EP4638088A1 - Slot die and methods of operating it - Google Patents

Abstract

Provided are slot dies and related methods of use. The slot dies comprise an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die. A flexible die lip provides the first slot die surface and is capable of independently adjusting a cross-sectional height of the fluid flow path. A plurality of actuators are spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, and the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width.

Description

SLOT DIE AND METHODS OF OPERATING IT

Field of the Invention

Provided are slot dies and related methods for manufacturing a continuous web using an extrusion process.

Background

In the field of extrusion, slot dies are used in the large-scale manufacture of a wide range of products. These slot dies include opposing die lips that form an applicator slot extending along the width of a moving web or the width of a roller that receives an extrudate, such as a film. As used herein, with respect to slot dies and slot die components, the term “width” refers to the transverse, or cross-web, dimension of a slot die or slot die component.

Slot dies can be especially useful in making extruded films, coatings, and other extruded articles. As one example, slot dies can be used in slot die coatings to apply a fluid material to a chilled roll, release liner, another extruded web, or other continuous substrate. Coating materials can be at ambient temperature or a controlled temperature. At ambient temperatures, the extruded composition can be a reactive composition that cures at some time after it is coated onto a substrate. When a coating material temperature is elevated to ensure that the coating material is melted or liquefied for processing, this is often referred to as hot melt coating or extrusion coating.

Alternative configurations are also possible. For example, the extruded film can be delivered directly into a nip, or biased to contact one or the other roll just prior to the nip for various process considerations. The slot die could be oriented vertically, horizontally, or at some other angle. The nip orientation can also be adjusted. The rolls that form a nip can be of different diameters and composition, and can be metal, release coated, rubber covered, embossing patterning or smooth, chilled or use a heated temperature controller.

Extruded coating compositions can include a diluent, such as a solvent. Useful solvents for this purpose include water, organic solvents, and any suitable fluid that dissolves or disperses components of a coating. Solvents are typically removed in subsequent processing such as by drying. A coating can include single or multiple layers, and some slot dies may be used to apply multiple layers simultaneously. A coating can be a continuous coating across the width of the die or instead be comprised of strips, with each strip extending across only a portion of the width of the die and being separated from adjacent strips. An extruded film might be subsequently processed by length orienting or tentering operations.

The thickness of an extruded film or coating is largely dependent upon the flow rate of the extrudate through the slot die. As mentioned above, the slot die includes a flexible die lip that can be used to adjust the local height of the applicator slot, or “slot height,” to control the flow rate of the extrudate from the applicator slot and provide a desired web thickness profile.

A slot die may include a plurality of actuators spaced along the width of the applicator slot in order to manipulate the thickness profile. For example, each actuator can be configured to provide a local positional adjustment of the flexible die lip. During an extrusion process through the slot die, the cross-web profile of an extrudate is systematically measured. Each actuator can then be collectively or individually adjusted to provide a desired thickness profile, typically a uniform thickness, for the extrudate across the width of the applicator slot.

A significant technical problem encountered in extrusion is draw resonance, defined as a sustained periodic nonuniformity of web gauge or weight in the machine direction. In manufacturing, this is typically manifested as excessive oscillation of width and thickness of the extruded web between the slot die and moving substrate. Draw resonance can occur in any melt drawing process such as spinning, melt embossing, or extrusion coating.

Summary

The provided slot die and methods thereof represent a novel solution to the problem of draw resonance based on the concept of a negative neutral slot height and/or a negative zero-pressure slot height. Use of a slot die having a negative neutral slot height has been dismissed by experts in the extrusion arts as impossible. Yet, the provided slot die and methods have proven to be effective in eliminating draw resonance, increasing line speed, and reducing extruded film thickness. This, in turn, can enable a substantial reduction in overall manufacturing cost.

Note that while the term “negative slot height” is sometimes used herein, the actual operating slot height is never actually negative. With molten extrudate flowing through the slot die, internal fluid pressure forces open the closed slot to a pressure-deflected applicator slot. If flow is stopped, then the slot may physically close to a zero gap in which the die lips contact each other with a compressive spring force. In their “neutral” (or relaxed) state, there would be spatial interference between the die lips.

Switching from a conventional positive neutral die slot to a negative neutral die slot has the technical benefit of recovering the “spring force” of the die lip to enable a greater reduction in the operating die slot than otherwise possible. Due to physical constraints, the actuators used to adjust the shape of the flexible die lip can only elicit a limited force, individually or collectively. With the system effectively constrained by a limited force budget, it is preferred to avoid wasting this force budget to simply bend the lip itself closed when such forces could be built into the slot die.

Reduction of the slot height can help address the problem of draw resonance, a major issue across many different manufacturing operations with multiple materials. This includes any operation with drop dies such as film extrusion, extrusion coating, extrusion replication, and hot melt drop dies. Draw resonance directly impacts the profitability of products for several reasons. First, draw resonance can limit maximum manufacturing line speed, reducing productivity. Second, draw resonance degrades web thickness uniformity, leading to waste and inferior product in some cases. Third, draw resonance limits the minimum web thickness, impacting product cost related to use of materials, secondary costs of processing and handling additional material, and increased energy costs for heating and melting polymer. Finally, draw resonance reduces product market viability and profitability.

