Linear flow equalizer for uniform polymer distribution in a spin pack of a meltspinning apparatus

A linear flow equalizer for distributing thermoplastic material to a spin pack of a meltspinning apparatus that provides for uniform apportionment of a flow of a flowable thermoplastic material at least vertically and in a cross-machine direction of the spin pack. The linear flow equalizer includes an inlet plate with multiple liquid passageways equidistantly spaced in the cross-machine direction that each provide flowable thermoplastic material to a set of equalizer plates. Elongated slots extending through alternating equalizer plates are registered with throughholes extending through adjacent plates in the equalizer plate set. Each throughhole in an upstream equalizer plate is registered with the center of a corresponding slot and each throughole in a downstream equalizer plate is registered with one of opposed closed ends of a corresponding slot.

FIELD OF THE INVENTION

The present invention relates generally to melt-spinning apparatus and methods, and more particularly to a linear flow equalizer for a spin pack of a melt-spinning apparatus and methods of forming non-woven webs with a melt-spinning apparatus incorporating the linear flow equalizer of the invention.

BACKGROUND OF THE INVENTION

Non-woven webs are incorporated into a diversity of consumer and industrial products, including disposable hygienic articles, throwaway protective apparel, fluid filtration media, and household durables. Generally, non-woven webs are formed using melt-spinning technologies, such as spunbonding processes and meltblowing processes, that form continuous filaments or fibers composed of one or more thermoplastic polymers. Spunbond non-woven webs are relatively strong in both the machine and the cross-machine directions because of drawing that aligns the polymer molecules. The continuity of the filaments also contributes to the observed strength of spunbond non-woven webs. Spunbond non-woven webs also resist abrasion, have a high porosity, and may be soft and conformable.

Spunbonding processes generally involve pumping one or more molten thermoplastic polymers through a spin pack that distributes, filters, combines, and finally extrudes continuous filaments of the constituent thermoplastic polymer(s) through hundreds or thousands of spinneret holes or orifices in a spinneret. After extrusion, the filaments are cooled or quenched to increase their viscosity and then drawn or stretched by an impinging high-velocity airflow generally capable of orienting the molecules of each constituent thermoplastic polymer if the air velocity is sufficiently high. The airflow propels the drawn filaments toward a forming zone to form a non-woven web on a moving collector.

The spin pack distributes a flow of each constituent thermoplastic polymer from a few inlet ports to individual outlet ports that span the width of the spin pack. Specifically, the molten thermoplastic polymer from each inlet port is directed into a shared lateral flow passageway and individual portions of the incoming thermoplastic polymer are allocated from the lateral flow passageway to the outlet ports for subsequent distribution to the orifices in the spinneret plate. Because all of the inlet ports share a single lateral flow passageway, thermoplastic material streaming from adjacent inlet ports into the lateral flow passageway intersects, collides and mixes before arriving at the outlet ports. The intersecting streams of molten thermoplastic polymer may experience hold-ups, dead spots or stagnation zones, and/or recirculation within the lateral flow passageway. The individual streams of the polymer(s) from the outlet ports are ultimately supplied to the orifices in the spinneret.

The inability to uniformly divide the incoming stream of the molten thermoplastic polymer in the machine direction and in the cross-machine direction with uniform flow characteristics to the outlet ports causes unacceptable variations in the non-woven web formed by the spunbonding process. For example, non-uniform distribution of the molten thermoplastic polymer in cross-machine direction may cause the basis weight of the non-woven web to fluctuate in the cross-machine direction, which produces perceptible strips of varying basis weight extending parallel to the machine direction. In particular, the basis weight of the non-woven web originating from filaments extruded from spinneret orifices receiving thermoplastic polymer from outlet ports directly downstream of an inlet port has been observed to be significantly larger than the basis weight of the non-woven web originating from filaments extruded from spinneret orifices receiving thermoplastic polymer from outlet ports near the mid-point between adjacent inlet ports. The fluctuation in the basis weight is believed to arise from unequal flow path lengths in the shared lateral flow passageway. This results in non-uniform residence times and pressure drops for different portions of the non-Newtonian thermoplastic polymer exiting the outlet ports from the lateral flow passageway. The non-uniform flow path lengths also result in disparate shear histories for different portions of the thermoplastic polymer flowing in the lateral flow passageway reflected in the polymer properties and the characteristics of the non-woven web formed therefrom.

It would be desirable, therefore, to provide a spin pack for a melt-spinning apparatus capable of forming a non-woven web having improved basis weight uniformity in the cross-machine direction.

