Sheet Imposition Formula

ReflexBlueHorror

Active member
Hi all,

I'm coming at this as a graphic designer --- someone without imposition software for offset printing, or enough knowledge (yet!) on sheet imposition.

I'm looking at formulas for determining the possible Section Folds+Trim Sizes for different sheet sizes from different mills (eg 530x760mm GS; 660x1016 GL; 720x1020 GL etc)

The formula I've been given from a book production manual is to:
1. Divide the sheet according to the section folds (eg 1020mm x 720mm GL folded to make a 32pp section = 180mm x 255mm Portrait)
2. Subtract trimming: 6mm height (-3mm top and -3mm bottom) and -3mm edge. Subtract a further -3mm from the width if the book is to be perfect bound.
The result is the maximum Trim Size for that imposition: a 1020 x 720 sheet can make 32pp section 177mm x 249 Portrait (Case sewn)

I can't see how this is right. At the very least it doesn't account for Gripper Margins. It doesn't mention bleeds either. Surely there must be a 'safe' standard allowance for the Gripper Margin too.

I'm posting here hoping that someone might know a better formula!

Thanks
 

PrintingFools

Well-known member
http://www.imposeonline.com/ is a free online impose program. I find that when teaching at our shop , this has been a great tool. You are able to impose and download your own files. Clicking through the options really teaches you a lot about how inposition works, after you see your results. Then going back and changing those options that you chose to get a different result helps you to understand. Not to mention that if you like the results you’re able to download the file for free and actually use that to print from or make plates for offset or print ready for digital.

At our shop we are lucky to have two professional imposition software programs, as well as our owner ( that’s me LOL ) knows how to impose all jobs manually old-school if ever needed. But, I highly recommend this program for anyone new anyway to learn and understand now impose works
 

SteveSuffRIT

Well-known member
Defining terms such as Work & Back, SheetWise (SW), Work & Turn (W/T)), Work & Flop, Work & Tumble, is important.
On a sheetfed press, a perfecting impo will need allowance for 2 gripper edges.
Many sheetfed deliveries also need accomidation for slow down wheels or star wheels that might mark, scuff, smear, scratch fresh wet in.
Dont forget about room for quality color bars.
Steve Suffoletto
 

Glenn McDowall

Well-known member
Hi all,

I'm coming at this as a graphic designer --- someone without imposition software for offset printing, or enough knowledge (yet!) on sheet imposition.

I'm looking at formulas for determining the possible Section Folds+Trim Sizes for different sheet sizes from different mills (eg 530x760mm GS; 660x1016 GL; 720x1020 GL etc)

The formula I've been given from a book production manual is to:
1. Divide the sheet according to the section folds (eg 1020mm x 720mm GL folded to make a 32pp section = 180mm x 255mm Portrait)
2. Subtract trimming: 6mm height (-3mm top and -3mm bottom) and -3mm edge. Subtract a further -3mm from the width if the book is to be perfect bound.
The result is the maximum Trim Size for that imposition: a 1020 x 720 sheet can make 32pp section 177mm x 249 Portrait (Case sewn)

I can't see how this is right. At the very least it doesn't account for Gripper Margins. It doesn't mention bleeds either. Surely there must be a 'safe' standard allowance for the Gripper Margin too.

I'm posting here hoping that someone might know a better formula!

Thanks
At Stage 1 take off the Gripper Margin(s) and Colour bar (Height may also include a white margin top and bottom) from your sheet size.
as an example this could be 12mmGrip + 6mm Colour Bar +1mm +1mm (White Margins) = 20mm off your 720mm sheet.
or as a 2nd example for a Perfecting Press 10mm bottom grip + 6mm Col Bar +1mm +1mm (white margins) + 10mm Top Grip = 28mm off your 720 sheet.

At Stage 2 your 3mm extra on 3 edges Trim Margins would also typically be your bleed. You are adding an extra 3mm for Perfect Bound because 3mm will also be ground off the spine.
Stage 2 for for Stitched Jobs is another level complicated, and reduces TPS, due to lips or laps, your final folded section requires at least 6mm more on one side of the printers-pair than the other, and all sections typically need to have the lip on the same side. So for example (a 16page section) if you have 3mm trimming/folded on the outside of pages 16,15-14,13 you'll want at least 9mm on the outside of pages 1,2-3,4. You can't just calculate this with a formula because it depends on how many folds AND you can also use the Gripper Margins and Colour Bars to get the extra paper for the lips.
 

ReflexBlueHorror

Active member
At Stage 1 take off the Gripper Margin(s) and Colour bar (Height may also include a white margin top and bottom) from your sheet size.
as an example this could be 12mmGrip + 6mm Colour Bar +1mm +1mm (White Margins) = 20mm off your 720mm sheet.
or as a 2nd example for a Perfecting Press 10mm bottom grip + 6mm Col Bar +1mm +1mm (white margins) + 10mm Top Grip = 28mm off your 720 sheet.

At Stage 2 your 3mm extra on 3 edges Trim Margins would also typically be your bleed. You are adding an extra 3mm for Perfect Bound because 3mm will also be ground off the spine.
Stage 2 for for Stitched Jobs is another level complicated, and reduces TPS, due to lips or laps, your final folded section requires at least 6mm more on one side of the printers-pair than the other, and all sections typically need to have the lip on the same side. So for example (a 16page section) if you have 3mm trimming/folded on the outside of pages 16,15-14,13 you'll want at least 9mm on the outside of pages 1,2-3,4. You can't just calculate this with a formula because it depends on how many folds AND you can also use the Gripper Margins and Colour Bars to get the extra paper for the lips.
On a sheetfed press, a perfecting impo will need allowance for 2 gripper edges.
Steve Suffoletto
Thanks all and thanks Glenn especially. I've penciled your comments into my print production books! Obviously as you say there's more considerations involved but this is far better help than the formula I was provided with. In my search for answers on this topic, I also came across references to the need for folded panels to be slightly unequal, and the outer-most fold of a section requiring additional space on the spine edge. I don't know what the term for that is (it's where the section identifier is printed). I think for anyone visiting this thread it's good to know those other aspects might also fall into consideration. Thanks also Steve for the pointer about a perfecting press having 2 grippers. I'm assuming that a gripper edge is what is implied when a plant list species a sheetfed press with a 72x100 sheet size, but a 71x100 printable area.
 

Glenn McDowall

Well-known member
"folded panels being slightly unequal" I would think is referring to a 6pp roll fold finished size A4 would be typically set up as a spread with panel sizes 209-210-210 or 208-210-210 whereas a concertina or Z fold would typically be set up with equal panel sizes 210-210-210 If you have a throw out 6pp in an A4 stitched book (imagine the spread for OBC-OFC-MaxSizeFlap) you'd go something like 210-208-206 for three knife trimming. The Same Cover perfect bound would be something like 210-208-200 to accommodate the 6mm Glue Hinge as well as three knife trim. "outer most fold requiring additional space on the spine edge" probably refers to creep/shingling, as a designer you don't need to worry about this. "section identifiers" are usually printed in the gutter ( or Grind-Off) of a Perfect bound spine, also known as Collation Marks. It would be unusual to add these to sewn or stitched work (though I have been asked to put them on) because they would show on the finished piece. "72x100sheet size but 71x100 printable area" would indicate to me that they can't print in the bottom 10mm of the sheet because of a single gripper, perfecting presses would more likely have 20mm removed from the sheet size because of 2 grippers.
 

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