Screw for twin-shaft non-meshing extruder and twin-shaft non-meshing extruder

Abstract

According to one embodiment, a screw 14 has an upper-stage-side screw portion 31 and a lower-stage-side screw portion 32 having different flight cross-sectional shapes, and a curvature radius of the flight cross-sectional shape of the lower-stage-side screw portion 32 is larger than a curvature radius of the flight cross-sectional shape of the upper-stage-side screw portion 31.

Description

Screw for twin-screw non-intermeshing extruder and twin-screw non-intermeshing extruder

[0001] The present disclosure relates to twin-screw non-intermeshing extruders.

[0002] Extruders that extrude molten resin are known. The extruders can constitute a granulator that produces resin pellets. Patent Document 1 describes a granulator that includes an extruder and a cutter that cuts the molten resin extruded from the extruder.

[0003] The extruder has a cylinder into which the raw resin is supplied and a screw that is rotated within the cylinder. The raw resin supplied into the cylinder is melted within the cylinder and kneaded and transported by the screw.

[0004] Japanese Patent Application Laid-Open No. 2022-071886

[0005] It is required that the molten resin does not remain in the cylinder of the twin-screw non-intermeshing extruder.

[0006] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

[0007] According to one embodiment, a twin-screw non-intermeshing extruder screw has an upper screw section having a first flight profile and a lower screw section having a second flight profile, the second flight profile having a radius of curvature greater than the radius of curvature of the first flight profile.

[0008] According to one embodiment, retention of molten resin in the cylinder of the twin-screw non-intermeshing extruder is suppressed.

[0009] FIG. 1 is a schematic diagram showing the configuration of a granulator according to one embodiment. FIG. 2 is a cross-sectional view showing the structure of an extruder according to one embodiment. FIG. 3 is another cross-sectional view showing the structure of an extruder according to one embodiment. FIG. 4 is a perspective view showing the appearance of a screw according to one embodiment. FIG. 5 is a perspective view showing an enlarged view of a portion of an upper-stage flight according to one embodiment. FIG. 6 is a perspective view showing an enlarged view of a portion of a lower-stage flight according to one embodiment. FIG. 7 is a partial cross-sectional view of an upper-stage flight taken along line a-a in FIG. 5. FIG. 8 is a partial cross-sectional view of a lower-stage flight taken along line bb in FIG. 6. FIG. 9 is a table showing the results of a comparison test.

[0010] An embodiment will be described in detail below with reference to the drawings. In all drawings used to describe the embodiment, components and devices having the same or substantially the same functions are designated by the same reference numerals. Furthermore, as a general rule, components and devices that have already been described will not be described again.

[0011] 1 is a schematic diagram showing the configuration of a granulator 1A according to this embodiment. The granulator 1A includes a twin-screw non-intermeshing extruder 2, a gear pump 3, a foreign matter removal section (screen changer) 4, a die head 5, and a cutter (pelletizer) 6.

[0012] <Extruder> The twin-screw non-intermeshing extruder 2 has a main motor 10, a reducer 11, a cylinder (barrel) 12, a hopper 13, and a twin-screw non-intermeshing extruder screw 14. In the following description, the twin-screw non-intermeshing extruder 2 may be abbreviated as "extruder 2," and the twin-screw non-intermeshing extruder screw 14 may be abbreviated as "screw 14."

[0013] The main motor 10 is an electric motor. The rotational driving force output from the main motor 10 is input to the screw 14 via the reducer 11, causing the screw 14 to rotate. The reducer 11 reduces the speed of the rotational driving force output from the main motor 10 and increases the torque of the rotational driving force input to the screw 14.

[0014] The hopper 13 is disposed at one longitudinal end of the cylinder 12. The cylinder 12 forms a conveying path (flow path) through which the raw resin or molten resin supplied via the hopper 13 is conveyed.

[0015] The raw resin charged into the hopper 13 passes through the hopper 13 and falls into the cylinder 12. The raw resin that has fallen into the cylinder 12 is transported from one end of the cylinder 12 in the longitudinal direction, where the hopper 13 is located, to the other end of the cylinder 12 in the longitudinal direction.

