Abstract:
A blender for delivery of blend additives to a plastics extrusion device has a mixing chamber for reception of materials to be blended. Batches of blend materials are delivered to the mixing chamber from material supply bins via a weight hopper. Within the mixing chamber, a mixing screw circulates and mixes the blend materials prior to discharge through an outlet to an extrusion process line. As material is discharged, a material level sensor is uncovered and calls for a further batch of blend materials to be discharged from the weigh hopper into the mixing chamber. The screw has a pair of mixing blades of opposed pitch to direct material inwardly from outer ends of the screw towards a center of the screw located above the outlet. Excess material above process demand requirements is delivered to the outlet by the screw. This excess material is directed upwardly away from the outlet and recirculated towards outer ends of the screw and remixed without material within the mixing chamber.

Description:
FIELD OF THE INVENTION 
     This invention relates to a blending apparatus and method. 
     The invention particularly relates to a batch blending apparatus for controlled mixing and delivery of particulate blend additives into a material processing device, such as a plastics extruding device, or, in processes requiring blending of material. 
     BACKGROUND OF THE INVENTION 
     It is known to provide a blender having a mixing chamber for reception of two or more materials to be blended. A mixing paddle is mounted within the mixing chamber to stir the materials within the mixing chamber for blending the materials within the mixing chamber prior to discharge to a process. A disadvantage with these types of blender is that the mixing tends to be fairly coarse and vigorous within the mixing chamber and thus it can be difficult to achieve an accurate blending of the material additives with the process materials. Also, in these blenders, because of their constructions, there may be dead areas in the mixing chamber where unblended or only partially blended materials collect. Further, these blenders are sensitive to the density and particle size of the material to be blended. It is also difficult to process relatively small quantities of material and achieve the required blend. Thus, these blenders are somewhat limited in the range and quantities of materials that can be successfully blended. 
     The present invention is directed towards overcoming these problems. 
     SUMMMARY OF THE INVENTION 
     According to the invention, there is provided a blender for mixing particulate materials prior to discharge into a process line, the blender comprising a mixing chamber, the mixing chamber having an inlet for reception of additive materials to be mixed and a blended material outlet, mixing means for mixing additive materials within the mixing chamber, delivery means for delivery of blended material to the outlet at a rate greater than the rate of discharge of blended materials through the outlet, and means to direct excess blended material away from the outlet for remixing within the mixing chamber. 
     In one embodiment, the mixing means is operable to generate and maintain a substantially level top surface in the additive materials within the mixing chamber above the outlet. 
     In a preferred embodiment, the mixing means comprises a mixing screw rotatably mounted within the chamber, the mixing screw operable to mix two or more materials delivered into the chamber, prior to discharge of the blended materials through the outlet. 
     Advantageously the use of a mixing screw facilitates good and even blending of the material additives in a controlled manner which minimises agitation of the materials which facilitates more accurate monitoring and control of additive material throughput to the process. 
     In one embodiment of the invention, the mixing screw has a rotatable shaft with a pair of mixing blades of opposite pitch extending outwardly from a central portion of the shaft which is located adjacent the outlet. Thus, conveniently the mixing screw is operable to direct material towards the outlet of the mixing chamber, while at the same time circulating the material within the mixing chamber for blending of the additive material. 
     Preferably the mixing screw is demountably secured within the mixing chamber. 
     Conveniently the mixing screw is movable axially through an opening in a side wall of the mixing chamber for removal. This facilitates easy cleaning of the screw and the mixing chamber as desired. 
     In a further embodiment there is provided a blender comprising a mixing chamber for reception of two or more materials to be blended, the mixing chamber having a material inlet and a blended material outlet, a mixing screw mounted within the mixing chamber adjacent the outlet for circulating and mixing of materials within the mixing chamber and material level sensor mounted within the chamber, said level sensor connected to a controller which is operable to regulate the supply of fresh materials to the blending chamber for mixing. 
     Preferably the blender further comprises a weigh hopper having a material inlet and a material outlet, the outlet arranged to discharge material from the weigh hopper to the mixing chamber. 
     In another embodiment a discharge gate on the weigh hopper outlet is operatively connected to the controller. 
     In a further embodiment the blender further comprises raw material supply bins, each bin having a discharge valve mounted at an outlet of the material supply bin, each of said discharge valves being operatively connected to the controller to regulate discharge of material from the bin to the weigh hopper or the mixing chamber. 
     Preferably, each discharge valve comprises a ram operated plate slidably mounted on a complementary track, the plate engagable with a downwardly depending discharge collar at the outlet to close the outlet. 
