A method of making gypsum wallboard in which a portion of the conventional foamed gypsum slurry for forming the wallboard core is fed through a relatively high speed agitator, wherein a special addition of material is thoroughly admixed therewith to increase the fluidity without adversely affecting the density or setting characteristics and this portion of the slurry is then disposed along the edge portion of the wallboard being formed, whereby a hard-edge wallboard is produced under conditions including improved uniformity of core edge setting time, density and dimensions and reduced plug-ups of feed lines through which the core-edge material is fed.

This invention relates to a method of providing improved hard-edges on 
gypsum wallboard using a method which reduces production problems. 
In the Teale U.S. Pat. No. 2,762,738, methods are described for providing a 
gypsum wallboard unitary monolithic core having edge portions of greater 
density than the main central portion of the monolithic core; which is 
also an object of the present invention. The Teale patent describes a 
preferred embodiment in which a pair of relatively small auxiliary mixers 
each deposit a stream of mixed gypsum slurry which is the same, as to 
ingredients and proportions, as the main stream of gypsum slurry from a 
main mixer, excepting only that the foam or foaming agent is omitted. 
In addition to this description of a preferred embodiment, the Teale patent 
further vaguely refers to eliminating the cellular nature of the board at 
its edge portions, and in this respect the means of accomplishing this, as 
described, is to deposit all of the cementitious slurry on a paper facing 
sheet in a foamed state and then subsequently eliminate the foam from the 
edge portions of the deposit by agitation, the use of defoaming agents, or 
otherwise. 
Subsequent to the Teale patent, it became common to divert a portion of the 
foamed gypsum slurry from a main mixer into one or two small mixers 
whereat the foamed mix was subjected to a high speed rotary agitator which 
caused the foamed slurry to become relatively denser. This denser material 
was then deposited along the edges of the wallboard core being formed. 
Several problems were common in using this high speed rotary agitator. One 
primary problem was the excessive amount of continuous accumulation of 
settable gypsum in the hose which conveyed the relatively densified foamed 
mix from the high speed rotary agitator to the edges of the wallboard core 
being formed. A second problem, which stems in part from the first 
problem, is the need to use a relatively larger hose than would be best 
for controlling the placement of the densified mix, whereby the size of 
the hard-edge portion of the board core cannot always be reduced to 
optimum desired dimensions. A third problem, which also stems in part from 
the first problem, is the degree to which the densified mix is non-uniform 
and sometimes not densified, caused by variations in the time of passage 
of different portions of material through a partially clogged hose, with 
some portions being more advanced in the setting reaction, and with some 
set accelerating effects by the presence of set material in the hose. 
One improvement over the above-described common method of forming 
hard-edged wallboard is set forth in my copending joint application, with 
Donald J. Petersen, Ser. No. 120,566, filed Feb. 11, 1980, now U.S. Pat. 
No. 4,279,673, in which the densified mix is formed in a relatively low 
speed rotary agitator, while admixing a defoaming agent therewith. 
The present invention involves admixing a freshly added quantity of a 
material which will increase the fluidity of the agitated, defoamed 
material, such as a gypsum set control retarder or a water reducing agent. 
It has now been recognized that the high speed agitators were creating an 
excessive accelerating effect on the settable gypsum present. The present 
invention involves a recognition of the fact that an effective quantity 
of, preferably, gypsum set control retarder or, alternatively, a water 
reducing agent can be added to the mix subjected to high speed agitation, 
and disposed along the edge of the wallboard, without creating wallboard 
edge portions which are still unset as the central portion of the core 
becomes set. The presence of this fresh additive in the densified mix 
eliminates or substantially reduces the accumulation of settable gypsum in 
the hose through which it is pumped, providing many advantages in the 
manufacture of hard-edge gypsum wallboard. 
It is an object of the present invention to provide a novel means for 
preventing overdrying and/or underdrying of the edges of gypsum wallboard 
while in a wallboard dryer, thus avoiding soft calcined edges, wet edges 
and cupped tapers. 
It is a further object to provide a method of increasing wallboard edge 
densities while reducing manufacturing problems, including machine wear. 
It is a still further object to provide a method of increasing wallboard 
edge densities under conditions permitting greater control and resulting 
in an improved quality wallboard, all at reduced cost.

Referring to FIGS. 1 and 2, there is shown a standard continuously moving 
conveyor 10 of a gypsum wallboard machine, upper master roll 11, lower 
master roll 12, and a lower wallboard cover paper facing sheet 13, being 
conveyed along on conveyor 10. A main mixer 14 deposits a stream of mixed 
calcined gypsum, water and foam on sheet 13 in the usual manner. 
