WO2009040235A1

WO2009040235A1 - Extruder screw for machines extruding plastics material

Info

Publication number: WO2009040235A1
Application number: PCT/EP2008/061852
Authority: WO
Inventors: Antonio Gumina; Stefano Paoletti
Original assignee: Samp S.P.A.
Priority date: 2007-09-27
Filing date: 2008-09-08
Publication date: 2009-04-02
Also published as: CN101848799A; EP2203294A1; ITBO20070658A1; CN101848799B

Abstract

Extruder screw (3), of the single type, for a machine (1) for extruding plastics material, said machine (1) comprising a barrel (2) heated and temperature-controlled by suitable means and having an inner surface (8) inside which rotates said screw (3) which comprises a primary thread (4) forming a primary compartment (10) and comprises a secondary thread (5) forming a secondary compartment (11) and having a ridge (9) positioned at a certain distance (d) from the inner surface (8) of said barrel (2) to form a transfer passage for the plastics material, wherein said secondary thread (5) comprises supplementary transfer means (6) formed entirely within the body of said secondary thread (5), through which more plastics material can flow from one face (14) to the other (13) of the secondary thread (5).

Description

TITLE: "EXTRUDER SCREW FOR MACHINES EXTRUDING PLASTICS MATERIAL"

DESCRIPTION The present invention relates to an extruder screw for machines for extruding plastics material, particularly for plastics material containing coloured fillers and/or other materials dispersed in the plastics material, such as: polyethylene (PE), polyvinyl chloride (PVC), thermoplastic materials in general, cross-linked polyethylene (XLPE), thermoplastic polyester elastomers known as HYTREL®, ester- and/or phenol-based polymers such as polyphenylene oxide (PPO), HFFR (Halogen Free Flame Retardant), and others.

A problem encountered in extrusion machines for plastics material containing coloured fillers and/or other dispersed particles is the lack of uniform mixing of the additives in the extruded plastics material, which may give rise to non-uniform colouring and/or differences in the characteristics of the extruded plastics material. This lack of homogeneity can be caused by various factors, for example insufficient blending and/or heating of the plastics material containing additives, insufficient reduction of the dimensions of the additives, or sedimentation or settling of the additives in the extrusion chamber.

In machines for processing plastics material using a single extruder screw, the use of different extruder screws with different thread shapes has been proposed with the aim of eliminating this kind of problem, the screws being changed according to the type of plastics material to be extruded. However, this solution is costly, because it is necessary to provide a certain number of different extruder screws for each type of plastics material to be extruded, and the construction of the extruder screws is complicated and expensive; furthermore, this solution requires a certain amount of time and machine stoppage for changing the extruder screws. Attempts have therefore been made to find other solutions, for example by producing extruder screws having special shapes, enabling a single screw to process different types of plastics material. Screws of this type have, for example, areas of composite shape such that they blend and disperse the fillers and particles contained in the plastics material being extruded.

US 2 680 879 describes a machine for mixing plastics material, provided with a rotor having a plurality of helical blades or threads. The threads define one or more channels within which the plastics material is made to advance, and the threads are also provided with oblique grooves which enable a certain amount of plastics material to pass from one compartment to another. These grooves are arranged to form a spiral with a pitch different from that of the helical blades or threads, and said grooves therefore intersect the thread obliquely, at an acute angle.

EP-1 028 839 B1 describes a single screw machine for extruding plastics material, comprising a barrel in which the extruder screw rotates. The screw is provided with a central shaft or core and with at least one thread which includes a front pushing face and a rear face. The profile of each front pushing face acts jointly with the inner surface of the barrel and has progressively narrowing passages on its periphery, through which the plastics material passes, such that any agglomerations are broken down and blended uniformly in said plastics material.

In these known solutions, because of the perimetric and open arrangement of said recirculating grooves or passages, the plastics material is subjected to high elongation and shear stresses, as a result which it is not always possible to process materials having different densities or viscosities within the limits of the chemical and physical characteristics of the various components of the mixture of plastics material to be extruded.

The object of the present invention is to provide an improved type of extruder screw for a machine for extruding plasties material, this screw being capable of blending and advancing the plastics material in an optimal way along the body of an extruder, without encountering said drawbacks of the known art.

Another object of the invention is to provide an extruder screw of the single type which can be used for a wide range of plastics materials with or without additives, and which provides optimal blending and dispersion of the additives in various types of plastics material in all cases.

Yet another object of the invention is to provide an extruder screw which is simple and inexpensive to produce and which is also highly reliable in technical terms.

