Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2001-08-16
2003-04-15
Dunn, Tom (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S900000
Reexamination Certificate
active
06546991
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for manufacturing semi-finished products and molded articles of metallic material incorporating an extruder for producing a metal flow and appliances connected thereafter for molding the semi-finished products and the molded articles.
BACKGROUND OF THE INVENTION
A device of this type for die-casting preforms is known from EP 0 080 787, wherein a metallic material having dendritic properties, a magnesium alloy for example, is converted into a thixotropic state in an extruder. In this state, the metallic material has a mud-like or pasty consistency and can be processed so as to form metallic molded articles in the molding appliances following the extruder.
From EP 0 080 787, it is known to use a die-casting unit connected after the extruder for the molding or shaping operations, or else to process this material directly in a conventional injection molding machine without previously processing the metallic material in an extruder. There however, processing in an injection molding machine is disregarded because of the superimposition of rotational and translational movements by the injection worm (reciprocating screw) and the increased problems of sealing arising thereby in comparison with the use of the extruder (merely rotational).
The process of converting the metallic material (e.g. the magnesium alloy) into a thixotropic mass in the extruder is effected, in the manner described in EP 0 080 787, by feeding the material in granular form into a pre-heated feed hopper, whereby the size of the granular particles is made such that they can be easily processed by the screw in the extruder. The heating of the granules is effected at a temperature which is close to or above the solidus temperature whereby the heating process may take place either prior to and/or in the extruder.
The metallic material is in any case subjected to further heating in the extruder by means of external heating devices that are effective via the screw cylinder, and also as a result of frictional heat (shear stress). Hereby, the heating process in the extruder is controlled in such a manner that the temperature of the metallic material will remain below its liquidus temperature.
Due to the maintenance of a temperature in the range between the solidus and the liquidus temperatures and also due to the shear stress, the effect achieved in the extruder is that the dendritic structures of the metallic material will be broken down and a solid-liquid metal alloy in a thixotropic state will emerge from the output of the extruder.
In this device for producing a solid-liquid thixotropic metal alloy which is known from EP 0 080 787, there is a feed channel in the form of a continuous helical channel between the flanks of the screw from the start of the screw up to the end thereof.
Basically, the underlying principle of the conveying process in an extruder is that the material being moved experiences friction against the cylinder walling of the extruder and glides over the so-called base of the screw. When processing metallic materials, the problem arising as a result of the high thermal conductivity is that there is a build up of a smelt film on the cylinder walling, said film being of very low viscosity and considerably reducing the friction between the material being moved and the cylinder walling thereby leading to a drastic reduction in the performance of the conveying process. Moreover, the mixing process also suffers to a considerable extent whereby a growing temperature gradient over the cross-section of the interior of the extrusion cylinder, which gradient increases from the exterior to the interior thereof, cannot be effectively dissipated.
As a consequence of these conditions, there arise inhomogeneities between the solid and liquid components and the stability of the conveying process becomes extremely unsatisfactory whilst the build up of pressure is highly erratic. Continuously altering process states thereby arise whereby the resultant non-reproducible quality of the components has to be accepted.
It is therefore desirable to improve the construction and functionality of a device of the type mentioned in such a manner that reproducible component-qualities can always be produced.
SUMMARY OF THE INVENTION
According to one aspect of the invention, an extruder for producing a flow of metal and appliances connected thereafter for shaping the semi-finished products and the molded articles includes a screw system consisting of two or more meshing screws.
In the case of the device in accordance with the invention, the processing of the metallic material, for example, starting from the granular state up to the thixotropic solid-liquid material or the liquid material states thereof, is effected in such a manner that, taken with reference to the axial length of the extruder, the processing steps will generally be consistent and the material will be continuously advanced. The negative consequences of fluctuations in temperature and the irregularities of viscosity inherent therein together with the proportional composition of the liquid material components are thereby reduced to a negligible amount.
It has been found that the previously described problem encountered in single screw extruders can be eliminated by means of an extruder including a screw system comprising two or more meshing screws, although the problems described above still have to be taken into account initially even with this type of extruder. Here however, there is a counteracting mechanism at work which, surprisingly, is of sufficient extent as to allow the material being conveyed to be transferred from one screw onto the adjacent meshing screw. It is evident thereby, that when processing metallic materials, an adequately large mixing and transportation effect is achieved which ensures continuous advancement of the metallic material being conveyed in addition to the dissipation of the temperature gradient. Stable feeding of fresh material into the inlet zone of the extruder has also been observed in addition to the increase in mixing performance.
This has a particularly positive effect when processing materials in granular or chip-like form since the bulk density thereof of typically approximately 0.5 to 0.8 g/cm
3
has to be doubled or brought up to the still higher densities of approximately 1.7 g/cm
3
or more of the solid-liquid stream of material. This is difficult if not impossible in the case of the reduced conveyor performance of an extruder.
Heating strips or heating devices functioning inductively are used conventionally for the purposes of introducing heat when processing metallic smelts.
However, inductive heating devices are very expensive. Classical heating strips are mounted around the periphery of the extruder cylinder engine and tend to become heavily oxidized at the high temperatures prevailing when processing metallic smelts, this leading to scaling of the cylinder surface and hence reducing thermal transfer between the heating body and the cylinder. Moreover, precautions have to be taken when using heating strips so as to retain them in continuous contact with the surface of the cylinder in order to achieve adequate thermal transfer.
Another disadvantage associated with heating strips is the large spacing between the heating strips mounted externally on the extruder cylinder and the smelt present in the interior of the cylinder in the face of the necessarily high heat flow densities and temperature gradients of up to 200° C. and more which occur in operation.
If the ratios involved even in the case of a single screw extruder are not particularly favorable, then the heat introduction ratio is still less favorable in the case of two and more screw extruders since the spacing between the outer cylinder surface and the inner walls thereof is inevitably increased here due to the geometrical considerations.
In accordance with the invention, so-called heating cartridges, which comprise resistance heating elements arranged in a usually cylindrical housing, are of assistance here.
Bürkle Erwin
Dworog Andreas
Wobbe Hans
Zimmet Rainer
Zwiesele Jochen
Dunn Tom
Feiereisen Henry M.
Krauss-Maffei Kunststofftechnik GmbH
Lin I.-H.
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