This blog is a follow-up to the “6 Steps to Making Paperboard” blog. We will briefly discuss the types of wood used to make virgin paperboard, the most important characteristics of paperboard, and the properties of the various grades of paperboard.
Virgin paperboard can be made from two types of wood – softwood from pine trees and hardwood from deciduous trees (lose their leaves every year). Each has different characteristics that lend themselves to different applications. The primary difference is that softwood has longer fibers than hardwood does. Those long fibers lend themselves to better tear resistance and better stiffness since the longer fibers have more surface area to bond together.
However, hardwood’s short fibers lend themselves to a smoother surface because, if on occasion fibers do not lay completely flat on one another, they stick up slightly toward the surface of the sheet. Short fibers stick up less than long fibers do, resulting in a smoother sheet. All coated paperboard is smooth because of the coating applied to the top of the sheet, but paperboard from short fibers is smoother.
The eight most important characteristics of paperboard:
1. Machine direction (MD)– the direction in which the fibers naturally line up in the paperboard making process, effects stiffness and tear strength.
2. Cross direction (CD)– perpendicular to the machine direction, also impacting stiffness and tear strength.
3. Smoothness– how smooth the sheet is, affecting its printability.
4. Tear strength– long fibers “grab” each other better than short fibers. For example, beverage can cartons need strong handles that won’t tear easily. Tear strength is greater in the cross direction (across the fibers) than in the machine direction (parallel to the fibers).
5. Stiffness– long fibers provide more stiffness, again because of the greater surface area. Stiffness is important in gluing cartons and in high-speed filling because those operations stress the cartons. Stiffness is greater in the cross direction (across the fibers) than in the machine direction (parallel to the fibers).
6. Basis weight– the weight of 1,000 square feet of paperboard.
7. Caliper– the thickness of paperboard in thousandths of an inch. Typically paperboard for folding cartons ranges from .0014” to .026”.
8. Brightness– how white and bright the sheet is.