EP1105237B1 - Method for process monitoring during die casting or thixoforming of metals - Google Patents

Abstract

The invention relates to a method for process monitoring during die casting or vacuum thixoforming of metals in a mold, wherein the time lapse of temperature (T) is continuously measured on at least one point of the system and the temperature progression of the system is calculated in real time using a program. The time lapse of the heat flow (W) is calculated from the temperature progression of the system and the time lapse of the energy (U) of the system and the solidification heat portion (UE) of the metal solidified in the mold are calculated from the heat flow. At a given time, the calculated values are used as monitoring parameters.

Description

The invention relates to a method for process monitoring in die casting or thixoforms of metals in a vacuum in a mold.

The automotive industry is placing increasing demands on tolerances and the mechanical properties of die cast and thixiform parts posed. To achieve these high quality requirements is one monitoring the process parameters and their parameters as completely as possible Reproducibility is of great importance. An essential factor that is direct the mechanical properties of a die casting or thixoforming manufactured part is determined, the solidification course of the metal in the Shape.

The invention has for its object a method of the aforementioned To create the way with which the production of die-cast and thixoform parts continuously and reliably monitored under production conditions can be.

The temporal course leads to the achievement of the object according to the invention the temperature is continuously measured at at least one point in the system and by means of a program the temperature curve of the system in Real time is calculated, and that from the temperature curve of the system the temporal course of the heat flow and from the heat flow the temporal Course of the energy of the system and the amount of heat of solidification of the in the shape of solidified metal is calculated, at a specified time calculated values can be used as characteristic values for monitoring.

The amount of heat per unit time between the metal to be cast and exchanging the mold halves determines the rate of solidification of the part produced by die casting or thixoforming. Since the Characteristics of this exchange directly the mechanical properties of the die-cast or thixiform part is the monitoring of Solidification of the metal in the form to maintain a high quality standard essential.

The detection of the amount of solidification heat dissipated via the mold enables it u.a. determine whether the solidification is completely within the shape takes place whether pre-solidification occurs or what liquid-solid ratio is present in a thixomaterial.

Most of the heat exchanged during solidification comes from the latent heat released during solidification. The The amount of latent heat depends on the liquid metal content Filling the mold cavity. The amount of dispensed over the mold halves latent heat in turn depends on the metal to be cast or on the alloy used and can further by the temperature of the mold or the mold halves, by the pressure exerted, by the piston speed as well as the thickness of the lubricant layer.

The heat exchange that occurs during the various phases of solidification takes place is calculated using a program. The calculations lie Based on temperature measurements on the mold, preferably the temperature measured in the mold wall and the temporal course of the temperature is calculated on the shaping surface of the mold. To do this Sensors used in the mold wall of the mold halves at a distance of, for example, 1 mm to the surface. The program takes into account inverse heat conduction and calculates the temperature in real time the shaping surface of the mold halves and the heat exchange between the solidifying metal and the shape. With those arranged in this way Temperature sensors can ensure the uniformity of the cooling process and the thermal equilibrium on the mold surface in the different successive phases of casting and cooling in Be monitored in real time. The sensors are therefore preferred in places arranged where the thermal equilibrium and solidification are good too are recorded.

A characteristic value for the amount of solidification heat dissipated at a specified time is preferably between about 20% and 100%, in particular between about 50% and 100% of the maximum amount of solidification heat.

In practice, it has proven to be useful as a characteristic value for Die casting the amount of solidification heat at a fixed time of 0.1 up to 2 s, preferably 0.3 to 0.8 s and in particular about 0.5 s.

As a further characteristic value, it can be used immediately before each shot for the mold surface calculated temperature can be used.

From the time course of the temperature, the time course of the Heat exchange coefficients can be calculated. The one at a particular Time calculated value for the heat exchange coefficient, e.g. the maximum values in the solidification or cooling phase, or also the entire curve, can be used as additional parameters.

From the time course of the energy of the system, the difference between the energy values at the beginning of the mold filling for successive Shots also serve as an additional characteristic.

The time course of the solidification length can be derived from the time course of the temperature be calculated. The solidification length is that of Mold surface understood from the measured thickness of the solidified metal. The The solidification length calculated at a specified time can be used as a further additional length Characteristic value can be used.

Other possible characteristic values are the minimum pressure that results from the measurement the time course of the pressure in the mold cavity is determined, and the minimal relative measured immediately before a shot in the mold cavity Humidity.

The calculated or measured characteristic values can be used for process monitoring are compared as actual values with corresponding target values, whereby it can be provided that in the event of an impermissibly large deviation of the actual values an alarm is triggered by the setpoints within a tolerance range and if the tolerance range of the die casting or thixoforming process is exceeded is interrupted.

The setpoint for the amount of solidification heat removed is, for example given as the mean with a standard deviation. The standard deviation can be set as the first tolerance limit, for example Exceeded by the actual value triggers an alarm.

