Note: Descriptions are shown in the official language in which they were submitted.
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REGISTRATION SYSTEM FOR PHASING SIMULTANEOUSLY
ADVANCING WEBS OF MATERIAL HAILING VARIABLE PITCH LENGTHS
IE D
The present invention relates to a registration system for phasing
simultaneously advancing, webs of material having variable pitch lengths. More
specifically, the present invention relates to a registration system useful
for
controlling and correcting the phase and position of simultaneously advancing
continuous webs of a pre-printed polymer backsheet material characterized by
significant pitch variation, and a partially fabricated consumer product, such
as a
catamenial or diaper product.
BACKGROUND
In the manufacfure of disposable absorbent articles such as diapers, it is
a common manufacturing practice to initially partially fabricate a plurality
of
individual articles that are consecutively spaced in interconnected positions
as a
continuous web. Individual articles are thereafter formed by combining the
partially fabricated articles with a water-impervious backsheet material and
then
directic~g the finished product web into a cutter to cut between the spaced
articles, usually as the final cutting operation in the manufacturing process.
Registered graphic designs or characters are desirably used in
connection with disposable articles such as diapers to enhance their outward
appearance for greater consumer acceptance. The positioning of a web pre-
printed with registered graphics such that the graphics are properly placed in
relation to the rest of the diaper product is desirable, e.g., in order to
provide a
large-sized graphic without cutting it. at an incorrect location, but has
typically
been problematic. This is important not only for desirable aesthetic
appearance,
but also for example, because young children may become disturbed at seeing
graphic representations of animals cut up into pieces (e.g., a head cut off).
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In addition, breathable polymer films that are particularly useful as
backsheet materials for disposable absorbent articles typically have good
surface characteristics that make them suitable for the application of multi-
colored, high resolution graphics, which are consumer-preferred, but such
films
tend to be mechanically unstable with a particular tendency toward themnal
shrinkage in the machine direction. Such instabilities contribute to the
difficulty
in correctly positioning the pre-printed polymer web on the product web such
that
the graphic is correctly positioned, or phased, to the product web.
Various methods and apparatus have previously been used for combining
such components in a desired relationship. For example, in conventional
"discrete phasing" operations, the product is built around the printed
graphic, by
adjusting the timing position of the machinery in response to a correction
signal.
However, it cannot be used in operations where more than one web bearing a
preprinted (or other phased object) is required for the process, since the
machinery can move in response to the graphics on one web only. See, e.g.,
International Publication No. WO 96129966.
In "non-continuous web placement° operations, the graphic-printed
web is
discretized, or cut, into segments. Each segment is then placed on the target
web or product in the desired position. This does not, however, ensure the
centering of the graphic on the target web; furthermore, it limits the overall
product design. The length of the discretized segments must be less than the
length of the product, which may allow leakage in the areas where the product
is
longer than the discretized segment, for example, in the waist region of a
diaper.
Other control systems and apparatus for registration have been
described, see International Publications WO 97124094 and WO 97124283.
However, none of the existing systems or apparatus for registration addresses
a
system in which inherent significant material instabilities exist, nor do they
address a system in which the pitch length of the preprinted graphic is non-
uniform, particularly if the preprinted pitch length is longer than the pitch
length
of the product under production. Similarly, a control system such as that
disclosed in US 5,766,389 requires a fixed target pitch. In practice, the
target
pitch can be subject to the same instabilities as that of the registered
printing.
Based on the foregoing, there is a need for a registration system for
phasing simultaneously advancing webs having variable pitch lengths, one of
which has consecutively spaced preprinted objects (e.g., registered graphics),
and the other of which (e.g., a diaper product web) contains consecutively
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spaced target objects. Furthermore, there is a need for a registration control
system that can account for the material specific instabilities that
inherently exist
in such webs and that present small but significant variations resulting
therefrom,
which make it difficult to combine such webs. In addition, there is a need for
a
system in which registration of incoming webs having either longer or shorter
pitch lengths than the pitch length of the product under production is
possible.
None of the existing systems provides all of the advantages and benefits of
the
present invention.
