Figuring Out Flash-Curing Parameters

Flashing an underbase on a dark garment in order to allow lighter colors to pop is a process garment screen printers have used for more than 25 years. At first, it was a process that printers only needed on rare occasions. But flash curing quickly became a day-to-day part of garment screen printing.

Even though flash curing is somewhat standardized in most garment-printing facilities, a learning curve awaits those who are new to the process and industry. This month, we'll look at the assorted considerations to keep in mind if you want to avoid headaches when flash curing plastisol inks.

Engineering artwork

One of the first considerations to address in underbase flashing is properly engineering the artwork for the process. Each graphic needs to be addressed on an individual basis in order to determine the best times and which colors to flash.

Keep in mind that flash curing only applies to about two-thirds of the colors you'll print on dark garments. Many novice printers work under the assumption that printing a multicolor graphic on a dark garment requires a solid underbase for every color. Many will get into trouble here. One of the first rules is to avoid wet-on-wet overlays when printing onto a solid, flashed underbase; otherwise, the result will be excessive mottling.

When printing onto a virgin fabric, you have the absorbency of the fabric that allows for the application of wet-on-wet colors. Once you flash an underbase, you then switch to printing on a dried sheet of non-absorbent plastic that doesn't allow for the wet-on-wet overlay process. For this reason, you want to minimize any wet-on-wet overlays. The exception to this rule is when you work with photographic separations and have a halftoned underbase. In that case, you have a percentage of open area (available fabric) that allows for wet-on-wet printing.

Another consideration when working with solid colors is to use underbase gapping. Many multicolor graphics involve colors that butt-register together. This is fine, except for the fact that most presses offer a minimal amount of registration variation. That means the butt-registered colors will typically begin to bleed together and mottle after a few prints. In this case, you'll want to produce your underbase by combining your respective colors and pull each one back a half point. This will produce a 1-pt gap in the underbase where the butt-registered colors meet. The result here is an absorbent gutter that gives the colors somewhere to go when they meet each other on the underbase. This technique allows you to maintain better resolution where the colors meet and avoid mottling.

Another simple trick for overprinting onto a flashed underbase is to allow for a slight overlay of the overprint onto the underbase. The easiest way to achieve this is to either spread the overprint (or choke the underbase) 1.5 to 2.0 points. The very slight overlay produced here will hide any registration variations your press may have and make the printing process that much simpler.

Mesh selection

Proper mesh selection can make a big difference when underbases are involved in a garment-printing job. The mesh can make the difference between a garment with a soft, bright print and a bullet-proof graphic that has a hard and stiff feel. Even though the graphic determines the actual final sequence of colors and meshes, your objective should be to produce the brightest graphic possible while employing the finest mesh counts possible. The "more is better" philosophy leads to bullet-proof prints, use of more ink than necessary, longer flashing times, and potential curing issues. Table 1 presents some basic guidelines for mesh selection based on the ink type being used. However, remember that the right mesh also can vary according to the application, your equipment, and other factors.

Table 1 Mesh Selection According to Ink Type

Ink type	Underbase mesh	Overprint mesh
High-opacity white	110-156 threads/in.	156-230 threads/in.
High-opacity colors	156-200 threads/in.	200-255 threads/in.
Transparent colors	110-156 threads/in.	156-230 threads/in.

Ink selection

Ink selection plays a crucial role. The underbase ink that you'll flash will set the printing conditions for the rest of the run. When flashing on dark garments, you want an underbase that has good opacity and good bleed resistance on synthetics, is fast flashing, and, most importantly, offers good after-flash characteristics.

There are some inks in our industry that are simply not intended to be flashed. Most notable are high-opacity fluorescents and metallics. These inks either have a lot of plasticizers (high-opacity fluorescents) or reflective properties (metallics) that resist the flashing process. Although these formulations are the exception and not the rule, you will still want to find the best white or color to employ as an underbase to allow for maximum productivity and minimal waste.

Flashing parameters

Once you've figured out which inks to flash and the meshes to use, your next challenge is to determine the actual flashing parameters. Based on your flash unit, these will include the distance of the unit to the garment surface, time the garment is under the unit, and the actual temperature setting of the unit itself. Many printers, especially those who use manual equipment, own flash units that do not have adjustable temperatures. If you own such a unit, your parameters are limited to time and distance.

The overall objective here is to flash the underbase with as little time and temperature as possible. Flashing hotter does not mean that you are necessarily flashing faster. You should be able to use a three- to five-second flash to make the surface of the underbase dry to the touch and not tacky.

One of the greatest mistakes made in this process is overcooking the underbase (at temperatures of 230-260ºF or higher). This results in poor intercoat adhesion. The underbase is cured, and the overprinted colors can't properly fuse during the curing process. When this happens, the overprint may separate from the underbase in just a few wash cycles. That's why you only want the underbase to reach 125-250ºF during the flashing process. You can use a non-contact pyrometer (temperature gun) to ensure that the proper temperatures are reached.

The guidelines presented here should help you develop effective flash-curing procedures in your shop. Each printing operation is different, however, so remember to test your procedures and document the settings you establish and the steps you take.