Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C15/00—Moulding machines characterised by the compacting mechanism; Accessories therefor

Definitions

• 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.
• the binder was typically clay, but today, different kinds of organic and chemical binders have also been used.
• the binder e.g. clay
• 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.
• the inflowing melt washes with great strength over the imprint surface.
• 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.
• the finished castings are given a corresponding rough, uneven surface which is unacceptable for many purposes.
• 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.
• 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 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.
• 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.
• ultrasound can advantageously be used ultrasound of frequencies between 8 and 80 kHz, especially between 18 and 50 kHz, and amplitudes between 1 and 100 ⁇ , especially between 5 and 20 ⁇ .
• 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.
• the strength of the binder is increased by drying and/or heating the mold after the pattern has been removed.
• 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.
• 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/cm 2 , preferably between 0,5 and 8 kg/cm 2 and especially between 1 and 5 kg/cm 2 .
• the specific static pressure can be transmitted to the molding sand via the pattern.
• 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 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.
• the ultrasonic generator can advantageously be connected to the core box.
• 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
• fig, 7 is the same at the final stage.
• 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.
• 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.
• 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.
• the space between the sand grains is sufficiently large to, with certainty, allow gasses and vapours from the pouring process to pass.
• 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 pattern 7 an ultrasonic generator or a horn 8
• a base for supporting the apparatus 10.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ .
• the pattern was pressed into the molding sand, as shown in fig. 4, by a static pressure which varied from 1 to 14 kg/cm 2 , an d the density of the molding sand was measured with and without the use of ultrasound.
• 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Mold Materials And Core Materials (AREA)
• Casting Devices For Molds (AREA)

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• 1997
  • 1997-10-13 EP EP97943794A patent/EP0949980A1/de not_active Withdrawn
  • 1997-10-13 JP JP51792698A patent/JP2001504394A/ja active Pending
  • 1997-10-13 WO PCT/DK1997/000445 patent/WO1998016333A1/en not_active Application Discontinuation
  • 1997-10-13 AU AU45507/97A patent/AU4550797A/en not_active Abandoned
  • 1997-10-13 PL PL97332780A patent/PL332780A1/xx unknown

Non-Patent Citations (1)

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