GB2579100A

GB2579100A - Spinneret block with readily exchangable nozzles for use in the manufacturing of meltblown fibers

Info

Publication number: GB2579100A
Application number: GB1819098.3A
Authority: GB
Inventors: Zampollo Fabio
Original assignee: Teknoweb Materials SRL
Current assignee: Teknoweb Materials SRL
Priority date: 2018-11-23
Filing date: 2018-11-23
Publication date: 2020-06-10
Also published as: GB201819098D0; US20210310156A1; JP2022507154A; EP3884088A1; MX2021005985A; CN113574216A; KR20210091808A; BR112021009905A2; WO2020104190A1

Abstract

A die block for forming meltblown filaments comprising a molten polymer supply, air supply, an air distribution plate comprising openings, an exterior airplate comprising openings, a cover plate, securing means and a spinneret block comprising a spinneret body with an upper plate, comprising a polymer cavity and upper plate openings, a lower plate with through holes matching the holes in the upper plate wherein the through holes have an upper portion, optionally with chamfering, and a lower portion wherein said upper portion is larger than the lower portion; and a plurality of nozzles also exhibiting upper and lower sections with a capillary through hole, the upper nozzle portion fitting in to the upper portion of the lower plate . The spinneret, air distribution plate, exterior air plate and cover strip are mounted in that order such that the nozzles protrude through openings in the air distribution plate and exterior air plate such that the polymer passageways are formed for molten polymer passing from the polymer supply side of the spinneret through the nozzles and air passageways are formed from air passing from the air supply through opening in the air distribution plates. The nozzles may form an array which may further comprising stationary pins exhibiting the same outer diameter and length as the nozzles and positioned on the periphery of the array. Also included is a method for forming a non woven web using the equipment described.

Description

SPINNERET BLOCK WITH READILY EXCHANGABLE NOZZLES FOR USE IN THE MANUFACTURING OF MELTBLOWN FIBERS

Field of the invention

The present invention relates to an equipment adapted to create filaments of the meltblown type for forming nonwoven materials of superior quality.

Background

Spunmelt is a process where fibers are spun from molten polymer through a plurality of nozzles in a die head connected to one or more extruders for being formed into web, such as nonwoven webs or as components thereof The spumuclt processes are well known in the arc and may include meltblowing, see e.g. US8017534 (K-C, Harvey), spunbonding, see e.g. US59355I2 (K-C, Haynes).

From such technologies, a "hybrid" technology, often referred to as "spunblowing" has been developed and described in e.g. W02015/171707 / US 9303334 (Biax, 2014). This technology provides a number of benefits with regard to the fiber and web properties, but also with regard to the equipment and process for the manufacturing. However, the equipment is quite inflexible with regard to replacement of individual nozzles, as in case of uneven wear of the nozzles over time, or if varying filament diameter were to be executed.

Thus, from US6364647 it is known to construct a similar spunblowing apparatus wherein the nozzles are removable. Whilst this provides certain improvements with regard to operation but also with regard to product flexibility, such a design carries a disadvantage of sharp transition from the polymer supply system into the nozzle capillary, which results in turbulences of the polymer flow, not only reducing smoothness of the flow through the capillary, but also increasing the propensity for polymer depositions around the nozzle opening that may require more frequent cleaning.

Henceforth, it is an object of the present invention to overcome problems of the spunblowing technology.

Summary

In a first aspect, the present invention is a die block for forming meltblown filaments, the die block comprising -2 -molten polymer supply; air supply; a spinneret block comprising an upper plate comprising a polymer supply side, and a lower plate, and a plurality of nozzles; an air distribution plate comprising openings; an exterior air plate comprising openings; a cover strip securing means.

The spinneret block, the air distribution plate, the exterior air plate, and the cover strip are mounted in this order and secured by the securing means such that the nozzles protrude through corresponding openings in the air distribution plate and further through corresponding openings in the exterior air plate, and such that polymer passageways are formed for molten polymer passing from the polymer supply side of upper plate through the nozzles; and such that air passageways are formed for air passing from the air supply through openings in the air distribution plate and the exterior air plate; The openings in the exterior plate and the nozzles are adapted so as to allow molten polymer exiting the nozzles and air flowing through the openings of the exterior air plate are essentially parallel.

