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Tips for Direct Projection

(August 2003) posted on Fri Aug 08, 2003

Learn about the intricacies of direct-projection exposure systems, how to keep your mesh from deteriorating, and the secrets of pad printing on inflatable objects.

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By Carol Swift, Peter Kiddell

Another critical aspect of direct-projection is the alignment between the lens on the system and the screen to be exposed. One printer we worked with was having problems with poor dot reproduction on the right half of every screen. Our first thought was that this problem was caused by the projection system's optics. However, we found it to be an alignment problem. The distance from the lens to the plane of the mesh was supposed to be 5000 mm. It was correct on one side of the frame, but at the other end, the distance was 5005 mm. That 5-mm difference was enough to throw off the focus on the right-hand side of the image, leading to poor dot definition. Once we realigned the frame, the problem disappeared.


Mesh deterioration


The need to remove haze and ghost images from the mesh causes an all-too-common problem that many screen printers don't recognize--reduced mesh life. Ghost images are usually caused by stencils that were underexposed, leaving the emulsion tacky and very difficult to remove from the mesh. Ink residue also can remain on the mesh after reclamation, leaving ghost images behind.


The problem is that the haze removers some printers use are very hard on mesh. Although environmentally friendly, non-caustic haze removers are now available that are very effective, many formulations still contain a fairly high concentration of caustic chemicals. These agents can very quickly degrade polyester mesh. Chances are you spend a lot more on your mesh than on screen chemicals, so be careful when using these products, keeping in mind that different formulas contain varying amounts of caustic chemicals.


One shop we visited was experiencing a problem with large-format screens that were ripping for no apparent reason. We traced the cause back to the haze-removing formula, which had an alkalinity that was higher than normal, resulting in a very aggressive mix that weakened the mesh. We switched to a different product with a lower concentration of caustic chemicals and found that it worked well without harming the mesh, thus eliminating the problem. We are not chemists and may not be using the correct jargon, but our point here is that you need to avoid using too aggressive a stencil cleaner on your mesh.


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