Note: Descriptions are shown in the official language in which they were submitted.
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Press belt in a paper-making machine
The invention relates to a press belt for a shoe-press device, having the
features
of the preamble of claim 1. The invention furthermore relates to a method for
manufacturing a plastic matrix for a press belt of a shoe-press device, having
the
features of the preamble of claim 12.
Press belts which may be configured, for example, as a closed sleeve of a shoe-
press roller or as a transfer belt which is routed as a continuously revolving
belt
between the fibrous web and the sleeve of the counter roller, are exposed to
high
mechanical, thermal and also chemical stresses. Press belts of this type
typically
are composed of a polyurethane matrix which is fiber-reinforced. The
polyurethane
matrix here may be configured so as to be single-layered or multi-layered,
such
that the press belt may display a plurality of plies or layers, respectively.
The outer
surface of the respective press belt may be provided with a structure, such as
grooves, blind holes, or similar, in order for dewatering in the press to be
optimized. On account of the high mechanical stresses, fissures may develop in
the press belts, wherein further growth of the fissures may arise, likewise on
account of the high mechanical stress. This occurrence of fissure growth may
increasingly arise also in press belts having grooves or blind holes. On
account of
fissure growth, structural and/or functional failure of the press belt may be
incurred. Moreover, the press belts are also exposed to enormous mechanical-
cum-dynamic stresses, such that the press belts additionally are subjected to
high
abrasion. Moreover, on account of the various media present in the paper-
making
machine, the press belts are exposed to intense chemical stresses in the paper-
making machine. In this way, the press belts may be in contact with, for
example,
water, oil, acids, bases, solvents, or similar, and are at least partially
corroded by
these media.
For example, a press sleeve is thus disclosed in DE19702138 A1, the hardness
and resistance to wear of said press sleeve having been increased by additives
from rock flour, ceramic, or carbon. It is proposed in DE4411620 A1 to provide
1
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only an outer layer of the press sleeve with additives which increase
resistance to
wear.
US2005287373 and US20060118261 disclose press belts which display
polydimethyl siloxane. The paper-making machine belts of W02005090429 and of
US2008081179 display nanoparticles, in order to improve resilience in relation
to
fissure growth, hardness or resilience to abrasion, for example. Paper-making
machine belts which display silicon dioxide microparticles are described in
EP2330249.
Despite the embodiments which already exist, there is ongoing demand for press
belts for a shoe-press device, having improved resilience in relation to
mechanical,
thermal, and chemical stress.
The present invention focuses on the object of providing for a press belt of a
shoe-
press device, and for a method for manufacturing a plastic matrix for a press
belt
of this type, an improved or at least an alternative embodiment which is
distinguished, in particular, by higher resilience to abrasion, a tendency
toward
fissure formation and toward fissure growth which is lower or at least not
worse,
and/or by a lower sensitivity in relation to media which are present in a
paper-
making machine.
According to the invention, this object is achieved by the subject matter of
the
independent claims. Advantageous embodiments are the subject matter of the
dependent claims.
In one aspect of the invention, a press belt for a shoe-press device for
dewatering
or smoothing a fibrous web, in particular a paper, cardboard, or tissue web,
is
proposed, in which the press belt displays a fiber-reinforced plastic matrix.
The
fiber-reinforced plastic matrix here may at least in a part-region display a
polyurethane material, and polydimethyl siloxane and silicon dioxide
microparticles
as additives.
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Advantageously, chemical resistance in relation to the media present in the
paper-
making machine can be increased on account of the combination of the additives
polydimethyl siloxane and silicon microparticles. Moreover, on account of the
listed
additives, resilience to abrasion can be improved, and the tendency toward
fissure
formation and toward fissure growth can be held low. Advantageously, by way of
adding the combination of additives, the swelling behavior of the press belt
is not
or only slightly modified.
In comparison with employing polydimethyl siloxane alone, the combination of
additives leads to improved resilience to abrasion and to increased chemical
resilience. In contrast, employing silicon dioxide microparticles alone leads
to
worsened dispersibility of the reactant of the plastic matrix and, on account
thereof, optionally to an increased tendency toward fissure formation in the
finished press belt. Moreover, it was determinable that press belts having
only
silicon dioxide microparticles as an additive displayed significantly worsened
resilience to abrasion.
