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THE DENSITY OR WEIGHT AFFECTED BY HOT PRESSING AND COLD PRESSING OF THE PLYWOOD
Weights of Plywood varies depends on the wood species, however, as a guide approximate weight Depends what the “ply” value is,the number of layers glued together and density of a plywood,Moisture content of the plywood and the wood species ….also Due to the difference in manufacturing methods for the various plywood products, they can vary in weight considerable,like cold and hot pressing methods …
Mainly plywood density is 450kg/CBM to 650kg/CBM, but some hardwood plywood density can beyond 1,000kg/CBM.
450 kilogram/cubic meter = 28.092 582 378 pound/cubic foot
650 kilogram/cubic meter = 40.578 174 547 pound/cubic foot
1 000 kilogram/cubic meter = 62.427 960 841 pound/cubic foot
Now, applying the rule of science,
Weight = Volume*Density
Volume = L*B*H
Volume = 4*8*(0.5/12)
Volume = 1.33 ft^3
*Let us consider all 3 cases of different densities:
1.Weight = Volume*Density
Weight = 1.33*28.092 582 378 pound
Weight = 37.36313456274 pound.
2.Weight = Volume*Density
Weight = 1.33 * 40.578 174 547
Weight = 53.97 pound
3.Weight = Volume*Density
Weight = 1.33 * 62.427 960 841 pound
Weight = 83.03 pound
For chinese poplar and chinese eucalyptus and other hardwood or soft wood plywood,I will make a WEIGHT list soon .
When thin plywood or thick plywood are cold pressed, twenty or more panels may be placed together between the colded plates.
The veneer core are cold pressed ,after facing the face/back veneer ,then cold pressing again .
After cold and hot pressing ,the density of the plywood is more than the original glue veneer core without pressing .The heavier the plates and the hotter of the teperature and the longer of the time ,the density of the plywood are bigger and thickness are thinner .
That is why more and more importers trying to request the UNIFORM DENSITY (OR WEIGHT) OF THE PLYWOOD now for their good quality plywood.
If the same thickness ,same wood species,core construction and glue ,the weighter the better strength of the quality .
Many factories using less pressing time and pressure for their cold pressing and hot pressing to make THICKER plywood ,means the inside strength of the plywood is not good ,they just wanted to make the plywood more thick using less pressure .
Everbody knows that plywood is being glued in hot presses, particularly when the glues are used. At the present time, all gluing with film glues, practically all gluing with phenol-resin glues, and much gluing with urea-resin glues is done in hot presses.
When thin plywood is hot pressed, two or more panels may be placed together
between the heated plates. Only one thick panel is pressed in each opening. Hot
presses usually have many plates and openings between them, so that a number
of panels can be glued in one pressing operation. Particularly when panels
with thin faces are being glued, the press must be closed promptly after the
panels are inserted in order to avoid partial setting (precure) of the glue before
pressure is applied. Automatic loading and unloading equipment is widely used
to reduce time lags in hot pressing and to reduce such precure.
The time required in the hot press depends on the thickness of the material
being glued and on the glue being used.
The pressing time depends on the distance the heat must travel from the plates to reach this glueline. The time may vary from 2 or 3 minutes for very thin panels to an hour or more or panels 2 or 3 inches thick. The time required can be calculated by
the use of mathematical formulas that consider wood thickness, species, moisture
content, press temperature, and setting temperature of the glue. The rate of
heating of a panel has difference.
The amount of pressure required in hot-press gluing varies with the kind of
wood being glued. Heavy, dense woods can withstand higher pressures than lighter,
softer woods. In any panel assembly, the maximum pressure to be used is
controlled by the species of lowest density in the assembly. For woods of low
density, such as hardwoodwood, poplar, and pine, pressures of 100 to 150
pounds per square inch are used. With medium-density woods, such as okoume paulownia , the pressures to be used lie between 150
and 200 pounds per square inch, and forhigh-density woods, such as yellow birch
and hard maple, the pressures may be from 200 to 250 pounds per square inch
or even higher. In any case, the pressure must not be so great as to crush the
wood or produce excessive compression in panels under the conditions of
heat and moisture prevailing in the panel, nor so low that the glue will not be
pressed out into a thin, continuous film in complete contact with the surfaces to
be joined Precautions must be taken to be certain that total pressure is adequate
and that it is uniformly distributed over the entire joint area.
Prepressing is now being used in some softwood plywood mills as a separate
operation preceding hot pressing of panels. In this process a stack of panels is
assembled and pressed for a few minutes in a cold press. Later the panels are
then individually pressed in a conventional hot press. Advantages of prepressing
claimed are: better transfer of glue from the coated to the uncoated surface,
reduction in slippage of individual veneers in the panel assemblies during sub-
sequent loading and pressing, and reduction in the amount of opening (“daylight”)
between platens because assemblies are already partially bonded and are there-