A shredding apparatus and a method for shredding thermoplastic waste

Abstract

A shredding apparatus and a method for shredding thermoplastic waste is disclosed In one characterising aspect, a flexible cover is mounted on the housing above the reciprocating slider (2) in a cantilever manner where the overhang portion of the flexible cover (7) covers the area only exposed to the clearance between the reciprocating slider and fixed plate due to which there is no direct contact between the plastic waste and reciprocating slider at the clearance zone. The provision of the flexible cover (7) prevents the shredded material from jamming between reciprocating slider (2) & side walls (11) of the housing of shredding device. A set of rotating and counter knives (4, 5) are arranged along the periphery of the rotating shaft in a helix manner. Proportional valve air jets (9) are placed in the lower region of the conveyor tube to push the material towards the end of the conveying region.

Description

[0001] A SHREDDING APPARATUS AND A METHOD FOR SHREDDING THERMOPLASTIC WASTE

[0002] Field of Invention

[0003] The present invention relates to a shredding apparatus for shredding the thermoplastic waste and method for shredding the thermoplastic waste. The present invention specifically is related to a shredding apparatus whereby the material jamming between the reciprocating slider & side walls of the housing of shredding apparatus is prevented while shredding the thermoplastic waste.

[0004] Background of Invention

[0005] To produce the production-grade thermoplastics from thermoplastics waste, firstly the waste thermoplastic is crushed in a shredding device and this crushed material is transported through a screw-type conveying zone to an extruder where melting and beneficiation of crushed thermoplastic is performed.

[0006] The shredding device generally comprises of a rotating shaft, on the outer periphery of which several knives are mounted in a circular or helix manner. Another set of spatially fixed counter knives are also present. The placement of the spatially fixed counter knives greatly enhances the cutting process at the start of shredding. Therefore, simultaneous cuts can be performed easily. The thermoplastic waste is crushed due to the action of rotating knives and fixed counter knives. The rotating shaft is placed in a horizontal manner and the waste thermoplastics is fed through a hopping device. A reciprocating slider is placed in order to facilitate the feeding process. The hopping device which is placed upright to the shaft axis feeds the material in a direction perpendicular to the axis of the rotating shaft i.e., in the area between the reciprocating slider and the fixed counter knives. The motion of the slider is mutually perpendicular to both the rotating shaft axis and the direction of the material feed. This slider is a reciprocating charging device, with a hydraulic force cylinder pressing on the waste thermoplastic and guiding it towards the rotating knives. The axis of rotation of the shaft and the direction of slider’s motion are perpendicular to each other.

[0007] The crushed waste thermoplastic is conveyed through a screw-type conveying region. At the lower end region of the conveying region, there is a filler opening to the extruder screw, where the plastic material is melted and beneficiated to finally obtain production-grade thermoplastic.

[0008] In conventional shredding systems as shown in figure 1, material is fed through the hopping device (1) placed upright to the shaft axis (12). The reciprocating slider (2) is used to push the material, which is collected on the base plate (3) towards the rotating knives (4) to facilitate the cutting of the plastic waste. However, the disadvantage of the conventional shredding system is that during long hours of operation, the waste material jam occurs between the reciprocating slider (2) and the fixed plate (3a) as there is a clearance (19) for efficient sliding of the slider (2) of the housing (11) thereby leading to jamming of the slider (3) movement. Hence, the machine needs to be stopped and cleaned manually to remove the stuck plastic waste (18). This is hazardous and increases the downtime of the machine and efficiency of the shredding device is affected.

[0009] As shown in figure 1, the pre-shredded plastic waste is supplied to the base plate (3) which is pushed by the front surface of the slider (2d) in the forward direction towards the knife (4) which is housed in the housing (11). During reverse motion of the slider (3) the waste (20) which is there on the top surface (2a) of the slider (2) tends to go inside the clearance between slider and fixed plate (19) and creates jamming of the slider (3) movement. So eventually operator has to clean the jammed plastic waste (18) to run the machine smoothly.

[0010] U.S. Pat. No. 6,126,100 describes a shredding device with a reciprocating slider and a rotating shaft with fixed knives attached on its periphery. The material is cut with the combined action of rotating knives and fixed counter knives. The disadvantage of such a system is that the jamming of material occurs in the clearance between the reciprocating slider and housing side walls. Due to which there is a downtime in the machine.

[0011] The plastic waste is cut with the combined action of fixed counter knives and rotating knives. Air jets are placed in the lower region along the inner walls of conveyor tube in a helical manner in order to guide the shredded plastic towards the end of the conveyor tube with the help of air pressure. The air jets work on on / off principle i.e., when excess shredded material is collected in the lower region of conveyor tube, the air jet pushes the material in the axial direction.

