WO2008156660A1

WO2008156660A1 - Apparatus and method for incorporating recycled plastic

Info

Publication number: WO2008156660A1
Application number: PCT/US2008/007383
Authority: WO
Inventors: Anthony Genova, Jr.; Joel Thomson
Original assignee: Community Enterprises Llc
Priority date: 2007-06-15
Filing date: 2008-06-13
Publication date: 2008-12-24

Abstract

Devices and methods for combining at least two thermoplastic materials having at least two screw flights, each screw flight plasticizing a respective material in a respective melt stream, such that the materials are combined in an accumulation space into a combined melt stream which is then passed through a mixer.

Description

APPARATUS AND METHOD FOR INCORPORATING RECYCLED PLASTIC

Cross Reference To Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 60/934,659 filed June 15, 2007.

Background of the Invention

[0002] Thermoplastic materials are widely used in short-lived products such as packaging. A vast amount of these materials are available for recycling, either in the form of post-consumer products or industrial scrap. Material from both of these sources has experienced at least one cycle of melting and solidification, which is known as a "heat history". Depending on the material type, plastic which has experienced one or more heat histories may have degraded properties. Physical properties such as stiffness and impact strength may be reduced, and optical properties such as clarity and color may suffer. Mechanisms on the molecular level such as chain scission, which lowers molecular weight, thermal and oxidative degradation, which can introduce unwanted byproducts, are usually the cause of the reduced properties.

[0003] Because the recycled material is already prone to reduced properties, and forming that material into new products introduces yet another heat history, steps must be taken to maintain product quality whenever recycled material is used. One simple way is to dilute the percentage of recycled material with new material, with the resulting properties of the product somewhere between the properties of new and recycled plastic. This is usually done by dry-blending pellets of new plastic with flakes of ground-up recycled plastic. The resulting mixture is then processed under the same conditions as for new plastic. This method has some drawbacks. For instance some properties such as optical yellowing of clear materials are very hard to mask even when the recycled percentage is very low. Degradation products can also affect taste and odor, in the case of food packaging, in very minute amounts. As a result of these drawbacks, the use of recycled material in critical applications such as food and beverage packaging is usually limited to low percentages, ten percent or less.

[0004] There are other ways to incorporate recycled material. One such method is co-injection molding, wherein the recycled material is molded as an intermediate layer surrounded by new material in the finished part. In this method the new material on the outside carries the mechanical, visual and chemical requirements of the product and the recycled material remains hidden. One disadvantage of co-injection is the complexity and cost of the machinery and molds. Another disadvantage is the fact that any visual difference between the materials will be noticeable on clear products such as bottles.

Summary of the Invention

[0005] One object of the present invention is to provide devices and methods to incorporate recycled plastic material into an injection molding process while minimizing the amount of degradation undergone by the recycled material in a re-molding operation. Such degradation is minimized by subjecting the recycled material to less heat history, that is, the amount of heat or temperature with respect to time, than the new material into which the recycled material is mixed.

[0006] According to one embodiment of the present invention, two independent flights process the new material and the recycled material separately, and the flight geometry and barrel heat settings are independently adjusted to produce less degradation on the recycled material. This embodiment can be implemented using a device similar to the device described in U.S. Patent No. 7,004,739 to Thomson, the- entire content of which is incorporated herein by this reference, in which the independent flights are arranged axially along a single screw shaft within a single barrel. While the Thomson patent describes two melt streams being conveyed separately to a common accumulation space located downstream of the screw flights, that device produces a semi-random cosmetic effect similar to marble or tortoise shell, whereas the combined material of the present invention requires mixing of the recycled material with the new material. Accordingly, one object of the present invention is to incorporate a dynamic mixing device to produce a substantially homogeneous mixture of the two melt streams prior to injection. [0007] A further object of the present invention is to have the dynamic mixing device on the same shaft as the two plasticating screw flights powered by the same drive and motor. This allows the process to be retrofitted to a standard model injection molding machine which is equipped with a single drive and motor for plasticating functions.

[0008] A further object of the preferred embodiment of the present invention is to have the mixing of the two melt streams occur upstream of whatever shut-off valve is used to prevent reverse flow into the screw flights during injection. In the case of a reciprocating screw machine, the shut-off valve can be a ring check valve affixed to the end of the screw, downstream of the mixing section. In the case of a non-reciprocating screw with a separate accumulator, the valve can be located within the outlet passage between the plasticating extruder and the accumulator, which is also downstream of the mixing device.

[0009] An alternative embodiment of the present invention can use a static or powered mixing device downstream of the check valve in either the outlet passage of the plasticating extruder or the outlet of the accumulation space, for the purpose of homogenizing the melt mixture.

Brief Description Of The Drawings

[0010] Figure 1 shows a partial cross-sectional view of the single screw device configured to plasticize two materials in axially separated zones according to one embodiment of the present invention;

[0011] Figure 2 shows a cross-sectional view of a screw tip for randomly mixing the two materials in the melt phase and a static mixer for homogenizing the melt mixture according to one embodiment of the present invention;

[0012] Figure 3 shows a cross-sectional view of a screw tip incorporating a mixing device and check valves upstream of where the melt streams meet according to one embodiment of the present invention;

[0013] Figure 4 shows a cross-sectional view of a screw tip incorporating a mixing device and check valves downstream of the where the melt streams meet and downstream of the mixer according to one embodiment of the present invention; and

[0014] Figure 5 shows a cross-sectional view of a screw tip incorporating a mixing device with a separate accumulation and injection device according to one embodiment of the present invention .

