Note: Descriptions are shown in the official language in which they were submitted.
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CUTTING STRING
FIELD OF THE INVENTION
This invention refers to a cutting string for cutting per-
manently elastic adhesive bonds. Said cutting string is pulled
crosswise to the longitudinal axis of the bead of adhesive to
~ be cut through the same, thereby cutting it lengthwise.
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BACKGROUND OF THE INVENTION
A cutting string is a relatively simple cutting tool, and
thus is used to a large extent fcr taking out windows which are
glued, by means of a permanently elastic adhesive, to a frame,
e.g. vehicle windows, and particularly the front and rear win-
dows of motorcars.
Such cutting strings may be a repair tool which is used in
the case of need. In this case, said cutting string is pushed
crosswise through the bead of adhesive. Thereafter its ends are
provided with pulls, and ~he cutting string is manually pulled
on the inner and outer surface of the window along the bead of
adhesive.
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According to another proposal, i.e. the teaching of
DE-A2-32,15,892, the cutting string is applied to the periphery
of the glued window along the bead of adhesive and then manual-
ly or mechanically operated in the manner of a rip cord. In a
variant of this method~ the cutting string is industrially ap-
plied, ready for use, to the window before its insertion into
the frame, e.g. by embedding it at the edge of the bead of ad-
hesive or into the latter or into a neighboring rubber profile.
After taking out the window, the part of the bead of adhesive
remaining on the frame is left there and can be used as a base
for gluing the replacing window.
The conventional cutting strings, such as disclosed e.g. in
said DE-A2-32,15,892, have a circular cross section. In view of
the relatively high cohesive resistance of conventional per-
manently elastic adhesives, e.g. polyurethane based adhesives,
the cutting string should have a diameter as small as possible
and have a tensile strength (tear resistance) as high as pos-
sible, in order to produce a good cutting effect for a rela-
tively low cutting force. However, in practice such a cutting
string made from commercial string material of high tensile
strength is to be compared with a blunt knife. This means that
the string diameter is to be made relatively large in view of
the limited tensile strength. However, increasing the string
diameter provokes, due to the increased cutting resistance, an
increase of the tensile force to be applied. This makes a manu-
al cutting more difficult. Moreover, when increasing the string
diameter, the cutting resistance, and therewith the tensile
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force to be applied, increase to a larger extent than the
string diameter does. This makes that, when increasing the
string diameter, the tensile force to be applied to the cutting
string approximates the tear strength. The expression "tear
strength" is to be understood as meaning the tensile force ne-
cessary for breaking the cutting string, whereas the expression
"tensile strenght" means said tensile force divided by the
string sectional area.
A further disadvantage of a cutting string having a circu-
lar cross section is that, due to the high cutting resistance,
the cutting string warms up. This warming up reduces the tensi-
le strength. Moreover, a clean cut of the adhesive cannot be
produced with a string having a circular cross section, since
its diameter is relatively large as compared with the struc-
tural elements of the adhesive to be cut. Rather, a crumbly cut
surface is produced, which is not suitable as a base for being
combined with new adhesive, since said surface is inhomogenous
and to small. On excessive warming up of the cutting string
during the cutting process, the cut surface undergoes irre-
versible chemical modifications which further deteriorate the
adhesive properties of said part of the bead of adhesive left
on the frame. The same unfavorable effect is produced when
lowering the cutting force by heating the cutting string is
tried.
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OBJECTS OF THE INVENTION
It is an object of the present invention to eliminate the
disadvantages of the prior art described above, and to provide
a cutting string which, while avoiding the disadvantages de-
scribed above, allows the application of lower tensile forces
and produces a clean and smooth cut edge which is suitable as a
base for a new adhesive bond, when reused.
It is a further object of the present invention to provide
a method for cutting permanently elastic adhesive bonds, and
particularly of permanently elastic adhesive bonds made of
polyurethane, which method, while avoiding the disadvantages
described above, provides the advantages just stated.