Preferred embodiments of provided slot die and methods can be advantageously deployed with the systems and methods for adjusting slot dies as described previously in U.S. Patent Nos. 9,044,894 (Loukusa et al.), 9,216,535 (Trice et al.), 9,579,684 (Yapel et al.), and 9,744,708 (Loukusa et al.). These technologies, however, can also have broader application across extrusion arts, particularly those in which draw resonance is a limitation.

In a first aspect, a slot die is provided. The slot die comprises an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, wherein the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width.

In a second aspect, a method of operating a slot die is provided, comprising an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, the method comprising: using the plurality of actuators, adjusting the first slot die surface to provide a slot height profile that is non-negative along an entire slot die width prior to extrusion; extruding through the applicator slot an extrudate to increase the cross-sectional height along some or all of the slot height profile; and using the plurality of actuators, further adjusting the first slot die surface during extrusion to achieve a target slot height profile, wherein the applicator slot has a slot height profile that is negative along at least some of the slot die width in absence of extrudate pressure effects.

In a third aspect, a method of operating a slot die is provided, comprising an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, the method comprising: using the plurality of actuators, adjusting the first slot die surface to provide a slot height profile that is non-negative along an entire slot die width prior to extrusion; extruding through the applicator slot an extrudate to increase the cross-sectional height along some or all of the slot height profile; and using the plurality of actuators, further adjusting the first slot die surface during extrusion to achieve a target slot height profile, wherein the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width. Brief Description of the Drawings

FIGS. 1A and IB are thickness profiles measured from a beta gauge showing a manifestation of draw resonance.

FIG. 2 is a cross-sectional view of a slot die according to an exemplary embodiment.

FIG. 3 is a bottom view of the slot die of FIG. 1, showing the plurality of actuators aligned along the width of the slot die.

FIG. 4 is a schematic view of an actuator assembly including a position sensor and a controller for selecting the position of the actuator assembly based on the output of the position sensor.

FIG. 5 is an enlarged fragmentary cross-sectional view of a slot die according to another embodiment, showing the die slot both in a neutral position (with upper die lip in dashed lines) and in an active position (with upper die lip in solid lines).

FIG. 6 is a block diagram showing an exemplary process for operating a slot die.

FIG. 7 is a chart showing various slot height profiles for slot dies having different configurations.

FIG. 8 shows a coating weight profile obtained using a slot die having a negative neutral slot profile.

FIG. 9 shows ambient temperature tensile strength curves for an extrudate.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

Detailed Description

As used herein, the terms “preferred” and “preferably” refer to embodiments described herein that can afford certain benefits, under certain circumstances However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

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RECTIFIED SHEET (RULE 91) ISA/EP As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or “the” component may include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements.

It is noted that the term “comprises”, and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably herein. Relative terms such as left, right, forward, rearward, top, bottom, side, upper, lower, horizontal, vertical, and the like may be used herein and, if so, are from the perspective observed in the particular drawing. These terms are used only to simplify the description, however, and not to limit the scope of the invention in any way.

Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described relating to the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention.

The technical problem of draw resonance can be observed using a scanning web thickness gauge as an oscillation in web thickness because the high frequency periodic surging (waves) of the machine direction appear as an artifact manifested by waves in the cross direction. FIGS. 1A and IB show the absence and presence of draw resonance, respectively. The provided slot dies and methods provide a unique technical advancement that can mitigate or eliminate the undesirable effects of draw resonance.

FIGS. 2 and 3 illustrate the operation of an exemplary slot die hereinafter referred to by the numeral 50. Slot die 50, shown in cross-section for clarity, includes an upper die block 52 and a lower die block 54. The upper die block 52 combines with lower die block 54 are mating halves that come together to form a fluid flow path through slot die 50. In fluid communication with each other are entry 56, die cavity 58 and applicator slot 60. The applicator slot 60 is bounded between a fixed upper die lip 61 and flexible lower die lip 62 and extends longitudinally across the die width. In this embodiment, the upper die lip 61 is integral with the upper die block 52 and the flexible die lip 62 is integral to lower die block 54. It is to be understood, however, that either or both of the die lips 61, 62 could be modular and replaceable.

The die lips 61, 62 extend across the width of the applicator slot 60. A plurality of actuator assemblies 70 are mounted on a common mounting bracket 72 and are also aligned across the width of slot die 50 as shown in FIG. 2. In some instances, the mounting bracket 72 may be segmented, where the mounting bracket 72 includes separate structures for each actuator assembly 70. Each actuator assembly 70 is operable to adjust a height of the fluid flow path at its respective location along the width of slot die 50 to provide a local adjustment of fluid flow through applicator slot 60. This adjustment occurs by changing the shape and/or position of flexible die lip 62 within the fluid flow path of the extrudate within slot die 50.

During operation of slot die 50, an extrudate enters slot die 50 at fluid flow path entry 56 (as shown by the arrow) and continues through the fluid flow path of slot die 50, including die cavity 58, until the extrudate exits through applicator slot 60 and is deposited into a nip region between counter-rotating rollers 98, 99. The extrudate can be delivered into the nip directly, or biased to contact one or the other roll just prior to the nip depending on process considerations. While not shown here, a continuous substrate can be conveyed into the nip region and provide a carrier for the extruded web.