[0016] That is, one longitudinal end of the cylinder 12 on which the hopper 13 is disposed is the upstream side of the conveying path, and the other longitudinal end of the cylinder 12 is the downstream side of the conveying path.

[0017] From another perspective, one longitudinal end of the cylinder 12 where the hopper 13 is located is the upstream side of the flow path FP (FIG. 2) through which the molten resin flows, while the other longitudinal end of the cylinder 12 where the hopper 13 is not located is the downstream side of the flow path FP (FIG. 2) through which the molten resin flows.

[0018] Fig. 2 is a cross-sectional view showing the structure of the extruder 2. Fig. 3 is another cross-sectional view showing the structure of the extruder 2. The cylinder 12 of the extruder 2 is hollow, and the aforementioned flow path FP is formed by the internal space of the cylinder 12.

[0019] Two screws 14 are arranged in parallel within the cylinder 12 (flow path FP), and both ends of each screw 14 are rotatably supported by bearings.

[0020] However, the two screws 14 are not intermeshed with each other. Furthermore, the two screws 14 are rotationally driven in different directions within the cylinder 12 (in the flow path FP). In other words, the two screws 14 are rotationally driven in opposite directions.

[0021] That is, the extruder 2 is a "twin-screw counter-rotating extruder" in which two parallel screws 14 are driven in opposite directions. More specifically, the extruder 2 is a twin-screw counter-rotating non-intermeshing extruder.

[0022] In the extruder 2, which is a twin-screw counter-rotating non-intermeshing extruder, the two screws 14 do not come into contact with each other, and the screws 14 do not come into contact with the cylinder 12. Therefore, the extruder 2 can rotate the screws 14 at high speed to strongly knead the molten resin.

[0023] Furthermore, the extruder 2 has high mechanical reliability because the cylinder 12 and the screw 14 are not subject to wear due to contact with each other. The screw 14 of the extruder 2 will be described later.

[0024] <Gear Pump, Foreign Matter Removal Unit> Referring again to Figure 1, the gear pump 3 supplies (pressure-feeds) the molten resin extruded from the extruder 2 to the die head 5 through the foreign matter removal unit (screen changer) 4. The gear pump 3 is driven by a motor 3a. The rotational driving force output from the motor 3a is input to the gear pump 3 via a reducer 3b. The foreign matter removal unit 4 removes unnecessary components, residue, contaminants, etc. from the molten resin passing through it.

[0025] <Die Head> The die head 5 is composed of a die holder, a die plate, a die plate cover, etc., and has multiple nozzles from which molten resin is extruded. The molten resin that has passed through the foreign matter removal section 4 and is supplied to the die head 5 is extruded from the nozzles of the die head 5 and formed into a strand shape.

[0026] <Cutting Machine> The cutting machine 6 has a cutting processing section that receives the strand-shaped molten resin extruded from the nozzle of the die head 5. The cutting processing section is provided on a flow path of water (pellet transport water) circulating within the manufacturing system including the granulator 1A.

[0027] During operation of the manufacturing system, the cutting processing section of the cutter 6 is filled with pellet transport water. That is, the molten resin formed into strands by the die head 5 is extruded into the water (pellet transport water).

[0028] A rotationally driven cutter head is provided in the cutting processing section of the cutting machine 6. A plurality of cutter blades are attached to the cutter head.

[0029] <Method for Producing Resin Pellets> Next, an example of a method for producing resin pellets using the granulator 1 A will be outlined. In the production method described here, resin pellets are produced by the following steps.

[0030] First, raw resin is charged into the hopper 13 of the extruder 2. More specifically, thermoplastic resin powder is charged into the hopper 13. For example, polyethylene (PE) or polypropylene (PP) powder is charged into the hopper 13.

[0031] Generally, polyethylene has a higher hardness and viscosity than polypropylene, and therefore, in many cases, polyethylene needs to be kneaded more strongly than polypropylene.

