Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method for controlling the curl of paper and a paper or
board machine line that applies the method
The invention concerns a method for controlling the curl of paper, in which
method
steam treatment and/o:r moistening of the paper web and/or equivalent
operations are
employed in order to control the curl of the paper web.
Also, the invention concerns a. paper or board machine, which comprises at
least a
headbox, a former, a press, and a dryer section, in which paper or board
machine
steam boxes and/or moistening devices and/or a one-sided drying impulse and/or
equivalent devices for the control of the curl are employed.
As is known from the prior art, in multi-cylinder dryers of paper machines,
twin-
wire draw and/or single-wire draw is/are employed. In twin-wire draw the
groups
of drying cylinders comprise t:wo wires, which press the web one from above
and
the other one from below against heated cylinder faces. Between the rows of
drying
cylinders, which are usually horizontal rows, the web has free and unsupported
draws, which are susceptible of fluttering, which may cause web breaks, in
particu-
lar so when the web i;~ still relatively moist and, therefore, of low
strength. This is
why, in recent years, ever increasing use has been made of said single-wire
draw,
in which each group of drying cylinders includes just one drying wire, on
whose
support the web runs through the whole group so that the drying wire presses
the
web on the drying cylinders against the heated cylinder faces, whereas on the
reversing cylinders or rolls between the drying cylinders the web remains at
the side
of the outside curve. Thus, in single-wire draw, the drying cylinders are
placed
outside the wire loop, and the reversing cylinders or rolls inside said loop.
It is known from experience that, if paper is dried unequalsidedly, the result
is a
tendency of curling of the sheet. When paper is dried by means of normal
groups
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with single-wire draw from the side of its bottom face and if such asymmetric
drying
is extended over the entire length of the forward dryer section, the drying
takes
place so that first the bottom-face side of the paper web is dried and, when
the
drying makes progress, the drying effect is also extended to the side of the
top face
of the paper web. Under these circumstances, the dried paper is usually curled
and
becomes concave, seen from above.
As is known from the prior art, the tendency of curling of paper is already
affected
in connection with the web formation, in particular at the sheet formation
stage by
means of selection of the difference in speed between the slice jet and the
wire and
by means of selection of the former and by means of the mode of its running,
as
well as by means of other running parameters. As is known from the prior art,
for
example, in the case of copying paper, by means of unequalsidedness of drying
in
the after-dryer a suitable initial curl form is regulated for the sheet in
order that the
curling of the paper after one-sided or double-sided copying could be
optimized. In
the case of copying paper, the reactivity of curling, i.e. the extent to which
curling
occurs per unit of change in moisture content, is affected to a greater extent
by
means of a mufti-layer structure of the paper, which is produced in connection
with
the web formation in the wet end.
With respect to the prior art related to the present invention, reference is
made to the
applicant's FI Patent No. 91, 900 (equivalent to US Pat. No. 5, 416, 980), in
which a
method is described in the dryer section of a paper machine, in particular for
reducing the tendency of curling of paper, in which method the paper web is
dried
by means of drying cylinders, against whose heated faces the paper web is
pressed
by means of a drying wire, and in which dryer section groups of drying
cylinders
are used in which twin-wire draw and/or single-wire draw is/are applied. In
said
method it has been considered novel that in the dryer section hot water steam
is
supplied substantially over the entire width of the paper web, by means of
which
steam the tensions that have arisen or that tend to arise in the fibre mesh in
the paper
web are relaxed by means of heat and moisture in, or substantially directly
after, the
area of formation of said tensions.
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In the applicant's FI .Patent Application 963734 (equivalent to US Provisional
Patent
Application 60/030, fi93) a method is described for drying a surface-treated
paper
web or equivalent in an after-dryer of a paper machine as well as an after-
dryer for
a paper machine for applying; the method, wherein, in view of compensating for
a
tendency of curling of the paper web, in the after-dryer the paper web is
dried in a
dryer group/groups snaking use of a normal single-wire draw, and that, in
connec-
tion with or after the drying, the paper web is treated by means of a
device/devices
in order to compens~~te for a. tendency of curling of the paper web, said
devices
being, for example, a steam box, a blow unit, a moistening device, and/or a
soft
calender.
