Note: Descriptions are shown in the official language in which they were submitted.
~25~
RIGID P~PE~BOAR~ CON TIMER AND METHOD
AND APPARATUS FOR PRODUCING 'THE SAME
TECHNICAL FIELD
This invention pertains generally to the field of
processes and apparatus for forming pressed paper board
products such as paper trays and plates and to the products
formed by such processes.
BACKGROUND ART
Formed fiberboard containers, such as paper plates
and trays, are commonly produced either by molding fibers
from a pulp slurry into the desired form of the container
or by pressing a paper board blank between forming dies into
the desired shape. The molded pulp articles, after drying,
are fairly strong and rigid but generally have rough
surface characteristics and are not usually coated so -that
they are susceptible to penetration by water, oil and
other liquids. Pressed paper board containers, on the
other hand, can be decorated and coated with a liquid--
proof coating before being stamped by -the forming dies
into the desired shape. Large numbers of paper plates and
similar products are produced by each of these methods
every year a-t relatively low unit cost. These products
come in many different shapes, rectangular or polygonal as
well as round, and in multi compartment configurations.
Pressed paper board containers tend to have somewhat
less strength and rigidity than do comparable containers
made by the pulp molding processes. such of the strength
and resistance to bending of a plate-like container made
by either process lies in the side wall and rim areas
53~
which surround the center or bottom portion of the
container. In plate-like structures made by the pulp
molding process, the side wall and overturned rim of the
plate are unitary, cohesive structures which have good
resistance to bending as long as they are not damaged or
split. In contrast, when a container is made by pressing
a paper board blank, the flat blank must be distorted and
changed in area in order to form the blank into the
desired three dimensional shape. Score lines are
sometimes placed around tile periphery of blanks being
formed into deep pressed products to allow the paper board
to fold or yield at the score lines to accommodate the
reduction in area that takes place during pressing.
However, the provision of score lines, flutes, or eon-
Russians in the blank may result in a formed product with natural fault lines about which the product will bend more
readily, under less force than if the product were
unfold. Shallow containers, such as paper plates, may
also be formed from paper board blanks which are not scored
or fluted, but the pressing operation will cause wrinkles
or folds to form in the paper board material at the rim and
side walls of the container at more or less random
positions; these folds, again, act as natural lines of
weakness within the container about which bending can
occur.
In the common process for pressing paper board
containers from flat blanks, a sheet or web of paper board
is cut to form the blink circular shape for a plate--
and the blank is then pressed firmly between upper and
lower dies which have die surfaces conforming to the
desired shape of the finished container. The paper board
web stock is usually coated with a liquid-proof material
on one surface and may also have decorative designs
printed under the coating. The surfaces of the upper and
lower dies have typically been machined such that, when
they begin to compress the shaped paper board blank between
them, the die surfaces will be generally spaced uniformly
apart over the entire surface area of the formed paper-
board. The lower die is spring mounted to limit the
maximum force applied to the paper board between the dies;
and this force is distributed over the entire area of the
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paper board if the spacing between the dies is uniform. In
practice, the machining of the dies is such that random
high and low spots are commonly formed on the die
surfaces, resulting in random, localized areas of the
paper board which are highly pressed while other areas are
unpressed. The dies are also generally heated to aid in
the forming and pressing operation. Paper board plates
produced in this manner have good decoration quality and
liquid resistance because of the surface coating, and are
suited to high production volume with resulting relatively
low unit cost. however, as noted above, the plates suffer
from a lower than desired level of rigidity and are
subject to greater bending during normal household use
than it perhaps most desirable.
While problems with the rigidity of pressed paper board
containers have long been known, there has heretofore been
limited success in improving the rigidity qualities of
these products in a commercially practical manner. One
example of a process intended to increase the rigidity of
pressed paper plates is shown in the Us patent to Bonnier, et
at, 3,305,434 issued February 21,1967. A process is disclosed
therein in which paper board having very highr~oisture content, in the range
of 15~ to 35% by weight, is pressed between heated forming
dies which are specially designed to allow escape of the
water vapors driven off during the pressing operation.
The paper board blank stock is thus relatively soft and
easily formed into shape. Distortion of the shape of the
soft and plowable fiberboard is prevented by driving the
forming dies to a stop at which the surfaces of the dies
are uniformly spaced apart a distance approximately equal
to or slightly less than the desired thickness of the
formed container. The shaped fiberboard material dries
under the heat and pressure applied by the dies and the
fibers within the material build up internal bonds upon
drying which help -to maintain the strength and rigidity of
the deformed portions of the paper board material. The
apparent limitations of such a process are the complex
dies required to allow release of the water vapors from
the pressed fiberboard, handling problems with high
moisture fiberboard, and slower production -times required
because of the time necessary to allow removal of the
--'1 --
! water vapor from the paper board during the pressing
operation, thereby all contributing Jo increased pro-
diction costs.
