Fiberboards 2787 



Table 23-5. — Production capacities of single-cylinder forming machines producing '/2- 

 inch insulation board (Lyall 1969) 



Daily 



Cylinder diameter Cylinder (24 hours) Tons 



and length (feet) area production' per day 



Square feet Tons 



8.0 X 9.0 225 200,000 70 



11.5x10.0 350 300,000 100 



14.0 X 9.0 396 350,000 120 



14.0 X 13.3 572 600,000 200 



14.0 X 15.0 660 700,000 250 



These values are minimums; greater production is possible. 



In a double-cylinder forming machine (fig. 23-31 bottom), the two cylinders 

 are geared to run at the same speed but in opposite directions. Each cylinder 

 forms one-half the total mat thickness, the two halves being merged and lami- 

 nated in the nip between the cylinders, where hydraulic pressure forces them 

 together and helps dewater the mat. Doubling of drainage area contributes to the 

 higher productivity of this machine. Also, cylinder construction is simplified 

 because the pressure differential needed to form the mat is created by a water- 

 drop leg (see fig. 23-22 top for illustration of drop leg) rather than a vacuum 

 system. Resulting positive water pressure requires sealing of the vat where the 

 two cylinders enter the pulp, and around the ends of the cylinders. Air blown 

 into the nip keeps water in the vat away from the outgoing mat. 



The double cylinder machine has an important advantage over all other 

 forming machines because it produces a mat of symmetrical fiber structure. All 

 single-cylinder wet-forming machines tend to deposit coarser fractions of stock 

 first and finest fractions last. The two surfaces of the board will therefore have 

 different appearance and properties, a condition only partially alleviated by 

 agitating the stock in the vat during forming. Laminating the two halves of the 

 mat mates top surfaces from each cylinder and presents identical bottom surfaces 

 outside. Bond quality depends on the fiber characteristics; if the surface fibers 

 are too fine or too coarse, the sheet may delaminate during or after drying. 



The caliper of the laminated mat, besides being a function of the thickness of 

 the two halves, is controlled by the force applied to the mat in the nip. A force of 

 400 pounds per lineal inch of nip is not uncommon. Double cylinder machines 

 produce boards with exceptional uniformity of caliper and density. 



Cylinder machines produce a mat of relatively low water content (less than 80 

 percent), and are capable of handling slower stock than Fourdrinier forming 

 machines at the same forming speed and on the same forming surface area, but 

 they are not suitable for very free draining stock. 



Cylinder machines have a tendency to orient fibers in the machine direction 

 (direction of rotation). Eustis^ estimates this bias as 60/40 which means that 

 properties of the finished board measured in the two principal directions will 

 have a ratio 60/40. Bending strength would be greatest in the machine direction, 

 and the linear expansion coefficient would be largest perpendicular to the ma- 

 chine direction. 



