Weighing and mixing system

Abstract

The invention is a weighing and mixing system for the preparation of mixtures of components required by processing machines for the manufacture of plastic products. In particular the invention is suitable for the preparation of weighed mixtures comprised of at least two different masterbatches in order to obtain the color shade desired for a specific plastic product.

Claims

1. A weighing and mixing system for mixing at least two original masterbatches, comprising: at least two weighing units, with at least one separate weighing unit for each one of the at least two original masterbatches of, each weighing unit comprising: a material hopper comprising: at least one first opening, a first feeding mechanism for each first opening, at least one second opening, and a second feeding system for each second opening; a weighing hopper comprising a flap at the bottom; and a load cell for weighing original masterbatch contained in the weighing hopper; a chamber configured to receive weighed original masterbatch from each of the at least two weighing units and to produce a homogeneous mixture; and a computer configured to control operation of each of the separate weighing units, to supply a predetermined weight of each of the individual original masterbatches to the chamber, to: open and close each of the at least one first feeding mechanism for the at least one first opening to allow a first controlled amount of original masterbatch that is less than a predetermined weight required for a final batch to fall by gravity from the material hopper through the at least one first opening into the weighing hopper for weighing by the load cell; after each of the at least one first feeding mechanism is closed, activate the at least one second feeding system of the at least one second opening to cause a second controlled amount of original masterbatch to fall into the weighing hopper for weighing by the load cell; and when a total weight of original masterbatch weighed by the load cell is equal to the predetermined weight required for the final batch, halt activation of the at least one second feeding system and open the flap at the bottom of the weighing hopper to cause the weighed original masterbatch to flow by gravity into the chamber for mixing with final batches from the other individual weighing units, wherein the computer is further configured to control operation of the separate weighing units to weigh each of the at least two original masterbatches in parallel, wherein at least one second feeding system comprises a feed tube and a motor configured to rotate the feed tube, the feed tube comprising a hollow interior section having a tightly fitted compression spring inserted into a front end and an opening to allow granules of original masterbatch that fall from the material hopper through the second opening to enter the hollow interior section, wherein a diameter of a wire of which the spring is made and a diameter of the interior of the spring are chosen such that when the motor is activated causing the feed tube to rotate about a longitudinal axis, granules of original masterbatch in the hollow interior section of the feed tube travel in single file in a spiral path through the interior of the spring until they reach the end of the spring and drop off one at a time into the weighing hopper, wherein the feed tube is surrounded by a cylindrical sleeve and a threaded section is created on an outer wall of the feed tube, with threads in the threaded section formed in an opposite direction to that in which the feed tube is configured to rotate in order to push any granules of original masterbatch that attempt to enter the space between the feed tube and the sleeve back towards the opening of the feed tube.

2. The weighing and mixing system of claim 1, wherein each of the at least two weighing units is configured to achieve the predetermined weight required for the final batch with an accuracy of less than 0.03 g.

3. The weighing and mixing system of claim 1, wherein each of the at least two weighing units feeds weighed original masterbatch to the chamber through a common funnel.

4. The weighing and mixing system of claim 1, wherein each feed tube comprises an opening at a position on a circumference of the feed tube, the feed tube opening being positioned to rotate with the feed tube and to align once on each rotation with a second opening of the material hopper so that granules of original masterbatch flow by gravity from the material hopper and into the feed tube.

5. The weighing and mixing system of claim 1, wherein the computer is configured to: close the at least one first feeding mechanism for the at least one first opening when a weight of the original masterbatch in the weighing hopper is determined, by the load cell, to be 0.1g to 20g less than the predetermined weight required for the final batch; activate and halt the at least one second feeding system to deliver the second controlled amount of original masterbatch to the weighing hopper to achieve the predetermined weight required for the final batch, as determined by the load cell.

6. The weighing and mixing system of claim 5, wherein the computer is configured to activate and halt the at least one second feeding system to deliver the second controlled amount of original masterbatch to the weighing hopper to achieve the predetermined weight required for the final batch with an accuracy of less than 0.03g.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic perspective view of the system of the invention;

(2) FIG. 2 schematically shows a weighing unit of the invention;

(3) FIG. 3 schematically shows a conventional auger;

(4) FIG. 4 schematically shows a conventional auger having a very fine pitch;

(5) FIG. 5 schematically shows a side view of the feed tube of the invention; and

(6) FIG. 6 schematically shows a cross sectional view of the feed tube of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(7) The invention is a weighing and mixing system for the preparation of mixtures of components required by the processing machine for the manufacture of plastic products. In particular the invention is suitable for the preparation of weighed mixtures comprised of at least two different masterbatches in order to obtain the color shade desired for a specific plastic product.

