Metal founding – Process – With measuring – testing – inspecting – or condition determination
Reexamination Certificate
2002-01-28
2003-04-29
Dunn, Tom (Department: 1725)
Metal founding
Process
With measuring, testing, inspecting, or condition determination
C164S900000, C164S113000
Reexamination Certificate
active
06554057
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for process monitoring during diecasting or thixotropic moulding of metals. The invention also relates to a metal diecasting or thixotropic moulding device.
In the car industry in particular, increasingly high demands are made on tolerances and on the mechanical properties of diecastings and thixotropic mouldings. In order to meet these high quality demands, the most comprehensive degree of monitoring of the method parameters and their reproducibility are of great importance.
In order to monitor a diecasting or thixotropic moulding process, firstly the condition of the metal introduced into the casting chamber and secondly the parameters of the diecasting or thixotropic moulding process are decisive. In order to optimise the diecasting or thixotropic moulding process and to evaluate all parameters which are critical for process stability and reproducibility, as far as possible all parameters which can affect the process need to be covered.
A key factor for achievement of high reproducibility and process stability is the condition of the thixotropic metal rod and the diecasting alloy on introduction into the casting chamber, whereby the temperature of the thixotropic rod or the diecasting alloy represents a very important parameter.
In order to check and monitor the diecasting or thixotropic moulding process, temperature measurements may be undertaken in the alloy melt and inside the thixotropic metal rod during the heating process, whereby the temperature distribution may for example be determined using thermo-elements at various melt or rod positions (inside the rod and around the rod edge), Habitually the heating curves, i.e. temperature as a function of the heating time, relevant for the individual measuring positions, are determined.
Whereas for monitoring alloy melts for diecasting, in essence temperature measurement is used, measurement of the electrical heating energy applied during preheating constitutes a further option for monitoring the state of the rod during thixotropic moulding.
For monitoring a thixotropic moulding process, metallographic tests to determine the distribution of the liquid proportion can be undertaken on the thixotropic rod, for example by cutting the rod at various longitudinal positions across its longitudinal axis and determining the liquid proportion in the rod cross-section, for example as a function of the distance from the centre of the rod. The aim of such investigations is to optimise the heating curve in such a way that a predetermined liquid proportion is achieved as homogeneously as possible within the entire thixotropic rod in the shortest possible time. Calorimetric measurements can also be performed in order to determine the mean liquid proportion.
With respect to the parameters of the diecasting or thixotropic moulding process, the temperatures of the casting chamber, the sprue channels and the mould cavity are normally measured, and the pressure and humidity in the evacuated mould cavity are ascertained.
The formerly habitual determination of the parameters with respect to the diecasting or thixotropic moulding material and of the diecasting or thixotropic moulding process is complex and unsuitable for monitoring the diecasting or thixotropic moulding processes under production conditions.
SUMMARY OF THE INVENTION
The invention seeks to solve the problem of creating a method for monitoring the process during diecasting or thixotropic moulding of metals, with which the manufacture of diecastings or thixotropic mouldings can be reliably monitored under production conditions.
This problem is solved according to the invention in that the temporal development of the moulding pressure p(t) is measured and the time-related speed of the casting piston v(t) is determined, and the energy E(t) supplied by the casting piston as a function of the process time t, and the total energy E
tot
supplied by the casting piston during the diecasting or thixotropic moulding process, are calculated on the basis of the time-related development of the moulding pressure p(t) and the casting piston speed v(t), and the total energy E
tot
is used as a parameter for monitoring the diecasting or thixotropic moulding process.
The method according to the invention is especially suitable for diecasting or thixotropic moulding of aluminum alloys or magnesium alloys.
The method according to the invention is especially suitable for horizontal thixotropic moulding installations and horizontal diecasting plants, i.e. devices in which the casting chamber lies horizontal.
The method according to the invention is based on the fact that the total energy supplied through the casting piston represents an extremely relevant checking parameter for the entire diecasting or thixotropic moulding process. The method according to the invention to determine the energy supplied through the casting piston and the use in particular of the total energy value as a parameter for process monitoring is also known as the RTIM process (Real Time Injection Monitoring).
The preheating temperature and the corresponding temperature distribution in the metal rod, and the measure of the energy supplied through the casting piston in the case of thixotropic moulding, are especially relevant, since a specific liquid proportion which remains within a narrow variation range must be observed. For example, in the case of thixotropic moulding, it may be concluded from a high total energy supplied by the casting piston, that the viscosity of the thixotropic material is too low, which can be caused either by a too small liquid proportion or too low shearing forces during the thixotropic moulding process.
The method according to the invention permits better process stability, optimisation of the process parameters, improvement of product quality and a reduction in the reject rate.
The method according to the invention is used with particular preference for thixotropic moulding. Here, it serves in particular to determine the optimum liquid proportion of the thixotropic metal rod under production conditions. The optimum mean liquid proportion in the thixotropic metal rod in this case is 40-55% by weight. If the liquid proportion is too high, the thixoforming of thixotropic material takes place virtually under the same conditions as the diecasting of liquid metal alloys, so that for example the benefit of a low shrinkage of thixotropic material during cooling in the mould cavity is lost, or else the shearing of the oxide skin surrounding the thixotropic rod is made more difficult or impossible. Moreover, the dimensionally stable insertion into the casting chamber of a thixotropic rod with a high liquid proportion is difficult and in most cases is not reproducible.
A further important factor in the case of thixotropic moulding is the homogeneity of the thixotropic condition, i.e. the distribution of the liquid proportion over the length of the rod and the rod cross section, whereby this homogeneity is generally better, the slower the preheating process is undertaken; on the other hand, the shortest possible heating time is desired for economic reasons.
During the inventive activity it was found for thixotropic moulding, that by determining the total energy supplied through the casting piston to the thixotropic material during a thixotropic moulding process, the liquid proportion existing after the preheating and its homogeneity within the thixotropic rod can be indirectly monitored.
The method according to the invention is further suitable in particular for monitoring the preheating furnaces, i.e. by determining the total energy for each charge, i.e. for each complete diecasting or thixotropic moulding process, with thixotropic rods or diecasting material from a specific preheating furnace, it is possible to ascertain and monitor the regularity of this furnace. Furthermore, by determining the total energy for each charge with thixotropic rods or diecasting material from different preheating furnaces, the regularity of the heating power of the furnace
Arnold Grégoire
Bagnoud Christophe
Plata Mirsolaw
Alcan Technology & Management Ltd.
Dunn Tom
Lin I.-H.
Stoffel Klaus P.
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