On the other hand, in the applicant's FI Laid-open Publication 98, 387
(equivalent to
US Patent Application No. 08/705, 059), a method is described for the
manufacture
of a paper to be surface-treated, in particular of fine paper, as well as a
dry end of
a paper machine for applying the method. The paper web, which has been
dewatered
by pressing, is dried in a fonNard dryer section, in which drying energy is
applied
to the paper web over the entire length of the forward dryer section
asymmetrically
in the z-direction from the side of the bottom face of the web. Said stage is
carried
out by means of a number of successive groups with single-wire draw that are
open
towards the bottom, on support of a drying wire. In this way, shrinkage of the
web
both in the machine ~~irection and in the cross direction is substantially
prevented,
which shrinkage tends to occur with an increase in the dry solids content. In
connection with a web break, the paper broke is removed from below the dryer
groups that are open towards the bottom, substantially by the effect of
gravity, onto
the broke conveyor placed underneath. The paper web, which has a tendency of
curling because of the. asymmetric forward drying, is passed into an after-
dryer, in
which it is after-treated while, at the same time, moistening and/or
plastically
working, so that the tendencies of curl that arose in it in the forward drying
stage
are eliminated. As e:Kamples could be mentioned, in the after-dryer, groups
with
twin-wire draw and regulation of steam as well as steam boxes that have been
fitted
in view of controlling; the curl, as well as infra and airborne web dryers.
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In the applicant's FI Patent Application 964830 (equivalent to US Provisional
Patent
Application No. 60/032, 405), a method is described for drying of paper as
well as
a dry end of a paper machine. The method for drying of paper comprises the
following steps: the paper web to be dried is passed from the press section
into a
forward dryer section, in which the paper web is dried from the side of its
bottom
face in dryer groups that apply a normal single-wire draw, said forward dryer
section comprising exclusively single-wire groups with normal single-wire
draw, and
from the forward dryer section the paper web is passed into a finishing
section, in
which the paper web is coated/surface-sized by means of a coating/surface-
sizing
equipment, dried in an after-dryer section, in which the paper web is dried in
at least
one dryer group that applies a normal single-wire draw, after which the paper
web
is calendered in a calender and passed to a reeling station, in which the
paper web
is reeled into a machine reel, and in the method the curling of the paper web
is
controlled by means of elements and/or by means of assemblies and combinations
formed out of said elements in the area of the forward dryer section andlor of
the
finishing section. On the other hand, the dry end of a paper machine comprises
a
forward dryer section and a finishing section, which finishing section
comprises a
coating/surface-sizing equipment, an after-dryer, a calender, and a reeling
station.
The dry end of the paper machine comprises elements and/or assemblies and
combinations formed out of said elements in view of controlling the curling of
the
paper web in the area of the forward dryer section and/or of the finishing
section.
The elements for controlling the curling comprise, among other things, blowing
of
hot moist air through the wire in the forward dryer section, steam boxes
employed
in the after-dryer, a combination in which steam-treatment by means of a steam
box
is combined with a cooling cylinder, a lower support belt or support wire in
the
after-dryer, twin-wire groups employed in the after-dryer, through-drying
through
the wire in connection with at least one cylinder in the after-dryer, the
ratio of
cylinder diameter, spraying of water against the web in the after-dryer,
infrared
boxes before the calender, transfer of moist air from the forward dryer to the
after-
dryer to be blown against the web, and mechanical working of the web by means
of
a spreader bar.
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With respect to the prior art, reference is also made to the US Patent S, 557,
860, in
which a dryer section. is described which has been accomplished by means of
dryer
groups that make use of a normal single-wire draw and of a moistening device
fitted
after the dryer group:, by whose means the curl is controlled.