These and other objects are achieved by the present invention with an extruder screw of the single type for an extrusion machine for plastics material as claimed in the attached Claim 1 and in the subsequent dependent claims, in which the machine comprises a barrel having an inner surface within which the screw rotates, said extruder screw comprising a primary thread forming a primary compartment and a secondary thread forming a secondary compartment, said secondary thread having a ridge positioned at a certain distance from the inner surface of said barrel and forming a recirculation passage for the plastics material and being characterized in that said secondary thread is provided with supplementary transfer means contained entirely within the body of this thread, through which the plastics material can flow in countercurrent mode from one face to the other of the secondary thread, in order to achieve the desired objects of homogeneous blending of the plastics material with the additives and retention of the material in the body of the extruder to undergo the necessary heating, even if the length of the extrusion machine is limited.

Further objects and advantages of the present invention will be made clear by the following detailed description of some preferred embodiments thereof, illustrated, solely by way of non-limiting example, in the figures of the attached sheet of drawings, in which

Figure 1 is a partial view in longitudinal section of an extrusion machine, showing the mixing and dispersion area of the screw according to the invention;

Figure 2 shows the extrusion machine on a larger scale and in partial section taken along the line M-Il of Figure 1 ;

Figure 3 shows enlargements of other details in a view taken along the section line Ill-Ill of Figure 1.

In the drawings, the number 1 indicates schematically part of an extrusion machine for plastics material, comprising a barrel 2 heated and temperature-controlled by suitable known means (not shown), within which an extruder screw 3 for extruding plastics material rotates. Using known arrangements, which are not shown in detail, the machine 1 comprises a single extruder screw 3, which in turn comprises an initial feed section to which is supplied, through an initial feed inlet of the barrel 2 (not shown), the plastics material in the form of pellets or granules, with any necessary additives, all these substances being fed into a suitable feed hopper which is of a known type and therefore is not shown.

The extruder screw 3 also comprises an intermediate section which serves to compress and amalgamate all the components, including fillers, pigments and any other particles which may be present in the paste of plastics material, and to advance this material along the barrel 2 at a speed and with a contact time such that the material receives the necessary heat for fluidization from the barrel 2, even if the length of the screw 3 and of the whole extrusion machine is limited.

The screw 3 also comprises a final section which essentially advances the melted and blended plastics material towards the outlet aperture of the machine 1 , which extrudes the material through a die or other similar device, which is not shown since it is known and is not essential to the understanding of the invention. Figure 1 shows only the intermediate section of the screw 3, which is the section designed to advance, blend and disperse the components of the plastics material with any additives, and is also designed to retain the material in the body of the barrel 2 for the time required to heat and plasticize the material to be extruded. As shown in Figure 1 , the plastics material is advanced by the extruder screw 3 from the right to the left as shown in this drawing, in the direction of the arrow F.

The extruder screw 3 comprises a primary thread 4, having a ridge 7 which, in combination with the inner lateral surface 8 of the barrel 2 and the core of the screw 3, forms a primary compartment 10, and as a result of the rotation of the screw 3 this thread 4 produces an initial blending of the plastics material and advances it in the direction of the arrow F along the barrel 2 so that it receives from the barrel the heat required to develop the specified fluidity. In known solutions, the thread 4 can have different diameters from the start to the end of the screw 3, these diameters usually increasing in size so as to accentuate the blending function initially and essentially provide the transport function thereafter. In the final part, the distance of the ridge 7 of the primary thread 4 from the inner surface 8 of the barrel 2 is very small but still sufficient to allow the screw 3 to rotate freely in the barrel 2, without any substantial passage or transfer of plastics material between the ridge 7 and the inner surface 8 of said barrel 2.

Figure 2 also shows that, in order to promote the advance and the blending of the plastics material, the primary thread 4 which forms the primary compartment 10 has a first side with a flat face 12 forming the pushing side of the thread 4, this face 12 being connected to the base of the compartment 10 with a fillet having a small radius, while the opposite side has a rounded face 13 with a larger fillet radius. The flat face 12 promotes the pushing of the thread 4 against the plastics material, and consequently the advance of the plastics material, while the rounded face 13 promotes the recirculation and blending of the plastics material within the compartment 10. The intermediate section of the extruder screw 3, shown in Figure 1 , also has a secondary thread 5, also called the recirculation or transfer thread, which is initially in contact with a section of the spiral of the primary thread 4 and has the same screw direction as the latter, and which is spaced progressively farther apart from this primary thread in the longitudinal direction and then progressively approaches it again until it joins it in an area R.

In order to obtain this configuration, the secondary thread 5 can have a pitch which varies with respect to that of the primary thread 4, and/or the pitch of the primary thread can be made to vary with respect to that of the secondary thread. In any case, in order to achieve the objects in question, a variation of the pitch of the two threads with respect to each other is required in the section in which both threads are present. It should be noted that the area where the thread 5 starts is located on the right of the intermediate section of the screw 3 shown in Figure 1 , this area not being shown in Figure 1 since it is identical to the illustrated area R where the two threads

4 and 5 join.

