SPINNERET BLOCK WITH UNITARY SPINNERET BODY AND NOZZLES FOR USE IN THE MANUFACTURING OF SPUN-BLOWN FIBERS

Field of the invention

The present invention relates to an equipment adapted to create filaments of the spun-blown type for forming nonwoven materials of superior quality, as well as methods to manufacture and operate such an equipment.

Background

Spunmelting is a process where fibers are spun from molten polymer through a plurality of nozzles in a die block, also referred to as die head, connected to one or more extruders for being formed into web, such as nonwoven webs or as components thereof. The spunmelt processes are well known in the art, and include meltblowing, see e.g. US8017534 (K-C), and spunbonding, see e.g. US5935512 (K-C).

From such technologies, a“hybrid” technology, often referred to as“spun-blowing” has been developed and described in e.g. US 9303334 (Biax,). 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, when implementing such technology, several hurdles have been encountered. As will be discussed in more detail herein below, it is difficult to precisely align the individual nozzles, first, relative to other nozzles, which results in difficulties at the assembly stage, but also with regard to air blowing holes, such that an annular shrouding air flow may become eccentric and deteriorate filament and further web properties, such as by undesired bundling of adjacent filaments.

Henceforth, it is an object of the present invention to overcome problems of the spun- blowing technology. Summary

In a first aspect, the present invention is a die block for forming meltblown filaments, this die block comprising

at least one molten polymer supply;

air supply;

- a spinneret block comprising a spinneret body comprising a polymer supply side, and

a plurality of nozzles forming an array of nozzles;

an air distribution plate comprising openings;

an exterior air plate comprising openings;

a cover strip

securing means,

wherein the spinneret block, the air distribution plate, the exterior air plate, and the cover strip 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 the spinneret body 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;

whereby 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 and wherein the spinneret body and the nozzles are unitary.

The die block may satisfy 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 L/d 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. The present invention also relates to a die block comprising a unitary spinneret body and nozzles, further comprising pre-holes in the spinneret block, the pre-holes

extending from the upper surface positioned towards the supply of the molten polymer towards the capillaries

in the form of countersinks to the capillaries of the nozzles,

preferably with a chamfering angle of between 30° and 60°,

preferably exhibiting a diameter of between 1.5 to 4 times the inner diameter of the capillaries, preferably exhibiting a length of more than about 2 mm, preferably more than about 4 mm,

preferably 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.

In a preferred embodiment, the transition of the nozzles to the base of the spinneret block exhibits a radius of more than about 0.1 mm, preferably more than about 0.3 mm.

The array of nozzles in the die block may comprise at least two sub-arrays comprising nozzles differing from nozzles of a different sub-array in at least one of the dimensions selected from the group consisting of

- inner diameter of the nozzle;

- outer diameter of the nozzle,

- length of the nozzle.

The array of nozzles may further comprise at least two sub-arrays, each of the sub-arrays are connected to a separate_polymer supply system adapted to supply molten polymer to said sub-arrays differing in at least one of the features selected from the group consisting of

- polymer type;

- polymer flow rate;

- polymer pressure;

- polymer temperature.

In another aspect, the present invention is a method for the manufacture of a die block comprising a spinneret block comprising a spinneret body and a plurality of nozzles, an air distribution plate, and an exterior air plate, comprising the steps of

providing a single piece of die block precursor, preferably steel;

machining from the single piece of die block precursor

o a spinneret block comprising

^■ nozzles;

^■ air inlet and distribution chamber;

o an air distribution plate;

o an exterior air plate

wherein the machining is high precision CNC treatment.

In another aspect, the present invention is a method for the manufacturing of a spinneret block comprising a spinneret body and a plurality of nozzles for being used in a die block, comprising the steps of providing a single piece of spinneret block precursor, preferably steel;

machining from the single piece of spinneret block material

nozzles;

air flow channels;

wherein the machining is high precision CNC treatment.

In yet a further aspect, the present invention is a process for cleaning a melt blowing apparatus, comprising the steps of providing a spinneret or a die clock as described, burning of polymer residues, ultrasonically removing burnt residues, blowing pressurized water / steam through the nozzles.

In yet a further aspect, the present invention relates to a process for forming a nonwoven web comprising meltblown fibers, comprising the steps of

providing equipment as described in the above,

providing a thermoplastic polymer for forming meltblown fibers, exhibiting a MFI from 30 to 2000,

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 spun-blowing equipment according to prior art.

Fig. 2 A to H depict particular features of the present invention.

Fig. 3 4A to D and 5 depict further features of the present invention.

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

Detailed description

The present invention relates to a particular execution of a die block for a spun-blowing process for forming a fibers or filaments that may further form a fibrous web or nonwoven comprising such a formed fibrous 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 hybrid process as described hereinafter in more detail, also referred to as spun-blowing.