If desired, the extrudate can then be threaded over and under a series of rollers to allow the extrudate to cool. As a further option, additional processes can be performed on the extrudate downstream of the rollers 98, 99. While not germane to this disclosure, such processes include, but are not limited to, stretching, coating, texturing, printing, cutting, rolling, and laminating. In some processes, production release liners can be removed and release liners added, or one or more additional layers (such as a laminated transfer tape) can be added. Any required curing steps could also occur, such as exposure to e-beam, a heated oven, or an ultraviolet irradiation chamber.

The extrudate is not particularly restricted and can include any conventional extrudable thermoplastic or curable composition. Useful extrudable thermoplastic compositions include, for example, polyolefins such as polypropylene (PP), high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and various known hot melt adhesives, such as styrenic block copolymers and blends thereof.

FIG. 3 shows the slot die 50 with five actuator assemblies 70. It is to be understood that more or fewer actuator assemblies are also possible, depending on the width of the slot die 50. Each actuator assembly 70 is attached to, or otherwise engaged with, the flexible die lip 62 as shown, and actuator assemblies 70 are spaced evenly along its width. Each of the actuators is operable to control the height of the fluid flow path at its location by providing a local adjustment of the position of flexible die lip 62 within the applicator slot 60 of slot die 50.

As will be discussed in further detail with respect to FIG. 4, each of actuator assemblies 70 can be motorized and can also include a precision sensor, such as a linear variable differential transformer (LVDT) or a linear encoder to quantify movements of an output shaft. The output shafts of linear actuator assemblies 70 are spaced along the width of flexible die lip 62 such that each linear actuator assembly 70 is operable to adjust the local position of the die lip. The positions of each linear actuator can be independently controlled to provide a desired cross-web profile of an extrudate. In addition, the positions of linear actuator assemblies 70 can be precisely coordinated to provide a desired die cavity pressure within die cavity 58 during the operation of slot die 50 based on the cross-sectional area of the fluid flow path adjacent to the flexible die lip 62 within slot die 50. In other examples, the positions of each actuator assembly 70 may be actively controlled to create an extrudate with patterned features, such as repeating or randomized features. As referred to herein, references to the position of an actuator or actuator assembly are intended to more specifically refer to the relative positioning of the actuator output shaft.

FIG. 4 illustrates an assembly including an actuator assembly 70, zero-backlash coupler 74 and controller 76. Actuator assembly 70 includes a motor 78, a linear actuator 82 coupled to motor 78, and a position sensor 80. The motor 78 rotates a shaft that is mechanically coupled to linear actuator 82. A sensor 80 measures the position of the linear actuator 82. For example, sensor 80 may be a LVDT sensor or a linear encoder. Sensor 80 is secured to output shaft 84 of linear actuator 82 with a clamp 86 and precisely measures the relative position of output shaft 84 of linear actuator 82. In other examples, the sensor 80 could measure the position of the coupler 74, die actuator linkage 88, or even the flexible die lip itself. Controller 76 receives position inputs from both the motor 78 and the sensor 80. For example, the motor 78 can be a stepper motor and provide an indication of the number of “steps” the stepper motor has taken from a known reference position of the stepper motor. The sensor 80 can provide more precise position information to controller 76 than that provided by the motor 78. The controller 76 provides instructions to the motor 78 to drive the output shaft 84 of actuator assembly 70 to a preselected position. In some examples, controller 76 may control a set of actuator assemblies 70, either simultaneously or sequentially. Here, controller 76 can be used to control each of the actuator assemblies 70 in slot die 50 shown in FIGS. 2 and 3.

Referring again to FIG. 4, the zero-backlash coupler 74 includes two halves that are fastened together — bottom half 90 and top half 92. The bottom half 90 is directly attached to die actuator linkage 88 using a screw or bolt. In addition, the zero-backlash coupler 74 includes a stacked protrusion assembly that bolts onto the end of the output shaft 84 of the actuator assembly 70. The stacked protrusion assembly includes two metallic discs 94 surrounding an insulative disc 96. The insulative disc 96 may comprise a ceramic material. The bottom half 90 and top half 92 combine to encircle the stacked protrusion assembly, including the metallic discs 94 and the insulative disc 96, which are bolted onto the end of the output shaft 84. With the top half 92 securely fastened to the bottom half 90, the output shaft 84 is effectively registered with the zero-backlash coupler 74 and the die actuator linkage 88.

The zero-backlash coupler 74 functions to thermally isolate the actuator assembly 70 from the slot die. Within this assembly, the insulative disc 96 significantly limits the metal-to-metal contact path between output shaft 84 of actuator assembly 70 and die actuator linkage 88 and protect the actuator assembly 70 from damaging heat emanating from the slot die 50. Slot dies commonly operate at temperatures in excess of 300°F (149°C), whereas the components of actuator assembly 70, including motor 78 and sensor 80 may experience limited functionality or even permanent damage when subjected to temperatures to in excess of 130°F (54°C). For this reason, zero-backlash coupler 74 preferably helps in keeping the temperature of actuator assembly 70 at 130°F (54°C) or less.