In conventional wallboard manufacture, at least in one common mode of 
procedure, the side edges of lower facing sheet 13 are turned up at acute 
or right angles, preferably but not necessarily before sheet 13 arrives at 
the main mixer 14. Subsequently, and commonly just before the sheet 
reaches the master rolls, these upstanding edge portions of sheet 13 are 
turned inwardly over the top of the deposited core and immediately 
thereafter a top cover sheet is applied just as the facing sheet 13 and 
the deposited core reach and begin to pass between the master rolls. 
These instrumentalities for effecting the upfolding and the infolding of 
the facing sheet 13 are entirely conventional and, since they do not 
affect the principles of the present invention, they are not illustrated 
in the drawing. 
Mixer 14 may be a ball mixer, a pin mixer, a combination of both, or of any 
other suitable type. In FIGS. 1 and 2, mixer 14 is shown schematically as 
depositing a single relatively wide stream of slurry designated 15. In 
actual practice, a common expedient is to deposit several separate streams 
across the table, the plastic streams merging by lateral gravitational 
flow before reaching the master rolls. 
A pair of relatively small slurry agitators 16 and 17 are shown disposed 
just below the main mixer 14, between the main mixer 14 and the master 
rolls 11 and 12. Agitators 16 and 17 are shown depositing streams 18, 19 
of mixed slurry through hoses 20, 21, each along one marginal edge of the 
main stream or body of slurry 15. In the alternative, a single small 
slurry agitator, located wherever convenient, may be provided with two 
discharge openings or a single discharge opening with two hoses 20 and 21 
for depositing the two edge streams. Hoses 20 and 21 are preferably of 
about 3/4 inch inside diameter. 
The agitators 16 and 17 receive gypsum slurry from main mixer 14 through 
hoses 22, 23. Feed pipes 25, 26 feed gypsum set retarder into the 
agitators 16, 17, closely adjacent the entering gypsum slurry. 
The gypsum slurry from main mixer 14 contains a substantial amount of foam 
as it becomes mixed with the retarder and as it enters agitators 16 and 
17. The agitators 16 and 17 act immediately to break the foam cells in the 
slurry, releasing much of the air that was entrapped in the foam cells. As 
the slurry is agitated in the agitators 16 and 17 and as it proceeds 
therefrom, down the hoses 20 and 21, it gives up much of the contained air 
within it, becoming progressively denser. 
Considering the lengthwise direction of the apparatus of FIGS. 1 and 2, the 
agitators 16 and 17 need not be located as there shown, but may be closer 
to the main mixer 14 and, in fact, may be to the left of main mixer 14 as 
viewed in FIGS. 1 and 2, so that the side streams of slurry 18 and 19 are 
deposited in advance of the deposit of the main stream 15. 
The upstanding edges of the facing sheet 13 are folded in over the 
deposited slurry between the agitators 16 and 17 by conventional means not 
shown and an upper paper facing sheet 27 is applied over the slurry 
streams 15, 18 and 19 and over the inturned edges of facing sheet 13 at 
the master rolls 11 and 12 in a conventional manner. The subsequent 
apparatus and operation of the entire wallboard machine is entirely 
conventional in the manner in which the boards are dried and cut to 
length. 
However, because of the greater density of the gypsum and water mix at the 
board edges, the drying of the already set gypsum core is somewhat slower 
than it would be if it hadn't been defoamed and thus, even though edges 
have greater surface exposure, than the board center portion, and are 
thinner than the center, the drying at the edges is as slow as in the main 
cellular body of the board and preferably even slower, whereby there is no 
recalcination of the gypsum at the board edges. In the form of the 
invention here described, the superior quality and strength of the board 
edges is due jointly to the greater density at such edges and the added 
strength arising directly therefrom, as well as to the greater strength 
which arises from the fact that the slurry at the edges, by the relative 
slowness of its drying is not recalcined and thus is completely hydrated 
calcium sulphate. 
In the general method of wallboard manufacture referred to herein by way of 
example, the wallboards are fabricated with their front or outer faces 
down so that lower facing sheet 13 is the front facing sheet of the 
ultimate wallboard or panel, as appears from a consideration of FIG. 3 
which shows one side edge portion of such a wallboard in cross-section. It 
will be noted from FIG. 3 that the slurries 15, 18 and 19 form a single 
monolithic core of the same set cementitious material, although the edge 
portions thereof, as at 19, are denser and harder than the cellular 
central portion indicated at 15 in FIG. 3. 
