DD293971A5 - METHOD FOR FORMING CASTINGS - Google Patents

Abstract

The invention relates to the molding of cast pieces using a non-bonded (read) molding medium, for example a model evaporation die casting method. In order to achieve a very significant improvement in the rate of cooling and solidification of the piece of castings, according to the invention, the air normally present in the interstices of the non-bonded molding medium is replaced by a gas having a higher heat conductivity than air. Helium has been found to be particularly useful for this purpose. non-bonded molding medium; Guszstuecke; Cooling / solidification rate; Forming medium Zwischenraeume; Gas; thermal conductivity; Helium}

Description

Casting into a malleable medium

This invention relates to molding into a moldable medium, and more particularly to a casting method in which a non-baked molding medium contains a high thermal conductivity gas filling the gaps. Conventionally, quartz sand was the molding medium used in the molding of various metals and their alloys. Among the known casting methods there may be mentioned the sand casting, in which metal is poured into a mold made of sand and a binder, CO ₂ -GuB, in which the binder (water glass) is brought to react with CO ₂ -GaS, to activate it, cast in the wax-out method, in which the mold is made by covering a lost model with a refractory sludge, and mold mask casting, wherein the mold is made by sticking together sand particles to make a mold mask who adopted the contours of the Matall model. Another casting process of particular interest is the foam-evaporation casting process wherein a foam model, generally comprising polystyrene foam, is the article from which the casting is made. This foam model is coated with a suitable refractory sludge, placed in a Gußkasten or Gußrahmen and surrounded with unglued quartz sand as a molding medium. A sprue of foam extends from the model to the top surface of the forming medium, which forms a channel for entry of the molten metal. The casting box is vibrated to achieve maximum consolidation and density of the sand. The molten metal is then poured into the casting box over the sprue where the molten metal vaporizes the sprue and model displacing it.

The result is a casting that perfectly reproduces the shape of the model. Gases formed from the vaporized polystyrene penetrate the mud and sand and escape through vents in the casting box. Various metallic materials have been used as molding media to provide increased thermal conductivity. For example, in the USSR, studies have been made on the use of pig iron or steel shot either as a casting or in fragments as a ferromagnetic forming medium to be used in conjunction with the foam evaporation process. These studies were reported in the reports of a symposium "Life po Gazifitsiruemym Modelyam" published in 1979 by the Institute of Foundry Problems of the Ukrainian Soviet Republic, Academy of Sciences, Kyiv, USSR.

However, while the above materials provide the desired thermal conductivity, they are very heavy materials which tend to distort the polystyrene patterns used in the foam evaporation process, resulting in inaccurate castings. In addition, such heavy forming media can not be handled with the conventional equipment used for quartz sand.

British Patent Application 2183517, published June 10, 1987, describes the use of zircon sand as a molding medium in the foam evaporation process. Since zircon sand has a bulk density higher than quartz sand, approximately equal to that of the molten metal being cast, it is believed that the hydrostatic forces acting on the forming features are reduced, thereby greatly improving dimensional stability, and Hereby the final accuracy of the casting is greatly improved. At a temperature of 600 ⁰ C the other hand, the thermal conductivity of zircon, 0.83 W / m ° K, only the two times that of quartz (quartz sand), 0,54W / m ° K. Since the degree of heat removal is approximately proportional to the square root of the thermal conductivity of the molding medium, zircon provides an increase in cooling rate of about 24%. Another method for improving the rate of solidification is described in Ryntz et al., U.S. Patent 4,520,858, issued June 4, 1985. In this patent, a metal chiller serving as a potential heat sink is attached to a foam evaporation model. When metal is poured into the mold, the cooling part accelerates the cooling and solidification. However, the attachment of a cooling member to each model is an expensive ancestor and provides only a very limited increase in solidification rate.

It has also been proposed to improve the molding media by coating the particles with a refractory layer. Such a process is described in Rikker, U.S. Patent 4,651,798, issued March 24, 1987, where silica sand, clay, zirconia, or glass particles are coated with such a refractory layer. This layer also modifies the shapes of the particles to make them more spherical so that they are more evenly wrapped around the model, thereby improving accuracy. Again, these materials do not have the high thermal conductivity required to increase the rate of consolidation.