In alternative embodiments, the metallic discs 94 can be replaced by discs of nonmetallic materials to avoid any metal-to-metal contact between the output shaft 84 and the die actuator linkage 88. Such configurations further thermally isolate actuator assembly 70 from the slot die housing. As a further example, the surface area of the coupler 74 can be configured as a passive heat dissipator to keep the temperature of the actuator assembly 70 at 130°F (54°C) or less. This might be used independently or in combination with the insulative disc 96. In further examples, active thermal control can be used to cool the zerobacklash coupler 74, output shaft 84 or actuator assembly 70.

In contrast to slot-die designs that utilize differential bolts as actuation mechanisms, the zero-backlash coupler 74 couples the output shaft 84 of actuator assembly 70 to die actuator linkage 88 with little or no backlash. While a differential bolt mechanism can display a backlash of more than one-hundred micrometers, zero-backlash coupler 74 may provide almost no backlash, such as less than ten micrometers, or even less than five micrometers, such as about three micrometers.

In a slot die utilizing a set of differential bolts to control applicator slot width, the relatively large backlash of each differential bolt means that adjusting the position of one differential bolt may change the height of the fluid flow path at other bolts. In slot die 50, the position of output shaft 84 of actuator assembly 70 directly corresponds to the local position of flexible die lip 62. For this reason, slot die 50 facilitates repeatable, precise positioning not available in slot dies utilizing differential bolts as actuation mechanisms.

Other actuation mechanisms are also possible. For example, the actuation mechanism can include one or more of thermally adjustable bolts, differential bolts, piezoelectric actuators, pneumatic actuators, and hydraulic actuators. In some embodiments, the applicator slot can be adjusted by applying a pressing load or tensile load to a flexible die lip using a lever supported by a rotating shaft as a fulcrum, along with an operating rod displaced in an axial direction by the body of the slot die. Rotational force of the lever can be converted into a force in the axial direction of the operating rod, and the force in the axial direction becomes a pressing load or a tensile load exerted on the flexible die lip. The lever directly can apply a force to the operating rod at the point of action of the lever.

In another example, thermally adjustable bolts automatically regulate the applicator slot using a plurality of adjusting pins, coupled to respective thermoelements disposed on a flexible die lip. In these configurations, the applicator slot can be adjusted through the action of mechanical force applied to the flexible die lip by the corresponding adjusting pin through expansion or contraction of the thermoelements. As a further option, the actuation mechanism can include providing at least two adjusting pins and/or thermoelements that are simultaneously adjusted. Aspects of the foregoing and other variants are described in U.S. Patent No. 9,700,911 (Nakano) and International Patent Publication No. WO 2019/219724 (Colell et al.). Each of these actuation mechanisms is capable of controlling the flexible die lip by applying pushing or pulling forces at precise locations spaced along the flexible die lip.

FIG. 5 provides a fragmentary view of a slot die 100 having upper and lower die blocks 102, 104 and an applicator slot defined as the space between fixed upper die lip 111 and flexible lower die lip 112. In this figure, remaining components of the slot die 100, including actuator assemblies and control systems, are omitted for the sake of simplicity.

In practice, many forces simultaneously bear upon the upper and lower die lips 111’, 112 and contribute to the actual slot height profile obtained. Besides occlusion forces that prevent overlap between the upper and lower die lips 111’, 112, there are also forces created by the flow of molten fluid (i.e., extrudate pressure effects) through the slot die 100 that tend to widen the applicator slot. Finally, there are active forces applied by the plurality of actuators used to adjust web thickness, such as the actuator assemblies 70 in FIGS. 2 and 3.

As used herein, the slot height reflects the spatial separation between opposing slot die surfaces on the die lips defining the applicator slot, and can be physical and/or theoretical. In FIG. 5, the slot die 100 is shown in two distinct applicator slot configurations. The first configuration, set out in solid lines, is represented by an upper die lip 111 and flexible die lip 112, resulting in a positive slot height h. The second configuration is represented by a fixed upper die lip 111’ set out in dashed lines and the flexible die lip 112, resulting in a negative slot height h

While the negative slot height h ’ is theoretical and not physical, it is nonetheless a useful mathematical construct that can be defined and calculated given certain assumptions. For example, h ’ can represent the degree of overlap between the fixed upper die lip 111’ and flexible die lip 112 along the slot height dimension that would occur if these components could pass through one another in absence of any external forces. This defines the neutral slot height, with the neutral slot height values across the width of the slot die 100 collectively defining the neutral slot height profile.

Alternatively, h ’ can represent the degree of overlap between the fixed upper die lip 111’ and flexible die lip 112 along the slot height dimension that would occur if these components could pass through one another in absence of internal extrudate pressure effects but including any forces applied on flexible die lip 112 by the plurality of actuator assemblies 70. This measurement is referred to as the zero-pressure slot height, with the zero-pressure slot height values across the width of the slot die 100 collectively defining the zero-pressure slot height profile. Extrudate pressure effects can be modeled, as described in International Patent Publication No. WO 2022/123355 (Secor et al.) or otherwise calculated empirically.

