Patent Application
20140312151
The invention relates to an apparatus according to the preamble of claim 1.
The prior art reveals numerous similar apparatuses of varying design, comprising a receiver (receiving container) or cutter compactor for the comminution, heating, softening and treatment of a plastics material to be recycled, and also, attached thereto, a conveyor or extruder for the melting of the material thus prepared. The aim here is to obtain a final product of the highest possible quality, mostly in the form of pellets.
By way of example, EP 123 771 or EP 303 929 describe apparatuses with a receiver and, attached thereto, an extruder, where the plastics material introduced into the receiver is comminuted through rotation of the comminution and mixing implements and is fluidized, and is simultaneously heated by the energy introduced. A mixture with sufficiently good thermal homogeneity is thus formed. This mixture is discharged after an appropriate residence time from the receiver into the screw-based extruder, and is conveyed and, during this process, plastified or melted. The arrangement here has the screw-based extruder approximately at the level of the comminution implements. The softened plastics particles are thus actively forced or stuffed into the extruder by the mixing implements.
Most of these designs, which have been known for a long time, are unsatisfactory in respect of the quality of the treated plastics material obtained at the outgoing end of the screw, and/or in respect of the quantitative output of the screw. Studies have shown that the requirements placed upon the screw downstream of the container, mostly a plastifying screw, differ during the course of the operation, and that this is attributable to container residence times that are longer for some batches of the product to be processed than for other batches. The average residence time of the material in the container is calculated by dividing the weight of the charge in the container by the amount discharged from the screw per unit of time. However, this average residence time is—as mentioned—generally not valid for large portions of the material to be processed, but instead there are irregular substantial upward and downward deviations from this average value. These deviations may be attributable to differences in the nature of the batches of product introduced successively into the container, e.g. differences in the nature or thickness of the plastics material, e.g. foil residues, etc., or else uncontrollable variations.
For material that is thermally and mechanically homogeneous, there is usually a quality improvement in the product obtained at the outgoing end of the screw when the flight depth of the metering zone of the screw is very large and the screw rotation rate is kept very small. However, if it is desirable to increase the quantitative output of the screw or to improve the performance for example of a shredder-extruder combination, the screw rotation rate must then be raised, and this means that the shear level is also raised. However, this causes the screw to subject the processed material to higher mechanical and thermal stress, and there is therefore the risk of damage to the molecular chains of the plastics material. Another disadvantage that can arise is greater wear of the screw and of its housing, in particular during the processing of recycling material, by virtue of the contaminants present in this material, e.g. abrasive particles, metal parts, etc., which cause severe wear of the metal parts as they slide across one another, in the screw or in its bearings.
However, an effect that occurs both with slow-running and deep-cut screws (large flight depth) and with fast-running screws is that, as previously mentioned, differences in quality of individual batches of material introduced to the screw, e.g. differences in flake size and/or differences in temperature of the plastics material, have a disadvantageous effect with regard to inhomogeneity of the plastics material obtained at the outgoing end of the screw. In order to compensate for this inhomogeneity, the temperature profile of the extruder is in practice raised, and this means that additional energy has to be introduced into the plastic, thus subjecting the plastics material to the thermal damage mentioned and increasing the amount of energy required. Another result here is that the viscosity of the plastics material obtained at the outgoing end of the extruder is reduced, and this makes the material more free-flowing, with concomitant difficulties in the further processing of this material.
It can be seen from this that the process parameters that are advantageous for obtaining material of good quality at the outgoing end of the screw are mutually contradictory.
In an initial attempt to solve this problem, the diameter of the cutter compactor was increased in relation to the diameter of the screw. This enlargement of the container in comparison with conventional sizes improved the mechanical and thermal homogeneity of the plastics material pretreated in the container. The reason for this was that the ratio by mass of the continuously added untreated “cold” portions of material to the amount of material present in the container and already to some extent treated was smaller than under the conditions that usually prevail, and that the average residence time of the plastics material in the container was substantially increased. This reduction of the ratio by mass had an advantageous effect on the thermal and mechanical homogeneity of the material entering the screw housing from the container, and with this had a direct advantageous effect on the quality of the plastified or agglomerated material at the end of the extruder screw or of the agglomerating screw, since the product initially introduced to the screw was at least approximately of identical mechanical and thermal homogeneity, and therefore the screw itself was not required to achieve this homogeneity. The theoretical residence time of the treated plastics material in the container was approximately constant. Furthermore, this type of system with enlarged container was less sensitive than the known systems in relation to the accuracy of input portions.
