Fig. 40.—Single Cylinder or Yankee Machine.
Many of the fine wrappings of the tissue class and the somewhat heavier papers known as M. G. Caps are manufactured on the single cylinder machine, which produces a paper having a highly polished surface on one side and a rough unglazed surface on the other side.
In the single cylinder machine the beaten pulp passes from the stuff-chest on to the wire of the ordinary Fourdrinier machine and through the press rolls, but instead of being dried over a number of cylinders the paper is led over one single cylinder of very large diameter which is heated internally with steam. The paper is usually pressed against the surface of the cylinder by means of a heavy felt, which is, however, sometimes omitted. The side of the paper coming into contact with the cylinder becomes highly polished, the surface in contact with the felt remaining in an unfinished rough condition. This paper is said to be machine glazed and is known as an M. G. paper.
Fig. 41.—Section of Wet Press, or Board Machine.
Boards.—Cards, millboards, middles, boxboards, carriage panels, and similar paper products are manufactured either on a single board machine, by means of which single sheets of any required thickness can be obtained, or on a continuous board machine, which is capable of producing cards and plain or duplex boards of moderate thickness.
The raw material used consists, as in the case of browns and wrappers, of every conceivable fibrous substance mixed with mineral matter and then suitably coloured. The preliminary processes for the treatment of the pulp are exactly the same as those employed in the case of brown papers up to the point at which the beating has been effected.
Single Board Machine.
The beaten pulp, diluted with large quantities of water, is pumped continuously into a large wooden vat of rectangular shape. Inside this vat revolves slowly a hollow cylindrical drum, the circumference of which is covered with wire gauze of fine mesh. The drum is not completely immersed in the mixture of pulp and water, so that as it revolves the water passes through the wire, while the pulp adheres to the surface. The water flows regularly into the interior of the drum and runs away through pipes fitted at each side of the vat near the axis of the drum, and the pulp is brought up out of the water until it comes into contact with a travelling felt. The thin moist sheet of pulp adheres to this felt, passes through squeezing rolls which remove part of the water, and is finally carried between two wooden or iron rollers of large diameter. The pulp adheres to, and is wound up on the upper roller, the felt being carried back by the lower roller to the vat. When the sheet on the upper roller has attained the desired thickness, it is immediately cut off and transferred to a pile of similar sheets, a piece of coarse sacking or canvas being interposed between every wet board. The dimensions of the full-sized board are determined by the diameter of the upper roller and its length. A roll 74 inches wide and 14 inches diameter will give a board 74 inches by 44 inches.