Manufacture of Hardboard Out of Wood Waste

The component fibers of hardboard are bonded to form an almost homogeneous mass, which when cut with a knife, leaves a sharp smooth edge. The bonding may result from chemical changes that take place in the composition of the fibers, or it may be brought about by the addition of extraneous fibers. The binding is always done under heat and considerable pressure. A true hardboard retains its thickness with very little swelling when it gets wet. A good hardboard will have a modulus of rupture of 5,000 psi. It will expand in water less than one-tenth as much as lumber. Its density is about 1.0. An increase in density over 1.0 results in a rapid rise in MOR, so that hardboards of 1.20 density frequently have a MOR of 10,000 psi. Until modern synthetic resins became available to coalesce wood granules all hardboard had to be made of fibers obtained froth solid wood. Major processes for manufacture of hardboard are: 1) continuous wet process such as well-known Masonite Process (total mfg. cost – $21.70 per M sq. ft. of 1/4-inch hardboard) 2) batch wet process using wood chips from Pacific Coast sawmill waste or waste veneer reduced by conventional refining machines and a small percentage of phenolic resin added as the binder (total mfg. cost – $36.80 per M sq. ft. or 1/4-inch hardboard). 3) semi-dry batch process in Pacific Northwest in which the fibers, appropriately coated with phenolic resin, are deposited on screens. They contain only enough moisture to permit grinding, 4) continuous wet felting with dry pressing using cordwood for the raw material rather than waste solid wood because of existing patent limitations, 5) dry process for producing granular hardboard in which wood particles passing a 20-mesh screen are mixed with powdered resin in a rotary mixer and then mechanically spread on steel, cauls and hot pressed at 200 psi for 5 minutes (total mfg. cost is $44.00 per M sq. ft. of 1/4-inch hardboard).

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