Further, the spinneret body comprises an upper plate comprising a polymer supply cavity and upper plate openings exhibiting at least at their lower end an upper plate opening diameter; a lower plate comprising lower plate through holes concentric to the upper plate through holes exhibiting an upper portion with an upper portion diameter, optionally with an upper portion chamfering, and an upper portion length and a lower portion with a lower portion diameter and a lower portion length,whereby the upper portion diameter is larger than the lower portion diameter.

The nozzles comprise an upper section exhibiting an upper section outer diameter -3 -and a lower section exhibiting a lower section outer diameter: a capillary as an inner through hole through the nozzle.

The upper sections of the nozzles fit removably into the upper portions of the lower plate, preferably without protruding therefrom, Further, the upper plate opening diameter and the lower plate opening diameter or the chamfering exhibit a difference of less than 50 [un, preferably less than 20 p.m whereby the term "upper" refers to the polymer supply oriented side, and the term 'lower" to the opposite.

Preferably, a die block satisfies one or more of the conditions selected from the group consisting of the inner diameter of the nozzle being less than about 1.25 mm, preferably less than about 0.8 mm; - the outer diameter of the nozzle being less than about 2 mm; the nozzle exhibiting a length of less than about 50 mm: -the nozzle exhibiting a length of more than about 10 mm; - the nozzle exhibiting a Lid ratio of less than about 50: the die block exhibiting a CD width of more than 250, preferably more than 1500, even more preferably of more than about 2000 mm or even more than 5000 mm.

It is further preferred that the openings in the upper plate satisfy one or more of the conditions selected from the group consisting of comprising a chamfering h a chamfering angle of between 30° and 60'; exhibiting a diameter of between 1.5 to 4 times the inner diameter of the capillaries; exhibiting a length of more than about 2 mm, preferably more than about 4 mm; exhibiting a length of less than about 20 mm, preferably less than about 14 mm more preferably less than about 8 mm, and most preferably a length of about 6 mm, tapering from the polymer supply side towards the opposite side.

The nozzles may form an array and further the die block may comprise stationary pins, preferably exhibiting the same outer diameters and lengths as a nozzle, whereby the stationary pins may be positioned at the periphery of the array of nozzles.

In another, die present invention is a process for forming a nonwoven web comprising meltblown fibers, the process comprising the steps of providing equipment as described, providing a thermoplastic polymer for forming meltblown fibers, exhibiting a MFI from 30 to 2000 for 10 minutes at 2.16 kg at appropriate material class temperature, preferably at 210°C for polypropylene and 190°C for polyethylene, and -4 -forming filaments by applying a pressure of less than 70 bar, preferably less than 50 bar more preferably less than 45 bar at the polymer supply.

Brief description of the Figures

Fig. 1 shows a spunblowing equipment according to prior art.

Fig. 2 shows an spunblowing equipment according to prior art with removable nozzles.. Fig. 3 A to D depict schematically particular features of the present invention.

Same numerals in the figures depict same or equivalent features. Figures are not to scale.

Detailed description

The present invention relates to a particular execution for a die block for meltblowing process for forming fibers or filaments that may further form a meltblown web or nonwoven comprising such a formed meltblown web, e.g. as a layer in a multi-layer composite web.

Spunmelt is a process where fibers are spun from molten polymer through a plurality of nozzles in a die head connected to one or more extruders. The spunmelt process may include meltblowing, spunbonding and the process as described hereinafter in more detail, also referred to as spunblowing.

Meltblown is a process for producing very fine fibers typically having a diameter of less than about 10 microns, where a plurality of molten polymer streams are attenuated using a hot, high speed gas stream once the filaments emerge from the nozzles. The attenuated fibers are then collected on a flat belt or drum collector. A typical meltblowing die has around 35 nozzles per inch and a single row of nozzles. The typical meltblowing die uses inclined air jets at each side of the row of nozzles for attenuating the filaments.