From the prior art, it is also known to moisten the air that surrounds a
winding
device, in which case; drying and shrinkage of the paper in the winding device
are
prevented. An excessively love relative humidity of the air surrounding the
winding
device results in uncontrolled drying and shrinkage of the paper, which again
causes
cutting off of the paper at the knife blades and makes the slitting more
difficult and
also makes the making of splices more difficult. Under excessively dry
conditions,
there may also be difficulties in meeting the requirements of dimensional
precision
imposed on a roll. This prior-art moistening system comprises moistening
nozzles
and air devices installed below the winding device. By means of these devices,
the
air that rises to the winding device is moistened and directed at the knife
blades
under controlled conditions. If' necessary, the moisture level can be adjusted
continu-
ously. Thus, in this prior-art solution, control of the curl of paper is not
aimed at.
In the prior-art solutions described above, the devices for the regulation of
curl and
the other, corresponding arrangements are fitted in the dry end of the paper
machine
before the calender. It has teen assumed that, if curl regulation operations
are
carried out after the r_alender., they have a detrimental effect on the
surface prop-
erties of the paper. However, it is a drawback of a curl regulation device,
such as
a steam box or a moistening device, fitted before the calender that, at the
same time,
the calendering result is affected to a great extent, i.e. the smoothness of
the faces
and the bulk of the paper. Thiis quite often prevents full control of the
curl. On the
other hand, the efficiency of a moistening/steam-treatment device suffers from
the
high temperature of the web. Devices fitted in the interior of the hood of the
dryer
section must also be designedl in consideration of the hot and moist
environment.
Also possible servicing work must be carried out at times of standstills.
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The curl regulation devices fitted before the calender must be placed near the
end of
the dryer section, at which time the temperature of the web and of the
surrounding
air is highest. In such a case, for example, the use of a steam box is not so
efficient
(without cooling of the web), for steam does not condense into the hot
(70...80°C)
web and, thus, does not equalize the 2-way moisture profile. When cooling of
the
web, for example cooling cylinders, is employed, the length of the dryer
section is
increased, which is not economically advisable. Also, at the end of the dryer
section,
owing to the hot web, larger quantities of steam or water are needed for
correcting
the 2-way moisture profile.
Thus. the object of the present invention is to approach the problems related
to the
control of curl from a new point of view and to suggest novel solutions
contrary to
conventional modes of thinking for said problems.
An object of the present invention is to provide a method for the control of
the curl
and a paper or board machine in which the operations necessary for controlling
and
regulating the curl can be carried out after the calender.
In view of achieving the objectives stated above and those that will come out
later,
the method in accordance with the invention is mainly characterized in that,
in the
method, the operations for controlling the curl of the paper web are carried
out after
the calendering process carried out in the calender.
On the other hand, the paper or board machine in accordance with the invention
is
mainly characterized in that the steam boxes and/or moistening devices and/or
equivalent curl control devices for the control of the curl of the paper web
are placed
after the calender.
According to the invention, the curl is regulated after the calendering
process by
means of a steam box or a moistening device or by means of any other,
equivalent
curl control arrangement in itself known, for example, in connection with a
winder,
slitter-winder, intermediate calender, Pope-type reel-up, etc. The operations
necess-
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ary for controlling the curl can be carried out, for example, as steam blowing
carried out in connection with a reel-up, in connection with an unwind stand
it is
possible to turn the reel around, in which connection the curl regulation
effect can
be applied to the opposite face of the paper web, by means of steam boxes
placed
before the reel-up, by means o~f a steam box fitted in connection with the
reel that is
being formed, by means of a combination of moistening and steam treatment
before
the reel-up, by moistening before the reel-up, by fitting, for example, an
infra heater
or an equivalent device by whose means the web is dried from the opposite
side.
By means of these novel arrangements, it is possible to act upon the curl of
the
paper web and to regulate the curl as desired. Since the curl control
operations are
not carried out until after the c:alender, the advantage is obtained that the
control of
the curl takes place as close to the final product as possible, and thereby
the effects
of all of the preceding process factors can be taken into account effectively.
The
temperature of the web is also rather low, in which case the effect of the
steam
treatment is high. Also, the devices can be placed so that they can also be
serviced,
at least partly, while the machine is in operation. Nor does a regulation of
curl
carried out after the calender, for example one-sided steam treatment, affect
the
calendering process and, thus, produce differences in the final product
between the
top face and the bottom face, for example roughness and glaze.
