Cavityless Casting Mold

My present invention relates to a cavityless mold for the casting of a metallic article by the so-called full-mold method.

In producing articles of steel or other metal by this known method, a heat-destructible insert or core having the shape of the desired casting and termed a pattern is imbedded in a mass of free-flowing refractory particulate matter such as sand, the pattern consisting of a preferably thermoplastic low-melting material such as foam polystyrene which burns or vaporizes under the heat of the molten metal introduced into its site. As the vapors diffuse through the surrounding sand, the metal completely fills the void left in the mass and, upon hardening of the casting, can be readily extracted therefrom.

In order to maintain the surrounding sand mass in a sufficiently compacted state, it is customary to place it under a pressure differential by subjecting it to atmospheric or superatmospheric pressure from above, via the open top of the mold, while generating (if necessary) a partial vacuum within the mold to hold the sand particles firmly together.

With certain types of casting, e.g. those of T or H profile in which a broad horizontal web or flange shields lower areas of the pattern from the exerted pressure, this unidirectional compaction of the flowable mass is unsatisfactory and may result in an objectionable deformation of thin-walled parts of the pattern leading to corresponding shape deviations of the finished casting. Conversely, the lack of sufficient density of the particulate mass at shielded locations may allow the molten metal to expand beyond the boundaries of the vaporizing pattern and to penetrate into the mass so as to produce undesirable excrescences on the surface of the casting. If the pattern is originally encased in an antibonding layer or coating designed to prevent adhesion between the metal and the surrounding sand, that layer could be locally ruptured by the penetration of the metal flow into the insufficiently compacted mass, thus further impairing the finished article.

The object of my present invention, therefore, is to provide an improved mold for casting such articles with avoidance of the aforestated drawbacks.

In accordance with my present invention, a casting mold is provided with at least two movable wall members formed by airtight membranes, with establishment across each of these membranes of a pressure differential tending to compact the mass from opposite sides around the destructible pattern prior to introduction of the molten metal into the mold. As discussed above, the pressure differential may be provided by the exertion of a superatmospheric pressure from without (e.g. by placing the mold in a high-pressure chamber) or, advantageously, by the creation of suction within the mold.

In principle, the membranes forming the wall members may consist of a wide variety of deformable sheet materials including paper, metal, rubber and solid or cellular plastics. In order to minimize the risk of burning or smoke evolution, they may be made from solid particles such as powdered alumina held together by an elastomeric binder, e.g. silicone rubber; the binder, aside from bonding the solid particles to one another, also fills the intervening interstices to provide the necessary airtightness.

The above and other features of the invention will now be described in detail with reference to the accompanying drawing in which:

FIGS. 1A and 1B are cross-sectional views of a mold according to my invention, prior to and during the application of suction, used in forming a T-profile casting;

FIGS. 2A and 2B are views similar to FIGS. 1A and 1B, respectively, showing the same mold employed for producing an H-profile casting;

FIGS. 3A and 3B are analogous views of an improved mold particularly designed for castings of the general type shown in FIGS. 2A and 2B; and

FIG. 4 is an isometric view of a flexible liner for a mold of the type shown in FIGS. 3A and 3B.

In FIGS. 1A and 1B I have shown a mold with a rigid shell 1, in the form of a four-sided prismatic box, whose bottom is formed by a rigid plate 1a with an array of uniformly distributed perforations 8 and by an overlying first membrane 2a of the aforedescribed character; a second, similar membrane 2b spans the open top of the mold to form a space which is sealed against the outside except for an exhaust pipe 5, leading to a suction pump not shown, and one or more inlets 9 through which molten metal can be introduced. The space within the mold is occupied by a binderless sand mass 4 in which a core 3 of foam polymer such as polystyrene, constituting a heat-destructible pattern, is imbedded.

Upon the application of a partial vacuum to pipe 5, as indicated by an arrow 6 in FIG. 1B, a pressure differential symbolized by arrows 7 is simultaneously developed across both membranes 2a, 2b so that the mass 4 is compacted from opposite sides around the pattern 3 which in this case has a T-profile with the web transverse to the two membranes. It will thus be apparent that there are no dead corners within the mold which would be shielded from the exerted pressure; the sand mass 4 is therefore uniformly compressed to maintain the shape of the pattern 3 and of the casting subsequently taking its place as hot, molten metal is introduced into the site of that pattern via the inlet or inlets 9.

FIGS. 2A and 2B depict the same mold shell 1 with a pattern 3' having the shape of an H-profile, the web of the profile being again perpendicular to the membranes 2a, 2b. In this instance, however, compaction may be insufficient between the flanges of the "H", resulting in a deformation of the pattern 3' as exaggeratedly illustrated in FIG. 2B. This drawback is obviated by the improved construction of FIGS. 3A and 3B where the prismatic shell 1 has its sidewalls provided with internal vertical ribs 1' and with perforations 8' opening into the intervening grooves or flutes whereby atmospheric pressure acts not only upon the lower and upper membranes 2a, 2b but also upon two similar membranes 2c, 2d lining these sidewalls so that, with the application of suction to the interior of the mold via pipe 5, a pressure differential 7 is developed on four sides. In this way, as seen in FIG. 3B, a deformation of the H-profile core 3' is avoided. The perforations 8' may be omitted, since the flutes are vented at the top of the mold, but their presence insures a more uniform pressure distribution.

In like manner, the two remaining sidewalls of the prismatic shell 1 can be fluted or ribbed, as indicated at 1", and provided with nonillustrated perforations, if desired, in order to establish a pressure differential across two further membranes (not shown in FIGS. 3A and 3B) constituting a continuous flexible liner with membranes 2a, 2c and 2d to which they may be secured adhesively or by other suitable means. The upper membrane 2b, topping the liner, should be at least partly detachable therefrom in all instances to facilitate insertion of the sand mass 4 and the pattern 3 or 3' as well as the removal of the finished casting. The ribs 1', 1" may be omitted in the case of uniformly perforated sidewalls; on the other hand, the mold bottom 1a could also be corrugated, i.e. provided with ribs and flutes, for more effective pressurization.

Naturally, my invention is also applicable to differently shaped molds, e.g. those with a cylindrical shell centered on a vertical axis. With downwardly pointing exhaust pipes 5, for example, liners in the form of prismatic or cylindrical bags made from flexible membranes may be bodily extracted from the shell for filling and emptying. A four-sided prismatic bag of this type, fitting into the shell 1 of FIGS. 3A and 3B, has been illustrated in FIG. 4 (without the top membrane 2b); it will be understood that this bag, being flexible, assumes the illustrated shape only when inserted as a liner into a correspondingly shaped mold shell.

The several membranes, though shown flat in their unstressed state as illustrated in FIGS. 1A, 2A and 3A, could also be preshaped with external concavities so as to be only inwardly deformable.