The retarder used for retarding the set of the gypsum in the densified mix 
may be any of the known set retarders for gypsum. The preferred retarder 
is a hydrolyzed proteinaceous material, essentially as described in U.S. 
Pat. No. 2,865,905, and sold by National Gypsum Company as Gold Bond 
Retarder. The slurry 15 may also contain a set retarder, which may be of 
the same composition or a different composition; however, it is essential, 
in accordance with the invention, that if the densified mix is made from 
the foamed slurry from mixer 14, and it already contains some retarder, 
then a special addition of retarder must still be made just prior to or 
simultaneous with the foamed mix going into the agitators 16 and 17. 
Agitators 16 and 17 may be of any of many varied forms and construction, so 
long as they are able to agitate the gypsum slurry passing therethrough 
adequately to break down the foam and cause substantial increase in the 
slurry density. Typically a 4000 r.p.m. high speed rotary agitator, as 
shown in FIG. 4, will remove a very substantial proportion of the foam in 
a foamed gypsum slurry. Agitator 16 consists of a hollow cylindrical body 
30 having an inlet 31 at one end and an outlet 32 at the opposite end. An 
entirely closed cylindrical body 30 has a length which is about equal to 
the body radius, for example about three inches. Agitation of material 
passing through the hollow cylindrical body 30 is accomplished by a 
multi-paddle wheel 33 with a shaft 34 which extends through the wall of 
cylindrical body 30. Paddle 33 and shaft 34 are rotatably driven by a 
motor 35, preferably a variable drive having a speed of about 3000-7000 
r.p.m. 
The standard form of gypsum wallboard 28 is four feet wide and having a 
thickness of 3/8 to 5/8 inch. Normally the wallboard edge is tapered, with 
the edge of the board thinner than the center of the board, as shown in 
FIG. 3. The side streams of slurry 18 and 19 are each preferably about two 
to four inches wide, however as little as about 1/2 inch, if it could be 
kept uniform, could be effective in providing the benefits sought in 
making hard edges. With a tapered portion which is three inches wide, a 
three-inch wide stream of higher density is advantageous. The degree of 
defoaming accomplished by the invention is not critical; however, an 
increase in density by defoaming of about 10% will produce a wallboard of 
significant superiority to a wallboard having uniformly foamed core 
throughout. 
It will be understood that with different strength retarders, different 
amounts will be required. A suitable amount of Gold Bond hydrolyzed 
proteinaceous retarder for use in the two agitators 16 and 17, during the 
manufacture of a thousand square feet of 1/2 inch gypsum wallboard is 
considered to be about 3 grams. 11/2 in each agitator; however the 
advantages of the invention are expected to be provided in various degrees 
over a wide range of amounts of retarder. 
In place of a gypsum set retarder, a freshly added water reducing agent can 
be used to increase the fluidity of the defoamed mix, allowing the smaller 
3/4" inside diameter discharge hoses 20 and 21 to be used, from the 
agitators 16 and 17. Either a set retarder or a water reducing agent will 
increase the fluidity of the defoamed mix and can so function without 
adversely affecting the density of the setting characteristics. 
Suitable water reducing agents include Marasperse C-21, a powdered 
sodium-calcium lignosulfonate of American Can Company; Marasperse B-22, a 
powdered calcium lignosulfonate of American Can Company; Norlig G, an 
aqueous solution of calcium-sodium lignosulfonate of American Can Company; 
Lignosite, a powdered, or aqueous solution of, calcium lignosulfonate of 
Georgia-Pacific Corporation; or Orzan AL-50, an aqueous solution of 
ammonium lignosulfonate of Crown Zellerbach. 
Water reducing agents are commonly used to permit a decrease in the amount 
of water needed in an aqueous dispersion; however, in the present 
invention the water reducing agents function to increase the fluidiy of 
the defoamed mix, permitting faster flowing characteristics, and thus a 
smaller hose than the common prior one inch inside diameter discharge 
hoses. With the faster flowing characteristic, there is provided a more 
complete self cleaning and, consequently, little if any setting action by 
the settable gypsum and build-up of pre-set gypsum as it passes through 
the hoses. 
Having completed a detailed disclosure of the preferred embodiments of my 
invention so that those skilled in the art may practice the same, I 
contemplate that variations may be made without departing from the essence 
of the invention or the scope of the appended claims.