design, or partially cure thick ink deposits to prevent smearing when overprinted can create an environment that’s ideal for dye migration if proper flash-cure 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.

You can control the heat delivered by your flash unit in a couple of different ways. If the unit supports temperature adjustment, you can turn it down as the press platens heat up. Or, you can speed up production once the platen is warm, exposing each platen to a shorter period beneath the flash unit. A less practical option is to increase the distance between the flash unit and platens. This will reduce the heat to which your platens and prints are exposed, but it also tends to slow production significantly.

Testing fabric
If you’re uncertain about the quality of the fabric you plan to decorate, perform a simple test that involves wrapping a clean piece of white fabric around your finger, rubbing the piece of white fabric two or three times across the same area of the test fabric, and checking for excess dye that is not bound to the polyester. Repeat the test, but use a wet piece of white fabric. If you see a significant amount of color transfer with either test, you should consider rejecting the garment.

If you see very faint coloration from either test, the fabric is probably fine to decorate, but you should still print a test garment and place it in a warm area for at least 24 hours. If you see no dye migration after the test period, you shouldn’t encounter any issues when printing the full job run.

KEYS TO CONTROLLING DYE MIGRATION

Minimize ink deposit to decrease flashing/curing time
• Use high screen tension
• Use higher mesh counts
• Use lower off-contact
• Use sharper squeegees

Minimize fabric exposure to heat
• Reduce flash temperature as press platens warm up (platens should not become too hot to touch)
• Adjust press speed to provide shortest possible dwell time under flash units
• Use flash timers in manual operations to prevent prolonged exposure to high temperatures
• Measure and adjust dryer temperature and belt speed to safest production levels

Maximize staff training
• Staff must understand how dye sublimation/migration occurs
• Staff must understand the influence of flash curing and press and dryer settings on dye sublimation/migration
• Staff must test bleeding potential of new or unfamiliar garments