(8) Due to its accuracy, the system can produce a mixture of all the molded components, e.g. masterbatches, additives and resins, that will be fed to the processing machine through its free fall hopper, or only a mixture of the masterbatches and additives that will be fed to the machine through a single-component gravimetric or volumetric feeder located at the throat of the machine. The first option is more suitable for small sized throughput applications.

(9) Unlike a prior art gravimetric blender, in which each component is dispensed separately into a single weighing chamber and then all components are dropped into a mixing chamber which delivers a homogenous blend to the processing machine, the system of the invention is equipped with a weighing chamber for each component. Therefore the weighing will be done in parallel, thus enabling the use of much smaller load cells and accordingly much better accuracy, yet with a relatively high throughput.

(10) In order to achieve extreme accuracy and maximal speed the weighing unit first dispenses a weight that is close to but slightly less than the specified set point. A second and final dispensing is performed using a feeding system comprising an auger, screw, conveyer belt, or dosing cylinder, or a vibratory mechanismeach of which is able to dispense a small amount of material at a timeand a motor that rotates the auger, screw, or dosing cylinder or drives the conveyor belt or vibratory mechanism.

(11) FIG. 1 is a schematic perspective view of the system of the invention. The system 100 is a modular one that can be comprised of one or more weighing units 10 arranged in a way that allows the material weighed in each of the units to fall through individual chutes via a common funnel into a chamber 14, that combines the output of all of weighing units 10 into a uniform mixture that can be fed into a processing machine. Each of the weighing units 10 contains a different type of resin, masterbatch, or additive in granular form. A computer (not shown in the figures) controls the operation of each of the separate weighing units 10 in order to supply the exact weight of each of the individual components to the mixing machine. To prepare a given mixture, the computer will activate only the weighing units 10 that contain the material required for that mixture according to a formulation that has been preloaded into the computer. Herein the individual weighing units 10 of system 100 will be called identical since they are structurally and functionally identical but in a commercial application may be of different sizes to accommodate different quantities of material.

(12) FIG. 2 schematically shows one of the weighing units 10 of the invention. In many ways weighing unit 10 is similar to conventional weighing systems that are well known in the art; therefore most of the components of the unit 10 will not be described in detail herein.

(13) Weighing unit 10 comprises a vacuum (Venturi) feeder 16 that draws one type of material, i.e. resin, masterbatch, or additive, into material hopper 20. Hopper 20 has a first opening 24 with a feeding mechanism e.g. a flap, an auger, a screw, a vibratory mechanism, or paddles, at its bottom and a second opening 22. In a first weighing stage, the first opening 24 is opened and closed and the feeding mechanism is activated when required by an element (not shown in the figures) that is controlled by a system computer (not shown in the figures) to allow a controlled amount of material to fall by gravity into weighing station 30 where it is weighed on load cell 32. When the weight of material that has entered weighing station 30 is a little less than the weight required for the final batch, the computer causes flap 24 to close.

(14) In embodiments of the system, for example to allow faster flow of material from material hopper 20 into weighing station 30 during the first stage of the weighing procedure, the material hopper has two or more first openings 24 with feeding mechanisms.

(15) Second opening 22 in the bottom of hopper 20 allows material to fall by gravity from hopper 20 into the throat 40 of a feeding system 25 and from the throat of the feeding system into the interior of a cylindrical sleeve 26. In a second stage of the weighing procedure, a motor 28, controlled by the system computer causes an auger, screw, or feed tube to rotate within the cylindrical sleeve 26 to force granules of material that have entered sleeve 26 to be pushed towards the end of the tube where they fall into the weighing hopper 30. When the total weight of the material introduced into weighing hopper 30 from opening 24 and through feeding system 25 and weighed by load cell 32 is equal to the required weight, then the system computer stops the motor 28 and opens flap 34 to allow the material to flow by gravity through funnel 12 into chamber 14, where the material is joined by material of other types that has been weighed in parallel by other independent units identical to weighing unit 10.

(16) Embodiments of the weighing unit may comprise two or more second openings 22 in the bottom of hopper 20 each of which allows material to fall by gravity from hopper 20 into the throat 40 of a separate feeding systems 25.

(17) The auger or screw inside the cylindrical tube 26 can have many different forms depending on the weighing accuracy desired and/or the dimensions of the material being weighed. For example, FIG. 3 shows a conventional auger having a course pitch of about 20 mm and FIG. 4 an auger with a fine pitch of about 3 mm that is designed for use in weighing powders. Each rotation of the auger shown in FIG. 3 would push a large number of typical granules used in the plastic industry into the weighing hopper (estimated number is about 200 granules weighing 0.02 gram/granule); therefore it could be used in applications in which the absolute accuracy need not be high or in which each batch is comprised of large quantities of material so that the relative accuracy would be sufficient to obtain the desired mixture. In another embodiment the feeding system might comprise a conveyor belt, dosing cylinder, or vibratory mechanism.

