US2772458A - Method of making smooth-surfaced sand-resin molds - Google Patents

Description

METHOD OF MAKING SMOOTH-SURFACED SAND-RESIN MOLDS Donald J. Henry, Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application May 29, 1951,

Serial No. 228,996

1 Claim. (Cl. 22---193) This invention relates to the production of sand cores and molds for foundry practice and particularly to a sand-resin composition and to a method for forming the same, wherein thin-walled molds and cores are provided with exceptionally smooth surfaces.

Recently developed techniques in foundry practice incorporate the use of thin-walled dispensable molds and cores composed of sand and a plastic binder. These procedures, often generally referred to as shell molding processes, employ molds and cores which are particularly suitable for the production of precision castings in a wide variety of metals.

In these shell molding processes, the advantage obtained by the use of thin-walled sand-resin molds and cores in casting metal parts to close dimensional tolerances requires that these castings have very smooth as-cast surfaces. A principal object of this invention, therefore is to provide a means for forming uniform sand resin molds and cores to produce improved surfaces on precision or semi-precision cast parts.

It will be understood that the term mold, as hereinafter used, is generally applied in its generic sense to mean a casting form which includes both molds and cores, this invention in no manner being limited only to the former. Similarly, where not otherwise designated, the word pattern is used herein as including both mold patterns and core boxes.

Essentially, the shell molding process consists of using a thermosetting plastic or resin as a binder for the sans. grains to form rigid molds having high gas permeability, good surface smoothness and dimensional stability. The molding material, which is generally a dry mixture of a major proportion of silica sand and a minor proportion of the plastic binder, is used in powdered form with no water added. Phenol formaldehyde and melamine formaldehyde resins are typical examples of the type of thermosetting resin binders preferably used. The sand employed is preferably free of metal oxides, clay, moisture and organic matter.

These sand-resin molds are prepared by allowing the dry mixture of sand and resin powder to remain in contact with a hot metallic pattern for a short period of time. A layer of the mix adheres to the metal surface due to the heating of the resin which entraps the sand with which it is intimately mixed, thereby accurately reproducing pattern details. Pattern temperatures in the range between 250 F. and 350 F. are typical, but temperatures up to 600 F. may be advantageously employed under particular conditions. The pattern temperature and the length of time that the molding material is allowed to remain in contact with the hot pattern surface determine the resulting thickness of the mold. Mold buildup times ranging from a few seconds to approximately one minute are appropriate for various applications. After this short time interval the excess dry mix is removed, and the closely adhering soft sand-resin layer is preferably cured by heating to a temperature within the range of approximately 300 F. to 600 F. for a short period of time, usually from thirty seconds to live minutes, while in contact with the metal pattern. This baking operation results in the conversion of the resinous material to a hard insoluble plasticbinder which securely bonds the sand grains together. The formed molds are, in effect, thin shells which have sufficient strength and stiffness to make them suitable for many casting operations.

Metal patterns must be employed because they are subjected to such elevated temperatures. The half patterns, gate and runner usually are all permanently fixed on the metal plates.

It will be appreciated that the degree of smoothness of the mold surface is determined largely by the screen analysis of the sand used in the dry mix and the particle distribution in the portion of the sand which first strikes the hot pattern plate. If the sand has a uniformly graded screen analysis from very fine to coarse particles, or if all the sand contains grains of approximately the same size, the smoothness of the mold depends entirely on the aforementioned particle distribution. Using only extremely fine powders, such as silica flour, which is a form of finely divided silica, I have produced molds having exceptionally smooth surfaces. However, such molds not only require an extremely high plastic content but also possess low transverse strength due to the formation of cracks on its back surface. On the other hand, a mixture of purely coarse sand with a thermosetting resin results in the formation of relatively strong molds, but castings resulting from the use of such molds have rough surfaces due to the coarse grain pattern on the mold surface.

