Hi there! My husband and I have a garage setup where we print our own stuff as well as that of clients. We have been printing for quite some time and have worked out most of the kinks. However, we have a job that is giving us an absolute headache. Usually, I can find our answer here, but this time I can't seem to find anyone with a similar issue.
We usually print graphic type images, but have a longtime client that is wanting some t-shirts silkscreened with an older photograph in black and white. I had a graphic artist friend convert the image to halftones and it looks great on the print out. But when we go to burn the screen we are getting these lines or ripples running from top to bottom of the image. They are barely noticable on the screen, but they are obvious on the test prints. We have never had this issue before.
Could it be our exposure unit (homemade, but never had an issue in the last 5 years!)?
We have tried exposing the screens longer and this seems to eleviate some of the lines, but they are still unacceptable. Do halftone images need to be exposed longer? We are using a 305 mesh screen and plastisol ink- does that sound right for a half tone photo image? Or perhaps there is an issue with the halftone job? although, the vellums look good?!?!? ARRRGgggghhh!
Any suggestions would be greatly appreciated.
Thanks!
Sounds like the wonderful moire effect. My understanding is that it's caused by the holes in the mesh being just out of line with the halftone dots. While my coworkers were at a trade show this weekend, they were told to set the halftone to an angle instead of vertically. We haven't had a chance to try it out yet. I don't know what software you're using, but I'm sure someone here can help you out with that.
I could be wrong, too. It definitely sounds like a moire pattern messing up your print. Hopefully someone can be more specific on fixing this.
Its moire.The angle is important but the mesh count is more so.dpi 55 at least a 305 or 355.45dpi 230 or up.Printing on a low grade fabric will also enrich that lovely pattern.
Sorry,didn't read the whole post.I can see the problem with using vellum.
Last edited by INKFREAK; March 20th, 2009 at 09:12 AM.
Reason: correcting myself
Stencil sensitizer doesn't know if it is bold block letters or 65 line halftones. The stencil needs to be completely cured/hardened to prevent it breaking down.
You should study the effect of undercutting - where light strikes the positive at an angle and makes the ink opening smaller than the positive. http://www.ulano.com/FAQ/FAQexposure.htm#Q2
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How are you measuring? retired Ulano Technical Support Screen printing since 1979 - SGIA Academy Member
Vellums from a laser printer tend to use 45 degree angles.
Screens, if stretched square to the frame have angles at 0, 90 and at 45 degree.
Moire is an interference pattern between the mesh and the positive, therefore rotating the angles by 7.5 degree usually gets rid of the patterning.
You can also get moire between films and between the cloth weave and screens/films etc.
Thank you all! Adjusting the angle has made a big difference. However we are still struggling with the image being "muddy". I believe this is just due to the quality of the old sepia photos we were provided with. Still playing with contrast and brightness. THANKS again.
halftone photo...lines on my screen 
Re: halftone photo...lines on my screen 