(18) FIG. 5 schematically shows a side view and FIG. 6 a cross-sectional view of a feed tube 36 that has been designed by the inventors for dosing one pellet in each rotation. The rear section 42 of feed tube 36 is solid and machined to couple with motor 28 configured to rotate feed tube 36 about its longitudinal symmetry axis. The interior 44 of the remainder of the length of feed tube 36 is hollow. On each rotation of feed tube 36 opening 41 will be aligned with the throat 40 of the feeding system below an opening 22 in the bottom of material hopper 20 (see FIG. 2). When this happens granules of material enter into the hollow interior 44 of feed tube 36.

(19) At the front end of the feed tube 36 is tightly fitted a compression spring 46. The diameter of the wire of which spring 46 is made and the diameter of the hollow interior of the spring are chosen such that a single granule of the material that enters the interior of the front end of the feed tube 36 will fit into the roughly triangular space between adjacent coils of the spring. As feed tube 36 rotates, granules of material travel in single file along a spiral path through the interior of spring 46 until they reach the end of the spring and drop off one at a time through funnel 18 and from there into the weighing hopper 30 where they are added to the weight of the material on the load cell.

(20) In the weighing unit 10 the feed tube 36 is surrounded by a cylindrical sleeve 26. In order to prevent granules of material from moving between the outer side of the wall of feed tube 36 and the inner wall of the sleeve 26 and eventually hindering or preventing feed tube 36 from rotating, a threaded section 38 is created on the outer wall of feed tube 36. The threads in threaded section 38 are created in the opposite direction to that in which the feed tube 36 is rotated in order to push any granules that attempt to enter the space between the feed tube and the sleeve back towards the opening 41 of the feed tube. When the feeding system 25 comprises a feed tube 36, the computer causes flap 24 to close ending the first weighing stage when the weight of material that has entered weighing station 30 is equal to, for example, 0.1-20 grams less than the weight required for the final batch.

(21) Since the weight of one pellet of typical masterbatch is about 0.02 g, the system using feed tube 36 is able to achieve the desired weight with an accuracy of less than 0.03 g. This accuracy is essential in order to produce a mixture of masterbatches in order to obtain a desired shade in particular in case of small mixtures of component of about few kilograms.

(22) The accuracy of about 0.03 g for each component is far beyond the accuracy of any existing gravimetric blender or any other system used to prepare a mixture of components for a processing machine in the plastic industry.

(23) The product that leaves the chamber 14 is a uniform mixture of the individual pellets that were weighed in the individual weighing units. This mixture, after melting in a processing machine, will achieve the desired color due to formulas that were formulated in advance in order to achieve a desired shade of color from a combination of specified weights of each of a number of masterbatches.

(24) The invention can be used to prepare a mixture of the granules from two or more masterbatches. This mixture can then be melted in an extruder and then passed through a pelletizing machine to produce pellets of a masterbatch that will have a color that is a combination of the original two or more colors. This new masterbatch is sold to a manufacturer who feeds it together with resin and other additives, e.g. to give UV resistance, to a processing machine to manufacture a product having a desired color.

(25) The advantage of this to the masterbatch producer is that he does not have to keep a large inventory of every conceivable shade of masterbatch available but can quickly provide his customers with masterbatches having the color of their choice that he is able to produce from a limited number of color masterbatches. The advantage to the customer is that he receives a masterbatch order for small quantities much faster and at a reduce price to be used at his manufacturing facility.

(26) For a processing company that uses a lot of color additives (masterbatches) the advantage of implementing the new method of the invention for producing the desired shade by either adding a selected combination from a given number of masterbatches into the processing machines or to produce a new homogeneous masterbatch by passing a selected combination of masterbatches through an extruder and pelletizing machine to form new pellets with the desired shade is lower cost made possible by reducing the inventory of many shades of masterbatches, some of which are very expensive since they are ordered in small quantities and rarely ordered.

(27) In the basic configuration of the system, the dispensing is controlled by preset amounts of masterbatches according to the required recipe.

(28) In an alternative configuration, in order to improve the accuracy of the resulting color with respect to the required reference, the dispensing is controlled by external feedback on the color of the resulting product. This can be achieved by using various means such as spectrophotometers, color sensors and similar devices.

(29) The invention has been described above for use in the plastic industry; however it can be used in other industries, for example the chemical industry, that have similar requirements. In addition the system can be configured for use with powders by using an auger having a non-conventional fine thread having a pitch of, for example, 3 mm, such as shown in FIG. 4, in the feeding system.

(30) 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.