Whether slot height h ’ represents the neutral slot height or the zero-pressure slot height, the slot height h ’ can be from -2500 micrometers to 3500 micrometers, from -1300 micrometers to 1650 micrometers, from -1300 micrometers to 500 micrometers, or in some embodiments, less than, equal to, or greater than -2500 micrometers, -2000, -1500, -1300, -1000, -500, 0, 500, 1000, 1500, 1650, 2000, 2500, 3000, or 3500 micrometers.

With extrudate flowing through the slot die, the pressure forces apply outward forces that open up the closed slot to a pressure-deflected die lip configuration. If flow is stopped, then the applicator slot physically closes to a zero gap with the die lips in mutual contact. Would this occur, the flexible die lips would be in a stressed (i.e., non-relaxed) state, reflecting an equilibrium where compressive spring forces from the die lips urge against each other in opposition.

Advantageously, the change from a conventional positive neutral die slot to a negative neutral die slot can recover available spring force of the flexible die lip to provide a greater reduction in the operating die slot. This is technically significant because there is a limited actuator force budget, and this force budget is not wasted on simply bending the flexible die lip towards its closed (i.e., zero slot height) position.

Use of an applicator slot that has a negative neutral slot profile along at least some of the slot die width can provide substantial technical advantages over conventional slot dies, particularly in suppressing undesirable draw resonance. Draw resonance is reflected by a sustained periodic nonuniformity of gauge or weight in the machine direction, and can present a significant technical problem across many manufacturing processes and for many extrudate compositions. Manufacturing processes adversely impacted by draw resonance include, but are not limited to, operations with drop dies such as film extrusion, extrusion coating, extrusion replication, and hot melt drop dies. Reduction or elimination of draw resonance can enable increased line speed, reduced extruded web thickness, and overall reduction in web manufacturing cost. Direct contact between the upper and lower die lips 111’, 112 can cause brinelling of the slot die 100, or permanent deformation at its surface. It is generally preferable that extrudate pressure effects and/or actuator forces are sufficient at all times to avoid such contact. This in turn can require special considerations and deviations from techniques used to operate conventional slot dies. These modes of operation will be discussed further below.

Other die configurations are also possible. Although not shown, for example, both of the opposing die lips could be independently adjustable by separate adjustment mechanisms. Such configurations, along with related options and advantages, are described in International Patent Publication No. WO 2022/123294 (Yapel, et al.).

FIG. 6 shows an exemplary workflow 150 using a control loop to achieve a slot height adjustment in a provided slot die, such as slot die 50. Computational prediction of a slot die adjustment is possible given the desired actuator positions (and by inference slot height profile), along with extrudate target thickness profile and the current measured web thickness profile. Given certain assumptions, the relationship between slot height profile and web thickness profile can be calculated, making it possible to predict a slot height profile suitable to obtain the target web thickness profile.

At the outset, the manufacturing process adopts initial actuator setting profiles (block 151). These actuator settings may or may not be pre-determined. These initial profiles can be saved based on historic data based on similar process conditions, or lacking such data, these profiles could be set to default values based on user input. The actuators therefore assume initial positions according to these pre-determined settings at the outset of a control loop. Advantageously, the initial actuator settings can provide an applicator slot that has a non-negative slot height profile along the entire slot die width. As mentioned above, this configuration can help avoid damage to the die lips incurred by collisions between these parts. In a preferred embodiment, the pre-determined settings for the actuators are constant, or flat, across most or all of the slot die width.

With an extrusion process underway, block 152 shows a first step of providing a profile measurement for the web extruded from the applicator slot of the slot die. As described previously, any known measurement technique can be used. The profile measurement can be taken as cast or after some the extrudate has been oriented or otherwise altered. In the former case, measurement of web thickness profile occurs close to where the extruded web exits the applicator slot (“as cast”). The thickness measurement could be carried out shortly after the molten web is solidified, or alternatively could take place at a location after the cast web has been post-processed (“as converted”).

Some gauging devices scan back and forth across the width of the moving web to collect data on web thickness. In many instances, these measurements can involve a very large number of discrete measurements, which are not only scattered across the entire width of the web but may be taken at different points in time. To manage this, a controller can process the raw discrete measurements into a smaller number of data values, that uses a form of cross-web and down-web averaging to capture a virtual snapshot of cross-web thickness profile at a particular location along the continuous web.

The provided methods can be used for both continuous full-web coated extrudates and stripe coated extrudates. In a full-web coating, the multitude of measured values represent the entire coated width. In the case of stripe coating, measured thickness values corresponding to a plurality of coated regions spaced apart each other along a cross-web direction, such that the measured values only cover the coated stripes and not the uncoated lanes in-between.

Block 154 represents the next step of performing a web-to-die mapping of these profile measurements. Certain simplifying assumptions can be made when mapping the gauge measurement to the die. First, it can be assumed that the data received from the profile measurements are taken from edge-to-edge, which is generally the case for modern gauging systems that have edge detection. Second, it is assumed that gauge data directly corresponds to actual extrudate weights/thicknesses. If the web is disposed on a release liner, the basis weight of the release liner can be subtracted such that the data represents the coating weight only. Finally, it can be assumed that any neck-in from both edges of the extruded web occurs symmetrically.

