Fiberboards 2839 



The third unique element of the medium-density fiberboard process is radio- 

 frequency curing of the resin during pressing. Unlike conductive heating be- 

 tween hot press platens, radio-frequency heating raises temperature uniformly 

 regardless of the distance from the platen surface. In theory, every part of the 

 mat should have the same time-temperature relationship during the compression 

 period. Thus, compressibility of the mat should be uniform over its thickness at 

 any given time, and density should not vary over the cross section of the finished 

 board. 



A uniform density over the board cross section is not always desirable. Many 

 manufacturers produce boards with high face densities to increase bending 

 strength and stiffness. In medium-density fiberboard, on the other hand, radio- 

 frequency heating in the press produces uniform density and a solid edge, which 

 allows smooth machining and finishing. 



Urea-formaldehyde resins regularly used in the particleboard industry are also 

 used for medium-density fiberboard, and about half of the existing plants use 

 regular steam heated presses rather than radio frequency. The pressurized re- 

 finer, however, is still the typical pulping machine for medium-density 

 fiberboard. 



Drying, resin binder application, and forming. — Drying of fiber for medi- 

 um-density fiberboard is accomplished in tube suspension dryers or flash dryers 

 exactly like those used in other dry fiberboard processes (fig. 23-55). 



Resin is generally applied to the dry fibers in short retention mixers (figs. 23- 

 27 and 23-69). These mixers were developed for the particleboard industry, a 

 significant advance from large, low velocity trough-type blenders (Knapp 

 1971). Short-retention mixers rely on rapid agitation to transfer resin from one 

 fiber to another throughout the furnish. In more recent machines spray nozzles 

 have been replaced by liquid adhesive delivery through shaft and rotating pad- 

 dles or through injection into the rotating ring of furnish (fig. 23-69 center). 

 Retention times are only a few seconds. Mixers of this type have been equipped 

 with agitators especially suited for handling fiber (fig. 23-69 bottom). 



With fiber furnish these machines sometimes distribute resin unevenly, devel- 

 oping resin spots visible on the finished board. Being considered as an alterna- 

 tive is injection of the urea-formaldehyde resin into blow pipes from the refiners. 

 This is common practice in dry process hardboard manufacture where phenol- 

 formaldehyde resins are used. Additional benefits (besides absence of glue 

 spots) are: no tackiness, clean pneumatic ducts and cyclones, higher moisture 

 content at dryer output (14 percent vs. 4-5 percent with blender), and no formal- 

 dehyde odor. Resin consumption, however, increases by about 10 percent (Hay- 

 lock 1977). 



Forming of the medium-density fiberboard mat is similar to forming of thin 

 dry-formed hardboard mats. The vacuum former (fig. 23-58) is standard. Mat 

 thickness is, of course, much greater, so that it may be questionable whether or 

 not upper mat layers benefit from the vacuum. 



A more sophisticated forming machine, the Rando-Wood-MDF former (fig. 

 23-70), which attempts to form the entire mat thickness simultaneously, was 

 described by Wood (1976). A separator assembly separates the fibers from the 



