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Smart impositions can make the difference between winning or losing a job, making or losing money on a job, and whether or not the customer is happy with the finished project.
I'm sure that in your shop you have standard impositions for common trim size books. They were created by your forefathers (or mothers) and are used as standard operating procedures. The real need to understand imposition occurs when dealing with odd-size trims, multiple-up impositions, and as a means of avoiding extra "ink problems" (see below). Many times your trade binder has uncommon folder capabilities (sections, plates, split guides, and spot gluing) that can lead to a more efficient imposition by saving press and folder time and paper waste.
Imposition is the positioning of the pages on the press sheet so that when the sheet is folded and trimmed, the pages fall in the correct order, in the correct orientation (i.e., right-side up), and with the correct margins.
The individual who has accumulated the knowledge and experience to understand each of the subjects below is just as important as your top salesperson! (At most companies, the critical decisions regarding planning are made by the estimator.) There are more than 30 ways to fold a 16-page signature on a buckle folder. Which ones are right for your particular project? Consider these factors and you'll find out!
There are many things to consider when it comes to planning imposition:
Press sheet size
Number of pages or size of project
Length of run
Weight of stock
Trim size and trim allowances (plus lip lap or leg if saddle stitched)
Mechanical capabilities of folding machines
Grain of paper
Type of binding
Ink problems - Printed elements demanding different ink requirements that fall "inline" on the press sheet or ghosting potentials
Your printing and folding/binding capabilities
Cost to operate the above equipment plus plates and paper, etc.
Because of limited space, I'll just touch on a few points that the student of sheet-fed imposition should learn. There are several types of impositions-sheetwise, work and turn, work and tumble-all for printing both sides of the sheet. A signature is a sheet of paper, that when folded and cut, forms a section of a book, magazine, or pamphlet. As a unit, the signature will contain from 4 to 96 pages. The most common sizes are 4-, 8-, 16-, and 32-page signatures.
Fitting the page to the sheet requires determining how many pages can be imposed on a given size of paper.
A - Paper size is 25 x 38
B - Size of 2-page spread is 12 x 9
C - Size of spread with trimming allowances is 12 1/2 x 9 1/4
Divide C into A and it equals 8 double spreads or 16 pages to a side or 32 pages to a sheet.
It is relatively easy to determine what press sheet size is needed for standard trim sizes. Following is a chart showing the most common trim sizes and the standard sheet sizes that provide the most economical fit.
(see chart at end of article)
When dealing with an uncommon trim size, determine your maximum press sheet size and divide this by your 2-page spread (with trims). Take the maximum number out and multiply it by two. This equals the number of pages to a side. Multiply it by two again and this will give you the number of pages to a sheet. Eliminate any unneeded paper from your maximum press sheet size and you have your desired press sheet size. If your schedule and number of pounds permit, you can get a make order. (One of our customers recently imposed a book as 18-page signatures, rather than standard 16-page signatures, on making size sheets for a large run. Imagine the cost savings on press and in folding!) If not, the closest standard sheet size will have to do, you may have to alter your trim size slightly. The more you go through this exercise, the better and faster you'll become. But remember, just because you can print it, does not mean you can fold it, consult with your folding department or trade shop! And remember, the signature you plan must be compatible with your binder's equipment.
I have a great imposition puzzle you might want to try, just fax me a request and we'll send it your way (courtesy of the BFA - Binders & Finishers Association).