[0032] The raw resin charged into the hopper 13 is supplied to the cylinder 12 via the hopper 13. The raw resin supplied to the cylinder 12 is transported from the upstream side to the downstream side of the flow path FP by the rotating screw 14. The raw resin transported in the flow path FP is melted and kneaded during the transport.

[0033] More specifically, the raw resin is melted by heat generated by a shearing action caused by the rotation of the screw 14. The raw resin is also kneaded (dispersively mixed and distributively mixed) by the shearing action and elongation action caused by the rotation of the screw 14.

[0034] If necessary, the raw material resin or the molten resin may be heated by a heating means such as a heater, and the raw material resin or the molten resin may be cooled by a cooling means.

[0035] The kneaded molten resin is supplied to a gear pump 3. The molten resin supplied to the gear pump 3 is supplied (pressurized) to a die head 5 through a foreign matter removal section 4. In the foreign matter removal section 4, foreign matter that has entered from the outside is removed.

[0036] The molten resin pressure-fed to the die head 5 is continuously extruded from a plurality of nozzles provided in the die head 5. The molten resin passes through the nozzles of the die head 5 and is formed into a strand shape (string shape, rope shape).

[0037] The strand-like molten resin extruded from the nozzle of the die head 5 is cut to a predetermined length by the cutter head of the cutter 6 and solidified. In other words, the molten resin extruded in the form of a strand is cut into pellets. As a result, resin pellets of a predetermined size (length and diameter) are produced.

[0038] As described above, the cutting section of the cutting machine 6 is filled with water (pellet transport water), and the molten resin is cut underwater (pellet transport water). This method of cutting molten resin underwater is sometimes called "underwater cutting."

[0039] Depending on the characteristics of the raw resin (particularly the melting point), resin pellets may be produced without circulating pellet transport water. In this case, the molten resin extruded from the nozzle of the die head 5 is cut in a cutting processing section where there is no pellet transport water. In other words, the molten resin is extruded into the air and cut. This method of cutting molten resin in the air is sometimes called "hot cutting."

[0040] Returning to the explanation of the resin pellet manufacturing method in which underwater cutting is performed, the mixture (slurry) of resin pellets and pellet transport water sent out from the cutting processing section is separated into resin pellets and pellet transport water.

[0041] Thereafter, the resin pellets separated from the pellet transport water are sent to a pellet silo or the like and stored therein. The resin pellets are sent to the pellet silo by an air current generated by a blower, for example.

[0042] The resin pellets separated from the pellet transport water may be dried using a centrifugal dehydrator, etc. The resin pellets may also be separated based on size before being stored.

[0043] <Screw> The screw 14 provided in the extruder 2 will be described in more detail. Fig. 4 is a perspective view showing the appearance of the screw 14.

[0044] As described above, two screws 14 are disposed in the cylinder 12 of the extruder 2, and each screw 14 has the same shape and structure. In other words, a description of one screw 14 also applies to the other screw 14.

[0045] The screw 14 has a shaft portion 20, a screw portion 30, and a kneading adjusting portion 40. However, the shaft portion 20, the screw portion 30, and the kneading adjusting portion 40 are integrally molded from a metal material (for example, stainless steel).

[0046] The shaft portion 20 has a cylindrical or columnar shape as a whole, and includes a base end portion 21 located at one axial end and a tip end portion 22 located at the other axial end. The base end portion 21 is provided with a connecting hole 23 into which the output shaft of the reducer 11 is inserted.

[0047] The kneading adjustment portion 40 is provided at the center or approximately the center in the axial direction of the screw 14. As a result, the shaft portion 20 can be divided into a region located on one side (base end portion 21 side) of the kneading adjustment portion 40 in the axial direction and a region located on the other side (tip end portion 22 side) of the kneading adjustment portion 40 in the axial direction.

