Patent Publication Number: US-2010117258-A1

Title: Apparatus and method

Description:
This invention relates to apparatus for and a method of producing particles for inclusion in a media bed. 
     Compound extrusion (or compound pelletizing) machines are widely used within the plastics industry to produce pellets of thermoplastics material suitable for subsequently forming into specific shapes in moulding machines. The feedstocks for these extrusion machines may be powders, or chopped material made of either newly produced plastics material or recycled plastics material derived either from plastics processing or from used plastics products. 
     A typical compound extrusion machine consists of an Archimedean screw along the inside of a heated barrel. The feedstock is introduced at the low pressure end of the barrel as a solid. The feedstock progressively melts and forms into a continuous stream as it is pumped along the barrel and the melting occurs owing to heating elements on the outside of the barrel. As the temperature rises the viscosity of the plastics falls, allowing very high pressure to be developed at the outlet end of the barrel. Passage along the screw impeller inside the barrel also intensively mixes the melted plastics which allows plastics with different properties to be blended to produce a material of uniform and consistent properties. A debris filter is provided at the end of the impeller if the source material is recycled plastics. It is also common practice to vary temperature profiles along the barrel to obtain optimum homogeneity of the particular plastics mix. 
     At the end of the barrel the plastics melt is forced through a plate with multiple apertures forming jets of molten plastics. This perforated plate is referred to as a die head and the apertures as die holes. 
     The jets of molten plastics issuing from the die head are cooled and then chopped by rotating blades to form solid pellets. The jets can be chopped immediately upon issuing from the die head into a cooling bath of water where they solidify to form the pellets. This is known as ‘face cutting extrusion’ and is followed by the pellets being separated from the cooling water and often dried before being collected into drums or sacks for subsequent processing into plastics products. 
     One use of this process is to change washed and chopped re-cycled or reprocessed plastics into small pellets which can then be used for further extrusion or injection moulding. The pellets vary in size and shape but normally range from 2 mm to 10 mm in diameter and length. 
     Alternatively, the molten jets are pulled through the air by rollers which cool and solidify the jets. The solid jets or strings are then chopped by rotating blades to form the pellets. This method produces more odour than the above-mentioned underwater face cutting method and is known as ‘drawn extrusion’. 
     A known product, “BIOBEAD”®B is produced by the compound extrusion method and used in media beds in wastewater treatment systems. The raw materials are high density polyethylene (HDPE) and other plastics, to produce solid pellets with roughened surfaces. The pellets produced also contain various impurities found in recycled plastics. The die holes in this case are plain 2.5 mm diameter holes but, owing to the shear and strain stresses of the melt when it leaves the die holes under pressure, the diameter of the plastics leaving the die head swells to 4.5 mm diameter during cooling. This is a finished product which is not re-processed into any other shape or form. 
     A further known product is the “BIOBEAD”®M, produced by taking the “BIOBEAD”®B product and re-extruding it to form hollow cylinders with cruciform internal webs. This shape requires the ‘drawn extrusion’ type of machine. The double processing (compound extrusion followed by drawn extrusion) of the plastics has been necessary as drawn extrusion machines cannot handle the mix of shredded raw re-cycled feedstock materials required for the “BIOBEAD”®M products. Drawn extruders are usually shorter and smaller barreled machines than compounding extruders and they do not usually include in-line filtration as the feedstock used has previously undergone filtration. 
     To extrude such hollow tube-type product requires a die mandrel to be positioned centrally of the die hole. This mandrel is fitted with a vent to release the vacuum formed as the plastics tube leaves the die head. Without this vent the extruded tube would collapse and lose its hollow feature. Owing to the release of pressure as the tube leaves the die head, the material, as already mentioned, expands on its exit from the die hole. Thus the tube is pulled out of the hole continuously at a higher speed than that it which it issues from the hole. This draws down the diameter of the tube and also secures the integrity of the cruciform web by compressing it. The extruded tube is then passed through a second profiling die to stabilise the tube diameter before entering a cooling bath which is at a slight pressure and which solidifies the plastics to set the diameter before the tube is cut to length. 
