When it comes to direct-to-garment printing, the ink needs to be very thin to spray through the tiny nozzles of a print head. Because the think ink, the fabric being printed needs a pretreatment to bind it properly. Absorption starts to occur if pretreatment is skipped.
Pretreatment creates a surface that the ink can print on and not be absorbed. When the ink encounters the pretreatment, a chemical reaction causes the ink to gel. The reason for this reaction is to help the ink itself become more stable and rigid for printing on top.
If this process didn't occur, the ink would bleed together creating an unwanted swirl of color and white ink. The final stage for pretreatment is during the curing phase. When the print is being heat pressed, the pretreatment becomes a bonding agent that helps bind the ink to the fabric fibers. When the correct temperature, time, and pressure are used, you should have washfastness on par with screen printing or any other garment decoration process.
After application, the pretreat must be dried. A heat press is the most common method for this, although higher production needs require a conveyor dryer.
Source: Paul Green