It may thus be established that a balanced stress profile can be created only
by
way of the combination of additives, such that both resistance to chemicals
and
the tendency toward fissure formation and toward fissure growth, and
resilience to
abrasion, are improved in a mutually balanced ratio, without any deterioration
of
one of these properties or in the further required properties, such as the
swelling
behavior, for example, occurring.
A press belt is to be understood a belt or a sleeve which, together with a
fibrous
web, is routed through a shoe-press nip which is formed between a static press
element, the so-called press shoe, and a cylindrical counter roller. The press
shoe
is supported on a fixed yoke and is hydraulically pressed against the counter
roller.
In addition to the fibrous web and the press belt, one or a plurality of
continuously
revolving felts and/or further continuously revolving press belts may be
routed
through the press nip.
The press belt may be implemented as a press sleeve of the shoe-press roller,
i.e.
as a closed sleeve said press belt is held by two lateral tension disks and
rotates
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about the fixed press shoe. In order to reduce the friction of the press belt
on the
press shoe, oil for lubricating is applied to the inner side of the press
belt. Instead
of being guided by the two lateral tension disks, the press belt may be routed
over
the press shoe and a plurality of guide rollers, as is the case in open shoe
presses. The surface of press sleeves may be provided with grooves and/or
blind
holes.
The press belt may also be implemented as a transfer belt which, in order to
convey the fibrous web through the shoe-press nip, is routed as a continuously
revolving belt between the fibrous web and the sleeve of the counter roller.
After
the shoe-press nip, the fibrous web is then taken off the press belt with the
aid of a
suction roller, transferred to another clothing, and supplied to the
downstream
machine group. It is thus advantageous for the surface of the transfer belt to
have
adequate adhesion in relation to the fibrous web in order to reliably guide
the
latter, and for the surface of the transfer belt to have good smoothness and a
low
tendency toward marking. On the other hand, it is likewise advantageous for
the
fibrous web to be capable of being easily taken off again.
A fiber-reinforced plastic matrix is to be understood as a plastic matrix in
which
one-, two-, or three-dimensional fiber structures are embedded. The term one-
dimensional fiber structure here comprises fibers, endless fibers, yarns,
fiber
bundles, fiber strands, filaments, filament bundles, rovings, or hybrid forms.
The
term two-dimensional fiber structure comprises woven fabrics, knitted fabrics,
warp-knitted fabrics, non-woven fabrics, cross-laid structures,
unidirectionally
deposited fiber layers, multiaxial cross-laid structures, mats, knitted goods,
woven
spacer fabrics, braided hoses, embroideries, sewn goods, peel plies, or hybrid
forms. The term three-dimensional fiber structure is to be understood as being
substantially a plurality of two-dimensional fiber structures which are
layered so as
to be on top of one another. The two-dimensional fiber structures here may be
configured in different manners. In this way it is conceivable, for example,
that a
unidirectional fiber layer is followed by a non-woven fabric as the next
layer, while
a woven fabric may complete the three-dimensional fiber structure. However,
unidirectional two-dimensional fiber structures may also be exclusively used
for
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constructing a three-dimensional fiber structure. The unidirectional two-
dimensional fiber structures here may be identically oriented or be variably
oriented with respect to their direction. In the event of the latter, a
multiaxial cross-
laid structure is present.
Materials which may be employed for fiber structures are glass fibers, carbon
fibers, plastic fibers, aramid fibers, PBO fibers, polyethylene fibers,
polyester
fibers, polyamide fibers, natural fibers, basalt fibers, quartz fibers,
aluminum oxide
fibers, silicon carbide fibers, or hybrid forms.
Additives are to be understood as materials which are added to the plastic
matrix
in order to modify the properties of the latter in the desired way and manner.
