Patent Publication Number: US-2022212366-A1

Title: Self-cleaning gravimetric and volumetric dosing apparatuses

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of and priority to Israel Patent Application No. 268544 filed Aug. 6, 2019, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention is from the field of adding raw material to production machines for producing products by means of industrial processes. Specifically the invention is relates to the addition of small quantities of masterbatch to production machines that produce products made from plastic. 
     BACKGROUND OF THE INVENTION 
     In the modern world plastics are the material of choice for the manufacture of a seemingly unlimited number of products. These products are produced by a variety of industrial processes, e.g. injection molding, blow molding, extrusion, and 3-D printers. The raw material that is fed into the machines used to produce the final products is a mixture consisting of: polymers (called resin in the industry) in the form of small beads, colorants and other additives, e.g. UV inhibitors. The colorants and other additives are supplied as masterbatches, which are concentrated mixtures of pigments and/or additives encapsulated during a heat process into a carrier resin which is then cooled and cut into a granular shape. 
     Herein the terms “bead”, “pellet”, and “granule” are used interchangeably to refer to a small piece of material typically having a weight in the range of 0.01 g-0.04 g. 
     In the majority of present day plastic product production facilities, the resin is fed in controlled amounts directly into the production machines and the desired color of the plastic product is achieved by feeding the masterbatch, which is the color additive and has the same shade as the required shade of the final product, by relatively small gravimetric or volumetric dosing system installed on the throat of the processing machine. 
     U.S. Pat. No. 6,911,607 describes a typical gravimetric dosing system of this type. The system comprises a container, which is filed with an additive material. The additive material is dosed into and mixed with a basic flow that is feed into a processing device by a dosing mechanism comprised of a motor that rotates a worm screw in a cylindrical insert that projects out of the container. A load cell weighs the container, dosing mechanism, and the additive material in both of them. Loss-of-weight measurements are used to calibrate the rotation speed of the worm screw to provide the desired dosing rate. As explained in the application, the design of the system results in weighing errors that are compensated for by use of calibration graphs determined by experiment for each different additive material. The weighing errors are caused by shifting center of gravity of the container and dosing mechanism caused when material is added to or removed from the container and non-linearity between the rotation speed of the motor and the discharge of the dosing mechanism in mass per unit time. One type of weighing error that cannot be compensated for is caused by noise and vibration, e.g. of the motor in the dosing mechanism, processing device, and persons moving in the vicinity of the dosing system, or by someone or something touching the container. 
     U.S. Pat. No. 6,688,493, to the inventor of the present invention, describes a gravimetric dosing system that provides solutions to the weighing errors that are inherent in the system described in U.S. Pat. No. 6,911,607.  FIG. 1  illustrates a schematic block diagram of the system of U.S. Pat. No. 6,688,493. The system includes a container  10 , a material hopper  11  located inside the container  10 , a load cell  16  coupled to the material hopper  11 , a controller  13  that calculates weight according to the load cell  16  information and commands the motor  14  of the screw feeder  12  to dispense portions of material into a molding machine  15 . The inclined and parallel walls of the hopper  11  minimize weight errors and facilitate weight calibration by keeping the material-pressure in the hopper  11  constant. 
     The material must be fed to the molding machine in accurate-weight portions and each portion must be fed at given time and for a given duration. To achieve this target the controller  13  commands the motor  14  of the screw feeder  12  to start rotating with a specific spin at the given time for the given duration. Since the given time and duration of feeding are determined by the molding machine  15 , the rotation of the screw feeder  12  is the only variable that can be used to control the weight of the fed portion. 
       FIG. 2  illustrates the cross-section of the system with material in the material hopper and the system in a work or ready to work configuration. The material  17 , which fills the material hopper  11 , is fed through an outlet  18  of the material hopper  11  to a space  19  created between outlet  18  of the material hopper and an outlet  20  of the container  10 . From this space  19  the material  17  is fed through the outlet  20  of the container  10  into the screw feeder  12 . The outlet  18  of the material hopper is shifted from the outlet  20  of the container to enable keeping the material in the outlet-shaft  21  constant as long as the material level  22  is higher than the upper end of the outlet-shaft  21  and to isolate the material hopper  11 . The part of the material  17  located in the space  19  acts as a buffer, which isolates the material hopper  11  and limits noise and shaking coming from the screw feeder  12  and its motor  14 , to enable an accurate weighing of the material hopper  11  by the load cell  16 . The controller  13  commands a refill means  23  to refill the material hopper  11  when the material level  22  is reduced to a threshold level. 
     All known gravimetric and volumetric dosing systems designed for directly adding small quantities of additive, e.g. masterbatch, directly into the throat of a production machine have essentially the same structure and operate in the same manner as that described with respect to  FIG. 1  and  FIG. 2 . It is noted that a volumetric dosing system is also manufactured by the applicant of this application. The volumetric system is identical to the gravimetric one shown in  FIGS. 1 and 2  with the exception that the volumetric system does not comprise a load cell and the feeding during production runs is dependent on the assumption that the properties of the material, e.g. the specific gravity, and/or of the material streaming properties do not change after the initial calibration of the feed screw rotation rate is made. 
     One of the practical disadvantageous of working with dosing systems of the type described above is that a considerable amount of time is required to empty the material hopper and clean all traces of material from the hopper and screw each time that the material has to be changed in order to produce products having a different color. 
     It is a purpose of the present invention to provide a solution to this problem in the form of components added to the conventional system that can be activated to perform a self-cleaning function. 
     Further purposes and advantages of this invention will appear as the description proceeds. 
     SUMMARY OF THE INVENTION 
     Described herein is a self-cleaning dosing apparatus that comprises:
         a) a material hopper containing pellets of material;   b) a feed screw;   c) a first material outlet through which pellets of material fall by gravity onto a first end of the feed screw;   d) a cylindrical insert that surrounds the feed screw; and   e) a motor that is connected to the first end of the feed screw and rotates the feed screw inside of the cylindrical insert to push pellets of masterbatch to a second end of the feed screw where the pellets of material fall out of the cylindrical insert.       

