EP0949980A1

EP0949980A1 - Method and apparatus for manufacturing a sand mold

Info

Publication number: EP0949980A1
Application number: EP97943794A
Authority: EP
Inventors: Benny Dall
Original assignee: Brodrene Gram Invest AS
Current assignee: Brodrene Gram Invest AS
Priority date: 1996-10-14
Filing date: 1997-10-13
Publication date: 1999-10-20
Also published as: PL332780A1; WO1998016333A1; JP2001504394A; AU4550797A

Abstract

A method and an apparatus are serving for manufacturing a sand mold for pouring a metallic melt of e.g. cast iron. A pattern (7) having a configuration complementary to the castings wished to be cast, forms an imprint in a volume of molding sand (5), as the molding sand during this is acted upon by both a static pressure from a squeeze plate (9) and by ultrasound from an ultrasonic generator (8). In the sand mold can be cast castings with fewer fins and a more even and smoother surface than known per se.

Description

Method and apparatus for manufacturing a sand mold

The invention concerns a method of manufacturing a sand mold for pouring a metallic melt of e.g. cast iron, whereby a pattern in a volume of molding sand forms an imprint having a configuration complementary to the castings wished to be cast.

Casting e.g. iron in molds of sand is a technique which has its origins far back in history. From the beginning, it was obvious to attempt making the molds of sand. Sand was abound many places and could, in wet condition, easily be formed to a good imprint of the employed pattern. Sand could also reasonably stand the high molding temperatures, and there was the necessary passage between the sand grains to allow the gasses and vapours generated during molding to escape into open air. And finally, the mold, in dry condition, had sufficient strenght to hold the melt without collapsing.

Molding sand consists primarily of sand grains of especially quartz and a binder making the grains stick together. Earlier, the binder was typically clay, but today, different kinds of organic and chemical binders have also been used.

A considerable part of the mold manufacture consists in compressing the molding sand thereby obtaining a sufficiently great strength and an even, smooth surface of the imprint. The original method for this purpose was to ram the sand up by hand. In modern foundries, hand ramming however is largely abandoned in favour of machine molding where the molding sand is compressed by means of centrifuging, shaking, oscillating or pressing.

In compressed, dry condition, the binder, e.g. clay, will form a kind of skeleton which binds the sand grains together. This binder skeleton has difficulties in standing up to the pressure from the inflowing melt when it is heated to the very high temperatures present at the surface of the imprint during casting. Thereby, the melt can penetrate and enclose a part of the sand grains which thereby are cast into the surface of the castings. This phenomenon, normally known as burning, poses a recurring and very serious problem to the foundry industry. Attempts at solving the problem have been made by adding coal dust to the molding sand and by blackening the mold with a suitable agent having, as an example, a high content of graphite. However, the obtained results have not been entirely satisfactory.

In the case of heavy burning, it normally is more profitable to discard the castings completely. In the case of light burning, these can be chiselled and/or ground away, but this is a process which increases the manufacturing costs of the castings considerably.

During pouring, the inflowing melt washes with great strength over the imprint surface. At the same time, the binder skeleton is weakened, as described above, and the melt flow will therefore be inclined to tear up the sand surface and make it rough and uneven. Thereby, the finished castings are given a corresponding rough, uneven surface which is unacceptable for many purposes. In these cases, it is necessary to finish the castings by means of difficult, costly processes, for example grinding, sandblasting or tumbling.

The mold is built up of sand in wet condition, where the sand only has modest strength. During forming, fine details and sharp edges in the molding sand can therefore easily be lost. If it was to be avoided that, in two-piece molds, flashes and fins were made on the castings at the transition of the imprint to the joint face of the mold pieces, the edges would have to be able to maintain their sharpness on exactly this spot, but in practice, it has instead turned out that the soft edges are often deformed when the pattern is removed.

To this is added that the mentioned edges also are inclined to collapse during the pouring itself. The reason for this is chiefly that the binder skeleton is weakened the most at the sharp edges where the temperature stress is the greatest. Flashes and finns are costly to remove. In many cases, the work must be done manually by chiselling and grinding.

The object of the invention is to improve the method mentioned in the opening paragraph of manufacturing a sand mold so that there in the mold can be cast castings with less burning, fewer flashes and a more even, smoother surface than known per se .

The novel and unique features according to the invention, whereby this is achieved, is the fact that the molding sand is acted upon with ultrasound during the forming of the imprint, whereby the molding sand obtains an unprecedented dense and strong structure.

The effect of the ultrasonic action upon the molding sand can, according to the invention, be further increased when it is overlaid by a static pressure on the molding sand.