[0012] Patent IN201817014661 describes the facilitation of shredded plastic waste with the help of regulation in air stream. The opening and closing of valve of air jets depend on the extruder load. If the load on the extruder is less than the set defined value, then the air jets are in operation, such that more shredded material is fed towards the filler opening of the extruder. The rotating shaft is divided into two sections in the axial direction. In first section, the rotating knives are placed along the periphery of the shaft. In the second section, an augur screw is placed to convey the shredded plastic waste towards the end of the shaft. In the end, the shredded plastic is conveyed into a filler opening of the extruder screw. Objects of Invention

[0013] The primary object of the present invention is to provide a shredding apparatus for shredding the thermoplastic waste and method for shredding the thermoplastic waste which prevents the material jamming between the reciprocating slider & side walls of the housing of shredding device so as to provide effective shredding of the plastic waste.

[0014] A further object of the present invention is to effectively convey the shredded plastic waste towards the filler opening of the extruder screw in an efficient manner with the help of proportional valve air jets.

[0015] Yet another object of the present invention is to provide a shredding device which is more efficient and effective for preventing the material jamming.

[0016] Summary of Invention

[0017] The present invention discloses a shredding apparatus for shredding the thermoplastic waste and method for shredding the thermoplastic waste which prevents material jamming between the reciprocating slider & side walls of the housing of shredding device so as to provide effective shredding of the plastic waste. In the shredding apparatus disclosed herein, a flexible cover is mounted on the housing above the reciprocating slider in a cantilever manner where the overhang portion covers the area only exposed to the clearance between the reciprocating slider and fixed plate due to which there is no direct contact between the plastic waste and reciprocating slider at the clearance zone. A major part of the feed material falls on this flexible cover due to the action of gravity and the motion of slider, the material accumulates in-front of the slider. This material is pushed with the help of the slider during its motion from one extremum to another extremum. The knives are arranged along the periphery of the rotating shaft in a helix manner. Several knives are placed such that the force on the individual knives is decreased. The plastic waste, after shredding gets conveyed along with the knives in the axial direction of the shaft. In order to facilitate the conveying process, proportional valve air jets are placed in the lower region of the conveyor tube to push the material towards the end of the conveying region. At the end of the conveying region, there is a filler opening to the extruder screw, where the shredded waste plastic is finally melted and beneficiated to produce production grade thermoplastics. Brief description of Figures:

[0018] Figure 1 shows the general arrangement of the shredding device as per prior art

[0019] Figure 2 shows the slider movement and material entanglement at various surfaces of the slider of the shredding device as per prior art Figure 3 shows the material jamming in the conventional system between reciprocating slider and fixed wall of housing. Inset figure represents an enlarged view of jammed plastic waste.

[0020] Figure 4 shows the apparatus of the present invention with flexible member preventing material jam. Figure 5 shows the arrangement of knife in helical fashion.

[0021] Figure 5 a shows the knife and counter knife arrangement in perspective view

[0022] Figure 6 shows the air jet nozzles and their arrangement. List of Parts:

[0023] 1. Hopping device 2d. Front surface of slider

[0024] 2. Reciprocating slider 3. Base plate

[0025] 2a. Top surface of slider 3a. Fixed plate

[0026] 2b. side surface of slider 3b. Side plate 2c. bottom surface of slider 35 4. Knife 5. Counter knife 17. Clearance between

[0027] 6. Conveyor tube 15 reciprocating slider &

[0028] 7. Flexible cover side walls

[0029] 8. Shaft / Rotating Shaft 18. Jammed plastic waste 9. Air jet 19. Clearance between slider

[0030] 10. Opening of air jet & fixed plate

[0031] 11. Housing 20 20. Pre-shredded waste

[0032] 12. Shaft axis material

[0033] 13. Supply Valve 21. Shredded waste 14. Axial direction 22. Support plate for counter

[0034] 15. Augur screw knife

[0035] 16. Filler opening of 25 23. Bottom plate for counter extruder screw knife

[0036] Detailed Description of Invention As discussed earlier, one of the major objects of the present invention is to provide a shredding apparatus for shredding the thermoplastic waste and method for shredding the thermoplastic waste which prevents the plastic waste material from jamming between the reciprocating slider (2) & fixed plate (3a) of the housing (11) and effective shredding of the plastic waste material. This is achieved with the help of an inventive shredding apparatus.

