Wet-end to Dry-end of the Paper Machine
WE GOT PAPER
As paper leaves the head box it is either dragged by the wire part (fourdrinier) or rushes and sprays on the wire fast than the wire is moving. This can affect the orientation of the fibers and the formation of the paper. Fiber travels from the breast roll near the head box to the couch roll at the end near the press section. The breast rolls is solid and supports the wire while the couch roll is perforated for high-vacuum power to help with dewatering.
Drainage along the wire is driven by hydraulic pressure gradients. As the fire passes over the foil, suction is produced. This is called white water since it is cloudy looking from debris coming off the wire. The water falls into the various silos so bits of pulp can be recycled into the process again.
The beginning of the wire is supported by the forming board because of the high force of the jet flow from the head box.
Towards the couch roll, there is the broke system which would take paper that is partly manufactured and re-pulp it back to stock and send it to the head box.
The Press Section
The main objective the press section which literally presses the sheet between two rolls is to remove water from the sheet and consolidate the web of fibers. Consolidation is important because many fiber to fiber bonds are formed as water is removed and the fibers are pressed. Generally the sheet and felt are pressed in the nip between two rotating rolls with the top roll determining the pressure used on the sheet/felt.
The Drying Section
Water is further removed by evaporation in the drying section. This is the largest part of the paper machine and has the largest capital cost. Performance is measured by the evaporation drying rate measured as pounds of water evaporated per hour per square foot of dryer surface contacted. You don’t want to dry the paper out too much because once enough water is removed for the fibers, the fibers can collapse in on themselves in the process called hornification. Here the pulp goes from 50–60% moisture to about 7% moisture after the sheet passes through the calendaring section.
After the drying section, paper is pressed again to obtain a smooth surface for printing and improve cross-direction uniformity.
Reeling and Winding
Paper is then reeled into a large roll moved to the winder where it can be cut up into smaller rolls. You now have paper that is read to ship to a box plant of for whatever your paper need might be for.
This concludes my series following wood chips as they become paper. There are many more steps that I neglected to mention and chemical processes were neglected as well since I only wanted to follow the pulp to keep it simple. Thanks for reading!