Compliance with the characteristic values leads to a consistently high quality standard. Deviations of the actual values from the target values are in real time recorded so that appropriate corrective action can be taken quickly can be.

A particularly interesting field of application of the method lies in Die casting and thixoforming, in particular of aluminum and magnesium alloys, for example for the production of safety components for the Vehicle industry.

• Fig. 1 den zeitlichen Verlauf des Wärmeflusses;
• Fig. 2 den zeitlichen Temperaturverlauf während eines Druckgiesszyklus;
• Fig. 3 den zeitlichen Verlauf der Energie während eines Druckgiesszyklus;
• Fig. 4 den zeitlichen Verlauf der Erstarrungswärmemenge im Bereich A von Fig. 2;
• Fig. 5 den zeitlichen Verlauf der Temperatur bei der Abkühlung der Form im Bereich B von Fig. 2;
• Fig. 6 den zeitlichen Verlauf des Wärmeaustauschkoeffizienten bei der Abkühlung der Form im Bereich B von Fig. 2;
• Fig. 7 den zeitlichen Verlauf des Drucks im Formhohlraum;
• Fig. 8 die Erstarrungswärmemenge als Kennwert für drei verschiedene Aluminiumlegierungen bei zunehmender Schusszahl;
• Fig. 9 eine Druckgiessanlage mit Prozessüberwachung.

Further advantages, features and details of the invention result from the following description of test results using the example of die casting and with reference to the drawing; this shows schematically in

• 1 shows the time course of the heat flow;
• 2 shows the temperature profile over time during a die casting cycle;
• 3 shows the time course of the energy during a die casting cycle;
• 4 shows the time course of the amount of solidification heat in area A of FIG. 2;
• 5 shows the time course of the temperature when the mold is cooled in area B of FIG. 2;
• FIG. 6 shows the course over time of the heat exchange coefficient during the cooling of the mold in region B of FIG. 2;
• 7 shows the time course of the pressure in the mold cavity;
• 8 shows the amount of solidification heat as a characteristic value for three different aluminum alloys with increasing number of shots;
• 9 shows a die casting system with process monitoring.

t

Zeit

T_o

berechnete Temperatur an der formgebenden Oberfläche

T

gemessene Temperatur in der Formwand, 1 mm unter der Oberfläche

T_v

Oberflächentemperatur der Form unmittelbar vor dem Schuss

U

Energie

ΔU

Energiedifferenz zwischen Beginn der Formfüllung und nach Abkühlung der Form

U_E

Erstarrungswärmemenge

U_e1s,i

Soll- bzw. Istwert der Erstarrungswärmemenge bei der Zeit t₁ = 0.5 s

W

Wärmefluss

h

Wärmeaustauschkoeffizient

p

Druck im Formhohlraum

rH

relative Feuchtigkeit im Formhohlraum

n

Schusszahl

1 to 7 show the course over time of the parameters calculated by a program-controlled computer on the basis of the temperature and pressure measurements. It means:

t

time

T _o

calculated temperature on the forming surface

T

measured temperature in the mold wall, 1 mm below the surface

T _v

Surface temperature of the mold immediately before the shot

U

energy

.DELTA.U

Energy difference between the start of mold filling and after the mold has cooled

U _E

Solidification heat

U _{e1s, i}

Setpoint or actual value of the amount of solidification heat at time t ₁ = 0.5 s

W

heat flow

H

Heat transfer coefficient

p

Pressure in the mold cavity

rh

relative humidity in the mold cavity

n

number of shots

8 shows the result of a series of tests with three different aluminum alloys. 128 identical parts were cast on the same die casting machine, namely 79 parts made of alloy 1, 35 parts made of alloy 2 and 14 parts made of alloy 3. In the illustration, the setpoint for the amount of heat of solidification U removed at the time t ₁ = 0.5 s is shown _{E1s plotted} as the mean with the standard deviation. The standard deviation defines a first limit value, which includes a tolerance range R with a second limit value. The second limit delimits the tolerance range R from the error range S. If two successive actual values U _E1i for the _amount of solidification _{heat fall} within the tolerance range R, as is the case with _sections 76 to 79 (area X), an alarm is triggered and the corresponding correction is initiated. In the case of the shots in the area X - these show an excessively high value U _E1i - the surface temperature T _{v of} the mold immediately before the shot was about 30 ° C. lower than the average value of the previous shots. For shots 123 to 125 (area Y), the actual values U _E1i for the _amount of solidification heat dissipated were in the error area S. The reason was that the melt temperature was too low, which led to pre-solidification outside the mold and subsequently to a lower value for the mold dissipated heat of solidification led. In this case, an interruption in production and the implementation of corrective measures is advisable.

