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)
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.
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:
"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.
"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."
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.