Screw extruder for continuous extrusion of materials with high viscosity

The present invention relates to a screw extruder for the continuous extrusion of materials with high viscosity, in particular metals such as aluminium and its alloys, the extruder including an Archimedes screw provided in a screw housing with an inlet for the feeding of the material to be extruded, a compacting or extrusion chamber and an extrusion die assembly with a die which forms the shape of the desired extruded product.

Extrusion of materials with high viscosity such as aluminium requires a considerably high pressure to force the aluminium through the die block and the die, typically 100 to 500MPa. The state of the art as regards aluminium extrusion is today and has been for the last 100 years, dominated by ram extrusion. Ram extrusion is a batch process in which a billet is loaded into a container and forced (pressed) through the die by means of a moving piston. One mentionable method for continuous extrusion of aluminium is the so-called CONFORM® process which is based on the use of a single revolving wheel as the driving force in an extrusion process and which enables the manufacture of products of unlimited length. In its simplest form the wheel has a single groove in its periphery which accepts the feedstock and transfers the material to the extrusion zone and die.

Another continuous screw extrusion process is used in the production of lead and lead alloy profiles which is based on the Robertson Hansson extruder, US 3693394. In this

process the lead is fed to the extruder in liquid state and solidifies during the extrusion process.

Lead behaves differently from aluminium since it has "sliding" friction, i.e. the friction between the lead and the container material (steel) is proportional to the pressure. Aluminium and most other metals have, however, sticking friction at the extrusion temperatures, i.e. the aluminium welds to the container and the screw material. As a result of this behaviour, screw extrusion of aluminium and other sticky metals with high viscosity have been difficult and non-practical due to the enormous forces required to overcome the frictional forces between e.g. aluminium and steel.

With the present invention is provided a new and improved screw extruder which is not encumbered with the above problems of sticking and which provides a consistent, continuous process for the extrusion of materials with high viscosity such as aluminium.

The extruder according to the invention is characterized by the features as defined in the accompanying independent claim 1.

Preferred embodiments of the invention are further defined in the attached dependent claims 2 - 5.

The invention will be further described in the following by way of example and with reference to the drawings where:

Fig. 1 shows schematically a cross sectional view of an extruder according to the present invention,

Fig. 2 shows a side view of the screw shown in Fig. 1 , further viewed in sections,

Fig. 3 shows a perspective view of the compression chamber of the extruder as ς shhnowwnn i inn F Fiing. 11.

When extruding viscous and sticking materials such as metals in a continuous process by means of screw extruders, an increasing pressure is produced due, to the dragging of the material which sticks to the liner and the screw surfaces along the extruder, reaching the pressure necessary to force the material through the die at the end of the housing. Unlike the typical screw extruder used in the polymer industry where the viscosity of the polymer is very much lower than in metals, the pressure zone has to be substantially reduced in order to avoid too high torsion forces on the extruder. This is achieved by a special design of the screw in combination with a extrusion chamber designed in order to restrict rotational flow of the metal. Also the temperature profile along the extruder has to be controlled. The total length of the screw is therefore also considerably shorter, typically 1.5-2.5 times the screw diameter.

The extruder. is preferably fed with granular metal at T<Ts, where Ts is the temperature at which the metal will have sticking friction. The granular metal may be heated in the extruder by contact with the screw and container wall, or it may be preheated to the desired temperature. As sticking friction occurs, the metal is further heated by frictional heat and deformation. As the semi compacted metal reaches the pressure generating zone, the metal sticks to an already compacted "wing" of metal and becomes kneaded and fully compacted.

The extruder according to the invention includes, as shown in Fig. 1 , a screw housing or container 4 with an inner liner 2 within which is rotationally provided an Archimedes screw 1with a helical tread or flight. The liner 2 is provided with cooling channels 3 to control the temperature under the extrusion process. The up-stream end of the container or housing 4 is provided with an inlet 11 for the supply of granular material to be extruded. At the outlet end of the container/liner, 2, 4, by the downstream end of the screw 1 , is provided a extrusion chamber 5, and further at end of this chamber is provided a die assembly 6 which form the shape the profile 7 to be extruded. The screw 1 is preferably stabilized by a thrust bearing assembly 8 fitted in a support plate 9 realisably mounted to the container 4. A screw shaft 10 connects the screw 1 to a drive unit through a suitable connection.

