Device for measuring bending strength of sheet material

A device for measuring the natural bending strength of a sheet material, and for measuring the bending strength of a creased or scored portion of the sheet material, and for dynamically analyzing such test results. The device has a rotatable clamp, a load sensor, a data processing unit, and a data displaying unit. The load sensor is positioned to directly contact the sheet material being held by the clamp. The clamp and sheet material can be rotated so that the sheet material presses against the load sensor, and the data processing unit will then determine the resistance force detected by the load sensor and will determine the amount of rotation associated with that force, and the resulting data are then processed, stored, and can be displayed as output data in a predetermined form.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for measuring the bending 
strength of sheet material such as corrugated fiberboard or paperboard, at 
a creased, scored, or blank portion thereof. 
Typically, to manufacture containers, suitable sheet material can be 
subjected to a printing process and cut into a desired shape by means of a 
die board. The die board can also make slits, scores or creases, which aid 
in the formation of a container by reducing the force necessary to bend 
the sheet into a desired configuration. Typically, the creased portions of 
a sheet material can be guide lines for folding the sheet material and 
creating flaps from portions of the sheet material, which can then be 
configured or folded to make a box or other type of container. 
Regarding the various means of creasing or scoring the sheet material, the 
warping force of the creased portion varies with the methods selected for 
making the crease. For example, when a crease is prepared as a series of 
holes by means of a laser beam, the warping force of the creased portion 
varies with the differences in size of each hole and with the differences 
in spacing between adjacent holes. The warping force of the creased 
portion also varies with the clearance between the edge portion of the 
sheet and the portion to be folded or creased. In addition, the warping 
force of the creased portion also varies with the materials used in the 
corrugated fiberboard and with the conditions of the formed crease. 
As a consequence of the aforementioned variations in the warping force of 
the creased portion, the following problems can occur. In a case where the 
boxes are used for packing a wide variety of products on packing lines 
under automatic control, the step of fixing the top and bottom flaps of 
each box together by adhesive tape is performed as a last step in the 
packing process. The height of the empty box depends on the warping force 
of the creased portion, the box's own weight, and the symmetry or 
proportion of the box's shape or dimensions. When the warping force of the 
creased portion is too high, each flap of the corrugated box tends to 
extend higher than the acceptable height or width of the box with respect 
to the packing line. If the flaps of the corrugated box extend too high, 
it is difficult to place the box correctly on the line and to prevent it 
from becoming entangled with equipment along the line, which may cause the 
packing line to be stopped. 
Recently, automatic high-speed packaging apparatuses have been introduced 
in the food and drink industries, and containers made of paperboard or 
corrugated fiberboard used in those fields can suffer serious damage 
depending on the quality of the creases formed thereon. Accordingly, 
manufacturers of paper containers and corrugated fiberboard have tried to 
develop a quality control method with a particular emphasis on the 
strength of the crease. 
The strength of any portion of the sheet material, except the creased 
portion, is directly related to the strength of the resulting container. 
It is therefore important to understand the bending strength of the sheet 
material independent of the bending strength of the creased portions of 
the sheet material, so that the relationship between the two can be 
established for each type of sheet material, and from the point of quality 
control. 
SUMMARY OF THE INVENTION 
The present invention provides a device for measuring the bending strength 
of various sheet materials and for measuring the bending strength of 
creased or scored portions thereof. In addition, the present invention 
provides a portable device for measuring the bending strength of sheet 
material. 
Specifically, the present invention provides a device for bending sheet 
material and measuring the force required to naturally crease the material 
as it is bent, comprising: a clamp for securing one end of the sheet 
material, the clamp having a tip that extends perpendicularly to the 
predetermined bending direction; a load sensor positioned to contact a 
portion of the sheet material; a means for rotating the clamp; a means for 
determining the angle of clamp rotation necessary to cause the sheet 
material to crease; a means for measuring the force required to naturally 
crease the sheet material by dynamically analyzing data obtained from the 
load sensor and the means for determining the angle of clamp rotation; and 
a means for displaying the data. The present invention also provides a 
portable device as described above. 
By using the device of the present invention, bending strength of 
corrugated fiberboard, paperboard, or a creased or scored portion thereof, 
can be easily measured in any location at any time. Accordingly, the 
device of the present invention contributes to the control of qualities of 
these sheet materials and also facilitates sales thereof. The above and 
other objects, effects, features, and advantages of the present invention 
will become more apparent from the following description and the 
accompanying figures.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be described in detail with respect to preferred 
embodiments, and changes and modifications may be made without departing 
from the invention in its broader aspects. Accordingly, it is the 
intention to cover all such changes and modifications as fall within the 
true spirit of the invention. 
FIGS. 1 to 3 illustrate a first embodiment of a device of the present 
invention for measuring bending strength of a sheet material. In these 
figures, FIG. 1 is a top plan view, and FIGS. 2 and 3 are front 
elevational views of an embodiment of a device of the present invention. 
The device shown in these figures is compact and lightweight so as to be 
easily carried or moved to any place at any time. 