     Compared to compound extrusion the drawn extrusion process is comparatively slow. The dies are also extremely expensive when, for example, four-pronged mandrels per tube are required for cruciform webbed tube. Venting a four-pronged mandrel so as not to create a vacuum is technically challenging. When using a mixture of plastics with impurities and different melt temperatures, the extrudate often pits and collapses during cooling. This requires time-consuming re-starts of the process that are wasteful of material and processing costs. To prevent this, the feedstock for drawn extrusion is first produced by compound extrusion, thus requiring two processing procedures. 
     A further problem in producing the finished particles from a compound extruder is that of swell, especially when using recycled plastics. As previously explained, when the tubular extrudate leaves a conventional drawn extrusion die head it is drawn down in a second profiling die to prevent swell. Without this operation the extrudate swells in diameter and in doing so stretches the internal webs, which thin and can even snap, creating an unacceptable product. 
     According to a first aspect of the present invention, there is provided a method of production of tubes comprising, on a compound extruder, extruding molten plastics through die formations of a die head so as to extrude tubes each including at least one internal longitudinal web, and cooling the extruded tubes. 
     According to a second aspect of the present invention, there is provided compound extrusion apparatus comprising a die head including a die formations arranged to form tubes each including at least one internal longitudinal web; and a cooling device arranged to cool the extruded tubes. 
     Owing to these two aspects of the present invention, the finished tubes can be produced by a system having only a single extrusion step. 
     In this way, manufacturing costs of the tubes can be significantly reduced and production volume of the finished particles can be significantly increased. 
     The cooled tubes may be chopped into pieces to form hollow particles each substantially cylindrical and each including at least one internal longitudinal web. The particles so formed are particularly suitable for use as particles of a media bed. 
     According to a third aspect of the present invention, there is provided a method of production of a tube, comprising advancing molten plastics through a die hole of a die head of a die body, said die head including a die mandrel having a first portion attached to said die body and a second portion extending in said die hole so as to extrude said tube with at least one internal longitudinal web, and cooling the extruded tube, said advancing including conveying molten plastics along a flow passageway in said first portion of said die mandrel towards a central longitudinal region of said die hole at said second portion. 
     According to a fourth aspect of the present invention, there is provided extrusion die apparatus for use in production of a tube including at least one internal longitudinal web, said apparatus including a die body, a die hole in said die body, and a die mandrel having a first portion attached to said die body and a second portion extending in said die hole so as to extrude said tube, said first portion of said die mandrel including a flow passageway arranged to guide molten plastics to a central longitudinal region of said die hole at said second portion. 
     Owing to these two aspects of the present invention, it is possible to deliver enough molten plastics to the core of the mandrel to form at least a substantial proportion of at least one internal longitudinal web of a tubular product such that, upon cooling of the product, the web(s) do(es) not thin to a large degree or snap. 
     Advantageously, the first portion of each mandrel is readily detachable from the die body. 
     According to a fifth aspect of the present invention, there is provided extrusion die apparatus for use in production of tubes, said apparatus including an extruder barrel, an extruder screw for advancing molten plastics towards a die head, said die head comprising a die body having a die mandrel locating device arranged at the upstream side of said die body and for locating a die mandrel centrally of a die hole of said die body. 
     According to a sixth aspect of the present invention, there is provided a method of production of tubes, comprising, prior to fixing a die head relative to an extruder barrel, positioning by means of a die mandrel locating device, a die mandrel longitudinally centrally of a die hole in a die body of said die head. 
     Owing to these two aspects of the present invention, a die mandrel can be appropriately positioned relative to a die body. 
     According to a seventh aspect of the present invention, there is provided extrusion die apparatus for use in production of tubes, said apparatus including a die head having die formations through which molten plastics is advanced, and a cooling fluid delivering device for directing cooling fluid directly on to a downstream surface of said die head to cool the extruded tubes exiting from said die formations at said surface. 