[0037] The shredding apparatus of the present invention consists of a flexible cover (7) attached on the fixed plate (3a) on the housing (11) above the reciprocating slider (2) in a cantilever manner such that the length of the said flexible cover(7) can be varied to the extent that it covers fully the clearance (19) between reciprocating slider (2) & fixed plate (3a) as shown in figure 4. The overhang length of the flexible cover (7) should be at least twice the height (X) of the fixed plate (3a). The overhang length is the length of the loose portion of the flexible cover (7) between the points where it is attached to the fixed plate (3a) and its loose end. The plastic waste (20) fed from the hopper (1) which is shown in figure 1 falls on the flexible cover (7) as shown in figure 4. Some plastic waste (20) also falls on the area between the rotating knives (4) and the reciprocating slider (2). This plastic waste is pushed towards the counter knives (5) with the help of reciprocating slider (2) during its motion in forward direction. During the reverse movement of the reciprocating slider (2), the flexible cover acts as a barrier such that the waste (20) does not enters into the clearance (19). The flexible cover (7) material may be in the from of metallic strips or stiff plastic sheet, or fabric depending on the process and space available. If the waste material to be shredded is of high denier or have lumps of high weight the use of metallic elements is preferred such that waste material does not degrade the flexible cover (7). These metallic elements can be flexible chains, micro perforated flexible mesh, or a high flexible sheet metal which have very less friction of coefficient or can be of any flexible metallic element which is known to a person skilled in the art. The metal may be made from steel, copper, aluminium, alloys or any other material that a person skilled in the art finds suitable for this purpose. However, the preference will be flexible sheet which can be made of Polypropylene, polyethylene or can be of high strength plastic having both wear resistant property and flexible in nature. Since it is attached to the reciprocating slider (2) so forward and backward movement of the reciprocating slider (2) tends to degrade the flexible cover (7) in short span of time only. Therefore, it is recommended to use cover (7) which is flexible in nature with high tensile and compressive strength for long lasting.

[0038] The benefit of this process and apparatus is that there is very less direct contact between the plastic waste and the reciprocating slider (2) when compared with the conventional shredding system. Thus, there is no stoppage of the machine and the same time an uniform throughput rates are achieved. In conventional shredding system, the stuck material (18) needs to be removed manually as described in Figures 1 and 2. Thus, in the present invention, the downtime of the machine is eliminated and there is no risk for the operator and hence fully safe.

[0039] In another embodiment of the present invention as shown in figure 5, the rotating knives (4) are arranged in a helix manner or can follow curved path on the outer periphery of a shaft (8) that is rotatable around its central axis. The material (20) is fed by the slider (2) towards the knives (4) where it is cut into finer granules (21) and supplied to the extruder through filler opening of extruder screw (16). The rotating knives (4) cut the plastic waste material against the spatially fixed counter knives (5) as shown in Figure 4. Such kind of arrangement of rotating knives (4) decreases the cutting force on individual knives as in each cycle of rotation different knives are in contact with the waste material for shredding due to the helical arrangement of the knives (4), hence the input feed of the plastic waste to the rotating shaft (8) can be varied according to the throughput required.

[0040] For example, if the shredding machine is operating to deliver the 650 kgs of output and at every 2 mins the material from hopper is delivered to the slider (2) and in that case the effort required by the knives if they are placed in the normal manner (without helical) will be more and also they will not be able to transfer or convey the material to towards the extruder. Such 2mins time can be reduced to 1 min and the overall efficiency of the system is also increased if the rotating knives are placed helically.

[0041] As described earlier, the rotating knives (4) are placed along the periphery of the rotating shaft (8) forming a helix and spaced at regular intervals. The rotating knives (4) arrangement also creates a conveying action in the axial direction (14) for the plastic waste after shredding, whereby shredded plastic waste is conveyed away from the gap (19). The spacing or interval between the consecutive rotating knives (4) is determined in such a way that only one rotating knife (4) is in contact with the sheet to be shredded at any given time. The first rotating knife (4) of the helix is placed just after the last rotating knife (4) of the previous helix. Thus, at a time only one rotating knife (4) performs a cut as shown in the zoomed view of Figure 5.

[0042] Figure 5a, shows the knife and counter knife (4,5) arrangement in perspective view. The knifes (4) which are embossed or bolted to the shaft (8) are parallelly arranged to the counter knife (5) such that material (20) which comes between the knifes (4,5) are shredded easily. The counter knife (5) is sandwiched between the bottom plate for counter knife (23) and support plate for counter knife (22) by mechanical joints such as by fasteners like screws or bolts such that during high vibrations of shredding, the knife (5) are not loosen. Further both bottom plate (23) for counter knife (5) and support plate (22) are affixed to the base plate (3). This joining of the bottom plate (23) with the base plate (3) can be done by welding or screwing by bolts as the case may be to form a rigid structure.