SUMMARY
The present invention is directed to a registration system for phasing
simultaneously advancing webs of material having variable pitch lengths
comprising: (a) means for feeding a continuous target web containing
consecutively spaced target objects along a path at a first velocity, the
target
web having a target web pitch length between consecutive target objects; (b)
means for feeding a continuous controlled web of material preprinted with
consecutively spaced preprinted objects along a path at a second velocity, the
preprinted web having a controlled web pitch length between consecutive
preprinted objects; (c) means for adjusting the controlled web pitch length
such
that the controlled web pitch length is approximately equal to the target web
pitch
length; (d) detection means for detecting a preselected feature of the
controlled
web; (e) means for generating an error signal based upon the detection of the
preselected feature of the controlled web; and (f) means for adjusting the
second
velocity such that the preprinted objects are shifted toward the target
objects on
the target web.
These and other features, aspects, and advantages of the invention will
become evident to those skilled in the art from a reading of the present
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing out and
distinctly claiming the invention, it is believed that the present invention
will be
better understood from the following description which is taken in conjunction
with the accompanying drawings, in which like designations are used to
designate substantially identical elements, and in which:
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Figure 1 is a plan view of a preferred embodiment of a disposable diaper
of the present invention in a flat, uncontraeted position showing a desired
location for a preprinted graphic object;
Figure 2A is a schematic diagram showing simplified representations of a
portion of a continuous diaper backsheet web having consecutively spaced
graphics printed thereon and a portion of a diaper product web;
Figure 2B is a top view of a diaper product web including a representation
of the diaper core area;
Figure 3 is a graphical representation of the pitch length variation typically
experienced by preprinted microporous materials that have been subjected to
typical conditions of winding and warehouse storage;
Figures 4A and 4B are perspective views of portions of preferred
embodiments of laminates of microporous material and nonwoven material,
illustrating the impact of running the microporous material at a pitch length
different from that of the nonwoven material;
Figure 5 is a schematic diagram showing the process of combining two
simultaneously advancing webs;
Figure 6 is a schematic diagram showing a preferred registration control
system of the present invention;
Figure 7 is a front view of a preferred embodiment of a disposable
absorbent article having registered graphics; and
Figure 8 is a simplified plan view of another preferred embodiment of a
disposable absorbent article having registered graphics.
DETAILED DESCRIPTION
All references cited herein are incorporated herein by reference in their
entireties. Citation of any reference is not an admission regarding any
determination as to its availability as prior art to the claimed invention.
As used herein, the following terms are defined as follows.
"Machine direction" or "MD" means the direction of movement of the
product along a manufacturing line. The madiine direction is shown in Figures
1
and 5 as an arrow labeled MD.
"Phase" refers to the machine direction position of an object expressed as
a degree of rotation. For example, the phase representation from 0° to
360° for
a flat, uncontracted diaper 20 is shown in Figure 1. The phase position of an
object can be described as any identifiable feature of the object. In Figure
1, X°
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is representative of an exemplary desired phase position of the graphic object
22.
"Product pitch length" means the length of material, on a relaxed basis,
that runs the full length of the product under production. A product pitch for
typical diaper products is illustrated as "PP" in Figure 2B.
"Registered process" means that iwo or more continuous webs having
phased features are combined in a correct phase relationship to one another.
"Registered graphics" refers to single or multiple color graphic objects that
are printed on a web close to a specified pitch length on a relaxed web basis.
The registration system of the present invention permits small but significant
variation from the intended product pitch length of the product delivered.
"Target web" or "independent web" is the main web which represents the
product under production. This target web may be a diaper chassis with the
leading edge of the absorbent core being a reference for the target object. It
is
to be expected that this reference target object may also have a spacing with
small but significant variations. In typical use of the present invention an
arbitrary
once per product "strobe" is used as the reference. Small but significant
variation
in strobe locus relative to the passing of the reference target object is to
be
expected.
"Target object" means the location on the target web to which the
preprinted object on the controlled web is intended to align. Note that
alignment
may take place at some standard offset from a easily recognizable feature or
"target object.
'Target web pitch length" means the nominal repeat length between
consecutively spaced target objects on the target web. The target web pitch
length may contain small but sign~cant variation during the use of the
registration system of the present invention, e.g., t 2°r6 between
consecutive
target objects.
"Controlled web" or "manipulated web° means the preprinted web which is
manipulated (extended or contracted) as needed to align the preprinted object
with the target object. Preferably, the controlled web is comprised of a
microporous polymer film suitable for use as, or as part of, a diaper
backsheet.
"Preprinted object" means the preprinted feature located on the controlled
web at consecutively spaced, but typically not statistically controllable,
locations
on the controlled web. The preprinted object is preferably high resolution,
multi-
colored graphics that are appealing to consumers. "Preprinted object°
as used
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herein may also refer to a collection of objects, see e.g., graphics 122 in
Figure
8, and includes pre-bonded, pre-applied, pre-cut, and pre-glued objects.