     Conveniently a vacuum loader is provided associated with each material supply bin, the vacuum loader operable to draw material from a material storage reservoir into the bin, said vacuum loader having a vacuum pipe connected to the bin with a filter mounted across said vacuum pipe, cleaning means being provided to clean the filter. 
     Preferably the cleaning means comprises an air jet operable to automatically back-flush the filter after each fill cycle. 
     In another embodiment, an auxiliary supply bin is provided with a discharge screw at an outlet of the auxiliary bin for delivery of material to the mixing chamber, said screw rotatably mounted within a complementary barrel, a non-return flap valve being mounted at an outlet end of the barrel. 
     In a further embodiment, an access door is provided on the mixing chamber, said access door operably engagable with switches for isolating electric and pneumatic power supply to the blender when the door is open. 
     In another embodiment, a complementary pair of capacitor plates are mounted in each material supply bin, said plates being connected to a controller operable to determine the level of material in the bin. 
     In another embodiment, the blender incorporates a material bulk density measurement device. 
     Preferably, the bulk density measurement device comprises a chamber which is filled with material during operation of the blender, an associated pair of capacitor plates being mounted within the chamber and connected to a controller operable for determining the bulk density of material between the plates. 
     In another aspect, the invention provides a level measurement device for measuring the level of material in a bin or container, the device comprising a complementary pair of capacitor plates mounted within the bin, said plates being connected to a controller operable to determine the level of material in the bin. 
     In a still further aspect, the invention provides a bulk density measurement device comprising a chamber which is filled with material, the bulk density of which is to be measured, an associated pair of capacitor plates mounted within the chamber and connected to a controller which is operable for determining the bulk density of material between the plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which: 
     FIG. 1 is an exploded, partially cut-away perspective view of a blender according to the invention; 
     FIG. 2 is an elevational view of a material mixing screw shaft forming portion of the blender; 
     FIG. 3 is an end elevational view of the blender; 
     FIG. 4 is a front elevational view of the blender; 
     FIG. 5 is a plan view of the blender; and 
     FIG. 6 is a detail perspective view of a slide valve forming portion of the blender; 
     FIG. 7 is a perspective view of a vacuum chamber forming portion of the blender; 
     FIG. 8 is an underneath plan view of the vacuum chamber; 
     FIG. 9 is an underneath perspective view of the vacuum chamber with a filter in place; 
     FIG. 10 is an elevational view of another blender; and 
     FIG. 11 is a detail view of portion of the blender of FIG. 10; 
     FIG. 12 is a sectional elevational view of the blender of FIG. 10 in use in an off-line application; 
     FIG. 13 is a schematic illustration of the blender in another off-line application; and 
     FIG. 14 is a schematic illustration of another blender incorporating a diverter for cleaning. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and initially to FIGS. 1 to  9  thereof, there is illustrated a blender according to the invention indicated generally by the reference numeral  1 . The blender  1  has a mixing chamber  2  for reception of two or more materials to be blended. A mixing screw  3  is mounted within the mixing chamber  2  adjacent an outlet  4  of the mixing chamber  2  for circulation and mixing of materials within the mixing chamber  2  prior to discharge through the outlet  4 . A material level sensor  5  is mounted within the chamber  2  above the screw  3 . The level sensor  5  is connected to a controller (not shown) which is operable to regulate the supply of fresh materials to the mixing chamber  2  for mixing. In addition, the sensor  5  allows the calculation of the rate of use of material. 
     The mixing chamber  2  has an upper portion comprising upright side walls  10 ,  11  and associated front wall formed by an access door  12  and rear wall  13  extending between the side walls  10 ,  11 . A lower portion of the mixing chamber  2  tapers inwardly and downwardly towards the material outlet  4  which is centrally located in a bottom of the mixing chamber  2  which essentially forms a trough  15  for complementary reception of the screw  3 . Thus, because of the construction of the mixing chamber  2 , all material is delivered into the trough  15  and then directed by the screw  3  to the outlet  4 , There are no dead areas where material can collect. 
     The sensor  5  is mounted on a support  16  which has a tapered top to facilitate free flow of material downwardly within the bottom  15  of the chamber  2 . This facilitates maintaining an even, substantially level top surface generated by the action of the screw  3  in the material as it moves downwardly through the chamber  2  as it is discharged through the outlet  4 . 