The number and locations of the coarse-grained measurements is often different from that of the die lip actuators, and so, a generalized method of mapping the measurement values to actuator locations is preferred. Film neck-in calculations can consider the effects of different flow rates, draw distances, and web speeds. In one embodiment, coated web coordinates in physical units (inches or mm) are mapped to the coordinates in actuator units, (i.e., a die coordinate system based on sequential actuator number). This process enables mapping from a coating measurement array value to a physical coated web position, and then through the web-to-die mapping function to a physical die position. Here, the coating measurement array can represent, for example, the most recent array of thickness measurements generated each time a scanner traverses the moving web.

With a suitable die-to-web or web-to-die mapping function, it may be advantageous to utilize the function elsewhere in the profile control scheme. For example, the mapping could incorporate cross-web variation in the control parameters, such as controller gain or actuator interaction through the coating flow field. This could be done in an ad hoc fashion based on production experience or in a more scientifically-based manner using fluid mechanical models of the coating flow field.

Modeling the flow may use of any appropriate models characterizing fluid rheology. For example, modeling the flow may include finite element analysis or may more directly rely on one or more equations. Exemplary mathematical models are described elsewhere, for example in co-pending International Patent Application No. WO2022/123355 (Secor et al.).

In some embodiments, mapping this relationship is facilitated by scan-1 ocation-to- di e-bolt-location mapping already built into the measurement system. Various commercial systems are capable of performing web-to-die mapping, such as profile control solutions from NDC Infrared Engineering Inc., Irwindale, CA.

Proceeding then to block 156, a profile deviation for the web thickness is then determined. This can be provided by a residual array of values (i.e., vector) representing the degree to which the current web thickness profile deviates from a target web thickness profile (block 158) at each control zone. The control zone, as referred to herein, represents the segment of the slot die corresponding to a given actuator.

The initial target profile in block 158 can be defined at a location near to the extrudate exiting the slot die (e.g., near applicator slot 60 in FIG. 2) or, alternatively, a location downstream from where the web has been partially or fully converted. It can be of significant technical benefit to incorporate both types of measurements to fully understand, and correct for, deviations between desired and actual web thickness profiles that may be attributable to different sources in a given manufacturing line.

In subsequent iterations of workflow 150, the profile deviation determined in block 156 can be based on either the initial target web thickness profile set in block 158, or if desired, a revised target web thickness profile. Revisions to the target web thickness profile need not be particularly restricted and can be pre-determined using a computer algorithm or manually by input from an operator.

When the workflow 150 is initially executed, the target web thickness profile is provided. This profile can be based on a saved web thickness profile, such as an array of web thicknesses used in a previous iteration of the workflow 150 or other workflow conducted under similar conditions. For example, a previously used target web thickness profile when the same or similar slot die and similar flow conditions through the slot die are being used. Generally, a flat extrudate profile is desired, but there are situations where other profiles are preferred. For example, a “dog-bone” shaped target profile might be desired to control edge instabilities in the extrusion process, such as those relating to draw resonance or edge scalloping.

In block 156, a profile deviation is then determined based on the difference between the initial target web thickness profile from block 158 and a measured web thickness profile, if available. Initial iterations of the control loop in workflow 150 can be carried out before a complete set of web thickness measurements is obtained. If a measured web thickness profile is not yet available, adjustments in the slot die may be delayed until such time a sufficient number of web thickness measurements are available. To the extent that a partial profile measurement (block 152) might be obtained, the workflow 150 could be used to adjust the slot die along one portion of the applicator slot but not another.

With a profile deviation thus determined, an appropriate slot height adjustment can then be calculated (block 160). When predicting the adjustment to the actuators engaged to the flexible die lip suitable to achieve the initial target profile from block 158, it can be advantageous to apply a pressure die deflection model that enables the effects of fluid flow through the die to be decoupled from mechanical effects relating to the slot die in absence of fluid flow. A suitable model can predict the pressure profile across the applicator slot and corresponding amount of pressure deflection in the flexible die lip. In some cases, this pressure-based deflection is measured empirically.

The pressure deflection can be accounted for by adding it to a calculated slot height to obtain a target slot height profile. This target slot height profile can then be compared with a current slot height profile (block 162) to determine an appropriate adjustment. Notably, the current slot height includes some degree of pressure deflection (D) that is due to extrudate composition flowing through the applicator slot. The deflection D solely attributable to the fluid can be subtracted from the pressure-compensated slot height profile, H+D, to obtain the zero-pressure slot height profile (H), which can then be used to calculate the target actuator positions.

Mathematical models describing fluid dynamics and pressure effects in the slot die are described in detail elsewhere, such as in co-pending International Patent Application Nos. WO2022/123296 (Secor, et al.) and WO2022/123355 (Secor, et al.).

The target actuator positions can provide an applicator slot having a negative zeropressure slot height profile (i.e., slot height profile in absence of extrudate pressure effects) along at least some of the slot die width. In some embodiments, the target actuator positions can provide an applicator slot having a negative zero-pressure slot height profile (i.e., slot height profile in absence of extrudate pressure effects) along the entire slot die width. In some embodiments, the provided slot die configuration may include a negative neutral slot height profile along at least some of the slot die width. Optionally, the provided slot die configuration may comprise a negative neutral slot height profile along the entire slot die width.