Systems of this type were therefore in principle capable of effective use, and advantageous. However, although systems using containers or cutter compactors with large diameters, e.g. of 1500 mm or more, and with relatively long residence times, have good functionality, and although the quality of the recylate is high, they are not ideal in terms of space required and of efficiency, or they emit a large amount of heat.
Another feature shared by these known apparatuses is that the direction of conveying or of rotation of the mixing and comminution implements, and therefore the direction in which the particles of material circulate in the receiver, and the direction of conveying of the conveyor, in particular of an extruder, are in essence identical or have the same sense. This arrangement, selected intentionally, was the result of the desire to maximize stuffing of the material into the screw, or to force-feed the screw. This concept of stuffing the particles into the conveying screw or extruder screw in the direction of conveying of the screw was also very obvious and was in line with the familiar thinking of the person skilled in the art, since it means that the particles do not have to reverse their direction of movement and there is therefore no need to exert any additional force for the change of direction. An objective here, and in further derivative developments, was always to maximize screw fill and to amplify this stuffing effect. By way of example, attempts have also been made to extend the intake region of the extruder in the manner of a cone or to curve the comminution implements in the shape of a sickle, so that these can act like a trowel in feeding the softened material into the screw. Displacement of the extruder, on the inflow side, from a radial position to a tangential position in relation to the container further amplified the stuffing effect, and increased the force with which the plastics material from the circulating implement was conveyed or forced into the extruder.
Apparatuses of this type are in principle capable of functioning, and they operate satisfactorily, although with recurring problems:
By way of example, an effect repeatedly observed with materials with low energy content, e.g. PET fibres or PET foils, or with materials which at a low temperature become sticky or soft, e.g. polylactic acid (PLA) is that when, intentionally, stuffing of the plastics material into the intake region of the extruder or conveyor, under pressure, is achieved by components moving in the same sense, this leads to premature melting of the material immediately after, or else in, the intake region of the extruder or of the screw. This firstly reduces the conveying effect of the screw, and secondly there can also be some reverse flow of this melt into the region of the cutter compactor or receiver, with the result that flakes that have not yet melted adhere to the melt, and in turn the melt thus cools and to some extent solidifies, with resultant formation of a clump or conglomerate made of to some extent solidified melt and of solid plastics particles. This causes blockage on the intake and caking of the mixing and comminution implements. A further consequence is reduction of the throughput or quantitative output of the conveyor or extruder, since adequate filling of the screw is no longer achieved. Another possibility here is that movement of the mixing and comminution implements is prevented. In such cases, the system normally has to be shut down and thoroughly cleaned.
Problems also occur with polymer materials which have already been heated in the cutter compactor up to the vicinity of their melting range. If overfilling of the intake region occurs here, the material melts and intake is impaired.
Problems are also encountered with fibrous materials that are mostly orientated and linear, with a certain amount of longitudinal elongation and low thickness or stiffness, for example plastics foils cut into strips. A main reason for this is that the elongate material is retained at the outflow end of the intake aperture of the screw, where one end of the strip protrudes into the receiver and the other end protrudes into the intake region. Since the mixing implements and the screw are moving in the same sense or exert the same conveying-direction component and pressure component on the material, both ends of the strip are subjected to tension and pressure in the same direction, and release of the strip becomes impossible. This in turn leads to accumulation of the material in the said region, to a narrowing of the cross section of the intake aperture, and to poorer intake performance and, as a further consequence, to reduced throughput. The increased feed pressure in this region can moreover cause melting, and this in turn causes the problems mentioned in the introduction.
It is therefore an object of the present invention to overcome the disadvantages mentioned and to improve an apparatus of the type described in the introduction in such a way as to permit problem-free intake of materials by the screw, even of those that are sensitive or strip-shaped, and to permit processing or treatment of these materials to give material of high quality, with high throughput, while making efficient use of time, saving energy, and minimizing space requirement.