[0048] From another perspective, the screw 14 can be divided into an area AR1 between the kneading adjustment portion 40 and the base end 21 of the shaft portion 20, an area AR2 between the kneading adjustment portion 40 and the tip end 22 of the shaft portion 20, and an area AR3 between the areas AR1 and AR2.

[0049] As shown in Figure 2, the screw 14 is arranged in the cylinder 12 so that the base end 21 of the shaft portion 20 is located upstream of the flow path FP and the tip end 22 of the shaft portion 20 is located downstream of the flow path FP.

[0050] As a result, the regions AR1, AR3, and AR2 of the screw 14 are aligned in this order from the upstream side to the downstream side of the flow path FP within the cylinder 12. Therefore, in the following description, the region AR1 of the screw 14 may be referred to as the "upstream portion," and the region AR2 may be referred to as the "downstream portion." Also, the region AR3 of the screw 14 may be referred to as the "midstream portion."

[0051] However, the above names and distinctions are merely used for the convenience of explanation.

[0052] <Screw portion and flight> The screw portion 30 includes an upper screw portion 31 provided in region AR1 (upstream portion) of the screw 14 and a lower screw portion 32 provided in region AR2 (downstream portion) of the screw 14.

[0053] The screw section 30 is provided with a flight 50. More specifically, the upper stage screw section 31 is provided with an upper stage flight 51, and the lower stage screw section 32 is provided with a lower stage flight 52.

[0054] The flight 50 protrudes in a flange-like manner from the outer circumferential surface 24 of the shaft portion 20 toward the outside in the radial direction of the shaft portion 20 , and extends spirally along the axial direction of the shaft portion 20 .

[0055] Fig. 5 is an enlarged perspective view of a portion of the upper flight 51, and Fig. 6 is an enlarged perspective view of a portion of the lower flight 52. Fig. 7A is a cross-sectional view showing the cross-sectional shape of the upper flight 51 (flight cross-sectional shape), and Fig. 7B is a cross-sectional view showing the cross-sectional shape of the lower flight 52 (flight cross-sectional shape).

[0056] More specifically, Fig. 7A is a partial cross-sectional view of the upper flight 51 taken along line aa in Fig. 5. Fig. 7B is a partial cross-sectional view of the lower flight 52 taken along line bb in Fig. 6.

[0057] 5 and 6 are straight lines parallel to the axis of the screw 14. In other words, the cross-sectional shape of the flight is the shape of a plane that appears when the flight 50 is cut parallel to the axis of the screw 14.

[0058] The surface of the flight 50 is composed of an upstream side surface 53 facing the upstream side of the flow path FP within the cylinder 12, a downstream side surface 54 facing the downstream side of the flow path FP within the cylinder 12, and an upper surface 55 connecting the upstream side surface 53 and the downstream side surface 54.

[0059] The upstream side surface 53 rises from the outer circumferential surface 24 of the shaft portion 20 toward the upstream edge of the upper surface 55. On the other hand, the downstream side surface 54 rises from the outer circumferential surface 24 of the shaft portion 20 toward the downstream edge of the upper surface 55.

[0060] More specifically, the upstream side surface 53 includes a vertical portion 53a that is perpendicular or approximately perpendicular to the outer peripheral surface 24 of the shaft portion 20, and an arc portion 53b that is interposed between the vertical portion 53a and the outer peripheral surface 24 of the shaft portion 20.

[0061] Similarly, the downstream side surface 54 includes a vertical portion 54 a that is perpendicular or approximately perpendicular to the outer peripheral surface 24 of the shaft portion 20 , and an arc portion 54 b that is interposed between the vertical portion 54 a and the outer peripheral surface 24 of the shaft portion 20 .

[0062] In addition, in Figs. 5 and 6, a dot pattern is applied to the arc portion 53b in order to clearly distinguish it from other portions.

[0063] One side of the arcuate portion 53b of the upstream side surface 53 is connected to the outer circumferential surface 24 of the shaft portion 20, and the other side is connected to the vertical portion 53a. One side of the arcuate portion 54b of the downstream side surface 54 is connected to the outer circumferential surface 24 of the shaft portion 20, and the other side is connected to the vertical portion 54a.

