Aperture Forming Member for Gasifiable Patterns

^• Technical Field This invention relates to metal foundry tech¬ niques, and in particular to a gasifiable pattern assembly for forming cast articles by the full-mold or cavityless process.

Background Art The use of gasifiable foam patterns for pro¬ ducing metal castings is well known in the art. Such patterns can be performe ^' d ^' of expanded polystyrene or other suitable low-density organic plastic which vaporizes upon contact with molten metal. The method of making metal castings by the use of gasifiable foam patterns is known commercially as the full-mold or •cavityless process and consists of embedding a pattern having the exact shape of the casting, with allowances for shrinkage, in sand and then pouring molten metal into contact with the foam whereupon the foam gasifies and produces a casting that is a duplicate of the pattern. U.S. Patent 3,314,116 issued April 18, 1967 to A. Wiltmoser et al discloses such a gasifiable pattern. Usually suitable risers and gates are also formed of gasifiable foam materials and are attached to the casting pattern. The sand surrounding the foam patterns is vibrated or lightly tamped into place and may be unbonded or bonded by one of the cold pro- cesses such as the Furan no-bake system. Since the molding medium must be permeable to permit the escape of gases generated during vaporization of the foam pattern, the medium is restricted to coarse-grained

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sands, generally silicon, and, if used, limited amounts of binder. To overcome the rough surface finish re¬ sulting from the necessary use of large-grained sand, it is common to coat the patterns with a refractory coating. To assure permeability, these coatings are generally on the order of only 3 mm thick and do not significantly increase the strength of the shaping surfaces.

A problem in the prior art pattern assemblies has been the need for relatively large conduits pro¬ viding metal flow passages between feed risers, or headers, and the casting cavity. Owing to the inher¬ ently low strength of unbonded or minimally bonded large-grained sand which provides the casting shaping surfaces, the casting cavity and feed riser must be separated in order to provide sufficient wall thickness and strength. This results in a connecting passage of undesirable length between the strip portions. The problem is further compounded by the requirement for increasingly larger diameter connecting passages as the passage length increases in order to assure suffi¬ cient feeding of the casting during the solidification period.

It is also known to provide porous insulatiye coverings around risers and ^' feed gates to retard cooling of molten metal therein and enhance feeding of the primary casting cavity during solidification. How¬ ever, if insulation is placed between the feed riser and cavity, the two are accordingly spaced farther apart, at least by the thickness of the insulation, and thereby becomes even less desirable. As separation increases, so does the requirement for a progressively larger diameter interconnecting passageway. Use of such large diameter connecting passageways also signif- icantly increases the difficulty in removing risers and gates from the casting after removal from the mold.

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For removal purposes, these appurtenances should ideally be connected by a small diameter section of minimal length while maintaining the above described desirable functional properties. U.S. Patent 3,831,66,2 issued August 27, 1974 to J. R. Nieman et al discloses a stricture formed between the riser and a cost article in a conventional molding process.

Disclosure of Invention

According to the present invention, a re- fractory, aperture forming member is provided for a gasifiable pattern assembly and is positioned between and contacting first and second portions of the pattern.

The aperture forming member is sufficiently thin as to not adversely inhibit the rapid transfer of heat through the member.

Brief Description of the Drawings

Fig. 1 is a sectioned view of a mold having a gasifiable pattern assembly and the aperture forming members;

Fig. 2 is an enlarged fragmentary sectioned view showing the aperture forming member; and

Fig. 3 is an enlarged fragmentary sectioned side view showing an alternate embodiment of the aper- ture forming member.

Best Mode for Carrying out Invention

Referring to Fig. 1, the gasifiable pattern assembly of this invention is generally indicated by the numeral 10. Gasifiable patterns, as known in the art, are generally constructed of expanded polystyrene, but may be formed of other foam plastic or any suitable material that is vaporizable upon contact with molten metal and has low residual ash properties. The

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assembly 10 includes first and second portions 12, 14 that are respectively patterns of a desired casting and a feeder riser. Positioned between and con¬ tacting both the first and second portions 12, 14 is an aperture forming member 16 constructed of a re¬ fractory, non-porous material that is nonfusible at the temperatures encountered in metal casting and which is preferably frangible to allow it to be readily broken and separated from a resultant molded object. Fired ceramics are preferred examples of the material of member 16. The term "non-porous" as used herein refers to a material property characterized by an absence of voids providing channels for the passage of gases through the material. The aperture forming member 16 shown in greater detail in Fig. 2, has a generally web-shaped portion 18 with a first wall surface 20 contacting the first portion 12 and a second wall surface 22 contacting the first portion 14. Preferably, the first wall surface 20 is of a configuration sufficient to conform to the surface of the first portion 12 thereby providing a shaping surface for a casting upon displacement of the first portion by the molten metal. Uniform and rapid transfer of heat between the first . wall surface 20 and second wall surface 22 is provided by the web portion 18 preferably having a generally ^. uniform thickness no greater than necessary to provide structural integrity. Although the minimal thickness may vary somewhat for various material compositions, it has been found that for most fired ceramics a thick¬ ness of 3 mm (1/8 inch) provides sufficient strength for members having a web portion diameter up to 100 mm (4 inches) . Larger aperture forming members may re¬ quire a web portion thickness of up to 6 mm (1/4 inch) . Once the web portion to be used has been sized, one

skilled in the art can readily determine the thickness necessary to provide sufficient strength in the environ¬ ment to which it will be exposed.