"Preprinted" as used herein refers to printing or combining on a separate
independent process not control linked to the registration process.
"Controlled web pitch length" means the repeat length between
consecutively spaced preprinted objects on the controlled web. Like the target
web pitch length, the controlled web pitch length may contain small but
significant variation during the use of the registration system of the present
invention.
A preferred embodiment of the present invention is directed to a
registered process for phasing simultaneously advancing continuous target and
controlled webs, the target web containing target objects and the controlled
web
containing preprinted objects, while permitting all in-running continuous webs
to
sustain small but significant variations in the spacing of key objects (such
as
preprinted graphics). Such variations result for example, from climatic
conditions, machine conditions and variations, printing errors, and creep.
The present invention accounts for the variation in the spacing of key
objects that is inherent in the mechanically unstable microporous polymer
films
that are particularly useful as controlled webs herein, while providing gentle
handling conditions for these delicate webs. Although the description of
embodiments contained herein is primarily given in the context of a diaper
converting line, i.e., registration of preprinted graphic objects (preprinted
objects)
phased to diaper cores (target objects) on a diaper converting line, it will
be
understood by those of skill in the art that the registration process herein
may be
used to phase multiple simultaneously fed webs in any application.
In conventional unit operation registration processes, the position of only
a single phased web must be considered. The phase of the unit operation is
advanced or retarded as needed to place the function of the unit operation in
correct phase relationship to the single phased web. Such a system cannot be
used, however, to combine two or more incoming webs in correct phase, as the
preprinted pitch lengths of the target and preprinted objects on each will
rarely
be identical.
Where two or more webs each carrying phased objects must be combined
in a predetermined relationship, information pertaining to the phase position
of
the. target web, usually the product under production, e.g., the diaper
chassis
assembly, must be fed to the web handling process. In addition, the position
of
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the phased objects (e.g., a preprinted object, such as a preprinted graphic on
a
polymer backsheet web) on each incoming, controlled web which is to be
combined with the target web must also be known and fed. As previously noted,
the target web pitch length and the controlled web pitch length are rarely
identical. It will be understood by those of skill in the art that any
incoming fed
web may be designated as a target web or a controlled web, and processed
accordingly.
The continuous registration system of the present invention performs two
functions simultaneously in order to correctly combine two or more incoming
webs having different phase positions, as described above. First, it
continuously
corrects the preprinted pitch length, or spacing of the graphics consecutively
preprinted on the controlled web, to match the pitch length of the target web.
Second, it continuously synchronizes the phase positions of the two webs such
that they are combined in the proper position, e.g., such that the graphic 22
is
correctly located as desired on the diaper 20, for example, in substantially
the
positions shown in Figures 1, 7 or 8.
The target web preferably comprises that part of the diaper chassis itself
that is generally continuously comprised of .the core and topsheet assembly,
prior to the attachment of the backsheet web. The target web is represented in
Figures 2A and 2B as target web 40. The target web 40 contains target objects
42 consecutively but not equidistantly spaced thereupon at a target web pitch
length, "Ltp". Target objects 42 are preferably the absorbent cores of the
disposable diaper chassis. Ltp need not be identical to the product pitch
length
PP of the diaper product; in fact, the target web 40 will likely have small
but
significant variation in consecutive product pitch lengths PP due to climatic
conditions during storage and transport, thermally induced creep, and the
like.
In addition, it is noted that the pitch length Lpp of the controlled web 30
may be
either longer or shorter than Ltp.
It is noted that the target object position may be directly sensed or inferred
from the use of an electronic strobe. A preferred embodiment of the present
invention uses an electronic strobe. This signal happens once per machine
cycle at the same place in the machine cycle. For example, if the machine
cycle
is represented by 1000 counts, then the strobe may be arbitrarily set at 500
to
600 "on" and balance of cycle "off". The leading edge transition between "ofP
to
"on" marks a machine time fixed position to which a graphic is registered.
When
the system is first started up, the operator inputs an offset value using an
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°advancelretard" button while examining the position of the controlled
web
graphic using an actual strobe light. Once the offset value is set, the
registration
system will target the controlled web graphic to the same point in the machine
cycle.