     The screw  3  has a central shaft  20  with a pair of mixing blades  21 ,  22  of opposite pitch extending outwardly from a central portion  23  of the shaft  20  which is located directly above the outlet  4 . It will be noted that an inner end  25  of the screw projects through the side wall  11  for engagement with an associated drive motor  26 . The screw  3  is movable through an opening in the opposite side wall  10  for removal of the screw  3  from the chamber  2  for cleaning the screw and/or chamber  2 . A motor engaging end of the screw  3  has a quick release coupling for engaging with and release from the motor  26 . An opposite free end of the screw  3  has a mounting flange which is engagable with quick release clips on the side wall  10  to retain the screw  3  in position and facilitate rapid removal as required. 
     Mounted directly above the chamber  2  is a weigh hopper  30  which is suspended by a load cell hanging mechanism  31  of the type described in our previous Irish Patent Application No. 95 0706. The weigh hopper  30  has an open top and a discharge outlet  32  closed by a ram operated flap. 
     Mounted directly above the weigh hopper  30  on a support platform  35  are a number of raw material supply bins  36 . A lower end of each bin  36  has an outlet closed by a discharge valve  37 . Each of the discharge valves  37  are operatively connected to the controller to regulate the discharge of material from the bins  36  into the weigh hopper  30 . A discharge valve  37  is shown in more detail in FIG.  6  and comprises a ram operated plate  40  slidably mounted on complementary tracks  41 . The plate  40  is engagable with a downwardly depending discharge collar  42  at the outlet of a bin  36 . An upper face of the plate  40  engages against a rim of the collar  42  to securely close off the outlet from the bin  36 . The tracks may be made of any suitable material such as plastics material such as an acetyl material for standard use or a polycarbonate material for high temperature use. 
     A vacuum loader  48  associated with each material supply bin  36 . The vacuum loader  48  essentially comprises a vacuum chamber  49  mounted on top of each bin  36  and communicating by a pipe  50  with a vacuum pump to apply a vacuum to each bin  36  when a vacuum control valve in each vacuum pipe  50  is opened. A filter  51  is mounted across a bottom of the chamber  49  to prevent dust and fine materials being drawn to the vacuum pump. Mounted within the vacuum chamber  49  is a pneumatically operated ram  52  for operating a valve  53  which is engagable with an end of the vacuum pipe  50 . A shuttle valve  55  controls air supply to the ram  52  for opening and closing the valve  53 . The shuttle valve  55  also controls air supply to a pressurised air reservoir  56 . In use, to fill a bin  36  the ram  52  is operated to open the valve  53  and apply a vacuum to an interior of the bin  36 . This vacuum then causes material to be sucked in from a remote supply reservoir into the bin  36  through an inlet pipe  58 . A level sensor mounted within the bin  36  detects when material is required, and operates the vacuum filing system to fill the bin  36  up to the level sensor and continues filling for a preset period to fill the bin  36  to a desired level and then switches off the vacuum by operating the ram  52  to close the valve  53 . Alternatively, high and low level sensors could be used. Then air is vented back from the cylinder  56  through an inlet nozzle  59  at a top of the chamber  49  to back flush the filter  51  blowing down through the filter  51  into the bin  36  cleaning the filter  51 . 
     The front access door  12  is preferably of clear plastics material and is hingedly mounted on a side wall  10 , 11  of the mixing chamber  2  to provide an access door to the mixing chamber  2 . This access door  12  is operably engagable with associated switches (not shown) for cutting out electric and pneumatic power supply to the blender  1  when the door  12  is opened to prevent injury or damage when cleaning or maintaining the blender  1 . In addition to isolating the pneumatic power supply, the rams for the discharge valves  37  are vented so that they can be manually operated for cleaning. 
     In use, the screw  3  rotates and urges material in the chamber  2  inwardly towards the outlet  4 . Some mixed material will be discharged through the outlet  4  on demand to the process. Other materials will be recirculated within the chamber  2  for mixing. Preferably, the screw  3  speed is such that the material is circulated and mixed several times before being discharged through the outlet  4 . The weight hopper is charged with additive material. As the level of additive material within the chamber  2  drops and the sensor  5  is uncovered, the controller is operable to dump a batch of materials from the weigh hopper  30  into the mixing chamber  2 . Then materials from selected supply bins  36  are sequentially weighed into the weigh hopper  30  for subsequent discharge on demand into the chamber  2  when the level drops again for mixing within the chamber  2  prior to delivery to the process. 
     It will be appreciated that the design of the blender allows for easy removal and servicing of the mixing screw which also allows for ease of cleaning of the mixing chamber. 
     It will also be noted that the mixing chamber and mixing screw design also allow for the feeding of other materials directly into the mixing chamber without passing through the weigh hopper. 
