Prepress Compensation

[mazengh]

I have heard from someone about the existence of a formula that helps compensate between two presses when printing on press 1 then trying to fit additional colors (a fifth and sixth color for example) on press 2. Or in another scenario where you create a film for screen spot UV to be laid over printed material on an offset press. Anyone heard of such a formula?

Thanks in advance.

 

[rich apollo]

Not quite sure what you're asking. Are you trying to distort the image to fit on a second pass or on a spot coating?

 

[mazengh]

Not quite sure what you're asking. Are you trying to distort the image to fit on a second pass or on a spot coating?

exactly this is what i need to do.

 

[Paul Santer]

When I was an apprentice we did this regularly by cutting the film as most of the jobs were printed on a 2 colour press and the paper often stretched when passed through the machine.
I did do it once using a Web Growth module that allowed you to Rip separations in the same job with different scaling.
I have never seen anything that calculates the stretch automatically as it would be relevant to the stock, pressures and coverage. I'm sure you could work out your own formula with a spread sheet if you have access to all the variables.

 

[rich apollo]

Fan-out can be accounted for with web-growth, as Paul describes; but that can be inconsistent depending on the stock, pressures, running speed, water, sheet size, et cetera.

For a spot-coating, you have to calculate the change in the circumference of the cylinder. To do this, double the blanket thickness
multiply * pi (this number is the change in the length of print)
add the result to the length of the blanket
divide the length of the blanket (which is the circumference of the cylinder) by the new number

This final number should give you the percentage at which you need the image on the blanket. Keep in mind, however, that this is only in the direction around the cylinder. Across the cylinder the image will need to be 100%.

 

[buckeye]

I have never seen anything that calculates the stretch automatically as it would be relevant to the stock, pressures and coverage. I'm sure you could work out your own formula with a spread sheet if you have access to all the variables.

Agreed 100%. IMHO, there would be too many unknown variables for any software to be able to calculate a formula for this to work. It would have to be all internal.

Erik