Abstract:
An apparatus is disclosed for converting feed material into granules under the application of pressure. The apparatus includes a conveyor screw arranged about an axis of rotation with at least one screw spiral, a rotationally symmetric working chamber arranged about the axis of rotation, which chamber is formed by two annular walls arranged at an axial distance from one another and by an annular perforated die. The annular wall has a central opening arranged coaxial to the axis of rotation into which the conveyor screw discharges in order to feed the working chamber with feed material. The apparatus also includes a pressure element that rotates about the axis of rotation and has at least one pressure arm, which extends from the axis of rotation toward the perforated die delivers the axially supplied feed material to the perforated die in the radial direction, and compacts it while doing so.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) to German Patent Application No. DE 102005037177, which was filed in Germany on Aug. 6, 2005, and which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus for converting free-flowing feed material into granules, agglomerates, pellets, compacts, and the like.  
         [0004]     2. Description of the Background Art  
         [0005]     In the manufacture of compacts, pellets, compounds, composites, agglomerates, or granules, the feed material on which the processing is based generally has a free-flowing consistency. By means of a screw press, the feed material is brought to the processing tools, where it is appropriately processed depending on its later use.  
         [0006]     In order to manufacture simple products, such as compacts or pellets for example, it is sufficient to compact the feed material, e.g. wood (sawdust, wood fibers, shavings, wood flour) and press it through a molding tool. The qualitative requirements on the resulting product are fairly low in this context. The manufacture of high-quality granules and agglomerates from feed material of a pure type, such as PE granules for example, entails greater construction effort. To this end, the dry feed material is delivered to an agglomerator or an extruder after being compacted.  
         [0007]     All the aforementioned processing methods start with heavy compaction of the feed material, which causes a considerable development of heat as a result of the high pressure and intense frictional forces. During the processing of thermoplastic materials, the development of heat can be influenced in such a manner that plasticizing of the feed material occurs, followed by the formation of granules and agglomerates, which then may be used as raw materials for feeding extruders, injection molding machines, presses, and the like in later production processes. Additives such as sawdust or wood fibers can also be added to the plastics to create wood/plastic composites (WPC).  
         [0008]     During the manufacturing process, it is critically important to observe an upper temperature limit. If the temperatures in the apparatus exceed a material-specific limit value, thermal decomposition and thus chemical conversion of the feed material occurs. Such a damaged granulate is usable only for further processing into low-quality products on account of the associated degradation of chemical and physical properties.  
         [0009]     An apparatus suitable for this described purpose is known from DE 32 10 974 A1. This apparatus has a screw press, having an annular chamber and/or a perforated plate arranged at an end opposite the intake. During the course of the axial transport of the feed material to the annular chamber or to the perforated plate, a continuous pressure buildup takes place in the screw press. To this end, the screw press can have a pitch of the screw spirals that is variable over its length, and/or a conicity. The feed material reaches its maximum pressure immediately before and during passage through the annular chamber or perforated plate, where a considerable development of heat is present as a result of the compressive and frictional forces present. In the case of thermoplastic materials, the high temperatures cause them to soften, thus making possible their passage through the annular chamber or perforated disk.  
         [0010]     Apparatuses are also known in which the annular chamber or perforated plate at the end of the screw press is replaced by two conical grating elements arranged coaxially one inside the other while maintaining a grating gap; the surfaces of the grating elements that face one another are equipped with grating strips. Due to a relative motion of the grating elements, the feed material forced into the grating gap by the screw press is exposed to high shear forces between the grating elements, and it agglomerates as a result of the associated frictional heat.  
         [0011]     The disadvantage of these apparatuses resides in the fact that the feed material is exposed to high compressive and shear forces as early as in the screw, so that heating of the feed material occurs at a very early point in time. Due to the accumulation of heat energy as the feed material passes through the apparatus, the maximum temperature is often exceeded before the annular chamber, pressure plate, or grating elements are reached, with the above-described disadvantages. This problem is generally addressed by the integration of cooling devices. Nevertheless, regulation of the temperature within the apparatus is very difficult, so the temperature profile exhibits a wide range of variation, and consequently only granulate of nonuniform shape and quality are produced.  