[0064] From another perspective, the arcuate portion 53b is the lower end of the flight 50 on the upstream side, and the arcuate portion 54b is the lower end of the flight 50 on the downstream side.

[0065] As described above, the upper screw section 31 and the lower screw section 32 have similar flight cross-sectional shapes. However, the flight cross-sectional shapes of the upper screw section 31 and the lower screw section 32 are not identical.

[0066] Specifically, the radius of curvature of the arcuate portion 53b of the upper stage flight 51 provided in the upper stage screw section 31 is different from the radius of curvature of the arcuate portion 53b of the lower stage flight 52 provided in the lower stage screw section 32. In other words, the upstream lower end portions of the upper stage flight 51 and the lower stage flight 52 are both arc-shaped, but the radii of curvature are different.

[0067] Specifically, the radius of curvature r2 of the arcuate portion 53b of the lower flight 52 is larger than the radius of curvature r1 of the arcuate portion 53b of the upper flight 51 (r2>r1). More specifically, in this embodiment, the radius of curvature r1 is 40.0 mm, and the radius of curvature r2 is 90.0 mm. In other words, the radius of curvature r2 in this embodiment is 2.25 times the radius of curvature r1.

[0068] By making the radius of curvature r2 of the arcuate portion 53b of the lower flight 52 larger than the radius of curvature r1 of the arcuate portion 53b of the upper flight 51, retention of molten resin within the cylinder 12 is suppressed.

[0069] More specifically, the lower-stage flight 52, which has an arcuate portion 53b with a larger radius of curvature than the upper-stage flight 51, has a smaller contact area with the molten resin than the upper-stage flight 51. From another perspective, the resistance that the molten resin experiences when passing through the lower-stage screw portion 32 is smaller than the resistance that the molten resin experiences when passing through the upper-stage screw portion 31.

[0070] As a result, the molten resin is transported more smoothly downstream of the flow path FP where the lower screw portion 32 (lower flight 52) ​​is located, compared to the upstream side of the flow path FP where the upper screw portion 31 (upper flight 51) is located, and stagnation of the molten resin is suppressed.

[0071] Furthermore, as a result of suppressing the retention of molten resin on the downstream side of the flow path FP, the retention of molten resin is also suppressed on the upstream side of the flow path FP. Overall, the retention of molten resin in the flow path FP (in the cylinder 12) is suppressed, and resin burning and the like are prevented.

[0072] Resin burning is a phenomenon in which black spots appear on the surface of a resin molded product such as a resin pellet. One of the causes of resin burning is that the molten resin remaining in the cylinder 12 is excessively heated and scorched. Therefore, if the retention of the molten resin in the cylinder 12 is suppressed, resin burning can be prevented.

[0073] The radius of curvature of the arcuate portion 54b of the upper flight 51 is the same as the radius of curvature of the arcuate portion 54b of the lower flight 52. The upper flight 51 and the lower flight 52 also have the same outer diameter D.

[0074] More specifically, the outer diameter D of the upper flight 51 and the lower flight 52 is 450 mm. In other words, the radius of curvature r2 of the arcuate portion 53b of the lower flight 52 is equivalent to 0.2 times the outer diameter D of the flight 50.

[0075] However, the outer diameter D of the flight 50 is not limited to 450 mm. Furthermore, the radius of curvature r2 of the arcuate portion 53b is not limited to 0.2 times the outer diameter D of the flight 50. On the other hand, taking into account the various performance characteristics (such as kneading performance and conveying performance) required of the screw 14, it is preferable that the radius of curvature r2 of the arcuate portion 53b be 0.2 to 0.3 times the outer diameter D of the flight 50.

[0076] In the extruder 2 of this embodiment, in which retention of molten resin is suppressed, a change in the brand of raw material resin can also be completed quickly. More specifically, in the extruder 2 of this embodiment, when changing the brand of raw material resin, the previous brand of raw material resin can be reliably discharged from the cylinder 12 in a short time. In other words, the new brand is completely replaced with the old brand in a short time.

