Davis explains how flash units can influence productivity and describes problems you'll see when they fail to keep their cool.
By Rick Davis
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.
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