     According to an eighth aspect of the present invention, there is provided a method of production of tubes, comprising advancing molten plastics through die formations in a die head and directing cooling fluid directly on to a downstream surface of said die head where said molten tubes exit from said die formations. 
     Owing to these two aspects of the invention, cooling of the extruded plastics tubes can occur immediately as the tubes exit from the die head. 
     In this way, the time required to dismantle the mandrels from the die head for cleaning and maintenance of the die head is greatly reduced. 
    
    
     
       In order that the invention may be clearly and completely disclosed, reference will now be made by way of example, to the accompanying drawings, in which:— 
         FIG. 1  shows a cross section through an annular die head of a known drawn extrusion machine; 
         FIG. 2  is an end and a side elevation of a known media particle to be included in a media bed; 
         FIG. 3  is views similar to  FIG. 2 , but of a faulty known media particle; 
         FIG. 4  is a top plan view of a die body of for a compound extruder according to an example of the invention and for forming particles of the form of  FIG. 2 ; 
         FIG. 5  shows a section along the line V-V in  FIG. 4 ; 
         FIG. 6  is a side elevation of a slotted die hole mandrel for the die body of  FIG. 4 ; 
         FIG. 7  is an axial section through the mandrel of  FIG. 6 ; 
         FIG. 8  is an underneath plan view of the mandrel of  FIG. 6 ; 
         FIG. 9  is a top plan view of a part of the die body of  FIG. 4  with the mandrel attached to the die body; 
         FIG. 10  shows a section along the line X-X in  FIG. 9 ; 
         FIG. 11  is a fragmentary, underneath plan view of that part of the die body with the slotted mandrel in the die body; and 
         FIG. 12  is a diagrammatic representation of a cooling system for the compound extruder. 
     
    
    
     Referring to  FIG. 1 , a conventional die head  2  for a drawn extrusion machine comprises a die body  4  for attaching to the extruder barrel outlet (the direction of flow of molten plastics being shown by the arrow A), a die mandrel  6  having a shape to form the finished product, and an air vent  8  to prevent formation of a vacuum as the extruded plastics tube  10  leaves the die head  2 . 
     Referring to  FIG. 2 , the known particle  12  is of a tubular nature and is shown as having an external diameter D and a length L and two internal diametral webs  14  which form a cruciform web structure. This construction of the particle  12  is an example of the “BIOBEAD”®M product, mentioned hereinabove. 
     The “BIOBEAD”®M product is produced on a drawn extrusion machine from a good quality source pellet (such as “BIOBEAD”®B), which requires, in order to overcome the problem of swell, the extruded plastics tube to be drawn down through a second profiling die in a water bath to aid in the setting of the tube diameter. The problem of the swell is such that, if no profiling die were present on the drawn extrusion machine, the tubular product would not cool quickly enough (especially internally) to prevent the plastics material from swelling to a larger diameter D and in doing so stretching the webs  14 , which may snap, creating an unacceptable product, as shown in  FIG. 3 . 
     Referring to  FIGS. 4 and 5 , a die body  104  for a compound extruder has a plurality of through bores  106  for receiving bolts or the like for attachment to a downward outlet from the barrel of the compound extruder, a single central bore  108  for receiving the spigot of the compound extruder, and a plurality of die holes  110  (fifteen die holes being shown in this example) which are equi-spaced circumferentially around a so-called plastics flow ring  112  (which is an annular recess). The plastics flow ring  112  is bounded by outer and inner edges  114  and  116  respectively and aids in the even distribution of the molten plastics to the die holes  110 . The face of the die body shown in  FIG. 4  is that which faces upwardly, the direction of flow of the molten plastics through the die head being shown by the arrow B in  FIG. 5 . 