The design of the screw may be viewed in relation to three sections as shown in Fig. 2

- a feeding zone, A

- a transport zone, B and

- a pressure generating and compacting zone, C.

The screw is designed to ensure that the granules can be transported freely from the feeding zone A to the transport zone B until the material, by the building up of pressure at the end of the screw, stick to the metal in the pressure generating and compacting zone C. This can be done by reducing the screw core diameter from the feeding section to the pressure generating section in a linear, progressive or digressive manner, or by increasing the outer screw diameter or a combination of both.

In the feeding zone A, the material to be extruded is fed through the inlet 13 to the screw in the container/liner assembly 2, 3, either by the force of gravity or by other means. The material arrives at the open space between the screw and the liner and as the screw rotates, the material is transported further into the transport zone, B.

In the transport zone B the material are further transported and may, if not being preheated prior to feeding, be heated to T>Ts by contact between the liner wall and screw. Further, on entering the pressure generation zone C compression of the material take place, the material begin sticking (to each other, to liner and to screw surface) and are kneaded and are further heated, thus forming a plug of solid material towards the end of the screw before entering the extrusion chamber 5. Tests with different aluminium alloys proves that the required compaction necessary for extrusion takes place at the last part of the screw corresponding to up to 540°, or up to 1 , 5 turn of the screw flight length. However, some tests proved that required compaction took place at even shorter flight lengths, even below 360 ^α This represents, as indicated above, an important feature with the present invention. It is quite likely to believe that the reason why researchers formerly have not succeeded with the continuous extrusion of materials like aluminium, is that the design of the extruders they

have used, e.g. the long screws and compaction area, has caused redundant frictional work whereby more energy (much higher rotational momentum) would have to be supplied to the process. It is thus of outmost importance that the design of the screw and housing is such that the compaction takes place over a short distance (area) of the screw.

Further, it is necessary for the solid plug to stick to the housing (and the screw) in order for the forward dragging of the plug to occur. The sticking must be prevented from collapsing into a highly localized shear deformation zone adjacent to the liner wall surface with a resulting drastic reduction of the viscosity and a locally rapid increase in temperature.

Still further, it is proved by testing that the solid plug of metal in the extrusion chamber 5 must be restricted from rigid rotation to obtain the required compaction pressure. This can be done by designing the walls of the chamber with a shape or providing the surface of the chamber with a desired roughness, for instance with inwardly extending protrusions (not shown), such that rotation is avoided.

Fig. 2 shows an example of design where the front end of the extrusion chamber 5 has close to square cross section shape.

The compacted material contained in the pressure generating section of the screw is completely welded to the plug and liner wall, and is therefore also restricted from rotating. As the compacted material in the screw is restricted from rotating and the screw rotates, the rotational energy of the screw is transferred into a net forward flow of material into the extrusion chamber. As the volume of material fed into the screw per turn is smaller than the volume of the flight, part of the material flows back into the space between the flights from the extrusion chamber. This represents an important part of the working principle of the invention as it generates a high amount of shear deformation such that the material in the extrusion chamber becomes fully dense. The shear deformation breaks down the oxide layer on the granules making the "welds" between granules complete.

The shear deformation also aid in breaking down particles, such as iron containing particles in aluminium, thereby reducing them in size.

The extruder according to the present invention may be used for the extrusion of a number of materials with high viscosity such as:

All sticky metals (such as: light metal alloys, Al, Mg, Ti) Mechanical mixtures of metals Granular materials. Mixtures of metals and compounds.

Metal swarfs and fines, e.g. turnings.

The invention as defined in the claims is not restricted to the above examples. Thus, the screw and extrusion chamber may not include, heating in ,the transport zone B as described above, but in stead the material may be pre-heated before it is fed to the extruder. ■ <