The aperture forming member 16 has an outer rim 24 having at least one surface free from contact with the first and second portions 12, 14. The rim 24 is at least partially embedded in a molding medium 26 and functions to maintain the member 16 in place during the casting operation.

A centrally disposed opening 28 is defined ^' by the web portion 18 and provides communication be¬ tween the first portion 12 and second portion 14. The opening 28 is of a size sufficient to provide metal flow requirements during pouring and filling operations and maintain fluid communication through the opening during solidification. Owing to the advantageous heat transfer properties provided by the web portion 18 of the invention, the opening 28 can be of a significantly smaller size than heretofore utilized. The aperture .forming member also provides for contiguous placement of first and second portions 12, 14 of the gasifiable assembly with the area of commonality or mutual con¬ tact being defined within opening 28.

The gasifiable pattern assembly 10 can also include other preformed gasifiable elements. With reference again to Fig. 1, a runner 30 is attached be¬ tween the second portion 14 and a sprue 32. It is also generally desirable to provide additional feed reservoirs to prevent the formation of voids in a casting during solidification. Accordingly, an addi- tional second portion 14" or gasifiable blind riser and a second aperture forming member 16' are suitably attached to the first portion 12.

Fig. 3 illustrates an alternate embodiment of the aperture forming member 16' * . In this embodiment, the rim 24 ' * is offset from the web portion 18'' and

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provides greater rim contact area with the molding medium 26. This configuration is particularly suit¬ able for use with larger castings where additional support is required. 5 The respective elements of the pattern assembly 10 can be directly cemented together with the first portion 12 joined to first wall surface 20, and the second portion joined to second wall surface 22. Any one of several low-residue commercial cements

10 that are combustible at the temperatures encountered in metal casting can be used. One example of such cement is a low temperature hot melt glue produced by 3M Co. and marketed under the ^' name "Jet Melt Bonding System". The first and second portions can be con-

15 nected directly to each other by adhesively joining common contacting surfaces of the first portion 12 and the second portion 14 which extend into or through the opening 28. Fig. 3 shows a separate gasifiable connector 34 spanning the opening 28 and extending

20.. < into the first and second portions 12 and 14, res¬ pectively. The elements may either be cemented to each other or held in place by a nesting interference fit.

The aperture forming member can also be

25 placed in a pattern mold prior to the forming of the foam elements 12, 14. In this manner, the aper¬ ture forming member can be integrally incorporated into either the first or second portions 12, 14 or included as an element in a unitarily molded pattern.

30 Operation

In operation, the gasifiable pattern assem¬ bly 10 is inserted into a suitable flask 36 and sur¬ rounded by the molding medium 26. The molding medium is then vibrated or otherwise evenly distributed or

35 tamped about the pattern assembly 10 to insure uniform

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conformity with all external surfaces of the pattern assembly. If binding agents are incorporated in the molding medium, the mold is appropriately cured to set the binder and the prepared mold is thereafter ready to receive a charge of molten metal.

Molten metal is poured directly into the sprue 32. Upon contact with the molten metal the sprue vaporizes and the gases created are discharged into the atmosphere. The molten metal flows through the pattern assembly, vaporizes the polystyrene foam, and fills the void left by the gasified pattern. During solidification, shrink voids in the casting are prevented by transferring molten metal from the feeder cavities 14, 14' into the casting cavity 12. via the openings 28, 28' in members 16, 16'.

After the casting has solidified, it is re¬ moved from the mold, shaken to remove attached mold material, and separated from the risers and gating.

It is at this point in the process that the value of the present invention can be fully appreciated. As a result of the reduced sectional area and minimal length of the connection between the casting and risers, these now unwanted appendages can be easily and smoothly separated by a force of low magnitude. Quite often when castings are formed by this invention, risers and gates are separated from the casting during the shake- out operation.

The reduced section is also of such short length that the break predictably and smoothly occurs at the connection provided by the opening 28 and sub¬ sequent grinding to remove irregular break lines is virtually eliminated. This construction, therefore, results in avoiding a waste of labor and equipment.

This aperture forming member for gasifiable patterns contributes significantly to the casting art.

By overcoming disadvantages heretofore associated with full-mold or cavityless casting processes, a superior casting is provided in which defects due to voids re¬ sulting from insufficient feed during cooling are sub-*. stantially eliminated. In addition, the necessary pouring and filling attachments to the casting are now more easily and smoothly removed from the formed casting.

While preferred embodiments of the invention have herein been illustrated and described, this has been done by way of illustration and not limitation, and the invention should not be limited except as required by the scope of the appended claims.