2824 Chapter 23 



HIGH- AND MEDIUM-DENSITY HARDBOARDS 



Figure 23-54 illustrates a typical dry-process hardboard plant in the United 

 States. It shows the combination of three important elements: atmospheric Bauer 

 disk refiners, tube suspension dryers, and vacuum formers. This is by no means 

 the only practical or possible configuration, but it does represent common 

 industrial practice. The largest dry process hardboard plant in the country (Ma- 

 sonite at Towanda, Penn.), for instance, uses an Asplund Defibrator and a 

 secondary refiner in a combination pulping unit. Weyerhaeuser Company at 

 Klamath Falls, Ore., uses Asplund type D and L defibrators. The illustrated 

 plant uses roundwood as raw material. Today's mill would supplement or 

 replace the roundwood supply with purchased debarked pulp chips, whole-tree 

 chips, or mill residue. 



Drying. — To press hardboard without a screen, the moisture content of the 

 mat going into the press must be below specific threshold values which depend 

 on board density and other variables. Excessive moisture will be trapped as 

 steam in the board, resulting in steam blisters (blows) as the press is opened. 

 High temperatures and pressures generally require lower moisture content but 

 the drying schedule must not be severe enough to condense the resin adhesive, 

 which is generally applied prior to drying. The drying must, therefore, terminate 

 before the resin cures; at temperatures high enough to assure efficient water 

 removal, this limits drying time to a few seconds. 



Moisture content of furnish entering the dryer is around 50 percent. Target 

 moisture content of the dry furnish entering the forming machine is between 8 

 and 12 percent. 



The dryer most commonly used is a tube dryer, in which fibers are suspended 

 and transported by the drying medium — hot air or combustion gases. The ratio 

 of air to fiber is about 50 cu ft/pound and air speed about 3,000 feet/minute 

 (Rausendorf 1963). Air or gas temperatures at the wet end range between 500 

 and 650°F. Exit temperatures are about 150 to 190°F. At 650°F the curing time 

 for a phenol-formaldehyde resin is about 8 seconds. To prevent precure of the 

 resin, drying time should be limited to about 5 seconds which, at 3,000 feet/ 

 minute air velocity, would require a dryer length of 250 feet (Rausendorf 1963). 



Single- and double-stage tube dryers are diagramed in figure 23-55. Two 

 stages are advantageous when the moisture of the incoming furnish fluctuates 

 widely. The first stage is then used as a pre-dryer to equalize the moisture 

 content, and main drying occurs in the second stage. The final moisture content, 

 adjusted by control of dryer outlet temperature, must be kept within small 

 tolerances to provide proper venting in the press and to allow accurate control of 

 board density or board thickness. 



Two problems are inherent in high temperature drying of wood fibers: danger 

 of fire and explosions, and emission from the dryer of fibers, fiber fractions, 

 solid particles resulting from the combustion process, and small particles which 

 are condensation products of volatile materials evaporated from the furnish. The 

 installation of sensitive fire detection and control devices is therefore impera- 

 tive. The emission of larger particles such as fibers and fiber fractions can be 