         [0012]     In order to solve these problems, DE 38 42 072 C1, which corresponds to, U.S. Pat. No. 5,009,586, discloses an agglomerator having a conveyor screw with material intake that guides the thermoplastic feed material axially into a disk-shaped working chamber whose outer periphery constitutes an annular compaction chamber. The outer periphery of the compaction chamber is bounded by a perforated die through which the feed material is radially pressed by a rotating pressure vane after sufficient compaction and heating. Due to the sickle-shaped design of the pressure vane, a compression zone is formed that narrows opposite the direction of rotation, into which the feed material is drawn as the pressure vane rotates and is subsequently exposed to increasing compressive forces. At the same time, the forced mixing of the feed material by the rotating pressure vane leads to high shear and frictional forces, which on the whole results in a rapid autogenous development of heat in the compaction chamber and compression zone.  
         [0013]     The temperature rise is accompanied by a softening of the thermoplastic feed material, which in turn reduces its adhesion resistance during passage through the perforated die. Thus, when this state is achieved, no further increase in compressive and shear forces is possible, since the feed material counteracts these tendencies as a result of the associated temperature increase through a reduction in viscosity. An equilibrium state is thus established in which the temperature remains below the upper temperature limit of the feed material in question.  
         [0014]     The advantage of this apparatus thus includes an automatic regulation of the temperature in the apparatus, so that damage to the feed material resulting from thermal overstress is precluded.  
         [0015]     In practice, such apparatuses have proven their worth due to the constant high quality of the granules produced. However, a certain range of variation among the machine outputs of various different apparatuses is notable despite the use of identical components.  
       SUMMARY OF THE INVENTION  
       [0016]     It is therefore an object of the present invention to provide apparatuses of the generic type such that maximum machine output is achieved with consistently high quality of the final product.  
         [0017]     The invention is characterized by the compaction of the feed material at as late a point in time as possible within the conversion process. In this context, the conveyor screw performs the task of axial material transport to the pressure element without producing an appreciable pressure buildup. Steadily increasing compaction does not take place until after redirection of the stream of material in the working chamber to a radial direction. Thus, it is possible for the feed material to experience a constant increase in temperature, yet not reach its maximum temperature until passage through the openings of the perforated die. In this way, the thermal stress of the feed material is reduced to the necessary degree.  
         [0018]     These processes are greatly influenced by the material flow within the inventive apparatus and, in particular, in the area of the redirection toward the perforated die. The inventive design of this area achieves the result that, with the greatest possible flow cross-section of the feed material, an abrupt increase in volume occurs once the feed material reaches the annular compaction chamber following its redirection. As a result of the sudden increase in the flow cross-section by the cross-sectional area of the annular compaction chamber, a material vacuum arises in the direction of the perforated die, enhancing the radially outward material flow of the feed material. In this context, the large flow cross-section prior to entry into the compaction chamber permits a largely unimpeded afterflow of the feed material, minimizing pressure losses and material blockages.  
         [0019]     In the compaction chamber, a continuously ongoing compaction of the feed material takes place through volume reduction. At the same time, the feed material in the compaction chamber is subjected to a uniform, laminar circulating motion until passage through the perforated die. These conditions contribute, firstly, to the circumstance that the temperature profile of the feed material rises continuously as it travels through the apparatus, and reaches its maximum in the vicinity of the perforated die. Secondly, a constant and uniform material flow makes it easier to maintain optimal process parameters, thus permitting production with uniform high quality at maximum machine output and low energy consumption.  
         [0020]     In an embodiment of the invention, a relative position of the pressure element to the conveyor screw is such that the end of the screw spiral coincides with the intersection of the contact surface of the pressure arm and the central opening of the first annular wall. With this embodiment, a maximum flow cross-section is achieved in the vicinity of the redirection of the material flow, followed by an approximately uniform increase in compaction of the feed material.  
         [0021]     The greater the deviation from this optimal relative position by an angular offset toward or away from the direction of rotation, the greater the disturbances in the material flow become, since one deviates ever further from an essentially constant material flow for the feed material up to its entry into the working chamber. This is tolerable to a certain degree depending on the type of feed material. The limit is reached, however, when the inventive advantages no longer arise. This is the case when the material flow is significantly disrupted by clogging and blockages resulting from cross-sectional narrowing. In this sense, the values stated in the patent claims are to be understood merely as limit values, and individual values lying within the limit values, for example those shown in  FIG. 2 , are also to be considered as disclosed.  
         [0022]     A preferred embodiment of the invention provides that the front of the pressure arm serving as the contact surface is curved in the opposite direction of rotation. In this way, a progressively increasing compaction of the feed material in the direction of the perforated die is achieved with the advantage that the compressive forces exerted by the pressure arm are oriented essentially parallel to the passages. This minimizes pressure losses and prevents excessive generation of heat.  