In this
way, additives are added to the plastic matrix in order to, for example,
influence in
a targeted manner resilience to abrasion, a low tendency toward fissure
formation,
a low fissure growth, high resilience in relation to media present in the
paper-
making machine, such as, for example, water, oil, acids, bases, solvents, or
similar, desired surface properties, such as, for example the adhesive
capability in
relation to a fibrous web, hardness, or similar. Here, likewise properties
which are
achieved by way of the fiber reinforcement may be influenced by the additives.
In
this way, for example, pigments, microfibers, such as, for example, carbon
fibers,
glass fibers, or similar, powdered glass, carbon black, clay, montmorillonite,
saponite, hectorite, mica, vermiculite, bentonite, nontronite, beidellite,
volkonskoite, manadiite, kenyaite, smectite, bederite, silicon carbide,
silicic acid
salt, metal oxides, or arbitrary mixtures of the aforementioned compounds can
be
used as further addivitives.
A fibrous web is to be understood as a cross-laid structure or a random
structure
of fibers, respectively, comprising wood fibers, plastic fibers, glass fibers,
carbon
fibers, additives, auxiliaries, or similar. In this way, the fibrous web may
be
configured as a paper, cardboard, or tissue web, for example, which is
substantially composed of wood fibers, with small amounts of other fibers or
also
additives and auxiliaries being present.
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Furthermore, at least one further part-region of the fiber-reinforced plastic
matrix
may be configured as foam. On account of configuring a further part-region of
the
fiber-reinforced plastic matrix as foam, higher elasticity and softness of the
press
belt may advantageously be established. On account of the press belt
displaying
less hardness, the compressive force may be adjusted in a more exact manner.
Moreover, in the case of unevenness in the fibrous web or other components of
the paper-making machine, the compressive force fluctuates less intensely.
Foam
here is to be understood as bubbles which are separated by walls. If the foam
has
open pores, the walls are at least in part breached, while in a closed foam
the
individual bubbles are closed off by the walls.
The part-region which displays at least polyurethane, and polydimethyl
siloxane
and silicon dioxide microparticles as additives, or the further part-region
which is
configured as foam, may comprise a layer of the press belt, a surface layer of
the
press belt, a peripheral region of the press belt, or an inner layer of the
press belt,
for example.
If the part-region comprises a surface layer of the press belt, the press belt
may
thus be equipped with the desired surface property but, on account of layers
of the
press belt which lie on the inside and which are configured in various
manners,
may be equipped with further advantageous properties. By way of a surface
layer
which is designed in such a manner, abrasion resistance, an advantageous
fissure
behavior, and high resilience in relation to the media present in the paper-
making
machine may be achieved, for example, while sufficiently high elasticity and
tear
strength can be established by way of inner layers. In an analogous manner,
this
also applies to the peripheral regions of the press belt. An inner layer, for
example
configured from a foam, may positively influence elastic behavior and softness
of
the press belt, without the demanded high resilience of the surface of the
press
belt deteriorating.
A layer or a ply, respectively, of the press belt here is understood to be a
region
which is delimitable in the direction of thickness in relation to other layers
or plies.
Delimiting may be performed, for example, by the fiber reinforcement, by the
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construction of the plastic matrix, by the additive proportions and/or by
mechanical
properties.
Furthermore, the employed polydimethyl siloxane may display a viscosity of 100
to
100,000 mPa*s. Polydimethyl siloxane having a viscosity of 500 to 50,000
mPa*s,
optionally of 1000 to 10,000 mPa*s, in particular of 1500 to 5000 mPa*s, and
of
2000 to 3000 mPa*s, for example, may also be employed. This relates to a
temperature of 25 C.
On account of employing polydimethyl siloxane having a viscosity of this type
a
reduction of the surface interferences in the press belt may advantageously be
performed. Moreover, dispersibility of the reactants of the plastic matrix may
be
improved on account of a viscosity of the polydimethyl siloxane of this type.
The at least one part-region furthermore may display 0.1 to 10% by weight
polydimethyl siloxane. It is also conceivable that the at least one part-
region
displays 0.1 to 8% by weight, in particular 0.1 to 5% by weight, optionally
0.1 to
3% by weight, and 0.2 to 1.5% by weight polydimethyl siloxane, for example.