Another molding medium that enhances the model evaporative gypsum process is aluminum granules. This medium has been found to be highly effective in its ability to increase the rate of heat extraction while avoiding the problems of heavy metals as forming media. However, all of the above studies have focused on the properties of the solid phase of the molding medium and have not considered the influence of the gas phase in controlling the heat conduction properties of the molding medium occupying the interstices between the particles.

There are already previous proposals to use helium gas to the Z.veck to modify the rate of heat transfer. For example, Soviet Patent 369972, published on November 15, 1973, discloses a method of freezing sand molds, presumably to bind the particles of the media prior to casting, in which, to increase the freezing rate, the molds are filled with a gas which is higher Thermal conductivity coefficient has as air. However, the patent concerned only the cooling of molds to temperatures below ⁰ ° C and not the casting of molten metal.

Russian Patent 1161224, published June 15, 1985, relates to a mold having a porous core whose porosity varies from fine pores at the surface to coarse, deeper penetrating caverns in the middle. These coarse cavities of the core may be filled with different cooling media, including helium, to change both the heat storage capacity of the core and the cooling rate of the casting which is in contact with the core.

US Pat. No. 4,743,027, issued June 7, 1987, describes the use of a helium film between the molten metal and the front surface of a casting belt in a continuous casting machine to produce a metal stem. However, the purpose of helium is merely to make a gas film between the metal and the tape.

S.Engler and R. Ellerbrok, "Influence of Various Gas Atmospheres and Gas Pressures in Some Casting Characteristics in Example Alloy Al Si 12.8" ("Influence of Different Gas Atmosphere and Gas Pressure in Some Casting Properties in the Example of the Al-Si Alloy 12.8") , Foundry 64 (9), 227-230 (1977), describe the effect of argon and other gases present in the atmosphere surrounding the molten metal in a smelting furnace, in a transfer spoon, and when poured from the spoon into a mold , The purpose of this gas was to reduce the cooling rate of the metal during the melting and transfer. The article teaches that reducing the pressure of each gas and replacing air with argon achieves the goal of reducing the rate of cooling, i. H. to increase the duration of solidification.

It is an object of the present invention to provide an improved molding process with greater heat transfer through the molding medium.

In accordance with this invention, it has been discovered that by replacing air normally present in the interstices of a non-bonded (loose) molding medium with a gas of higher thermal conductivity (such as helium), a much greater cooling rate and solidification rate can be achieved.

Thus, in its broadest aspect, the present invention relates to a casting molding method comprising the steps of producing a model of the product to be cast in a pouring box by means of a non-adhered forming medium, for example non-adhered particles of heat-resistant material, and a filling of molten one Pour metal into the casting box to create a casting in the shape of the model within the molding medium. According to the novel feature, the air normally present in the interstices of the non-bonded (loose) molding medium is replaced by a gas having a higher thermal conductivity than air. A preferred feature of the invention relates to a process for forming castings comprising the steps of generating a model of the product to be cast from a material which is gasifiable without residue substantially when exposed to the molten Gußfüllung confused ^1, and a figure corresponding to the product to be cast, surrounding the model in a mold box with a molding medium comprising unbonded particulate material and pouring a filling of molten material into the casting box to vaporize the model and cast it into a mold to create the shape of the model. The novel feature involves the step, the air, which is normally in the interstices of the

particulate shaping material is replaced by a gas having a higher thermal conductivity than the air.

Helium is the preferred gas because it is inert, non-toxic, non-corrosive and relatively cheap. There are also other gases with high thermal conductivity, especially hydrogen and neon, but the practical limitations of their use in terms of hydrogen safety and neon cost are readily apparent. Blends of helium with other non-reactive gases with low thermal conductivity provide benefits in certain applications, where carefully selected cooling rates faster than those obtained by air but slower than those obtained by helium are required. For these applications, the required cooling rate is obtained by using mixtures of helium and air, or helium and nitrogen, or helium and argon, or helium and any other gas which reacts neither with the molten or pre-solidified metals nor with the molding medium. The use of such selected blends provides "tailoring" of the cooling and solidification rate.

In one embodiment of the invention, the interspaces of the particulate molding medium are simply filled with highly conductive gas before the casting operation is begun. However, the mold may also be filled with the molten metal prior to introducing the high thermal conductivity gas to completely fill the mold under conditions of low heat release rate and subsequently to increase the cooling rate by introducing the gas, such as helium, or by adding helium Used air mixtures described above to obtain average rates of heat extraction.