The secondary thread 5 forms, in combination with the leading spiral of the primary thread 4, a secondary compartment 11 which initially has the maximum length of the primary compartment 10, and whose length decreases progressively until it reaches zero in said joining area R. The secondary thread 5 also has a leading surface with a flat face 14 forming the pushing side of this thread 5. Unlike the leading surface of the primary thread, the leading face 14 of the secondary thread 5 is connected to the core of the screw 3 with a suitable fillet 16. On the other hand, the rear surface of the secondary thread 5 has a well-rounded face 15, similar to the face 13 of the primary thread. In this case also, the flat leading face 14 promotes the pushing of the thread

5 against the plastics material, while the rounded rear face 15 promotes the recirculation and blending of the plastics material within the primary compartment 10. The secondary thread 5 moves away from the primary thread 4 in the longitudinal direction and then approaches it again, thus running across the whole of the longitudinal extension of the primary compartment 10, and forcing the plastics material being extruded to pass from one side of the secondary thread 5 to the other, and therefore to pass from the secondary compartment 11 , which is upstream, to the primary compartment 10, which is downstream. For this purpose, the secondary thread 5 has a ridge 9 positioned at a suitable distance "d" from the inner surface 8 of the barrel 2 (Fig. 2). This distance "d" can be fixed throughout the length of the composite intermediate section of the screw 3, or can be variable. Clearly, owing to the presence of the secondary thread 5, the plastics material is subjected to efficient blending which ensures the homogeneous dispersion of any additives in it, ensures a distributed laminar contact of the plastics material with the hot inner surface 8 of the barrel 2, and retards the passage of the plastics material through the extrusion machine, in order to achieve the necessary blending and heating of the material by the machine, even if the length of the machine is limited.

In order to improve the operation of the extrusion machine to suit the range of plastics materials to be processed, it is possible to vary the aforesaid parameters of the extruder screw 3, especially the following parameters: - The length of the intermediate section of the screw which participates in the heating, blending and dispersion of the plastics material.

The depth of the primary compartment 10 and the fillet radius of the rounded face 13.

The depth of the secondary compartment 11 and the radius of the leading fillet 16 and of the rounded rear face 14 of the secondary thread 5.

The length of the section of the screw 3 in which the spacing and joining of the threads 4 and 5 take place owing to the relative variation of pitch between the primary thread 4 and the secondary thread 5.

The diameter of the secondary thread 5 and consequently the distance "d" between the ridge 9 of this thread and the inner surface 8 of the drum 2. It should be noted that, in order to vary the depth of the compartments 10 and 11 formed by the threads 4 and 5, the diameter of the core of the extruder screw 3 can be varied; in other words, the core of the screw can be partly cylindrical and partly conical, or cylindrical or conical throughout but with varying diameters.

An important feature of the present invention is that the secondary thread 5 has further transfer means 6, formed entirely within the body of the secondary thread 5, through which more plastics material can flow from one side of the thread 5 to the other, and therefore from the secondary compartment 11 to the primary compartment 10. These transfer means comprise at least one transfer duct 6 formed in the body of the secondary thread 5, and, in the version shown in the drawings to which reference is made, the thread 5 comprises a plurality of these transfer ducts 6 placed at predetermined angular intervals which are variable or equal. In the example shown in Figure 3, the thread 5 comprises four transfer ducts 6 placed at angular intervals of ninety degrees, although it is to be understood that the ducts 6 can be present in any number and arrangement, which can vary along the longitudinal extension of the thread 5.

As shown in Figures 2 and 3, the transfer ducts 6 are tapered, with a larger cross section facing the secondary compartment 11 and a smaller cross section facing the primary compartment 10. This configuration is based on the fact that different pressures are present in the compartments 10 and 11 , and therefore the taper promotes correct filling and blending in the primary compartment 10, since the flow of the plastics material being transferred increases in speed.

The positions of the outlet apertures of the transfer ducts 6, close to the base of the primary compartment 10, promotes the complete filling of the primary compartment and the blending of the material with that which arrives from the ridge of the secondary thread 5, and also prevents the settling of this material next to the core of the screw 3. The diameter and the taper of the ducts 6 are determined so as to allow the plasties material to pass in a satisfactory way from one compartment to another; for guidance, the value of this taper can vary in accordance with the diameter of the screw and the inside diameter of the barrel, and as a function of the constructional features of these components, and can easily be determined experimentally.

An important feature of the transfer ducts 6 is that the axes of these ducts are perpendicular to the surface of the flat face 14 of the secondary thread, on which is located the larger-diameter inlet aperture of the duct. Thus the pressure exerted by the plastics material on the flat face 14 is uniformly distributed over the perimeter of the inlet of the ducts 6, preventing the sedimentation or settling of plastics material in the secondary compartment 11. However, it is to be understood that the ducts 6 can be orientated differently, for example in such a way that their axes converge towards the downstream primary compartment 10.