DISCLOSURE OF THE INVENTION
S The paper board container of the present invention is
formed from fibrous substrate stock in such a way that the
raised areas of the container are substantially free of
the type of fault lines which are found in paper- board
containers pressed in a conventional manner. Exemplary of
products formed in accordance with the invention is a
container having a bottom wall, an upturned side wall
extending from the bottom wall, and a rim extending from
the side wall. The bottom wall of the formed container is
substantially equal in thickness and density to the blank,
whereas the rim is preferably somewhat denser general
than the blank and is substantially denser in those areas
where folds are formed in the rim during initial shaping.
Those portions of the paper board which are folded up
during forming are substantially the same thickness as the
rest of the container, although containing more fibrous
material, and the entire surface of the rim area is
essentially smooth. The upturned side wall, or a portion
thereof, may also be densified, particularly in the areas
of the folds formed therein. The container may be formed
in the various geometric shapes used for pressed paper-
board products. The risk preferably has a downtrend edge
portion, compressed and densified, which is found to
particularly enhance the rigidity of the container
structure. The paper board stock may be coated in a
conventional manner to provide decoration and liquid-
proofing. because of the lack of voids and other fault
lines, the container of the invention will have a rigidity
at least I and often 100% greater than conventional
containers pressed from the same paper board stock.
In the method for forming a paper board blank into the
container described above, the blank material is selected
to have a snoisture content before forming in the range of
I% to 12~ by weight, and preferably 9.5% to 10.5% by
I I
weight The blank is then pressed between a pair of
mating dies having die surfaces generally conforming tote shape of the formed plate, but with the adjacent
surfaces of the dies at the rim area being closer together
than at the bottom wall area as the die Syracuse
approach. During the forming operation, the surfaces of
the two dies engage the paper board blank between them and
distort the blank into the general shape of the formed
product. Louvre, as the die surfaces continue to
approach, the more closely spaced die surfaces at the rim
engage the paper board in the area of the rim between them
before the paper board in the bottom wall portion of the
blank is firmly engaged; as a result, extremely high
compression forces are applied in the rim area and, in
particular, at any downwardly extending portions of the
rim. Compression force may also be applied to the
upturned side wall to press out wrinkles and voids created
therein during initial shaping of the container. The
¦ moisture in the paper board helps to weaken the fiber bonds
¦ 20 within the ~a~erboard, thereby allowing the fixers to
I disengage from one another and flow under the intense
! compression force applied to the rim area, particularly at
the folds. The flowing of the fibers within the fiber-
! board under pressure causes the wrinkles and other fault
¦ 25 lines within the rim to be substantially eliminated so
that, after the dies are removed from the paper board and
the bonds between fibers are reformed, the rim area of the
! formed container is a substantially integral structure.
- Under preferred conditions, the dies are maintained at
a temperature between 250 F. and 320 F. These
temperatures are found to yield the best conditions of
fiber flow and distortion under the intense pressures
applied by the dies without overheating the blank and
causing surface blisters or scorching of the paper board.
As moisture is driven out of the heated paper board, bonds
between fibers are reformed in their compressed-posi-
lions. The dies are mounted in a conventional manner,
such that the motion of the die surfaces toward one
another is stopped only by the compression of the
paper board material between them. The force applied to
the dies is limited by the spring mounting of the lower
6 ~53~2
./
die, typically at a force of at least 6,000 pounds and
preferably 8,000 pounds or more for containers in the
common 9 to 10 inch diameter range. Most of the force
between the dies is applied to the rim area of the formed
plate, yielding typical pressures in the rim area of at
least 200 pounds per square inch and even greater
localized pressures at the areas where the paper board is
initially folded.
Further objects, features and advantages will be
apparent from the following detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Fig. 1 is a perspective view of a plate-like
paper board container in accordance with the invention.
Fig. 2 is a cross-section of the container of Fig. 1
taken generally along the lines 2-2 of Fig. 1.
Fig. 3 is a cross-section of the upper and lower dies
used to press the container of Fig. 1, showing a flat
blank in position between the dies.
Fig. 4 is a simplified schematic view illustrating the
clearances between the upper and lower die surfaces of
Fig. 3 when they are adjacent and pressing the paper board
blank between them.
Fig. 5 is a photomicrograph-~140X) of a cross section
through the bottom wall portion of a prior commercially
produced paper board plate.
Fig. 6 is a photomicrograph (80X) of a cross-section
through the center of the rim portion of a prior
commercial paper board plate.
Fig. 7 is a photomicrograph (80X) of a cross-section
at a position adjacent the edge of the rim portion of a
prior commercial paper board plate.
Fig. 8 is a photomicrograph (140X) of a cross-section
through the bottom wall portion of a paper board plate
formed in accordance with the invention.