A wide variety of particulate materials can be used as forming media comprising silica sand, zircon sand, chromium-magnesite sand, steel shot, silicon carbide, clay, aluminum granules, etc. A wide variety of metals can also be formed by the process of this invention which include such materials as aluminum, magnesium, zinc and their alloys.

Preferred embodiments of the invention are illustrated by the following non-limiting examples.

example 1

Foam Verdampfungsguß

Expanded polystyrene models were prepared (38.1 mm χ 50.8 mm x 152.4 mm) and coated with a mold coating consisting of Styro-Kote 250.1 (trademark of The Thiem Corpor:.; Ion). These were packed in various forming media (aluminum granules -20 / + 80 mesh, SiC # 24 grit and foundry sand), and castings were made by casting an Al-4.5% Cu orientation at 750 ° C on the model , A heat sensor was placed in the middle section of the casting, and the cooling times were recorded under the conditions indicated in Table 1.

Table 1

Cooling times for a cast of Al-5.4% Cu alloy at 750 ° C. The times represent seconds elapsed between the liquid transformation point and the designated temperature.

Table 1

, the atmosphere

Aluminum granulate air He

Sand air

He

SiC air

liquid-solid transformation point 330

liquid-400 ° C 580

liquid-300 ° C 955

Weight of the casting (g) 703

220 430 720 800

420

760

1300

659

220 430 740 810

1105

It can be seen that under these conditions the cooling rates in sand and helium were equivalent to those achieved in aluminum granules and helium and better than those achieved with either aluminum granules or silicon carbide in air. The use of helium has approximately doubled the cooling rate of the parts cast in sand and in air.

Example 2

A separate series of experiments was conducted to estimate the effect of helium on the rate of heat removal under conditions closer to conventional sand casting. During these experiments, the forming media were packed around unused empty can bodies. Metal (Al-4.5% -Cu) was poured directly into the cans at 700 ⁰ C and an insulating cover was placed over the mold. Temperature-time recordings were obtained to compare relative cooling rates under various casting conditions.

The results obtained are shown in Tables 2 and 3. Review of these tables reveals that the presence of helium had a greater (enhancing) effect under all conditions studied and that the introduction of helium into quartz sand is a very effective measure to increase the cooling rate of the castings.

Table 2

Cooling times (s), AI-4.5% Cu, T _{E! NouB} = 700 ⁰ C, uncoated form

form medium air al He sand air He air SiC He the atmosphere Time (min) 2.9 2.2 8.25 3.6 6.25 3.05 liquid-solid 3.6 2.6 10.25 4.6 8.0 3.9 liquid 500 6.0 4.5 17.75 8.0 13.5 7.0 liquid 400 8th 6 23.5 10.75 17.5 9.25 liquid 350 10.8 8.25 30.5 15.0 23.25 12.7 liquid 300 601 557 684 589 467 713 Sample weight (g)

Table 3

Cooling times (s), AI-4.5% -Cu, T _E | _nguB = 100 ° C or 700 ⁰ C, coated form

form medium air - al He air - sand He air - SiC He the atmosphere 12.5 19 13 Time (min) 6.4 22 3.3 9.25 33 3.3 6.5 22.5 3.5 liquid-solid 649 - 752 - 683 - liquid 500 6.3 6.5 7.3 liquid 400 12 12 13.8 liquid 300 630 430 614 Sample weight (g)

Example 3

A series of experiments were carried out using the foam model evaporation molding technique with various different molding media.

