Droop Rigidity CD: Droop rigidity measures the stiffness of the paper or board, more often applied to lighter weight grades. CD refers to cross direction, and MD to machine direction, Droop rigidity is higher in the machine direction. The higher the value the stiffer the paper.
Droop Rigidity MD: Droop rigidity measures the stiffness of the paper or board, more often applied to lighter weight grades. CD refers to cross direction, and MD to machine direction, Droop rigidity is higher in the machine direction. The higher the value the stiffer the paper.
In addition to the various weights of paper, you can use watercolor board, which is paper adhered to illustration board. It doesn’t buckle when wet and is available in 20-x-30-inch pieces or by the case.
To use the Weight Conversion tool, please select a Type of Paper from the pull down menu, then enter a value in pounds and hit calculate. To reverse the calculation simply select and enter a gsm value.
The density of paper is expressed by basis weight. Basis weight is the weight in pounds of 500 sheets of the paper at a standard sheet size, which in the case of book papers is 25″ x 38″. For uncoated book papers, 50-lb., 55-lb., and 60-lb. are typical basis weights. Common coated stock basis weights are 70-lb., 80-lb., and 100-lb. In Asia and Europe, metric grammage is used instead of basis weight; instead of pounds, the unit is grams per square meter (gsm). By way of comparison, for text paper 60-lb. is roughly equivalent to 89-gsm.
Example: Let Outer Diameter of Roll (Do) is 40", Outer Diameter of Core (Di) is 6.5", thickness is 8 Pt., Width of roll is 40" and Basis weight is 100 g/m2
The thickness of paper is called caliper. Caliper is important because it determines the width of the spine of a book, and therefore the precise layout of the cover. Caliper is measured in pages per inch (PPI). The thinner the paper, the higher the PPI. For example, a 600 page book on 300 PPI paper will be 2″ thick, excluding the thickness of the cover (600 / 300 = 2). A 240 page book printed on 360 PPI stock will be .66″ thick, excluding the cover (240 / 360 = .66). The thickness calculated this way is often referred to as bulking.
Book paper should be consistently manufactured from batch to batch to a specified PPI, because the thickness of the paper determines the width of the spine. If the PPI changes significantly between batches when the book is reprinted the width of the cover spine will have to be changed to fit the altered bulking. This will take time and money, and may require another proof cycle, losing more time and introducing the potential for error.
If you know the outer diameter of the roll and the core on which paper is wound and the thickness of the paper and basis weight, you can calculate the weight of paper on roll using following equations.
Some uncoated stocks are manufactured to be thicker relative to their weight than standard stocks. These are called “high-bulk” stocks. High-bulk stocks are used to keep the weight and cost of the paper down while maintaining the same spine width as a the heavier stock. For example, a 50-lb. high-bulk stock is 456 PPI, almost the same as a standard 60-lb. stock at 436 PPI. A book printed on the 50-lb. high-bulk has almost the same thickness as if printed on the 60-lb. stock, but weighs about 17 percent less.
Basis weight also affects the thickness (caliper) of paper, although other factors enter into this as well, such as the method and amount of calendering used in the manufacture of the stock.
Example: Let Outer Diameter of Roll (Do) is 100 cm, Outer Diameter of Core (Di) is 16 cm, Width of roll is 100 cm, basis weight is 100 g/m2 and thickness is 200 micrometer.
Each paper weight has its advantages, and the weight you choose depends on what you want to accomplish. Beginners can get a good start with 140-pound cold-press, 100 percent cotton rag, acid-free paper in any brand. (For more on cold press and hot press, see the next section.)