Again referring to Figure 2A, the controlled web 30 preferably comprises
a continuous film material, preferably a breathable microporous polymer film,
that
is used as part of the diaper backsheet. The controlled web 30 has preprinted
objects 22, consecutively but not equidistantly printed thereupon at a
particular
controlled web pitch length, "Lpp". Lpp is invariably something other (either
longer or shorter) than both the product pitch length PP and the pitch length
of
the target web Ltp due to climatic conditions, thermally induced creep, and
the
like. An exemplary Lpp is shown in Figure 2A. Preferably, preprinted objects
22
are registered graphics that are colorful, high resolution designs that are
appealing to the consumer, who is typically a care-giver to the wearer of the
diaper, or who in other cases may actually be the wearer.
As noted above, in most cases, Ltp is not equal to Lpp; in fact, exact
matches are highly improbable. This difference is a major source of the
difficulties associated with proper phasing of the controlled web 30 and the
target web 40. As can be most clearly seen in Figure 2A, a pitch delta C
(e.g.,
Ltp minus Lpp) usually exists and must be corrected in order to insure proper
product phasing. It is additionally noted that the pitch delta C associated
with
Ltp and Lpp typically accumulates for each subsequent target object 42, and
thus a greater amount of correction is generally needed. This error
accumulation
of pitch delta is represented in Figure 2A as C, C2, C3, C4.
The corrections of pitch delta C, C2, etc. associated with the registration
system of the present invention may not actually be necessary for product
functionality; however, they are critically important for consumer acceptance
of
the diaper products produced. And, in some cases, children wearing a
disposable article herein that has an improperly phased graphic, e.g., an
animal
with its head cut off, may be upset.
Figure 3 illustrates the instabilities in controlled web pitch length Lpp that
typically arise due to the inherent thermal instability of the preferred
microporous
polymer films of the present invention. Such thermal instability leads to
differential shrinkage of the microporous film, and significant variations in
Lpp
frequently result. Referring to Figure 3, the position along a wound roll
(i.e., a
continuous, controlled web 30) of microporous polymer film that has been
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preprinted with registered graphics is represented on the X axis. The start of
the
roll is indicated at the origin of the axes, while moving to the right along
the X
axis indicates moving toward the core of the roll, corresponding to unwinding
the
roll.
On the Y axis, preprinted pitch length Lpp is represented in millimeters.
As can be seen, differences in preprinted pitch lengths Lpp of up to about 10
mm
may be seen between various locations on the same web of material.
The registration system of the present invention is thus designed to
overcome the phasing problems associated with combining the target web 40
and the controlled web 30 where Ltp is not equal to Lpp, where Lpp is
inherently
variable, and where both webs are simultaneously and continuously fed to a
combining operation. The process has the ability to on-goingly adjust the
pitch
length of the controlled web 30 as delivered to match or to more closely
approximate that of the target web 40. In addition, the system of the present
invention makes such correction in a gentle, non-abrupt and gradual fashion so
as to account for the delicate nature of the web, which is preferably a
microporous polymer film, without damaging it. Alternatively, if conventional
higher strength webs are used, the system is capable of making more rapid and
abrupt adjustments that would not be acceptable with the preferred microporous
polymer films.
As described more fully below, this is accomplished by the use of web
tension transients to~ change the preprinted pitch length Lpp of the
preprinted
objects on the controlled web 30. Changing the tension in a web causes a
change in the pitch length of the web. Thus, more tension stretches the
material
of controlled web 30, causing the pitch length Lpp to increase, while less
tension
relaxes the controlled web 30, causing the pitch length Lpp to decrease. The
absolute tension in the controlled web 30 should not approach zero tension, as
control of the controlled web 30 would be lost. Conversely, more or less web
may be metered to the process, which also changes the level of tension in the
web.
The system of the present invention thus introduces small tension
transients, which provides on-going correction. This serves to maintain the
integrity of the delicate, mechanically unstable polymer films that are useful
for
disposable absorbent articles herein. It also serves to correct the phase
relationships of objects on the each incoming web in increments that are too
small to be detected by the web handling process, which means that there is
little
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possibility for the amount of error to become large enough to trigger the shut-
down functions of the web handling process. In addition, it allows for
manufacture of consecutive products that may have insignificant differences in
the locations of the graphic objects on each product.
Any control algorithm that creates such small on-going transients is useful
herein. The control system prepares small transient corrections in a timely
fashion that intervene in the control of incoming web handling drives to
prevent
mismatches between the positions of the target object and the preprinted
object
from drifting out of the zone that the consumer would find acceptable.
