DE2120492A1 - Thermoplastics extrusion using screw with auxiliary thread - Google Patents

Abstract

A single or multi-thread extruder screw for extruders with one or more non-meshing screws for processing plastics, esp. highly crystalline polymers, e.g. polyamide or polyethyleneterephthalate, has a cylindrical or conical shaft with projections alongside the conventional thread. - These projections start from one or both sides of the main thread and have a different pitch angle, thus dividing the channel between successive threads into two or more spaces, which vary along the length of the screw. - The shaft may also comprise an axial ventilating duct in its initial part. - The advantages are improved quality and output.

Description

Extruder screw

The invention relates to an extruder screw in single and multi-screw extruders with non-meshing screws for Plasticizing, homogenizing and extrusion of plastic masses, especially. Highly crystalline polymers, such as Polyamide, polyethylene terephthalate, etc.

The known screws for processing plastic masses in screw presses with one or more screws have the most varied of structural features that can be implemented by the respective with these screws procedural processes are determined. In the case of single and multi-screw extruders with solid material, ζ · Β. in the form of granules or powder, the processes are conveying, preheating and precompaction the solid starting material; Melting and compressing; Homogenizing and pressing out, i.e. extruding the melt, to realize. Will the extruders get hotter

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If the melt is charged, then only the processes are conveying perform the melt for the purpose of pressure build-up in the same and extrusion.

The purpose of the invention is to use a material adapted to fast and gentle plasticizing a high throughput rate as well as a homogeneous melt to guarantee to achieve a good quality and dimensional accuracy of the product.

A screw construction consisting of 5 zones has already been created consists, proposed This screw has a cylindrical filling zone, a compression zone, which consists of a conical, a cylindrical and a second conical section, which is the cylindrical metering zone connects. With this screw geometry a quick achievement of the necessary for the plasticization of the material is to be achieved Temperature can be made possible, but the material as a result of a gentle treatment - in several Stages - are subjected to avoid overheating and damage to the same.

It has also already been proposed to use a screw that has a cylindrical conveying zone, a conical one ! Compression zone, a short cylindrical metering zone and has a deep-cut cylindrical discharge zone. With this screw construction should also

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be effected that on the one hand the processing temperature Reached quickly and, on the other hand, the melted and homogenized material is gently discharged. aside from that is intended with the large aisle depth of the discharge zone, to avoid the throttling effect of the flat-cut metering zone, which is common in plasticizing screws, and a to achieve high output.

Another proposal relates to a screw with a multi-thread profile in the core surface of the metering zone is incised. The low intermediate webs are intended to ensure a better homogenization of the plastic mass Secure short-term shear over the web.

It has also been specifically proposed for the processing of materials whose viscosity at the processing temperature is below 8000 poise that a three-zone screw, as mentioned above, have a metering zone length of at least 13 times the screw diameter and make up at least half the screw, while the metering zone thread depth is at least 3 »5%» more advantageous but (4 ... 5) % of the screw diameter.

Other screw developments also show the most diverse Built-in components such as mixing rings, mating threads, eccentric sections, kneading elements, damming bodies, corrugated threads, Kneading gears, openwork webs, etc., all of which

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to serve the individual existing in the worm threads to influence laminar flows in their direction and thereby to achieve better mixing. With these mixing screws, the discharge rate can be cut deeper and thus the temperature increase in the melt caused by shear effects is limited to a minimum will.

A screw has also been proposed which allows the melted part to be removed during the melting process spatially separate the material from the unmelted. This separation takes place after or during the passage of the melted material part through the gap between Screw flight and inner wall of the cylinder. The melt arrives in a collecting chamber, which is passed through a second screw flight is formed, which is not connected to the loading opening. The flow area of the to the loading opening connected worm gear decreases continuously to zero in the conveying direction, while the cross-section of the worm gear serving as a collecting space increases continuously in the same direction from zero. The space of the unmelted The material is separated from the melting chamber by a screw flight.

