Those of us who have been in the garment screen-printing business for a while know that flash curing during the printing process was, at one point in time, a specialty application. But today, the flash unit is a tool as common to the process as the screen itself. Be that as it may, there are still parts of the garment-printing process that can make or break a printer's day when they're not clearly understood. In this installment, we will review the different considerations associated with flash curing during the garment-printing process, as well as the way in which flash-cure units and drying systems work together.
In previous columns, I have mentioned the fact that many printers attempt to flash at temperatures that are detrimental to the process in the name of productivity. It is true that fast flashing times, specifically on automatic presses, require a higher temperature. The trick is to determine how fast you can properly flash the printed ink film without overheating to the point of damaging the underbase.
The best flash units offered today for controlling the ink-film temperature during the flash-curing process are quartz flash systems that reach a specific temperature in a short period of time and cool down immediately. These units offer a greater degree of control than most other flash setups. You also can use what are called black-panel IR flash units. Their specific temperature-control settings offer the next best degree of control.
Another option available to you is the black-panel unit that has no temperature setting and simply runs when you flip the on/off switch. The only real way to control or adjust these types of units is to simply raise or lower the height of the unit from the platen or garment surface. This method of adjustment poses the greatest challenge as you typically can only decrease the flash time as your platens heat up after printing for a period of time. Only you can determine which system will work best for your shop, but I think it's very important in the long run to have the ability to adjust your temperature and production proceeds.
Any number of problems can occur when the platens reach too high of a temperature as a result of prolonged exposure to hot flash-curing units. The problems include poor intercoat adhesion between ink-film layers, sublimation of polyester dyes on 50/50-cotton/poly fabrics or 100%-polyester fabrics, blistering of the underbase when printing with bleed-resistant inks, excessive afterflash tack on the underbase surface, and fabric to screen adhesion, which eventually leads to the screen actually pulling the garment from the platen surface. This typically occurs when printing on 100%-cotton fabrics in an environment with high humidity. The heated moisture contained within the fabric develops an affinity for the emulsion coating on the screen, allowing the fabric surface to stick to the screen's surface. When the garments are pulled from the platens, production must stop so that screen can be wiped. It's a big waste of time.
Although most printers will resort to spraying the screen with silicone, the real trick here is to minimize your flashing temperatures while achieving the 125-250°F needed to properly flash the color (or colors) in question. Platens absorb heat from the flash-cure units during production and get hotter as the printing cycle progresses. Depending on the type of flash units used, it may be possible to achieve an optimum setting (time, dwell, and distance) that will allow for continuous production without the need to adjust the unit. Those who do not have that luxury may be required to adjust one or more of the unit's settings in order to minimize the operating temperature during production.
One aspect of the flashing and curing process that many do not consider is the relationship of the ink-film temperature when switching from a flashing to a non-flashing production run. In general, you have your dryer set to bring a screen-printed ink film from room temperature to 320°F in order to properly cure the ink film.
When you print a production run that requires the use of a flash, you can send your shirts into the dryer with ink films that are already as hot as 125°F. In such cases, the temperature of the garment's surface combines with the environment you have set in your dryer and can easily bring the ink-film temperature beyond the 320°F curing point to more than 360°F. Once a plastisol ink film reaches 360°F, the ink can start to re-melt.
This problem is very evident when printing a bleed-resistant underbase. The bleed-resistant components within the ink film will begin to boil off, leaving small voids or craters in the overprinted ink films. This scenario typically occurs in dryers that have short infeeds because the flashed ink film does not have the time to cool prior to passing through the drying chamber. In many cases, larger shops have dryers with longer infeeds, allowing the temperature of the flashed ink films to drop. Garment-printing facilities that lack the luxury of long infeeds have a much greater need to ensure that the garments are flashed at as low a temperature as possible in order to minimize the potential for some of the problems described here.
Exposing garments and ink films to excessive heat also can trigger polyester dyes to sublimate in 50/50 fabrics. This effect may not be immediately evident at the end of the dryer, over time the polyester dyes can noticeably sublimate into the ink film and, consequently, bleed out. The sublimation and bleeding can occur anywhere from one day to six weeks, depending on the polyester dye content within the fabric and the degree to which the dyes sublimated during the curing process.
The role of the flash
The flash unit's job is only to set or gel the underbase ink film. The same holds true when you flash at the end of a printing sequence prior to adding a special effect. Although flash units are referred to as flash-curing units, curing the ink film really is the last thing you want them to do. A full cure would prevent ink layers from adhering to one another. All you want to do is to prepare one ink film to receive the ink that you'll deposit during the next print stroke.
Once you've determined the point at which the ink film is adequately set by the flash unit, you'll want to maintain that temperature carefully throughout the run. If necessary, you can combat the effect of platen temperature on the process by preheating your platens prior to production and then determining the minimum temperature needed from the flash unit to properly flash your underbases. The philosophy of more is better when it comes to setting flash temperatures in order to achieve faster production often causes many more problems than opportunities for you to increase efficiency.
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