It is a further object of the present 1nvention to provide
a method for taking out windows glued along their edge into a
frame by means of a bead of adhesive, and particularly of mo-
torcar windows glued into a flange.
SUMMARY OF THE INVENTION
In order to meet these and other objects, the present in-
vention makes use of a cutting string which has an edged pro-
file, and which is preferably made of a massive material having
a tensile strength of at least 1000 N/mm ! particularly of a
V2A stainless steel alloy having a tensile strength of 1700 to
2000 N/mm2.
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Preferably, use is made o~ a profile having a regular form
and as few edges as p~ssible, thus e.g. a square profile. It
was surprising to find that a cutting string having such a pro-
file, upon applying a certain pressure when starting the cut-
ting operation, by itself takes a position in which one of the
edges is pointing in the cutting direction. For this reason,
the cutting string according to the present invention acts to a
much higher degree as a knife than circular cutting string of
the same sectional area, thus requiring less power for cutting
and producing a much cleaner cut. With increasing number of
profile edges, this favorable effect gradually decreases and
approximates that of the cutting string having a circular pro-
file.
The experimental comparison of cutting strings having cir-
cular and square profiles, re~pectively, shows a remarkable
difference with respect to the course of the cutting force dur-
ing the cutting process. The course of the cutting force o~ a
cutting string having a`circular profile is uneven, showing
local force peaks`pointing to a jerky motion of the cutting
string. In contrast to this, the course of the cutting force of
a cutting string having a circular profile is regular, i.e.
without force peaks. This effect is due to said self-orienta-
tion and said leading of the cutting edge. B~ this, the danger
of breaking is much reduced.
Similarly good results can be accomplished with trigonal
and hexagonal profiles. However, in practice, the square pro-
file is preferred, since it can be produced at a much lower
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price than trigonal or hexagonal profiles, though a cutting
string having a trigonal profile actually has better cutting
properties, and cutting strings having a hexagonal profile has
a larger sectional area for the same circumcircle of the pro-
file. Thus, the choice of a square profile is an optimal cen-
tral path.
Known cutting strings in the form of ropes or cords may
deviate from the circular profile and on principle have a
higher flsxibility. However, they do by far not reach the cut-
ting effect obtainable with a edged cutting string, since e.g.
the profile edges obtainable by stranding a plurality of
threads have a bending radius which is too large if a small
number of threads is used, and lie too closely together if a
large number of threads are used. In both cases, they almost
act like a cutting string having a circular p~rofile.
The improvement intended by using a cutting string profiled
according to the present invention, as compared with a cutting
string having a circular profile, can be obtained if a string
material having a tensile strength of at least 1000 N/mm2 is
used. A further improvement can be obtained if a string materi-
al having a higher tensile strength, preferabl~ in the range of
1200 to 2500 N/mm2, is chosen. By this, the diameter of the
profile circumcircle can be diminished, thus providing suffi-
cient security reserve even if the string deflection during the
cutting process is important.
Advantageously, a highly alloyed steel, e.g a V2A stainless
steel alloy, is used as string material. Thereby, tensile
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strengths of 1700 to 2000 N/mm2 and more, are reached. Thus,
the string cross section can be diminished accordingly, and the
cutting force can be further reduced. The more the string cross
section is reduced, the higher is the flexibility of the cut-
ting string. This not only makes its pre application and its
handling during the taking out of the window more easier but
also allows higher bending loads.
Instead of metal strings, cutting strings made of plastic,
fiber-reinforced if necessary, having the abovementioned pro-
perties may be used.
Of course, the low cutting force entails a smaller input
for the pull to be applied to the cutting string. The advantage
of a distinctly lower tensile force is of particular importance
when the cutting is done manually, especially in the regions of
the window corners. In these regions, the danger of causing a
defect in paint is particularly serious, since the guiding of
the cutting string cannot sufficiently controlled if important
strenuous efforts are involved, particularly if it is addition-
ally tried to improve the cutting effect by supplemental move-
ments of the cutting string.
