Gig Posters

Posters: 154911 | Bands: 135050 | Designers: 11471                 
   
       RSS Feeds

Username:   Password: 
Register      

Social Networking Activity                 



 Bands  Designers  New Arrivals  Top Lists  Forums  Buy Posters  Submit  Merch Store  Advertise  Widgets  Help

Page 1 of 2 12 LastLast
Results 1 to 10 of 15
  1. #1

    Join Date
    Nov 2009
    Location
    Austin, Texas
    Posts
    3
    Comments
    0

    Default Mesh Interference Moire? Primary Moire in Halftone Images

    I just finished a 2 color halftone poster, and I got a screen interference moire in both layers.

    Here's the print and some detail showing the moire:

    moire3 | Flickr - Photo Sharing!

    moire2 | Flickr - Photo Sharing!

    moire1 | Flickr - Photo Sharing!

    I printed with 230 mesh, 40lpi, 15 and 30 degrees, respectively.

    The faint ghost striping showed up in each burn (it wasn't in the films).
    I'm using thicker vellums than usual from a different supplier.
    I double wet coated the bottom of the screen only. Didn't coat the top side.
    There was no moire created between the two layers.

    Could the problem be one of the following?

    1. Not enough emulsion. (Next time double coat both sides.)
    2. Underexposed. (Next time increase exposure time.)
    3. 40lpi is somehow interfering with the 230 mesh for some strange mathematical reason. (Next time change the lpi to 52 or some other number.)
    4. Screen angles bad juju. (Next time try different angles, like 37 and 67)

    Or all of the above.

    Any help would be very - helpful.
    Thanks!

  2. #2
    Premium Member
    El Roacho's Avatar

    Join Date
    May 2009
    Location
    one mile high, colorado.
    Posts
    841
    Comments
    147

    Default

    i would suggest adjusting your angles.

  3. #3

    Join Date
    Nov 2009
    Location
    Austin, Texas
    Posts
    3
    Comments
    0

    Default

    Thanks.
    I'll probably try everything on my list!

    I've had the exact same thing happen to me before with a fine mezzotint file - which is what leads me to believe it might have something to do with exposure / emulsion.
    I think both times the emulsion was getting to the bottom of the bucket. The emulsion seems to get dryer / tackier by then, which might prevent it from reaching full screen penetration.

    As a first test, I might try double coating both sides of a screen and re-burning the same vellum, burning for 25 percent longer.

  4. #4
    Premium Member
    mikeage's Avatar

    Join Date
    Jul 2008
    Location
    midwest
    Posts
    3,428
    Comments
    87

    Default

    you might want to try a doing a search of the forums. i remember finding some threads on it before including, i believe, a calculation for what angle you should use based on the screen mesh count.

  5. #5

    Join Date
    Nov 2009
    Location
    Austin, Texas
    Posts
    3
    Comments
    0

    Default

    I searched around the forum and found some different opinions about numbers, angles and LPI.

    A few good starting points:

    22 52 82 112
    . . .
    Each layer 30 deg. away from each other, with Y 15 degrees away.
    . . .
    C 7.5 K 37.5 M 67.5 Y 82.5
    . . .
    Yellow / Light 22
    Red / Magenta 82
    Cyan / Blue 112
    . . .
    Divide screen mesh by 3.5 to get LPI
    . . .
    Divide screen mesh by 4.4 to get LPI
    . . .
    For one color halftone, set angle to 22 deg.
    . . .
    For one color halftone, set angle to 45 deg.
    . . .
    Always start at 22.5 deg.

    Here's a GigPosters thread about LPI, mesh count and degrees:

    http://www.gigposters.com/forums/scr...t-degrees.html

    In it, bdix says:

    "Screen mesh is based on a grid where one set of threads is angled at 0˚, and another set at 90˚. Therefore you should avoid setting your halftone angles to anything that may interfere with that pattern. Basically you're avoiding 0˚, 15˚, 30˚, 45˚, 60˚, etc. So what do you set your halftone angles at? The rule of thumb is to split the 15˚ difference in half, which would be 7.5˚. Since you can't enter half a degree, you round down to 7˚. So instead of a 45˚ angle you add 7˚, making it 52˚. Instead of 15˚, you add 7˚ to get 22˚, etc, etc."

    Screenthree said: "(I usually divide my mesh by 4 to determine the lpi). For whatever reason 22 degrees is the easiest for me to shoot without getting a dot on screen moire. So if it's one color, I'll output my dots that way. When doing 4 colors I use increments of 22 degrees, Cyan 22, Mag 44, Yellow 66, Black 88. Also, if I'm worried about a screen moire I will use a lighttable to line up the film and screen before coating."

    Others noted that 44 is so close to 45 that it's flirting with screen moire.

    Anyways my angles were 15 and 30 which seems like a bad idea according to what these guys said.

    I've found lots of differing opinions online.
    Here's an article that talks about angles:
    Separations for Four-Color Process Printing | Printwear

    He says:

    "There are two schools of thought when selecting screen angles (those that coincide with the mesh threads and the mesh knuckles). The first is to select a single angle for all colors. This is done to minimize moiré and other variables.

    The two best angles (mathematically) for moiré reduction are 22.5 and 67.5 degrees. The worst angles for moiré are 0, 45 and 90 degrees as they present the highest likelihood of threads or knuckles blocking the halftone dot opening. The two best angles are exactly halfway between these conflict angles.

    The second choice is to select an angle set that will form a rosette pattern when all the colors are printed. The rosette pattern increases the range of color you can reproduce. If you are a real pro, and have critical colors and transitions, this is the choice I would recommend.
    The angle choices for optimum rosette formation and minimum moiré are as follows:
    Yellow 5 degrees
    Magenta 20 degrees
    Cyan 80 degrees
    Black 50 degrees

    If you experience moiré in the black screen, it’s acceptable to swap angles with the cyan; resulting in black at 80 degrees and cyan at 50 degrees."

  6. #6
    Premium Member
    Join Date
    Dec 2004
    Location
    Kingston Ontario Canada
    Posts
    969
    Comments
    3

    Default

    22.5 degrees.

  7. #7
    Premium Member
    Andymac's Avatar

    Join Date
    Aug 2003
    Location
    Van Isle BC Canada
    Posts
    11,687
    Comments
    277

    Default

    stochastic
    Andymac

    services www.squeegeeville.com
    equipment www.tmiscreenprinting.com

    Todo es empezar.

  8. #8
    squeegeethree's Avatar

    Join Date
    Dec 2006
    Location
    Brooklyn, NYC
    Posts
    6,601
    Comments
    62

    Default

    You can spot a moire before you shoot your screen simply by putting the screen on top of the film and moving the screen until the moire is the least. Mark the screen with pencil where your reg marks are then coat your screen. When it comes time to shoot tape the film in registration to the pencil mark.
    There is always a moire it's just a matter of making it so minimal that nobody notices it.

  9. #9
    RichieGoodtimes's Avatar

    Join Date
    Mar 2002
    Location
    San Frangeles, Califas
    Posts
    27,041
    Comments
    5093

    Default

    Quote Originally Posted by Andymac View Post
    stochastic
    I've seen stochastic moire.

  10. #10
    Premium Member
    Andymac's Avatar

    Join Date
    Aug 2003
    Location
    Van Isle BC Canada
    Posts
    11,687
    Comments
    277

    Default

    me too, but not near as bad as from halftones, and super easy to avoid - angles are out of the equation, only thread count.
    Andymac

    services www.squeegeeville.com
    equipment www.tmiscreenprinting.com

    Todo es empezar.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •