Patent Application
20060016359
This invention relates to flexographic printing on extensible substrates.
Printing on a variety of surfaces is well known in the art. Printing has been done on paper, fabric, wood, and other surfaces for generations. Printing on newer synthetic materials, such as polymer films, is also known. Printing allows colors, graphic designs, and text to be placed on the material of interest.
Printing on polymer film and other extensible materials can present challenges, though, due to the extensible nature of the material. Extensible materials such as polymer films can stretch and deform when stressed, even if the material is not considered to be elastomeric.
For example, continuous webs such as films undergo stresses when being wound on a roll. These stresses on the web may vary depending on the depth of the web on the wound roll. For instance, material close to the roll core may experience a great deal of tension due to the force required to initially start winding the roll. The material closer to the middle of the roll depth may experience less tension as the roll is smoothly wound. The material near the outer portion of the roll may experience increased tension. Thus, as an extensible material is wound on a roll, these varying tensions can cause the material to stretch slightly to a lesser or greater extent. Some winders have the ability to adjust the winding speed and tension over the profile of the roll in order to compensate somewhat for this effect.
Another effect that extensible materials, particularly polymer films, can experience is relaxation during aging. This relaxation process is sometimes referred to as “snapback”. When a film is first extruded, the polymer chains may be aligned and stressed somewhat during the extrusion process. As the film cools, and particularly as it ages for a few days, these stresses are gradually released and the film relaxes. During this relaxation, the film will tend to retract (i.e., shorten) slightly. However, because of the varying web tensions within the roll itself, varying degrees of retraction will be observed within the roll.
These two problems, winder tension variability and snapback, can cause a printed extensible materials to vary significantly in print repeat length. A pattern printed repeatedly on a strip of extensible material can be distorted by as much as 1% in length as the printed material is wound and later as it ages. This distortion, particularly if cumulative, can result in misaligned or miscut product when the printed material is later unwound for converting.
Today's consumer has come to expect high-quality, detailed graphics on products from packaging films to shrink-wrap seals to disposable hygiene products. There is thus a continuing need to improve the printing repeat-length control of polymer films in order to manufacture materials that can meet this expectation.
This present invention provides methods of printing a repeating pattern on a web of extensible material. In one embodiment, this method comprises the steps of:
The adjusted print repeat length profile is determined such that, when the roll of printed material is subsequently unwound, the variability of the print repeat length along the length of the printed web will be less than it would be had the print repeat length not varied along the length of the web.
This method may further include the steps of measuring the actual print repeat length of the printed indicia printed on the extensible material; comparing the actual print repeat length measurement to the adjusted print repeat length profile; and controlling the printing step in response to the results of the comparing step. The measuring step may be performed automatically by a repeat length measurement device, such as an optical device (e.g., a camera capable of measuring images). Any of a variety of printing systems and devices may be used, such as a gearless, flexographic printing press.
The methods of the present invention may be performed on a variety of extensible materials, such as a polymeric film. Suitable polymeric film materials include, for example, polyolefins, polyesters, nylons, copolymers of one or more of the foregoing materials with one another or with another polymer-forming monomer, and mixtures thereof. The methods of the present invention may also be used for printing fabrics, such as nonwoven fabrics.
The method described above may further include the step of winding the extensible material into a roll after the printing step.
The present invention also provides a printed web of an extensible material produced in accordance with the methods described herein, as well as a printed sheet of extensible material cut from a web produced in accordance with the methods described herein. By way of example, a printed label may be cut from a web of extensible material which has been previously printed in accordance with the methods described herein. Similarly, a backsheet for a disposable diaper may be cut from a web of extensible material which has been previously printed in accordance with the methods described herein, and the disposable diaper then assembled in a manner known to those skilled in the art.
The present invention also provides a freshly printed web of extensible material having a repeating pattern printed thereon, the repeating pattern comprising printed indicia which is repeated along the length of the web, wherein the print repeat length of the printed indicia varies along the length of the web.
Similarly, the present invention also provides a wound and aged web of extensible material having a repeating pattern printed thereon, the repeating pattern comprising printed indicia which is repeated along the length of the web, wherein the print repeat length variability of the printed indicia is less than 0.2% (or even less than 0.1%) when the web is unwound. The printed webs of extensible material produced in accordance with the various embodiments of the present invention may include any number of repeating patterns printed thereon, such as 100 or more repeating patterns, or even 1000 or more repeating patterns printed on a single continuous web.
The following detailed description will be more fully understood in view of the drawings in which:
a-3d illustrate representative PRL profiles of an exemplary printed material without and with PRL adjustment at the press;
a-4d illustrate representative PRL profiles of another exemplary printed material without and with PRL adjustment at the press;
The embodiments set forth in the drawings are illustrative in nature and are not intended to be limiting of the invention defined by the claims. Moreover, individual features of the drawings and the invention will be more fully apparent and understood in view of the detailed description.
One embodiment of the present invention provides a method of compensating for distortions experienced by printed extensible materials (e.g., polymer film) during processing, such as winding stresses and snapback. Applicant has surprisingly found that these winding and aging effects are predictable for a given material composition. A target print repeat length (“PRL”) profile may thus be established for a given material composition and dimension (e.g., film thickness), and the printing system can be controlled in order to adjust the print repeat length in accordance with the target PRL profile. In this fashion, the printing process may compensate for the forces previously discussed and yield a wound, aged film with little PRL variability (e.g., ±0.2% or even ±0.1%).
For the purpose of this patent application, the following terms are defined as follows:
“Film” refers to material in a sheet-like form where the dimensions of the material in the x (length) and y (width) directions are substantially larger than the dimension in the z (thickness) direction. In general, films have a z-direction thickness in the range of about 1 μm to about 1 mm.
“Extensible” refers to polymer materials that can be stretched at least 130% without breaking, but retract to greater than 120% of their original dimension and therefore are not elastomeric. For example, an extensible film that is 10 cm long should stretch to at least about 13 cm under a stretching force, then retract to a length greater than about 12 cm when the stretching force is removed.
“Print repeat length” or “PRL” refers to the measured distance between identical places on two successive printed patterns on a printed material. The PRL will include both the printed pattern and any unprinted space between each printed pattern.
“Actual print repeat length” or “actual PRL” refers to the measured PRL of a printed material.
“Target print repeat length” or “target PRL” refers to the PRL value that is desired by the end user of the printed material.
“Print repeat length profile” or “PRL profile” refers to a descriptive or graphical representation of the PRL over the length of a printed material containing a multiplicity of repeated printed patterns. A common way to present a PRL profile is by way of a graph. If presented as a graph, the PRL profile may be presented in a number of ways. The x-axis (independent variable) is typically the distance from the beginning of the printed material, measured in appropriate units of distance such as lineal meters. The y-axis (dependent variable) will be some value of the measured PRL at a given point on the x-axis. The dependent variable may be the actual PRL, the raw variance of the measured PRL from the target print length, the absolute value of said raw variance from the target print length, the percent variance of the measured PRL from the target print length, or other such appropriate value.
“Freshly printed” refers to material immediately after it has been printed.
“Aged” refers to material that has been printed and held for any interval of time longer than one second after completing the printing process.
“Wound and aged” or “wound, aged” refers to material that has been printed and wound into a roll, then held for any interval of time longer than one second after the material has completed the winding process of that individual roll of printed material.
Flexographic printing is one of the simplest methods of mechanically printing on a continuous web of material. In flexographic printing system 10 shown in
Originally, flexographic printing was synchronized through mechanical means. The impression plates were gear-driven by a central motor in order to synchronize the printing steps so that the colors would superimpose on one another and form a pleasing image. Setting up the synchronization was a difficult and time-consuming task requiring highly skilled press workers. It was an energy-intensive operation, due to frictional losses through the multiple gears. Also, the gears tended to wear, which would result in the gradual loss of synchronization and hence print quality.
In recent years, flexographic printing has been simplified through the development of gearless printing.
In a gearless printing press, the rolls 22 and 32 on which the impression plates are mounted may be independently driven, such as by servo motors controlled by a controller 60 in order to maintain high registration accuracy. The gearless press is more energy efficient, and it experiences no mechanical wear to the drive components. Because of gearless printing presses and other developments, flexographic printing control has improved. Flexographic printing now rivals rotogravure printing in the precision and detail that can be obtained.
These improvements in flexographic printing have not solved another problem, though. This is the problem of print repeat length variability. When printing on an extensible material, such as a polymer film, the size of the printed pattern and the distance between repeated patterns can be closely controlled. However, after the printed extensible material is wound into a roll and stored for a period of time, at least two forces come into play that cause the extensible material to stretch or retract to varying degrees. This stretching or retraction results in a significant change in the distance between repeated printed elements. The change in the distance between repeated printed elements is problematic when the printed film is later cut to form various products. In other words, although the print repeat length of the freshly printed film may match (or closely match) the target print repeat length with little or no variability, when the end-user later attempts to unwind and cut a roll of the printed film, the end-user will discover that the print repeat length has changed. Not only will the print repeat lengths not match the target, there will be considerable variability in print repeat length throughout the roll.