SUMMARY

In one aspect, the invention is directed to an apparatus for distributing thermoplastic material supplied to a spin pack of a meltspinning apparatus. The apparatus includes a linear flow equalizer having a plurality of flow passageways of substantially equal length that divide a flow of a thermoplastic material supplied from a plurality of liquid inlet ports into individual streams having a spaced relationship in a cross-machine direction.

In one specific embodiment of the apparatus of the invention, the linear flow equalizer includes an inlet plate having a plurality of liquid passageways spaced substantially equidistantly in a cross-machine direction of the meltspinning apparatus, a first equalizer plate positioned downstream from the inlet plate, and a second equalizer plate positioned downstream from the first equalizer plate. The first equalizer plate has elongated slots each centered about one of the plurality of liquid passageways. Each of the first plurality of elongate slots extends in the cross-machine direction and includes opposed closed ends substantially equidistant from one of the plurality of liquid passageways. The second equalizer plate has throughholes each substantially registered in alignment with one of the opposed closed ends of a corresponding one of the first plurality of elongated slots.

Another aspect of the invention is directed to a method of distributing thermoplastic material supplied to a spin pack to form a non-woven web. To that end, a flow of thermoplastic material is divided in a cross-machine direction of a spin pack among liquid passageways of substantially equal path length to form individual streams of thermoplastic material spaced in the cross-machine direction. The individual streams of thermoplastic material are shaped or formed into filaments, which are quenched, drawn, and collected to produce the non-woven web.

In accordance with the principles of the invention, the flows of thermoplastic material within the linear flow equalizer are partitioned homogeneously and symmetrically in the cross-machine direction and vertically in a downstream direction. The basis weight of the non-woven web produced by a melt spinning apparatus incorporating the linear flow equalizer of the invention is more uniform in the cross-machine direction. The improved uniformity in the basis weight is believed to arise from equal or nearly equal flow path lengths in the spin pack, which results in more uniform residence times and pressure drops for different divided portions of the thermoplastic polymer and approximately equal shear histories. As a result, the properties of the non-woven web are substantially independent of the lateral location of the outlet port from the final downstream equalizer plate relative to the individual inlets in the inlet plate. In accordance with the principles of the invention, the linear flow equalizer of the invention optimizes the flow distribution of the thermoplastic polymer(s) while achieving a uniform shear rate and a minimum residence time in the die pack.

These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIGS. 1 and 2, a spin beam assembly, generally indicated by reference numeral10, for forming filaments includes a chassis12holding drive pumps14,15,16,17each driven by a corresponding one of a set of motors18,19,20,21. The motors18–21are suspended from the chassis12by an open framework of beams22and generally overlie the drive pumps14–17. Extending from each of the motors18–21is a drive shaft18a,19a,20a,21athat supplies a drive coupling with a corresponding one of the drive pumps14–17. The spin beam assembly10is incorporated into a melt-spinning apparatus that includes conventional components, such as a filament-drawing device for attenuating the filaments and a moving collector located on a forming table, for forming a non-woven web.

Drive pumps14and16receive a flow of a first polymer (Polymer A) furnished by a supply line23from an extruder (not shown) and drive pumps15and17receive a flow of a second polymer (Polymer B) furnished by a separate supply line24from another extruder (not shown). The invention contemplates that the drive pumps14–17may be supplied by a single supply line communicating with and service by a single extruder. The first and second polymers may differ in composition, such as polyethylene and polypropylene, or may constitute two polymers of identical composition that differ with respect to a property such as melt flow rate or the presence or absence of an additive. The two polymers are heated to a temperature sufficient to produce a liquid or semi-solid material having a viscosity suitable for flow through an arbitrary set of passageways.

With continued reference toFIGS. 1 and 2, a pump plate26attached to the chassis12supports the pumps14–17. Extending through the pump plate26is a plurality of liquid passageways28, of which two liquid passageways28are shown inFIG. 2, arranged in rows such that each is coupled in fluid communication with an outlet of one of the drive pumps14,16. Also extending through the pump plate26is a plurality of liquid passageways30, of which one liquid passageway30is shown inFIG. 2, each coupled in fluid communication with an outlet of one of the drive pumps15,17. Accordingly, each pump14,16outputs a stream of polymer A to the liquid passageways28and each pump15,17outputs a stream of polymer B to the liquid passageways30.

