Title of Invention

A DIE ARRANGEMENT FOR EXTRUDING VISCOELASTIC MATERIALS AND A METHOD THEREOF

Abstract

The invention relates to a die arrangement for extruding viscoelastic materials, in particular polymers, pasta, etc., having a casing for receiving and conveying the viscoelasti~ material along an axial conveying direction through the die, the material received in an area located above (upstream) of die channels extending through the die from an inlet hole to an outlet hole, the die channels having an inlet area at the upstream end of the die channel, an inner area inside the die channel, and an outlet area at the downstream end of the die channel, all die channels being mutually parallel along the axial conveying direction of the material through the die, with adjacent inlet holes, characterized in that in the area located above the die channels at least one separating wall extending radially inward from the casing is located between adjacent inlet holes, which separating wall has a cutting edge at its upstream end, whereby the' outlet area of the die channel is expanded from the inner area to the outlet hole over a length along the axial conveying direction, and/or the inlet area of the die channel is conically expanded from the inner area to the inlet hole opposite the conveying direction along a length. The invention also relates to a method for extruding viscoelastic materials, in particular polymers, pastas, etc., by a die arrangement as described herein.

Full Text

EXTRUSION DIE FOR VISCOELASTIC MATERIALS (OUTLET WIDENING) The invention relates to a die arrangement or a method for extrusion for viscoelastic materials according to the preamble of claim 1 or the preamble of claim 17. Die arrangements for extruding viscoelastic materials, in particular polymers, pasta, etc., are known in the art. As a rule, they are fitted with mutually parallel, identical die channels, which extend through the die from a respective inlet hole to a respective outlet hole, wherein the respective die channels have an inlet area at the upstream end of the die channel and an outlet 'area at the downstream end of the die channel, both along the axial conveying direction of the material. The inlet holes are adjacent to each other. During the extrusion of viscoelastic materials, e.g., pastas, polymers, these materials are molded. However, this molding process requires that the material flow. The material can also tear at specific locations. The elastic properties of the viscoelastic material give rise to mechanical stresses that continue in the molded material given deformations and tearing in such a viscoelastic material. After the process of molding the material is complete, this can result in additional, apparently spontaneous deformations. Reference is often made to "dimensional memory" in this conjunction, because the mechanical stresses of the material exiting the molding device creates the impression that it "remembers" a previous shape, and wants to return to it. During the extrusion of pastas or polymers via die arrangements, this can cause a crimping of the strands exiting the individual die channels. The stresses are here introduced into the material during the separation and distribution of the material to the various die channels on the one hand, and during the expansion of the material inside the die channels on the other. The stresses that arise in the material due to material separation and distribution likely have the most disruptive influence owing to their asymmetry relative to the generated strands. However, the material stresses and resultant tendency of the strands to change direction in the die channels can also produce an asymmetrical wall friction that further enhances these material stresses under certain conditions. Be that as it may, such strands consisting of viscoelastic materials exhibit a tendency to crimp as mentioned when exiting the die arrangement. In addition, a relatively large amount of energy or a high pressure differential must be used for the die arrangement during extrusion from viscoelastic materials in order to separate the material, distribute it to the molding die channels, and finally press it through the molding die channels, during which the material is expanded. In other words, the conventional die arrangements for molding viscoelastic materials have a relatively high die resistance for such materials. This is particularly problematical for pastas, since, as opposed to classic polymers like polyesters or rubber, there . is here only a limited possibility to reduce the die resistance of the die arrangement and the stresses introduced into the material via an at least local temperature elevation in the die arrangement, even one limited primarily to the surface of the material. The object of the invention is to minimize such material stresses in viscoelastic materials while molding them into material strands, and lower the energy outlay and pressure differential, i.e., die resistance, required for strand extrusion. This object is achieved according to the invention in the die arrangement described at the outset by expanding the outlet area from the die channel inner area to the outlet hole over a length LA along the axial conveying direction F. If a viscoelastic material, e.g., a polymer material or pasta, etc. , arrives at the die channel of the die arrangement according to the invention, the product stream divided into several partial streams is pressed through the several die channels. Stresses arise in the material as it enters the die channel and is molded in the inlet area. Stresses that have built up in the material at the cutting edges in the cutting process and/or in the die channels during an expansion and have not yet abated are then practically completely relaxed in the expanding outlet area. As a result, the several small product strands exit the respective die channels under virtually no stress in this die geometry. The expanded outlet area allows the product to relax in both an axial and radial direction. This prevents corrugation ("shark skin") on the surface of the viscoelastic product strands exiting the die channels. The outlet area is advantageously expanded from the die channel inner area to the outlet hole over a length LA along the axial conveying direction F in the shape of a bell, wherein the widening angle of the outlet widening measured between the axial conveying direction and the inner wall channel outlet area preferably increases gradually along the axial conveying direction. In particular, the widening angle can rise steadily along the axial conveying direction, wherein the widening angle increases from 0° inside the die up to 9 0° at the downstream end of the die body. In this case, the expansion can follow an arc in longitudinal section, for example, whose bending radius RA is greater than the radius RK of the die channel inner area. This curved, expanded outlet area replaces the edge of conventional outlet holes with a curved, continuous transition from one vertical tangent inside the die channel to a tangent that is inclined, in extreme cases horizontal, relative to the vertical at the downstream end of the outlet area. In a special embodiment, the die channel inner wall can have a higher surface roughness in the outlet area over a length LR along the axial conveying direction than the remainder of the die channel inner wall. This makes it possible to influence the surface of the product in a targeted fashion by selecting the roughness and/or the material of the roughened area. Therefore, the axial length LR of the rough area is preferably smaller than the axial length LA of the outlet widening, smaller than the axial length LE of the inlet widening, and smaller than the axial length Ls of the cutting edges. However, it is also advantageous to provide several consecutive roughened areas that satisfy the condition VF > IJR/TRELAX over large axial partial areas of the die channels. This makes it possible to influence the interplay between wall adhesion and wall sliding (stick/slide effect). The periodicity or spatial frequency of the rough axial wall sections of length LR combined with the flow rate enable the targeted generation of more wall tears per unit of time, i.e., the alternating relatively rough and relatively smooth wall sections "artificially" force a higher frequency stick/slide' effect. The advantage to this is that no excessively high_ material stresses can build up, so that little or no tears arise on the product. In a particularly advantageous embodiment, the upstream end of the die channel has a separating wall that runs parallel to the axial conveying direction between a respective two adjacent inlet holes and has a cutting edge at its upstream end. If a viscoelastic material, e.g., a polymer material or a pasta, etc., hits the die arrangement according to the invention, the product stream introduced into the housing along conveying direction F is divided into several partial streams, which each respectively flow through one of the several die channels. The sharp cutting edges cut the product stream introduced to the die arrangement into several partial streams already as it enters into the several die channels. Since each of the cutting edges represents only a very small working surface for the product, a very large force acts on the impacted viscoelastic product locally at the cutting edge. This gives rise to a locally concentrated shearing force along the cutting edge, which divides the product. However, before the viscoelastic product brought"to the cutting edges is torn off at the cutting edges, it deforms up until reaching its breaking stress and breaking elongation, wherein the viscoelastic material stores potential energy that is passed on to several partial streams. As a whole, however, the stresses introduced into the material as the cutting edges separate and distribute the viscoelastic material to the several die channels are distinctly lower than in a conventional die arrangement without sharp cutting edges, so that far less content makes its way into the dimensional memory of the viscoelastic material already as the viscoelastic material brought to the die arrangement according to the invention is being distributed into several partial strands, thereby significantly reducing both the tendency of the product strands to deform (crimp, etc.) while exiting the die channels, and the die resistance. These positive effects are particularly pronounced in the case of pasta die arrangements. The area arranged upstream from the inlet area of each adjacent inlet hole is preferably completely enveloped by separating walls running parallel to the axial conveying direction, whose upstream end is designed as a respective cutting edge. As