JPH0441066A - Method for controlling die temperature - Google Patents

Abstract

PURPOSE:To decrease the internal defects arising from solidification down to the extremity by comparing and computing a preset die cooling curve and an actual die temp. and supplying a cooling medium to the prescribed position of the die while continuously controlling the flow rate thereof by PID control. CONSTITUTION:The preset die cooling curve and the actual die temp. are compared and computed from the final stage of packing the molten metal into the cavity 2 in the die 1 or after the completion of the packing at the time of executing the die casting method to pack the molten metal into the cavity 2. While the flow rate of the cooling medium, such as water or air, is controlled by PID control continuously in accordance with the results of the computation, the cooling medium is supplied to the prescribed position of the die 1. The fluctuation in the temp. change is eliminated according to this temp. control and the die temp. change approximated as far as possible to the optimum cooling curve is reproduced according to this temp. control. The external and internal defects arising from the solidification are decreased and the casting having extremely good quality is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a mold temperature control method, and more specifically, it is possible to obtain high quality cast products by always keeping the cooling curve of the mold constant. This invention relates to a mold temperature control method in a mold casting method.

[Conventional technology]

Casting methods for obtaining cast products using molds can be broadly classified into gravity mold casting, low-pressure casting, and high-pressure casting.

To briefly explain these, gravity mold casting method is
The molten metal is allowed to flow into the cavity in the mold by gravity to fill it. In the low pressure casting method, the molten metal is held in a closed container and pressurizes the surface of the metal melt field with gas pressure, thereby causing molten metal to flow into the mold cavity and fill it.

In addition, high-pressure casting methods include those that fill the mold cavity with molten metal at high speed and high pressure (die casting method), and those that fill the cavity with molten metal at low speed and then apply high pressure to the molten metal (molten metal forging method). . The selection of each of these casting methods is made in consideration of productivity, quality, cost, alloy material, etc.

In each of the above casting methods, the alloy is solidified and cooled by heat extraction from the mold to form a casting, but in the casting cycle of closing the mold, pouring molten metal, solidifying, and opening the mold, natural cooling and forced cooling are used. Either of these are adopted.

FIG. 8 shows the mold temperature change due to the natural cooling method, and FIG. 9 shows the mold temperature change due to the forced cooling method.

The forced cooling method in this case is an example in which cooling is started from the final stage of pouring. It is assumed that the vertical and horizontal scales of both figures are the same. Further, each symbol in the figure is as follows.

t0 to t1 - Casting cycle time To: Mold temperature at the start of pouring Tmax: Maximum mold temperature ΔT - Final mold temperature variation As can be seen from both figures, in the natural cooling method of Fig. 8, t0 to t
i (casting cycle) is long and is a slow cooling type. Therefore, in terms of casting quality, coarsening of the structure and internal defects in the wall thickness variation portion occur, making it difficult to obtain a sound casting.

On the other hand, in the forced cooling method shown in Fig. 9, to=tI is shortened,
It can also be seen that the maximum mold temperature Trnax is suppressed by starting forced cooling.

The forced cooling method is used not only to shorten the casting cycle but also to prevent internal defects (such as cavities) in the casting by ensuring directional solidification.

In other words, local forced cooling is performed in areas where the mold temperature is particularly high, areas where cooling is slow due to thick walls, or areas that require rapid cooling to increase strength.

Conventionally, these forced cooling methods have been carried out by setting an arbitrary time period in the casting cycle and introducing a fixed amount of water or air into the mold.

[Problem to be solved by the invention]

By the way, in the above-mentioned conventional forced cooling method for molds, in which a certain amount of water or air is passed through the mold,
Although it is true that the above effects can be achieved, the following problems remain.

That is, in both of the above figures, what is indicated by ΔT in the figures is the variation in mold temperature during the molten metal solidification process in the casting operation that is repeated many times. These variations in mold temperature depend on the mold temperature just before pouring, the pouring temperature, the pouring amount, the pouring speed, and the surrounding environment such as daily and seasonal changes in temperature. The cooling curve cannot always be constant. As shown in the figure, variations in mold temperature, that is, variations in temperature during solidification and contraction of the molten metal, directly result in variations in product quality. Furthermore, this variation in mold temperature cannot be solved even with the conventional forced cooling method described above, and the temperature of the cooling medium itself, such as water or air, is also affected by the surrounding environment, as shown in Fig. 9. As described above, the conventional forced cooling method may actually increase the mold temperature variation ΔT.