The spin beam assembly10further includes a spin pack, generally indicated by reference numeral32, supported by support brackets34,36within a housing38of chassis12. The spin pack32receives separate flows of the two polymers from the liquid passageways28,30in pump plate26. The spin pack32is an assembly that incorporates, in order from a top or upstream side to a bottom or downstream side, a linear flow equalizer40, a combining plate42, and a spinneret plate44. A major or long axis of the spin pack32is aligned generally parallel to a cross-machine direction45(FIG. 1), which is generally orthogonal to a machine direction46. A collector (not shown) collects the filaments discharged from the spinneret plate44of spin pack32.

With reference toFIGS. 2 and 3, the linear flow equalizer40is an assembly constituted by an inlet plate48and three equalizer plate sets50a–c. The inlet plate48includes inlet ports or passageways52, visible inFIG. 3, arranged in three spaced linear rows to coincide with the locations of liquid passageways28,30. Adjacent inlet passageways52in each of the rows are spaced at equal centerline-to-centerline intervals, or a uniform pitch, across the width of the inlet plate48. In one specific embodiment of the invention, the inlet plate48features three rows of eight inlet passageways52.

Inlet passageways52in the center row are registered for fluid communication at an upstream surface54of inlet plate48with the liquid passageways30in the pump plate26. Similarly, inlet passageways52in the two rows flanking the center row are registered in fluid communication at the upstream surface54of inlet plate48with the liquid passageways28in the pump plate26. Accordingly, each inlet passageway52in the center row receives an output stream of polymer B from one of pumps15,17and each inlet passageway52in the rows flanking the center row receives an output stream of polymer A from one of pumps14,16. The rows of inlet passageways52in inlet plate48adjacent the front and rear edges of the spin pack32distribute respective output streams of Polymer A to the equalizer plate sets50a,50c. The central row of inlet passageways52distributes an output stream of Polymer B to the center equalizer plate set50b.

In accordance with the principles of the invention, the fluid pathways in the linear flow equalizer40define approximately equal length lateral and vertical flow paths and, preferably, equal length flow paths, for each polymer stream in a flow path extending from the downstream side of the pump plate26to the downstream side of each of the equalizer plate sets50a–c. The approximately equal lengths of the lateral and vertical flow paths in the linear flow equalizer40result in approximately uniform residence times and shear histories characterizing the polymer flows through the linear flow equalizer40. Preferably, the lateral and vertical flow paths for the polymers in the linear flow equalizer40are equal in length for providing optimum filament properties. Consequently, material properties of the resultant non-woven, such as basis weight, possess an improved uniformity in the cross-machine direction45.

With reference toFIGS. 3–5, the inlet plate48includes shallow rectangular recesses or cavities56,57,58partitioned from one another by dividing walls59,60. Each of the cavities56,57,58is dimensioned to receive one of the equalizer plate sets50a–c. A downstream surface of each cavity56,57,58includes a series of shallow multi-segment channels62each centered about an outlet of one of the inlet passageways52. The channels62define a second stage or level of lateral and vertical thermoplastic material distribution in the linear flow equalizer40.

Each channel62includes a linear segment64extending in the cross-machine direction45and centered or symmetrical about inlet passageway52. Linear segment64terminates at each opposed open end in fluid communication with the center of a corresponding one of a pair of linear segments66each extending in the machine direction46. The linear segments66are equidistant in the cross-machine direction45from the corresponding inlet passageway52. Each of the linear segments66is centered or symmetrical about the intersection with linear segment64and terminates at each open end in fluid communication with a slotted linear segment68. Each slotted linear segment68extends in the cross-machine direction45and includes a pair of opposed curved terminal or closed ends69,70. Each slotted linear segment68is centered or symmetrical about the intersection with the corresponding one of the linear segments66. Therefore, the flow path length for the flowable thermoplastic material in each channel62is substantially equal and, preferably equal, from the inlet passageway52to the closed ends69,70of each slotted linear segment68.

As each of the equalizer plate sets50a–chave identical constructions, only one equalizer plate set50ais shown inFIG. 3and is described herein. Equalizer plate set50aincludes a plurality of, for example, five equalizer plates72,74,76,78and80, a sheet-forming plate82, removable mesh filters83,84, and85, a filter support plate86, and a seal87arranged in juxtaposition from the top or upstream side to the bottom or downstream side. The filter support plate86has a peripheral rim88surrounding a generally rectangular recess that captures the filters83,84,85in the set assembly. The equalizer plates72,74,76,78and80are secured together and fastened to the inlet plate48by conventional fasteners90extending from countersunk openings in the inlet plate48through appropriately aligned bolt holes formed in each of the equalizer plates72,74,76,78and80and secured by nuts91situated in countersunk openings on the downstream side of the sheet-forming plate82.