As shown in FIGS. 1 to 3, the device comprises a base 11 in which a data 
processing unit (not shown) is embedded. The data processing unit is 
provided for processing the results obtained by measurement to print them 
as characteristic values including a yielding value (gf), a yielding angle 
(deg), an initial gradient (gf/deg), and a graph representing the 
relationship between a revolution angle (deg) and a resistance value (gf). 
On one portion of the base 11, there is provided a clamp 12 for clamping 
one end of the corrugated fiberboard D as a test sheet. The clamp 12 is 
rotatably connected to a shaft 13 as the center of the rotation. In 
addition, the shaft 13 is rotatably supported by bearing pads 14, 15 which 
are fixed on both sides of the clamp. The clamp 12 comprises a fixed click 
12a being secured to the shaft 13 and a movable click 12b that performs 
closing and opening with respect to the fixed click 12a. The movable click 
12b is supported by a pair of pins 16 and a clamp-adjusting screw 17 so as 
to be movable only in the direction of opening and closing. Thus the 
movable click 12b can be opened or closed by revolving the clamp adjusting 
20 screw 17. The clamp 12 of the present embodiment is a ratchet type 
clamp that can be adjusted so the force exerted on the movable click 12b 
toward the fixed click 12a does not exceed a predetermined level. The 
adjustment of the clamp prevents the corrugated fiberboard D held therein 
from becoming compressed or deformed. 
As shown in FIGS. 1 to 3, one end portion of the shaft 13 protrudes from 
the bearing pad 14 and is operatively connected to a handle 18 for 
revolving the clamp 12. The handle 18 can be fixed to an end portion of 
the shaft 13 in the direction of revolution by means of a handle-fixing 
screw 19. Near the center of the base 11, there is provided a load sensor 
20 having a load sensing portion 20a. As shown in the figures, the load 
sensor 20 protrudes upward from the surface of the base 11, while the load 
sensing portion 20a extends in the direction corresponding to the width of 
the test sheet. Furthermore, the load sensor 20 is secured on a mobile 
platform 21 which can be moved along a guide rail 22 toward either side of 
the device. After the movement, the mobile platform 21 can be fixed at a 
predetermined position by means of a fixing screw 23. 
For measuring bending strength of the corrugated fiberboard by using the 
device described above, a sample test sheet of corrugated fiberboard, with 
a size of 50 mm in width and a measuring length of 40 mm or more in 
length, was prepared. Then one end thereof is held by the clamp 12. In 
this condition, the handle 18 is in the position shown in FIGS. 2 and 3, 
while the free end portion of the test sheet D is in the position above 
the load sensing portion 20a of the load sensor 20 so as to be spaced 
therefrom (see FIG. 3). Then the handle 18 is revolved in a predetermined 
angle between about 90.degree. and about 180.degree. in a counterclockwise 
direction. As the handle 18 starts to revolve, the end of the test sheet D 
touches the load sensing portion 20a of the load sensor 20. The 
measurement can be completed by further revolving the handle in the 
predetermined angle from that position. A force f (a resistance value) 
applied on the load sensor 20 is measured so as to correspond to the 
revolving angle, which can be determined by an angular displacement 
sensor, not shown. 
According to the present embodiment, as shown in FIGS. 1 to 3, there is 
provided a printer 24 on the opposite side of the base 11 with respect to 
the handle 18 equipped thereon. The printer 24 is able to print out the 
results processed by the aforementioned data-processing unit in accordance 
with the data obtained by the measurement. The results may be printed out 
in the form of a table showing the characteristic values or as a graph 
showing the relation between the revolution angle (deg) and the resistance 
value (gf). The printer 24 is detachable and attachable for portability. 
The connector for the printer 24 is in the type of RS-232C for general 
purpose use, so that it can be connected to a personal computer for the 
purpose of not only easily displaying at-a-glance charts of the 
characteristic values and the characteristic graphs as shown in the 
figures, but also for editing, comparing, and storing the data, and for 
easily carrying out an advanced analysis thereof. 
For the sake of attaining the objectives of the present invention, with or 
without the printer, the device is preferably equipped with a means for 
displaying the data in any location at any time. For example, a data 
displaying device such as a liquid crystal display can be used. 
Measurements for determining the bending strength of the corrugated 
fiberboard were performed using the device of the present invention. In 
addition, a plurality of obtained results of several samples were entered 
into a personal computer for data processing to make an assessment of 
bending strength. For further consideration, several examples of the 
output from the personal computer are shown in FIG. 4 to 6. The results 
obtained from the bending measurements are shown in the tables of FIGS. 4 
to 6 and include a yielding value (gf), a yielding angle (deg), and an 
initial gradient (gf/deg). In addition, FIGS. 4 to 6 show a graph 
representing the relationship between a revolution angle (deg) and a 
resistance value (gf). 