In other words, although the above-mentioned conventional forced cooling method can shorten the casting cycle and eliminate the harmful effects of slow cooling, it has not yet achieved homogenization of the cast product.

Therefore, as a means to solve such problems, a method disclosed in Japanese Patent Application Laid-Open No. 1-148449 entitled "Mold Temperature Control Method in Low Pressure Casting Method" has been attempted.

This mold temperature control method actually measures the temperature of the mold after filling the cavity in the mold with molten metal, and enables directional solidification of the molten metal based on this mold temperature and casting conditions. In this method, the amount of cooling water supplied to the mold is controlled by comparing the cooling curve of the mold set in advance, and the mold is cooled in accordance with the cooling curve.

In the mold temperature control method described above, the actual mold temperature is brought close to the preset reference mold temperature, so it is possible to prevent the above-mentioned mold temperature variations as much as possible even though it is a forced cooling method. .

However, in the mold temperature control method described above, the measured mold temperature is divided into stepwise temperature zones, and the flow rate is set in advance for each temperature zone. Since it is a method of Therefore, although it is certainly possible to control the actual mold temperature so that the temperature is within a predetermined temperature zone, it is not possible to control the temperature within each zone. Each temperature zone naturally has a certain width, and the impossibility of temperature control within the zone makes it difficult to realize directional solidification when taking into account the fine structure. , strict temperature control is not performed. That is, in casting, which requires very strict temperature control to achieve directional solidification, even the above temperature control method is still insufficient.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to maintain the mold temperature at the time of solidification of the molten metal with variations (always in a constant state of change), and to keep it in close agreement with the desired cooling curve set in advance. It is an object of the present invention to provide a mold temperature control method that can control the mold temperature to a constant state of change, thereby minimizing internal defects caused by solidification.

[Means to solve the problem]

The mold temperature control method according to the present invention, when performing a mold casting method in which a cavity in a mold is filled with molten metal, starts the temperature control method from the final stage of filling the molten metal into the cavity or at the time of completion of filling. A comparison calculation is made between the mold cooling curve and the actual mold temperature, and based on the calculation result, a cooling medium such as water or air is supplied to a predetermined position of the mold while its flow rate is continuously controlled by PID control. It is characterized by:

[Effect]

By comparing and calculating the preset mold cooling curve with the actual mold temperature, and supplying the cooling medium to a predetermined position of the mold while continuously controlling its flow rate using PID control, the mold temperature can be adjusted. It is possible to continuously change the mold cooling curve almost in agreement with the ideal mold cooling curve without being influenced by external factors.

At that time, the amount of water or air supplied for cooling is controlled by continuous PID control rather than so-called ON10FF control or stepwise control, so it closely approximates the preset mold cooling curve. The cooling curve can be reproduced.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which the present invention is applied to gravity die casting.

In the figure, numeral 1 is a mold, IA is an upper mold, IB is a lower mold, 2
is the cavity formed by the upper mold IA and the lower mold IB,
3 is a sprue for pouring molten metal into the cavity 2, and 4 is a riser formed in the upper part of the upper mold IA.

A conduit 5.6 for passing cooling water is formed at a predetermined position close to the cavity 2 in the mold l. As shown in FIG.
Water supplies 05a and 6a and drain ports 5b and 6b are formed, respectively, and one end of a water supply line 7.8 is connected to each of the water supply ports 5a and 6a. The other ends of the water supply lines 7.8 are both connected to a flow control device 10.

Said flow control device! 0 controls the amount of cooling water supplied to the water conduit 5.6. In the flow control device 10, reference numeral 11 is a main water supply line, and reference numeral I2 is a stop valve. The main water supply line 11 is connected to a first control system 13A connected to one of the water supply lines 7 and a second control system 13B connected to the other water supply line 8 after the stop valve 12. Branched out. Both control systems 13A and 13B each have a stop valve 1516
, control valves 19.20, and flow meters 21.22. The control valves 19, 20 are each actuated by a valve control device 17, 18.