In accordance with my invention, I have obtained accurate castings having very smooth as-cast surfaces by using especially prepared sand-resin mixes for precision shell-type molding in which the ceramic or refractory filler portion is composed of two constituents having widely different screen analyses. Each of these refractory constituents should have sand grains of fairly uniform size, but the average particle size of the two constituents preferably should vary to a considerable extent. Thus I have found that a molding mix containing a coarse refractory filler material and a fine refractory powder or flour meets these requirements. Accordingly, with a dry mixture of silica flour, sand, and the required amount of binder I can produce a shell-type mold having the exceptional surface smoothness which I could otherwise obtain by mixing the resin with only silica flour. More over, a mold formed in this manner also possesses the strength and hardness obtainable only from coarse sand. A thermosetting resin, which may be of the phenolic type, is preferably added in amounts ranging from 4% to 20% by weight, depending upon the particular mix, and has proved highly satisfactory as the plastic binder.

Accordingly, I have obtained excellent results by using a sand-resin mix containing 8% to 40% by weight of silica flour, 50% to 88% by weight of sand and 4% to 20% by weight of binder. Where exceptionally smooth surfaces on the molds are desired for precision work, however, the silica flour content may advantageously be raised to as high as approximately 50% by weight. Under these conditions the amount of sand used may be reduced to as low as approximately 40% of the total weight of the molding mixture. Similarly, there the sand employed is less coarse than those which would normally be used in accordance with this invention, smooth-surfaced molds may be obtained with as little as 3% by weight of silica flour and as high as 93% by weight of sand. In fact, most semi-precision castings may be satisfactorily produced using molds having silica flour contents near the lower end of the above. ranger.

As an example of specific mixtures of refractory materials to be used, useful ratios of coarse and fine refractory materials include a mixture of 60% by weight of coarse sand having an American Foundrymens Society number of 40 and 40% by weight of No. 140 silica flour. In this molding mixture, the No. 14-0 silica flour is of such a fineness that 85% of it passes through a 300 mesh screen while 99% passes through a 140 mesh screen. The A. F. S. No. 40 sand is of sufiicient average" coarseness and particle size uniformity so that only about of this sand passes through a 100 mesh screen and 60% passes through a mesh screen. Substantially none of this sand will pass through a 140 mesh screen.

Similarly, commercially available so-called silica bond and banding sands may be used as the coarse refractory constituent. For example, a bond sand having an American Foundrymens Society fineness number of approximately has proved highly satisfactory. This sand is still sufficiently coarse so that about 67% or" the sand will be retained in a 70 mesh screen, and only about 10% of it will pass through a 100 mesh screen. When silica banding sand is used as the coarse filler constituent, a mixture containing to 88% by Weight of this sand having an A. F. S. fineness number of approximately 76, 8% to 30% by weight of the aforementioned silica flour and 4% to 20% by weight of a thermosetting resin binder produces excellent results. This banding sand contains grains of such size and has a limited particle distribution to permit only approximately 7% of the sand to pass through a 140 mesh screen and to pass through a 70 mesh screen. 73% of this sand will be retained by a 100 mesh screen.

Superior results are also obtained with a refractory mixture comprising, by weight, to of a finer silica float sand and 5% to 15% of silica flour to which is added an appropriate amount of a thermosetting plastic binder. As an example of the use of this type of sand, a mixture of the aforementioned binder with a refractory filler consisting of approximately 90% by weight of silica float sand having an A. F. S. fineness number of 107 and 10% by weight of No. 140 silica flour results in molds having very smooth surfaces. A screen analysis of this particular float sand indicates that 49% of the sand will be retained by a mesh screen and only about 13% of it will pass through a 200 mesh screen. This sand and the coarse sands used in the two preceding examples are preferably commercially pure silica.

It will be noted that the above float sand is less coarse than the other sands previously described. If the flour content were to remain unaltered, the amount of thermosetting resin used would generally have to be raised somewhat for best results. However, the use of the somewhat finer sands frequently permits the use of a lesser amount of flour, which in turn allows a reduction in resin content. Of course, if exceptionally smooth surfaces are required for castings a higher silica flour content may be preferable. In this latter instance it should be realized that as the over-all fineness of the sand mix increases, the plastic requirement becomes accordingly higher.