Each of the equalizer plates72,74,76,78and80is formed by milling or drilling a thin rectangular sheet of a suitable material using computer numerically controlled (CNC) machining. For example, equalizer plates72,74,76,78and80may be formed by CNC machining from sheets of a metal alloy, such as 17-4 stainless steel, having thermal expansion characteristics compatible with the surrounding metal environment of the spin pack32. The equalizer plats72,74,76,78and80may also be fabricated by alternative manufacturing techniques, such as by laser or chemical machining or by stamping.

With reference toFIG. 3, equalizer plate72is positioned downstream of the inlet plate48and includes a plurality of flow passageways in the form of circular bores or thoughholes92extending vertically through the thickness of plate72from an upstream inlet to a downstream outlet. Contact between the equalizer plate72and the inlet plate48closes the channels62to define flow paths in equalizer plate72to the throughholes92. The throughholes92are arranged in two spaced linear rows such that each throughhole92is registered on an upstream surface93of plate72in substantial vertical alignment with one of the closed ends69,70of one of the slotted linear segments68in equalizer plate72. Adjacent throughholes92in each of the rows are spaced at equal centerline-to-centerline intervals, or a uniform pitch, across the width of the equalizer plate72. The throughholes92receive flowable thermoplastic material from the channels62in inlet plate48and define individual liquid inlets supplying flowable thermoplastic material to equalizer plate74. The channels62and the throughholes92collectively define a second stage or level of lateral and vertical thermoplastic material distribution in the linear flow equalizer40.

Equalizer plate74is positioned downstream of equalizer plate72and includes a plurality of slotted flow passageways94extending vertically through the thickness of plate74from an upstream inlet to a downstream outlet. A major axis of each slotted flow passageway94is aligned generally in the cross-machine direction45. The center of each slotted flow passageway94is registered on an upstream surface99of equalizer plate74in substantial vertical alignment with one of the throughholes92in equalizer plate72. Throughholes92and channels62cooperate to also divide the flow of thermoplastic material into two separate laterally-extending rows. As a result, opposed curved terminal or closed ends96,98of each slotted flow passageway94are substantially centered or symmetrical in the cross-machine direction45relative to the corresponding throughhole92.

With continued reference toFIG. 3, equalizer plate76is positioned downstream of equalizer plate74and includes a plurality of flow passageways in the form of circular bores or thoughholes100extending vertically through the thickness of plate76from an upstream inlet to a downstream outlet. Each throughhole100is registered on an upstream surface101of equalizer plate76in substantial vertical alignment with one of the closed ends96,98of one of the slotted flow passageways94in equalizer plate74. Adjacent throughholes100in each of the rows are spaced at equal centerline-to-centerline intervals, or a uniform pitch, across the width of the equalizer plate76. The throughholes100in equalizer plate76and the slotted flow passageways94in equalizer plate74define a third stage or level of lateral and vertical thermoplastic material distribution in the linear flow equalizer40.

Equalizer plate78is positioned downstream of the equalizer plate76and includes a plurality of slotted flow passageways102extending vertically through the thickness of equalizer plate78from an upstream inlet to a downstream outlet. A major axis of each slotted flow passageway102is aligned substantially in the cross-machine direction45. The center of each slotted flow passageway102is registered on an upstream surface108of equalizer plate78in substantial vertical alignment with one of the throughholes100in equalizer plate76, which define individual liquid inlets supplying flowable thermoplastic material to equalizer plate78. As a result, opposed curved terminal or closed ends104,106of each slotted flow passageway102are substantially centered or symmetrical in the cross-machine direction45relative to the corresponding throughhole100.

With continued reference toFIG. 3, equalizer plate80is positioned downstream of equalizer plate78and includes a plurality of flow passageways in the form of circular bores or thoughholes110extending vertically through the thickness of equalizer plate80from an upstream inlet to a downstream outlet. Each throughhole110is registered on an upstream surface112of equalizer plate80in substantial vertical alignment with one of the opposed closed curved ends104,106of one of the slotted flow passageways102. Adjacent throughholes110in each of the rows are spaced at equal centerline-to-centerline intervals, or a uniform pitch, across the width of the equalizer plate80. The throughholes110in equalizer plate80and the slotted flow passageways102in equalizer plate78define a fourth stage or level of lateral thermoplastic material distribution in the linear flow equalizer40.