The effect of the ultrasound is only optimum to a limited debth into the molding sand from the coupling facing of the ultrasonic sound waves. This fact is, according to the invention, advantageously utilized for letting the ultrasonic sound waves emanate from the pattern. Thereby, the imprint is given a dense and strong surface area which can stand up to the high casting temperatures and the pressure from the inflowing melt. The rest of the molding sand will then be able to maintain the airiness needed to allow the gasses and vapours from the casting process to pass. For this purpose, there can advantageously be used ultrasound of frequencies between 8 and 80 kHz, especially between 18 and 50 kHz, and amplitudes between 1 and 100 μ, especially between 5 and 20 μ.

When the molding sand in the surface area of the imprint is excited with ultrasound within these limits, momentary frictional heat is generated at the borders where the particles of the molding sand touch each other. The heat changes the consistence of the binder. Depending on the frequency of the ultrasound and the amplitude of the ultrasonic sound waves, a typical binder, such as clay, can in this way dry or sinter. In each of these conditions, the clay has a far greater strength than in its wet forming condition.

In many of the conventional methods of manufacturing a sand mold, the strength of the binder is increased by drying and/or heating the mold after the pattern has been removed. By means of the method according to the invention, this increase in strength now takes place as an integrated part of the actual molding process, i. e. when the pattern still is in the mold. The pattern can therefore be removed without much risk of thereby damaging or deforming fine details and sharp edges of the imprint. Thereby, the advantageous new effect is surprisingly obtained in which fine details can be reproduced perfectly in the castings, and where these, better than before, can be made without flashes and fins.

The specific static pressure can advantageously be between 0,2 and 15 kg/cm², preferably between 0,5 and 8 kg/cm² and especially between 1 and 5 kg/cm². When the static pressure is transmitted to the molding sand via a separate squeeze plate, the airiness of the sand can be regulated by adjusting the pressure load. In another advantageous embodiment of the method according to the invention, the specific static pressure can be transmitted to the molding sand via the pattern. When the molding sand is excited by the ultrasound via the pattern, the molding sand next to the pattern will become almost fluid while the rest of the molding sand maintains its non-fluid condition. Now, when the pattern also acts on the molding sand through a static force, the fluid sand close to the pattern will be displaced while the rest of the sand is only compressed. This fact can, according to the invention, be utilized for building up the mold with prime quality sand in the surface region of the imprint and inexpensive filling sand in the rest of the mold. Another advantage is that a relatively low specific pressure can be used, as the molding sand next to the pattern is fluid.

The advantages obtained by means of each of the two above-mentioned methods can jointly be obtained by, in combination, transmitting the pressure to the molding sand via both a separate squeeze plate and the pattern.

The invention also concerns an apparatus for manufacturing a sand mold for pouring a metallic melt of e.g. cast iron in an imprint formed in a volume of molding sand by a pattern, said imprint having a configuration complementary to the castings wished to be cast. The novel and unique features of this apparatus is that it comprises an ultrasonic generator for acting on the molding sand with ultrasound, and that the ultrasonic generator is connected to the pattern.

In an embodiment where the mold has a core and the pattern a core box, the ultrasonic generator can advantageously be connected to the core box.

The invention will be explained in greater details below, describing only exemplary embodiments with reference to the drawing, in which fig. 1 is a cross-sectional view of the structure in an area near the surface of the imprint in a sand mold made by means of the method according to the invention,

fig. 2 is a schematic view of a first embodiment of an apparatus according to the invention,

fig. 3 is a schematic view of a second embodiment of an apparatus according to the invention,

fig. 4 is a schematic view of a third embodiment of an apparatus according to the invention,

fig. 5 is a schematic view of a fourth embodiment of an apparatus according to the invention,

fig. 6 is a schematic view of a fifth embodiment of an apparatus according to the invention at an initial stage of the molding process, and

fig, 7 is the same at the final stage.

In the following, the invention is described on the assumption that the molding sand contains clay binder, and that the castings to be cast in the mold are of cast iron. It is to be noted that this is only to be understood by way of example, as many other kinds of binders can be used for carrying out the method, just as the formed molds can be used for many other kinds of metals than iron.

Fig. 1 shows, on an enlarged scale, the structure of the sand in a sand mold, generally designated 1. An imprint with a surface 2 is made in the mold by means of a pattern (not shown) . The mold is built up of molding sand primarily consisting of sand grains 3 of especially quartz and clay 4. The clay has the shape of a skeleton surrounding the sand grains and binding them together.

The shown sand mold has been acted upon with ultrasound transmitted to the molding sand via the pattern. The ultrasound has penetrated the surface region A, but has not been able to reach the subjacent region B. Therefore, the ultrasonic sound waves have mainly been able to excite the molding sand of the surface region A, where the sand grains now lie very close to each other. In the highly reduced spaces left between the sand grains after the ultrasonic action, there still is a fully intact clay skeleton for binding the sand grains together.