         [0023]     It also proves to be advantageous when the section of the front in the region ahead of the intersection with the central opening runs in a straight line. This results in a rapid conveying of the feed material into the annular compaction chamber.  
         [0024]     The end face of the pressure element facing the conveyor screw can have depressions in the region of the screw spiral, so that the flow cross-section of the feed material, which, in the final section before its redirection, is formed by the pressure element and the screw spirals, is as large as possible.  
         [0025]     A preferred embodiment of the invention provides pressure elements with two pressure arms, each of which is associated with its own screw spiral. The result is an extremely compact and effective compression and compaction unit in which the regions for redirection and the regions for compaction of the feed material stand in a balanced relationship to one another.  
         [0026]     Another embodiment of the invention has a pressure arm in which the rear side of the pressure arm in the direction of rotation is offset by approximately 90° opposite the direction of rotation from the radial line through the point S. This achieves the result that the feed material arriving axially from the conveyor screw does not immediately encounter the end wall of the disk-shaped working chamber, but instead strikes the pressure element. As a result, the pressure arm protects the relevant end wall from excessive wear. For this purpose, the end face of the pressure element can have a depression in this area.  
         [0027]     Since the relative position between pressure element and conveyor screw of an inventive apparatus is unchangeable, it proves to be advantageous to arrange the conveyor screw and pressure element on a common drive shaft. In this way, the synchronism of the conveyor screw and pressure element is ensured, and only one drive unit is needed to operate an inventive apparatus.  
         [0028]     In a further embodiment of the invention, the pressure element and at least the last section of the conveyor screw associated with the pressure element are designed as a single piece. This ensures that, during assembly of the apparatus, the relative position of pressure element to conveyor screw is observed.  
         [0029]     Alternatively, a two-piece design is possible, in which the relative position of pressure element to conveyor screw is ensured, for example, by centering pins.  
         [0030]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0032]      FIG. 1  is a longitudinal section through an inventive device;  
         [0033]      FIG. 2  is a cross-section through a subsection of the apparatus along line II-II shown in  FIG. 4 ;  
         [0034]      FIG. 3  is a cross-section through a subsection of the apparatus along line III-III shown in  FIG. 4 ;  
         [0035]      FIG. 4  is a longitudinal section through a subsection of the apparatus shown in  FIG. 2  along line IV-IV; and  
         [0036]      FIG. 5  shows a profile of the flow cross-section of the feed material during passage through the apparatus. 
     
    
     DETAILED DESCRIPTION  
       [0037]      FIG. 1  shows a longitudinal section through an apparatus according to an embodiment of the present invention. Shown is an approximately drum-shaped housing  2  surrounding an axis of rotation  1 , whose end faces are closed off by a front wall  3  and back wall  4 . The housing  2  is subdivided by a partition  5  arranged between the front wall  3  and a back wall  4 . The front wall  3  has a central opening, which can be sealed by a pivoting housing door  6 . The back wall  4  and the partition  5  in the vicinity of the axis of rotation  1  together form a horizontal bearing region to accommodate and pass a drive shaft  8 . The end of the drive shaft  8  located outside the housing  2  bears a multiple groove pulley  9 , which is connected by belts (not shown) to a drive.  
         [0038]     On the end of the drive shaft  8  inside the housing  2 , a hub  10  sits in a rotationally fixed manner; extending from the hub are diametrically opposed sickle-shaped pressure arms  11  ( FIG. 2  and  3 ). The hub  10  and pressure arms  11  constitute the pressure element  12 , which is set in rotational motion about the axis  1  by the drive shaft  8 .  
         [0039]     Extending from the partition  5  and rigidly attached thereto is a cylinder bearing  13  that coaxially encloses the drive shaft  8  and bears a flange  14  arranged in a radial plane at its end facing the pressure element  12 . A likewise coaxial annular wall  15  adjoins the flange  14  in the axial direction toward the pressure element  12 . The flange  14  and annular wall  15  together constitute a sliding connection to the hub  10 .  
         [0040]     The housing door  6  has, in the region of the axis of rotation  1 , an annular opening which is adjoined by the outer sleeve  16  of a likewise coaxially arranged conveyor screw  17  with a vertical material intake  18 . The conveyor screw  17  can thus be pivoted to the side during the course of opening the housing door. Visible on the inside of the housing door  6  is a second annular wall  19 , which is spaced axially apart from and faces the annular wall  15 , and which has a concentric opening  40 . The two annular walls  15  and  19  thus constitute a coaxial, disk-shaped working chamber  20 , which is bounded on the circumferential side by a perforated die  21  with passages  22 , and in which the pressure element  12  rotates. The larger diameter of the opening  40  as compared to the outside diameter of the hub  10  produces an annular gap  23  through which the working chamber  20  is fed with feed material.  