a result, the material entering the respective die channels is cut practically everywhere that it must still be separated, so that extremely little stress is introduced into the material. The cutting edges can form an angle differing from 90° relative to the axial conveying direction of the material. An acute angle is preferred, however. This is because, the more acute the angle relative to the conveying direction, the greater the length LS measured in along the conveying direction of the area in which the material is cut in a radial direction perpendicular to the conveying direction, e.g., from radially outside to radially inside. The radially outside streaming areas of the material are then cut first, for example, while the radially inside streaming area of the material is cut later. However, the radially outside areas will here already have had the time to break down the stresses introduced into the material during the cutting process. As a result, the cutting process once again causes less overall stress to be introduced into the material distributed to the die channels than would be the case given cutting edges running at a right angle to the direction of flow (simple "punch-out" principle). it is also advantageous to expand the inlet area of the die channels from the inner area to the inlet hole opposite the conveying direction F along a length LE, wherein the widening angle of the inlet widening measured between the axial conveying direction and the inner wall of the channel inlet area ranges from 5° to 45°, but preferably measures from 8° to 25°. From a production standpoint, it is particularly simple for the widening angle to be constant from the inner area toward the inlet hole, i.e., for a conical inlet widening to be present. Even when the material is conveyed through the die arrangement at relatively high speeds, a "soft" expansion, i.e., one slow enough for the viscoelastic material, can be achieved, so that the relaxation time of the viscoelastic material is less than the duration of material expansion in the inlet widening. The die arrangement is best designed in such a way that the die channel has a circular cross section over its entire length. As a result, the same boundary conditions exist everywhere on the walls, yielding a uniform, as symmetrical as possible of an expansion. A compact layout of the die arrangement is characterized in that the axial length of the channel inlet area ranges between 50% and 80% of the overall length of the die channel. At least partial areas of the inner walls of the die channel consist of Teflon or a similar material in order to minimize adhesion of the viscoelastic material to the inner walls and sliding friction thereon. In the extrusion method for the mentioned viscoelastic materials, in particular polymers, pastas, etc., using the die arrangement described above, a pressure gradient Ap between the upstream end and the downstream end of the die arrangement presses the viscoelastic material through the die arrangement. According to the invention, the pressure gradient Ap is here selected in such a way that the flow rate VF of the viscoelastic material along the conveying direction F in a respective axial partial area of the die arrangement where at least a portion of the material molding required for extrusion takes place satisfies the condition VF This ensures that the material will always have enough time to relax during the individual molding steps of the viscoelastic material necessary for strand extrusion, e.g., cutting along' a length Ls at the cutting edges, expansion along a length LE of the inlet widening, and final relaxation along the length LA of the outlet widening, so that the material has practically no more stresses as it exits the end of the die arrangement according to the invention. In order to make optimum use of the aforementioned roughened partial area, the flow rate VF of .the viscoelastic material is harmonized along conveying direction F with" the length LR of the roughened partial area of the die arrangement in the method according to the invention in such a way as to satisfy the condition VF > LR/TRELAX/ wherein TRELAX is the relaxation time of the viscoelastic material, and LR is the axial length of the roughened partial area. As already mentioned further above, this makes it possible to specifically influence the surface of the product by selecting the roughness and/or material of the roughened area. Additional advantages, features and possible applications of the invention can be gleaned from the following description of preferred exemplary embodiments based on the drawing, which is not to be regarded as limiting in any way. Shown on: Fig. 1 is a sectional view through a die arrangement according to the invention along the axial product conveying direction F; Fig. 2 is a top view of the die arrangement according to the invention on Fig. 1 along the product conveying direction F; Fig. 3 is a sectional view through a die channel according to the invention along the axial product conveying direction F; Fig. 4 is a sectional view through another die channel according to the invention along the axial product conveying direction F; Fig. 5 is a sectional view through a die channel according to prior art along the axial product conveying direction F; Fig. 6 is a sectional view through another die channel according to prior art along