Further, at a predetermined position close to the cavity 2 in the mold 1, that is, near the water passages 5 and 6, these water passages 5
．． Temperature sensors 23 and 24 are embedded on the cavity 2 side of 6, respectively. In the embodiment, these temperature sensors 23 and 24 are constructed from thermocouples. Additionally, these temperature sensors 23 and 24 are connected to the program temperature control device 2.
5.

The program temperature control device 25 receives the detection signal from the temperature sensor 23.24, performs predetermined arithmetic processing, and sends a control signal to the valve control device 17.18. This program temperature control device 25 includes:
The optimal cooling curve of the mold 1 when casting a predetermined alloy using the mold 1, that is, the relationship between elapsed time and mold temperature is input. This optimum cooling curve is a value determined in advance through previous experiments.

Next, a mold temperature control method according to the present invention will be explained based on the function of the mold 1 having the above structure and the temperature control device related thereto.

First, before carrying out the actual casting, we conducted a preliminary casting test.
(Preliminary experiment) to determine the mold cooling expansion curve that minimizes internal defects, that is, the optimal cooling curve. Here, the optimum cooling curve is determined as follows.

First, we create a casting material drawing that takes into account machining costs, casting plans, etc. from the casting product drawing, then decide on the mold division method and casting plan, then decide on the temperature measurement position and the cooling target position, and create the mold. do. Using this mold, we conducted a casting test while varying the cooling pattern and measuring the mold temperature.
We determine the exact optimal cooling curve by observing the appearance of the casting and inspecting its internal defects. Internal defect inspection is performed, for example, by transmission X-ray inspection, cross-sectional color check, or cross-sectional/microstructure observation.

Once the optimal cooling curve for the mold l has been determined through the preceding experiment, the optimal cooling curve, that is, the relationship between elapsed time and mold temperature, is input to the program temperature control device 25. Note that the elapsed time here is the elapsed time after the mold temperature reaches the set predetermined temperature.

Now, in actual casting, as shown in FIG. 3, the mold temperature rises from t0 at the start of pouring, and when it reaches T (time elapsed), cooling of the mold 1 is started.

The mold temperature T is set to appear at the final stage of filling the cavity 2 with molten metal or immediately after filling is completed.

The mold 1 is cooled by supplying cooling water from the main water supply line 11 to the first control system 13A and the second control system 13B, and further through both water supply lines 7.8 into both water conduits 5.6. This is done by passing.

In addition, when the cooling water is turned on, the mold temperature T is detected by the temperature sensor 23, 24, and based on the temperature T, the program temperature control device 25 is transferred to the valve control device 17.
．． It begins by sending a signal to 18 and opening control valves 19,20.

By the way, as mentioned above, the optimum cooling curve of the mold 1 when casting a predetermined alloy with the mold 1 is manually input into the program temperature control device 25. As soon as cooling is started as described above, the actual mold temperature is compared with this optimum cooling curve, and the following control operations are executed.

That is, the actual mold temperature (substantive mold temperature) is input into the program temperature control device 25, and the optimum cooling curve temperature is TM'.The difference between the actual mold temperature TM and the optimum cooling curve temperature TM' is TM-TM') is θ, water conduit 5
, 6, the flow rate Q is controlled by: Kp: proportional operation coefficient TI: integral time TD: differential time.

In practice, the flow rate Q is controlled by a control signal sent by the programmed temperature controller 25 to a valve controller 17 , 18 which drives the control valve 1920 . In other words, the flow rate Q is the actual mold temperature TM
Based on the comparison between and the optimum cooling curve temperature TM', PID
It is continuously controlled by (proportional-integral-derivative) control. Here, the PID control is performed by a PID control controller included in the program temperature control device 25.

Here, in the equation expressing the above PID control, the right side is Kpθ The term represents P action (proportional action).

Since this P operation causes a residual deviation (offset) between the target value and the measured value, it is necessary to manually reset this offset. Therefore, by adding the (1) operation (integral operation) to this and making the PI operation (proportional-integral operation), that is, (2) Kp(θ+1θdt) I, the above offset is automatically reset.

