Controlling Residual Platen Impressions
Davis examines the most influential parameters and considers the steps needed to reduce or prevent the occurrence of these unwanted marks.
Residual platen impressions have hampered production since the introduction of the flash-curing unit. Residual platen impressions refer to the impression of the platen that is "pressed" into the garment during the printing process. The problem is most apparent on printing dark youth garments.
The factors that contribute to residual platen impressions are numerous. In this installment, we will examine the most influential parameters and consider the steps needed to reduce or prevent the occurrence of these unwanted marks.
Causes and effects
Although platen impressions plague printers on garments of all colors and sizes, the problem is most prevalent on dark youth garments. Residual platen impressions are more visible on dark fabrics than on light ones. And garment size is a factor because of the interaction between a youth-sized platen and an adult-sized squeegee; the combination of the heat from flashing applications along with the pressure of the squeegee depressing the fabric over the side of the platen creates a visible impression of the platen edge.
The platen edge marks appear as lines of lighter color in the fabric. What causes them is that the fibers on the fabric's surface have been compressed and matted down into the fabric itself. The same effect can be seen when applying a heat transfer to a garment. The difference with the transfer application is that the platen edges are not so well defined since it is a platen-to-platen process, as opposed to a squeegee-to-platen process.
In most applications involving dark youth garments, the platen leaves a residual impression of some form, but it might be very faint. The degree to which the latent image becomes visible depends on a number of variables, including flash-curing temperature, squeegee durometer, squeegee speed and pressure, squeegee length in relation to platen width, and the nature of the fabric itself. The greater the surface nap on the garment fabric, the more apparent the effect can be.
You can take several steps to reduce the occurrence of residual platen impressions. The primary printing elements on which to focus include the screen, the squeegee, and the flash-curing unit.
The screen As you probably expected, screen tension and off-contact distance play crucial roles in controlling residual platen impressions. The tension and off-contact of the screen determine the squeegee pressure required to transfer the image, and as with so many other aspects of textile screen printing, less is better.
Higher screen tensions will require lower screen off-contact and lighter squeegee pressures. The greater the amount of force that the screen/squeegee combination places on the fabric's surface, the greater the potential for a residual impression to appear on the garment's surface. Residual impressions are good indicators of screen condition since the lower the screen tension, the more apparent the impressions will be.
The squeegee As I previously mentioned, a contributing factor in residual platen impressions is a squeegee length that is greater than the width of the platen holding the garment. Standard youth-platen widths are 12 in. wide, and the squeegees used to print them are typically 12 in. wide as well. Since the primary goal here is to ensure that the edge of the squeegee does not meet with the edge of the platen, it's a good idea to round off the squeegee ends. This will ensure that blade areas making contact with the screen are within the edges of the platen.
On high-loft garments, sharp squeegee ends can create a distinct border between the areas where the squeegee passed and where it didn't, an effect that appears similar to platen marks. Not only do rounded squeegee edges allow you to remain within the platen edges, they also reduce the chance of this residual impression.
The durometer of the squeegee also plays a role. Unless you are printing process-color images that demand a harder squeegee edge for the sake of definition, I suggest a 70 or 75 durometer blade edge to minimize residual images. I emphasize "edge" because I recommend triple-durometer squeegees--the 80-90 durometer centers help minimize squeegee rollover.
Blade rollover causes you to print with the face of the blade, which will lead to a loss of print definition. Contrary to the belief of most printers, you can actually increase the squeegee angle slightly to reduce the amount of rollover the squeegee experiences. However, if you currently print with your squeegee set at 60° or less, you will not want to increase the angle.
Flash curing Since residual platen impressions occur on dark garments, it stands to reason that the parameters of the flash-curing unit play a role in their occurrence. In most cases, flash units are set at temperatures that are higher than needed. The higher temperatures are used in an effort to flash faster and achieve higher production rates. But it's better to use lower temperatures and prevent a wide assortment of problems that high temperatures can promote--including platen impressions.
When printing and flashing on dark garments, higher heat levels create more potential for platen impressions. The effect is much the same as ironing a garment to remove wrinkles. As the garment fibers absorb heat, they soften and are far more susceptible to being depressed or flattened onto the fabric's surface. Since flashing is followed by subsequent squeegee strokes, the heated fibers are more likely to remain flat and leave platen marks. In effect, the press and flash unit behave like an iron. The best solution in these circumstances is to run your flash-curing unit at a cooler temperature--perhaps 650-800°F as opposed to 1000-1200°F.
It's possible that you may be in the midst of a dark-garment run right now and experiencing platen-impression problems. If the garments are still on hand and their surfaces aren't scorched, I can suggest a remedy: Spray the garment with starch and pass it through the dryer again. The impression should diminish. In effect, the heated starch forces the garment's surface fibers to stand back up. As a result, the formerly depressed areas will return to their original color.
Platen impressions can be a vexing problem. But like so many other challenges in screen printing, they can be overcome once you understand the variables involved and learn how to control them. Pay attention to screen conditions, squeegee parameters, and flash-curing units, and your platen impressions will disappear.