On account of a proportion of polydimethyl siloxane of this type the
aforementioned advantages may be advantageously achieved.
Furthermore, the silicon dioxide microparticles may display a mean particle
size of
2 to 800 pm. It is also conceivable for the silicon dioxide microparticles
which
display a mean particle size of 5 to 500 pm, in particular of 5 to 50 pm, for
example of 10 to 30 pm, and optionally of 10 to 20 pm, to be employed.
On account of a mean particle size of the silicon dioxide microparticles of
this type,
dispersibility of the reactants of the plastic matrix may be advantageously
improved.
Furthermore, the at least one part-region may display 0.01 to 10% by weight
silicon dioxide microparticles. It is also conceivable for the at least one
part-region
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to display 0.01 to 5% by weight, in particular 0.05 to 3% by weight,
optionally 0.05
to 0.5% by weight, and for example 0.05 to 2% by weight silicon dioxide
microparticles.
On account of a proportion of silicon dioxide microparticles of this type, the
aforementioned advantages may be advantageously achieved
Furthermore, silicon dioxide nanoparticles having a mean particle size of 10
to 80
nm may be employed in the at least one part-region. It is also conceivable for
sili-
con dioxide nanoparticles which have a mean particle size of 12 to 60 nm, in
par-
ticular of 14 to 40 nm, for example of 16 to 30 nm, and optionally of 18 to 25
nm, to
be employed.
On account of employing silicon dioxide nanoparticles, the tendency toward
fissure
formation may be advantageously reduced. Possibly, the tendency toward fissure
growth likewise may be reduced. Employing silicon dioxide nanoparticles alone
does in turn improve the tendency toward fissure formation, but leads to a
deterioration in the resilience to abrasion. By combining the additives,
resilience to
abrasion of the at least one part-region is increased.
Furthermore, the at least one part-region may display 0.01 to 10% by weight
silicon dioxide nanoparticles.
By means of a proportion of silicon dioxide nanoparticles of this type in the
at least
one part-region, the aforementioned advantages may be achieved.
Furthermore, the at least one polyurethane material may be manufactured from a
polyurethane polymer and a cross-linking agent. The polyurethane polymer here
may be configured as an MDI prepolymer and/or as a PPDI prepolymer having
polyether and/or polycarbonates and/or polycaprolactones as a polyol
component.
On account of a configuration of the polyurethane component of the plastic
matrix
of this type, the desired durability and resilience in relation to the wear of
the press
belt may be advantageously established. Moreover, a plastic matrix of this
type is
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distinguished by high resilience in relation to media present in the paper-
making
machine.
Furthermore, the cross-linking agent may contain at least one polyether
polyol. It is
also conceivable for linear polyether polyol, and also linear polypropylene
etherpolyol, for example, to be employed.
On account of cross-linking agents of this type, the properties of the plastic
matrix
with respect to elasticity, hardness, and resilience to media present in the
paper-
making machine may be advantageously influenced.
In a further aspect of the invention, a method for manufacturing a plastic
matrix for
a press belt of a shoe-press device for dewatering or smoothing a fibrous web,
in
particular a paper, cardboard, or tissue web as described above, is proposed.
Here, the plastic matrix is manufactured from at least one polyurethane
prepolymer, at least one cross-linking agent, polydimethyl siloxane, and
silicon
dioxide microparticles.
On account of a method of this type, press belts which display the
abovementioned advantages may be advantageously manufactured.
Furthermore, prior to cross-linking of the at least one polyurethane
prepolymer,
silicon dioxide nanoparticles having at least part of the cross-linking agent
are
mixed to form a nanoparticles mixture which contains 20 to 60% by weight
silicon
dioxide nanoparticles. It is also conceivable for the nanoparticles mixture to
contain 25 to 55% by weight, for example 30 to 50% by weight, in particular 35
to
45% by weight, and optionally 38 to 42% by weight silicon dioxide
nanoparticles.