[0043] As shown in the Figure 6, in order to efficiently facilitate the conveying process, air jets (9) are placed along the inner wall of a conveyor tube (6) in a helix manner. The opening nozzle (10) of the air jet (9) is placed along the axis (12) of the rotating shaft (8). The air jets (9) are disposed in the helical sense forming a sector in the lower region of the conveyor tube (6). The air jets (9) push the plastic waste after shredding towards the exit of the conveyor tube (6). In conventional shredding systems, air jets (9) operate with an on / off function depending on the extruder load. In another embodiment of the present invention, the air jet (9) operation is controlled with the help of an air supply valve (13). The air supply valve (13) can be of various types like these can be proportional valves which are always in ON condition (i.e., supplying air is at low pressure) but when load on the shredding device is increased the pressure increases drastically. As soon as the load on the knives increased a signal is sent to the proportional valve to operate at full.

[0044] Generally during experiments, it was found that at machine full output of 650kg, the air supply valves (13) provided output pressure of 4-5bars. A proportional valve regulates the flow of air that is proportional to an electronically controlled input. It is a continuously operated valve, which controls the air flow from low to high at regular intervals of time. Thus, the material is continuously fed towards the filler opening of the extruder.

[0045] A method for shredding the thermoplastic waste (20) using the shredding apparatus which prevents jamming of material between the reciprocating slider (2) and side walls of the housing (11) comprises following steps: a) Feeding the thermoplastic waste feed (20) to the hopper (1). b) Passing the thermoplastic waste from hooper for shredding the plastic waste (20) fed from the hopper (1) falls on the flexible cover (7) attached on the fixed plate (3a) on the housing (11) above the reciprocating slider (2) in a cantilever manner and some of the plastic waste (20) falls on the area between the rotating knives (4) and the reciprocating slider (2). c) Pushing the plastic waste towards the counter knives (5) with the help of reciprocating slider (2) during its motion in forward direction. d) Further facilitating the shredded waste material to the conveying region with the help of air jet valves (9) where the shredded waste material is melted for further production of high grade thermoplastic. While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

Claims

Claims:

1. A shredding apparatus for shredding thermoplastic waste (20), characterised in that said apparatus comprises: a hopper (1) to feed said thermoplastic waste (20); a reciprocating slider (2); having a top surface (2a), a side surface (2b), a bottom surface (2c), and a front surface (2d) a base plate (3); a fixed plate (3a) having height (X); a shaft (8) that is rotatable about its central axis wherein rotating knives (4) are arranged in a helical manner on its cylindrical surface; a set of counter knives (5) sandwiched between a bottom plate (23) and a support plate (22), said both bottom plate (23) and support plate (22) being attached to said base plate (3) air jets (9) having opening nozzles (10) placed along the inner wall of a conveyor tube (6); wherein, a flexible cover (7) with length of its overhang at least twice the height of said fixed plate (3a) is attached on the fixed plate (3a) on a housing (11) above the reciprocating slider (2) in a cantilever manner, and said reciprocating slider (2) is capable of traveling orthogonally to the axis of said shaft (8) between its positions A and B, and said air jets (9) being supplied with air under pressure through an air supply valve (13).

2. The shredding apparatus as claimed in claim 1, wherein said flexible cover (7) is in the form of metallic strips or stiff plastic sheets, or fabric.

3. The shredding apparatus as claimed in claim 1, wherein said flexible cover is made from metallic elements such as flexible chains, micro perforated flexible mesh, or a high flexible sheet metal, wherein said metal is steel, copper, aluminium, or alloys.

4. The shredding apparatus as claimed in claim 2, wherein said plastic is Polypropylene or polyethylene.

5. The shredding apparatus as claimed in claims 1 to 3, wherein said air jets (9) are disposed in the helical configuration forming a sector in the lower region of the conveyor tube (6).

6. The shredding apparatus as claimed in claim 1, wherein said counter knives (5) are attached to said support plate (22) and bottom plate (23) with fasteners like screws or bolts.

7. The shredding apparatus as claimed in claim 1, said is joined to said base plate (3) by welding or screwing using bolts to form a rigid structure.

8. The shredding apparatus as claimed in claims 1 to 7 wherein a supply valve (13) is provided for operating air jets so as to regulate the flow of air that is proportional to an electronically controlled input.

9. A method for shredding the thermoplastic waste (20) using the shredding apparatus as claimed in claims 1 to 8, which prevents jamming of material between the reciprocating slider (2) and side walls of the housing (11) characterised in that said method comprises the steps of(a) feeding the thermoplastic waste feed (20) to the hopper (1),(b) passing the said thermoplastic waste from hooper for shredding characterised in that the plastic waste (20) fed from the hopper (1) falls on the flexible cover (7) attached on the fixed plate (3a) on the housing (11) above the reciprocating slider (2) in a cantilever manner and some of the plastic waste (20) falls on the area between the rotating knives (4) and the reciprocating slider (2),(c) pushing the said plastic waste towards the counter knives (5) with the help of reciprocating slider (2) during its motion in forward direction,(d) further facilitating the shredded waste material to the conveying region with the help of air jet valves (9) where said shredded waste material is melted for further production of high grade thermoplastic.

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