the axial product conveying direction F; Fig. 1 is a sectional view through a die arrangement 1 according to the invention designed especially for dough used in noodle production along the axial product conveying direction F. The die arrangement 1 having a total of four die channels 2 (see Fig. 2) is accommodated in a cylindrical housing 7. An inlet hole 3 located at the upstream end of each die channel 2, and an outlet hole 4 is situated at the downstream end of each die channel 2. The inlet area 2a of each die channel 2 that adjoins the inlet hole 3 is c'onically expanded, while the outlet area 2c is cylindrical. The widening angle a (see Fig. 3) measures about 10,-20°. Located at the upstream end of the die arrangement 1 are four separating walls 5 (see Fig. 2) , which run parallel to the axial conveying direction F, and divide the area upstream from the inlet holes 3 into four partial areas, which are each located upstream from an inlet hole 3 . The edges of the separating walls 5 facing opposite the axial conveying direction F are each designed as an inclined cutting edge 5a, which extends from the inner wall of the housing 7 both radially inward and in the conveying direction F. Fig. 2 is a top view of the die arrangement 1 according to the invention on Fig. 1 along the product conveying direction F (see Fig. 1) . It shows the four die channels 2 with their respective conically expanded inlet area 2a, along with the separating walls extending radially inward from the cylindrical casing 7, which divides the area above the die arrangement 1 into four partial areas. The four sharp cutting edges 5a extend at an inclination opposite the conveying direction F. If a viscoelastic material, e.g., a polymer or pasta, etc., now arrives at the die arrangement 1 according to the invention, as diagrammatically shown on Fig. 1 with flow profile V(r) , the product stream in the housing 7 passed along the conveying direction F is divided into" four partial streams, which each flow through a respective one of the four die channels 2. The sharp cutting edges 5a cut the product stream supplied to the die arrangement 1 into four partial streams, already before entry into the four die channels 2. Since each of the cutting edges 5a represents only a very small working surface on the product opposite the conveying direction" F, a very large force acts locally on the . viscoelastic product hitting the cutting edge 5a at the cutting edge 5a. A locally concentrated shearing force that 'divides the product comes about along the cutting edges 5a. However, before the viscoelastic product supplied to the cutting edges 5a tears away at the cutting edge, it deforms until reaching its breaking stress, wherein the viscoelastic material stores potential energy, which is relayed further to the four partial streams, and results in a partial relaxation in these four partial streams, before further deformation or molding of the viscoelastic material takes place in the four product partial streams as the material enters the respective die channels 2. Stresses are also encountered here in the material as it enters into the die channels 2 and becomes molded in the respective inleu areas 2a. However, these are lower than at the cutting edges 5a, and do not result in the product tearing off. In comparison to conventional die arrangements without cutting edges and conical expansion with an widening angle according to the invention of about 10-20°, laying out the cutting edges 5a of the separating walls 5 ar.d the inlet areas 2 a of the die channels 2 as described in the invention reduces the scope of the stresses in the material conveyed through the die arrangement 1 according to the invention and molded therein, along with the flow resistance of the die arrangement 1. This is because, in the die arrangement 1 according to the invention, the molding associated with the buildup of TTianerial stresses from a large into four small product strands essentially takes place in two steps. In a first step, the large product strand is cut into four small partial strands at the cutting edges 5a. In a second step, the four partial strands are then expanded in the conical inlet areas 2a. Immediately after the first step (cutting at cutting edges 5a) and just before the second step (expansion' in the inlet area 2a), an at least partial relaxation (stress abatement, decrease in potential energy) takes place in the material as it slides along the separating walls 5. If the product then expands in the conical inlet areas 2a, material stresses also build up, whereupon an at least partial relaxation again takes place in the adjacent cylindrical outlet areas 2c. As a result, the viscoelastic material divided into four small product strands exits the outlet holes 4 of the die channels 2 under practically no stress, so that the four exiting product strands exhibit no noteworthy deformations (e.g., crimps). Since the product tears away already at far lower product shearing forces due to the cutting edges, the flow resistance of the die arrangement 1 is also clearly lowered. Hence, the die arrangement 1 according to the invention enables operation at a lower pressure differential relative to conventional die arrangements, i.e., at a lower pressure gradient in the product along the die arrangement 1, and with a practically complete "deletion" of dimensional memory in the