The multi-zone screws described above, such as two-, three- and five-zone screws, but also screws of this basic principle with additional and special homogenizing zones or sections or with openwork webs have inc-

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special deodorant disadvantage of the uncontrolled and therefore uneven Energy input that comes about that already melted material due to the unfavorable geometric design of the passage cross-section with the treated with the same intensity, d. H. the thermal contact with the heated cylinder and the hot screw as well as the is subjected to external and internal friction, as is still unmelted Material that is found on the pressure flank of the screw flight as a result of the melting process Melting material that collects can pass through the existing Mix the cross flow again with the not yet melted material. The cross flow is through the relatively large passage depth, as in the plasticizing zone by the presence of unmelted material in the Usually required, extraordinarily favored.

Through this mixing, the as yet unmelted Particles are surrounded by melt and thus by the hot cylinder wall and the hot screw flights and screw base thermally insulated. In addition, the unmelted particles are enveloped in melt reduces the friction that occurs when unmelted particles slide past each other. This in turn reduces that which is produced in the material itself and is extremely important for the melting process of various types of material Priiction heat.

Both disadvantages lead to an unfavorable development of the plasticizing process. The thermal insulation of the una

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Molten particles from the biting metal surfaces of the cylinder and the screw on the one hand slows down the plasticizing process and, on the other hand, leads to further heating due to the contact of the metal surfaces with the melt particles the latter and thereby an uneven introduction of energy into the material.

The reduction in the development of frictional heat also has the effect of slowing down the melting process. Except_ the deformation per melt particle is greater if melt particles are attached to unmelted, i.e. hard, material particles slide past than when melt particles slide against each other. This fact leads to another uneven Energy loading of the individual melt parts.

The disadvantages outlined and their effects thus lead to a prolonged effective melting section of the Screw that can extend to the end of the screw, which means that unmelted or insufficiently melted ones can be pressed out Particles can result. For this reason, as well as because of the irregularities explained above The material particles are charged with energy by the melt both thermally and structurally inhomogeneous. This inhomogeneity in the melt logically leads to a Product with deteriorated and inadequate properties Dimensional accuracy.

All mentioned construction principles of the multi-zone screw 10 9 8 4 8/1710

with core progressive compression is also characteristic that the dividing lines between the different zones run perpendicular to the screw axis. That means the transitions between the individual states of the to be processed Material would have to run abruptly. Since this is not feasible in practice, especially since different screw speeds, Temperature programs and sometimes also different materials are used in a screw construction ' the above-described cross-mixing of melted and unmelted material particles occurs. H. if the parting lines are in an unfavorable position, it occurs "Especially with these multi-zone screws with progressive core compression, there is easy flow rate fluctuations per unit of time. An "elastic" way of working while at the same time being optimal technological parameters in a wide speed range and / or with several temperature programs and / or even when using different materials or different modifications of one and the same material can hardly be guaranteed with such screw constructions.

The core progressive screw sections set due to the stronger adhesion of the material to the screw core surface This is countered by a stronger 'hydraulic resistance, which causes a reduction in throughput performance

In the case of the screw construction with an overflow web, a quite a number of the disadvantages mentioned above with regard to their

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Effects are dampened. This construction already separates the melted particles from the not yet melted material and thereby largely avoids inhomogeneities in the finished melt.

The design of this screw, however, has two main features Disadvantages on. There are only two snail zones, which are intended for the "solid" and "liquid" forms are. A geometric section that corresponds to the "viscoelastic" transition state of the material to be processed, is not present. This fact can lead to overuse and thus damage to the not yet complete lead liquid material as it flows through the gap between the screw flight and the inner wall of the cylinder. When overflowing the overflow web, there is a considerable Pressure drop in the melt, which causes a reduction in throughput. The latter will in particular noticeable in those materials which are called "wetting" and which, like the latest experimental ones Investigations have shown that they do not produce any independent melt conveyance. Due to the spatial isolation of the Both screw sections lack the pressure on the part of the necessary to achieve a high throughput rate Fixed material conveyor sections

It is the object of the invention to design the screw appropriately so that a uniform introduction of energy into the material to be melted by utilizing the frictional heat generated in the material itself and the heat generated by the 109848/1710

external heating takes place. and the material a. his physical. State corresponding space within the duct volume assigned to a relatively short snail "will.