The ultrasound sets, at high frequency, the sand grains in reciprocatory movement, whereby the simultaneously exerted static pressure has innumerable chances of getting the sand grains into positions where their surfaces lie close together. However, if the mold is acted upon merely by the static pressure, the sand grains will chiefly maintain their original orientations and will, therefore, necessarily be transmitting the compressive force point by point.

The excitation has furthermore resulted in a momentary heat generation in the boundary surfaces of the particles touching each other. The heat has dried or, in some cases, sintered the clay, and this change in the condition of the clay results in the clay skeleton being able to bind the sand grains together with a far greater strenght than if the clay had merely kept its original wet condition.

In this way, the clay skeleton does not obtain, as is the case in the conventional methods, an increased strengt only at the forming, but already while the pattern is still in the mold. The pattern can therefore be removed without great risk of damaging and deforming fine details and sharp edges in the mold.

The high density of the sand grains in the surface region furthermore has the effect of the imprint surface being extremely even and smooth, and contributes moreover, together with the strengthened clay skeleton, to making the surface region A strong and stout against the pressure of the inflowing iron melt. The problem of the sand grains burning onto the surface of the castings is furthermore minimized as a result of the spaces between the sand grains being reduced to a very small size which does not give the melt much chance of penetrating and surrounding the sand grains.

In the region B, the space between the sand grains is sufficiently large to, with certainty, allow gasses and vapours from the pouring process to pass.

In a sand mold, made by means of the method according to the invention, it is, as a result of the above advantageous qualities of such a sand mold, possible to cast e.g. iron castings with no appreciable flashes and fins and a minimum risk of burning. At the same time, the castings obtain an extremely even, smooth surface reproducing even the finest details.

Fig. 2 - 7 show different embodiments of an apparatus for carrying out the method according to the invention. Identical parts are in all cases designated by the same reference number. A volume of molding sand is thus designated 5, a flask for accommodating the molding sand 6, a pattern 7, an ultrasonic generator or a horn 8, and a base for supporting the apparatus 10.

As can be seen, the horn 8 is, in all embodiments, connected to the pattern. However, within the scope of the invention, embodiments are possible in which the horn is connected to other parts of the apparatus.

In fig. 2 and 3, the separate sqeeze plate 9 is pressed towards the molding sand 5 by an external static force of e.g. a pneumatic or hydraulic cylinder, indicated by the arrow P in the figures. The airiness of the molding sand can be regulated by adjusting the load of the external static force. The two arrangements only differ, in principle, from each other by the fact that the weight of the molding sand in fig. 3 contributes to compressing the molding sand near the pattern 7, while the opposite is the case in fig. 2.

In fig. 2 and 3, the static pressure for compressing the molding sand at the pattern must be transmitted through the total sand volume. As an important part of the external static force acting upon the squeeze plate 9 is absorbed by the walls of the flask 6 on account of the tendency of the sand to slide, rather high specific pressures of e.g. 10 - 15 kg/cm² are required in these cases.

These high pressures mean that the apparatus must be dimensioned proportionately powerful, and that rather solid pneumatic or hydraulic cylindres must be used to generate the pressure. This disadvantage is eliminated by the arrangement shown in fig. 4, where the external static force is transmitted to the molding sand via the pattern 7. In this case, the static pressure is compressing the molding sand directly against the pattern without first having to pass the total sand volume, as is the case in fig. 2 and 3. It has turned out that, using this embodiment, good results can be obtained with specific pressures of around 1 kg/cm².

Fig. 5 shows an embodiment equivalent to the one shown in fig. 4 with the addition that there now also is a squeeze plate 9 for compressing the molding sand at a greater distance from the pattern. Thereby, it is possible to optimally control the compression of the total sand volume and regulate the airiness of the molding sand according to requirements.

In fig. 2 - 5, the molding sand is filled over a pattern placed in the flask beforehand, after which the pattern is pressed into the sand by means of the external static force, the sand is during this displaced and compressed at the same time. In fig. 6 is seen an initial stage of this process and in fig. 7 is seen the final stage.

The precondition of this method succeeding is that the surface region next to the pattern 7 is excited by the ultrasound from the horn 8, so that the molding sand close to the pattern becomes fluid and can be displaced by the static pressure. It is essentially only the upper part of the molding sand which takes part in this process, and this fact can advantageously be utilized for using prime quality molding sand 11 at the top of the flask 6 and inexpensive molding sand 12 at the bottom.

The arrangement shown in fig. 6 and 7 requires that the flask is filled very precisely with molding sand if a complete imprint of the pattern is to be formed. This problem can be solved by additionally using a separate squeeze plate (not shown) as in fig. 5.

The invention is shown in the drawing and described above as though the mold was for pouring positive castings. However, the term mold is also to be understood as a core for casting an opening or a recess in positive castings, just as the term pattern also includes a core box.