Exemplary
control algorithms herein include manual control, a control algorithm based on
statistical process control, or a PID control system.
It is believed that control systems relying on statistical analysis of past
history such as running average are simply too sluggish to stay ahead of the
significant variation observed in the position of graphics on the in-running
controlled web 30. To avoid the complexity of controlling multiple drives to
position the graphic, a preferred embodiment herein is a PlD control algorithm
which controls the feed rate of the controlled web 30 only as necessary to
minimize the strobed distance between a fixed position of converter machine
time with user-set offset and the sensed mark. If the space between the sensed
marks of the controlled web 30 ( Lpp), is longer than the target pitch length
Ltp,
more web 30 is fed to the process, reducing the web tension and allowing the
space between graphics to contract. Conversely, if the space between the
sensed marks of the controlled web 30 is too short, less web is fed, causing
an
increase in web tension causing the space between marks to increase. In this
way, not only is the pitch of the web brought close to the pitch of the
product, but
the position of the graphic is brought close to the target position.
As it is often not possible to have a visible timing mark, and the use of
marks printed with fluorescent inks have the disadvantage of both adding cost
and creating "glow in dark" marks on diapers under certain conditions of home
lighting, a preferred embodiment herein uses a direct sensing approach to
determine the position of the graphic on the incoming controlled web 30. A
sensor is selected which detects differences between light passing through the
web via different printed colors. The color of the target web object, the
position
of the target web object relative to other objects of the same color, and the
size
of the target object combine to create the means for generating a clean signal
whose position is strobed once per machine cycle.
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Alternative means of establishing position of graphics include: 1 ) sensing
a visible timing mark on a part of the web that is later removed by the
process; 2)
sensing a fluorescently-printed or other invisible-to-the-human eye mark; or
3)
using a grid of sensors to look for a recognized pattern of light diffusion.
The PID control system shown in Figure 6 seeks to minimize the error
distance which. is observed once per product. The proportional gain (Kp) 71
sends a correction signal to the controlled web feed drive that is
proportional to
the size of the error. The integral gain factor (Ki) 74 is set to eliminate
the steady
state offset typical of purely proportional control. While a differential gain
factor
may also be used, it is believed that best results are obtained by setting
this
equal to zero. it is observed in Figure 6 that the error signal trims the
operation
of the feed drive which is typically running at "gear ratio" relative to rate
of
product production. Although speed and .position may vary, on average one
preprinted object must be processed per target product.
The registration system of the present invention preferably adjusts the
pitch Lpp of the controlled web 30 by on-goingly adjusting the tension in the
controlled web 30. The Lpp is adjusted to "Lc", which is used herein to mean
"length at combining." Lc is equal or approximately equal to Ltp, i.e., the
pitch
length of the controlled web 30 is made approximately equal to that of the
target
web 40 so that the preprinted object, preferably graphic 22, can be positioned
in
the correct phase relationship to the product. This tension acts through the
web
modulus, Ew, of the controlled web 30, according to the following equation:
Lc = ( 1 + E) x Lpp = ( 1 + TIEw) x Lpp, whece
s = strain;
T = controlled web tension; and
Ew = web modulus of controlled web.
The foregoing equation acts for all webs being combined together. Thus
in the case of two webs:
Lc1 = (1 + T11Ew1 ) x Lpp1 and Lc2 = (1 + T2IEw2) x Lpp2.
As noted above, at the point of registered combining under steady state
conditions, Ltp is equal to Lc. Thus, the ratio of the relaxed lengths
combined at
any time is given by the equation:
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Lc11Lc2 = (1 + T2IEw2)I(1 + T1IEw1 ).
The tension variables T as well as the modulus variables Ew each have a
range of variation, so the ratio of the Lc values wilC always vary somewhat
from
unity. The difference between the Lc ratio and unity is referred to herein as
"puckering": .
Puckering°~ _ (Lc11Lc2 -1 ) x 100.
The .puckering value can assume positive or negative values depending
which web has the least relaxed length. After the finished product is cut
apart,
the combined web is pulled to the length of the incoming web with the least
relaxed length. This causes the other web to "pucker." Above a certain
tolerance level, puckering has a noticeable impact on the appearance of the
product. For a diaper ~backsheet formed from a nonwoven material and a
polymer film preprinted with graphic designs, puckering levels up to about t
2°~,
while noticeable', are generally not perceived as distorting the appearance of
the
product.