exiting partial strands of product. Fig. 3 is a sectional view through a die channel 2 according to the invention also designed especially for dough used in noodle production along the axial product conveying direction F. This die channel 2 can be used as a replacement for the die channels 2 shown on Fig. 1. In place of the cylindrical outlet area 2c of the die channel 2 on Fig. 1, a relatively short cylindrical inner area 2b initially follows downstream from the inlet area 2a, followed by a bell-shaped, expanded outlet' area 2c. This outlet area 2c replaces the edge of the outlet hole 4 (see Fig. 1) with a curved, continuous transition from a vertical tangent in the inner area 2b of the die channel 2 to a horizontal tangent at the downstream end of the outlet area 2c. The bending radius RA of the outlet widening tapers continuously toward the outlet hole 4, i.e., there is a bell-shaped expansion with a tapering curve toward the outlet hole 4. If a viscoelastic material, e.g., a polymer or pasta, etc. , is now supplied to the die arrangement according to the invention as described on Fig. 1, the product stream divided into four partial streams is pressed through the four die channels 2 (see Fig. 1 and 2). As on Fig. 1, stresses are encountered in the material as it enters the die channel 2 and while it is molded in the inlet area 2a. Stresses in the material that built up during the first step (cutting at the cutting edges 5a) and/or during the second step (expansion in the inlet area 2a) and have not yet relaxed are here also relaxed practically completely in the expanding outlet area 2c. As a result, the four small product strands exit the die channels 2 under practically no stress in this die geometry as well. However, one special advantage to the expanded outlet ' area 2c is that it allows the product to relax in both an axial and radial direction. This makes it possible to avoid corrugation ("shark skin") on the surface of the viscoelastic product strands exiting the die channels 2, which is practically always encountered given a sharp-edged outlet hole 4 at a cylindrical outlet area 2c (see Fig.. 1) - The axial length of the relaxation areas shown on Fig. 1 and Fig. 3, which are formed essentially by the axial length Ls of the cutting edge 5a and the axial length LA of 'the outlet area 2c, and the maximum flow -rate VF of the viscoelastic material along the product conveying direction F, are preferably adjusted to the relaxation time TRELAX of the product material in such a way as to give the material enough time to decrease the stresses it previously built up as it passes through the respective relaxation areas, i.e., VF x TRELAX Using the die channels 2 with the conical inlet area 2a and the bell-shaped outlet area 2c on Fig. 3 in the die arrangement 1 equipped with cutting edges 5a enables not just a lower pressure gradient in the product along the die arrangement 1 and a practically complete "deletion" of the volumetric dimensional memory in the exiting partial strands of product, but also a "deletion" of the surface dimensional memory of these product strands. Another advantage to the bell-shaped outlet area 2c of the die channels is that it enables a smooth transition from the stream present inside the die channels 2 with a parabolic velocity profile to the "stream" with constant velocity profile present outside the die channels 2, i.e., to the moved strand. This makes it possible to prevent tearing on the surface of the strands exiting the die channels 2. Fig. 4 is a side view through another die channel 2 according to the invention also designed especially for dough used in noodle production along the axial product conveying direction F. The inlet area 2 a of the die channel 2 adjoining the inlet hole 3 is expanded like a bell, while the outlet area 2c is cylindrical. The bending radius RE of the inlet widening is smallest at the inlet hole 3, and increases along the die channel 2 with rising penetration depth, tangentially going over to the cylindrical outlet area 2c. As in the bell-shaped outlet area, the bell- shaped expanded inlet area 2a helps ensure that the product is treated gently. Smoothly accelerating the product in the bell-shaped, expanded inlet area 2a prevents abrupt changes in velocity, which most often cause tears in the product, so that a smooth transition takes place from one stream with a constant velocity profile upstream from the die channels 2 to a stream with parabolic velocity profile inside the die channels 2 here as well. Fig. 5 is a sectional view through a die channel 2 according to prior art along the axial product conveying direction F. The die channel is designed as a cylinder with constant radius RK from its inlet hole 3 up to its outlet hole 4. Fig. 6 is a sectional view through another die channel 2 according to prior art along the axial product conveying direction F. The inlet area 2a has a much larger widening angle a in comparison to the invention, and has a significantly shorter length LE than in the invention. Reference List 1 Die arrangement 2 Die channel 2a Inlet area of the die channel 2b Inner area of the die channel 2c Outlet area of the die channel 3 Inlet hole of the die channel 4 Outlet hole of the die channel 5 Separating wall 5a Cutting edge 7 Housing F Conveying direction Ls Axial expansion of the cutting edge LE Axial expansion of the inlet widening