Fig. 9 is a photor,licrograph (140X) of a cross-section
- through the center of the rim portion of a paper board
plate formed in accordance with the invention.
. I ~253~
/ rig. 10 is a photomicrograph (lox) of a cross-section
. at a position adjacent the edge of the rim portion of a
paper board plate formed in accordance with the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to the drawings, a paper board container
in the form of a plate is shown in perspective at 10 in
Fig. 1. This container structure will be described to
illustrate the invention, although it will be readily
apparent that -the invention can be incorporated in many
other container geometries. The form of the plate 10 is
typical of commercially produced plates now distributed in
the mass market: it has a substantially flat, circular
! bottom wall portion 11, an upturned side wall portion 12
¦ which serves to contain food and particularly juices on
¦ 15 the plate, and an overturned rim portion 13 extending from
the side wall. the plate portions 11, 12, and 13 are
formed integrally with one another. The distinctions
between the portions may be best illustrated with respect
to the cross-sectional view of Fig. 2. The flat bottom
wall 11 of the plate extends to about the position in the
plate denoted at 15, at which the side wall 12 begins
rising upwardly; the upturned side wall 12 terminates a-t
: about the position marked 16 in Fig. 2, at which the
paper board begins to curve over and downbeat a smaller
25 , radius to form the overturned rim 13 which terminates at
a peripheral rim edge 17.
The rim 13 serves a number of purposes in the paper
plate product.. It provides a more anesthetically pleasing
appearance than would a plate which simply had an upturned
side wall -terminating in an edge, and i-t provides a
generally lateral area which can be gripped by a-user when
carrying the plate. From the standpoint of the structural
integrity of the plate, the most important function. of the
rim 13 is to make the plate rigid and resistant to bending
when held by a user. us is apparent from an examination
of the cross-sectional view of Fig. 2, the vaulted shape
of the overturned rim 13 provides a structure which is
naturally resistant to bending about any radial axis
extending from the center of the plate. If the paper board
3~2
forming the rim portion 13 is unitary and cohesive, the
plate will resist bending in the hand of a user until the
I plate is loaded so heavily that the paper board in the rim
13 is under tensile stress sufficient to cause the
¦ 5 paper board to yield and buckle. The maximum tensile stress
in the plate under normal loading will lie across a
i generally radial cross section through the rim area.
While the theoretical maximum load carrying
capabilities of a paper plate are related to the tensile
strength of the paper board of which -the plate is made,
¦ plates made by the conventional blank pressing process are
¦ found to have much lower load carrying capabilities than
¦ might be expected, due to folds and wrinkles formed in the
rim. These folds and wrinkles naturally occur in the
incipient rim during forming to accommodate the decrease
in area of the rim as it is being drawn radially inwardly
during formation of the upwardly turned wall 12. The
wrinkles or folds extend radially over the rim and usually
extend through a portion of the upwardly turned side wall
12, which is also somewhat shrunk in surface area. The
I wrinkling or folding of the rim material produces a
disruption of the fiberboard material at the fold,
breaking many bonds between fibers, and results in a
radial fault line in the Rome natural hinge which is
much less resistant to stresses produced by loads on -the
plate than -the original paper board. Since such wrinkling
is inevitable in normal pressing processes, it has
heretofore not been considered feasible to significantly
increase the rigidity of plates pressed from flat
paper board blanks. Paper board blanks, especially those to
be deep pressed, are commonly provided with plurality of
radial score lines to control the number and position of
the wrinkles in the formed product, but such score lines
do not increase the rigidity of the final product and, in
fact, usually -lend to decrease rigidity in shallow pressed
products compared to containers which are not scored.
The paper board plate lo of the invention is also
formed from a unitary flat blank of paper board stock,
either scored or unsquared, and thus must also undergo
I folding in the side wall 12 and rim 13. The resulting
fold lines are shown for illustrative purposes at 20 in
I
Fly. 1. However, the plate 10 is produced in such a way
that the paper board in the vicinity of the rim portions of
the folds 20 is tightly compressed and essentially bonded
toc3ether so that the folds 20 in the rim do not present
natural hinge lines or lines of weakness and, in fact,
haze a tensile strength substantially similar to that of
the integral paper board. As described further below, the
! paper board material in the rim 13 is downside at the
¦ folds, and any voids or disruptions formed in the rim
areas of the folds 20 during the pressing operation are
compressed out and new bonds are formed between the
tightly compacted fibers in these areas. The entire rim
¦ is preferably densifiec1 and slightly reduced in thickness
¦ compared with the bottom of the plate. As shown in the
cross-sectional view of Fig. 2, in which the dimensions
are exagc3erated for pursues ox illustration, the thick-
news of the plate 10 at the flat bottom wall 11 and the upturned side wall 12 is essentially the same as that of
¦ the nominal thickness of the unpressed blank from which
j 20 the plate is made. Louvre, beginning at about the point
denoted in Thea intersection between the side wall
portion 12 and the rim portion Thea paper board density
increases and the thickness of the paper board decreases
out to the rim edge 17. In particular, the entire
downwardly extending portion of the rim the portion of
the rim from the top 21 to the edge issue thus preferably
compressed to a thickness somewhat less than the thickness
of the bottom wall. The material of the rim is common-
surately denser than the paper board material in the
remainder of the plate, and the areas of the folds 20 are substantially denser than the bottom wall. Generally the
paper board of the blank preferably has a nominal caliper
in the range of 0.010 inch to 0.040 inch with a basis
weight in the range of approximately 100 pounds to 400
pounds per 3,000 square feet. The density of the
paper board in the bottom wall and side wall portions is
preferably in the range of 10.3 pounds per 0.001 inch
caliper per ream (3,000 square feet).