     The die body  104  also includes laterally outwardly and inwardly extending recessed portions  118  in the edges  114  and  116  respectively. Two of these recessed portions  118  are located on respective opposite inner and outer sides of the die holes  110 . Each pair of recessed portions  118  forms die mandrel locating recesses and each recess is provided with a bore  120  for receiving an attaching device, such as a screw. In this way, die mandrels can easily be attached to the die body  104 . Such a die mandrel  122  is shown in  FIGS. 6 to 11  and has a generally T-shaped elevational profile. The upper horizontal section  124  is a fixing plate and the lower vertical section includes four elongate quarter-circle sectors  126  with a cruciform void thereamong (as shown in  FIG. 8 ). Respective opposite end portions in the form of locating lugs  128  of the fixing plate  124  are provided with through-bores  130 . The elongate sectors  126  are for inserting into the die hole  110  with the lugs  128  of the fixing plate  124  closely fitting into the recessed portions  118 . 
     In this way, the mandrel  122  can be secured accurately in the centre of the die hole  110  by screws where the through-bores  130  align with the bores  120  for receiving the screws, or by a compressive fit, when the bores  120  and  130  can be dispensed with. The width of the fixing plate  124  is kept to a minimum so as to ensure that sufficient flow of molten plastics passes the outside edges of the mandrel  122  via semi-circular inlet spaces  132  (see  FIG. 9 ) into the die hole  110 . The free ends of the elongate sectors  126 , when the mandrels  122  are fixed in position, are in the plane of the die body outlet face to ensure a clean cut by one or more rotating blades which chop the extruded plastics into the finished particles. 
     Each mandrel  122  also comprises a central feed passage  134  in the fixing plate  124 , which, when the mandrel  122  is fixed in place, is co-axial with the die hole  110 . The feed passage  134  forms a direct passageway for molten plastics to reach the voids between the elongate sectors  126  at the core of the mandrel, which plastics will form at least a significant proportion of the internal web structure of the finished particle similar to that shown in  FIG. 2 . In effect, the presence of the feed passage  134  creates a pressure of the molten plastics in the central void of the mandrel  122  greater than conventional, and delivers more molten plastics into the voids among the elongate sectors  126  than if the molten plastics were to enter the die hole  110  through only the inlet spaces  132 . Thus, upon cooling of the extruded plastics when it leaves the die head, when swell occurs, the excess material delivered into that central void results in a higher density of material which allows for stretching of the formed webs without thinning or snapping. 
     It may be necessary to enlarge the diameter of die holes  110  compared to those conventionally used on a compound extruder, in order to equip it with the mandrels  122 . The diameter of the die holes largely depends upon pressure, temperature and land length (i.e. thickness of the die head), but by judicial adjustment of these parameters the level of swell can be controlled sufficiently to produce an acceptable final product. The size of the feed passage  134  is determined by calculations based on operating conditions so as to improve the distribution of molten plastics as it enters the die head. The compound extrusion process can then readily be optimized by adjustment of:—
         flowrate of molten plastics along the extruder barrel   temperature profile along the barrel   filtration aperture size   pressure at the die head   size of the die head.   temperature in the die head   number of cutter blades at the die head   spacing of cutter blades to produce designed length of product   die head cutter speed   level of water cooling in a die head bath to solidify the product.       

     The required length of the mandrel  122  is related to the size of the central feed passage  134  and is accurately calculated. The mandrel length (and thereby also the land length) dictates the stress levels, which determine the extent of swell. 
     The dimensions of the fixing plate  124  of the mandrel  122  are precisely calculated as are the dimensions of the feed passage  134 , including the two locations along that passage where the cross-sectional area of the passage is reduced, in order to obtain the optimum flow of molten plastics into the voids among the elongate sectors  126 . The length of the elongate sectors is calculated to obtain optimum laminar flow along the voids. If the length of the elongate sectors  126  and the land length of the die head are too large, the flow of molten plastics along the voids experiences excessive friction against the external surfaces of the elongate sectors and the bounding surfaces of the die holes  110 , which has an effect on the degree of swell of the extruded plastics leaving the die head, resulting in an unacceptable final product. 