         [0041]     The cylinder bearing  13  constitutes a horizontal pivot bearing for a blade holder hub  24 , which coaxially surrounds it, and from which two diametrically opposed blade holders  25  extend in the radial direction, and which serve for adjustable and replaceable fastening of cutters  26  on the outer circumference of the perforated die  21 . By means of the V-belts (not shown), the blade holder hub can be set in rotation, causing cutters  26  to pass over the outer circumference of the perforated die  21 . The perforated die  21  is surrounded in turn by an annular channel  30 , in which the granules are collected after passing through the perforated die  21  and are drawn out of the apparatus through a material outlet  32  located on top.  
         [0042]     The conveyor screw  17  is designed as a double lead screw. The screw core  27  corresponds to the axial extension of the drive shaft  8 , and is rotatably mounted in a bearing  28  in the rear end of the housing of the conveyor screw  17 . In the plane of contact, the screw core  27  and the drive shaft  8  are detachably connected to one another in a force-transmitting manner by centering pins  31 . Seated on the screw core  27  are the two screw spirals  29 , which wrap around the screw core  27  with a uniform pitch, thus conveying the feed material through the annular gap  23  into the working chamber  20  by means of its contact surface  43 . The contact surfaces  43  of the two screw spirals  29  terminate in the radial plane between annular wall  19  and pressure element  12 , where they taper off.  
         [0043]     The flow of material through the apparatus is indicated by the arrows  33 . Feeding takes place through the material intake  18  in the conveyor screw  17 , which also conveys the feed material axially to the working chamber  20 . After the material enters the working chamber  20 , a redirection radially to the outside, toward the perforated die  21 , takes place. After radial passage through the perforated die  21  and comminution by means of the cutters  26 , the granulate first reaches the annular channel  30 , whence it is carried out of the apparatus through the material outlet  32 . The section after entry into the working chamber  20 , where redirection of the material flow with subsequent compaction takes place, proves to be a critical area on this path.  
         [0044]      FIGS. 2, 3  and  4  show this area in different segments, where the relative position of the screw spirals  29  and pressure element  12  is particularly evident.  FIG. 3  shows a cross-section facing in the direction of material flow, while  FIG. 2  shows the opposite direction of view.  
         [0045]     Visible on a larger scale are the perforated die  21  with radial passages  22 , which coaxially surrounds the axis of rotation  1  and in doing so encloses the disk-shaped working chamber  20 , which is bounded in the axial direction by the annular walls  15  and  19 . The annular walls  15  and  19  may be equipped with cooling channels  34  and  35  in order to influence temperature development within the apparatus. On the feed side, the outer sleeve  16  seals the conveyor screw  17  to the central opening  40  of the annular wall  19 . Inside the outer sleeve  16 , the conveyor screw  17  rotates the screw core  27  with the screw spirals  29  spiraling around it.  
         [0046]     The pressure element  12  is seated in a rotationally fixed manner with its hub  10  on the last section of the drive shaft  8 . Extending radially from the hub  10  are two blade-like pressure arms  11 , which are located opposite one another with radial symmetry relative to the axis of rotation  1 , and which occupy approximately half of the working chamber  20 . The other half is divided into a compaction chamber  38  between the opening  40  and perforated die  21 , and a redirection region  41  adjoining the hub  10  in the same radial plane, in which region the screw spirals  29  of the conveyor screw  17  terminate at their contact surfaces  43 . A circulating motion of the compaction chamber  38  and redirection region  41  results during the course of the rotation of the pressure element  12 .  
         [0047]     The pressure arms  11  of the pressure element  12  have a front side  37  which is effective for compaction in the direction of rotation  36 , and extends in a straight line in the redirection region  41 . In contrast, the outwardly adjacent region of the front. side  37  located in the compaction region  38  has a curvature opposite the direction of rotation  36 . In this way, the pressure arms  11  in the region of the compression chamber  38  form two wedge-shaped compression zones  39 , shown with cross-hatching in  FIG. 2 , which revolve during the course of the rotation.  