Detailed Description Of The Invention

[0015] As shown in Figure 1, one embodiment^' the present invention pertains to a device for plasticating (melting) two streams of thermoplastic material. In particular the device may be used to plasticize one stream of new, virgin material in one screw flight (1) and a second stream of recycled .thermoplastic material in a second screw flight (2) . Both flights are positioned within a single heated barrel (3) and are rotated by a single drive (4) . Material that is fully melted in the upstream zone is conveyed through a passage (5) past the downstream flight (2) to meet another material that is processed in the downstream zone. The combined melt stream passes through a mixer (8) and continues to a point downstream of the screw (6) where both melt streams are accumulated prior to injection into a closed mold. The mixer can be a static or a dynamic mixer. Furthermore, in one embodiment especially suitable for retrofitting a conventional molding machine, the dynamic mixer can be powered by the same drive and/or motor as the screw flights.

[0016] As shown in Figure 2, according to one embodiment of the present invention, one melt is conveyed from the upstream flight through passage (5) to enter the common accumulation space (6) through one or more openings (7) so as to form a substantially random mixture with another melt . This random mixture is then conveyed through a mixing device, here shown as a static mixer (14), downstream of the openings where the melt streams meet (7) .

[0017] As shown in Figure 3, according to one embodiment of the present invention, the mixing device (8) may have plurality of rotating pins, protrusions, or flight sections on the screw designed to divide and recombine the melt streams to form a substantially homogeneous mixture. This is known as a dynamic distributive mixer. Alternatively, the mixing device (8) can have one or more narrow spaces through which the melt is forced under increased shear and velocity, a device known as a dispersive mixer. Any combination of these two general types of mixer, or other mixers, may also be employed.

[0018] To prevent backward flow of melt, devices such as check valves (9), (10), (11) may be incorporated either upstream of the point (7) where the melt streams combine as shown in Figure 3 , or downstream of that point and as shown in Figure 4. Such check valves may be located either upstream or downstream of the mixing device (8).

[0019] The present invention can be employed with a reciprocating screw machine wherein the screw assembly moves back and forth axially within the barrel (3) as it respectively plasticizes a shot of material and injects it into a mold. However, in another embodiment of the invention, as shown in Figure 5, the screw assembly may remain axially fixed with respect to the barrel (3) and the plasticized melt can exit the extruder via passage (12) which can be further conveyed to a separate accumulation and injection device (13). In such an embodiment check valves are not required as the one-way flow of melt material is controlled by mechanical valves within the passage ( 12 ) .

[0020] While the present invention has been described with respect to preferred embodiments, those skilled in the art will appreciate that various adaptations and modifications of the embodiments may be made without departing from the scope and spirit of the invention. For example, the recycled material could be introduced partway along the length of a plasticating screw, using an additive stuffer as is frequently done in the process of compounding extrusion, or two separate plasticating screws and barrels could feed melted new material and melted recycled material to a common mixing device. In the latter example, the screw dedicated to recycled material could have a shorter length, deeper flights, lower rotation speed, and lower temperature settings, all of which would allow for better control and would contribute to minimizing the negative effects of heat history on the recycled material. Therefore, it is to be understood that the invention may be practiced other than as specifically described above, within the scope of the appended claims .

Claims

ClaimsWhat is claimed is:

1. A device for plasticizing and combining at least two thermoplastic materials comprising: a first screw flight for plasticizing a first material in a first melt stream; a second screw flight for plasticizing a second material in a second melt stream; an accumulation space for combining the first material from the first melt stream and the second material from the second melt stream into a combined melt stream; and a mixer for mixing material in the combined melt stream.

2. The device of claim 1, further comprising: at least one check valve in the first melt stream.

3. The device of claim 1, further comprising: at least one check valve in the second melt stream.

4. The device of claim 1, further comprising: at least one check valve in the combined melt stream.

5. The device of claim 4, wherein the check valve is positioned upstream of the mixer.

6. The device of claim 4, wherein the check valve is positioned downstream of the mixer.

7. The device of claim 1, wherein at least one the first material and the second material is a recycled material.

8. The device of claim 1, wherein the first melt stream subjects the first material to a different amount of heat than the second melt stream.

9. The device of claim 1, wherein the first melt stream subjects the first material to a different amount of heat with respect to time than the second melt stream.

10. The device of claim 1, wherein one screw comprises the first screw flight and the second screw flight.

11. The device of claim 1, wherein the mixer is a dynamic mixer.

12. The device of claim 11, futher comprising a drive providing power to at least one of the screw flights and to the dynamic mixer .

13. A method of plasticizing and combining at least two thermoplastic materials comprising the steps of: plasticizing a first material along a first screw flight in a first melt stream; plasticizing a second material along a second screw flight in a ^'second melt stream; combining the first material from the first melt stream and the second material from the second melt stream into a combined melt stream; and mixing material in the combined melt stream.

14. The method of claim 13, wherein the step of the mixing material in the combined melt stream homogenizes the material.

15. The method of claim 13, wherein the first material is subjected to a different amount of heat than the second melt stream.

16. The method of claim 13, wherein the first material is subjected to a different amount of heat with respect to time than the second melt stream.