Alternatively, different puckering levels can intentionally be created, or
puckering may othervvise be controlled to create materials with different
tactile
feels as desired. Note that even below t 2°~ that puckering can be
controlled
and located on the product according to the present invention to minimize its
impact on product appearance. To do this, the laminated material furthest away
from the consumer is overfed and pulled back, thus hiding the puckering from
the
consumer.
The present invention has the capability of registering controlled webs
having pitch lengths that are not uniform, and which can be either longer or
shorter than the machine product pitch length. The ability to register
materials
with pitch lengths longer than the product under production allows the
designer
of such a product to tailor the surface texture of the product. This is
demonstrated in Figures 4A and 4B, where two materials are laminated together
by passing through a nip point. The material which is longer than the finished
product pitch is pulled back by the other material. By making the layer of the
laminate that is furthest away from the eye of the consumer the material that
is
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pulled back, the impact of the registration process on the texture of the
finished
product is mitigated.
In Figure 4A, the pitch length of the nonwoven web 32 (Ltp) which may
form a portion of the diaper backsheet is longer than the pitch length (Lpp)
of the
polymer film (controlled web) 30. In this case, after the final cut that
separates
the consecutive diaper products, the polymer film pulls back the nonwoven
material. This creates a roughened texture on the exposed. surface of the
laminate which the consumer can see and feel.
Figure 4B shows the surface texture impact of the pitch lengths (Lpp) of
the polymer film (controlled web) 30 longer than the pitch length (Ltp) of the
nonwoven material web 32. In this case, the nonwoven material pulls back the
polymer film material after the final cut. Thus, the impact of the unequal
combining is manifested on the inside of the product and is hidden from the
eye
of the consumer. Thus, it is generally preferable to overfeed the controlled
web
of polymer film material so that~the consumer does not see or feel the
roughened
texture.
The nonwoven material 32 may be fed into the system as an independent
web having phased objects with variable pitch length. fn such a case, the
nonwoven web and preferably the controlled web are phased as described
herein, and combined; then the combination laminate material is phased and
combined to the target web (diaper product web) as described herein.
Assuming that a puckering tolerance level of up to about t 2°~ is
acceptable, the available adjustment rate for registered graphics is limited.
It is
important to design the registered printing such that a reasonable tolerance
of
graphic position is accommodated. The graphic position tolerance should be
greater than or equal to the web to web position shift typically seen at the
time of
web splicing; that is, at the time that a subsequent roll of preprinted web is
supplied to the web combining process at the time that the previous roll is
used
up by the process. The roll to roll error is different and generally greater
that the
normal variation.within a single roil. Thus, if random splicing were used,
phasing
corrections of up to 180 degrees (half of a full length) would sometimes be
necessary. During such times, all product produced would need to be scrapped.
The splice tolerances of the present invention are controlled in concert with
the
tolerances of preprinted object position to reduce splice time rejected
product to
ideally zero (other than rejection of the splice tape itself).
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The selection of the raw material to be used for the backsheet, i.e., the
controlled web 30, is somewhat constrained by both the product design and the
registration system of the present invention. The key parameters that are
unique
to registered graphics are impacted by the needs of product functionality. For
diaper backsheet, a preferred material is a breathable microporous polymer
film.
A preferred microporous polymer film is fomned from a mixture of polyethylene
and calcium carbonate, and titanium dioxide if needed to increase the white
appearance of the film, since whiteness is a film characteristic that is
necessary
for widespread consumer acceptance. More preferably, the microporous
polymer film has high thermal stability characteristics in order to support
the
printing of high resolution color graphics thereon. Preferably the film meets
a
maximum of t 1°r6 repeat pitch variation, i.e., mismatch, but more
typically t 2°r6
is likely to be seen. It is believed that puckering ratios ranging from about
0.97
to about 1.015 may successfully be provided using this system, making it
possible to handle films with an even wider range of pre-printed pitch length
variation.
Other preferred characteristics of the microporous polymer film that are
important in assuring good processing with the registration system of the
present
invention include: web modulus (preferably from about 4000 to about 13,000);
coefficient of friction (preferably high enough so that the system metering
nips
can act on the web with a sustainable level of web tension); and thermal
enthalpy (preferably high enough to have sufficient heat resistance during hot
melt glue lamination, but not increased to the point that the stiffness of the
film
increases to the point of discomfort for the wearer).
The process of combining the two webs 30 and 40 is schematically
represented in Figure 5. The controlled web 30 is initially fed at a velocity
Vo.