Containers formed in accordance with the invention
have much treater rigidity than comparable containers
formed of similar paper board blank material in accordance
f
I
.,~ --I o--
with the prior art processes. To provide a comparison of
the rigidity of various plates formed in the configuration
¦ of the plate 10, a test procedure has been used which
measures the force that the plate exerts in resistance to
¦ 5 a standard amount of deflection. The -test fixture
¦ utilized, a Marks II Plate Rigidity Tester, has a wedge
¦ shaped support platform on which the plate rests. A pair
of plate guide posts are mounted to the support platform a-t
¦ positions approximately equal to -the radius of the plate
from the apex of the wedge shaped platform. The paper
plate is laid on the support platform with its edges abutting
the two guide posts so that the platform extends out to the
center ox the plate. A straight leveling bar, mounted for
up and down movement parallel to the support platform,
is -then moved downwardly until i-t contacts the top of the
rim on either side of the plate so that the plate is lightly
held between the platform and the horizontal leveling bar.
The probe of a movable force gauge, such as a Hunter Force
Gauge, is then moved into position to just contact the
top of the rim under the leveling bar at the unsupported
side of -the plate. The probe is lowered to deflect the
rim downwardly one-half inch, and the force exerted by the
deflected plate on the test probe is measured. For typical
prior commercially produced 9 inch paper plates similar in
shape to the plate 10, rigidity readings made as described
above generally averaged about 60 grams or less (using the
Blunter Force Gauge), whereas the plate 10 as shown in
Figs. 1 and 2, and formed in the manner described below,
can be produced with average rigidity readings of a-t least
30 90 grams and generally over 100 grams.
Fig. 3 shows a cross-section of -the upper die 25 and
lower die 26 which are utilized to press a flat, circular
paper board blank 27 in-to the shape of the plate 10. The
construction of the dies 25 and 26, and the equipment on
which they are mounted is substantially conventional; for
example, as utilized on presses manufactured by -the
Peerless Manufacturing Company. To facilitate the holding
and shaping of the blank 27, the dies are segmented in the
manner shown. The lower die 26 has a circular base
I portion 29 and a central circular platform 30 which is
mounted -to be movable with respect to -the base 29. The
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platform 30 is cam operated in a conventional manner and
urged toward a normal position such that its flat top
forming surface 31 is initially above the forming surfaces
32 ox the base 29. The platform 30 is mounted for sliding
S movement to the base 29, with the entire base 29 itself
being mounted in a conventional manner on springs (not
shown). Because the blank is very tightly pressed at the
peripheral rim area, moisture in the paper board which is
driven therefrom during pressing in the heated dies cannot
readily escape. To allow the release of this moisture, at
least one circular groove 33 is provided in the surface 32
of the base, which vents to the atmosphere through a
passageway 34.
Similarly, the top die 25 is segmented into an outer
lo ring portion 35, a base portion 36, and a central platform
37 having a flat forming surface 38. The base portion has
curved, symmetrical forming surfaces 39 and the outer ring
35 has curved forming surfaces I The central platform
37 and the outer ring 35 are slidingly mounted to the base
39 and biased by springs (not shown) to their normal
position shown in Fig. 3 in a commercially conventional
manner. The die 25 is mounted to reciprocate toward and
away from the lower die 26. In the pressing operation,
the blank 27 is first laid upon the flat forming surface
31, generally underlying the bottom wall portion 11 of the
plate to be formed, and the forming surface 33 makes first
contact with the top of the blank 27 to hold the blank in
place as the fornling operation begins. Further downward
movement of the die 25 brings the spring biased forming
surfaces 40 of the outer ring 35 into contact with the
edges of the blank 27 to begin to shape the edges of the
blank over the underlying surfaces 32 in the areas which
will define the overturned rim 13 of the finished plate.