Spunbond is a process for producing strong fibrous nonwoven webs directly from thermoplastics polymers by attenuating the spun filaments using cold, high speed air while quenching the fibers near the spinneret face, individual fibers are then laid down randomly on a collection belt and conveyed to a bonder to give the web added strength and integrity. Fiber size is usually below 250 gm and the average fiber size is in the range of from between about 10 microns to about 50 microns. The fibers are very strong compared to meltblown fibers because of the molecular chain alignment that is achieved during the attenuation of the crystallized (solidified) filaments. A typical spunbond die has multiple rows of polymer holes and for a conventional polymer of the polypropylene type the polymer melt flow index (MF1) is usually below about 500 grams/10 minutes at 2.16 kg load.

The present invention is related to a hybrid process between a conventional meltblown -5 -process and a conventional spunbond process, using a multi-row spinneret similar to the spinneret used in spunbonding except the nozzles are arranged to allow parallel gas jets surrounding the spun filaments in order to attenuate and solidify them. Each of the extruded filaments is shrouded by pressurized gas and its temperature can be colder or hotter than the polymer melt. Optionally, the periphery around all of the filaments may be surrounded by a curtain of pressurized gas.

In order to explain the general principles of the spunblowing equipment and process for forming filaments and further webs, such as nonwoven webs or components of such webs, express reference is made to US9303334 describing such a technology in greater detail.

Thus, Fig. 1 depicts a die block for such a "hybrid" meltblowing process Overall, a die block 26 comprises as elements a spinneret body 52, an air distribution plate 70, an exterior plate 78, and a cover strip 88. Further, nozzles 58 extend from the spinneret body 52 through openings of the distribution plate 70 and exterior plate 78, respectively, such that molten material can pass through the capillary 60 of the nozzle 58 to form filaments 86 at the tip of the nozzle 96.

Typically the order of the elements is such that the spinneret body 52, the air distribution plate 70, the exterior plate 78, and the cover strip 88 are arranged along gravity, and for the purpose of the present explanation, the spinneret body 52 is positioned above and secured to the air distribution plate 70, which is positioned above and secured to the exterior plate 78, which is positioned above and secured to the cover strip 88, with securing means not shown.

Fig. I shows a cross-sectional view of the die block 26. When positioned into a manufacturing equipment for forming nonwoven, this view corresponds to an x-zdirectional view, with the x-direction 12 denoting the manufacturing direction, i.e. the direction of movement of the resulting web, and the z-direction 15 corresponding to the height (along gravity). In the execution as depicted, the three nozzles 58 represent one "column" of the "multi row" (here three-row) die block 26. The die block comprises a plurality of columns positioned y-directionally adjacently 18 (i.e. perpendicularly to the plane of drawing and indicated by the circle) such that the columns and rows of nozzles form an array of nozzles of a die block. A spiimerette body 52 can contain from as few as ten nozzles 58 to several thousand nozzles 58. For a commercial size line, the number of nozzles 58 in the spinneret body 52 can range from between about 500 to about 10,000. The number of rows can vary as well as the number of columns. Typically, the number of rows will be more than I, often more than 5, and will be less than about 30, or even less than 15. Typically, the number of columns will be more than 50, but can be more than about 200, and may be less than 3500.

As described in USl334, the nozzles 58 are formed of capillary tubes that are inserted through openings in the spinneret body 52 to form a passageway for the molten polymer. Each of the nozzles 58 has an inside capillary diameter and an outside diameter. The inside diameter can range from between about 0.125 mm to about 1.25 mm. The outside diameter of each nozzle 58 should be at least about 0.5 mm. The outside diameter of each nozzle 58 may range from between about 0.5 mm to about 2.5 mm.

Typically, the length of a nozzle 58 ranges from between about 0.5 to about 6 inches.