     The optimum flow of molten plastics is further promoted by the shape of the die hole  110 . As can be seen from  FIGS. 5 and 10 , the die hole is wider at its upper end. Working progressively down the die hole  110 , the bounding wall of the die hole is substantially circular cylindrical where the die hole has its largest diameter, then has a portion which is substantially frusto conical with the die hole progressively reducing in diameter, and a final portion which is substantially circular cylindrical and has the smallest diameter of the die hole. The semi-circular inlet spaces  132  (see  FIG. 9 ) are those gaps left at the opening of the largest diameter portion of the die hole  110  when the mandrel  122  is fixed in place in the die hole. Inwardly of the secured fixing plate  124 , the portion of the die hole having the largest diameter and that portion which progressively reduces in diameter form an annular void  136  about the part of the mandrel  122  where the elongate sectors  126  extend downwardly from the fixing plate  124 . The volume of the annular void  136  is closely related to the width of the fixing plate  124  to obtain the optimum flow of molten plastics through the semi-circular spaces  132  and into the portion of the die hole  110  of the smallest diameter in order to surround the elongate sectors  126  and subsequently form the tubular wall of the final product. 
     As the extruded plastics leaves the die head in its tubular form with the internal diametral webs it has a consistency similar to that of syrup so that it is necessary, as is conventional, to cool and then chop the tubular product to form the final particles. With conventional compound extruders, the exact form of the pellets produced is not that critical since they will later be re-extruded. However, for a desired finished tubular particle with internal diametral webs to be produced on a compound extruder, the form is critical to obtain an acceptable product. Therefore, the extruded plastics leaving the die head is cooled as rapidly as possible in order to control the swell and set the desired shape. A known system of cooling requires spraying water, which is recycled, at a temperature of approximately 60° C. to be circulated within a housing surrounding the die head of a compound extruder. A problem with use of this known system in the present compound extruder would be that, whilst the outside wall of the tubular part of the extruded product would set, the internal diametral webs of the extruded product would not be cooled rapidly enough to set the desired shape, so that continued expansion would produce an unacceptable product. Referring to  FIG. 12 , a cooling system for the present compound extruder comprises a shielding  200 , a cooling fluid spraying device  202  and a water bath  204 . The shielding  200  surrounds the downward outlet from the extruder barrel and its associated cutting blade  206 , and has an opening  208  where finished particles leave the compound extruder and fall into the cooling water bath  204 . The finished particles are subsequently separated and dried, as is conventional. The cooling fluid spraying device  202  sprays jets of cooling fluid, preferably cold water at less than approximately 15° C., directly on to the exposed surface of the die head. Thus, rapid cooling of both the external and internal surfaces of the extruded plastics leaving the die head occurs, setting the shape of an acceptable final product to be chopped into particles. 
     The preferred dimensions of an acceptable final particle are between substantially 8 mm and substantially 30 mm, and most preferably between substantially 12 mm and substantially 16 mm, in diameter, between substantially 1 mm and substantially 25 mm in length, with tubular wall thickness and a thickness of the internal diametral webs of between substantially 0.5 mm and substantially 2 mm, but most preferably being substantially 0.7 mm. 
     An advantage of the present arrangement is when the die head of the compound extruder needs cleaning, owing to the readily removable nature of the mandrels  122  from the die holes  110 , the amount of down-time, when the compound extruder is not running, is significantly reduced, therefore increasing production time. 
     The particles produced are particularly useful in systems for the treatment of wastewater. Advantageously, the particles, which have roughened, pitted surfaces, are used to support the growth of micro-organisms, in, for example, a Submerged Aerated Filter (SAF) system. 
     The cooling system illustrated in  FIG. 12  is usable in any appropriate extruder, but in particular in a compound extruder.