JPS6319253B2 - - Google Patents

Description

[Detailed Description of the Invention] Conventionally, in the production of molds, inorganic or organic binders such as water glass or synthetic resins have been used to maintain the mold with a certain level of strength. If used, it would be impossible to reuse the sand and it would be uneconomical.

In addition, recently, the dumping of sand that cannot be reused has become a type of industrial waste pollution that requires countermeasures.

In response to these demands, a few years ago we began forming molds using foundry sand mixed with water and starch or bentonite, and injecting low-temperature liquefied gas (liquid nitrogen, liquefied carbon dioxide, liquid helium, etc.) into the molds. A mold manufacturing method was developed in which molds were then frozen (see JP-A-54-41229).

However, in this known method, as mentioned above, it is preferable to use starch or bentonite in addition to foundry sand. According to the above publication, it is stated that ``a relatively small amount of substance is mixed in that does not affect sand regeneration and can provide a preliminary bond strong enough to separate the fastened sand mold from the model.'' It goes without saying that it is highly preferable to use no binder other than water for sand reclamation.

The present invention focuses on these points and provides a method for manufacturing precise frozen molds using sand mixed with only water.

The method of the present invention is characterized in that molding sand mixed with 1 to 7% water is frozen in two stages: initial freezing and final freezing. More specifically, initial freezing is performed with the model placed in the sand mold, then the model is removed and final freezing is performed with the sand mold assembled.

Hereinafter, one embodiment of the method of the present invention will be described in detail.

A sand mold is made using No. 6 silica sand mixed with 4% water and a wooden mold, and the mold is initially frozen in a split mold state with the wooden mold inside. The initial freezing may be such that the surface of the sand mold in contact with the wooden mold is frozen and sand is not entrained when the wooden mold is peeled off. This initial freezing allows the wooden mold to be easily removed without using any binder other than water, and is particularly effective in the case of wooden molds with complex shapes.

Next, remove the wooden mold from the initially frozen sand mold,
If necessary, insert the core and assemble the sand mold, then
Perform final freezing. Finish freezing involves lowering the temperature to -100°C or lower, thereby producing frozen molds.

A mold frozen at such a low temperature still has sufficient strength even after several minutes have elapsed before pouring.

Traditionally, to produce frozen molds, the wooden mold was removed and a low-temperature liquefied gas (for example, liquid nitrogen) was injected into the sand mold to freeze the moisture in the sand. and have an unfavorable effect on cast products. That is, since the injected liquid nitrogen is impregnated into the sand, the impregnated liquid nitrogen continues to vaporize even after the injection is stopped. Therefore, after the mold is assembled, the inside of the cavity is filled with nitrogen gas and the pressure becomes higher than atmospheric pressure, making it difficult for hot water to enter during pouring. This phenomenon is particularly noticeable when the cavity has a complicated shape, and prevents hot water from entering every corner, making it impossible to obtain a product with an accurate desired shape. Furthermore, during pouring, even after pouring, liquid nitrogen continues to vaporize and is released into the cavity, resulting in the formation of air bubbles on the casting surface of the product, which impairs its appearance.

In order to eliminate the above-mentioned drawbacks, the present inventors decided to perform initial freezing and final freezing using cold gas obtained by vaporizing liquid nitrogen, without directly injecting liquid nitrogen into the sand mold. Specifically, an apparatus such as that disclosed in Japanese Patent Publication No. 50-33260 was used. That is, a porous material such as furnace bricks is placed on the ceiling of a typical liquid nitrogen injection type tunnel freezer, and liquid nitrogen is supplied to the material to impregnate it. When the sand mold to be frozen is transferred under the porous material from the inlet side to the outlet side using a transfer device such as a belt conveyor, the impregnated liquid nitrogen is absorbed by external heat (radiant heat from the sand mold). As it evaporates and releases cold nitrogen gas into the tunnel, the sand mold is frozen by this cold gas. Initial freezing and finishing freezing can be performed using separate tunnel freezers, or the tunnel in the middle of one tunnel freezer can be interrupted to provide a section for removing wooden molds and inserting cores. It can also be done continuously.