The characterizing features of claim 1 achieve this object in an apparatus of the type mentioned in the introduction.
A first provision here is that the imaginary continuation of the central longitudinal axis of the conveyor, in particular extruder, if this has only a single screw, or the longitudinal axis of the screw closest to the intake aperture, if the conveyor has more than one screw, in a direction opposite to the direction of conveying of the conveyor, passes, and does not intersect, the axis of rotation, where, on the outflow side in the direction of rotation of the implements, there is an offset distance between the longitudinal axis of the conveyor, if this has a single screw, or the longitudinal axis of the screw closest to the intake aperture, and the radial that is associated with the container and that is parallel to the longitudinal axis and that proceeds outwards from the axis of rotation of the mixing and/or comminution implement in the direction of conveying of the conveyor.
The direction of conveying of the mixing implements and the direction of conveying of the conveyor are therefore no longer in the same sense, as is known from the prior art, but instead are at least to a small extent in the opposite sense, and the stuffing effect mentioned in the introduction is thus reduced. The intentional reversal of the direction of rotation of the mixing and comminution implements in comparison with apparatuses known hitherto reduces the feed pressure on the intake region, and the risk of overfilling decreases. In this way, excess material is not stuffed or trowelled with excess pressure into the intake region of the conveyor, but instead, in contrast, there is in fact in turn a tendency to remove excess material from that region, in such a way that although there is always sufficient material present in the intake region, the additional pressure exerted is small or almost zero. This method can provide adequate filling of the screw and constant intake of sufficient material by the screw, without any overfilling of the screw with, as a further consequence, local pressure peaks where the material could melt.
Melting of the material in the region of the intake is thus prevented, and operating efficiency is therefore increased, maintenance intervals are therefore lengthened, and downtime due to possible repairs and cleaning measures is reduced.
By virtue of the reduced feed pressure, displaceable elements which can be used in a known manner to regulate the degree of filling of the screw react markedly more sensitively, and the degree of filling of the screw can be adjusted with even greater precision. This makes it easier to find the ideal point at which to operate the system, in particular for relatively heavy materials, for example regrind made of high-density polyethylene (HDPE) or PET.
Surprisingly and advantageously it has moreover been found that operation in the opposite sense, according to the invention, improves intake of materials which have already been softened almost to the point of melting. In particular when the material is already in a doughy or softened condition, the screw cuts the material from the doughy ring adjacent to the container wall. In the case of a direction of rotation in the direction of conveying of the screw, this ring would instead be pushed onward, and removal of an outer layer by the screw would not be possible, with resultant impairment of intake. The reversal of the direction of rotation, according to the invention, avoids this.
Furthermore, the retention or accumulation phenomena formed in the case of the treatment of the above-described strip-shaped or fibrous materials can be resolved more easily, or do not occur at all, since, at the aperture edge situated in the direction of rotation of the mixing implements on the outflow side or downstream, the direction vector for the mixing implements and the direction vector for the conveyor point in almost opposite directions, or in directions that at least to a small extent have opposite sense, and an elongate strip cannot therefore become curved around, and retained by, the said edge, but instead becomes entrained again by the mixing vortex in the receiver.
The overall effect of the design according to the invention is that intake performance is improved and throughput is markedly increased. The stability and performance of the entire system made of cutter compactor and conveyor is thus increased.
Experiments have moreover allowed the applicant to discover and recognize that, surprisingly, the diameter D of the container can be reduced without any need to accept loss of quality or long residence times. The diameter D of the container here has the following relationship to the diameter d of the screw:
D=10·3√{square root over (K·d2)},
where
The effect of this specific design of a cutter-compactor/conveyor system is that—contrary to previous expectations—it is also possible to use containers or cutter compactors with relatively small diameters, and that it is possible to achieve high throughput performance levels and high quantitative output even with low residence time.
Because of the opposite direction of rotation of the mixing implements, the intake behaviour of the screw is not aggressive, and it is therefore possible, in the cutter compactor, to use implements that are more aggressive and that introduce more energy into the material. This also reduces the average residence time of the material in the cutter compactor. Accordingly, the cutter compactor can be operated at relatively high temperature, a consequence of which is in turn better homogeneity. Containers with relatively small diameters and residence times can therefore also be used successfully to prepare the material.
Another unexpected consequence of this type of combination of cutter compactor and extruder is improved melting performance of the material in an attached extruder. This provides compensation for possible inhomogeneity, and the material that passes from the container into the screw housing and is then compressed and melted has high thermal and mechanical homogeneity. The final quality of the plastified or agglomerated material at the end of the extruder screw or of the agglomerating screw is correspondingly also very high, and it is possible to use screws which—because of the pretreatment and the intake—treat the polymer in a non-aggressive manner and introduce a particularly small amount of shear into the material in order to melt the same.
For comparison, and to demonstrate the technical effect, comparison was made between a cutter-compactor/extruder combination (A) known from the prior art with a container or cutter compactor with relatively large diameter, i.e. high K value, and with direction of rotation of the mixing and comminution implements in the same sense, and an analogous apparatus (B) according to the invention, as shown in
Examples of values for a system of conventional design are:
The theoretical residence time calculated from these values is Zi:Ea=49:355=8.3 min. This conventional design corresponds to a value of K=207.
If, in a design which is otherwise the same, the diameter of the container and the height of the container are reduced respectively to 1000 mm, and the direction of rotation of the implements is reversed (the design of the system being as described in
The theoretical residence time calculated from these values is 5.7 min. This design, in accordance with the above formula, corresponds to a value of K=156.
Although residence time is markedly lower, and although container diameter is reduced by about 10%, a marked increase of about 20% in quantitative output is obtained for the same screw rotation rate and with, in each case, very good quality of the final material.
This can be seen from the table below, and also from the graph according to
The lower curve describes, for the known apparatus A, the standardized throughput, which is dependent on the temperature and, respectively, on the compaction standardized on the basis of the prior art. At a compaction of 0.71, in this case at a temperature of about 75° C. in the cutter compactor, this apparatus achieves about 30% of the maximum throughput, and at a compaction of 1, in this case at about 105° C., i.e. in the ideal usage range, it achieves about 100% of its maximum throughput. After this, the curve falls sharply. The situation for the upper curve for the apparatus B according to the invention is different. Here, the throughput is always higher at comparable temperatures, and at a compaction of 1, at about 105° C., it achieves values that are greater by about 20%.
The apparatus according to the invention therefore operates more rapidly and more efficiently and requires less space.
For a K value of 156 in apparatus B, the above formula gives the following container diameters D (in mm) for prescribed conventional screw diameters d (in mm):
If it is now desirable to achieve an additional increase in quantitative output or improvement of performance of the cutter-compactor/extruder combination mentioned, it is then possible, with extruders of the same size, i.e. with screws of the same diameter, to enlarge the screw flight depth and raise the rotation rate of the screw, while not subjecting the processed material in the screw to any relatively high shear level. This effect is also amplified by the non-aggressive intake due to the opposed rotation of the mixing and comminution implements. The melt temperature of the plastified material is thus reduced, as also therefore is the total energy requirement for the treatment procedure. This is particularly important for the processing of thermally unstable thermoplastics.
Although an increase in the height of the container (while diameter is unchanged) increases the volume of the container, it has practically no effect in improving the quality of the processed material. The reason for this is that the implements circulating in the container produce, in the circulating product, a shape (mixing vortex) which does not exceed a certain height (effective height), i.e. once the circulating plastics material, along the container wall, has reached the maximum height it falls back inwards and downwards into the region of the centre of the container. For these reasons, the container height is usually selected to be the same as the container diameter.
Further advantageous embodiments of the invention are described via the following features:
Operation of the apparatus is particularly advantageous when the constant K is in the range from 90 to 170. At these K values, or at the container sizes and residence times associated therewith, the implement transfers the material particularly effectively to the conveyor, and there is a particularly good balance between the other features that are to some extent negatively correlated: the container size, the residence time, the intake behaviour or the throughput, and the quality of the final product.
According to one advantageous development of the invention, it is envisaged that the conveyor is arranged on the receiver in such a way that the scalar product of the direction vector (direction vector that is associated with the direction of rotation) that is tangential to the circle described by the radially outermost point of the mixing and/or comminution implement or to the plastics material transported past the aperture and that is normal to a radial of the receiver, and that points in the direction of rotation or of movement of the mixing and/or comminution implement and of the direction vector that is associated with the direction of conveying of the conveyor at each individual point or in the entire region of the aperture or at each individual point or in the entire region immediately radially prior to the aperture is zero or negative. The region immediately radially prior to the aperture is defined as that region which is prior to the aperture and at which the material is just about to pass through the aperture but has not yet passed the aperture. The advantages mentioned in the introduction are thus achieved, and there is effective avoidance of all types of agglomeration in the region of the intake aperture, brought about by stuffing effects. In particular here, there is also no dependency on the spatial arrangement of the mixing implements and of the screw in relation to one another, and by way of example the orientation of the axis of rotation does not have to be normal to the basal surface or to the longitudinal axis of the conveyor or of the screw. The direction vector that is associated with the direction of rotation and the direction vector that is associated with the direction of conveying lie within a, preferably horizontal, plane, or in a plane orientated so as to be normal to the axis of rotation.
In another advantageous formation, the angle included between the direction vector that is associated with the direction of rotation of the mixing and/or comminution implement and the direction vector that is associated with the direction of conveying of the conveyor is greater than or equal to 90° and smaller than or equal to 180°, where the angle is measured at the point of intersection of the two direction vectors at the edge that is associated with the aperture and that is situated upstream in relation to the direction of rotation or of movement, in particular at the point that is on the said edge or on the aperture and is situated furthest upstream. This therefore describes the range of angles within which the conveyor must be arranged on the receiver in order to achieve the advantageous effects. In the entire region of the aperture or at each individual point of the aperture, the forces acting on the material are therefore orientated at least to a small extent in an opposite sense, or in the extreme case the orientation is perpendicular and pressure-neutral. At no point of the aperture is the scalar product of the direction vectors of the mixing implements and of the screw positive, and no excessive stuffing effect occurs even in a subregion of the aperture.
Another advantageous formation of the invention provides that the angle included between the direction vector that is associated with the direction of rotation or of movement and the direction vector that is associated with the direction of conveying is from 170° to 180°, measured at the point of intersection of the two direction vectors in the middle of the aperture. This type of arrangement is relevant by way of example when the conveyor is arranged tangentially on the cutter compactor.
In order to ensure that no excessive stuffing effect occurs, the distance, or the offset, between the longitudinal axis and the radial can advantageously be greater than or equal to half of the internal diameter of the housing of the conveyor or of the screw.
It can moreover be advantageous for these purposes to set the distance, or the offset, between the longitudinal axis and the radial to be greater than or equal to 5 or 7%, or still more advantageously greater than or equal to 20%, of the radius of the receiver. In the case of conveyors with a prolonged intake region or with grooved bushing or with extended hopper, it can be advantageous for this distance or the said offset to be greater than or equal to the radius of the receiver. This is particularly true for cases where the conveyor is attached tangentially to the receiver or runs tangentially to the cross section of the container.
It is advantageous that the outermost flights of the screw do not protrude into the container.
It is particularly advantageous if the longitudinal axis of the conveyor or of the screw or the longitudinal axis of the screw closest to the intake aperture runs tangentially with respect to the inner side of the side wall of the container, or the inner wall of the housing does so, or the enveloping end of the screw does so, it being preferable that there is a drive connected to the end of the screw, and that the screw provides conveying, at its opposite end, to a discharge aperture which is in particular an extruder head, arranged at the end of the housing.
In the case of conveyors that are radially offset, but not arranged tangentially, it is advantageous to provide that the imaginary continuation of the longitudinal axis of the conveyor in a direction opposite to the direction of conveying, at least in sections, passes, in the form of a secant, through the space within the receiver.
It is advantageous to provide that there is immediate and direct connection between the aperture and the intake aperture, without substantial separation or a transfer section, e.g. a conveying screw. This permits effective and non-aggressive transfer of material.
The reversal of the direction of rotation of the mixing and comminution implements circulating in the container can certainly not result from arbitrary action or negligence, and it is not possible—either in the known apparatuses or in the apparatus according to the invention—simply to allow the mixing implements to rotate in the opposite direction, in particular because the arrangement of the mixing and comminution implements is in a certain way asymmetrical or direction-oriented, and their action is therefore only single-sided or unidirectional. If this type of equipment were to be rotated intentionally in the wrong direction, a good mixing vortex would not form, and there would be no adequate comminution or heating of the material. Each cutter compactor therefore has its unalterably prescribed direction of rotation of the mixing and comminution implements.
In this connection, it is particularly advantageous to provide that the manner of formation, set-up, curvature and/or arrangement of the frontal regions or frontal edges that are associated with the mixing and/or comminution implements, act on the plastics material and point in the direction of rotation or of movement, differs when comparison is made with the regions that, in the direction of rotation or of movement, are at the rear or behind.
An advantageous arrangement here provides that, on the mixing and/or comminution implement, implements and/or blades have been arranged which, in the direction of rotation or of movement, have a heating, comminuting and/or cutting effect on the plastics material. The implements and/or blades can either have been fastened directly on the shaft or preferably have been arranged on a rotatable implement carrier or, respectively, a carrier disc arranged in particular parallel to the basal surface, or have been formed therein or moulded onto the same, optionally as a single piece.
In principle, the effects mentioned are relevant not only to compressing extruders or agglomerators but also to conveying screws that have no, or less, compressing effect. Here again, local overfeed is avoided.
In another particularly advantageous formation, it is provided that the receiver is in essence cylindrical with a level basal surface and with, orientated vertically in relation thereto, a side wall which has the shape of the jacket of a cylinder. In another simple design, the axis of rotation coincides with the central axis of the receiver. In another advantageous formation, the axis of rotation or the central axis of the container have been orientated vertically and/or normally in relation to the basal surface. These particular geometries optimize intake performance, with an apparatus design that provides stability and simple construction.
In this connection it is also advantageous to provide that the mixing and/or comminution implement or, if a plurality of mutually superposed mixing and/or comminution implements have been provided, the lowest mixing and/or comminution implement closest to the base has been arranged at a small distance from the basal surface, in particular in the region of the lowest quarter of the height of the receiver, and also that the aperture has been similarly arranged. The distance here is defined and measured from the lowest edge of the aperture or of the intake aperture to the container base in the edge region of the container. There is mostly some rounding of the edge at the corner, and the distance is therefore measured from the lowest edge of the aperture along the imaginary continuations of the side wall downwards to the imaginary outward continuation of the container base. Distances with good suitability are from 10 to 400 mm.
It is also advantageous for the processing if the radially outermost edges of the mixing and/or comminution implements almost reach the side wall.
It is particularly advantageous, in this connection, if the distance A of the radially outermost point of the lowest mixing and/or comminution implement, or the distance A of the circle described by this point, from the inner surface of the side wall of the container, is greater than or equal to 20 mm, in particular from ≧20 mm to 60 mm. This gives particularly effective and non-aggressive intake behaviour.
The container does not necessarily have to have a cylindrical shape with circular cross section, even though this shape is advantageous for practical reasons and reasons of manufacturing technology. When container shapes that deviate from the cylindrical shape with circular cross section, examples being containers having the shape of a truncated cone or cylindrical containers which, in plan view, are elliptical or oval, a calculation is required for conversion to a cylindrical container which has circular cross section and the same volume capacity, on the assumption that the height of this imaginary container is the same as its diameter. Container heights here which are substantially higher than the resultant mixing vortex (after taking into account the distance required for safety) are ignored, since this excess container height is not utilized and it therefore has no further effect on the processing of the material.
The expression conveyor means mainly systems with screws that have non-compressing or decompressing effect, i.e. screws which have purely conveying effect, but also systems with screws that have compressing effect, i.e. extruder screws with agglomerating or plastifying effect.
The expressions extruder and extruder screw in the present text mean extruders or screws used for complete or partial melting of the material, and also extruders used to agglomerate, but not melt, the softened material. Screws with agglomerating effect subject the material to severe compression and shear only for a short time, but do not plastify the material. The outgoing end of the agglomerating screw therefore delivers material which has not been completely melted but which instead is composed of particles incipiently melted only at their surface, which have been caked together as if by sintering. However, in both cases the screw exerts pressure on the material and compacts the same.
All of the examples described in the figure below depict conveyors with a single screw, for example single-screw extruders. However, it is also possible as an alternative to provide conveyors with more than one screw, for example twin- or multiscrew conveyors or twin- or multiscrew extruders, in particular with a plurality of identical screws, which at least have the same diameters d.
Further features and advantages of the invention are apparent from the description of the inventive examples below of the subject matter of the invention, which are not to be interpreted as restricting, and which the drawings depict diagrammatically and not to scale:
Neither the containers, nor the screws nor the mixing implements are to scale, either themselves or in relation to one another, in the drawings. By way of example, therefore, the containers are in reality mostly larger, or the screws longer, than depicted here.
The advantageous cutter-compactor/extruder combination depicted in
Arranged at a small distance from the basal surface 2, at most at about 10 to 20%, or optionally less, of the height of the side wall 9—measured from the basal surface 2 to the uppermost edge of the side wall 9—is an implement carrier 13 or a level carrier disc orientated parallel to the basal surface 2, which carrier or disc can be rotated, in the direction 12 of rotation or of movement indicated by an arrow 12, around a central axis 10 of rotation, which is simultaneously the central axis of the container 1. A motor 21, located below the container 1, drives the carrier disc 13. On the upper side of the carrier disc 13, blades or implements, e.g. cutter blades, 14 have been arranged, and together with the carrier disc 13 form the mixing and/or comminution implement 3.
As indicated in the diagram, the blades 14 are not arranged symmetrically on the carrier disc 13, but instead have a particular manner of formation, set-up or arrangement on their frontal edges 22 facing in the direction 12 of rotation or of movement, so that they can have a specific mechanical effect on the plastics material. The radially outermost edges of the mixing and comminution implements 3 reach a point which is relatively close to, about 5% of the radius 11 of the container 1 from, the inner surface of the side wall 9.
The container 1 has, near the top, a charging aperture through which the product to be processed, e.g. portions of plastics foils, is charged by way of example by means of a conveying device in the direction of the arrow. The container 1 can, as an alternative, be a closed container and capable of evacuation at least as far as an industrial vacuum, the material being introduced by way of a system of valves. The said product is received by the circulating mixing and/or comminution implements 3 and is raised to form a mixing vortex 30, where the product rises along the vertical side wall 9 and, approximately in the region of the effective container height H, falls back again inward and downward into the region of the centre of the container, under gravity. The effective height H of the container 1 is approximately the same as its internal diameter D. In the container 1, a mixing vortex 30 is thus formed, in which the material is circulated in a vortex both from top to bottom and also in the direction 12 of rotation. By virtue of this particular arrangement of the mixing and comminution elements 3 or the blades 14, this type of apparatus can therefore be operated only with the prescribed direction 12 of rotation or movement, and the direction 12 of rotation cannot be reversed readily or without additional changes.
The circulating mixing and comminution implements 3 comminute and mix the plastics material introduced, and thereby heat and soften it by way of the mechanical frictional energy introduced, but do not melt it. After a certain residence time in the container 1, the homogenized, softened, doughy but not molten material is, as described in detail below, removed from the container 1 through an aperture 8, passed into the intake region of an extruder 5, and received by a screw 6 there and subsequently melted.
At the level of the, in the present case single, comminution and mixing implement 3, the said aperture 8 is formed in the side wall 9 of the container 1, and the pretreated plastics material can be removed from the interior of the container 1 through this aperture. The material is passed to a single-screw extruder 5 arranged tangentially on the container 1, where the housing 16 of the extruder 5 has, situated in its jacket wall, an intake aperture 80 for the material to be received by the screw 6. This type of embodiment has the advantage that the screw 6 can be driven from the lower end in the drawing by a drive, depicted only diagrammatically, in such a way that the upper end of the screw 6 in the drawing can be kept free from the drive. The discharge aperture for the plastified or agglomerated plastics material conveyed by the screw 6 can therefore be arranged at this upper end, e.g. in the form of an extruder head not depicted. The plastics material can therefore be conveyed without deflection by the screw 6 through the discharge aperture; this is not readily possible in the embodiments according to
There is connection for conveying of material or for transfer of material between the intake aperture 80 and the aperture 8, and in the present case this connection to the aperture 8 is direct and immediate and involves no prolonged intervening section and no separation. All that is provided is a very short transfer region.