The target web 40 is fed to the combining stage 50 at a velocity Vt. Detection
of
the position of a preprinted object (e.g., graphic 22) moving in the machine
direction MD occurs at preprinted object detection stage 34. Similarly,
detection
of a target object (e.g., diaper core 42) moving in the machine direction MD
occurs at the product detection stage 44 or alternatively via an optiGresolver
50a
(automatically) or an operator (manually).
As previously noted, the process control dilemma that the registration
system of the present invention must correct is two-fold.
First, pitch mismatch must be corrected. There is a difference in pitch
between the online made web (target web 40) and the offline made web (control
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PCTIUS98I27135
web 30). This difference in pitch is shown in Figures 2a and 5, as the
difference
between Ltp and Lpp, where Ltp - Lpp = pitch delta C). As described in
connection with Figure 2a, this pitch delta C accumulates as the number of
products accumulates. Thus, for the first product, the pitch delta is about C;
for
the second product, 'the pitch delta is about C2; for the third product, the
pitch
delta is about C3, and so on.
Second, phase correction is needed. Phase error may be caused when a
transient change occurs ~in the controlled web 30 via a control web splice or
some phase upset in the position of the preprinted object 22 in relationship
to the
target object 42.
In a preferred PID control system of the present invention, error signal 70,
see Figure 6, is generated based upon recording of position of incoming timing
mark by resolver strobe. . This error signal is transformed into a correction
signal
via a PID algorithm. In a preferred algorithm, integral gain is used to
attenuate
steady state error, and differential gain .is preferably set at zero. This
correction
signal forces a programmable drive or differential correction shaft to speed
up or
slow down the axis driving the controlled web 30 at a metering point 30b.
Thus,
Vo is adjusted to Vc (see Figure 5) by gain factors 71 and 74, see Figure 6,
to
maintain the position of the preprinted object graphic 22 and the target
object
core 42, by changing flow rate of the controlled web 30.
Referring to Figure 6, a preferred registration control system 60 is shown.
The main reference signal is provided by the resolver 50a and is referred to
as
the converter position reference signal 62. The converter position reference
signal 62 is the resolver change in position over time, i.e., a velocity
reference
signal (preferably one revolution of the resolver per target object 42). -This
signal
62 is equal to the pitch length of the product times the converter rate. The
registration control system 60 provides a trim correction factor, which is
typically
a small percentage of the servo velocity reference signal 76, to the converter
position reference signal 62, based on the pitch mismatch and phase correction
described above, by automatically calculating an error signal 70 and adjusting
the servo system velocity via the velocity loop 78, to change the flow rate of
the
control web 30. This is accomplished by the controlled web 30 preprinted
object
22 being sensed by optic 34 that electronically strobes resolver 50a providing
actual position data 50b (feedback), and comparing it to the fixed target
position
constant 44. This error signal 70 is manipulated to maintain registration due
to
pitch mismatch and phase correction.
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16
The. pitch mismatch aspect of the control dilemma is used to limit the
accumulating offset, i.e., pitch delta C, C2, C3, and so on. Integral gain Ki
(auto
correction 74) is multiplied by the base reference signal 75 to yield a servo
velocity signal 76 to the velocity loop of the servo drive 78, thus matching
the
pitch of the controlled web 30 to the target web 40. The proportional gain Kp
71
is needed to stabilize the system and provide enough gain to maintain the
target
position 44:
The phase correction aspect of the control dilemma, i.e., the process
control issue, is used to phase the system for several reasons. First, initial
set-
up of the registration system 60 online requires phase correction. When the
registration system 60 is first started up, the position of the preprinted
object 22
assumes an arbitrary but controlled position relative to that of the target
object
42. This arbitrary position may be out of phase, requiring the operator to
change
the fixed target position constant 44 to initially set the preprinted object
22 to the
target object 42 (from this time on no additional.manual intervention is
needed).
Proportional gain 71 is needed to drive the system 60 to new setpoint, target
constant 44. The error signal 70 is multiplied by a proportional gain factor
71
and then added to a converter position reference 50a to generate a base
reference signal 75.
In addition, web splicing of control web 30 causes a phase error, requiring
phase correctiori. Proportional gain 71 is needed to drive the system to the
setpoint target constant 44.
Previous design efforts targeting registered graphics have been based
upon separate control systems for incoming graphics pitch length correction
and
phase position control. However, the system of the present invention
incorporates simultaneous correction based upon the relationship between pitch
length and the velocity. It will be understood by those of skill in the art
that
position has an integral relationship to velocity and can therefore be
controlled
using the same system that controls velocity.