[0077] <Kneading Adjustment Section> The kneading adjustment section 40 provided in the region AR3 (midstream section) of the screw 14 adjusts the pressure acting on the molten resin. The pressure adjustment by the kneading adjustment section 40 controls the degree of kneading of the molten resin (the degree of kneading).

[0078] From another perspective, the kneading adjustment portion 40 has the role of adjusting the cross-sectional area of ​​the midstream portion of the flow path FP so that the molten resin passing through the flow path FP is subjected to a desired pressure. Note that the smaller the cross-sectional area of ​​the midstream portion, the higher the pressure the molten resin is subjected to. Also, the part of the flow path FP where the kneading adjustment portion 40 is located is sometimes called a "gate portion."

[0079] <Comparative Test> Figure 8 is a table showing the results of a comparative test conducted by the present inventors. In this test, six extruders M1 to M6 were prepared, each having substantially the same configuration as extruder 2. However, the outer diameter of the screw 14 attached to each extruder M1 to M6 is different. Therefore, each extruder M1 to M6 is equipped with a cylinder 12 having an inner diameter corresponding to the outer diameter of the attached screw 14.

[0080] Furthermore, for each of the extruders M1 to M6, three types of screws 14 were prepared, each having a different ratio (r2 / r1) between the radius of curvature r2 of the arcuate portion 53b of the lower flight 52 and the radius of curvature r1 of the arcuate portion 53b of the upper flight 51.

[0081] That is, for one extruder, three types of screws 14 (screw No. 1, screw No. 2, screw No. 3) were prepared, each having the same outer diameter but different ratios (r2 / r1) of the radius of curvature r2 to the radius of curvature r1.

[0082] The ratio (r2 / r1) of screw No. 1 is 1.0, the ratio (r2 / r1) of screw No. 2 is 2.0, and the ratio (r2 / r1) of screw No. 3 is 5.0.

[0083] In other words, the radius of curvature r2 of the arcuate portion 53b of the lower flight 52 of screw No. 1 is 1.0 times the radius of curvature r1 of the arcuate portion 53b of the upper flight 51. The radius of curvature r2 of the arcuate portion 53b of the lower flight 52 of screw No. 2 is 2.0 times the radius of curvature r1 of the arcuate portion 53b of the upper flight 51. The radius of curvature r2 of the arcuate portion 53b of the lower flight 52 of screw No. 3 is 5.0 times the radius of curvature r1 of the arcuate portion 53b of the upper flight 51.

[0084] Next, screw No. 1, screw No. 2, and screw No. 3 were sequentially attached to extruders M1 to M6, and molten resin was extruded under the same conditions. After that, the extruded molten resin was evaluated for the presence or absence of resin burning.

[0085] More specifically, 18 combinations of six extruders and three types of screws were visually inspected for the presence or absence of resin burning and the possibility of resin burning. Furthermore, combinations in which resin burning was not observed were evaluated as "Good," and combinations in which resin burning was observed or the possibility of resin burning was observed were evaluated as "Poor." The table in Figure 8 shows the results of this evaluation.

[0086] In this test, no resin burning was observed in any combination where the ratio (r2 / r1) of the curvature radius r2 to the curvature radius r1 was 2.0 or 5.0. On the other hand, resin burning or the possibility of resin burning was observed in all combinations where the ratio (r2 / r1) of the curvature radius r2 to the curvature radius r1 was 1.0.

[0087] This test confirmed that when the radius of curvature r2 of the arc portion 53b of the lower flight 52 is at least 2.0 times or more and 5.0 times or less the radius of curvature r1 of the arc portion 53b of the upper flight 51, retention of molten resin is suppressed and resin burning can be prevented.

[0088] The invention made by the present inventors has been specifically described above based on embodiments and examples, but it goes without saying that the present invention is not limited to the above embodiments or examples, and various modifications are possible within the scope of the gist of the invention.