Table 4 Experimental Materials

designation

description

Foundry sand tabular Al ₂ O ₃

tabular Al ₂ O ₃

Silicon carbide χ 200 mesh

χ 54 RA

Aluminum granules NMI

AMPAL

Toyal

AFS No. 26

14x28 mesh

delivered by Kaiser Corporation,

Pleasantown, CA, United States

the same as above but glued

with 5% w / w sodium silicate

Sodium (40-42% Be; Hardened

with CO ₂

Mixture of 4 parts

Part 200 mesh

supplied by White Abrasives Inc.,

Niagara Falls, ON, Canada

Delivered by Canadian

Carborundum, Niagara

Falls, ON, Canada

AA1100 Al powder

25 x 40 mesh, supplied by Nuclear

Metal Inc., Concord, NJ, USA

AM PAL 603

delivered by Atomized

Metal Powders Inc.,

Flemington, NJ ₁ USA

Grade 5600 Al powder

delivered by AclanToyo

America Inc.,

Joliette, IL

20 x SO mesh Al powder

supplied by Johnson &

Mathey Limited

110 Industry Street

Toronto, ONM6M4M1

non-glued, porous, non-metallic non-glued, porous, non-metallic

glued, non-porous, non-metallic

non-glued, porous, non-metallic

non-glued, porous, metallic

Cylindrical expanded polystyrene models (density = 22.5 kg / m ³ ) measuring 38.1 mm in diameter by 152 mm in length were obtained from Lost Foam Technologies, Sheboygan Falls, Wisconsin. The metal weight required to fill these models was 0.5kg.

Coated models were prepared by immersing them in a Styro-Kote 250.1 mud, the specific gravity of which was adjusted to 1.56 to obtain a coating thickness of 0.2 mm. After dipping, the models were either air-dried overnight or dried in a microwave oven. Prior to wrapping the model in the molding medium, a thermal probe was inserted at the midpoint along the length extent to the depth of the centerline of the cylinder. The model was then inserted into a molding box and filled with the molding medium while shaking the entire assembly. To prevent heat loss through the bottom of the mold box, an insulating layer (either 2.7 mm fiberboard or two layers of Fiber Frax paper [trademark of Carborundum Corporation]) was placed at the bottom of the mold box. For experiments in which the gas phase was changed, a perforated stainless steel gas distributor was placed in the bottom of the mold box and connected to the gas supply and used to rinse the particle bed prior to casting. For the rinse, 2.7SLPM (nominal liters per minute - l / min) of helium was injected for two to three minutes. Immediately prior to casting, the gas flow was reduced to about 0.3 l / min to maintain the gas atmosphere during cooling. Samples were cast of a binary Al-4.5% -Cu alloy at a casting temperature of 700 ⁰ C, and the temperature was monitored using a strip chart. This alloy was chosen because it has a well-defined eutectic transformation point at 548 ° C, which allows easy detection of solidification time. When the casting was held at 700 ⁰ C, ^ ann was the metal that has reached the heat sensor, gera.de at the liquid temperature and the cooling rates were thereby calculated, that the area (100 ⁰ C) between the liquid temperature to has divided to the eutectic transformation temperature through the time that elapsed between the infusion and the time of the end of the eutectic transformation.

a) Results were recorded for solidification rates when air was replaced by helium in foam evaporation models without coating. These results are shown in Table 5 and the rate of dissolution was higher when helium was present.

b) Another experiment was performed in which air was replaced by helium in foam evaporation models with coating. These results are shown in Table 6 and again show that the solidification rate was higher when helium was present.

c) Another experiment was conducted to show the solidification rate with static air and flowing argon, both of which have lower thermal conductivity than helium. The results of this experiment are shown in Table 7, and it can be seen that the solidification rates observed with both air and argon were significantly lower than those observed with the use of helium.

d) Another experiment was carried out in which a larger casting based on a foam evaporation model was molded using 8 kg of metal. The results are shown in Table 8, and the same improvement in solidification rate and subsequent cooling to 445 ° C and 395 ° C was reached when air was replaced by helium.

Table 5

Solidification rate in air and helium, no coating on the model solidification rate ° C / s (standard deviation)

in air in he Air / He ratio foundry sand 0.34 (0.02) 0.74 (0.06) 2.2 Al-granules NMI 0.80 (0.06) 1.24 (0.08) 1.6 J & M 0.71 (0.03) 1.10 (0.05) 1.5 AMPAL 0.83 (0.04) 1.29 (0.04) 1.6 Toyal 0.58 (0.07) 0.99 (0.09) 1.7 TABULARA 1203 0.42 (0.01) 0.98 (0.03) 2.3 silicon carbide 80 χ 200 mesh 0.56 (0.0i) 0.96 (0.04) 1.7 36 x 54 RA 0.43 (0.03) 1.02 (0.10) 2.4