The sheet-forming plate82includes opposed concavely-curved surfaces114,116that integrate or merge the individual liquid flows streaming from the throughholes110of equalizer plate80. Sheet-forming plate82effectively eliminates gaps between adjacent streams of molten thermoplastic polymer exiting the throughholes110to form a substantially uniform sheet of flowable thermoplastic material that is provided to the combining plate42. The flowable thermoplastic material is subsequently filtered by the downstream filters83,84,85before being supplied to openings86aextending through the filter support plate86.

With reference toFIG. 5A, each of the equalizer plate sets50a–cmay be provided in an equalizer plate71in which a set of channels62ais formed. Each of the channels62aincludes multiple linear segments, of which only linear segment64ais shown, arranged similarly or identical to channels62(FIGS. 4 and 5). Channels62aare intended to replace channels62in inlet plate48(FIGS. 4 and 5). Consequently, an inlet plate48ais modified to include three rows of inlet passageways52aeach of which supplies thermoplastic material to the center of one channel62afor subsequent distribution to downstream equalizer plate72. Equalizer plate71is installed in recess56aof inlet plate48abetween equalizer plate72and inlet plate48aand also in the other two recesses in inlet plate48a(not shown but similar to recesses57and58inFIG. 3).

The invention further contemplates that additional pairs of equalizer plates (not shown) may be disposed between equalizer plate80and sheet-forming plate82to provide additional symmetrical and equal divisions of the flowable thermoplastic material in the flow path through the linear flow equalizer40. The number of symmetrical and equal divisions will depend, among other variables, upon the width of the spin pack32in the cross-machine direction and, therefore, the width of the nonwoven web being formed by the spunbond system (not shown) with which spin beam assembly10is operative coupled.

With renewed reference toFIG. 3, seal87provides a fluid-tight junction between a downstream side of the filter support plate86and an upstream side of the combining plate42. The combining plate42has internal liquid passageways118(FIG. 2) configured to receive the sheet-like flows of flowable thermoplastic materials from each of the linear flow equalizers40and to combine the flows to generate a bicomponent filament arrangement, such as a sheath/core arrangement or a side-by-side arrangement. In a sheath/core arrangement, for example, the flow path within the combining plate42of one of the two polymers is interposed and brought into coaxial alignment with the flow path of the other of the two polymers and directed the spinneret plate44. The spinneret plate44has multiple spinneret holes or orifices120(FIG. 2) registered with liquid outlets in the combining plate42from which bicomponent filaments122(FIG. 2) are extruded for subsequent solidification, attenuation and collection as a non-woven web.

With reference toFIG. 6, the operation of the linear flow equalizer40will be further explained. The flow path for a flowable thermoplastic material124through the linear flow equalizer40in a downstream direction from each inlet passageway52in inlet plate48to each throughhole110in equalizer plate80is substantially equal to or, preferable equal to, all other flow paths for the flowable thermoplastic material in the linear flow equalizer40. Therefore, the linear flow equalizer40divides the flow evenly among all flow paths so that the residence time of any arbitrary volume of flowable thermoplastic material124flowing between inlet passageway52and the corresponding throughholes110is approximately equal and, preferably equal, and so that the properties (e.g., shear history) of the flowable thermoplastic material124exiting from each throughhole110are substantially identical and preferably equal.

In the exemplary embodiment, the flowable thermoplastic material124entering the inlet passageways52is divided by inlet plate48into eight substantially equal portions, each of which is further subdivided by equalizer plates72,74into two substantially equal portions. It is understood that the number of substantially equal portions created by inlet plate48is dependent upon the width of the inlet plate48and equalizer plate sets50a–cin the cross-machine direction. Equalizer plates76,78further subdivide the portions received from equalizer plate74again into two substantially equal portions and directed through equalizer plate80to the combining plate42(FIG. 2). In the combining plate42, the thermoplastic material124, for example, Polymer A is combined with another thermoplastic material126, for example, Polymer B, which is subdivided uniformly in the linear flow equalizer40in a manner substantially similar to thermoplastic material124. The combined thermoplastic materials124,126form bicomponent filaments122, such as the sheath/core arrangement illustrated inFIG. 6, that are discharged from the spinneret orifices120in the spinneret plate44as a curtain of filaments122for subsequent collection. The invention contemplates that additional thermoplastic materials may be combined with the thermoplastic materials124,126to form multicomponent filaments122with more than two constituent thermoplastic materials and that the constituent thermoplastic materials may have other configurations, such as side-by-side.

While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, the principles of the invention may be applied for the formation of filaments composed of a single polymer or of filaments formed from more than two polymers. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept. The scope of the invention itself should only be defined by the appended claims.