Ilowever, because the ring 40 is spring biased, the
paper board material in the rim area is not substantially
compressed or distorted by the initial shaping since the
force applied by the forming surfaces 40 is relatively
light and limited to the spring force applied to the die
segment 35. Eventually, the die 25 moves sufficiently far
down so that the platform segments 30 and 37 and the ring
segment 35 are fully compressed such that the adjacent
.
/ -12-
portions of forming surfaces 38 and 39 are coplanar and
the adjacent portions of surfaces 39 and 40 are coplanar,
and, similarly, that the forming surface 31 is coplanar
with the adjacent portion of the forming surfaces 32. The
upper die 25 continues to move downwardly and thus drives
the entire lower die 26 downwardly against the force of
the springs (not shown) which support the die 26. At the
full extent of the downward stroke of the upper die 25,
the dies exert a force on each other, through the formed
blank 27 which separates then, which is equal to the force
applied by the compressed springs supporting the die 26.
Thus, the amount of force applied to the formed blank 27,
and distributed over its area, can be adjusted by changing
the length of the strolls of the upper die 25.`
In a conventional manner, the dies 25 and 26 are
heated with electrical resistance heaters (not shown), and
the temperature of the dies is controlled to a selected
I level by monitoring tile temperature of tile dies with
I thermistors (not shown) Mounted in the dies as close as
possible to the forming surfaces.
In the standard prior paper plate pressing operations,
the dies 25 and 26 were machined such that the forming
surfaces 38, 39 and 40 of the die 25 were nominally
substantially parallel to the forming surface 31 and 32
of the lower die 26 at a selected spacing approximately
equal to the thickness of the blank being pressed. From a
consideration of the geometry of the die surfaces, it can
be seen that the upturned sidewall and any downturn on the
rim would receive the greatest compressive forces
initially if the selected spacing at which the Ate
surfaces are parallel is less than the blank thickness;
whereas the top of the rim and the bottom wall would
receive substantially all the compressive force if the
selected parallel spacing is greater than or equal to the
blank thickness. In either case, the force between the
dies will be distributed over the entire area of the
paper board between the dies, including the bottom wall
Welch comprises more than half the area of the pressed
plate, except where irregularities in the machining of the
die surfaces cause high or slow spots. As inquietude above,
plates pressed utilizing uniform die forming surface
Jut I
f clearances had relatively low reedit, primarily due to
the severe disruption of -the fibers at the wrinkles in the
rim of the plate.
In accordance with the present invention, the forming
surfaces 38, 39 and 40 of the upper die 25 are no-t
entirely parallel to the forming surfaces 31 and 32 of the
lower die 26 at any spacing. The preferred spacing of -the
die surfaces in accordance with this invention is shown in
the view of FicJ. 4, which illustrates a cross-section of
the two dies closely adjacent to one another -- sub-
staunchly in the position that they would be in with a
paper board blank between them during the pressing
operation. Of course, the relative spacing between
the die surfaces will depend upon the thickness of
the paper board blank being formed. However, the typo-
graph of the die surfaces can be specified, in general,
by assuming that a-t the circumferential position 41 in the
die surfaces at which -the side wall of -the plate ends and
the rim begins, the die surfaces are spiced apart a
thickness substantially equal to the nominal thickness of
the paper board blank. The die surfaces are preferably
formed such that the spacing between the surfaces
decreases gradually and continuously from such reference
position toward the rim edge of the paper board plate
formed between the dies. The location in -the die surfaces
which corresponds to the rim edge is denoted a-t 42 in Fig.
4, and the location in the die surfaces corresponding to
the -top of the rim in the formed plate is denoted a-t 43 in
Fig 4. o'er paper board plate stock of conventional
thicknesses, i.. e., in the range of 0.010 to 0.040 inch, it
is preferred that the spacing between -the upper die
surface and the lower die surface decline continuously
from the nominal paper board thickness at the location 41
to at least 0.002 inch less than the nominal thickness at
the location 43 and to a-t least 0.003 inch less -than the
nominal thickness at the rim edge location 42. The
spacings between the upper and lower dies at other points
not on -the rim, such as at the midpoint 44 of the side
wall area, a-t the middle 45 of the bend between -the bottom
wall and the side wall, at -the beginnincl 46 of the side
Wylie, and at tile bottom well 47, are preferably at- least
12~5.;~
as great as the nominal thickness of the paper board
blank. In particular, the spacing between the die
surfaces at the bottom wall is substantially greater than
the thickness of the paper~oard blank so that the bottom
wall area receives little pressure. As an example, for a
paper board blank having a nominal thickness of 0.016 inch,
satisfactory die surface spacings are: position 42, 0.013
inch; position 43, ~.014 inch; position 41, 0.016 inch;
position 44, 0.019 inch; and at positions 46, 47, and 48,
at least 0.02 inch The actual die clearances can be
measured by laying strips of solder radially across the
surface of the bottom die, pressing the dies together, and
measuring the height of the solder at various positions on
the die surface after pressing.