On account of process management of this type good dispersibility may be
advantageously achieved. If silicon dioxide nanoparticles which have been
created
by a sol-gel process, wherein the OH groups on the surface of the particles
may
be blocked by means of silanization, are employed, dispersibility of the
reactants
of the plastic matrix may be further improved.
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Furthermore, the nanoparticles mixture in the downstream method steps may
replace the cross-linking agent entirely or by 5 to 40%. Furthermore, it is
also
conceivable for the nanoparticles mixture to replace the cross-linking agent
by 6 to
35%, in particular by 7 to 30%, for example by 9 to 30%, and optionally by 10
to
25%.
On account of process management of this type, dispersibility of the reactants
of
the plastic matrix likewise may be further enhanced. Furthermore, better
mixing of
the individual reactants of the plastic matrix is also possible on account
thereof.
Furthermore, prior to cross-linking of the at least one polyurethane
prepolymer, the
silicon dioxide microparticles can be mixed with the polydimethyl siloxane
and, if
applicable, with further additives, to form an additive mixture. The latter
may
subsequently be mixed with at least part of the cross-linking agent. It is
conceivable here for silicon dioxide nanoparticles to have already been
previously
mixed into the cross-linking agent.
On account of process management of this type, a homogenous mixture of the
reactants of the plastic matrix may advantageously be achieved and both
dispersibility and the mixing behavior may be improved.
The mean particle size may be determined, for example, by way of laser
scattered
light methods or by means of dynamic image analysis. By means of dynamic
image analysis, particle sizes from 1 pm to 30 mm may be determined. The laser
scattered light methods allow an analysis of particle sizes from 0.3 nm to 1
pm.
Here, the mean particle size is defined according to the measuring method
employed according to the respective size range
Further important features and advantages of the invention are derived from
the
dependent claims, from the drawings, and from the associated description of
the
figures by means of the drawings and the example. Preferred exemplary
embodiments of the invention are illustrated in the drawings and are explained
in
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more detail in the following description, wherein identical reference signs
relate to
identical or similar or functionally identical components.
In the drawings, in each case in a schematic manner:
fig. 1 shows a view of a shoe press having a press sleeve according to one
exemplary embodiment of the present invention, and
fig.2 shows a view of a press section of a paper-making machine, comprising a
shoe press and a conveyor belt, according to one exemplary embodiment of
the present invention.
In fig. 1 a shoe press 10 which comprises a shoe roller 12 and a counter
roller 14
is illustrated. While the counter roller 14 is composed of a rotating roller
which is
designed in a cylindrical manner, the shoe roller 12 is assembled from a shoe
16,
a static yoke 18 carrying said shoe 16, and a press sleeve 20. The shoe 16
here is
supported by the yoke 18 and by way of hydraulic press elements (not
illustrated)
pressed onto the press sleeve 20 revolving around said shoe 16. On account of
the concave design of the shoe 16 on that side which is opposite the counter
roller
14, a comparatively long press nip 22 results. The shoe press 10 is
particularly
suitable for dewatering fibrous webs 24. During operation of the shoe press a
fibrous web 24 having one or two press films 26, 26' is routed through the
press
nip 22, wherein the fluid which, on account of the pressure exerted in the
press nip
22 on the fibrous web 24, leaks from the fibrous web 24 and which, apart from
water, contains dissolved and undissolved compounds, such as, for example,
fibers, fiber fragments, additives and/or auxiliaries, is temporarily received
by the
press felt or felts 26, 26', respectively, and by depressions (not
illustrated) which
are provided in the press sleeve surface. After leaving the press nip 22 the
fluid
which has been received by the press sleeve 20 is thrown off from the press
sleeve 20, before the press sleeve 20 again enters into the press nip 22.
Moreover, water received by the press felt 26, 26' is removed by way of
suction
elements, after having left the press nip 22.
On account of the comparatively long press nip 22, which is due to the concave
design of the shoe 16 on that side which is opposite the counter roller 14,
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considerably better dewatering of the fibrous web 24 is achieved by such a
shoe
press as compared with a press composed of two rotating rollers, such that
subsequent thermal drying may be correspondingly curtailed. In this way,
particularly gentle dewatering of the fibrous web 24 is achieved.
