Ghost images can be problematic for all screen printers. You can print textiles, plastics, glass, or metal and end up with ghost images. You can run the job on manual or automatic presses and see ghosting. We know ghost images can appear independently of substrate and press selection, but why are some print runs more susceptible to producing distinct ghost images than others? How can you have ghosting after you print a sample run of two dozen but experience none after a 2000-piece job?
We can come up with a good answer by taking into consideration the primary causes of ghosting and what steps we can take to minimize their effects. This month, we'll take a look at how inks and screens influence the occurrence of ghost images on our jobs and discuss some of the simple ways we can keep them at bay.
The effects of inks
We can trace the generation of ghost images to pigment particles from the ink becoming trapped between the knuckles of the mesh—the spot where two threads intersect. A number of ink-related factors determine the degree to which ghost images may appear.
The first aspect to consider is the quality of the inks you use. This may sound crazy at first, but the higher the quality of the ink, the greater the potential for ghosting to occur. Ink manufacture involves grinding pigments and mixing. These pigments pass through a mill that breaks the materials down to a predetermined particle size. The particle size of the pigments in the ink plays the greatest role in the appearance of ghost images. Fine particle sizes allow for a higher pigment load, which can yield rich, dense, and vibrant color in an ink. And even though the actual opacity of an ink is determined by a number of different factors, high-opacity inks are typically designed to have a high concentration of pigments. That attribute almost always contributes to the appearance of ghost images in our screens. So the more pigment you have, the greater the potential for ghosting to occur.
Very fine pigment particles always manage to wedge themselves into the mesh. It's a simple fact and a part of the process that we have to live with—especially for those of us who prefer a higher-quality ink over a possibly inferior product. Using top-quality ink and dealing with ghost images is preferable to using a low-quality formulation and losing customers.
Effects of the screen
My philosophy on screens is a simple one: quality in, quality out. I believe that the quality of your finished image is only as good as the quality of your screens. Screen tension not only plays a crucial role in the quality of your printing, but it also affects the potential for troublesome ghost images to appear.
The ink's pigments pass through the mesh during the screen-printing process. The mesh openings are filled with ink during the flood stroke, and inks transfer to the substrate during the print stroke, remaining on the substrate when the screen snaps away. Unfortunately, that very same squeegee pressure pushes the pigment particles into the knuckles of the mesh during production.
To eliminate ghost images, we must understand not only screen tension, but also how it relates to off-contact distance and squeegee pressure. High-tension screens are naturally more rigid than low-tension screens; therefore, high-tension screens allow the ink to more easily pass through the screen mesh. Additionally, high-tension screens require a shorter off-contact, which means you naturally need less squeegee pressure to transfer the ink through the mesh and onto the garments. As you can see, aiming for high screen tension yields benefits beyond improvements in image quality and repeatability on press.
Low-tension screens act as a sieve and therefore require a greater amount of force delivered in the form of squeegee pressure in order to pass the ink through the screen mesh. This is due to the fact that low-tension screens require a greater off-contact distance and thus a greater amount of squeegee pressure in order to practically force the ink onto the substrate. The extra force also is responsible for pushing the ink's pigment particles into the knuckles of the screen mesh, producing the phenomenon we call the ghost image.
The last aspect to address in regards to the tension of the screen and the effects of ghosting is the screen-reclaiming process. My favorite analogy here is simple: If you wipe ink onto a steel post, you can easily blast the ink off with a pressure washer because the post is rigid. But if you were to wipe ink on a curtain and use a pressure washer to blast the ink away, the curtain would simply blow aside, leaving the ink where you put it. Sure, my example is extreme, but the idea behind it holds true. The greater the tension of the screen—and thus the rigidity of the mesh—the easier it is for the pressure washer to remove the pigment particles from the knuckles of the mesh. Low-tension screens will buckle under the pressure from the high-powered washer, thereby hindering your ability to effectively and completely free the pigment particles from the knuckles of the mesh.
You will, at some point, have to treat your screens for ghost images. It's inevitable. Remember that the frequency at which you end up with ghost images is determined by the quality of your inks, the tension of your screens, and the processes and procedures you employ on press.
Manufacturers have made a significant number of products available for removing ghost images. Not one of these products is especially enjoyable to work with, and when it comes to ghost removers, some are harsher—more caustic—than others. The most effective products are those that contain sodium hydroxide (also known as caustic soda) as the active ingredient. The down side is that sodium hydroxide is very corrosive, so extreme care should be taken when working with ghost-removing products. You should also wear all recommended protective equipment during the screen-reclaiming process.
Finally, you should invest in a quality pressure washer. You should be able to easily find one in the 2000- to 2500-psi range. Pressures such as those will ease the reclaiming process and further ensure that you properly remove all residual chemicals from the surface and knuckles of the mesh.
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