According to the invention, this object is achieved in that the volume of the worm thread is increased by one or more elevations of the screw core in two or more depending on the physical state of the material to be processed corresponding rooms are divided

These increases begin after a certain number, which depends on a wide variety of technological factors of screw thread turns, calculated from the material filling opening, directly from the screw flight or from the screw flights - for multi-flight screws, they are on both the side of the screw flight and that of the filling opening of the material cylinder is facing away - rear side of the screw flight, as well as on the side of the Screw flight, which faces the filling opening, front side of the screw flight - can begin.

The elevations wind helically around the screw core, wherein they have a different pitch than that of the screw flight or the screw flights. The height of the slope of the elevation or elevations beginning on the front side of the worm thread have a smaller pitch than the screw flight. the

increases 109848/1710 beginning on the rear side of the worm gear

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Hung or incipient elevations, on the other hand, have a greater gradient than the spiral web. Through this After a series of turns, the spaces formed in this way cover the entire cross-section of the corridor, where the first room is "displaced" by the second, this in turn by the acitten, and so on. The last room then takes up the entire aisle volume.

The beginning of the elevations and the size of the slope height of these elevations depend on a whole range of technological Parameters, such as B. the type of material to be processed, its geometric nature, the product, the temperature program, the speed range, the melt temperature, the melt pressure, the drive power; they are to be adapted to the respective processing case.

Determine the sizes of the incline heights of the individual rooms the lengths of these spaces and. thus the type of compression of the material to be processed (short compression, long compression etc.). They have a decisive influence on the plasticizing process and the conveying stability. This influencing is due to the different heat transfer conditions, dynamic and kinematic conditions caused by the solution according to the invention will. - -

The conditions mentioned are also dependent on the size of the elevations, ie the radial differences between the 103848/1710

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same affects. The sizes of the elevations determine the volumes of the spaces formed and thus the level of compression. They depend on the technological parameters listed above and must be adapted accordingly.

The transition surfaces from the screw core to the elevation or between the elevations are vertical or sloping formed inclined surfaces to the outer surface of the screw core. The formation of the transition areas that are Just as the elevations themselves wind helically around the screw core, it particularly influences the heat transfer conditions in the material that is in the deeper

of the two neighboring rooms, but also the kinematic and dynamic conditions in this room.

The screw designed according to the concept of the invention can have a cylindrical or a conical screw core exhibit.

In order to have a better heat transfer from the heated cylinder to the To achieve the material to be melted, the screw core in the deep-cut space is conical.

For materials that are sensitive to shear, it is beneficial to finish the plasticizing or compression process to relieve the melt. For this purpose is at the beginning of the Application zone on the front and / or rear side of the A deep-cut space was introduced into the worm gear * analogous to the increase (s) in the corresponding

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Transition zone of this snail sntv / ickalt, the latter means that the deep-cut space begins on the side of the worm gear, where the elevation in the transition zone also begins had started. If elevations are provided in the transition zone on both sides of the worm thread, see above deep-cut steps are also provided on both sides in the discharge zone. The depressions twist helically around the core of the worm, whereby the stones ting height (s) to which the / of the tendon web (s) is / are so different that after a series of turns of the entire passage cross section is occupied by the depression (s). The latter can of course also be achieved that at the beginning of the discharge zone by means of an oblique which extends over the entire width of the passage cross-section Area where the transition from the transition zone to the deep-cut discharge zone is realized -

For better ventilation and suction of volatile constituents from the material to be processed, the screw has according to a further feature of the invention, an axial bore which extends over the length of the first deep-cut space in which the pesticide extraction takes place. From the bottom of this deep-cut screw step lead several radial ones distributed over the length of this step Holes in the axial hole of the worm.

By dividing the duct volume according to the invention into several the physical state of the material to be processed adapted rooms guarantee the well-known north

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Axial and radial degassing holes in the screw . an effective degassing of the material far into the area of the discharge zone.