[0089] 1 merely shows one example of the use of the extruder 2. In other words, the extruder 2 may constitute a device or system different from the granulator 1A.

[0090] The extruder 2 can also be used to process raw material resins other than polyethylene. For example, the extruder 2 can be used to process (melt, knead, etc.) polypropylene. From another perspective, even when a raw material resin other than polyethylene is processed using the extruder 2, retention of the molten resin in the cylinder 12 can be suppressed.

[0091] However, in many cases, polyethylene has a higher hardness and viscosity than polypropylene and requires stronger kneading, so extruder 2, which is a twin-screw non-intermeshing extruder, is more suitable for processing polyethylene when comparing only polyethylene and polypropylene.

[0092] In another embodiment, the screw section 30 including the flights 50 is formed by a plurality of elements attached to the shaft section 20 .

[0093] 1A... granulator, 2... twin-screw non-intermeshing extruder (extruder), 3... gear pump, 3a... motor, 3b... reducer, 4... foreign matter removal section (screen changer), 5... die head, 6... cutter (pelletizer), 10... main motor, 11... reducer, 12... cylinder (barrel), 13... hopper, 14... twin-screw non-intermeshing extruder screw (screw), 20... shaft portion, 21... base end portion, 22... tip portion, 23... connection Hole, 24...outer peripheral surface, 30...screw portion, 31...upper screw portion, 32...lower screw portion, 40...kneading adjustment portion, 50...flight, 51...upper flight, 52...lower flight, 53...upstream side surface, 53a...vertical portion, 53b...arcuate portion, 54...downstream side surface, 54a...vertical portion, 54b...arcuate portion, 55...upper surface, AR1, AR2, AR3...area, D...outer diameter, FP...flow path, r1, r2...radius of curvature

Claims

1. A screw for a twin screw non-intermeshing extruder comprising: an upper screw section having a first flight cross-sectional shape; and a lower screw section having a second flight cross-sectional shape, wherein a radius of curvature of the second flight cross-sectional shape is greater than a radius of curvature of the first flight cross-sectional shape.

2. A screw for a twin-screw non-intermeshing extruder as described in claim 1, wherein the upper screw portion and the lower screw portion are provided with flights that protrude radially outward from the outer circumferential surface of the shaft portion and extend spirally along the axial direction of the shaft portion, the flights having upstream and downstream side surfaces rising from the outer circumferential surface of the shaft portion, the upstream and downstream side surfaces including arc portions connected to the outer circumferential surface of the shaft portion, and the radius of curvature of the arc portions of the flights provided on the lower screw portion is greater than the radius of curvature of the arc portions of the flights provided on the upper screw portion.

3. The screw for a twin-screw non-intermeshing extruder according to claim 1, which has a kneading adjustment portion, the upper stage screw portion being provided on one side of the kneading adjustment portion in the axial direction, and the lower stage screw portion being provided on the other side of the kneading adjustment portion in the axial direction.

4. The screw for a twin-screw non-intermeshing extruder according to claim 1, wherein the radius of curvature of the second flight cross-sectional shape is 2.0 to 5.0 times the radius of curvature of the first flight cross-sectional shape.

5. The screw for a twin-screw non-intermeshing extruder according to claim 1, wherein the radius of curvature of the cross-sectional shape of the second flight is 0.2 to 0.3 times the outer diameter of the flight.

6. A twin screw non-intermeshing extruder comprising: a cylinder; and a screw disposed within said cylinder, wherein said screw has an upper screw section having a first flight cross-sectional shape and a lower screw section having a second flight cross-sectional shape, wherein a radius of curvature of said second flight cross-sectional shape is greater than a radius of curvature of said first flight cross-sectional shape.

7. A twin-screw non-intermeshing extruder according to claim 6, wherein the radius of curvature of the second flight cross-sectional shape is 2.0 to 5.0 times the radius of curvature of the first flight cross-sectional shape.

8. A twin-screw non-intermeshing extruder according to claim 6, wherein the screw conveys while kneading the polyethylene fed into the cylinder.

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