Furthermore, the term sand is not only to be understood as the above-mentioned quartz-based sand, but also as any other particulate material which is suitable for manufacturing a mold for pouring a metallic melt of e.g. cast iron. Example

A number of castings of cast iron were cast in molds which with and without ultrasound, were made in an apparatus corresponding to the one shown in fig. 4.

The flask was a pipe of a height of 250 mm and a diameter of 50 mm. Clay-bound sand was used which was conditioned with an admixture of water and a subsequent, intense mixing for 5 min. The water content was about 3,5%.

The pattern was of wood, and the tests with ultrasound took place with a power of the ultrasound crystal of 1 kW and booster 1:1. The horn was of aluminium and was connected directly to the pattern which thereby was made to vibrate with a frequency of 20 kHz and an amplitude of 17 μ.

The pattern was pressed into the molding sand, as shown in fig. 4, by a static pressure which varied from 1 to 14 kg/cm², ^and the density of the molding sand was measured with and without the use of ultrasound.

The result is shown in the diagram, fig. 8, and as it can be seen, a significant increase in the density of the molding sand was obtained when ultrasound was used in connection with the molding process.

In the absence of ultrasound, it was necessary to use a static pressure of close to 15 kg/cm² in order to obtain a reasonably smooth, even surface in the imprint formed by the pattern in the mold. However, the surface was not completely smooth, but had holes to an extent of over 15% of the surface area. Edges and fine details were indistinct. A certain resilement could be observed when the static pressure was relieved, whereby the surface structure of the mold was weakened. When ultrasound was used, there was no significant power loss in the pattern itself. The ultrasound could therefore excite the subjacent molding sand with, on the whole, full power. The debth of immersion depended to some extend on the load of the static pressure, but varied around a debth of 3 cm. When the static pressure exceeded about 7 kg/cm², an increasing tendency was noted in which the molding sand divided into an upper, strong, hard surface region and a lower back region with decreasing strength and increasing airiness downwards in the mold.

The surface of the imprint from the mold was hard and smooth with holes to an extent corresponding to between 2 and 8% of the surface area.

Edges and details in the imprint were of a high quality. Thus, details of a height of down to 0,3 mm could be seen clearly, and these details were, at the subsequent pouring, safely transfered to the finished castings. By way of comparison, it may be mentioned that the details of a similar mold made without ultrasound only showed up clearly at a height of over 1 mm.

When the mold had been acted upon with ultrasound, the resilement after relief of the static pressure was far less than without ultrasound. This stability meant that sharp edges in the imprint from the pattern were left unaffectedly.

The quality of the castings cast in the molds acted upon with ultrasound under varying conditions, is given in the table below, in which the quality is expressed by giving the number of burnt sand particles. Ultrasound Power Number of exposure Ultrasound pressure surface flaws (sec.) deposited kg/cm² % of area

2 30 2 50%

2 40 3 10%

2 60 4 30%

2 none 5 80%

2 60 5 3%

2 80 6 less than 1%

Claims

C l a i m s

1. Method of manufacturing a sand mold for pouring a metallic melt of e.g. cast iron, whereby a pattern in a volume of molding sand forms an imprint having a configuration complementary to the castings wished to be cast, characterized in that the molding sand is acted upon with ultrasound during the forming of the imprint.

2. Method according to claim 1, characterized in that the ultrasound is generated by at least one ultrasonic generator which acts upon the molding sand via the pattern.

3. Method according to claim 1 or 2 , characterized in that the molding sand is acted upon by an external static power during the forming of the imprint.

4. Method according to claim 1, 2 or 3 characterized in that the static power is transmitted to the molding sand via a separate squeeze plate.

5. Method according to claim 1, 2 or 3 characterized in that the static power is transmitted to the molding sand via the pattern.

6. Method according to claim 1, 2 or 3 characterized in that the static power is transmitted to the molding sand via both a separate squeeze plate and the pattern.

7. Method according to each of the claims 1-6, characterized in that the ultrasonic frequency is in the range of 8 and 80 kHz, especially between 18 and 50 kHz.

8. Method according to each of the claims 1-7, characterized in that the ultrasonic amplitude is in the range of 1 and 100 μ, especially between 5 and 20 μ.

9. Method according to each of the claims 1-8, characterized in that the specific static pressure is in the range of 0,2 and 15 kg/cm², preferably between 0,5 and 8 kg/cm² and especially between 1 and 5 kg/cm².

10 ■ Apparatus for manufacturing a sand mold for pouring a metallic melt of e.g. cast iron in an imprint formed by a pattern in a volume of molding sand and having a configuration complementary to the castings wished to be cast, characterized in that it comprises an ultrasonic generator for acting upon the molding sand with ultrasound, and that the ultrasonic generator is connected to the pattern.

11. Apparatus according to claim 10, where the mold is a core and the pattern is a core box, characterized in that the ultrasonic generator is connected to the core box.