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 pm 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 crystallizing (solidifying) 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 (MFI) is usually below about 500 grams/10 minutes at 2.16 kg load.

The present invention is related to spun-blowing, a hybrid process between a conventional meltblown 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 spun-blowing 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” spun-blowing 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. Whilst the present description has been explained by referring to a positioning of the die block such that the nozzle exhibits a vertical orientation, the skilled person will readily realize that this is not essential, but the die block may be tilted around a CD oriented axis such that nozzles may be oriented relative to the vertical at more than 5°, or more than 15°, or more than 30°, or more than 45° or even more, though typically less than 90° (i.e. horizontal).

Fig. 1 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-z- directional 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 adjacently y- directionally 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 spinnerette 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 1, 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 US’334, 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 extrader (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.

The die block 26 further comprises a cavity 30 and an inlet 28 connected to the cavity 30, positioned towards the polymer supply, and typically at the upper part of the spinneret body (i.e. against gravity). 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 therethrough 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 diameter of each of the stationary pins 62 should remain constant throughout its length. The dimension of the outside diameter can vary. The outside diameter of each of the stationary pins 62 may be at least about 0.25 mm, or at least about 0.5 mm, or at least about 0.6 mm, or even at least about 0.75 mm, and / or less than about 5 mm, or less than about 2 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.

Whilst the technology of US’334 can be employed well to provide very useful nonwoven materials, it has been observed that it carries certain limitations, mainly due to the inaccuracies induced by the insertion of the nozzle tubes into the spinneret body and the respective fixation. Even when adjusting the plurality of nozzle tubes, as may be several thousands, with utmost precision, at least the thermal dilatation upon inserting and welding, as well as the welding itself, if applied, tend to cause minute variations that can accumulate over the width of the spinneret block and then induce deterioration in handling of the equipment and / or quality of the resulting product:

If the capillaries are not exactly centered in the openings 82, the annular air passage way around the nozzles will be eccentric and hence the shrouding effect of the air is uneven which may disrupt the smooth airflow and result in bundling, i.e. adjacent filaments are blown towards each other and then may stick to each other, resulting in a larger fiber size distribution.

Further, even with sophisticated welding and machining tools, there is a high probability of burrs that disturb the smoothness of the flow into and through the capillaries.

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.

Considering the plurality of protruding capillaries and the tightly fitting air distribution plate, the difficulty of proper mounting increases with these variations, but also the number of nozzles in one mounting step. Thus there is presently a practical limitation of the y- directional extension of a single spinneret body - and consequently of the complete die block - of about 500 mm (approx. 20 in).

Thus, when employing such die blocks in large scale operations, where an overall y- directional extension may be well over 2 m, often 2.6 m, or even more 5 meter, multiple die blocks need to be employed, which not only increase handling complexity but that also can impact on the quality of the resulting product, at least because of y-directional edge effects of each block.

Further, considering the cleaning process, the relevance of which is aggravated by the increase in deposits around the capillary inlets, it only can be executed in one direction, namely along the normal process flow direction. Typically, the cleaning process comprises a pyrolytic treatment followed by treatment with pressurized steam or water. Also, ultrasonic cleaning, a generally desirable process for removing such residues, cannot be properly employed as it may very negatively interact with the connecting of the capillary tubes to the spinneret body, i.e. the welding or tight fitting.

Having thusly described the principle of the spun-blowing process as known from US’334, Fig. 2A to F depict the principle of the present invention, by showing a die block 126 comprising spinneret block 152, air distribution plate 170, exterior air plate 178 and cover strip 188 arranged in the same way as described in the context of Fig. 1. Further shown is one row of a plurality of five nozzles 158, also arranged in columns and rows forming an array of nozzles as described in the above. As described herein, the nozzles are oriented vertically along gravity. However, the orientation of the system may be such that the orientation of the nozzles is angled relative to gravity, and the skilled person will readily adapt relative positioning terms like“above” or“below” accordingly.

Optionally, the array of nozzles may comprise sub-arrays. Such a sub-array may include at least one row of nozzles, preferably, though not necessarily extending over the full width of a die block.

In a first execution of a die block comprising one or more sub-arrays as shown in Fig. 2G, the nozzles of at least one of the sub-arrays differ substantially from nozzles of a different sub-array in at least one of the dimensions selected from the group consisting of inner diameter of the nozzle, outer diameter of the nozzle, and length of the nozzle. Within the present context, the term“substantially different” refers to a difference in the respective dimension of at least 5%, often more than 10% thereof.

In a further execution as shown in Fig 2H, not exclusive to the other execution, the nozzles of one sub-array may be connected to a first, separate polymer supply system adapted to supply molten polymer to said sub-array differing from the molten polymer supplied to a different sub-array in at least one of the features selected from the group consisting of polymer type, polymer flow rate, polymer pressure, and polymer temperature. Optionally, the nozzles may be executed with co-axially positioned sub-capillaries so as to create bi- or multi-component fibers, wherein such sub-capillaries are supplied with different, respectively immiscible polymer types.