In a non-limiting example, given a configuration with four ducts 6 placed at 90 degrees and with a screw 3 having an outside diameter of approximately 80 mm, the ducts 6 can have a head diameter of approximately 4 mm and a tail diameter of approximately 2.5 mm. By decreasing the number of ducts 6, for example by using three ducts 6 at 120 degrees to each other, it is possible to produce ducts with a larger mean diameter; conversely, by increasing the number of ducts 6, it is possible to decrease their mean diameter, thus still providing approximately the same flow cross section through the ducts 6. The ducts 6 can be made, for example, by electroerosion, which is a rapid, precise and economical method.

The extruder screw according to the present invention can be used to process a wide range of plastics materials while still achieving good blending and dispersion, even at a low rotation speed of the screw 3. The system is self-balancing as regards the pressures applied, and reduces the stresses on the processed material and the corresponding overheating of this material. It is therefore possible to process materials having different densities and viscosities, subject to the chemical and physical characteristics of the components of these materials.

Said self-balancing of the pressures makes it possible to prevent localized overheating of the material due to localized pressure increases. When processing plastics material with self-extinguishing particles, this being a material that must never be allowed to reach excessively high temperatures which would degrade it, it is necessary at present to cool the extruder screw for certain types of plastics materials, entailing considerable complications in terms of construction. However, with the extruder screw according to the present invention it is possible to dispense with this cooling system, because of the presence of said supplementary transfer ducts, which enable the overheating of the plastics material to be prevented. In Figures 2 and 3, the screw is shown with an axial hole 17 which facilitates heat dissipation and which can be used in any way which promotes the conditioning and/or the temperature control of the screw.

Claims

1. Extruder screw (3), of the single type, for a machine (1 ) for extruding plastics material, said machine (1 ) comprising a barrel (2) heated and temperature-controlled by suitable means and having an inner surface (8) inside which rotates said screw (3) which comprises a primary thread (4), forming a primary compartment (10), and comprises a secondary thread (5), forming a secondary compartment (11 ) and having a ridge (9) positioned at a certain distance (d) from the inner surface (8) of said barrel (2) to form a transfer passage for the plastics material, characterized in that said secondary thread (5) comprises supplementary transfer means (6) formed entirely within the body of said secondary thread (5), through which more plastics material can flow from one face (14) to the other (13) of the secondary thread (5).

2. Screw according to Claim 1 , in which said supplementary transfer means comprise at least one transfer duct (6) formed entirely within the body of the secondary thread (5).

3. Screw according to Claim 2, in which said at least one transfer duct (6) is a tapered duct which has a larger cross section facing the secondary compartment (11 ) and a smaller cross section facing the primary compartment (10).

4. Screw according to Claim 2, in which said at least one transfer duct (6) is a tapered duct with its degree of taper correlated with the diameter of the screw (3), with the inside diameter of the barrel (2) and with the constructional characteristics of these components.

5. Screw according to claim 1 , in which said supplementary transfer means comprise a plurality of transfer ducts (6).

6. Screw according to Claim 5, in which the transfer ducts (6) are arranged on the circumference of the secondary thread (5) at equal angular intervals.

7. Screw according to Claim 5, in which the transfer ducts (6) are arranged on the circumference of the secondary thread (5) at equal angular intervals.

8. Screw according to claim 1 , in which each of the transfer ducts (6) has an outlet close to the base of the primary compartment (10).

9. Screw according to Claim 3, in which the transfer ducts (6) have their axes perpendicular to the surface of the flat face (14) on which the inlets of the ducts are located.

10. Screw according to Claim 3, in which the axes of the transfer ducts (6) have an inclination such that the material passing through them converges towards the core of the screw (3).

11. Screw according to claim 1 , in which the primary compartment (10) has a first side with a flat face (12) forming the pushing side of the primary thread (4), this face (12) being connected to the base of the compartment (10) with a fillet having a small radius, while the opposite side of the compartment (10) has a rounded face (13) with a large fillet radius.

12. Screw according to claim 1 , in which the secondary compartment (11 ) has a first side with a flat face (14) forming the pushing side of the thread (5), this face (14) being connected to the base of the compartment (11 ) with a fillet having a small radius (16), while the opposite side of the compartment (11 ) has a rounded face (15) with a large fillet radius.

13. Screw according to claim 1 , in which said secondary thread (5) has the same screw direction as the primary thread (4).

14. Screw according to Claim 13, in which the secondary thread is initially in contact with a section of the spiral of the primary thread (4) and one and/or the other of the two threads has a pitch which varies in such a way that the secondary thread runs across the whole longitudinal extension of the primary compartment (10) and terminates in contact again with a section of the spiral of said primary thread (4) in a specified area (R).