Metal founding – Process – With measuring – testing – inspecting – or condition determination
Reexamination Certificate
1999-07-26
2001-07-17
Nguyen, Nam (Department: 1722)
Metal founding
Process
With measuring, testing, inspecting, or condition determination
C164S454000, C164S483000
Reexamination Certificate
active
06260603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for automatic, vertical continuous casting of metals, in particular aluminium alloys, in a casting facility comprising several molds in which the liquid metal is fed from a furnace along a spout to the mold and guided, via nozzles with adjustable through-flow, to the mold which are initially closed off by dummy blocks arranged on a lowerable casting table. The metal through-flow volume of each nozzle is adjusted individually for each mold on the basis on an initial time and an initial metal level at which adjustment of the metal level begins, in such a way that the metal in all molds at a pre-set starting time is at substantially the same starting level at which the casting table starts to be lowered for the production of the metal strands.
2. Discussion of the Prior Art
Trouble-free operation of a multi-strand casting facility depends greatly on mastering the start-up phase of casting i.e. on achieving optimum control of the process of feeding metal to the individual casting units up to the actual start of casting which begins with the lowering of the casting table.
During the dummy-block/mold filling-phase it is normal to adjust the metal flow individually to each mold via the specific mold nozzles, the through-flow volume of which can be variably set by motor-driven plugging rods. At the same time, using known methods and means of measuring, the level of metal in each mold is continuously measured, compared with target values and the rate of metal flow into the mold adjusted via the difference between the target and actual values by corresponding control of the plugging-rod drive.
A means for adjusting the metal level in a multi-strand casting facility is known from GB-A-2 099 189 whereby, on reaching a first level of metal in one mold, a set-point value that increases linearly as a function of time is specified for a common second level for all molds. Adjustment of the metal level in the individual molds is made on the basis of the rising set-point value, as soon as this is reached in the individual molds.
The European EP-B-0 517 629 describes a method of the kind initially described above in which the individual adjustment of the metal level in each mold is made as soon as the inflowing metal has reached a prescribed minimum height above the dummy block. Starting from this minimum height above the dummy block, the adjustment of the metal level in each mold is made via individual, set-point curves comprising data points that increase linearly as a function of time, with the result that a predetermined, common level is reached simultaneously in all of the molds.
A significant disadvantage of the state-of-the-art method using different filling curves for each mold is the danger that the system can get out of control if one of the molds does not begin to fill or does so with delay as a result of premature solidification. Further, large differences in the kinetics of mold filling can occur on lowering the casting table if individual molds fill too late and consequently exhibit a steep filling curve towards the end of the filling process. Also, with different filling curves the algorithms become much more complex.
SUMMARY OF THE INVENTION
The object of the invention is therefore to provide a method by means of which the level of metal in the individual molds can be adjusted during the dummy-block/mold filling-phase in a simple manner and, in as short a time as possible, brought to a predetermined level for the start of the lowering of the casting table, this without danger of the metal freezing.
The object is achieved by way of the invention in that the metal level is adjusted in all molds simultaneously during filling, between the initial level at the initial time and the starting level at the starting time, and as a function of time in accordance with a set-point curve N=f(t) which is identical for all molds, whereby the slope dN/dt of the set-point curve is greater in a range starting from the initial level and smaller in a range approaching the starting level than the mean slope between the initial and the starting level.
By way of preference, the part of the slope of the set-point curve with the greater slope, starting from the initial level, extends over a range comprising approximately 10 to 70%, preferably 30 to 60%, of the whole increase in metal level. The part of the set-point curve with the smaller slope approaching the starting level extends preferably over a range comprising 10 to 40%, preferably 15 to 25%, of the whole increase in metal level.
In practice it has been found that control of sufficient accuracy can be achieved if the set-point curve is made up of 4 to 8, preferably 5 to 6, pairs of values.
A simple and practical manner of carrying out the method according to the invention is such that the adjusting of the metal level in all the molds begins simultaneously as soon as the initial level has been reached in one mold. Usefully, the lowering of the casting table with the dummy blocks also begins as soon as the starting level has been reached in one mold.
Also the level of metal in the spout is preferably kept constant from the start of filling the dummy blocks and molds, up to and including the stationary casting phase.
The feeding of metal to the moulds is preferably proportional-integral-derivative (PID) controlled on the basis of the set-point curve.
REFERENCES:
patent: 4567935 (1986-02-01), Takeda et al.
patent: 4730660 (1988-03-01), Tinnes et al.
patent: 5174361 (1992-12-01), Karsfeld
patent: 5409054 (1995-04-01), Moriceau
Caloz Etienne
Seppey Jean-Pierre
Vuignier Eric
Alusuisse Technology & Management Ltd.
Cohen & Pontani, Lieberman & Pavane
Lin I.-H.
Nguyen Nam
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