FI69412C - FOERFARANDE FOER SNABB AVKYLNING AV METALLGJUTSTYCKEN - Google Patents

Description

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Method for abrupt cooling of sand molds

The invention relates to a method for abruptly cooling metal castings cast in chemically binder-reinforced sand molds.

Many such methods are known, but with numerous methodological and metallurgical drawbacks.

Methods for cooling permanent metal molds by means of cooling channels in the molds are further known, as disclosed, for example, in CH Patent Publication No. 557,209. However, such a cooling method cannot be applied to sand molds.

The object of the invention is to eliminate this drawback. The invention as defined in the claims solves the task of rapidly providing a load-bearing surface layer to a solidifiable metal shaped body and a fine-grained structure, in particular the additional volume due to graphite separation in graphically solidifiable iron must be utilized to reduce inward solidification volume.

Of particular importance to the invention is the impregnation of the outer surfaces of the sand cup with a water-repellent substance, for example silicone, as a result of which metal-filled cups can be abruptly cooled from the outside, for example by water or water vapor, without cooling liquid penetrating the sand cup. iron.

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The advantages achieved by the invention are essentially seen in that, especially with graphitically solidifying iron, the above-mentioned volume compensation enables casting without a feed sink and the formation of gas bubbles in the metal with aluminum and its alloys can be prevented. Casting without a feed sink then means more efficient utilization of the metal and thus better economy.

In addition to the advantages already mentioned, the sudden cooling of the castings according to the invention also results in a significant improvement in the mechanical properties.

According to DIN 1725, a tensile strength of 140 N / mm with a tensile strength of 0.5% is required for aluminum alloy G AISi 8 Cu 3 cast in the conventional way in fresh sand.

The method according to the invention gives a strength value of 2,190 N / mm in the cast state with an elongation of 2%, after cold storage, strength values of up to 210 N / mm with an elongation of about 2.5%.

When the shells are removed in water, it is possible to further improve the strength to about 300 N / mm ^ with an elongation of 3.5%. Otherwise, such values can only be achieved by die casting.

However, compared to die casting, the method according to the invention is more cost-effective because expensive die casting molds are eliminated.

The good, die-casting-like mechanical properties of the castings produced by the process according to the invention are due to the fact that rapid heat dissipation results in a fine-grained structure and, after cooling,

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The shells can be made according to the prior art, for example by mixing a binder which hardens in the sand.

One such method is the Croning method, in which sand is bound with a phenolic resin.

The wall thickness of the shells is preferably at least 3 mm. The cup-like mold parts are assembled in the usual way into molds, in which case the impregnation, for example with silicone, is carried out before assembly, but preferably only after assembly, in order to avoid untreated points on the contact surfaces of the cups.

The sand cups, either supported or unsupported, can be sprayed with coolant or steam or also completely immersed in a cooling bath. Water or steam is preferably used as the cooling medium.

The timing of the addition of water is determined by the cast metal and the wall thickness of the casting. When casting aluminum or very thin-walled iron castings, for example when piston rods, the mold can already be poured with water during casting. When casting thicker-walled parts, for example a connecting rod, water is added a few seconds after casting.

Due to the solidification pressure and higher thermal load, thick-walled iron castings have recommended support plates, for example as described in CH patent 492,493.

Steel sand, quartz powder or quartz sand, as well as other granular masses or also fixed supports are particularly suitable as a support tool.

In addition to this, the coolant itself can also be used as a support in a closed tank.

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If the cooling medium is removed at a shell temperature of 600-700 ° C, the shells will glow more or less throughout and crumble upon disassembly. Alternatively, the peeling can be performed already in a water bath or by spraying with a cooling medium, which has the following advantages: a) more efficient cooling effect b) the peelings can be reused c) the formation of toxic decomposition gases from the used organic binder, e.g. phenolic resin, during casting and

As shown in Table 1 of CH Patent 583,758, for example, a phenol-formaldehyde binder generates not only 20-25% methane but also 25-30% carbon monoxide when heated to a casting temperature, which is very dangerous to humans.

However, the advantage mentioned in c) above can only be achieved when the shells are cooled below the decomposition temperature of the phenolic resin (about 300 ° C).

Claims (7)

A method for abruptly cooling metal castings cast in chemically binder-reinforced sand molds, characterized in that the sand shells are made water-repellent before or after assembly and the liquid metal-filled sand mold is cooled externally during filling or shortly thereafter with a liquid medium. Process according to Claim 1, characterized in that water is used as the liquid medium. Method according to Claim 1 or 2, characterized in that the entire sand mold is immersed in a cooling medium. Method according to Claim 1 or 2, characterized in that the sand mold is only sprayed with a cooling medium. Method according to one of Claims 1 to 4, characterized in that the sand molds are cooled to a temperature below the decomposition temperature of the chemical binder. 5,69412 Process according to Claim 5, characterized in that the molds reinforced with phenolic resin are cooled to a temperature of less than 300 ° C. Method according to one of Claims 1 to 6, characterized in that the sand molds are made water-repellent with silicone.