Table 6

Hardening rate in air and helium, coated models

Solidification rate ° C / s (standard deviation)

in air in he Air / He ratio foundry sand 0.35 (0.02) 0.59 (0.02) 1.7 Al-granules NMI 0.49 (0.02) 0.90 (0.07) 1.8 J & M 0.51 (0.03) 0.84 (0.05) 1.6 Toyal 0.45 (0.04) 0.72 (0.07) 1.6 silicon carbide 36 x 54 RA 0.36 (0.01) 0.71 (0.03) 2.0 80 x 200 mesh 0.41 (0.01) 0.64 (0.01) 1.6 TABULAR A1203 0.36 (0.01) 0.75 (0.04) 2.1

Table 7

Influence of helium and argon on the solidification rates measured on uptake into TOYAL aluminum granules

Solidification rate ° C / s (standard deviation)

Air ¹ » 1 static Helium ² » Argon ² * 0.58 (0.07) 2 flowing, 0.35 l / min 0.99 (0.09) 0.50 (0.06) , Thermal conductivity at 300 ⁰ K 1000 ⁰ K •Air 0,026WZm ⁰ K 0.067 W / m ° K • helium 0.151 W / m ° K 0.354 WAn ⁰ K •Argon 0,018W / m ° K 0.044 W / m ° K

Table 8

Cooling times as a function of molding medium and atmosphere for a large casting (8 kg)

forming medium gas Time (min) vomEinc'jßbiszu: 445 ⁰ C 395 ⁰ C Eutekt. conversion 38 60 foundry sand air 16 15.5 22.5 foundry sand He 7 20.5 32 Al-granules air 9 13.25 18 TABULAR A1203 He 7.25

It can be seen from the above examples that the solidification rate and cooling rate during the foam evaporation process can be significantly increased by the use of a high thermal conductivity / high heat capacity molding medium. The performance of these media is ultimately limited by the thermal resistance present at the object touch points. The use of a highly conductive gas, such as helium, substantially increases the solidification and cooling rate. For example, the use of helium with silica sand was even more effective at increasing the rate of solidification than the best aluminum granules in air used in the experiments. The refractory model coatings conventionally used in the foam evaporation process provide a barrier to heat flow that is significant when using helium in a highly conductive molding medium. Optimum results in solidification rates were obtained by combining helium with highly conductive media. The most effective approach has been found to be by using helium in combination with the conventional foam evaporation method. Although the use of other media could, in principle, result in yet a further increase in solidification rates, it will be appreciated that to achieve speeds above those of helium and sand, highly conductive model coatings or a coating-free process are required.

While the above detailed description refers primarily to the foam consolidation process, it will be apparent to those skilled in the art that the invention has a much broader application to other molding processes such as green sand molding, mold masking, lost wax, sand cores, and so forth.

Claims (9)

1. Method for molding castings, comprising the following steps: - Making a model of the product to be cast in a casting box by means of a nichtverklebten molding medium and Pouring a filling of molten metal into the casting box to create a casting in the shape of the model within the molding medium, characterized by the following step: one replaces air, which is normally present in the interstices of the molding medium, by a gas which has a greater thermal conductivity than air. 2. The method according to claim 1, characterized in that the non-bonded molding medium comprises loose particles of heat-resistant material. 3. The method according to claim 2, characterized in that the gas is helium. 4. The method according to claim 2, characterized in that the gas is a mixture of helium with air, nitrogen or a non-reactive gas. 5. The method according to claim 3, characterized in that the cast metal is aluminum or an aluminum alloy. 6. Method for molding castings with the following steps: Preparing a model of the product to be cast from a material which is gasifiable substantially without residue when it is exposed to the molten casting filling and has a shape conforming to that of the product to be cast, Surrounding the model in a casting box with a molding material containing unbound particulate material, and Pouring a charge of molten metal into the casting box to vaporize the model and produce a casting of the shape of the model, characterized by the following step: - Replaces the air normally present in the interstices of the particulate molding medium by a gas with a higher thermal conductivity than air. 7. The method according to claim 6, characterized in that the gas is helium. 8. The method according to claim 6, characterized in that the gas is a mixture of helium with air, nitrogen or a non-reactive gas. 9. The method according to claim 7, characterized in that the cast metal is aluminum or an aluminum alloy.