A final feature of the invention is that the ridges Have flow-influencing recesses of the most varied arrangement and geometry and that between compensating grooves are arranged on the elevations, which wind helically around the screw core «

The technical-economic effects of the solution according to the invention consist in the separation of the individual physical ones State forms of the material without separation of the space containing the material. This is how one exists direct connection between the filling opening of the machine and their tools. The pressure building up in the area of the solids conveying zone - according to modern knowledge extruder technology "the decisive phenomenon for stable process management - has an undiminished effect. This is already happening plasticized material located in the narrower spaces and thus creates the prerequisites for the process a stable extrusion process with optimal technological parameters.

Another essential advantage of the embodiment according to the invention is the achievement of large effective functional lengths. This advantage is created by the overlapping of the various elevations that make up the functional zones. The total screw lengths of such screw 1 0 9 8 A 8/1 7 1 0

ken can be kept shorter with the same throughput rates will. This saves material for screw production. With the same overall length, higher Throughputs can be achieved.

The invention is to be explained in more detail below using a few exemplary embodiments.

The accompanying drawings show: FIG. 1i a single-flight screw with a cylindrical screw core and an elevation beginning on the posterior side of the tendon duct;

Fig. 2: a single-flight screw with a cylindrical screw core and one on the front side of the screw flight incipient increase;

Fig. 3 * a screw according to Fig. 1 with two at the rear Side of the worm gear successively beginning elevations}

Fig. 4: a single-flight screw according to FIG. 1 with a deep-cut discharge zone;

5% shows a screw according to FIG. 1 with two elevations beginning on the rear or front side of the screw flight and a discharge zone _? which has two depressions beginning on the front and rear side of the worm thread.

Fig. 6? a screw according to Fig. 1 with vain axial and radial bore;

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7s shows a double-flighted screw with a cylindrical scan core and an elevation beginning on the front side of the worm gear}

Fig. 8: a twin screw with non-meshing screws, the gexveils one on the anterior side of the spiral duct show incipient increase;

9s shows a screw according to FIG. 1 with two "at the front Side of the worm thread successively beginning elevations with recesses and compensating grooves are provided;

Fig.10: a section through the screw according to Fig. 9 in the Level A-A;

11: a section through the screw according to FIG. 9 in FIG Level B-B.

In Fig. 1, a single-flight screw 1 is shown, the screw thread 3 by the on the rear side of the screw thread, i.e. on the side of the screw flight facing away from the material filling opening 2, directly from the screw flight 4 incipient increase 5 of the screw core 8 in the Rooms 6 and 7 are shared. The elevation 5 winds with it a slope height h ^. around the screw axis, where fcu im present example is so much greater than the pitch h of the screw flight 4 that after about five The elevation 5 turns over the entire width of the aisle. The transition area 9 between the two trees

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6; 7 runs at right angles to the toll surface of the screw core. The screw core 8 is cylindrical. By a conical in Fig. -1 not shown embodiment of the Screw core 8, the solid material is additionally pre-compressed and pressed against the hot cylinder surface. This further improves the heat transfer. The conicity, however, is only slight, by none to oppose substantial resistance to the conveyance of solids, as occurs in conventional core progressive snails.

The process flow in such a screw version is following:

The solid material occurs in the area of the filling opening 2 into screw 1. It is first in scan corner corridor 3 promoted, the increase 5 eats two rooms 6 and 7 teiit, the absolute gear volume being reduced. This compression corresponds to the reduction in volume of the material to be processed, which in the course of its delivery = to melt by frictional heat and externally supplied heat on the inner wall of the cylinder and by stripping by means of the rotating screw web 4 begins to collect on the pressure flank of the latter. Of the The pressure flank of the screw flight 4 starts the increase 5 * of the snail core 8. In the steadily growing tree, the one Collect increasing amounts of melt that make up the total amount of material until only melt is left.

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that is. The volumes shown here 6 and 7 must be Volumes of the solid and the melt and the change in volume the spaces 6 and 7 along the screw 1 of the change in volume of the solid and the melt along the screw correspond.

Such a screw 1 thus has considerable effective lengths of the solids conveying zone with a small overall length L-B and the discharge zone L .. Although the type of compression as short compression - due to the sudden transition of the melted material from the space 7 to the Room 6 - to be designated - the material is relatively long due to the overlap of the two zones Way over which it can melt to then move from room 7 in to flow into space 6.