     The self-cleaning dosing apparatus additionally comprises:
         i) a second material outlet located beneath the first end of the feed screw;   ii) a slide gate that blocks the second material outlet;   iii) a pneumatic piston that pulls the slide gate back to unblock the second material outlet;   iv) a Venturi pump located under the second material outlet; and   v) an air nozzle located opposite the second end of the feed screw and is configured to create and direct a stream of air through the cylindrical insert.       

     In the self-cleaning dosing apparatus:
         ai) the motor is configured to reverse the direction of rotation of the feed screw, thereby causing the pellets of material between threads of the feed screw to be transported backwards towards the second material outlet;   aii) the stream of air created by the air nozzle pushes pellets of material that have fallen to the bottom of the cylindrical insert back through the cylindrical insert towards the second material outlet; and   aiii) the Venturi pump pulls all pellets of material that fall from the material hopper through first material outlet and the second material outlet and the pellets of material that are pushed out of the feed screw and the cylindrical insert by the reverse rotation of the feed screw and the stream of air from the nozzle out of the apparatus;       

     As a result of the activities described in ai) to aiii) all traces of the material are completely removed from material hopper, the feed screw, and the cylindrical insert. 
     Embodiments of the self-cleaning dosing apparatus comprise:
         a) a container surrounding the material hopper; and   b) a load cell attached to the material hopper, the load cell is configured to weigh the material hopper and material inside the material hopper.       