As the molten polymer needs to pass only through the capillaries of the nozzles, US'334 describes the tubes to be tightly fitted and typically welded to the spinneret body, which represents an important difference, when compared to the present invention, as will be described in more detail herein below.

The molten material 22, as may be a thermoplastic polymer of the homopolymer type or a mixture of different polymers, is heated to a temperature well above its melting point, in case of propylene based polymers typically to at least about 170° C, often to about 210°C, upstream of the die block 26, usually in an extruder (not shown). Optionally, different polymers may be directed to respectively different groups of nozzles.

The polymer throughput through each nozzle 58 is stated in "gram per hole per minute" ("ghm"). The polymer throughput through each nozzle 58 can range from between about 0.01 ghm to about 4 ghm.

At its top, i.e. on the upper spinneret body side, the the block 26 has a cavity 30 and an inlet 28 connected to the cavity 30. The molten material 22 is conveyed along the polymer passageway from inlet 28 towards the upper portion of the spinneret body 52, and further via the nozzles downwardly. The spinneret body 52 also has one or more gas passages 32 formed thcrethrough for conveying pressurized gas (air) to an air chamber 54, which is essentially formed between the spinneret body 52 and the air distribution plate 70. The plurality of nozzles 58 extend downwardly from the spinneret body allowing molten material to flow through the capillaries 60 for exiting the nozzles and the die block downward of the exterior plate at nozzle tip 96 in the form of filaments 86.

Further, a plurality of stationary pins 62 may surround the array of nozzles, affixed to the spinneret body and extending through openings of the air distribution plate into the openings of the exterior air plate.

Each of the stationary pins 62 is an elongated, solid member having a longitudinal central axis and an outside diameter. Each of the stationary pins 62 is secured to the spinneret body 52 and usually they have a similar outside diameter to the polymer nozzles 58. The outside -7 -diameter of each of the stationary pins 62 should remain constant throughout its length. The dimension of the outside diameter can vary. Desirably, the outside diameter of each of the stationary pins 62 is at least about 0.25 mm. More desirably, the outside diameter of each of the stationary pins 62 is at least about 0.5 mm. Even more desirably, the outside diameter of each of the stationary pins 62 is at least about 0.6 mm. Most desirably, the outside diameter of each of the stationary pins 62 is at least about 0.75 mm.

An air distribution plate 70 is secured to the spinneret body 52 having a plurality of openings. Each one of first openings 72 accommodates one of the nozzles 58. If stationary pins 62 are employed, they are accommodated in second openings 74, and each of the third openings 76 is located adjacent to the first and second openings, 72 and 74 respectively.

When operating the process, pressurized gas, typically air, is flowing along air passageways from the air chamber 54 through openings 72, which are a thin annulus around the nozzles, openings 74, also a small annulus around the stationary pins, if present, and third openings 76 as a main passageway for the air.

An exterior air plate 78 is secured to the air distribution plate 70, away from the spinneret body 52. The exterior member 78 has a plurality of first openings 80 surrounding the nozzle 58. Second enlarged openings 82 surround each of the stationary pins 62, if present.

In operation, the molten material 22 (polymer) is extruded through each of the nozzles 58 to form multiple filaments 86 which are intended to be shrouded from the ambient air by the pressurized gas (air) emitted through the first enlarged openings 80, formed in the exterior member 78, at a predetermined velocity essentially parallel to the axis of the capillaries 60 and hence the flow direction of the filaments 86 at the nozzle tip 96 The pressurized gas (air) flow exiting the second enlarged openings 82 formed in the exterior member 78 around the stationary pins, if present, forms a further shrouding air flow, which is also oriented essentially parallel to the axis of the nozzles, and hence also essentially parallel to the filaments exiting the nozzles, aiming at isolating the filaments 86 from surrounding ambient air, as indicated in Fig. 1 with the arrow 94.