The first of the two forces which causes changes in print repeat length after winding and aging is known as “snapback”. This is a well-known effect in the printing of extensible material. For example, when an extensible polymer film is freshly extruded, the polymer molecule chains are stretched and roughly aligned with the direction of the extrusion flow. As the material ages, though, the polymer molecules slowly relax and retract, resulting in a small but predictable shortening of the film. Similarly, aged film will experience some stretching when it goes through the printing process. After printing, the material will “snap back” from this printer-induced stretching as it ages. When fresh or aged film is printed, the press operator knows that some snapback will occur. Accordingly, the press will be set to print a certain PRL with the anticipation that a given amount of snapback will occur. For instance, if the desired PRL is 300 mm, the press operator may actually set the PRL to 304 mm, in the anticipation that the material will experience 4 mm of snapback.
The second force in play, though, is the varying amount of tension experienced by an extensible material when wound into a roll. It is known that continuous webs undergo stresses when being wound. These stresses on the web will vary depending on the depth of the web on the wound roll. For instance, material close to the roll core may experience a great deal of tension due to the force required to initially start winding the roll. The material closer to the middle of the roll depth may experience less tension as the roll is smoothly wound. The material near the outer portion of the roll may also experience increased tension. If the web is an extensible material such as a polymer film, these varying tensions can cause the material to snapback to a lesser or greater extent, or even to stretch a bit. While winders are designed with the ability to adjust the winding speed and tension over the profile of the roll in order to compensate somewhat for the variable tensions the wound film may experience, they do not alleviate the problem entirely.
Prior to printing, whether an extensible material experiences snapback or stretching due to these forces is usually immaterial. After printing, however, these forces can cause the PRL of the printed extensible material to vary significantly. When the material is printed, the snapback effect is countered by the winding tension effect. Material near the core or outer edge of the film is under higher tension, and so the snapback experienced there is less. In the example above, the fresh film is printed with a PRL of 304 mm, anticipating snapback to 300 mm as the film ages. At the core and outer layers of the roll, where the winding tension is high, the material may snap back less, e.g. to only 302 mm, or even stretch slightly, e.g. to 306 mm. The material wound at the center depth of the roll, however, experiences less winding tension, and it is able to snap back to the full 300 mm. Hence, the aged printed material will have an actual PRL that varies from 306 to 300 mm.
For materials that are printed with a random pattern, this variability in the PRL is usually unnoticed or insignificant. However, for printed material where printed elements such as lettering and graphics must be precisely located for the end use (e.g., shrink-wrap labels), variability in the PRL can cause problems. When manufacturing the end product, this PRL variability can accumulate, resulting in the printed material being cut off-center or even cut into the pattern of the printing.
In order to compensate for PRL variability, a print shop may have to purchase excess material to ensure the correct number of items is printed. However, if the PRL variability can be controlled, the print shop can reduce the excess material purchased, and thereby save this wasted expense.
The present invention provides methods to correct for the variability in PRL when the print length must be precisely reproducible. Applicant has discovered that PRL variability is predictable for a given film composition and degree of aging. Once the PRL profile of a wound roll is known, the printing press system can be controlled to adjust the PRL, depending on the expected position of the printed extensible material on the roll, in order to yield a wound, aged film with reduced PRL variability. In some embodiments, PRL variability may be reduced to ±0.2%, or even ±0.1%.
a is the PRL profile of a freshly printed extensible material with a constant target repeat length, prior to being wound in a roll. This is the typical goal for a printed material, and the measured print length varies little from the target. However, once this exemplary printed extensible material is wound in a roll and aged, even for a brief period of time, the PRL profile will begin to change due to the snapback and tension forces discussed previously.
In contrast,
Applicant believes that the PRL profile will depend on the composition and structure of the extensible material being printed. For example, the PRL profile of a polyethylene film will differ from the PRL profile of a nylon film of similar thickness. The degree and reproducibility of the PRL profile over time and roll-depth position may be measured or estimated for a given composition or structure of the extensible material to be printed.
Surprisingly, applicant has also discovered that the PRL profile of a wound roll of a given extensible material will change in a predictable, repeatable manner. Also surprisingly, this PRL profile change occurs in a repeatable manner as the extensible material ages. In addition, applicant has unexpectedly discovered that the overall shape of the PRL profile remains essentially constant for a given extensible material that has aged for different time periods.