The operations that are carried out in accordance with the invention in order
to
control the curl are aF~plied to one side or to both sides of the web.
In the following, the invention will be described in more detail with
reference to the
figures in the accompanying drawing, the invention being, however, not
supposed to
be strictly confined to the details of said illustrations.
Figure 1 is a schematic illustration of an exemplifying embodiment of the
invention
for the control of the .curl in connection with a reel-up.
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Figure 2 is a schematic illustration of an exemplifying embodiment of the
invention
for the control of the curl in connection with an unwind stand.
Figure 3 is a schematic illustration of an exemplifying embodiment of the
invention
S for the control of the curl after the calendering process, in which
embodiment, in
view of controlling the curl, a steam box has been fitted before the reel-up.
Figure 4 is a schematic illustration of an exemplifying embodiment of the
invention
for the control of the curl after the calendering process, in which
embodiment, in
view of controlling the curl, two steam boxes have been fitted before the reel-
up.
Figure 5 is a schematic illustration of an exemplifying embodiment of the
invention,
in which, in view of controlling the curl, a steam box has been fitted before
the reel-
up and in connection with the paper reel that is being formed in the reel-up.
Figure 6 is a schematic illustration of an exemplifying embodiment of the
invention,
in which, in view of controlling the curl, two steam boxes have been fitted in
connection with the paper reel that is being formed in the reel-up.
Figure 7 is a schematic illustration of an exemplifying embodiment of the
invention
for the control of the curl after the calendering process, in which
embodiment, in
view of controlling the curl, both a moistening device and a steam device have
been
fitted before the reel-up.
Figure 8 is a schematic illustration of an exemplifying embodiment of the
invention
in which, in view of controlling the curl, a moistening device has been fitted
before
the reel-up.
Figure 9 is a schematic illustration of an exemplifying embodiment of the
invention
in which, in view of controlling the curl, two moistening devices have been
fitted
before the reel-up.
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Figure 10 is a schematic illustration of an exemplifying embodiment of the
invention
in which the curl control devices are fitted in connection with a
supercalender.
In the exemplifying embodiment of the invention that is illustrated
schematically in
Fig. 1, in view of controlling the curl, after the calender, in connection
with an on-
line or off line reel-up, the web W is unwound from the reel 27 over the rolls
22,23,24,25.26 into the reel 21. The frame constructions of the reel-up are
denoted
generally with the re Ference numeral 28, and the empty reel spools placed in
the
spool store are denoted with the reference numeral 29. Fig. 1 shows possible
different alternatives l~or placing the curl control device in connection with
the reel-
up 20. For example, below the reeling nip N between the reel 21 and the
reeling
drum 22, it is possible to place a steam blow box 22A or an equivalent device,
of
which a partial enlarged illustration is shown in the area A, from which it
comes out
that from the steam blow box 22A or from an equivalent source, steam blowings
can
be applied either into the nip N, blowing P2, or towards the reel 21 face,
blowing
P1. In accordance with further alternatives, a steam blow box or an equivalent
device 25A,26A can he fitted in connection with the alignment/guide rolls
25,26.
In the exemplifying embodiment of the invention illustrated schematically in
Fig. 2,
the paper web W is unwound in the unwind station 60 from the reel 61, and the
web
W is passed over the rolls 62,63,64 to the slitter 65, from which it is
further passed
over the rolls 66,67 to the dn;~m winder 70, in which the web W is wound into
the
paper roll 71 on support of the winding drums 68 and 69, and in connection
with the
winding also a rider roll 72 is employed. The reference numeral 73 denotes the
roll
transfer device. In view of controlling the curl, in connection with such an
assembly
of equipment, various; devices can be fitted so as to control the curl. For
example,
in connection with the: alignment roll 63 and/or 66, it is possible to fit
steam blow
boxes 63A,66A or equivalent .devices. In connection with the winder 70, in
connec-
tion with the winding drum 68, it is possible to arrange a steam box 68A, or
the
other winding drum 69 can be made into a steam roll, into which a steam blow
zone
69A is formed, or a steam pipe ? 1 A can be fitted in the space between the
winding
drums 68,69 and the paper roll 71 that is being formed. If necessary, in the
unwind
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stage, the paper roll 61 can be turned around, in which connection the effect
of the
curl regulation can be applied to the opposite side of the paper web W.