In US6364647 (hereinafter referred to as US'647) a system is described (see Fig. 2) wherein individual nozzles 58 of the array of nozzles are removable from the spinneret body 52. To this end, the nozzles 58 are equipped at their upper end (towards the polymer supply 22, which is further covered by upper cover 53) with shoulders 51, such that they can finnly rest in the wider portion of holes through the spinneret body 52. This provides a number of benefits, such as the flexibility of exchanging individual nozzles, when they are worn out. It also allows to implement and quickly introduce nozzles of differing capillary diameters.

During operation, the nozzles are pressed into the respective openings by the pressure of the molten polymer.

However, this system still carries a disadvantage of sharp transition from the polymer supply chamber 22 into the plurality of nozzle openings..

As a consequence thereof, deposits of polymeric material may form around the inlet of the capillaries, which may degrade over time and require more frequent cleaning.

Fig. 3A depicts the principle of the present invention, by showing in a schematic cross-sectional view a die block 126 comprising spinneret block 152, air distribution plate 170, exterior air plate 178, cover strip 188, and securing means 199 arranged in the same way as described in the context of Fig. 1. Also shown are an inlet cavity 130 for molten polymer 122, an air inlet and distribution chamber 132 (the air supply means not being shown). The spinneret block 152 comprises an upper plate 151 and a lower plate 155, with "upper" denoting positioning towards the polymer supply and "lower" away therefrom, as shall apply throughout the present description. Nozzles 158 are inserted into the lower plate 155 of the spinneret body, forming a polymer passageway that goes from the inlet cavity 130 through the capillaries 160 of the nozzles 158 towards the nozzle tip 196, where the filaments are formed.

Fig. 3B shows schematically an enlarged portion from Fig. 3A, focusing on the positioning of nozzles 158 in openings through the lower plate 155 and the relative positioning of openings through the upper plate 151, and Fig. 3C shows schematically an enlarged cross-sectional view of a single nozzle 158.

A plurality of nozzles 158, in Fig. 3A eight are exemplarily shown, may be arranged in columns and rows forming an array of nozzles as described in the above, each exhibiting an inner diameter 157, corresponding to the diameter of the capillary 160 for the molten fluid flow. The inside diametercan range from between about 0.125 mm to about 1.25 mm. The nozzle 158 is formed in two sections an upper section 331 and a lower section 335, differing in their outer diameter whilst the inner diameter of the capillary 160 remains essentially constant. At its upper end, the capillary 160 may have a chamfering 333, preferably with a chamfering angle of between 30° and 60°. The outside diameter 159 of the lower section 335 of the nozzle 158 should be at least about 0.5 mm, and may range from between about 0.5 mm to about 2.5 mm. The outside diameter 161 of the upper section 331 may be from about 0.5 mm to about 5 mm.

Typically, the overall length 330 of a nozzle 158 ranges from between about 20 mm to 35 about 150 mm. The length 332 of the upper section 331 may be from about 1 mm to about -9 - 50mm, and the length 334 of the lower section 335 may be from about lOmm to about 140mm.

Each nozzle 158 is fitted into an opening 360 of the lower plate 155 that has an upper portion 361 and a lower portion 365.

The upper portion 361 is adapted to receive the upper section 331 of the nozzle 158 and exhibits a diameter 362, which is not more than 2 mm wider than the outer diameter 161 of the upper section of nozzle 158. For a good fit, the diameter 362 is less than 50um or even less than 10 pm more than the outer diameter 161 of the upper section of the nozzle, and it may even be slightly smaller, such as less than 10pm smaller, such as when the nozzle is fitted with force or at lower temperature.

The lower portion 365 is adapted to receive the lower section 335 of the nozzle 158 and exhibits a diameter 366, which is not more than 2 mm wider than the outer diameter 159 of the lower section of nozzle 158. For a good fit, the diameter 366 is less than 5011m or even less than 10 p.m more than the outer diameter 159 of the lower section of the nozzle, and it may even be slightly smaller, such as less than 10um smaller, such as when the nozzle is fitted with force or at lower temperature.