As shown in
The aged, unadjusted PRL profile (e.g.,
Once the adjusted PRL profile has been established, by measurement and/or estimation, PRL variability can be reduced simply by printing the extensible material in accordance with this profile (i.e., the PRL closely matching the PRL indicated in the adjusted PRL profile). By way of example, in the exemplary embodiment of
The PRL of the freshly printed extensible material may be controlled by adjusting the print length during printing, yielding a freshly-printed film with a PRL that varies over the multiple images on the length of the material. After the printed film is wound and aged, the snapback and winder tension forces previously discussed will counteract the variability in the initially printed images.
By way of example, a gearless press may be controlled to variably adjust the PRL during the printing process in the manner described previously. The adjusted PRL profile may be input to the controller 60 in order to control PRL in accordance with the adjusted PRL profile over the length of the web. The PRL adjustment is based on the anticipated roll position of that section of film once it is wound onto a roll. This variable adjustment is designed to anticipate and correct for the anticipated snapback or tension that will occur at that location in the wound roll.
The system may also include one or more feedback control systems in order to ensure that the PRL corresponds to that indicated by the adjusted PRL profile. As shown in the exemplary gearless printing system of
As illustrated in
As was discussed previously for
In one embodiment of the present invention, the RLM computer 75 can be programmed to proactively adjust the PRL of the printed material based on the adjusted PRL profile so that, after the printed material is wound into a roll and ages, the PRL returns to a near-constant target value (i.e., little or no variability in PRL when the printed roll is later unwound for conversion into products). If an exemplary printed extensible material is known to exhibit the PRL profile of 3b upon winding and aging, for example, the RLM computer 75 can be programmed with the adjusted PRL profile of
Similarly, for a printed extensible material with a PRL profile like that shown in
As an alternative to controlling the PRL by adjusting the rotational speed of the rolls in which the impression plates are mounted, or in addition thereto, the tension applied to the extensible material in the printing zone may be adjusted in a manner known to those skilled in the art. For example, if the tension in the extensible material is increased in the region adjacent the impression plates 20 and 30, PRL will be reduced. Likewise, if the tension in the printing zone is decreased, PRL will be increased. In the same manner as described above and shown in
After printing, winding and aging, the printed roll of extensible material will typically be unwound and cut into individual sheets by the end-user for conversion into a final product. Such final products include, for example, a label, particularly a shrink-wrap label. The individual cut printed sheets may also be used in the manufacture of packaging materials, garments or even personal hygiene products such as diapers (e.g., a printed backsheet for a disposable diaper), a training pants, sanitary napkins, pantiliners and garments.
The present invention also provides a printed web of extensible material (e.g., 12′ in
The following examples are designed to illustrate particular embodiments of the present invention.
A polymer film composed of approximately 47% LLDPE, 4% LDPE, 45% ground calcium carbonate, and 4% minor ingredients (process aids, colorant, and antioxidant) is cast-extruded into an embossed film. The film is approximately 2 mils thick. The fresh film is printed with a standard, repeating print pattern at a PRL in the range of 300 to 600 mm. The snapback for this material is anticipated to be 1% of the given PRL. The printed material is then slit and wound into rolls containing approximately 10,000 lineal meters of film. The PRL of the freshly extruded and printed film is measured. The film is then set aside to age for predetermined intervals over several weeks.
After aging for predetermined intervals, the film is unwound, and the PRL of the aged material is measured. The PRL variability is plotted at a given age for the film for the position on the roll in lineal meters, where 0 is the outer surface of the roll and 10,000 is the core of the roll. The PRL variability of the aged film is shown in
A polymer film as described in Example 1 is prepared. The fresh film is printed with a standard print pattern at a PRL in the range of 300 to 600 mm. However, the tension of the film in the printing zone is controlled to compensate for the PRL variability noted in Experiment 1. The film is then slit and wound as in Example 1.
After aging for 21 days, the film is unwound and the PRL of the aged material is measured. The PRL variability of the aged film is plotted for the position on the roll, and shown in
A polymer film as described in Example 1 is prepared. The fresh film is printed with a standard print pattern at a PRL in the range of 300 to 600 mm. However, the RLM computer is programmed to adjust the press controller to vary the PRL of the freshly printed film to compensate for the PRL variability noted in Experiment 1. The film is then slit and wound as in Example 1.
After aging for 21 days, the film is unwound and the PRL of the aged material is measured. The PRL variability of the film is plotted for the aged roll, and shown in
While the invention has been described in detail, including specific embodiments thereof, it will be appreciated by those skilled in the art that other embodiments including variations and equivalents can be conceived. Accordingly, the scope of the present invention is that of the appended claims and any equivalents thereto.
This application claims the benefit of U.S. Provisional Patent Application No. 60/586,582, filed Jul. 10, 2004, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60586582 | Jul 2004 | US |