In FIGS. 4 to 6, the following characteristic values are defined as 
follows: 
Yielding value (gf) 
The yielding value represents a load at the time of bending the sheet 
material by revolving the handle from the position where the test sheet is 
horizontal or nearly horizontal after placing the test sheet in the 
measuring device. In addition, the yielding value is dependent on the 
stiffness of the material, so that higher the stiffness of the sheet 
material, the more likely it becomes that the higher yielding value has 
been attained. As for the corrugated fiberboard, the yielding value may 
vary according to, among other things, the effect of bonding conditions 
(i.e., bonding requirements, amount of applied paste, or the like). 
Yielding angle (deg) 
The yielding angle represents an angle corresponding to the yielding value. 
In the process of bending the blank sheet, the higher the stiffness of the 
sheet material, the more likely a lower yielding angle has been attained. 
Initial gradient (gf/deg) 
The initial gradient represents variations in the resistance from the time 
the measurement is started (i.e., at the time the revolution angle is 
almost at zero) to the time that the material yields to form a crease. The 
initial gradient is calculated from the gradient of the tangent to the 
line representing the test data on the graphs between the point at the 
beginning of the line representing where the measurement is started and 
the point on the line representing the formation of the crease, or when 
the yielding angle is attained. Therefore, the initial gradient can be 
obtained with respect to the bending of the corrugated fiberboard, 
paperboard, or the like, so that it represents the relative ease or 
difficult in bending (i.e., bending strength). 
FIG. 4 shows the data from the measurement of the bending strength of blank 
portions (not the creased portions) of ten test sheets (5.0 cm in width). 
The measuring length of the blank portions of the sample test sheets used 
to obtain the data in this Figure was 6.0 cm, which corresponds to the 
distance between the tip of the click portion 12a and 12b of the clamp 12 
and the load sensing portion 20a of the load sensor 20. The blank portion 
of each test sheet (class B) was scored in a direction perpendicular to a 
flute. 
The data shown in FIG. 5 corresponds to the results of the measurement of 
the bending strength of the creased portion of each test sheet (5.0 cm in 
width and 6.0 cm in measuring length), in which data No. 1 represents the 
measurement at the blank portion, while data Nos. 2 to 4 represent the 
measurement of the creased portions of the corrugated fiberboard (class 
B). 
The data shown in FIG. 6 corresponds to the results of the measurement of 
the bending strength of the scored portion of each test sheet (5.0 cm in 
width and 12.0 cm in measuring length) prepared from the corrugated 
fiberboard (class B). For the measurements detailed in FIG. 6, eight 
samples were used. 
For measuring the bending strength of paperboard, FIG. 7 shows a second 
embodiment of a device of the present invention. The measuring device of 
FIG. 7 is constructed in the same way as that of the aforementioned 
embodiment with the exception that follows. In the second embodiment an 
allowable load of the load sensor 20A is maintained within narrow limits 
and also the load sensor 20A is positioned at a point closer to the clamp 
12A, compared with that of the first embodiment. In addition, the clamp 
12A of the second embodiment is not a ratchet type but a normal clamp. 
FIGS. 8 to 11 represent the results of the measurement of the bending 
strength of samples of paperboard by using the device shown in FIG. 7. The 
characteristic values and definitions set forth with respect to FIGS. 4 to 
6 above are applicable to these figures. 
FIG. 8 shows the results of the measurement of eight test sheets of 
paperboard having a width of 2.5 cm and a length of 1.5 cm that were bent 
and measured using the device of FIG. 7. 
FIG. 9 shows the results of the measurement of nine test sheets having a 
length of 1.5 cm and a width of 2.5 cm that were bent and measured with 
the device of FIG. 7. In FIG. 9, the test sheets were bent so that a 
portion of each sheet was in a parallel position with respect to 
cross-grain. 
FIG. 10 shows the results of the measurement under the same conditions as 
that of FIG. 8 except that the results were obtained by measuring bending 
strength of the blank portion (data No. 1), creased portion (data Nos. 2 
and 3), and perforated portion (data Nos. 4 and 5) of the paperboard. 
FIG. 11 shows the results of the measurement under the same condition as 
that of FIG. 9 except that the results were obtained by measuring bending 
strength of the blank portion (data No. 1), creased portion (data Nos. 2 
and 3), and perforated portion (data Nos. 4 and 5) of the paperboard. 
In the device of the present invention, the handle 18 can be manually 
revolved to bend the sample material. However, the results of the 
measurement are not affected by the bending rate if the speed of 
revolution of the handle is less than about 50 degree/second. Just as in 
the case of the aforementioned embodiment, it is possible to make an 
assessment of yielding strength of the creased portion, warping force of 
the folded creased portion, mechanical characteristics of the creased 
portion, and so on. 
By accumulating and analyzing the data from the various tests, the bending 
strength of a sheet material can be determined. The bending strength of 
the sheet material can then be used to determine the strength of a 
container made from the sheet material, by using the yielding value of the 
bending of the blank portion. Thus a device of the present invention will 
provide information regarding the strength of a sheet material that can be 
used to set up and adjust a process whereby a container is formed from 
such sheet material and then filled. By providing a means of obtaining 
such information, the present invention can be used to reduce the 
incidence of containers becoming entangled with packaging equipment in an 
assembly line, and to reduce the incidence of jamming during the formation 
of containers from the sheet material.