Disposable diaper products typically include a topsheet material, an
absorbent core, and a backsheet material. The topsheet material is located to
be placed facing the body or nearest the body when the diaper is worn and is
generally provided with a liquid permeable region so that body exudates can
flow
through the topsheet to the absorbent core, where they are contained. The
backsheet material, which is placed away from the body during wear, is
typically
liquid impermeable so that outer clothing or other articles such as bedsheets
are
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17
not wetted by the body exudates. Such an exemplary diaper is disclosed in, for
example, Buell U.S. patent 5,569,234, directed to a "Disposable Pull-on Pant".
In addition to the basic topsheet, core, and backsheet components, it will
be understood by those of skill in the art that many other features for
disposable
absorbent articles are within the scope of the present invention. For example,
barrier cuffs as described in t_awson and Dragoo U.S. patents 4,695,278 and
4,795,454 are a desirable feature for disposable absorbent articles. In
addition,
skin care-type topsheets that are provided with lotion thereon for the purpose
of
reducing skin irritation and chafing are a desirable feature herein.
Referring to Figure 7, there is shown an exemplary disposable pull-on
diaper 150, which is generally pulled onto the body of the wearer by inserting
the
legs into the leg openings 162 and pulling the article up over the waist.
Generally, "pull-on diaper" refers to pull-on garments worn by small children
and
other incontinent individuals to absorb and contain body exudates. It should
be
understood that other pull-on garments such as training pants, incontinent
briefs,
feminine briefsr feminine hygiene garments or panties, and like, are included
herein.
Referring to Figures 7 and 8, the diaper 150 is generally comprised of a
backsheet 152, a topsheet 154 and an absorbent layer 156 (shown in dashed
lines in Figure 8) located between the backsheet 152 and the topsheet 154. The
backsheet 154 is the surface which faces away from the wearer's body, while
the
topsheet 152 is the surface which faces toward the wearer's body. Preferably,
the backsheet 152 is comprised of at least a microporous polymer film printed
with registered graphics, e.g., 120, 122, as described herein. The backsheet
152 may further comprise a layer of nonwoven material (see reference numeral
32, Figs. 4A, B) laminated or otherwise secured to the microporous film layer,
in
which case there is provided a more cloth-like and garment-like feel. In such
a
case, the nonwoven web may be fed to the web handling process as a
continuous incoming web and. may be combined in a desired relationship with
the controlled web 30 and the the target web 40.
In the case of a laminated nonwoven backsheet it is desirable that the
surface texture of the non-woven not be impacted by a laminated sublayer, e.g.
a
microporous polymer film sublayer, which pulls it back and causes it to
"pucker."
It is more desirable that any puckering that is required for the operation of
a
registration system be limited to the underlying polymer material, which is
neither
seen nor felt by the user of the product, see Figure 4B. The system of the
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18
present invention can be used in cases where the pitch length Lpp of the
microporous film web (controlled web 30j may be longer or shorter than that of
the product, Ltp. Overfeeding the target ~ web may lead to superior surface
texture. After the final cut, when Lpp is less than Ltp, the film material
pulls back
the nonwoven material that forms part of the product. This creates a roughened
texture of the exposed surface of the laminates where the consumer can see and
feel. This, when Lpp is greater than Ltp, the impact of the unequal combining
upon which the registration systems depends is hidden from the consumer. In
general, Lc should be greater than both Lpp and Ltp, to avoid the possibility
of a
zero-tension situation.
Elastically extensible . side panels 156 . are provided to ensure more
comfortable and contouring fit by initially conformably fitting the pull-on
diaper
150 to the wearer and sustaining this fit throughout the time of wear well
past
when it has been loaded with exudates. Leg elastics 158 and waist elastic
region 160 are also provided to enhance the fit around the legs and waist,
respectively.
Figure 7 shows the' front view of the diaper 150 with an exemplary graphic
120 positioned in about the upper region of the backsheet, on the front side
of
the diaper 150. In Figure 8, there is shown a simplified plan view of an
embodiment of a disposable absorbent article in its flat, uncontracted state
prior
to formation. In this embodiment, graphic 120 is shown in the back region of
the
diaper with graphics 122 additionally shown in the front region of the diaper.
It is understood that the examples and embodiments described herein are
for illustrative purposes only and that various modifications or changes in
light
thereof will be suggested to one of skill in the art without departing from
the
scope of the present invention.