It will be apparent from the consideration of the die
clearances discussed above that, as the dies 25 and 26
engage the paper board blank between them, all or
substantially all of the force between the two dies will
be exerted Oil the rim area of the pressed blank, which
lies generally between the positions labeled 41 and 42 in
Fig. 4. The springs upon which the lower die 26 is
mounted are typically constructed such that the full
stroke of the upper die 25 results in a force applied
between the dies of 6,000 to 8,000 pounds. For the common
9 inch diameter (after forming) paper plate, a force
between the dies of, e.g., 7,000 pounds, would, if
uniformly distributed over the area of the plate, result
in a pressure of about 110 pounds per square inch over the
entire plate area. However, the die shapes of the
invention, as shown in Fix. 4, wherein the rim areas of
the die surfaces are spaced more closely together,
concentrate most of the force on the plate at the rim. A
typical width for the rim the distance between the lines
41 and I -for a 9 inch plate would be approximately 1/2
inch. As an example, if 7,000 pounds of force applied to
the dies were concentrated in the rim area, the pressure
applied to the paper board in the rim would be
approximately 525 pounds per square inch. Because of the
inevitable slight misalignments between the upper and
lower dies, high and low spots in the dies, and variations
in the paper board thickness, the pressure applied to the
S3d.L;2
paper board at some points on the rim will be less than
this maximum amount but almost certainly at least 200
pounds per square inch, twice the pressure that would be
unlaced upon the rim if the compressive force were
distributed uniformly over the area of the pressed plate,
as has nominally been the case in prior paper board
pressing operations.
Toe compressive forces should be even greater at the
folds in the paper board, since these areas are raised
above the rest of the paper board and contain more fibrous
Inaterial. There folded areas will comprise a small
percentage of the area of the rim, e.g., 4 to 5 percent,
so that the compressive force concentrated in these areas
may attain many thousands of pounds per square inch. This
tremendous pressure serves to greatly density the fibrous
material at the folds in the rim.
The ideal die surface configurations given above would
preferably be maintained around the entire circumference
of- the dies, so that all the die surfaces were perfectly
symmetrical. of course, in the practical Michelin of the
die surfaces, it will be not be possible to maintain
perfect sylnmetry nor will it be possible to achieve, at
any radial cross-section through a practical die, the
exact, preferred vie surface spacings specified above.
The most critical tolerances are those within the rim area
from the position 41 to the position 42. It is highly
preferred that the die clearances in the rim be uniform
along any circumferential line around the rim so that all
folded areas in the rim receive the intense compressive
forces. A satisfactory radial gradient of die surface
spacing is, for nominal paper board thickness "N" at
position 41, N -0.002 inch at position 43, and N -0.003
inch at position 42. Satisfactory results have been
obtained with dies that have been measured to conform to
this gradient within plus or minus 0.002 inch, with best
results obtained with dies maintained within 0.001 inch,
provided that the spacing between the dies at the
positions 45-47 is at least as great as the nominal
paper board thickness N and preferably 0.003 to 0.008 inch
Jo treater than the nominal thickness N.
By utilizing the die surface configurations described
above, it is possible to apply compressive forces to the
/ -16-
r rim at a magnitude capable of Costello plastic deformation
of the rim area of the plate when the other conditions of
the process are satisfied, in particular, the moisture
content of the blank being formed and the temperatures of
the dies. Under the proper process conditions, the fibers
in the rim area, particularly at the folds, apparently can
break inter fiber bonds, compress together under the very
high applied stresses, and reform inter fiber bonds. The
use of these die spacings, with high die forces (e.g.
6,000 to 8,000 pounds), results in compression of the rim
area of 15% to 20~ or more of the blank thickness,
although the fibrous material will tend to spring back
toward the unpressed thickness after the pressure is
released. Although such high stresses might be expected
to cause rippinc3 or localized tearing of the paper board in
the rim area, such does not occur; rather, the plate stock
under the rim behaves as if it were a ductile, compress
sidle material. It is found that proper moisture levels
within the paper board are a condition for such ductility
or plastic behavior within the paper board. In addition,
the dies are maintained at high, though not excessive
temperatures to aid in the pressing process.
The paper board which is formed into the blanks 27 is
conventionally produced by a wet laid paper making process
and is typically available in the form of a continuous web
on a roll. The paper board stock is preferred to have a
basis weight in the range of 100 pounds to 400 pounds per
roam (3,000 square feet) and a thickness or caliper in the
range of about 0.010 inch to 0.040 inch. Lower basis
weight and caliper paper board is preferred for ease of
forming and economic reasons. Paper board stock utilized
for forming paper plates is typically formed from bleached
pulp furnish, and is usually double clay coated on one
side. Such paper board stock commonly has a moisture
(water) content varying from 4.0% to 8.0% by weicJht.