Although there is no spatial separation between the two rooms 6 and 7, there is a physical separation between the two States of the material due to their different laws of motion (melt and solid material - Granules or agglomerates or the like with dimensions that are greater than the height of the room tg and smaller than the height of the room tn are realized. However, this separation of the two physical states only means one Maintaining, as described above, yourself spontaneously separation of melted and unmelted material resulting from the plasticization and transport of the material Material in a worm gear.

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Important and cheap for the screw version shown is the fact that there is a direct connection between the material feed opening 2 and the screw end, In this way it is ensured that the material pressure which builds up in the first screw turns as a result of the pesticide conveyance is forward without any reduction can act on the melted material "and thus guarantees a high throughput. The transition area 9»

that in the Pig · 1 as one to the outer surface of the screw core is designed vertically arranged surface, can also be designed as an inclined surface. This will make it happen unmelted material is pressed to a greater extent on the hot inner surface of the cylinder and thus a better thermal contact created

In the case of the screw according to FIG. 2, the elevation 5 begins Screw core 8 on the front side of the screw flight.

Such a screw design is particularly suitable for non-wetting, highly viscous materials, v / ie e.g. Rigid PVC, etc., which in contrast to the wetting pla3ten - polyethylene, polyamide, etc. - the more homogeneous mass fractions on the passive flank of the screw duct.

In Fig. 3 is a screw with two elevations 5; 10 shown. As a result, the passage cross section 3 is divided into three spaces 6j 7; 11 divided, This version is ^ especially for mate-

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rials interesting, apart from the two physical ones. States "solid" and "viscous-liquid" an intermediate layer, like ζ · Β. "visco-liquid" an intermediate state, such as. B. "visco-elastic", have «

Bas solid material is first cut into the deep Room 7 funded. The material picks up during the plastification the visco-elastic. state to then continue in to pass the liquid state.

The individual geometric parameters of the screw are dimensioned in such a way that the visco-elastic material flows into space 6 and finally reaches space 11 after its final plasticization. The evolution of the volumes of the three rooms 6; 7; 11 along the screw axis corresponds to the development of the volumes of the three physical states - solid, visco-elastic, liquid (viscofluid). While initially only solid, i.e. unmelted material is present, later over a certain transition length there is solid and visco-elastic and finally solid, visco-elastic and visco-liquid material, the volume of the last screw becomes; in fertilize only filled with melt. The space 6, into which the viscoelastic material is transported and further plasticized, is dimensioned in such a way that smaller, unmelted material particles can also get into it. This space 6 thus forms a real transition zone It-, which is a gentle and elactiscl: -. Plasticization and transport of the material

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rials in a wide range of the most important technological parameters - speed, temperature program, pressure, etc. allowed.

In Fig. 4 is a screw as shown in Fig. 1, whose Discharge zone is deep-cut, with the depression over the entire width of the screw flight over the incline Area 12 is reached »

The screw of Figure "5 v / eist in the discharge zone L to * two W mutually arranged screw-Steg 4 wells 13, 14, which in principle proceed in the same manner as the ridges 5 in the solid conveying zone of the wormshaft Ln ,. The heights h ~ of the recess 13 and h-, the recess 14 can have the same or different values.

In the case of this screw, the material that was first melted and flowed into the space 6 is passed through the depressions 13; * also relieved first. The low tree height t, - ensures that no cross-mixing of the melted material rials with the unmelted entrance. This ensures that all of the material to be processed is the same undergoes thermal stress, which is only kept so large that the material is sufficiently plasticized that However, discharge through deeper-cut screw windings with lower shear stress and thus lower thermal stress took place

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FIG. 6 is a Schner.Ve as shown in FIG. 1, but which has an axial bore I5 ^UQ d radial bores 16. The latter connect the deep-cut space 7 so the zone in which only solid material is conveyed with the axial bore. In this way, volatile constituents can be removed from the solid material during preheating and precompression far into the area of the discharge zone (melt zone) L. These components are discharged to the outside via the bores 16 and the axial bore 15.