In fig. 2 a detail of a press section of a paper-making machine 30 which
comprises
a shoe press 10 is shown. As is also the case in the embodiment illustrated in
fig.
1, the shoe press 10 here comprises a shoe roller 12 which comprises a press
sleeve 20 and a press element or shoe 16, respectively, and a counter roller
14,
wherein a press nip is configured between the shoe 16 and the counter roller
14.
Moreover, this part of the paper-making machine comprises two suction rollers
28,
28' and two deflection rollers 30, 30'. During operation of the paper-making
machine a felt 26, which is guided by the suction rollers 28, 28' and which
receives
the fibrous web 24 on the suction roller 28, is routed through the press nip.
Moreover, a routed conveyor belt or transfer belt 32, respectively, is routed
through the press nip by the deflection rollers 30, 30' below the felt 26
which
guides the fibrous web 24, wherein the transfer belt 32 in the press nip takes
over
the fibrous web 24 from the felt 26 and conveys away said fibrous web 24 from
the
press nip via the deflection roller 30'. On account of the pressure exerted on
the
fibrous web 24 in the press nip, fluid leaks from the fibrous web 24, which
fluid
apart from water contains dissolved and undissolved compounds, for example
fibers, fiber fragments, additives and/or auxiliaries, and is temporarily
received by
the felt 26 and by depressions which are provided in the press sleeve surface.
After having left the press nip, the fluid which has been received by the
press
sleeve is thrown off by the press sleeve 20, before the press sleeve 20 again
enters into the press nip. Moreover, water which has been received by the felt
26
is removed after leaving the press nip by suction elements which are provided
on
the suction roller 28'. On account of the comparatively long press nip due to
the
concave design of the shoe 16, significantly better dewatering of the fibrous
web
24 is achieved by such a shoe press as compared with a press composed of two
rotating rollers, such that subsequent thermal drying can be correspondingly
curtailed. In this way, particularly gentle dewatering of the fibrous web 24
is
achieved.
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Example:
Mixture 0 HV AV AN VA VA RB
V
1% by weight polydimethyl smooth 93.0 29 26 26 47 0.40
siloxane-Si02 microparticles
Comparative plate 92.3 39 49 0.35
without additives
Caption:
HV hardness prior to water storage [ShA]
AV abrasion prior to water storage [mm]
AN abrasion post water storage 150h at 95 C [mm]
VAV improvement of resilience to abrasion prior to water storage [%]
VAN improvement of resilience to abrasion post water storage [%]
RB mean value of fissure growth at 1 million cycles in a flexural fatigue
machine
[mm]
In comparison with a comparative plate without additives, resilience to
abrasion is
significantly improved, in particular post water storage, on account of adding
polydimethyl siloxane ¨ silicon dioxide microparticles. The tendency toward
fissure
formation here is substantially unchanged. Thus, by way of adding polydimethyl
siloxane ¨ silicon dioxide microparticles, resilience to abrasion can be
improved
while the tendency toward fissure growth remains almost unchanged.
Manufacturing of the specimens:
An MDI-polyether-prepolymer having an NCO content of approx. 6% is employed.
MCDEA and PTHF200 are used as a cross-linking agent, and cross-linking is
performed at a temperature of 90 C.
The prepolymer, the MCDEA and the PTHF200 are separately degassed, using a
vacuum evaporator. Polydimethyl siloxane-Si02 microparticles are added to the
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cross-linking agent. Then all components are mixed in a vortex mixer. The
mixture
is poured into steel molds and tempered.
Determination of resilience to abrasion:
Resilience to abrasion determination was performed according to DIN 5316 and
ISO 4649. To this end, a specimen piece having a diameter of 16 mm was
impinged with a testing force of 10 N. The grinding length was 40 m at an
angular
speed of 40 revolutions per min.
Determination of the mean value of fissure growth:
Fissure growth determination is performed in a flexural fatigue machine. To
this
end, the specimen is flexed 1,000,000 times at a frequency of 7.5 Hz, at an
angle
of +/- 400. A section in the specimen displays a width of 6 mm and a depth of
2.5 mm..