It will be understood, of course, that my invention encompasses generally a sand-resin molding material for shell molding comprising a mixture of a refractory flour or powder, a thermosetting plastic binder and sands which have substantially all their grains of sufiicient size to permit the rapid seepage or flow of an appreciable portion of the molten binder and flour between the coarse sand particle to the surface of the formed mold-shell which abuts the hot metallic pattern. The details of the mechanics of this mold formation will be hereinafter more fully explained. To obtain this desirable result sands, such as those noted above-which possess relatively high particle size uniformity, are preferred. In accordance with my invention, I have found that coarse .sands having American'Foundrymens Society fineness numbers between 35 and 90 generally are preferable, although in some instances it may be desirable to use sands having fineness numbers as low as 25 and as high as 135. Generally any refractory flour or powder may be used with these sands to produce the desired smooth-surfaced sand-resin molds, but the flour employed preferably should be of such a fineness to permit at least 75% of it to pass through a 200 mesh screen.

It therefore will be noted that this invention is not restricted to refractory materials having specific mesh sizes or to merely silica sands and flours. Zirconium silicate sands and flours, for example, may be satisfactorily substituted for the silica refractories in most instances, the relatively high cost of these materials currently being the principal objection to their use.

When the dry mixture of coarse sand, silica flour and thermosetting resin contacts the hot metal pattern the mold formation occurs in the following manner. During the initial part of the molding period, the plastic binder melts and flows by gravity to the surface of this pattern plate, carrying with it the very fine silica flour with which it is intimately mixed. Bridging of the coarse sand particles occurs, thereby permitting the plastic-silica flour mix to seep to the pattern surface. Close examination of a transverse fracture of such a mold shows a very high percentage of the fine silica particles to be at or close to the mold surface. The mold surface or facing thus formed is extremely smooth, and reproduction of every minute detail of the pattern is thereby obtained. The reinforcing back portion of a mold formedin accordance with this process, on the other hand, possesses a layer of relatively strong resin-bonded coarse sand. The sand-resin shell-type mold thus produced has excellent permeability and transverse strength and shows no tendency to cause blowing on the casting when molten metal is poured into it.

Upon pouring the liquid metal into the mold or core cavity in the usual way, the hot metal, on coming into contact with the mold or core, burns the plastic binder to essentially carbon. The gases which are generated readily escape through the highly permeable sand-resin shell. As a result of this plastic breakdown, the shakeout is easily accomplished.

Among the numerous advantages of this process and the molds produced thereby is the fact that the molds and cores offer very little resistance to the expansion and contraction of the molten metal subsequent to pouring, thus minimizing the danger of the formation of cracks or hot tears. Moreover, the resulting castings have unusually smooth and clean surfaces, true dimensions and a minimum of fin at the parting line. The surfaces of these castings are free of residual mold material, thereby eliminating the necessity of shot blasting. This process and the molds formed in accordance therewith can 'be used to provide castings of extremely thin section because of the unusual smoothness and high gas permeability of the molding material. Moreover, the cured molds have no aflinity for water, are completely stable under atmospheric conditions, and may be stored indefinitely. Furthermore, these molds can be produced and'processed without dust formation.

As a consequence of the aforementioned desirable qualities, molds formed in accordance with my invention faithfully produce pattern details, maintain good dimensional tolerance and possess excellent surface qualities. Such molds permit the production of sound castings in a wide variety of metals and alloys over a large range of casting temperatures.

It is to be understood that while the invention has been described in conjunction with certain specific examples, the scope of the invention is not to be-limited thereby except as defined in the appended claim.

-I claim:

The process for forming a multilayer shell-type sandresin mold which comprises forming a dry mixture :of

coarse and fine refractory fillers and a thermosetting resin binder wherein said refractory fillers consist of approximately 85% to 95% by weight of a silica sand of substantially uniform particle size and 5% to 15% by weight of silica flour, said binder constituting 4% to 20% of the 6 References Cited in the file of this patent UNITED STATES PATENTS 805,144 Kuller Nov, 21, 1905 1 6 Dietert Sept. 16, 1941 Grossman Mar. 4, 1941 Grossman Mar. 4, 1941 Kleeman June 22, 1943 Whitehead Feb. 10, 1948 Feagin et a1 May 18, 1948 Barr Dec. 6, 1949 Weston Aug. 8, 1950 Vincent Dec. 19, 1950 FOREIGN PATENTS Germany Jan. 29, 1926 OTHER REFERENCES The Foundry, October 1950, pages 162, 164 and 168. FIAT, Final Report No. 1168, 6 pages.