However, if this PI operation is a control process or has a large dead time, the control result is likely to be oscillatory.
By adding the D operation (differential operation) to this, that is, the PID operation, the transient instability of the PI operation described above is stabilized.

However, the PID control referred to in the present invention refers to the above-mentioned PI
control, PD control (proportional derivative control), and even simple P control.

This is because most common controllers have three types of operation: P action (proportional action), 1 action (integral action), and D action (differential action). This is because it is adjustable. In other words, the D operation is unnecessary in a process with a small delay, and operations such as reducing the I operation when the delay is large can be performed arbitrarily and easily depending on the characteristics of the process at that time. It is.

FIG. 4 shows changes in the cooling water ilQ controlled by the above conditions, corresponding to FIG. 3.

It can be seen from FIG. 4 that the amount of cooling water Q is continuously controlled, that is, the amount of cooling water Q changes continuously and smoothly.

When repeatedly casting, the mold temperature is controlled by To until pouring for the next casting is started, so pouring can always be started at a constant mold temperature.

FIG. 5 is a graph comparing the above-mentioned FIG. 3 with the previous FIGS. 8 and 9, and is shown on the same scale as these FIGS. 8 and 9. In this figure, diagram A represents the optimal cooling curve,
Further, the line aperture and the line aperture indicate the upper limit line and lower limit line of the actual mold temperature curve, respectively.

From this Fig. 5, it can be seen that the entire casting cycle is further shortened compared to the simple forced cooling method shown in Fig. 9, and the actual mold temperature TM (diagram opening to opening') is the optimum cooling curve. It can be seen that the curve almost coincides with the temperature TM' (diagram A).

Also, a series of casting cycles t. -Lo' at t0
Regarding the reproducibility of ~L1 time (time from the start of pouring until the mold temperature reaches T max) and t,~Lo' time (time from mold opening to the start of the next pouring), the former is Although there are slight variations in the latter due to variations in working time due to temperature, etc., there is almost no variation in the time from t1 to ts, that is, in the solidification process of the molten metal, and overall, the reproducibility of the mold temperature change pattern and the casting cycle is extremely good. I understand something.

Therefore, according to the mold temperature control method, at least the part of the molten metal in the mold l corresponding to the part where the water conduit 56 is provided always undergoes a cooling process that almost matches the optimum cooling curve, and therefore the highest quality can be achieved. Excellent cast products can be efficiently obtained, and cast products of the same quality can always be obtained regardless of the number of castings or the surrounding environment. In particular, in the present invention, the amount of cooling water to be supplied is adjusted to the optimum cooling curve temperature TM.
' In comparison with ', the actual mold temperature T
The curve taken by M is approximated by the optimum cooling curve temperature TM', and the reproducibility of the optimum cooling curve is extremely high.

In this embodiment, the water conduits 5 and 6 are installed at two locations as shown in the figure. Needless to say, it is possible to provide the same water conduits in other parts as necessary to ensure directional coagulation in the same way as described above.

FIG. 6 and FIG. 7 each schematically represent another example of the control state of the mold temperature controlled by the present invention.

The one shown in Figure 3 or Figure 5 above starts water cooling (
1. The mold is cooled along a predetermined curve after (time elapsed), and the mold temperature is maintained at a certain constant temperature (To in the above example) for a predetermined time before opening the mold (51 hours elapsed). On the other hand, the one shown in FIG. 6 is a method in which the prescribed cooling is carried out continuously until the mold opening time (11 hours). In addition, in the case of the one shown in FIG. 7, after the mold temperature reaches T, the mold temperature is maintained at T for a required period of time, and then water cooling is started to cool the mold along a predetermined curve. This is what I did. Of course, these curves also have their respective optimum cooling curve temperatures TM.
This is the result of control to reproduce .

In this way, the flow jIQ of the cooling water to be supplied is adjusted as described above.
If PID control is used, it is possible to almost faithfully reproduce the optimal cooling curve, no matter what kind of curve it draws.