The transition from the upper portion 361 of the opening 360 to the lower portion 365 is preferably a sharp one, though a small radius or chamfering is acceptable. However, this transition should be matched with the transition from the upper section 331 of the nozzle to its lower section 335.

Most preferably, the length 332 of the upper section 331 of the nozzle 158 may be the same as the length 364 of the upper portion 36! of the opening 360, such that the differential 369 is zero, though the differential may be less than 10 pm, or less than about 2 p.m. A negative differential (i.e. the nozzle protruding out of the hole) is not preferred.

The length 368 of the lower portion 365 of the opening 360 may be from about 2nun to about 100inm.

The upper plate 151 of the spinneret body 153 comprises openings 370 that are aligned with the axis 339 of the capillary 160 of the nozzle 158, and hence with the openings in the lower plate 155. The opening 370 exhibits at its lower end a diameter 372 that matches as much as possible the diameter of the capillary 160 or -if present -of the chamfering 333.

Preferably, the difference between these diameters is less than about 20 Jim more preferably less than about 10 um. Also, the offset of the axis of the opening 370 from the axis of the capillary is most preferably essentially zero, but preferably less than 5 p.m or less than 50 pm. Optionally and often preferably, the opening 370 has a chamfering 375 at its upper end, preferably at an angle of 1°, 10°, 30°, 60° or less than 90° or any angle among these -10 -values. Thus, in the extreme the chamfering may extend over the full length of the upper plate, such that the cross-sectional view of the opening may correspond to a trapeze, as indicated by dotted lines 376.

During operation, the openings 370 function as pre-holes for the capillaries and preferably exhibit a diameter of 1.5 to 4 times the diameter of the capillary and a length corresponding to the thickness of the upper plate of from about 2 mm to about 20 mm.

Such a pre-hole provides a smoother flow from the cavity with the molten material into the capillary 160, which in turn will widen the opportunity for a wider process window for the process.

This process window is primarily dictated by pressure of the molten polymer in the cavity; temperature of the molten polymer; diameter of the capillary; length of the capillary; material properties of the molten polymer, as expressed by the Melt Flow Index (MFT), as may be determined by ASTM D123 8 and ISO 1133, and for polypropylene as a polymer that suitably can be processed with the current equipment and process, it is suitably expressed in units of gram per 10 minutes at 210°C and 2.16 kg load, whilst for other material classes the temperature is set to the appropriate temperature, e.g. 190°C for polyethylene..

As a comparative example, the equipment and the process as described in US'334 may exhibit a capillary inner diameter of 0.46 mm; a capillary length of 24 mm; hence an Lid ratio of about 52; and is preferably operated at a temperature of about 210 C° with a back pressure of 50 to bar for a molten polymer with an MFT of less than about 500 ig/ I OnTin 2.16 kg loadl.

In order to achieve comparable fiber dimensions and properties, the equipment of the present invention present invention may exemplarily exhibit a capillary diameter of 0.46 mm; a capillary length of about 18 mm.

a pre-hole diameter of about 1.2 mm: a pre-hole length (including a 60° chamfering at the inlet and at the transition to the capillary) of about 6 mm, henceforth an Lid ratio for the capillary of about 39 inch will further allow to employ a polypropylene polymer exhibiting an NIFI of about 500 Ig/lOmin (4 2.16 kg load] of a back pressure of significantly lower than 50 bar.

One benefit of subjecting the polymer to a lower backpressure is that the reduction of the mechanical stress results in allowing to produce stronger nonwovens.

In other terms, the present invention provides an equipment that can exhibit a lower Lid ratio, which is -for a given MFI -indicative of the flow resistance, mid thusly allows to operate at a wider process window for MFI mid backpressure.

Further, the smoother flow from the cavity for the molten polymer 130 to the pre-hole 370, preferably even more smoothed flow of polymer in the chamfering, significantly reduces 1 0 the turbulence of polymer around the inlet and hence also polymer residue deposition, allowing longer operating times without interruption for cleaning.