Whilst Fig. 2A depicts the elements in an assembled state (with securing means omitted), in Fig. 2B the spinneret block 152, the air distribution plate 170 and the exterior air plate 178 are shown in an exploded view, and Fig. 2C to 2F depict these individual elements.

It is a key feature of the present invention that the nozzles 158 are not formed separately as capillary tubes and inserted into holes of the spinneret body, as described in US’334. Rather, the spinneret body 153 and the nozzles 158 are“unitary” forming the spinneret block 152 as made from a single piece of material. This can conveniently achieved by modern CNC machining technology, such as employing laser cutting, flame and plasma cutting, hole-punching, drilling, milling, lathing, picking and placing, sawing, and other such technologies as known in the art for CNC treatments.

Referring to Fig. 2C, a piece of material, preferably exhibiting a homogeneous composition, preferably a metal and more preferably steel, is selected as a spinneret block pre-cursor 210, depicted by the dotted line, circumscribing the spinneret body and the nozzles to be formed integrally therefrom. It may be prepared at a size generally corresponding to the overall dimension of the later spinneret block, or its outer dimensions may be shaped in-situ.

Then, parallel or consecutively, though not necessarily in the listed order, the various features of the spinneret block are formed, especially though not limiting

- an inlet cavity 130 for molten polymer 122;

- air inlet and distribution chamber 132 (the air supply means not being shown);

- the array of nozzles 158, each exhibiting an inner diameter 157 , corresponding to the diameter of the capillary for the molten fluid flow, and an outer diameter 159;

- optionally further securing holes (199).

These elements may be machined with extreme precision, in particular with regard to the size, positioning and orientation of the nozzles 158. Further, once programmed, it is a well- known manufacturing process, just mounting the spinneret block precursor into the CNC machining tool and starting the machining program.

Each of the nozzles 158 has an inside diameter 157 and an outside diameter 159. The inside diameter may be more than aboutO.125 mm and / or less than about 1.25 mm. The outside diameter of each nozzle 158 is preferably more than about 0.5 mm, or more than 1 mm and/or less than about 2.5 mm. Typically, the length of a nozzle 158 is more than about 20 mm and/or less than about 150 mm.

Thus, the spin block comprises a polymer passageway that goes from the inlet cavity 130 through the capillaries of the nozzles 158 towards the nozzle tip 196, where the filaments are formed.

Further, because of this high precision, the nozzles 158 fit accurately though the respective nozzle openings 172 of the air distribution plate 170, and the nozzle openings in the 172 in the air distribution plate can be made only slightly larger than the outer diameter of the nozzle, down to an annular gap of less than 0.1 mm.

Thus, the air flow from the air chamber 132, which is delimited by the spinneret block 152 and the air distribution plate 170, may be through the plurality of air flow openings 176, as may be in a different row (i.e. not visible in the particular cross-sectional plane shown in Fig. 2), see dotted lines in Fig. 2E, optionally staggered row (see dotted lines in Fig. 2A). Further, the nozzle will also very precisely fit into the nozzle openings 180 of the exterior air plate and provide a very accurate and concentrically formed annulus around the nozzle 158. In operation, this results in a very even annular air shroud surrounding the filaments 186 as these exit the nozzle tip 196 essentially parallel to the filaments, thereby significantly reducing, if not preventing, bundling of the filaments.

Fig. 2 A further depicts two options (alternatively or jointly) for creating the outer perimeter curtain as described in US’334. In a first option, air passage openings 183 in the exterior air plate are positioned around the array of nozzles. In a second option, stationary and solid pins 162 also machined from the same spinneret block precursor extend from the spinneret body through openings 174 the air distribution plate into openings 182 of the exterior air plate 178, allowing also an annular air flow around these pins in analogy to the approach as described in US’334.

Further, the present approach allows much easier manufacturing when assembling the die block, by eliminating a practical restriction for the y-directional size of a die block, which now may well exceed the typical 500 mm of die block made according to the teaching of US’334, and may now be more than 1000 mm, even more than 2600 mm (as a typical size of a non-woven manufacturing unit into which such die block may be included) and even more than 5000 mm can be reached.

With the present invention allowing to employ single piece and unitary die blocks over much larger width, the fiber formation and fiber laydown will be cross-directionally more homogeneous as compared to having several less wide die blocks assembled together.

As depicted in Fig. 3, the present approach provides a further advantage in that the transition from the capillaries to the spinneret body may be executed with a radius 310, preferably of more than 0.1 mm, more preferably of more than 0.3 mm. This will increase the stability and hence also positioning of the nozzles compared to executions according to US’334.