In Fig. 7 there is a double-flighted worm, one in each worm thread on the rear side of the worm thread has incipient increase. Otherwise, the geometry of this screw corresponds to the design Fig. 1.

lig. 8 illustrates an embodiment of the invention in the form of a Double screw with non-meshing screws, each of the two screws in the manner already described above, one beginning on the rear side of the screw flight Has increase 5.

In FIG. 9, a screw is shown in accordance with FIG. 3 pictured, However, the two elevations 5 and 10 of which have recesses 17, between the elevations 5 and 10 and between the Elevation 10 and the screw flight 4, the grooves 18 and 19 are introduced, the cross-sections of which are preferably semicircles are executed.

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This construction designed as a homogenizing and mixing corner should guarantee the following process sequence:

The material moving in the deep-cut screw flight space 7 ″ is heated and partially melted. The material melt that forms on the hot inner surface of the cylinder is sheared off by the rotating first elevation 5 and collects on the front flank of this elevation 5. There this melt mixes with smaller ones solid and partially already melted material particles and flows into the space 6. Through the recesses 17 made in the elevation 5, the axis of which is inclined to the screw axis, a superimposition of two flows is achieved, one along the recesses 17 and the other tangential to Increase 5 »caused by the screw rotation. This creates high shear forces in the material, which are the prerequisite for good homogenization. The latter is reinforced by the mixing of the two laminar flows along the compensation groove 18 Mate rial melts completely, it flows into the space 11 of the second elevation 10. Also in this space there is a superposition of two currents, which are caused in turn by recesses 17 IA of the second ridge 10 and the screw rotation. The compensation groove 19, which is also due to the manufacturing process, also results in a flow compensation here. The recesses 17 in the second elevation 10, which after a few turns extends over the entire width of the screw flight, can be present both up to the screw tip and a few turns before the 109848/1710

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Stop the tip of the screw as shown in Fig. 8. The remainder of the discharge zone L. then corresponds to the geometric design customary for extruder screws.

For a better understanding of the geometry of the embodiment described, two sections are shown in FIGS. 10 and 11 through " the homogenizing screw shown in FIG. 9 is reproduced.

Such a homogenizing screw allows such complicated ones technological operations, such as coloring natural-colored materials with soluble and insoluble dyes and pigments can be realized with relatively little effort. This unite in such a snail the benefits of good homogenization with the benefits gentle plasticizing and a stable conveying process

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Claims (7)

Claims "| 1 · \ Single or multi-start extruder screw of a single or Multi-screw extruder with non-meshing screws for Processing of plastic masses, characterized in that a cylindrical or conical screw core (8) in addition to the known screw flight (4) has elevations (5ϊ ΊΟ) that are on one or both sides directly Beginning at the screw flight (4), it turns helically with a different pitch (b, -) wind the screw core (8) and divide the volume of the screw flights (3) into two or more spaces (6; 7 »11), which, in terms of volume, correspond to the plasticization and compression process progressing along the screw axis change. 2. Extruder screw according to claim 1, characterized in that the transition surface (9) from the screw core (8) to the Increase (5) and between the increases (5; 10) as perpendicular or as inclined, to the lateral surface of the screw core (8) inclined surfaces are formed. 3 extruder screw according to claim 1 and 2, characterized in that that the screw core (8) is made conical in the deep-cut space (7). 4. Extruder screw according to claim 1 to 3 »characterized in that that the discharge zone (L.) is cut so deeply is that the recess over the entire width of the 109 848/1710 -25- BAD OR (GiNAL Worm gear (3) is reached by an inclined surface (13). 5. Extruder screw according to claim 1 to 3, characterized in that the screw core (8) at the beginning of the discharge zone Oja) on one or both sides of the screw flight (4) beginning, helically with different heights to the latter (b ₂ , h ~) around the screw core (8) winding recesses (14) is designed so that after a certain 'number of turns they take up the entire flight volume. 6. extruder screw according to claim 1 to 5 »characterized in that that the screw (1) has an axial bore (15) which is deep-cut with the bottom of the first Space (7) and the screw core (8) is connected via radial bores (16). 7. Extruder screw according to claim 1 to 5, characterized in that between the recesses (17) having elevations (5; 10) of the screw core (8) helically around this winding compensation grooves (18; 19) are arranged. -.26 - 1098A8 / 171Ö