JPS5949106B2 - Pressure control method for low pressure casting equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-pressure casting apparatus, and particularly to a pressurization control method for controlling the rising rate of hot water.

Conventionally, castings manufactured by low-pressure casting methods usually weigh several kilograms.
Since it is a small item of about 2 out of 10 parts, the casting time is short, and the pressurization speed and casting speed can be considered to be almost the same. Satisfactory castings could have been obtained as long as the casting speed was not turbulent.

It was getting worse.

Here, ■ is the molten metal velocity at the minimum cross-section of the sprue, ■ is the volume of the sprue and the product part, a is the minimum area of the sprue, and t is the time.

For this reason, in the past, in order to prevent splash or foaming on the surface of the molten metal caused by the impact of the air injected into the furnace, it was necessary to pressurize at a constant rate, or to increase the pressure during the primary pressurization at the initial stage of pouring. Pressurization control was carried out to draw a simple pressure one-hour curve, in which the pressure was applied relatively slowly and the secondary pressurization was made fast.

However, when casting a casting that is significantly heavier than conventional castings, the casting time becomes longer, 20 to 40 seconds, and the conventional pressure control method based on the casting speed reduces the amount of molten aluminum in the mold. The drawback is that the molten aluminum surface remains in stagnant contact with the air for a longer period of time, which increases the generation of oxides and the entrainment of oxides into the casting, which increases the number of defects.

This is because when the pressurization rate is held constant, the volume per unit height of the casting increases at a certain position of the product where the surface area of the molten metal increases, and the supply of molten aluminum corresponding to this volume increase increases in volume. This is because it cannot follow the speed.

In view of these points, it is an object of the present invention to provide a pressurization control method for casting castings with few defects. It is constructed by changing the pressurization speed.

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

In FIG. 1, a holding furnace 1 for storing molten metal has a lid 2.
The lid 2 is closed with a gas hole 3.

Further, the upper end of a stalk 4 is fixed to the center of the lid 2, the lower end of this stalk 4 is immersed in the molten metal, and a mold 5 is placed on the lid 2.

As shown in the figure, a mold space 6 having an octagonal cross section is formed in the mold 5, and the points at which the average horizontal cross-sectional area change rate changes from the top end of the space 6 are indicated by A.

Assuming points B, C, and D, the horizontal cross-sectional area change rate increases between DC, remains unchanged between CB, and decreases between BA.

This state can be expressed in a graph as shown in FIG. 2, where the vertical axis shows the position in the height direction of the space 6, and the horizontal axis shows the average horizontal cross-sectional area change rate.

Note that the space 6 in Figure 1 and the graph in Figure 2 are schematic, and since the shape of an actual casting is complex, the horizontal cross-sectional area change rate between DC and BA is as shown in the figure. It is not always constant and may increase or decrease.

Now, the average horizontal cross-sectional area change rate between DC (hereinafter referred to as change rate) is a, the change rate between CB is S, the change rate between BA is C, and the optimal pressurization speed between CB is X.
shall be.

Note that the value of the rate of change between CBs does not substantially change even if the horizontal cross-sectional area increases in height, so it is 1 or a value close to 1, and this is taken as the reference rate of change.

Here, the pressurization speed between CD and BA is a/b, respectively.
-X will be affected.

First, the pressurization speed between CDs will be explained.

If the average rate of change a between the CDs is extremely large when casting, especially when casting aluminum that tends to generate oxides, a ripple phenomenon will occur on the surface of the molten aluminum, making it easy to generate oxides. If the horizontal cross-sectional area decreases in the middle (not shown in the figure), it is necessary to correct the pressurization rate because oxides will not be generated there even if the pressurization rate is not changed. An intermediate value between 1 and 10 is selected as the correction value.

Next, the pressurizing speed between BA will be explained.

The rate of change between BA is extremely small and becomes a change rate ratio, solidification of poured molten aluminum progresses, and the value needs to be 0.1 or more.

Furthermore, if there is a part where the rate of change increases in the middle between BAs (not shown), there will be no entrainment of oxides even if the local change in that part is not changed, and therefore there is no need to correct the pressurization rate. do not have.

That is, select an intermediate value between .

Note that even if the pressurization speed is corrected in this way, the effect will not be sufficiently apparent if the cross-sectional area of the stalk and the diameter of the air piping are small, so the correction value for the pressurization speed is based on the cross-sectional area of the stalk. , shall be determined taking into consideration the diameter of the air piping, and correction of the pressurization speed can be easily performed if the pressurization control is program controlled, and other pressurization control methods, such as electromagnetic valves and pressure reducing valves, can be corrected. This can also be done by a combination method or a method using an electromagnetic pump.

As mentioned above, using the optimal pressurizing speed X in a portion where the rate of change is approximately 1 (between CB in the figure) as a reference, the pressurizing speed in other portions is expressed as the rate of change ratio ( The ratio of the rate of change in that part to the reference rate of change) is set to a value multiplied by the rate of change, and the rate of change ratio of the part where the rate of change is greater than 1 (between the enlarged parts DC) is set to the intermediate value between 1 and 10. ,
If the change rate ratio of the part where the change rate is smaller than 1 (between the reduced part BA) is set to an intermediate value between 0.1 and 1, even if the casting becomes larger, oxides will be generated. It is possible to prevent the entrainment phenomenon.

As shown in the graph showing the relationship between time and molten metal surface rise in Figure 3, the molten metal surface rise according to the present invention is like the solid line graph X in the figure, and the molten metal surface rises at a constant speed, creating ripples. This is because the phenomenon or the cause of oxide generation is removed.

In contrast, the rise in conventional large castings is indicated by the dotted line y in the figure.
As shown in the figure, since the rising speed differs depending on each part, the generation of oxides and the ripple phenomenon cannot be prevented.

As explained above, in the present invention, the pressurization rate of the gas body to be pressurized is changed according to the average rate of change of the horizontal cross-sectional area of the mold space, so that even if the casting becomes large, it can be maintained at an optimal constant rate. The surface of the molten metal can be raised at a high speed, and a good casting without casting defects can be cast.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of the low-pressure casting apparatus, FIG. 2 is a graph showing the average horizontal cross-sectional area change rate of each part of the mold space in FIG. 1, and FIG. 3 is a graph of the rising state of the molten metal surface. 1... Holding furnace, 2... Lid, 3...
...Gas hole, 4...Stoke, 5...
Mold, 6...Mold space.

Claims (1)

[Claims] 1. A pressurization control method for a low-pressure casting apparatus, characterized in that the pressurization speed of a molten metal pressurized gas body is changed in accordance with the average rate of change of the horizontal cross-sectional area of a mold space.