     In embodiments of the self-cleaning dosing apparatus the material is masterbatch. 
     Embodiments of the self-cleaning dosing apparatus are configured to add small quantities of masterbatch to resin introduced into production machines to produce colored products made from plastic. 
     Embodiments of the self-cleaning dosing apparatus are configured to be used with injection molding, extrusion, and extrusion blow molding production machines. 
     In embodiments of the self-cleaning dosing apparatus the pellets of material removed from the material hopper, feed screw, and cylindrical insert by the Venturi pump are returned to a container used to supply the same material to the material hopper. 
     All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a prior art gravimetric dosing system; 
         FIG. 2  is a cross-sectional view of the system of  FIG. 1 ; 
         FIG. 3  schematically shows a self-cleaning gravimetric dosing apparatus; 
         FIG. 4  schematically shows a self-cleaning volumetric dosing apparatus; 
         FIG. 5  schematically shows an enlarged view the components of the apparatuses of  FIG. 3  and  FIG. 4  that are located beneath the material hopper; 
         FIG. 6  schematically shows the direction of air flow through the apparatus during the cleaning process; 
         FIG. 7  schematically shows the direction of motion of granules of masterbatch out of the apparatus through the outlet of the Venturi pump during the cleaning process; and 
         FIG. 8  schematically shows how the masterbatch removed from the apparatus  30  or apparatus  130  is returned to the supply container  72  during the cleaning process. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Described herein are self-cleaning gravimetric and volumetric dosing apparatuses. The apparatuses are designed to add small quantities of masterbatch to the resin introduced into production machines to produce colored products made from plastic. The self-cleaning apparatuses enable automatic removal of all pellets of a first type of masterbatch from their interiors before a second type of masterbatch is introduced into the apparatus. The apparatuses can be used with production machines for any type of plastic manufacturing process, e.g. injection molding, extrusion, and extrusion blow molding. 
       FIG. 3  schematically shows an embodiment of self-cleaning gravimetric dosing apparatus  30 . Apparatus  30  comprises a material hopper  32  located inside a container  34 . A load cell  36  is attached to material hopper to weigh material hopper  32  and masterbatch introduced into it by means of material loader  38 . In operation, pellets of masterbatch fall by gravity from material hopper through first material outlet  40  into feed screw  42 . A reversible motor  44  rotates feed screw  42  to push pellets of masterbatch to the end of feed screw  42  where they fall through the neck piece  46  (under the roof of the neckpiece  48 ) into the throat  50  of the production machine. Not seen in the figure is a controller that receives weight data from load cell to determine the weight of masterbatch that is dispensed by the apparatus by using the loss-in-weight method. After calibration, the controller activates the motor  44  and adjusts the speed of rotation of the screw to dispense accurate weight portions of masterbatch in a given time and for a given duration. If material loader  38  is for example a Venturi loader, then the controller activates it to refill the material hopper  32  upon receiving a signal from a sensor inside the hopper indicating that the level of masterbatch has fallen to a threshold value. 
       FIG. 4  schematically shows an embodiment of a self-cleaning volumetric dosing apparatus  130 . Apparatus  130  is identical to apparatus  30  in  FIG. 3 , with the exceptions that apparatus  130  does not comprise a load cell  36  and therefore does not require a container  34  to isolate the material hopper  32  from vibration and to improve the accuracy of the weights measured by the load cell. 
     The components of apparatus  30  and apparatus  130  described above are identical structurally and functionally to those found in the prior art gravimetric and volumetric dosing systems. A controller and the cables that connect it to sensors and electric valves at various locations in the system are not shown in  FIGS. 3 and 4  or the following figures because they are not relevant to the present invention. 
     The new components of apparatuses  30  and  130  that enable the self-cleaning function will now be described with reference to  FIGS. 3 and 4  and particularly  FIG. 5 , which is an enlarged view schematically showing the components of apparatuses  30  and  130  that are located beneath the material hopper  32 . 
     Seen in  FIG. 5  are material hopper  32  and a first material outlet  40  at the bottom of the hopper. In operation, masterbatch falls from material hopper  32  through first material opening  40  onto feed screw  42 , which is rotated by motor  44 . As feed screw  42  turns inside of cylindrical insert  45  pellets of masterbatch are moved one at a time to the end of the feed screw where they fall into a space below cover  48  of the neck piece  46  of the process machine to which apparatus  30  or  130  is attached. From the neck piece  46  the pellets fall through throat  50  into the process machine. 
     When it is desired to change from a masterbatch having a first color to one having a second color, motor  44  is reversed to reverse the rotation of feed screw  42  causing the pellets of masterbatch between threads of the feed screw  42  to be transported backwards towards second material outlet  52 . At the same time a signal is sent to solenoid valve  62  that opens to allow compressed air to enter normally closed pneumatic piston  56  to pull back a slide gate  64  opening a second material outlet  52 . Simultaneously with the opening of second material outlet  52 , compressed air is introduced into Venturi pump  54  via air inlet  52  and compressed air is introduced into neck  46  of the processing machine via nozzle  58 . Nozzle  58  creates a stream of air that enters the end of the feed screw  42  and pushes pellets of masterbatch that have fallen to the bottom of cylindrical insert  45  back through the cylindrical insert  45  towards second material outlet  52 . The Venturi pump  54  pulls all pellets of masterbatch, .i.e. those that fall from the material hopper  32  through first material outlet  40  and second material outlet  52  and the pellets of masterbatch that are pushed out of the feed screw  42  and cylindrical insert  45  by the reverse rotation of feed screw  42  and the stream of air from nozzle  58 , out of the apparatus in the direction indicated by arrow  60 . From the outlet of Venturi pump  54  the pellets of masterbatch will be returned to a container used to supply masterbatch to material hopper  32  (see  FIG. 8 ). Thus, the combined efforts of Venturi pump  54 , reversal of rotation of the feed screw  42  and the stream of air produced by nozzle  58  thoroughly remove all traces of the masterbatch from material hopper  32 , feed screw  42 , and cylindrical insert  45 . 
       FIG. 6  schematically shows the direction of air flow through the apparatus during the cleaning process. Note how during the cleaning process the slide gate  64  is pulled back to unblock the second material outlet  52 . Reference signs  68   a ,  68   b , and  68   c  schematically represent compressed lines from a compressor (not shown) to piston  56 , Venturi pump  54 , and nozzle  58  respectively. 
       FIG. 7  schematically shows the direction of motion of granules of masterbatch out of the apparatus through the outlet of the Venturi pump during the cleaning process. 
       FIG. 8  schematically shows how masterbatch is supplied to material hopper  32  and the masterbatch that is removed from the apparatus  30  or apparatus  130  is returned to a supply container  72  during the cleaning process. Container  72 , which is the container in which the masterbatch is supplied from the producer or in which it is stored in the manufacturing facility is normally located in a storage area a distance from the production machines and attached dosing apparatus  30 , 130 . During production runs, if the level of masterbatch  76  in material hopper  32  drops to a low threshold value, then material loader  38  is activated to draw masterbatch out of container  72  until level  76  reaches an upper threshold value when material loader is deactivated. After the production run is completed, the feed screw rotation is reversed and compressed air is introduced via air lines  68   a ,  68   b , and  68   c  to empty all pellets of masterbatch from the material hopper and feed screw  42  as described herein above. The pellets that exit through the outlet of the Venturi pump  54  are then transported to container  72  by any means known in the art, for example they are pulled through a tube connected to the outlet of Venturi pump  54  by a material loader, e.g. a Venturi pump, attached to container  72 . In  FIG. 8 , arrow  74  symbolically shows the direction of pellets of masterbatch falling off the end of the feed screw into the throat of a process machine. 
     Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without exceeding the scope of the claims.