In the housing 16, there is a screw 6 with compressing effect, mounted rotatably around its longitudinal axis 15. The longitudinal axis 15 of the screw 6 and that of the extruder 5 coincide. The extruder 5 conveys the material in the direction of the arrow 17. The extruder 5 is a conventional extruder known per se in which the softened plastics material is compressed and thus melted, and the melt is then discharged at the opposite end, at the extruder head.
The mixing and/or comminution implements 3 or the blades 14 are at approximately the same level as the central longitudinal axis 15 of the extruder 5. The outermost ends of the blades 14 have adequate separation from the flights of the screw 6.
In the embodiment according to
The distance 18 is somewhat greater than the radius of the container 1. There is therefore a slight outward offset of the extruder 5, or the intake region is somewhat deeper.
The expressions “opposite”, “counter-” and “in an opposite sense” here mean any orientation of the vectors with respect to one another which is not acute-angled, as explained in detail below.
In other words, the scalar product of a direction vector 19 which is associated with the direction 12 of rotation and the orientation of which is tangential to the circle described by the outermost point of the mixing and/or comminution implement 3 or tangential to the plastics material passing the aperture 8, and which points in the direction 12 of rotation or movement of the mixing and/or comminution implements 3, and of a direction vector 17 which is associated with the direction of conveying of the extruder 5 and which proceeds in the direction of conveying parallel to the central longitudinal axis 15 is everywhere zero or negative, at each individual point of the aperture 8 or in the region radially immediately prior to the aperture 8, and is nowhere positive.
In the case of the intake aperture in
The angle α between the direction vector 17 for the direction of conveying and the direction vector for the direction 19 of rotation, measured at the point 20 that is associated with the aperture 8 and situated furthest upstream in relation to the direction 12 of rotation, or at the edge associated with the aperture 8 and situated furthest upstream, is approximately maximally about 170°.
As one continues to proceed downwards along the aperture 8 in
An angle β, not included in the drawing in
The apparatus according to
The distance 18 here corresponds to about 5 to 10% of the radius 11 of the container 1 and to about half of the internal diameter d of the housing 16. This embodiment therefore represents the second limiting case or extreme value with the smallest possible offset or distance 18, where the direction 12 of rotation or of movement of the mixing and/or comminution implements 3 is at least slightly opposite to the direction 17 of conveying of the extruder 5, and specifically across the entire area of the aperture 8.
The scalar product in
This also represents a decisive difference in relation to a purely radial arrangement, since there would be an angle α<90° at point 20 or at the edge 20′ in a fully radial arrangement of the extruder 5, and those regions of the aperture 8 situated, in the drawing, above the radial 11 or upstream thereof or on the inflow side thereof would have a positive scalar product. it would thus be possible for locally melted plastics product to accumulate in these regions.
In order, in all these container-extruder combinations, to achieve ideal conditions in respect of the residence times of the plastic for the precomminution process, the predrying process and the preheating process for the plastics material in the container 1, the diameter D of the container 1 has the following relationship to the external diameter d of the screw 6: D=10·3√{square root over (K·d2)}, where D is the internal diameter in millimetres of the container 1, d is the diameter in millimetres of the screw 6 and K is a constant, this constant being in the range from 60 to 180.
As mentioned in the introduction, the specific ratio between the internal diameter D of the container 1 and the diameter d of the screw 6 ensures that, with a relatively low average residence time of the material, product of adequately constant thermal and mechanical nature is always passed into the intake aperture 80 of the housing 16, even when the product to be processed is difficult for this type of processing, an example being foil residues of differing nature (thickness, size, etc.). The mixing or comminution implements 3 provide, by virtue of their particular direction of rotation relative to the direction of conveying of the screw 6, non-aggressive intake of the material into the extruder 5, and ensure that it is possible to achieve a homogeneous melt at high throughput.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 1504/2011 | Oct 2011 | AT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AT2012/050161 | 10/12/2012 | WO | 00 | 4/14/2014 |