~ he diminished cutting force not only makes the handling of
the cutting string easier, but also warrants a smooth cut edge.
~oreover, the surface structure of the adhesive is not over-
heated and thus is much more suitable for a direct application
of a new bead of adhesive than cut edges produced by means of
conventional cutting strings of circular profile.
For usual adhesives and the usual forms of applications of
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said cutting strings, the diameter of its profile circumcircle
should not be larger than 1 mm. For the same diameter of the
profile circumcircle, the s~uare profile form has a larger sec-
tional area than the trigonal profile form. Moreover, producing
a cutting string of square profile is less expensive. In con-
trast to this, a cutting string of trigonal profile has a bet-
ter cutting e~fect and takes the position desired for the cut-
ting process already with a lower application pressure.
Some embodiments of the cutting string according to the
present invention and its operating method are schematically
shown in the annexed drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 show, in a sectional view and a view, re-
spectively, the manual cutting of the ad-
hesive bond between an motorcar window and
its frame;
- FIGS. 3 and 4 show the mechanical cutting of an adhesive
bond of the same type;
FIG. 5 shows the cutting place on the bead of ad-
hesive shown in FIG. 4 at an enlarged scale;
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g
FIGS. 6a to 6f show various phases of penetration of a
cutting string having a square profile into
a bead of adhesive;
FIG. 7 is a view of a testing apparatus for cut-
ting strings;
FIG. 8 is a sectional view along line VII-VII in
FIG. 4 at an enlarged scale;
IGS. 9 and 10 are diagrams of the course of force on cut-
ting strings having square and circular
profiles, respectively, during cutting pro-
cesses conducted in a testing apparatus
according to FIGS. 7 and 8; and
FIG. 11 . is a partial longitudinal section, at a
much enlarged scale, of a bead of adhesive
which was cut by means a cutting string of
circular profile, showing the surface
structure of the cut edge obtained in sty-
lized form.
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PREFERRED EMBODIMENTS OF THE INVENTION
Now, preferred embodiments of the present invention will be
described with reference to the drawings.
According to FIG. 1, a motorcar window 1 is nonpositively
bonded in its marginal region to a flange 3 of the window frame
4 of a car body by means of a bead of adhesive 2. The section
of adhesive 2 is made of a permanently elastic adhesive, e.g.
on the basis of polyurethane. In the present example, the cut-
ting device is a cutting string 5 having a length of e.g. 0.5 m
which, at the beginning, is pushed crosswise through the bead
of adhesive. Thereafter, each of its ends is provided with a
pull 6, one of which being situated at the outer side and the
other being situated at the inner side of said window 1 and
said car body, respectively. A freely rotatable chuck 7 is pro-
vided for clamping the cutting string 5 to the pulls 6, so that
the cutting string 5 may rotate to the position desired for the
cutting process.
By means of said pulls 6, the cutting string 5 is pulled
crosswise to the longitudinal axis through the bead of adhesive
2, thereby cutting it lengthwise. As shown in FIG. 2, the cut-
ting string 5 is alternately hold by one pull 6 and pulled off
by the other pull 6. From the position of the cutting device
shown in full lines, the pulling off the cutting string 5 is
started by means of the lower pull 6 the upper pull 6 being
held, until the cutting device reaches the position shown in
dot-dash lines. Thereafter, the cutting string 5 is held at its
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lower end and pulled off at its upper end.
Depending on the course of the bead of adhesive 2 it may be
appropriate to simultaneously pull off the cutting string 5 at
its both ends or to make short sawing movements, e.g. at the
corners of the motorcar window 1. In such cases, always the
same section of the cutting string is in action~
In the embodiment shown in FIGS. 3 and 4, for taking out
the same motorcar window, a cutting string 5 having a length of
several meters is applied to the outer rim of the bead of ad-
hesive 2. The two ends of the cutting string 5 are pulled
crosswise through the bead of adhesive 2 at a point 8, which in
the example shown is situated at the lower long side of the
window 1, and are connected via the deflection pulleys 9 to a
take-up unit 10. The latter is fixed to a crosshead 11 which is
secured to the inner side of the window 1, as shown in FIG. 4,
by means of suction cups 12. Also, each of the deflection pul-
leys 9 is secured to the window 1 by means of a suction cup 13.
