The heavy paper[paper?:paper sample maker cutting machine] used for milk cartons is categorized as a type of paperboard. It is typically made on a Fourdrinier machine, one of the oldest and most common types of papermaking equipment. The process begins with wood chips. The chips are heated and bathed in chemicals that soften them and break them into small bits of wood fiber. The pulp is bleached in a bath of oxygenated chlorine. The pulp is then washed and passed through several screens, to remove debris. Next, the pulp is fed through a machine called a refiner, which grinds the wood fibers between rotating disks.
The refined pulp flows into the headbox of the Fourdrinier machine. In the headbox, a mixture of water and pulp is spread across a continually moving screen. The water drains away below through the openings in the screen, leaving a mat of damp wood fiber. The mat is drawn through huge rollers that squeeze out additional water. Next, the paperboard is dried, by passing it over steam-heated cylinders.