In the embodiments shown in Figs. 3 to 9, the last two groups R1,R2 with
single-
s wire draw in the dry end 30 of the paper machine are seen, over whose group
gaps
the web W has a closed draw. In the illustrations the machine direction, i.e.
the
running direction of the web W is denoted with the arrow S. The single-wire
groups
Rl,R2 are so-called normal groups, in which the, for example, steam-heated
smooth-
faced drying cylinders 10 are placed in the upper horizontal row and the
reversing
10 suction cylinders 11 are placed in the lower horizontal row. Each dryer
group R1,R2
has a drying wire 15 of its own, which is guided by guide rolls 18. The drying
wires
press the web W to be dried, on the drying cylinders 10, against the smooth
heated faces of the cylinders, and on the reversing cylinders 11 the web W
remains
at the side of the outside curve on the outer face of the wire 15. On the
reversing
15 cylinders 11 the web W is kept reliably on support of the wire 15 against
the effect
of centrifugal forces by the effect of a vacuum present in the grooved faces
of the
reversing cylinders 11 or in the perforated mantle of an equivalent suction
roll, by
means of which effect cross-direction shrinkage of the web W is also
counteracted.
The reversing suction cylinders 11 that are used are particularly favourably
suction
cylinders marketed by the applicant with the trade mark VacRollT"', which have
no
inside suction boxes and in respect of the details of whose constructions
reference is
made to the applicant's FI Patent No. 83, 680 (eguivalent to US Patents Nos.
5, 022,163 and 5,172, 491 ) . The support contact between the web W and the
drying
wire 15 is kept adequate also on the straight runs between the drying
cylinders 10
and the reversing cylinders 11 by, at least on the runs from the drying
cylinders 10
to the reversing cylinders 11, using blow-suction boxes 17, by whose means
formation of pressures induced by the wire 15 is also prevented in the closing
wedge-shaped nip spaces between the wire 15 and the cylinder 11 mantles. The
blow-suction boxes 17 are understood as blow boxes whose blowing of air
produces
a vacuum, and said boxes 17 do not communicate with sources of vacuum. In
respect of the details of the constructions of these blow-suction boxes 17,
which are
marketed by the applicant with the trade mark "UnoRun BlowBox "T"', reference
is
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made to the applicant's FI Paxents Nos. 59, 637, 65, 460 and 80, 491
(equivalent to
US Patents Nos. 4, 44'1, 263, 4, 516, 330 and 4, 905, 380) . Blow box
constructions of
other types in themselves known are also included in the scope of the overall
concept
of the present invention. In the groups Rl,R2 with single-wire draw, in the
gaps
between the reversing; cylinders 11, blow boxes 16 are also used, by whose
means
said intermediate spaces are air-conditioned and evaporation taking place from
the
web W is promoted. 'The faces of drying cylinders 10 are kept clean by doctors
14.
Further, in the embodiments ~of the invention shown in Figs. 3 to 9, the
finishing
section includes a machine reelL-up 50, such as a Pope-type reel-up. The
machine reel
that is being made by means of the reel-up 50 is denoted with the reference
MRS,
and one complete machine reel with the reference MR. The web W is brought to
the
machine reel-up 50 tl~~rough the calender 40 from the after-dryer 30 over the
guide
and alignment rolls 3:5,45,46,47. After the last guide/alignment roll 47 a
measure-
ment device 48 is fitted for measurement of the properties of the web W before
reeling. The calender 40 may be favourably a so-called soft calender, in which
one
roll can be heated and the other roll has a soft coating. Of course, the
calender may
also consist of two hard rolls. There may also be several calender nips.