In this way, if freezing is performed with cold nitrogen gas without injecting liquid nitrogen into the sand mold, the air, moisture, etc. inside the cavity will condense during the final freezing process after removing the wooden mold and assembling the sand mold. The pressure inside the cavity is 100 to 150 mm of water column lower than atmospheric pressure when calculated from a standard mold test piece. As a result, in contrast to the liquid nitrogen injection method, it became easier to pour hot water and it became possible to spread the hot water to every detail. Also, during and after pouring,
Since no vaporized gas is released from the sand into the cavity, a product with a beautiful cast surface can be obtained.

In this example, the porous material is impregnated with liquid nitrogen, and the sand mold is frozen with the vaporized gas released from it, but there are other freezing methods that do not involve direct injection of liquid nitrogen (low-temperature liquefied gas). For example, there are the following methods.

(1) A cooling zone in which a horizontal partition plate is provided in the cooling zone of a tunnel freezer, a porous material is placed above the partition plate, impregnated with liquid nitrogen, and the partition plate is separated by the vaporized cold gas. This cools the air and freezes the sand molds transported thereby.

(See Figure 2 of Japanese Patent Application Laid-open No. 53-114548).

(2) In the same way as above, the cooling zone is divided by a partition plate, and liquid nitrogen is supplied to the upper part of the partition plate to form a liquid reservoir, which cools the air in the cooling section separated by the partition plate and is transferred. Freeze the sand mold.

(3) Place the sand mold in a metal case and freeze it by immersing it in a liquid nitrogen bath.

Among these, method (1) can be carried out in the same manner as in the above example, and is an effective method that consumes less liquid nitrogen (liquid nitrogen vaporizes in proportion to the heat entering from the outside). ). However, method (2) cannot create a temperature gradient within the tunnel and consumes relatively large amounts of liquid nitrogen. Method (3) also has disadvantages in that temperature control is difficult and it is unsuitable for continuous operation.

In the above embodiment, the final freezing was carried out with the wooden mold removed and the sand mold assembled, but this step can also be carried out with the split mold as it is by simply removing the wooden mold. However, in the latter case, it is inevitable that the degree of negative pressure inside the cavity will be smaller than in the former case.

If finishing freezing is performed with the wooden mold still in place, the mold release agent will also solidify, so the temperature cannot be lowered to a very low temperature, and a sufficiently strong frozen mold cannot be obtained.

The method of the present invention has the configuration as described above, and exhibits the following main effects and incidental effects.

(1) Freezing the sand mold is divided into initial freezing and finishing freezing, and the model is removed in the middle, so it is easy to release the mold and it is accurate even if it is a complex shape, without using any binder other than water. A mold that can be obtained.

(2) Since the water content to be mixed is small (1 to 7%, preferably 3 to 5%), there is almost no expansion due to freezing. Moreover, since the model is still inside during initial freezing, expansion can be further reduced.

(3) Sand grains contact each other in a state close to point contact;
Because the shape is such that ice forms at the contact area,
The mold has extremely good ventilation, allowing gas and water vapor generated during pouring to be easily released to the outside.

(4) Since the low-temperature liquefied gas is not injected directly, but is frozen by the vaporized cold gas, the pressure inside the mold cavity becomes negative, allowing the hot water to reach every detail of the cavity during pouring. .

(5) Since the sand is not impregnated with low-temperature liquefied gas and no vaporized gas is released into the cavity during or after pouring, a beautiful surface can be obtained without leaving bubble marks on the casting surface.

(6) Since there is no release of vaporized gas from the sand into the cavity, degassing during pouring becomes easier.

(7) Continuous freezing work is possible by using a special tunnel freezer.

Claims (1)

[Claims] 1. A method for manufacturing a mold by freezing molding sand mixed with 1 to 7% water, characterized in that the freezing step comprises an initial freezing step and a final freezing step. Mold manufacturing method. 2. The mold manufacturing method according to claim 1, wherein the initial freezing step is performed with a model placed in a sand mold. 3. The mold manufacturing method according to claim 1, wherein the finishing freezing step is performed with the model removed. 4. The mold manufacturing method according to claim 1, wherein the finishing freezing step is performed with the model removed and the sand mold assembled.