Variations of the fourdrinier machine forming section

The forming section type is usually based on the grade of paper or paperboard being produced; however, many older machines use a less than optimum design. Older machines can be upgraded to include more appropriate forming sections.A second headbox may be added to a conventional fourdrinier to put a different fibre blend on top of a base layer. A secondary headbox is normally located at a point where the base sheet is completely drained. This is not considered a separate ply because the water action does a good job of intermixing the fibers of the top and bottom layer. Secondary headboxes are common on linerboard.
A modification to the basic fourdrinier table by adding a second wire on top of the drainage table is known as a top wire former. The bottom and top wires converge and some drainage is up through the top wire. A top wire improves formation and also gives more drainage, which is useful for machines that have been sped up.The Twin Wire Machine or Gap former uses two vertical wires in the forming section, thereby increasing the dewatering rate of the fiber slurry while also giving uniform two sidedness.
There are also machines with entire fourdrinier sections mounted above a traditional fourdrinier. This allows making multi-layer paper with special characteristics. These are called top fourdriniers and they make multi-ply paper or paperboard. Commoly this is used for making a top layer of bleached fiber to go over an unbleached layer.