The take-up unit 10 is provided, in a manner which is conven-
tional and not shown, with two winding drums which can be
driven in opposite directions and can be alternatively fixed.
Each winding drum contains one of the two end sections 14 and
15 of the cutting string 5. A crank handle or a driving motor
is provided for operating said take-up unit 10. These means are
known and not shown.
At the beginning of the cutting process, one of the end
sections of the cutting string 5, e.g. the end section 14, is
held by the take-up unit 10 while the other end section 15 is
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wound up, thereby pulling the cutting string 5 crosswise
through the bead of adhesive 2, starting at point &. The de-
flection pulleys 9 take care of a favorable cutting angle. The
term "cutting angle" refers to the angle ~ between the two
string sections 17 and l8 originating from the cutting place
16. If possible, said angle should not be larger than 90. In
general, this can be achieved by placing one of the deflection
pulleys 9, each, at the two lower window corners. As soon as
one window half is cut by the cutting process described, the
wound-up end section 15 of the cutting string 5 is held, and
the other end section 14, so far held, is wound up in order to
cut the other window half in the same way.
FIG. 5 shows an enlarged view of the cutting place 16 of
FIG. 4. The cut edge produced by the cutting process is de-
signated as 19. The tensile forces P1 and P2 acting on the
string sections 17 and 18 are not equal when the cutting string
5 is unilaterally pulled off, i.e. the tensile force Pl applied
by the take-up unit 10 to the string section 17 is larger than
P2 and therefore reaches the limit of stability of the cutting
string 5 first. on the other hand, the cutting force S exerted
by the cutting string 5 to the adhesive 2 at the cutting place
16 depends on the cutting angle o~ in the sense thatl compared
with the tensile forces Pl and P2, it decreases when said cut-
ting angle increases. This means that the tensile forces Pl and
P2 are to be increased if the cutting angle is increased in
order to produce a constant cutting force S. As for the rest,
or course the cutting speed is influencing the force values as
well.
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Rotation of the cutting string 5, which e.g. is of square
profile, into the cutting position is self-effecting according
to the operational course shown in FIG. 6 as soon as said cut-
ting string 5 is penetrating into the bead of adhesive 2 under
the influence of the cutting force S which is acting in the
direction the arrow. At the beginning, the cutting string 5 is
placing one of its flat sides onto the surface of the bead of
adhesive 2 (FIG. 6a). When the applied pressure is increased,
the string profile starts tipping to one side, due to the un-
stable equilibrium (FIG. 6b,c), until it reaches a position in
which its leading edge 20 points in the cutting direction (FIG.
6d). On further penetrating of the cutting string 5 into the
bead of adhesive 2 (FIG. 6e,f), the string profile keeps its
position, so that its leading edge 20 cuts the bead of adhesive
2 in longitudinal direction like a knife.
The testing apparatus according to FIGS. 7 and 8 shows a
cutting device in which the cutting string to be tested is used
for cutting a bead of adhesive in a manner similar to the tak-
ing out of a motorcar window. In this way, various cutting
strings can be tested not only under identical conditions but
at the same time in an application which is relating to prac-
tice.
The testing apparatus shown essentially comprises a test
piece 21 consistiny of two metal bars 22, which are bonded to-
gether by a bead of adhesive 23, and of a traction gear 24 for
pulling off the cutting string 5 crosswise through said bead of
adhesive 23. The metal bars 22, e.g., are made of aluminium and
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have a length of 280 mm, a width of 25 mm and a thickness of 4
mm. The bead of adhesive 23, which e.g. consists of an adhesive
as used for gluing motorcar windows, is 260 mm long, and 15 mm
broad and 4 mm thick as well. An angular stand 26, to which the
test piece 21 can be clamped, is provided for holding the lat-
ter. Clamping is effected by means of screw bolts 27 fastened
to said stand 26, of two clamping clips 28, and of thumbs 29.