The examination results shown in FIG. 8 show that with the monitoring method according to the invention with regard to the solidification process a high quality standard can be achieved. deviations are displayed immediately online. The calculated values can, for example via a RS232 interface to a programmable machine, which controls the die casting machine. The data are checked, if necessary displayed and finally archived. Fall the calculated Values for the amount of solidification in the tolerance range R, see above an alarm can be triggered directly by the machine. With more deviating Values that fall within the range S can, for example, be automatic Production stop can be triggered.

Process monitoring can be carried out at various points in the mold halves Temperature sensors are arranged. The calculations are preferred carried out individually for the individual temperature sensors and also individually recorded as monitoring results. In this way it is possible to locate specific production problems on the mold. The recorded Monitoring results are conveniently archived and can later be used, for example, to prove the production quality of a particular Die-cast or thixiform part can be used.

Process monitoring is shown in the following description of FIG. 9 understandable.

A die casting system 10 has a filling chamber 12 with a filling chamber cavity 14 on. That from a furnace 18 via a feed line 20 for everyone Shot in the filling chamber cavity 14 filled with liquid metal a piston 16 via a sprue 22 from the filling chamber cavity 14 in one of a fixed mold half 24 and a movable mold half 26 formed mold cavity 28 shot.

The mold cavity 28 has one or more ventilation channels 30 which possibly combined into a collecting channel. In the fixed Mold half 24 is a control insert 32 with a control pin 34 arranged. The control bolt 34 has a locking head 36 for opening or closing the ventilation channel 30. The shift of the Control bolt 34 is carried out by means of an actuating cylinder 38. When this is done The mold is filled via the locking head 36 of the control bolt 34 the ventilation channel 30 is closed at the end of the mold cavity 28.

A vacuum line 40 connects to the control insert 32 Valves 42 connected to a vacuum container, not shown in the drawing is. Before the metal is shot into the mold cavity 28 this evacuates and the time course of the pressure in the mold cavity 28 over a pressure sensor 44 connected into the vacuum line 40 is measured.

Temperature sensors are located at different points in the two mold halves 24, 26 46 arranged. Not shown in the drawing is one with the Mold cavity related probe to measure relative Humidity.

The temperature sensors 46, the pressure sensor 44 and the one not shown Probe for measuring the relative humidity are connected to a program-controlled Computer 48 connected. This computer transfers the measured and calculated parameters to a data acquisition device 50 for monitoring and archiving. The triggering of an alarm or a production stop if tolerance values for individual or all characteristic values are exceeded takes place directly on the computer.

Claims (12)

1. Method of process monitoring during the die casting or thixoforming of metals in vacuo in a mould, characterised in that the variation of the temperature (T) with time is measured continuously at least at one point of the system and the changes in the temperature of the system are calculated in real time by means of a program and that the variation of the heat flow (W) with time is calculated from the changes in the temperature of the system and the variation of the energy (U) of the system and of the amount of solidification heat (U_E) of the metal solidified in the mould with time are calculated from the heat flow, calculated values being used as monitoring parameters at a specified time.
2. Method according to claim 1, characterised in that the temperature (T) in the mould wall is measured and the variation of the temperature (T_o) on the shaping surface of the mould with time is calculated.
3. Method according to claim 1 or claim 2, characterised in that a parameter for the amount of solidification heat (U_e1) removed at a specified time (t₁) during die casting is between 20 % and 100 %, preferably between 50 % and 100 % of the maximum amount of solidification heat (U_Emax).
4. Method according to one of claims 1 to 3, characterised in that the amount of solidification heat (U_E1) at a specified time (t₁) of 0.1 to 2 s, preferably 0.3 to 0.8 s and, in particular, approximately 0.5 s is calculated as a parameter during die casting.
5. Method according to one of claims 1 to 4, characterised in that the temperature (T_v) calculated for the mould surface immediately before each shot is used as a further parameter.
6. Method according to one of claims 1 to 5, characterised in that the variation of the coefficient of heat exchange (h) with time is calculated from the variation of the temperature (T) with time and the coefficient of heat exchange is used as a further parameter.
7. Method according to one of claims 1 to 6, characterised in that, from the variation of the energy (U) of the system with time, the difference (ΔU) between the energy values at the beginning of mould filling upon successive shots is used as a further parameter.
8. Method according to one of claims 1 to 7, characterised in that the variation of the solidification length with time is calculated from the variation of the temperature (T) with time and the solidification length calculated at a specified time is used as a further parameter.
9. Method according to one of claims 1 to 8, characterised in that the variation of the pressure (p) in the mould with time is measured and the minimum pressure (p_min) is used as a further parameter.
10. Method according to one of claims 1 to 9, characterised in that the minimum relative humidity (rH) in the mould immediately before a shot is measured and is used as a further parameter.
11. Method according to one of claims 1 to 10. characterised in that the parameters calculated or measured serve as actual values and are compared with corresponding setpoints.
12. Method according to claim 11, characterised in that an alarm is triggered if the actual values deviate in an impermissible manner from the setpoints within a tolerance range and the die-casting or thixoforming process is interrupted if the tolerance range is exceeded.

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