The effect of the compressive forces at the rim is
greatest when proper moisture conditions are maintained
withal the paper board: at least 8% and less than 12%
water by weight, and preferably 9.5% to 10.5%. Paper board
in this range has sufficient moisture to deform under
pressure, but not such excessive moisture that water vapor
US
/ interferes with the forming operation or that the paper-
board is too weak to withstand the high compressive
forces applied. To achieve the desired moisture levels
within the paper board stock as i-t comes off the roll, the
paper board is treated by spraying or rolling on a
moistening solution, primarily water, although other
components such as lubricants may be added. The moisture
content may be monitored with a hand held capacitive-type
moisture meter to verify that the desired moisture
conditions are being maintained. It is preferred that the
plate stock not be formed for a least 6 hours after the
moistening operation to allow the moisture within the
paper board to reach equilibrium.
Because of the intended end use of paper plates, the
paper board stock is typically coated on one side with a
' liquid-proof layer or layers. In addition, for aesthetic
', purposes, the plate stock is often initially printed
before being coated. As an example of a typical coating
material, a first layer of polyvinyl acetate emulsion may
be applied over the printed paper board with a second layer
of nitrocellulose lacquer applied over the first layer.
The plate stock is moistened on the uncoated side after all
of the printing and coating steps have been completed.
! on -the typical forming operation, the web of
paper board stock is fed continuously from a roll through a
cutting die (not shown) to form -the circular blanks 27,
which are then fed into position between the upper and
lower dies 25 and 26. The dies are heated, as described
above, to aid in the forming process. It has been found
that best results are obtained if the upper die 25 and
lower die particularly the surfaces thoroughfare
maintained a-t a temperature in -the range of 250 F. to
320 F. and most preferably 300 F. plus or minus 10 F,
These die temperatures have been found -to facilitate the
plastic deformation of paper board in -the rim areas if the
paper board has the preferred moisture levels. At these
preferred die temperatures, the amount of heat applied to
-the blank is apparently sufficient to liberate the
moisture within -the blank under the rim and thereby
I facilitate the deformation of the fibers without
overheatincJ the blarlk and causing blisters from liberation
/
of steam or scorching the blank material. It is apparent
that the amount of heat applied to the paper board will
vary with the amount of time that the dies dwell in a
¦ position pressing the paper board together. The preferred
die temperatures are based on the usual dwelt times
encountered for normal production speeds of 40 to 60
pressings a minute, and commensurately higher or lower
temperatures in the dies would generally be required for
hither or lower production speeds, respectively.
The characteristics of a paper container produced in
accordance with the present invention may best be compared
with prior paper board containers formed of similar
materials by exarilining the photomicroyraphs of Figs.
5-10. Figs. 5-7 show various cross-sections through a
paper board plate made in accordance with the prior come
Marshall practice in which the die surfaces are uniformly
spaced; whereas Figs. 8-10 are cross-sections through a
paper plate made in accordance with the present invent
toil. Both paper plates were formed of 170 pound per ream
(3,000 square feet), 0.016 inch caliper, low density
bleached plate stock, clay coated on one side, printed on
one surface with standard inks, coated with a first layer
of polyvinyl acetate emulsion and overreacted with a
nitrocellulose lacquer. The density of the paper board
stock, in basis weight per 0.001 illCh of thickness,
averages about 10.3, and the Tuber Stiffness of the
paper board ranges, with the grain, from about 110 to 300,
and across the grain, from about 55 to 165.
The view of Fig. 5 (140X) is through the center
portion of the prior plate structure. It may be observed
that there are numerous voids within the fiber structure,
indicating that the board is not substantially compacted,
although the fiber distribution is relatively uniform.
The thickness of the cross-section is about 0~016 inch.
Fig. 6 (80X) is a cross-sectional view through the rim
area of the prior plate, generally cut along a circus-
ferential line at about the top of the rim The
particular view of Fig. 6 is through one of the areas in
the rim which has a fold or wrinkle in it. As is
I graphically apparent from an examination of Fig. 6, the
paper board at the wrinkle has been badly disrupted,
I
/ -19-
leaving large voids between the fibers, with adjacent
fibers ripped apart, so that a fault line or very weak
area exists within the paper board at the fold. In
addition, it is clear that the surface of the paper board
at the wrinkle is discontinuous, with a large gap existing
between adjacent portions. The thickness of the cross-
section at the fold is about 0.026 inch and is greater
than the original thickness for some distance away from
the fold. Fig. 7 (80X) is a cut through the rim,
lo generally along a circumferential line at a position very
close to the edge of the rim. This cut shows the term-
nation of the one of the wrinkles running through the rim
in the prior plate. Again, in the area of the wrinkle
there are wide voids and a rough, discontinuous surface
structure. The thickness is about 0.020 inch maximum, at
the fold.