LA Axial expansion of the outlet widening RK Bending radius of the die channel cross section RE Bending radius of the inlet widening RA Bending radius of the outlet widening VF Flow rate of the viscoelastic material a Widening angle - CLAIMS 1. A die arrangement for extruding viscoelastic materials, in particular polymers, pasta, etc. , with at least one die channel extending through the die from an inlet hole to an outlet hole, which has an inlet area at the upstream end of the die channel, an inner area inside the die, and an outlet area at the downstream end of the die channel, all along the axial conveying direction of the material through the die, characterized in that the outlet area is expanded from the die channel inner area to the outlet hole over a length LA along the axial conveying direction F, 2. The die arrangement according to claim 1, characterized in that the widening angle of the outlet widening measured between the axial conveying direction and the inner wall channel outlet area increases gradually along the axial conveying direction. 3. The die arrangement according to claim 1 or 2, characterized in that there is a constant increase in the widening angle along the axial conveying direction. 4. The die arrangement according to claim 3, characterized in that the widening angle of 0° inside the die increases up to 90° at .the downstream end of the die body. 5. The die arrangement according to claim 4, characterized in that the widening follows an arc in longitudinal section. 6. The die arrangement according to claim 5, characterized in that the bending radius RA of the widening arc is greater than the radius RK of the die channel inner area. 7. The die arrangement according to one of the preceding claims, characterized in that the die channel inner wall has a higher surface roughness than the remainder of the die channel inner wall in the outlet area over a length LR along the axial conveying direction. 8. The die arrangement according to claim 7, characterized in that the axial length LR of the rough area is less than the axial length LA of the outlet, and less than the axial length LE of the inlet. 9. The die arrangement according to one of the preceding claims, characterized in that 'it has several mutually parallel, identical die channels with adjacent inlet holes, and that a separating wall runs parallel to the axial conveying direction F between a respective two adjacent inlet holes at the upstream end, and has a cutting edge at its upstream end. 10. The die arrangement according to claim 9, characterized in that the area situated upstream from the inlet area of each of the adj acent inlet holes is completely enveloped by separating walls extending parallel to the axial conveying direction, whose respective upstream end is designed as a cutting edge. 11. The die arrangement according to claim 9 or 10, characterized in that the cutting edges form an angle differing from 90° relative to the axial conveying direction F. The die arrangement according to one of the preceding claims, characterized in that the inlet area of the die channels is conically expanded from the inner area to the inlet hole opposite the conveying direction F along a length LE/ wherein the widening angle of the inlet widening measured between the axial conveying direction and the inner wall of the channel inlet area ranges from 5° to 45°, in particular from 8° to 25°. The die arrangement according to claim 12, characterized in that the widening angle of the outlet widening measured between the axial conveying direction and the inner wall of the channel outlet area gradually increases along the axial conveying direction. The die arrangement according to claim 12 or 13, characterized in that the die channel has a circular cross section over its entire length. The die arrangement according to one of the preceding claims, characterized in that the axial length of the channel inlet area ranges between 50 % and 80 % of the overall length of the die channel. The die arrangement according to one of the preceding claims, characterized in that at least partial areas of the inner walls of the die channel consist of Teflon. An extrusion method for viscoelastic materials, in particular polymers, pastas, etc., using a die arrangement according to one of claims 1 to 16, in which the viscoelastic material is pressed through the die arrangement by a pressure gradient Ap between the upstream end and the downstream end of the die arrangement, characterized in that the pressure gradient Ap is selected in such a way that the flow rate VF of the viscoelastic material along the conveying direction F inside the die channel satisfies the condition VF 18. The method according to claim 17, characterized in that the flow rate VF of the viscoelastic material along conveying direction F inside the die channel satisfies the condition VF

Documents:

2844-chenp-2005 abstract gratned.pdf

2844-chenp-2005 claims granted.pdf

2844-chenp-2005 description(complete) granted.pdf

2844-chenp-2005 drawings granted.pdf

2844-chenp-2005-abstract.pdf

2844-chenp-2005-claims.pdf

2844-chenp-2005-correspondnece-others.pdf

2844-chenp-2005-correspondnece-po.pdf

2844-chenp-2005-description(complete).pdf

2844-chenp-2005-drawings.pdf

2844-chenp-2005-form 1.pdf

2844-chenp-2005-form 18.pdf

2844-chenp-2005-form 3.pdf

2844-chenp-2005-form 5.pdf

2844-chenp-2005-pct.pdf