Micrometer
Before we have printed or finished a job we are frequently asked two things that are quite tricky to answer before the job is complete: “how much will it weigh?” and “how many will there be in a box?” Sometimes these are combined into the two espresso question: “how much will a box of the finished items weigh?”
“We don’t know yet. Shall we revisit this when the things actually exist?” would be the honest answer, but we do like to be helpful where we can, and it is in fact possible to get most of the way to a reasonably accurate answer to both questions – although I would always advise a customer that these are estimates only, and not commit to anything until the job is actually finished and in boxes IRL (in real life, according to my teenage son).
Calculating the weight of most printed items isn’t that difficult, since usually almost all of the weight lies in the paper or board the job is printed on. And since most paper is sold by weight, the information we need for the calculation is fairly easy to come by.
So if, for example, we want to work out the weight of a 32pp plus 4pp cover A4 portrait saddle stitched book, printed with 150gsm text pages and a 250gsm cover, the sums would go:
Text: 0.21 x 0.297 (the size of 1 page in metres) x 16 (the number of leaves of paper) x 0.15 (the weight of the paper in kg per metre square) = 0.149688 (weight in kg). Cover: 0.21 x 0.297 x 2 x 0.3 = 0.037422. Adding the two weights together gives 0.18711kg, or 187 grammes. If we allow a couple of grammes for ink and staples, we can estimate the weight will be approx 190g per book.
Calculating the number of copies per box is not so easy, since there are more variables at work. Material of different types but the same weight can have very different thicknesses; different binding techniques will slightly alter the thickness of the books; and even the preferences of whoever is boxing the job up may become a factor (it is an unusual finisher who will pack a job at 101 copies per box, even if this is theoretically the ‘optimal’ number.)
Still – we can get an approximation. What we need to know is the number of pages of each material, plus the binding technique and how thick the materials are, which will have to be established by measuring the sheets with a micrometer since they vary so much. For the example above, the sum would be:
Text: 16 (the number of leaves) x 0.15microns = 2.4mm. Cover: 2 x .3 = 0.6mm. Adding the two together gives us 3mm.
This will be roughly correct if the job is being saddle stitched or perfect bound. If the job is being burst bound or section sewn it is probably wise to add in a further 0.5 – 1mm to allow for glue. (If it is case bound it is a different kettle of fish entirely.) You should also make an allowance for the fact that stitched books are slightly thicker at the spine than the foredge, since the paper can never be completely flat as it changes direction.
Then, to answer the question of how many per box, we just need to know the size of the box. Most A4 cartons are 9” tall, or about 240mm. So our example will be 240mm/3mm = 80, less a notional allowance for spine thickness, so 75 copies per box.
And the weight of a box? 0.190kg x 75 copies = 14.25kg.
Coffee, anyone?