The view of Fig. 8 (140X) is a cross-section through
the approximate center of a plate made in accordance with
the present invention. A comparison of Fig. 8 with Fig. 5
shows that the structure of the paper board at the center
of the pressed plates is substantially similar in both
cases; both have relatively even surfaces and substantial
voids distributed throughout the matrix of fibers within
the-board which is characteristic of the unpressed, low
density paper board stock material from which the pressed
plates are made. The average thickness is about 0.016
inch. Fig. 9 (lox) it a photomicrograph taken along a
cut through the top of the rim portion of a plate made in
accordance with the invention, with the cut lying along a
circumferential line through one of the folded or wrinkled
areas of tile pressed plate. The contrast between Fig. 9
and Fig. 6 is significant. The paper board in the area
through which the section of Fig. 9 was taken is highly
compacted, leaving very little empty space between the
fibers; the structure of this folded region is in marked
contrast to the folded regions of Fig. 6 in which there
are gapping voids between fiberboard which account for the
badly weakened condition of the rim in this area. The
paper board in the rim shown in Fig. 9 has been compacted
and its density increased so that the paper board is
clearly denser than at the center region shown in Fig. 8.
-20-
/ The maximum thickness of this cross- section, occurring at
the two folds shown, is about 0.017 inch, substantially
the same as the bottom wall. Away from the folded areas,
the thickness of the rim is about the same as or somewhat
thinner than the bottom wall. Since the folded-over areas
contain substantially more solid fibrous material than the
rest of the paper board; perhaps 40 to 100% more, the
density of the folded areas is substantially greater than
the remainder of the paper board.
The surfaces of the paper board of Fig. 9 are
essentially smooth and continuous, in contrast again to
the discontinuity of surfaces shown in the view of Fig. 6,
and the folds within the paper board of Fig. 9 have been
turned back upon themselves and the folded-over surfaces
have been squeezed tightly together. The bottom surface,
in particular, of the slice shown in Fig. 9 is smooth and
continuous, rather than being disrupted at the wrinkle
lines as shown in Fig. 6. The coating which covers the
top surface of the plate is clearly visible in the view of
Fig. 9, and this coating well illustrates where the folds
began to occur in the rim of the plate as the plate was
being formed. Louvre, the extreme high pressure applied
to the rim of the plate has caused virtually all traces of
the fold to disappear at the bottom portion of the
paper board where the fibers of the paper have been
essentially bonded together, leaving only the vestigial
trace of the fold remaining in the top of the paper board
where the coating on the surface prevents the inter-
mingling of fibers. The heat and pressure applied during
I the forming process may be sufficient to cause some melting and surface adhesion between the abutting coated
surfaces which lie along the fold lines, although the
nitrocellulose outer coating is resistant to heat and
pressure.
cross-section through a plate of the invention taken
just inside of the rim edge is shown in Fig. 10 lucks).
Lowry again, it is seen that the fibers within the plate
are substantially compacted, and virtually all evidence of
the folds that existed in the rim area during the forming
Jo operation has disappeared, except for small areas where
the overreacted tops of the folded regions have been laid
-21- 12~53~
back upon themselves. The bottom of the paper board
surface is again smooth and unbroken, in sharp contrast to
the section through the prior art plate shown in Fly. 7.
was well illustrated in Fig. 10, the fibers are tightly and
Jo closely compressed together, leaving very few voids or air
spaces, and the overall structure is densified so that
even though the rim of the plate becomes progressively
thinner as the edge is approached, as illustrated in Fig.
2, the basis weight of the paper board in this region is
lo substantially uniform because of the compaction of the
fibers. The thickness of the paper board shown in Fig. lo
is about 0.0153 inch, about 4 to I thinner than the
bottom wall. The densification of the plate in the rim
area and the laying back of the folded surface areas on
themselves to reform the rim into a substantially integral
structure results in the marked increases in plate
rigidity that have been described above.
Of course, the successful manufacture of pressed
containers in accordance with the present process requires
attention to the details of the pressing processes in
accordance with good manufacturing techniques. In
particular, it is necessary to insure that the upper and
lower dies 25 and 26 are properly aligned so that they
engage the blank between them in the desired manner. Such
alignment techniques are a normal part of press Maine-
nuance. Observations of plates pressed with the dies can
be made to insure that the dies are properly aligned,
which is evidenced by a unifor1nity in the appearance of
the downtrend edge at the rim of the plate.
It is understood that the invention is not confined to
the particular construction and arrangement of parts and
the particular processes described herein but embraces
such modified forms thereof-as come within the scope of
the following claims.