One of the end sections 30 of the cutting string 25 is an~
chored in a chuck, here represented as a simple clamping plate
plate 31 with a clamping screw 32, while the other end section
33 of the cutting string 25 is clamped to the traction gear 24.
At the cutting place 34, between these end sections 30 and 33,
the cutting string 25 is pulled crosswise through the bead of
adhesive 23 of said test piece 21 and over one of the two de-
flection pulleys 35, 36 which are arranged on the stand 26 in a
manner that they can rotate around the shafts 37 and 38. In
total, the cutting string 25 to be tested has a length of about
700 mm.
- -The traction gear 24 is vertically moveable arranged linked
to a driving machine (not shown) which regularly moves the
traction gear 24 upwards from the position shown in FIG. 7. The
movement is executed along two vertical drag rods 39 on which a
crosshead 40 carrying the traction gear 24 is slidably mounted.
The drag rods 39 and the stand 26 fixed ko a common base 41
(only indicated in FIG. 8).
The testing procedure is carried out as follows: The trac-
tion gear 24 is lifted at a constant drawing-off speed of 500
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:
mm/min, and the tensile force F applied by the driving machine
is measured in function of the distance L covered by the trac-
tion gear 24. The cutting angle ~ and the cutting forcP can
easily be calculated from these data and the geometrical arran-
gement of the the testing apparatus. As for the rest, the tests
were continued until the cutting string 25 broke, and the ten-
sile force F necessary for such breaking, i.e. the tear force,
was measured.
The test results obtained under the same working aonditions
with cutting strings of square and of circular profile, re-
spectively, are shown in FIGS. 9 and 10. The cutting string
having a square profile was made of a V2A stainless steel alloy
having a tensile strength o 1700 to 2000 N/mm2, the length
of the profile edge being 0.60 mm. The cutting string having a
circular profile was made of spring steel C of a diameter of
0.60 mm and a tensile strength of 2400 to 2700 N/mm2 (accord-
ing to DIN 2076).
It turned out that in a selected range of the distance L of
100 to 300 mm the average tensile force F is practically con-
stant, independently of the varying cutting angle OC. This fact
allows comparing different cutting strings in a simple manner.
For this purpose, an average value of the tensile force over
the said distance range was calculated from the force peaks.
This average value was 244.2 N for a cutting string having a
square profile and 304.3 N for a cutting string having a cir-
cular profile. Furthermore, a tear force of 602.8 N for a cut-
ting string having a square profile and of 620.9 N for a cut-
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ting strinq having a circular profile was measured. From this
values follows a proportion of tensile force to tear force of
about 40% for the cutting strings having a square profile and
of about 49% for the cutting strings having a circular profile.
This means that, compared with a cutting string of circular
profile, for the cutting string having a square profile the
necessary tensile force is farer from the accompanying tear
force. Therefore the danger of a break of the string is smaller.
Moreover, the diagrams of FIGS. 9 and 10 show the different
course of force in the cutting process. Whereas the course of
the tensile force for a cutting string having a square profile
is rather constant, the tensile force for a cutting string hav-
ing a circular profile is subject to relatively heavy varia-
tions of the tensile force. Accordingly, important differences
of the cut picture were noted. Whereas the cutting string haY-
ing a square profile produced a very fine and smooth cut edge,
the cut edge produced by the cutting string having a circular
profile ~as coarse and crumbling, as schematically;shown in
FIG. 11. This figure shows the remaining part 42 of the bead of
adhesive. Projecting parts 43 were formed at its surface by the
cutting process using a cutting string of circular profile.
~hese projecting parts will prevent the new, generally viscous
adhesive 44 (dot-dashed line) from accessing to the surface
parties which are intermediate and partially lying underneath.
A consequence of this is that only an insufficient adhesive
area is available for the new adhesive bond.