         [0048]     In an axial projection, the opening  40  and the front side  37  of each pressure arm  11  intersect at point S ( FIGS. 2 and 3 ). The radial line originating at the axis of rotation  1  and extending through the point S is labeled  42 . Extending from the radial line  42 , opposite the direction of rotation  36 , is the compression zone  39 , which, with an angular dimension of −90° ends where front side  37  of each pressure  11  forms a sliding termination to the perforated die  21 . The region adjoining the radial line  42  in the direction of rotation  36  is composed jointly of the redirection region  41  and the compaction chamber  38 , and ends where the pressure blade  11  has its rear edge in the direction of rotation  36 . In the present example, that is the case at an angular dimension of approximately +80° originating from the radial line  42 . In the adjacent region to the annular gap  23 , moreover, each pressure arm  11  can have depressions  44  on the face surface facing the conveyor screw  17 , in order to make the flow cross-section as large as possible.  
         [0049]     In addition, each point S represents the end of the contact surfaces  43  of the screw spirals  29 . In this regard, the pitch of the screw spirals  29  in the end region can be chosen such that a tangential surface transition of the contact surfaces  43  to the annular wall  19 , and thus to the compaction chamber  38 , takes place.  
         [0050]     Below, the flow of the feed material through an inventive device is explained in detail with additional reference to  FIG. 5 . First, a flow cross-section is available to the feed material located in the conveyor screw  17  that is limited by the axial spacing of two adjacent screw spirals  29  and by the radial distance between the outer sleeve  16  and the screw core  27  of the conveyor screw  17 . This flow cross-section remains essentially constant over the length of the conveyor screw  17 , as the profile of the curve shown in section A-B in  FIG. 5  also shows.  
         [0051]     At the end of the conveyor screw  17 , the feed material is conveyed by the screw spirals  29  through the annular gap  23  into the working chamber  20 . In this regard, at the same time the feed material enters the redirection region  41  of the working chamber  20 , an abrupt increase in the flow cross-section takes place by the cross-sectional area of the compaction chamber  38 . This situation corresponds to a section in  FIG. 2  at the radial line +80° and in  FIG. 5  to the curve section B-C. The greatest flow cross-section is achieved when the flow cross-section is increased by the cross-sectional area of the compaction chamber  38 . However, at this point in time the feed material is not yet distributed uniformly over the entire flow cross-section, but instead is concentrated in the redirection region  41 . By way of contrast, a material vacuum exists in the compaction chamber  38 . These circumstances contribute to a change in the feed material direction from an axial direction of motion to a radial direction.  
         [0052]     At the same time as the maximum flow cross-section is reached, a continuous compaction of the feed material is started by reducing the flow cross-section. This is accomplished, firstly, by the continuous pitch of the screw spirals  29  to their terminations at the points S at the level of the plane of separation between the annular wall  19  and the pressure arms  11 . Secondly, the flow cross-section is additionally reduced by the profile of the contact surface  37 , which continually reduces the width of the redirection region  41  until the redirection region  41  has completely disappeared at the point S. In this way, the existing flow cross-section is steadily reduced to a cross-sectional area that, at the plane of the radial line  42 , is now only made available in the compaction chamber  38 . The compaction of the feed material up to this point corresponds to the curve segment C-S in  FIG. 5 , or the region between the radial lines of +80° to 0° in  FIG. 2 .  
         [0053]     The flow cross-section is continuously reduced further by persistent rotation of the pressure element  12  as a result of the curved contact surface  37 , until the flow cross-section assumes the value of zero in the region of the radial line −90°, where the pressure arm  11  forms a sliding connection to the perforated die  21 . The pressure and temperature increase occurring during the course of the continuing reduction of the flow cross-section causes the feed material to pass through the openings  22  of the perforated die  21 , during which process the feed material briefly transitions to a plastic state and agglomerates. The agglomerate is subsequently stripped off the outside of the perforated die  21  by the revolving blades  26 .  
         [0054]      FIG. 5  clearly shows the sudden increase of the flow cross-section when the feed material enters the redirection region  41 , by which means a redirection of the material flow is supported. This is followed by a continuous reduction in the flow cross-section in two stages with simultaneous temperature and pressure buildup in the working chamber  20 . The first segment concerns the region until the plane of intersection through the radial line  42  is reached, which terminates the axial conveyance of the feed material by the screw spirals  29 . The second region corresponds to the segment between the planes of intersection through the radial lines at 0° and 90°, where compaction only takes place between the perforated die  22  and the contact surface  37  of the pressure element  12 .  
         [0055]     In the present example, the curve profile shows a point of discontinuity in the transition region S between these two regions. The invention is not limited to such a profile, but rather also includes embodiments with a constant reduction in flow cross-section.  
         [0056]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.