Overprint not working - Adobe illustrator

We think we found a common factor. The color that we are making the stroke and/or the overprinted part has 10K in it. Any color that has no black in it will overprint fine. Any color chosen that has any percentage of black in it will not overprint with the black background. Why it doesn't just make it black I don't know.
Thanks for any input.

Overprint leaves any ink colour not part of the overprinted object alone when it is underneath. 10% Black overprinting 50% Yellow will produce 10% Black plus 50% Yellow. 10% Black overprinting 50% Black will leave Cyan, Magenta, and Yellow alone, but the 10% Black is what will print instead of the 50% Black.

What is the CMYK breakdown of the topmost colour that is set to overprint, and what is the CMYK breakdown of the colour underneath?

The top layer is a red that has 100M, 63Y and 12K and it goes up against a 100K background. The outcome will be black and that's what we want, but since it is screen printing there is a margin of error and we always trap over the black so that if the printing is off slightly no one will notice. What the file is doing is not letting any CMYK color overprint if the color has black in it as long as the background also has any black in it.

That's right. When you overprint Color A over Color B, any non-zero CMYK component in Color A will take precedence. 1% overprinting 100% will produce 1%. If you are printing this over just 100% black then there is nothing to worry about. The 10% Black will register perfectly with the 100% Black because they are on the same plate. If you want the 100% Black to print throughout the illustration, set Black in the overprinting object to either 0% or 100%.

But the problem is that the black will be to the correct shape, but the magenta and the yellow will not go beyond the black. It works for spot colors just not for 4 color process.

Sounds like you'll need to trap, not just overprint.

Sounds like you'll need to trap, not just overprint.




The stroke is splitting the fill and thus acts like a trap. (I know, you can make the stroke ride on the inside or outside of the path, but I don't believe Suzie is doing that)

Suzie,

Separate the stroke from the fill and set the stroke to multiply instead of overprint.

Suzie,

First, as Scott said (in different terms) in Post 4, if you misregister on press, the misregistration will not reveal a white sliver; it will reveal a 12%K sliver. If you feel the misregistration will be severe enough for that grey sliver to be a problem, then you need to increase the percentage of K in the trapping outline.

There is no need to mess with overprinting or transparency to create this trap. Simply:

1. Give the red object a stroke of the same red color as its fill (100M, 63Y, 12K).

2. Edit just the Stroke's color to include a higher percentage of K. (100M, 63Y, 100K).

3. Position the Stroke below the fill.

The red object's fill knocks out the 100%K of the inside half of its stroke. (Thus, the 100K component of the Stroke does not choke the red fill.) The stroke "knocks out" the black background, but includes the same 100% K anyway. Net result is the 100M and 63K bleed into the surrounding 100K by half the width of the stroke weight.

Second, if you do decide to use "Transparency" to create the trap as John suggested in Post 7, you don't have to separate the stroke from the fill in order to set its transparency mode to Multiply; you can apply the mode at the Stroke level.

JET

Thank you for all of your suggestions.

Suzie