It should be noted that within the array of nozzles the capillary diameter may vary, such that smaller and larger filaments may be created in different portions of the array.

Also, it is also with the scope of the present invention that the die block may -in addition to the nozzles as described in the above -comprise stationary pins, as described in the context of US'334 in the above. Fig.3D depicts schematically such a pin that with its outer dimensions may correspond to a nozzle 158, except for having no capillary but being solid.

Claims

-12 -Claims 1. A die block for forming meltblown filaments comprising molten polymer supply; air supply; a spinneret block comprising an upper plate comprising a polymer supply side, and a lower plate, and a plurality of nozzles; an air distribution plate comprising openings; an exterior air plate comprising openings; a cover strip securing means, wherein said spinneret block, said air distribution plate, said exterior air plate, and said cover strip are mounted in this order and secured by said securing means such that said nozzles protrude through corresponding openings in said air distribution plate and further through corresponding openings in said exterior air plate, and such that polymer passageways are formed for molten polymer passing from said polymer supply side of upper plate through said nozzles; and such that air passageways are formed for air passing from said air supply through openings in said air distribution plate and said exterior air plate; whereby said openings in said exterior plate and said nozzles are adapted so as to allow molten polymer exiting said nozzles and air flowing through the openings of said exterior air plate are essentially parallel, characterized in that said spinneret body comprises an upper plate comprising a polymer supply cavity and upper plate openings exhibiting at least at their lower end an upper plate opening diameter; a lower plate comprising lower plate through holes concentric to said upper plate through holes exhibiting an upper portion with an upper portion diameter, optionally with an upper portion chamfering, and an upper portion length -13 -and a lower portion with a lower portion diameter and a lower portion length, whereby said upper portion diameter is larger than said lower portion diameter, said nozzles comprising an upper section exhibiting an upper section outer diameter and a lower section exhibiting a lower section outer diameter; a capillary as an inner through hole through said nozzle; and wherein said upper sections of said nozzles fit removably into the upper portions of said lower plate, preferably without protruding therefrom, wherein further said upper plate opening diameter and said lower plate opening diameter or said chamfering exhibit a difference of less than 50 pm, preferably less than 20 pm whereby the term "upper" refers to the polymer supply oriented side, and the term 'lower" to the opposite.
2. A die block according to claim 1, satisfying one or more of the conditions selected from the group consisting of the inner diameter of the nozzle being less than about 1.25 mm, preferably less than about 0.8 mm; -the outer diameter of the nozzle being less than about 2 mm; the nozzle exhibiting a length of less than about 50 mm: the nozzle exhibiting a length of more than about 10 mm; the nozzle exhibiting a Lid ratio of less than about 50: the die block exhibiting a CD width of more than 250 preferably more than 1500, even more preferably of more than about 2000 mm or even more than 5000 mm.
3 A die block according to claim 1, wherein said openings in said upper plate satisfy one or more of the conditions selected from the group consisting of comprising a chamfering h a chamfering angle of between 30° and 600, exhibiting a diameter of between 1.5 to 4 times the inner diameter of said capillaries, exhibiting a length of more than about 2 mm, preferably more than about 4 mm, exhibiting a length of less than about 20 mm, preferably less than about 14 mm -14 -more preferably less than about 8 mm, and most preferably a length of about 6 mm; tapering from said polymer supply side towards the opposite side.
4. A die block according to any of the preceding claims, wherein said nozzles form an array, further comprising stationary pins, preferably exhibiting the same outer diameters and lengths as a nozzle, said stationary pins being positioned at the periphery of said array.
5. Process for forming a nonwoven web comprising meltblown fibers, comprising the steps of providing equipment according to any of the preceding claims, providing a thermoplastic polymer for forming meltblown fibers, exhibiting a TgFI from 30 to 2000 for 10 minutes at 2.16 kg at appropriate material class temperature, preferably at 210°C for polypropylene and 190°C for polyethylene, and forming filaments by applying a pressure of less than 70 bar, preferably less than 50 bar more preferably less than 45 bar at the polymer supply.