In Fig. 3, in view of ~controllimg the curl, a steam box 81 has been fitted on
the run
of the web W between the guide/alignment rolls 46,47, in which case, thus, the
curl
is affected after the c:alendering process before the reel-up 50. If it is
desired to
control the tendency ~of curling efficiently in both directions, a steam box
81A or
equivalent can also be fitted above the web.
In Fig. 4, a steam box 82 has been fitted between the alignment/guide rolls 45
and
46, and a steam box 81 has been fitted on the run of the web between the align-
ment/guide rolls 46,47, the control of curl being affected by means of both of
said
steam boxes on the ruin of the web W between the calender 40 and the reel-up
50.
In Fig. 5, besides the: steam box 81 fitted on the run between the
alignment/guide
rolls 46,47, a steam box 83 has been fitted in connection with the paper reel
MRS
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that is being formed, in which case, besides in connection with the run of the
web
between the calender 40 and the reel-up S0, the control of the curl is also
affected
in connection with the making of the reel MRo that is being formed. In
connection
with the reel MRp that is being formed, a displaceable steam box 83 has been
fitted,
which is illustrated by the arrow Sgt. The box 83 is shifted as the reel size
becomes
larger when the reeling proceeds.
In Fig. 6, in connection with the reel MRo that is being formed, two steam
boxes
83,84 have been fitted, both of which are displaceable Sg3,Sg4 as the reel
size is
increased with the progress of the reeling. In order to provide regulation of
curl
from the top side, of course, also in this connection, it is possible to use a
steam or
moistening device 81A fitted above the web.
In Fig. 7, besides the steam box 81 fitted on the run of the web W between the
alignment/guide rolls 46,47, a web W moistening device 85 has been fitted on
the
run of the web W between the alignment/guide rolls 45,46, in which case the
curl
of the web W is controlled after the calender 40 and before the reel-up 50
both by
means of a moistening treatment and by means of a steam treatment.
In Fig. 8, in view of controlling the curl after the calender 40, on the run
of the web
W between the alignment/guide rolls 46,47, a moistening device 86 has been
fitted.
The figure also shows an IR dryer 87, by whose means an impulse of thermal
energy can be applied to the top face of the web so as to affect the tendency
of
curling of the ultimate product. In stead of IR drying, it is also possible to
use some
other dryer device. Likewise, said devices can be fitted at both sides of the
web.
Regulation of curl by means of one-sided additional drying can be employed
either
alone or in combination with steam-treatment and/or moistening devices.
In the exemplifying embodiment shown in Fig. 9, moistening devices 85,86 have
been fitted on the runs of the web W between the alignment/guide rolls 45,46
and
46,47 so as to control the curl of the web W after completion of calendering
before
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the reel-up 50. Of course, it its also possible to employ one moistening
device only,
or the moistening devices can be placed at opposite sides of the web.
In the exemplifying embodiment shown in Fig. 10, the regulation of curl is
carried
out in connection with a supercalender 110. The web W runs from the unwind
stand
100 to the top portion. of the s~upercalender 110 and from the bottom portion
of the
supercalender 110 further to the reel-up 120. The supercalender 110 consists
of a
vertical stack of rolls, in which the web W glazing nips consist of a hard-
faced and
a soft-faced roll. In the figure, a few possible positions (91,92,93,94,95,96)
are
indicated in which it is possible to employ steam boxes andlor moistening
devices
and/or devices that he;~t the web face intensively. Thus, the devices can be
placed on
the run (91,92) of the web W between the unwind stand 100 and the
supercalender
110 and/or in connection with the take-out leading rolls 111 in the
supercalender 110
(93,94) and/or on the run of true web W between the supercalender 110 and the
reel-
up 120 (95,96).
Above, the invention has been described with reference to some preferred
exemplify-
ing embodiments of same only, the invention being, however, not supposed to be
strictly confined to the details of said embodiments. The features suggested
in the
different exemplifyine; embodiments of the invention can be combined in a
number
of different ways for the purpose of controlling the curl. Of course, in
accordance
with the invention, avfter the calendering process, it is also possible to use
other
arrangements in themselves known to a person skilled in the art in order to
control
the curl. Many variations and modifications are possible within the scope of
the
inventive idea defined in the following patent claims.