When making adjustments during an extrusion operation, a controller can predict a set of discrete actuator settings appropriate to arrive at the desired slot height profile. In a preferred embodiment, this set of discrete actuator settings is based on a plurality of actuator settings corresponding to a pre-selected cross-web profile. As will be described below, the plurality of discrete actuator settings can be obtained using a predictive model to determine an appropriate actuator adjustment (block 164).

In a preferred embodiment, the prediction of the discrete settings from the plurality of discrete settings and/or the zero-pressure current slot height, as derived from the current slot height (block 162), can be obtained through a mathematical construct called a stiffness matrix. The stiffness matrix transforms actuator settings to corresponding slot height profiles in absence of flow field physics effects. While not examined in detail here, International Patent Publication No. WO 2012/170713 (Secor, et al.) describes in greater detail an example of how empirical test data can be used to populate the stiffness matrix for a given slot die.

At a high level, the stiffness matrix and inverse stiffness matrix are used to convert actuator position changes to slot height changes, and vice versa. The stiffness matrix is represented by Equations 1 and 2 below. (Equation 1) (Equation 2)

In Equations 1 and 2, ho and h are vectors representing current and new slot heights, respectively, A and Ao are vectors representing of current and new actuator positions (in actuator units), respectively, K is a stiffness matrix, I is the identity matrix, and & is a proportionality constant that converts actuator position changes to slot height changes and is the same for each actuator.

The stiffness matrix K generally has NxN components (TV being the total number of active actuators) and physically represents the change in the slot height at a measurement point due to a change in the j^th actuator setting, Aj . The stiffness matrix can reflect significant mechanical interactions between actuators. Commonly, an actuator position change can produce a slot height change at a position several actuators away. In some embodiments, this is represented by a banded stiffness matrix. In many cases, the banded stiffness matrix can be assumed to be symmetric. For example, the matrix could have significant nonzero components only along 5 diagonals of the matrix - the main diagonal plus 2 diagonals on either side.

Once the stiffness matrix is determined, slot height measurements can be used in two ways. First, for any set of actuator settings, the resulting slot height profile can be computed by multiplying the stiffness matrix with the actuator displacements according to Equations 1 and 2 above. Secondly, the actuator settings that will result in a specified slot height profile are calculated by multiplying the inverse of the stiffness matrix with the desired change in slot height. For determining the appropriate set of actuator adjustments, Equation 3 is used:

(4 — i4₀) = K^-1 • (h — h₀ (Equation 3)

In block 164, the active actuators can then be adjusted to appropriate positions according to the calculated actuator adjustments above to achieve the desired slot height profile. In a preferred method to solve this linear algebra problem, a Gaussian Elimination is performed to obtain a matrix-based solution of stiffness matrix transformations.

With the actuator adjustments completed, the workflow 150 returns to block 152 where a new profile measurement is taken and the process described above starts anew. The steps in workflow 150 can progressively improve the actuator settings/positions based on real time web thickness measurements. Over time, through these stepwise adjustments, the desired target web thickness profile can be achieved.

In a comparison between (i) a first slot die (EX-1) having a 4 mil neutral slot height and (ii) a second slot die (EX-2) having a -10 mil neutral slot height, Table 1 reports characteristics of the extruded web under various extrusion conditions. Web thickness calculations assumed a polypropylene density of 0.9 g/cm³.

As shown, the latter enables a very significant reduction in pressure-adjusted slot height, from 1020 micrometers (40 mils) to 560 micrometers (22 mils). This is also reflected in the lower web thickness (19 gsm versus 32 gsm) and lower draw ratio (26.5 versus 28.6) produced using the EX-2. The two EX-2 runs in Table 1 represent webs made at two different extruder flow rates. Compared to the EX-1 run, there was a 45% decrease in draw ratio to achieve the same thickness and even a 7% decrease in draw ratio when making web of lower (19 gsm) thickness. In both cases, the lower draw ratio was found to significantly suppress draw resonance.

Table 1.

FIG. 7 shows both pressure-adjusted and zero-pressure slot height profiles for the dies EX-1 and EX-2 above. As can be observed in this chart, the zero-pressure slot height profiles for both EX-1 and EX-2 extended to negative values along at least a portion of the slot die width. EX-2, however, provides a much greater useable operating window for making low thickness web at potentially lower draw ratios to reduce or eliminate draw resonance. FIG. 8 shows the consistency of the thickness of the extruded web as cast onto a polypropylene backing having uniform thickness. This is captured through measurement of basis weight plotted against control zone index. Displaying an average total weight of 61 gsm at startup, good web thickness uniformity was obtained when manufacturing web from the EX-2 slot die. This is evidenced by only slight variance in the total weight of the material, with extrudate weight profile being well controlled at 32 gsm and manifesting a 3<j parameter of 2.16.