Further, as described above, the present invention does not prevent the control of the cooling water amount Q based on the optimum cooling curve temperature by PI sub-control, FD sub-control, or only P control; Needless to say, differential control) is the most preferable method. However, even if the flow rate Q is controlled only by these PI sub-controls PD control and P control, since the flow jIQ is controlled continuously, it is better than when the flow 11Q is controlled in stages. It is clear that the actual mold temperature curve becomes closer to the optimal cooling temperature curve.

In addition, in performing repeated casting in the above-mentioned FIG. 3, the mold temperature is T. Since the temperature is controlled at , the mold temperature can always be set at To when the next pouring starts. As a result, it is possible to improve the reproducibility of mold temperature changes not only in the solidification process but also in the pouring process, and it is possible to further improve the quality of cast products, and to achieve homogenization in a short solidification time.

Furthermore, although the above embodiments show examples in which the present invention is applied to gravity mold casting, the present invention is applicable not only to gravity casting, but also to other mold casting methods, such as low-pressure casting or high-pressure casting. It is also possible to obtain the same effect as described above even in the case of die casting other than the gravity casting method.

Further, in the embodiment, only an example in which water is used as the cooling medium has been described, but air or other cooling medium may be used instead of water, and the same effect as described above can also be obtained in that case.

In addition, if the mold temperature of the entire mold I drops too much during the series of casting cycles t0 to to', the entire mold I may be tempered by well-known means, such as by embedding a sheath heater or the like in the mold I. However, even in that case, if the area where the headrace 5.6 is installed is optimally cooled by performing local cooling based on the above conditions, the required area (in this case, the headrace 5.6) can be cooled optimally. 6) can be ensured.

In addition, although the present invention was made for the purpose of metal casting, it can also be applied to resin molding using a metal mold, if necessary.

〔Effect of the invention〕

As explained above, when performing a mold casting method in which a cavity in a mold is filled with molten metal, the mold cooling method is set in advance at the final stage of filling the cavity with the molten metal or from the time of completion of filling. A comparison calculation is made between the curve and the actual mold temperature, and based on the calculation result, a cooling medium such as water or air is supplied to a predetermined position of the mold while its flow rate is continuously controlled by PID control. This is what I did.

According to the mold temperature control method according to the present invention, it is possible to eliminate variations in mold temperature changes in target parts, and to maintain a target preset mold cooling curve by continuously controlling the mold temperature, for example, the optimal mold cooling curve. It is possible to almost faithfully reproduce mold temperature changes that approximate the cooling curve as much as possible. Therefore, the molten metal undergoes the most desirable cooling process, thereby ensuring the best solidification conditions in each part of the casting, minimizing external and internal defects caused by solidification, and obtaining castings of extremely high quality. Furthermore, cast products of the same quality can always be obtained regardless of the number of castings or the surrounding environment. Also,
This makes it possible to ideally shorten solidification time, and particularly in metal casting, it is possible to improve mechanical properties such as strength and toughness due to grain refinement. These advantages can provide excellent effects such as mass production of cast products that require very strict temperature control at a high level.

[Brief explanation of the drawing]

Figure 1 is a schematic configuration diagram showing a mold and its temperature control means according to an embodiment of the present invention, Figure 2 is a top view of the mold, and Figure 3 is a line showing the relationship between the casting cycle and mold temperature. 4 is a diagram showing the relationship between the casting cycle and the amount of cooling water, FIG. 5 is a diagram showing the relationship between elapsed time and mold temperature, and FIG. FIG. 7 is a diagram showing another form of the mold temperature cooling curve controlled by the present invention, FIG. 8 is a diagram showing the relationship between elapsed time and mold temperature in the natural cooling method, and FIG. 9 is a diagram showing the relationship between elapsed time and mold temperature in the natural cooling method. is a diagram showing the relationship between elapsed time and mold temperature in a conventional forced cooling method. ...Mold, ...Cavity 1. 6...Hirrace, 0...Flow control device, 3.24...Temperature sensor 5...Program temperature control device.

Claims (1)

[Claims] When performing a mold casting method in which a cavity in a mold is filled with molten metal, the preset mold cooling curve and the actual mold temperature are compared at the final stage of filling the molten metal into the cavity or at the time of completion of filling. A mold temperature control method characterized in that a cooling medium such as water or air is supplied to a predetermined position of the mold based on the calculation result while its flow rate is continuously controlled by PID control. .