Fig. 4A and C depict exemplary executions to reduce, if not to avoid these negative impacts of the sharp edges around the opening of the capillary of the nozzle.

In Fig. 4A as a first execution of a nozzle according to the present invention, the nozzle 158 may be a nozzle as described in general in the above cited US’647, exhibiting an inner capillary diameter 157 of the capillary 160 for the molten fluid flow. This inside capillary diameter can range from between about 0.125 mm to about 1.25 mm. At its upper end, the capillary 160 is chamfering in the chamfering section 333, preferably with a chamfering angle of between 30° and 60°, allowing a transition from the capillary to chamfering opening diameter 337. This chamfering opening diameter may range from more than about 2 mm or 4 mm to less than about 20 mm, or 14 mm, or less than 8 mm. 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.

Typically, the overall length 330 of a nozzle 158 ranges from between about 20 mm to about 150 mm. The length 334 of the lower section 335 may be from about 10 mm to about 140mm. The length of the chamfering section from the first diameter to the second diameter may range from about 2 mm to about 4 mm or more.

Optionally, and as depicted in Fig. 4B and C, the transition from the capillary diameter to the chamfering opening diameter may be executed as a radius or as another gradually transitioning curve, such as an elliptic or a parabolic shape. Within the present description, the term“chamfering” is intended to include such executions mutatis mutandis, and the chamfering angle is intended to correspond to the endpoints of the radius or smooth curve. Another execution according to the present invention is depicted in Fig. 4D, also showing a nozzle 158 with a capillary 160 exhibiting an inner capillary diameter 157 at the nozzle tip. Further, the nozzle has the chamfering opening diameter 337 towards the polymer supply, which chamfers towards a pre-hole diameter 341, representing a pre-hole of the capillary over a pre-hole length 344, further chamfering towards the inner capillary diameter 157. In this execution, the length of the nozzle is defined by the length of the capillary, the length of the pre-hole, if present, plus the length(s) of the chamfering section(s).

For one or both of the chamfering it may be executed as described for Fig. 4A, or be executed as a radius as described in Fig. 4B and C.

Optionally, and often preferably, the spinneret block 152 may comprise pre-holes 340, see Fig. 4D showing one nozzle only. These pre-holes are positioned upwardly and precisely aligned with the axis of the capillary 160 of the nozzle 158. The pre-holes may exhibit a diameter of 1.5 to 4 times the diameter of the capillary and a length of from about 2 mm to about 20 mm. Preferably, the transition from the pre-hole 310 to the capillary 160 is executed in a gradual way, e.g. at a chamfering angle of between 30° and 60° at the inlet and at the transition towards the capillary.

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.

Thus, in a second aspect, the present invention relates to operating the equipment as described in the above in a process with a wide process window, which 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 (MFI), as may be determined by ASTM D1238 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.

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 L/d ratio of about 52,

and is preferably operated at a temperature of about 210 C° with a back pressure of 50 to 70 bar for a molten polymer with an MFI of less than about 500 [g/lOmin @ 2.16 kg load]. In order to achieve comparable fiber dimensions and properties, the equipment of the present invention exemplarily exhibited

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 L/d ratio for the capillary of about 39, which will further allow to employ a polypropylene polymer exhibiting an MFI of about 500 [g/lOmin @ 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 L/d ratio, which is - for a given MFI - indicative of the flow resistance, and thusly allows to operate at a wider process window for MFI and backpressure.

Further, the smoother flow from the cavity for the molten polymer 130 to the pre-hole 340, preferably even more smoothed flow of polymer in the chamfering, significantly reduces the turbulence of polymer around the inlet and hence also polymer residue deposition, allowing longer operating times without interruption for cleaning.

In a further, and often highly preferred execution of the present invention, as depicted in Fig. 5, all three elements spinneret block 126, air distribution plate 170 and exterior air plate 178 are manufactured from a single block of material, indicated by dotted line 510, whereby in a first step slabs are separated, from which the respective elements are formed by CNC machining, as described in the above.

In a further aspect, the present invention allows simpler maintenance of the equipment. First, as the spinneret block (152) is now unitary and of the same material, it can readily withstand ultrasonic treatment without risking damaging the connection of the nozzles 158 and the spinneret body 153.

Second, as the nozzles and the spinneret body are much sturdier than e.g. a design as described in US’334, the equipment can also be cleaned with water and/or steam at a higher pressure and also not only along manufacturing direction, but also in the reverse direction (back-flushing).

Thus the cleaning of a spinneret block according to the present invention may comprise the steps of

- burning respectively pyrolyzing the residues;

- ultrasonically loosening the pyrolyzed residues;

- flushing the ultrasonically loosened pyrolyzed residues along the flow direction of the molten polymer, and/or optionally in the opposite flow direction with pressurized water or steam.