It was further discovered that draw resonance can be mitigated further through appropriate choice of extrudate composition. It can be beneficial, for example, to select an extrudate that has higher melt flow rate and a tensile strength curve less favorable to resonance, where possible. The limit deformation energy En_m represents the maximum amount of deformation energy that a polymer sample can tolerate before yielding or rupturing, and is equivalent to the area under the tensile strength curve from zero elongation up to the point at which the material yields or ruptures.

FIG. 9 illustrates various ambient temperature tensile strength curves for the extrudate and how they correlate with the likelihood of draw resonance. As shown, curve A displays a critical tension stress Gen, where there is a break in the tensile tension curve. Curve B shows a yield stress Gyieid representing a more severe break in the tension curve where irrecoverable deformation has occurred. Since it is impractical to obtain tensile strength curves and calculate limit deformation energy En_m at process temperatures, samples of extrudate material can be tested at ambient temperature instead. A higher value of En_m generally reduces the likelihood of draw resonance. In this hypothetical figure, curve A presents a En_m that tends to favor draw resonance, while curve C presents a comparatively higher value of En_m that would tend to disfavor draw resonance.

As shown, WA, WB, and Wc refer to triangular areas approximating values of En_m for curves A, B and C, respectively. En_m values can be material-specific. For example, useful polyolefin extrudate compositions can have an En_m at 25°C from 0.65 MJ/m³ to 1.87 MJ/m³, while styrenic block copolymers used at hot melt adhesive formulations can have an En_m at 25°C from 12 MJ/m³ to 206 MJ/m³, and preferably greater than 139 MJ/m³.

The techniques described in this disclosure may be implemented with hardware, software, firmware or any combination thereof. For example, various examples of the provided methods can be implemented within one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in controllers, user interfaces or other devices. The term “controller” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry.

Where implemented in software, the functionality ascribed to the systems and controllers described in this disclosure may be embodied as instructions on a computer- readable storage medium such as random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), FLASH memory, magnetic media, optical media, or the like. The instructions may be executed to cause one or more processors to support one or more examples of the functionality described in this disclosure. Various examples have been described in the foregoing passages.

All cited references, patents, and patent applications in the above application for letters patent are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.

Claims

CLAIMS: What is claimed is:

1. A slot die comprising: an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, wherein the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width.

2. The slot die of claim 1, wherein the neutral slot height profile is negative along an entire slot die width.

3. The slot die of claim 1 or 2, wherein the neutral slot height profile has neutral slot heights within a range of from -2500 micrometers to 3500 micrometers.

4. The slot die of claim 3, wherein the neutral slot height profile has neutral slot heights within a range of from -1300 micrometers to 1650 micrometers.

5. The slot die of claim 4, wherein the neutral slot height profile has neutral slot heights within a range of from -1300 micrometers to 500 micrometers.

6. A method of operating a slot die comprising: an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, the method comprising: using the plurality of actuators, adjusting the first slot die surface to provide a slot height profile that is non-negative along an entire slot die width prior to extrusion; extruding through the applicator slot an extrudate to increase the cross-sectional height along some or all of the slot height profile; and using the plurality of actuators, further adjusting the first slot die surface during extrusion to achieve a target slot height profile, wherein the applicator slot has a slot height profile that is negative along at least some of the slot die width in absence of extrudate pressure effects.

7. The method of claim 6, wherein the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width.

8. A method of operating a slot die comprising: an applicator slot extending across a slot die width and comprising opposing first and second slot die surfaces, wherein the applicator slot is in fluid communication with a fluid flow path through the slot die; a flexible die lip providing the first slot die surface and capable of independently adjusting a cross-sectional height of the fluid flow path; and a plurality of actuators spaced along the width of the first slot die surface, each actuator operatively coupled to the flexible die lip to adjust the cross-sectional height at its respective location to provide a local adjustment of fluid flow through the applicator slot, the method comprising: using the plurality of actuators, adjusting the first slot die surface to provide a slot height profile that is non-negative along an entire slot die width prior to extrusion; extruding through the applicator slot an extrudate to increase the cross-sectional height along some or all of the slot height profile; and using the plurality of actuators, further adjusting the first slot die surface during extrusion to achieve a target slot height profile, wherein the applicator slot has a neutral slot height profile that is negative along at least some of the slot die width.

9. The method of claim 8, wherein the applicator slot has a slot height profile that is negative along at least some of the slot die width in absence of extrudate pressure effects.

10. The method of any one of claims 6-9, wherein the extrudate comprises a recycled or degraded polymer.

11. The method of any one of claims 6-10, wherein the extrudate comprises a blend of compatible polymers.

12. The method of any one of claims 6-11, wherein the extrudate comprises one or more of polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene, and hot melt adhesive.

13. The method of any one of claims 6-12, wherein the extrudate comprises a polyolefin and has a limit deformation energy of from 0.65 MJ/m³ to 1.87 MJ/m³.

14. The method of claim 6-12, wherein the extrudate comprises a styrenic block copolymer and has a limit deformation energy of from 12 MJ/m³ to 206 MJ/m³.

15. The method of claim 14, wherein the extrudate has a limit deformation energy of from 139 MJ/m³ to 206 MJ/m³.

16. The slot die or method of any one of claims 1-15, wherein both first and second slot die surfaces are independently adjustable by separate adjustment mechanisms.