     If required, means may be provided for injecting liquid additives at the outlet or into the mixing chamber. 
     Furthermore, a modification in the mixing screw design detail at a point directly above the feed to the process allows the feeder to feed materials that might not flow naturally into the process—by “cramming” such materials via the flights on a modified screw or other means. Examples of such materials are low density re-processed film (fluff), etc. A particularly useful device for this is a helically wound wire screw rotatably mounted on a side wall of the mixing chamber  2 . 
     The controller regulates the amount of material filled from each material component and sequentially fills the required ratio of material from each material supply bin into the weigh hopper to achieve the blend required. The material level sensor within the mixing chamber determines when more material is required. The contents of the weigh hopper are then dumped into the mixing chamber. 
     The level sensor within the mixing chamber detects when the last batch of material has been used by the process which then enables the controller to calculate the usage rate in lbs/hour or Kg/hour and control same to keep the process output constant. 
     Furthermore, by monitoring the speed of the process (e.g. process screw speed and/or line speed) the controller can calculate the yield or weight per unit length and can also control same automatically by varying the speed set point for the screw drive. 
     It will be appreciated that the design of the mixing and feeding screw provides compact but positive mixing of the batch fed material without risk of separation of different materials. 
     Furthermore, it provides a stable material level which allows accurate and consistent level sensing which in turn determines the control accuracy and response time. 
     Referring now to FIGS. 10 to  12  there is illustrated another blender  70 . The blender  70  is largely the same as the blender previously described and like parts are assigned the same reference numerals. This case auxiliary supply bins  71  are mounted at each side of the mixing chamber  2 . Each bin  71  has a discharge screw  72  at an outlet of the bin  71  for discharging material into the weigh hopper  30 . When discharging material from the bin  71  the screw is rotated by an associated motor for a set period to deliver a desired quantity of material weigh hopper  30 . The screw  72  is mounted within a complementary barrel  73 . A flexible non-return flap valve  74  is mounted at an outlet end of the barrel  73 . Thus when vacuum filling the bin  71  the flap valve  74  closes over the end of the barrel  73  to stop drawback of material into the bin  71  which would adversely effect the accuracy of the delivery of material from the bin  71 . The flap valve  74  deflects, as shown in broken outline, to allow discharge of material from the end of the barrel when the screw  72  operates. 
     The blender  70  is shown in an off-line application. A pinch valve  75  is mounted directly below the outlet  4  controlling delivery of material on demand through a pipe  76  to a feed hopper  77  mounted on an extruder  78 . The pinch valve  75  is closed when the sensor  5  is uncovered and when the sensor  5  is covered, the pinch valve  75  opens to allow discharge of material to the hopper  77 . 
     It will be appreciated that any of the blenders herein described may be used in either an on-line application, that is directly mounted on an extruder, or an off-line application. 
     Referring to FIG. 10, a separate hopper  79  may be provided for feeding additive material directly through pinch valve  80  and line  81  to the outlet  4  so that the production line can be kept running during cleaning or repair of faults to the blender. Alternatively, the material from the hopper  79  could be delivered through a line  82  into the chamber  2  below the weigh hopper  30 . 
     Another off-line application for the blender  1 ,  70  is shown in FIG.  13 . In this case, the blender  1 ,  70  discharges blended material  95  into a storage hopper  96  having a pinch valve  97  at an outlet of the hopper  96 . Upon opening the pinch valve  97 , blended material is discharged into an extrusion line supply bin  98  having a number of take-off lines  99  for delivery to extruders or the like. 
     Referring to FIG. 14, a diverter flap  90  may be mounted on the side walls of the mixing chamber  2  which can be moved from a retracted position (shown in broken outline) into a position beneath the weigh hopper  30  to direct material out of the mixing chamber  2 . This would facilitate the cleaning of the bins  36 . 
     For measurement of the level of material in the bins  36 , a pair of spaced-apart capacitor plates may be provided connected to the controller which uses a measure of the capacitance to determine the level of material within the bin  36 . Either a pair of plates may be provided or one plate may be formed by the wall of the bin  36 . 
     In similar fashion, the bulk density of the material may be determined by providing a chamber, say at an outlet of one of the bins  36 ,  71 , which is continually full of material. A pair of capacitor plates is provided in the chamber and the capacitance used to determine the bulk density of the material being delivered through the chamber. This is particularly useful in set-up situations to get the process on line in a minimum time. 
     It will be appreciated that instead of a single screw with blades of opposite pitch, a pair of screws may be provided within the mixing chamber for direction of the material towards the outlet. 
     The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail.