US2836867A - Process of making mold - Google Patents

Description

June 3, 1958 x, BEAN 2,836,867

PROCESS OF MAKING MOLD Filed Dec. 4, 1950 Q Fm-51$.

, VENTORI XAe/FA Z. 354M ATTORNEYS United States i tent rnocnss or Martino MOLD Xarifa L. Bean, Yellow Springs, Ohio, assignor to Morris Bean & Company, Yellow Springs, Ohio, corporation of Ohio Application December 4, 1950, Serial No. 199,006

10 Claims. (Cl. 22193) This invention relates to the casting of molten materials and to the molds and patterns used therefor and to the making of patterns and molds and more particularly to the making of molds of highly refractory materials, such, for example, as silica sand, which are suitable for use in the production of smooth, accurate and sound castings of a wide variety of sizes and shapes, of iron, bronze and other relatively high melting alloys.

A mold for precision casting, especially one suitable for the precision casting of ferrous and other high melting alloys, must not only be refractory enough to withstand the pouring temperature of the metal but must present such a smooth surface as to give the desired surface to the metal as cast. The surface of the mold must also be sufficiently permeable and with a sufficiently permeable backing to support the metal at the desired surface but to provide for the ready escape of gases and vapors. Obviously such a mold should be relatively cheap, as to labor, material and equipment costs; also it should be easy to remove after the casting has cooled or set.

An important feature of the invention is the use, in the formation of the mold, of a disposable pattern of such composition that it is initially strong and dimensionally stable at room temperature, whereby it forms an effective and accurate support for the mold, which has substantially no initial strength at room temperature but acquires its strength by setting of the binder, the properties and relative strengths of the mold and pattern materials being such that the pattern retains strength to hold the mold surface and mold dimensions accurate during the bindersetting step to a point at which the mold has developed suihcient strength to hold its own form; but thereafter the pattern will be transformed in such a way that it can be easily removed from the mold or wasted without damage to the mold surface.

Another important feature in the cost of castings is the pattern cost or the cost of pattern molds. For practicable processes it is important that the capital investment tied up in expensive pattern equipment be made as productive as possible. This means of necessity that, for production quantities, it is possible to obtain several (10 to 30) impressions per hour a given pattern. it is also desirable that such a process should permit the use of flexible resilient patterns or molds whenever intricate shapes are concerned, especially those involving undercuts.

It is therefore one object of the invention to provide improved patterns and molds for casting of metals, and a method for making the same. It is an object to provide a high degree of accuracy, high strength and rigidity in the mold and in the pattern respectively, during use, but to provide a method by which the strength of each is terminated and its removal from the finished mold or casting is easy and inexpensive. It is an object to provide a method of accurately controlling permeability and heat transfer. It is also an object to provide in a refractory casting mold, for casting high temperature metals "ice such as ferrous alloys and bronzes, a surface as smooth and true as that produced by plaster molds with low temperature metals.

Another object of the invention is to provide an inexpensive and rapid method of reproducing molds for casting high temperature metals; and a further object is to provide an economical and rapid process for multiplying patterns for use in making casting molds without loss of precision in the multiplication process.

In general the invention aims to provide a simple process of preparing molds (and cores, which are here considered a type of mold) particularly of highly refractory materials, which are suitable for the production of castings of all sizes (as distinguished from the necessarily small castings made by investment molding over Wax or plastic patterns), in a Wide variety of metals, and par ticularly precision castings of ferrous and other high melting alloys, by which process such molds can be produced easily and quickly and at comparatively low cost for both materials and labor.

Other objects and the manner in which the objects are achieved will appear from the following description and claims.

To this end the invention relates to an improved process of building up on a pattern a mold of desired properties and dimensions from refractory mold material, such as quartz sand of suitable degrees of fineness, and with strength, permeability and surface character modified by use of a binder; and, if desired, of surfacing materials.

This invention also relates to patterns and to the treatment of patterns to facilitate their use in molding bonded granular materials such as sand and resin mixtures and to improve fidelity of surface reproduction. The invention is also concerned with the production of accurate patterns so inexpensive, and of such character that they can be disintegrated and wasted when the formation of the mold is complete, and with economical ways of disintegrating the patterns and cleaning the molds.

The invention also relates to a parting and method of parting molds from the patterns on which they are formed and cured.

The invention also relates to methods of making and processing molds so that uniformly successful results can be obtained, on molds of a wide variety of size and design, under commercially practical and economical processing conditions.

In the practice of the process in the preferred manner the pattern over which the mold is to be formed is made, as above suggested, of a material which initially has sufiicient rigidity to hold its form, surface character and dimensions with high fidelity at room temperature and and under necessary conditions of handling in foundry practice and under the heat to which the mold material must be subjected for setting the binder and until the mold has become self-sustaining. The preferred pattern material is also of such a nature that it is gradually transformed by the same treatment which sets up the bond so that at the end of the binder-setting treatment, the pattern material will have been disintegrated, or its disintegration initiated, e. g., by the pattern binder becoming fragile or friable (or sensitive to water or air or other readily avialable material) so that it can readily be disintegrated and removed from the mold.

It has been found in practice that a suitable pattern material for the purposes aforesaid is a plaster composition comprising a substantial portion (e. g. 20%-l00% calcium sulfate) with or without filler. The material used and the method of forming it into any desired shape, as described in U. S. Letters Patent to Morris Bean No. 2,220,703, granted November 5, 1940, have been found suitable for this purpose, and dimension stabilizing agents following advantages for the purposes with undercuts.'

gasses? .3 may be used, e. g.. those set forth in my Patent No. 2,391,855 and application Serial No. 125,645, filed vember'4, 1949; but the recrystallization treatments are not necessary. The use of resilient flexible materials for the pattern molds, such as the elastic vinyl polymer gels as set'forth in said Bean Patent No. 2,220,703, or syrithetic rubber thermal setting compounds,"is'iof important advantage. i V

More specifically, calcium sulfate plasters havethe aforesaid; ('1) Ease of forming (as a liquid slurry). (2) Rapidity of set (4 to 7 minutes). (3) High green strength (50 to 200 p. s. i.)., so that they can be drawn without damage otf flexible patterns (4) Good accuracy (plus or minus .0001" to .0005" per inch deviation when cast over a master pattern)- (5) Good dimensional stability, especially when mixed withja filler such as 'sand, at somewhat elevated temperatures (to400 deg. F.).

(6) A porous structure that contains free water, or can" be made to cotnain other fluids as a means of achieving the dejsired 'resu lts. Also a structure which will perma the introduction by driving or other simple means, of certain metallic blades, pins or other inserts later to be ineorporated as a part, of the .final casting.

' (7) Very easy wastability, especially if dehydrated by a suflicientheating process. It has been found that if a calcium sulfate bonded pattern or pattern mold is heated, especiallyto "t'he soluble anhydride condition, it is easily Wasted, either by an air jet (generally the preferred method), orby exposure to water. If sufficiently dehydrated plaste'nisimmersed in water it is immediately transformed to awet mass having the consistency of a loosemud, which is easily washed away, even by tap wate'igthough increased water pressures increase the speed of removal. (If set hydratedis immersed in water, it is not affected, and will maintain nearly its normal set strength for several days.) The next step in the process of making molds according to the present inventionis dependent somewhat upon aterial employed to coat the .p'atthe character of the m tern. .In the preparation of a mold for greatest smoothness of surface and minuteness and accuracy of detail,

three factors are important:

1) The particles on the surface must be of sufficient smallness of size to give-the surface quality desired.

(2) All particles must be packed closely together.

(3) Each;particle of the mold must .be bonded sufii- I ciently well to its neighbors so that pattern removal cannotdislodge it.

The extent to which conditions 2.and 3 are met determines the quality of result which can be obtained with particles which are small, but

gypsum plaster which is not de-.

still of such character that perfect bonding, it is necessary that the bonding 'agentbe present and effective (especially strong enough to resist damage in pattern removal) at the points of contact of'the sand grains. By proper selection of sand, binder, and method of distribution of binder, it is possible'by forming it, according to the present invention, against a disposable pattern such as that described below, to make a mold with a single kind of sand which will have surface smoothness, accuracy and castability (permeability and refractoriness) so that the casting obtained therein has the required properties as cast. For example, it has been possible and practical tomake castings of gray iron for tire molds by this method which meet the exacting requirements of the trade. I

The best binders for the purposes herein described are of the class known as thermo-setting resins whichare adhesives to sand. Several phenolic type resins have already come into use for the bonding'of sand in foundry molds, and we have found these verysatisfactory for the present invention. However, other drying or settable binders, whether set by heat or by catalyst, can be used in the invention.

The effectiveness of a binderin this invention, is not only a matter of its adhesiveness and strength, but also of its distribution. It is so difiicult to obtain, and particula'riy to maintain, satisfactory 'binder distributionfor the exacting requirements of castings of unusual quality that! have found it desirable, in utilizing this invention, to make a separate step of binder distribution which makes possible (1) complete distribution and'continuous coating of each grainof sand, (2) a dry, rec-flowing sand, easily distributed and closely anduniformly packed over a pattern, and capable of developing a strong bond the sand is coated with a lacquer-like coating of resin,

a mold body, if made out of' these same'particles, will have good permeability to gases.

perfect packing and bonding of that pass through a 100 mesh screen. Ordinarily the green bond which has to be reliedupon in conventionally drawing a sand mold'from a pattern is not sufficiently strong so that those grains which are packed into sharp corners or minute detail can be removed with the mold,

so that though surfaces. may be fairly smooth, sharp corners will have lost theirtrue sharpness. As has been explained, in the, process which weuse, the bond which very strong, especially It. is surprising the surface quality which can be obtained, for example, by. fraction of a washedround grainedsand, suchas an Ottawa'sand, which willby a heating process, I

so that it will flow like a fluid; and that it be made to ocpupy the minimum possible volume, asby ketone, alcohol, etc.

and the coated sand is again dry and free flowing. Good materials to use, for example, are a phenol formaldehyde resin such as the Bakelite Division of Union Carbide &

Carbon Corporations BR15401, or BR18554 or BR18478;

and a volatile solvent such asacetone, methylethyl In general, from 1%.to. 4% by weight of binder is used and. from 1% to 4% of solvent.

One ivery satisfactory formula is kg. of screened:

Ottawasand, 2 kg. BR18478, and-3000 cc. acetone. A

conventional sand muller works very wellfor the mixing operation.

Sand thus coated will bond perfectly, even if heated: in the presence of a large,; wet disposable pattern. If the resin is distributed infinely divided form by conven-- tional means (whether as a powder or suspended'in water;

or other non-solvent liquid, or as the commercial liquid binder) it is likelyto be subject 'to migration into the wastable pattern, so as to seriously interfere withthe; separation from the mold, or tosiutering without bonding, and not to remain effectively distributed The proper distribution of binder is very-important to the uniform, high quality resultsof this invention. Itis important also for securing the most eifectiveuse per unitcost of binder.

Although it is possible to make a out of a single kind of sand, such as described, it is frequently more satisfactory to use a fine sand for a facing layer, and a coarser sand for aback up. This is done; by taking a fine fractionof the coated sand (through a. for example) and or through a 200 mesh screen,

pouring it over the damp disposable pattern. The fine particles will stick as a very thinlayerto the damp surface of the disposable pattern, and the balance can be A 70 or a 50 meshsand can. then be poured poured out. in, thewhole result obtained: This pi'fo'cedure puts the fine sand on smooth, accurate mold mass vibrated, and a very satisfactory. total the surface of the mold, where it is needed for smoothness, sharpness, and reproduction of detail, and permits the use of a more permeable backing.

A very good result can be obtained by using a disposable plaster-bonded pattern just as it is made, without any further preparation. However, if desired, it can be dampened or saturated with (e. g. by dipping in) a nonsolvent liquid such as water, oil, or kerosene, or otherwise treated so that temporary adhesion of every particle of the fine sand layer is assured for smoothness and accuracy of surface reproduction. No difficulty is experienced in separating the finished, baked mold from the disposable pattern when a coated facing sand is used as described, as there is no opportunity for the resin to penetrate the surface of the disposable pattern in any way.

By the means just described, as fine and as refractory a surfacing layer as desired can be made, and also as permeable a mold body as is necessary for the conditions of casting.

The mold is made to any desired depth and dimensions. It has been found desirable, in general, to make molds of substantial thickness, so that distortion due to'bending during casting will not occur. Differential thermal expansion (between the side next to the metal and the side furthest removed) may cause mold distortions (which are reflected in the casting) greater than can be tolerated in castings which must meet high standards of accuracy. It has been found that any strongly bonded sand, such as conventional core sand, will distort amounts which are prohibitive for accurate work, unless corrective measures are used. This is a distortion similar to that which occurs in a slab of any material when it is suddenly heated on one side while the other remains cool. The addition to the sand mix of materials such as wood flour, or the incorporation in the mold of properly designed and placed steel reinforcings, will reduce the tendency to distort, but these methods are usually not as effective as increasing the mold depth or thickness to a point where the thermal stresses cannot induce bending.

The accuracy of molds made as we have described is very great, even in cases where there is a volume change (e. g. shrinkage) in the total mass of sand.

it has also been found that the presence of other binders, such as urea formaldehyde resins, or hydro-setting cements, which first bond the sand at a lower tem perature than the phenol formaldehyde binders, will set the mass as a constant volume before any shrinkage occurs, and accuracy, not only of surfaces supported or contacted by the wastable pattern, but of the whole volume of sand, will be the result. Such auxiliary binders are not necessary; and whether one is used will depend largely upon special requirements of the mold and casting to be made. Other thermo-setting binders and also low temperature setting binders may be used in lieu of the phenol-formaldehyde resins, but my experience has shown that advantage is gained by use of the type of resin specified above.

It is an important characteristic of this combination described that the sand mold is bonded so as to have its form fixed before any serious dimensional changes occur in the plaster pattern. When, at higher temperatures, such dimensional changes may tend to occur in the plaster, the mold is already stronger than the pattern.

An alternative method to the one we have described for obtaining a smooth, accurate mold surface against the disposable pattern is to make a liquid slurry of a fine refractory and binder. The most satisfactory slurry we have found to consist of finely divided silica sand, or silica sand and clay, a combination of dry powder and liquid phenolic binders in about equal amounts, and a liquid vehicle. it has been found that again the problem of resin distribution is very important, and that a water vehicle and a combination of the two commercial forms of phenolic binder give the most practical re-' sults. The combination binder, of both dry and liquid forms, will bond the fine facing sand much more efiec tively in the presence of a large, wet, disposable pattern, than either used alone. Another satisfactory method of making a slurry is to coat the fine refractory particles with solvent and dry resin as described above, and then add the water or other liquid vehicle. Since the dry resin is insoluble in water or other suitable liquid vehicle, it will remain effectively coated on the refractory particles in the presence of the vehicle, and during the curing operation.

If a wet slurry is used, it is usually necessary to apply a separating layer to the disposable pattern before the mold'is made. Otherwise adhesion occurs between the disposable pattern and the cured sand mold and it is very diilicult to remove the pattern from the finished mold except by disintegrating it with water and washing it off. Though goodv results can be obtained in this way, it is often awkward and messy, especially on large work, to disintegrate with water and requires separation of the plaster from the water in settling basins or tanks and redrying of the mold before casting.

The parting which we usually prefer to use is a solution of commercial flexible collodion in acetone, which is applied as a spray to the surface of the disposable pattern. Before the spray is applied, the disposable pattern is saturated with water so that the collodion solution cannot penetrate it, and in itself prevent the effective separation of the mold from the pattern. The collodion makes an excellent (continuous and extremely thin) separating film, and the finished mold when set can easily be freed from the disposable pattern by means of an air blast. The collodion film adheres to the binder in the slurry coat and is thus transferred to the mold in the baking step (the temperature of the baking in such case being kept below that at which the collodion coat would be destroyed) and thus it serves to protect the surface, during the cleaning and subsequent handling of the mold. It is quickly burned off in the casting step with no difficulty in the casting process or damage to the final casting.

Smooth films of refractory powders, especially of pigment grade, such as graphite, red iron oxide, aluminum, alumina, etc. can be used to form separating layers also, and frequently with important advantages, but means have to be provided to hold them in place on the surface of the disposable pattern so that the slurry does not dislodge or lift them. These are best applied as dry powder while the surface of the pattern is moist with water or other liquid which will hold a 'very thin layer and allow any localized excess to be redistributed by blowing or light brushing. A spray of potash soap or other soap applied over the pigment is an efiective means of doing this. The calcium soap produced where the soap solution meets the calcium plaster forms an insoluble film by which the powder is held, so that, during application of the slurry coat, the powder is held in place to perform its function as a separating layer. In the baking step, the refractory powder will become bonded to the mold surface by the bonding agent in the slurry, and will form the outermost surface layer of the finished mold, and most faithfully reproduce the sharpness and detail thereof. In cases where special surfacing layers are desired 0n molds, by reason of chemical, physical, economic or other factors encountered in making castings, this is an excellent method of achieving the desired result easily and efiectively, and especially without loss of surface detail or accuracy.

The slurry coat is preferably sprayed against the waste pattern, so as to avoid trapping of air, although it can be poured over the waste pattern and promptly poured out so that just a thin layer remains, if it is convenient to take the precautions necessary to avoid trapping of air, or in forms of molds such that that is no problem.

Following the application of the slurry coat, which as above pointed out is made with a very fine sand, or other refractory so that the face of the mold that forms the casting will be smooth andw'ill reproduce accurately the finer detail of the pattern, a rigid backing is formed there on, and this backing is made more permeable than the facing. The backing material may be introduced as a dry mixture of sand and binder, preferably a coated sand such as has been described, and whichhas the advantage of flowing freely and uniformly into all corners and making a completely continuous supportfor the slurry coat;

orit may be as a' wet mixture, which has the advantage -of' having some green strength and abiity to remain packed in position without the benefit of'cornplete external flasks or other support. The wet material is not as easy to distribute and pack uniformly as the dry material. The design and requirements of the mold will determine whichtype of backing is selected.

For a good'bond between the slurry coat and the backup, it is advantageous that the slurry coatbe wet (not air dried or robbed of water by the wicking .action of the disposable pattern) when the backing is applied. This permits individual grains of back-up sand to bed themselves in the slurry coat, which seems .to'be important if the best bond is to be developed. As noted previously, it is an advantage for the mold to be made for the complete setting of, the resin. varies somewhat 18. leaves .themold ready for immediate use after itsremoval from the, pattern, it is also-possible to effect removal of the pattern by soaking theinold and pattern in water and using a water jetto remove pattern particles from the deep recesseszof the mold,lbu t in'such case removal. must be completed promptly because the ,plaster will re-set before long, after it --has-been' wet. Another advantage .of the .air jet .method'is that it permits use of mold compositions, which would be deteriorated by water- Thus .in some cases onemayadvantageously use bentonite, cereals, or water soluble =or swellable binders, especially :in the backinglayer, applied-before or afterthe final treatment of :the mold face, and this could only be practicable if .the removal is done without-the useof water; l e I I When the casting to be made is such as requires inserts, as for example :machinedthreads, 'sheetmetalparts or wire, etc., toform narrow projections from the mass of the casting, this can the-done according to my invention by driving these into the plaster patternin the appropriate locations and depths, or byv placing them in the molding pattern and casting plaster around them, with the desired portions projecting for anchoring in the casting mold. .111 this manner, for example, We have successfully cast tire molds withinserts of 18-8 stain less steel strips formakingnon-skid divisions in the tire tread, although that .had previously been found impos sible in castitonmo-lds due'tozachange in thesteel from austenitic to martensitic.

, {From the foregoing description of the inventionsit'will be seen that .the practice -of the novel process of making V molds for precision casting of metals, and particularly with the size of the mold and runs'usually anywhere from 3 to 12 hours. In general it'is advantageous to bake the molds at a temperature which will not overcure part of the mold so long as other portions slower to heat up are not yet sufficiently baked.

It will be seen that, because of the Water in the pattern, the temperature of .the pattern and of the mold will not immediately be raised to the resin-setting tem-' perature for setting the phenolic type resin; and that as a matter of fact the temperature will not increase much above the boiling point of water or other saturant in the pattern until the pattern is dry and atleast partially dehydrated. It is this fact which makes it so important as explained previously that special means he provided for obtaining and maintaining etfective distribu tion of the resin, so that it cannot migrate under conditions of slowly rising temperature, or sinter without'bonding; and as an alternative, makes desirable the use of resins which set at lower temperatures and/or in. the presence of water vapor. evaporate the water as quickly and uniformly as possible, and therefore good circulation of dry air shouldbe maintained in the oven.

After the pattern is dried the temperature rises above thatat which the bonding resin is set or cured and 'above' that at which the disintegration of the pattern is initi-, atedin the case of plaster patterns by dehydration of the calcium sulfate. It will be seen that in the preferred operation, the dehydration of the plaster pattern takes place as a result of the heating used forsetting the resin, and this dehydration makes the pattern-relatively Weak and disposable, and as the calcium sulfate has been converted into, soluble anhydride, .it is in condition which can easily be further weakened by soaking with water. The pattern .is thus easily removed from the now set and self-sustaining mold e. g., by being broken away and by the use of a jet of compressed air to lift out any portions which remain embedded in mold recesses. Although the air blast method of cleaning the residual pattern material out of the molds is preferred, since this It is ordinarily desirable to for the precision casting of ferrous and other high melting alloys, which has been described :hereinabove results .in the :production of an accurate smooth mold which does .not depend atlall for its smoothness .on a wash, dip or other surface smoothing treatment, but reproduces with high'fidelity the surface of the original. over, such amen .can bemade with any desired permeabilitypover l0 :and advantageously from SO'to 200 as measured on .a standard A. F. A. test piece-in accordance with the standards setup by theAmerican Foundry mans Association. It willfurther be seen that by the present'inventionit is possible to make a one piece mold using a disposable patter-mas hereinabove described,not-

withstanding vundercuts such that, if a'rigid permanent pattern were used, it could notbe withdrawn therefrom- An important feature of the invention when practiced as hereinabove described is that, although the .mold, as it is built up on the pattern, hassubstantially no strength until the bonding resin is set, the pattern employed is initially strong at room temperature and maintains its form and dimension :stability during the binder setting operation and until; the mold has acquired suflicicnt strength toxhold its form and dimensions; whereas the pattern loses its strength so'that it can be readily disintegrated or broken away from the mold after the mold has thus gained its strength.

The mold produced by the process hereinabove described, that is the :mold thatis obtained after the baking operation and after removal of the disposable pattern, may, if it is desirable, be combined with other parts or incorporated as part of a larger mold before casting.

It may be used .in combination with green sand mold parts, conventional baked sand cores, chills, inserts or other foundry mold materials within the skill of the foundryman.

When the moldis'usedfor casting high melting alloys, for eirample those which are'cast above a temperature of 1200" 'C., the heat will cause disintegration of the binder. In other words, the binder will be charredto carbon and may be substantiallyoxidized,but not before the metal has set to the shape. of the mold. This burning. of the-binder makes the mold. very easy to remove from Morethe finished casting, and thus adds to the economy of the process.

Although I find it highly advantageous to use plaster patterns according to this invention, other aspects of my invention can be more broadly applied. For example, we can use my invention in a modification of the lost wax process, wherein the pattern is made of any material which is readily formed to the pattern shape and which is:

(l) Disintegrated by conversion to a pourable liquid or vapor at a temperature above the setting temperature of the thermo-setting bonding resin in the mold, so that it can hold its form and surface until the mold material is set up and thereafter can be poured out through the mold gating or drained off through the porosity of the mold;

(2) When melted, flows freely off from the mold surface so that any residual film does not clog the permeability of the mold;

(3) Can be repeatedly remelted and reformed or is cheap enough to waste.

(4) Does not dissolve or chemically attack the bonding material in the mold.

(5) Any residue retained on the mold does not objectionably attack the molten metal or other material which is to be cast into the mold;

(6) Is safe to use under plant conditions.

Higher melting point waxes (particularly the higher wax-like paraffins or so-called petroleum waxes) can thus be used in a lost-wax process using all of the mold making and metal casting features of my invention; and likewise patterns may be die-cast with die-casting alloys e. g. of tin with lead and/ or copper and antimony. Such alloys are available which melt at various temperatures from 380 F. up, so that one may be selected which melts a little above the curing temperature for setting up of the particular bonding resin being used. Thus, in the baking of the mold, it is first brought to setting temperature and after the mold is heated through and the bond has set, the temperature of the pattern is raised above its melting point and the molten alloy is poured out. We have also found that if the slurry coat and/or moist sand mixture with the bonding resin is molded on a wax pattern and then allowed to dry, particularly if the mixture contains a solvent for the resin, which does not dissolve the wax, the resulting mold will hold its form during careful heating even after the Wax is softened and fuses, and that a good mold can be obtained in this way.

Although we are giving below a specific example and are showing the same in accompanying drawings, and although we have given various specific preferences, recommendations and alternatives, it is to be understood that these are not exhaustive or limiting of my invention, but are illustrative and for the purpose of instructing others in the principles of the invention and how to modify it so that they may be able to use it in a variety of embodiments each best suited to the conditions and requirements of a particular use.

In the drawings:

Figure 1 shows a flexible resilient pattern mold ready for molding a disposable pattern therein;

Figure 2 shows the pattern made from the molding pattern mold in Figure l inverted and ready for the application of the mold material thereon;

Figure 3 is a view in vertical section of a mold built up thereon after baking and removal from the pattern and assembly in a green sand investment;

Figure 4 is a view in vertical section of the metal casting made in the mold of Figure 3.

As illustrating the practical utilization of the invention, I have shown the making of a steel mold for molding rubber or plastic (specifically shown as a tire mold).'

A suitable flexible resilient plastic is molded over the article or model (in this case a mold for rubber tires) to be reproduced, to produce an accurate negative im- 10 pression-lO of its surface. (In the case of a mold for plastic articles, as illustrated, this model will ordinarily be a negative of the plastic article made, for example, by casting plaster onto such article or an artists model of it). For this pattern mold, an elastic gel of vinyl chloride and a suitable plasticizer such as tri-cresyl phosphate has been found satisfactory. This pattern may also be made of a synthetic rubber, especially one of the class which is caused to vulcanize at room temperature by means of chemical accelerators. It has been found by experience that these synthetic rubbers have the advantage over the vinyl gel that they can be formed to greater dimensional accuracy and retain their dimensional stability over longer periods of time. Using the vinyl gels for example, the operation of making them is as described in the patents of Morris Bean Nos. 2,435,643, 2,404,528 and 2,349,806 excepting, of course, that here I make a negative pattern mold 10 (i. e., a negative impression with respect to the metal casting to be made) instead of a conventional pattern, which is generally a positive with respect to the metal casting.

The pattern mold 10 thus made can be used over and over, and plaster composition wastable patterns 14 taken off it in rapid succession, with negligible deterioration of the pattern mold from extended use.

The next step is to pour a plaster slurry into this pattern mold to make a positive pattern 14. This slurry may advantageously be made of a base composition containing 40 parts of molding plaster, 50 parts of Ottawa sand approximately 50 mesh in size (Bond Sand) and 10 parts of talc. If desired approximately /2 part each of sodium silicate and Portland cement may be added to increase the dimensional accuracy of the set composition. Also /2 to 2 parts of terra alba (calcium sulfate dihydrate in a finely divided powder form) may be added as an accelerator to bring the setting time into the 4 to 7 minute range which is so important to the economical commercial use of the pattern mold. The composition is made into a liquid slurry by the addition of approximately 54 parts of water to parts of the plaster composition.

This slurry is poured over the pattern mold which contains it until it sets. When the plaster in the composition has set, the disposable pattern is removed from the pattern mold, leaving intact an accurate impression of its surface. (The surface is formed entirely by the fine plaster and talc particles in the slurry, and the quality of the surface is not impaired by the presence of the sand grains in the mixture. The sand is added because it is a cheap filler, and also because its presence aids materially in easy disintegration in the disposal step). The disposable pattern is now ready to be used in the further step of mold making.

The pattern is, in the preferred method of mold making, immediately covered with fine silica the individual grains of which have been coated with a dry, lacquer-like coating of phenol-formaldehyde resin binder BR18478. The coated particles are selected for size so as to give the desired surface smoothness, e. g., a silica sand in the range of particle sizes which pass through a 100 mesh screen and are held on a 250 mesh screen, and advantageously in the range to 250 mesh. A uniform, thin layer 16 of these fine, coated particles will beheld to the disposable pattern by the water on its damp surface. The surplus, which does not stick, is then poured out. The thickness of this surface layer of fine coated sand is to be determined by permeability-the gas-level-duringpouring balance. The entire body of the mold can be made of sand fine enough to give a smooth surface with sharp definition, provided the interstices between grains are not filled by finer particles which plug them up. When a particle size too small for the desired permeability of the whole mold body is chosen for the surface covering, its thickness will ordinarily be kept at A; inch or less. If surface cracks or low surface permeability de- .1 1 ye opnn baking the t ck ess 0.5 .1. cove n hesldi reducedh Pa t i ne t c e e by ehody ..Q any s- 7 Percent On "40 1.0 Thru 40,.on 50 529.5 Thru 50, on 8 0 55.5 Thru 80, on l00 8.4 ThrulOO, .on 200 5.3 Thru 1200 .3

More-specifically,-the sands are prepared as follows: 15.0 to 250 mesh silica particles are mixed with 3% of ,dry powdered phenolic resin grade BR18478. 3 liters of acetone per lOOkg. of sand is introduced and mixing iscontinued until all of the acetone is evaporated. If desired the resin can be dissolved in-the acetone before mixing .and the solution mixed with the sand as above,

but in a single step.

. Ottawa Bond Band or Ottawa Banding Sand is prepared in a like manner for the principal body of the mold.

Another method is to take or make a single sand haying the range and percentage of particle sizes desiredfor the two; sands, coat :them simultaneously and then sieve or otherwi-seseparate out the desired fractions. This method assures that the final coated particles, especially forthe surface coat, are of the desired particle size.

The disposable pattern 14 and the mold material 16, 1-3 applied thereon as described above is next put into an oven and baked inan atmosphere held at about 425 F. for 3 to.1 2 hours with good circulation maintained in the atmosphere of the oven.

At :the end of this time, as soon as it is cooled (or before it is cooled if desired) the mold Zllwithits pattern 14-is ready for pattern removal. The pattern is therefore broken with alight hammer into as many pieces as necessary to effeetconvenient removal, and any remaining portions-lifted .or disintegrated by ;a jet of air.

. When the plaster pattern material is all removedit will be found that the particles of sand in the mold are individually'secured in that no surface detail has'been lost or in any way. disturbed by the removal step, and that-the surface "107 be reproduced in metal has been most ;accurately and faithfully reproduced.

A sandmold is preparedjin .theusual manner, in an ordinary flask and :witha standard pattern to form the rough back 21 of the mold cavity for the metal casting- 22, which in this-case isan iron mold forrubber tires, in which only the face needs; to have a true surface. If the. back .of the .casting is desired to --be smoothand a ccurate, 1116 back .of the moldalSo will be formed according ;to the invention.

The'baked molds made as above described from-the resin bonded sand-are then carefully positioned (by means of indexing orother locating means) and are invested with green sand or other refractory material. In designing the final mold, -it is gated and vented in accordance with known principles and. skillof standard foundry practice and thereupon is readyfor pouring the molten metal.

Th cas ing of th me l e a a t mperatu which will permit entry of the metal into all parts and recesses of the mold, :but which also permitquick solidification of the metalafter this has been-accomp s ed Th rear ng emper ture shoul b snfli m y is to burn the resin bond t a considerable stemadvantageously ,one to two inches, although -;a fraction T12 Qfaninehis mflicient oIzsu fifissf-nliremoval or the mold in many cases.

Aft wthr te has o i ified and cooled, ffthfiinul sand is broken away and blown off with air pressure, and the surface of the casting will befound to bea sub stantially perfect reproduction of the-pattern. Since a mold thusntade is substantially pure silica and :a combustible binder, there is no burning on of mold particles tothe surface of the casting (such as frequently occurs with more conventienal types of moldshfor highmelting al y n n l sti g wi san or other drastic cleanns meth d s n ces ary set th casting. su a W fectly clean.

T his example. may be modified in various ways:

Other refractories may be used besides silica, especia1-; 1y for the surfacing laye The disposable pattern may be dippedfor .a'few seconds in kerosene, and drained until thereis no surplus of kerosene onthe mold surfiace,ljust prior .to. the application of the dry, coated fines of the surfacing layer.- This treatment will aid in the adhesion :of. said .fines'to said disposable pattern surface.

More or less binder (than 13%); may-be ,used, depending on the -;conditions of the partbeing made. Ingener-I al, the binder percentage will range between /2% land- 5%. The resin coating on individual grains .of sand should be of'the order of .0001" to .001 ,.or stated in another way the volume of resin in the coatings, uniform.- ly distributed over the grainsurfaees, should be between .l-% and 5% .of the interstic al volume of the; s-and-witht the astin A s n l carefully. sele t d fine sandrnay beused for the whole mold, instead ofa surfacing layer-and a back up la flhe, disposable pattern may be removed by an..im

mersion in water-and a .washing process rather than by the means described. The mold is then redried toprep re it fo cast ng- Other binders may be substituted for BR1847.8, such. as a natural wood resin, or other suitable settablebinder' material.

Auxiliary binders, selected fortheir ability to mainin ll lume in .thesand mass .during curing; may be introduced, e. g. urea formaldehyde binders. These should be such fine powderor such thin suspension or solution J'acent particleswithout holding the particleswapart. Such auxiliary binder may be used by moistening the sand and then dusting with powder of the auxiliary. binder, or by wetting the sand will be by exposure to water, heating or other method depending on the binder selected.

:Other materials may be introduced building certain necessary .or desired foundry properties into the mold; e. g. :wood flour-for the purpose of redneingbendingdue to induced thermal stresses during PQunn a Y Other methods of curing the resin'rnay be used; e. g., the disposable pattern may be dried, so as to remove all the free water, but not so asto dehydrate it or change has the advantage of a very quick and uniform-cure, and

very close process control. 01' Qnevmay use .a watersetting resin :such as urea-formaldehyde of the so-called synthetic resin powderglues,.nrefcrablyid'issolving in an organic solvent and lacquering-the-sand grains therewith as ab e desc ibe vapp ying the dry g ams to the Pa rnv as before and-then expe ing-.;the la quered. sand body;

that'they bond in the crevices between ad-' with a slurry'of the binder. Setting" for the purpose of:

to water either by diffusion from the wet pattern or a slurry thereon, or by treatment with saturated steam.

Instead of the fresh plastf: used in the composition for the disposable pattern, second settle plaster can be used, or waste molds can be crushed and sifted and used, advantageously with a portion of fresh plaster, e. g. about /3 used material to /5 fresh material of the composition being used.

Various conventional means have been used to check the accuracy of molds made by the above described methods for making iron castings for tire molds, and they have been found to be accurate and true to a point heretofore only achieved with non-ferrous metals by plaster molding by the method described in U. S. Letters Patent to Morris Bean No. 2,220,703. The bonded sand molds are made, for example, so as to reproduce of the circumference of the tire (six such molds being set in a circle to form the mold for the entire ring casting) and it has been possible to make any quantity of such molds to meet the following test:

The finished bonded sand mold is fitted into a freshly made (green) disposable pattern made in the same pattern mold as the one used to make the mold being checked. Any deviation of linear dimension of any section cut through the assembly is .0007 per inch of length or less, and any deviation of surface contour at any point of any section so checked is .005" or less. Those familiar with previous possibilities for accuracy in a bonded sand mold will immediately recognize the outstanding advance in the art of casting thus achieved and the great practical importance of this in making practically available castings which could not be made by conventional means in molds of equivalent size and complexity.

When to the quality of this exceptional accuracy is added also the quality of fidelity of surface finish and detail, a result is obtained which is unique and of extraordinary practical significance. This is seen to be particularly true when it is realized that the result is obtained by methods which permit a very high degree of reproducibility and control in manufacturing and with materials which are cheap and commonly available and equipment which is standard in most foundries.

Example 2 The pattern in this case was made on a molding pattern of Korogel (i. e. a resilient elastic gel of vinyl chloride and a plasticizer) by casting therein a slurry composed of 50% second settle plaster (Detroit plaster heated at 200 C. for two hours on shallow trays) and 50% calcined and pulverized plaster composition from used plaster molds or patterns (containing 40 percent of calcium sulfate plaster, 50% Ottawa Bond Sand, and talc), all mixed with water to a consistency of 60.

This plaster slurry poured into the Korogel pattern mold is allowed to set and is then removed and immersed in water for minutes, or for such a length of time as will completely saturate it with water. The pattern is then removed from the Water, quickly drained, and immediately dipped in and out of acetone. It is then blown off with air. This method has the advantage of quickly and uniformly drying the mold surface while leaving the mold body completely saturated with water.

The surface of the disposable pattern is then promptly sprayed with a solution of equal parts of acetone and flexible collodion. A little aluminum powder, red iron oxide, or other suitable pigment may be introduced into the spray to lend visibility to the film being applied.

The surface is then sprayed, immediately, with a slurry consisting of that fraction of Albany Bank Sand which will pass through a 50 mesh screen (it is re-screened to extract any miscellaneous coarse particles), 100 parts, BR184782 parts, BRlS261-1.8 parts, and water to bring the mix to the proper consistency for spraying. It should be pointed out that Albany Bank Sand is very fine, and contains a considerable portion of clay, which lends plasticity to the slurry, and also facilitates the use of the dry and liquid resins combination. These liquid and dry resins have proven wholly unsatisfactory when mixed together in a conventional core sand, for example, because the two resins tend to combine into small taffylike lumps about in diameter, which it is not feasible to redistribute. In the presence of the very fine particles, such as clay, however, a homogeneous mixture of the two resins has been obtained.

The slurry coat is sprayed on to the surface of the disposable pattern to the desired thickness (advantageously .005 to .020" and in any case less than about 0.1) as uniformly as possible. Before the slurry coat has a chance to dry, it is backed up by pouring over it the desired depth or thickness of a coated sand such as described in Example 1, and the Whole assembly is vibrated to effect packing of the coated sand.

The disposable pattern, with the sand mold over it, is then ready for the resin curing, pattern disposal, and casting steps, which are carried out as in the preferred example.

This example may be modified in various ways:

Other parting coatings may be used, e. g., the surface of the disposable pattern may be sprayed with a fine suspension of graphite powder in water until the molding surface of the pattern is fully and uniformly covered, and presents a dense, black appearance. This layer is as thin as is consistent with complete and uniform covering of the surface (not over .003). This is then sprayed with a water solution of green soap which diffuses through the graphite layer and apparently reacts with the calciumat the surface of the mold to produce a calcium soap precipitate which bonds the graphite onto the face of the pattern. Red iron oxide, aluminum paint powder, alumina, and other fine, refractory powders may be used in like manner to form a parting layer, reproduce faithfully the surface of the pattern, and to form the outermost surface layer of the casting mold, since it is held by the resin in the slurry coat when the pattern material is disintegrated and removed.

Other finely divided refractories may be used in the slurry coat, e. g., a good slurry can be made with 10 parts pigment grade red iron oxide, parts silica flour, 4 parts BR18478, 3.6 parts BR18261, and water to bring the slurry to the right consistency.

Other liquid vehicles than water can be used in the slurry coat, though if it is desired to use the dry and liquid resins in combination, one has to be selected which is compatible with the liquids in the liquid resin. In cases where the dry resin is to be used alone (which is most acceptable in small molds or cores which can be baked out very fast) it may be advantageous to use a vaporizable liquid vehicle, such as kerosene or No. 2 diesel oil.

Instead of spraying the slurry coat onto the surface of the disposable pattern, it may be poured over the surface of the pattern, and then poured out. It will thus leave a uniform layer over the pattern surface, the thickness of which can be predetermined by the viscosity of the slurry. The addition of a wetting agent to the slurry, e. g., Tergitol 08, will aid in the distribution of the slurry over the pattern surface when thus applied, and will aid in the elimination of bubbles.

Instead of using a coated sand backup, it may be desirable in some cases to use a more conventional core mix which contains some liquid to give it some stickiness and consequent green strength. The damp backup will be harder to pack uniformly behind the slurry coat, though a small sand slinger has been found to be an aid in this step, but it will have the advantage of requiring less support to hold it in place, and other advantages familiar to experienced foundrymen.

Other modifications of Example 1 may obviously be used in this Example 2. Using the procedure of Example 2 and using a horseassasav 1-5 hide softball as the pattern, it was possible to reproduce every visible detail of the surface of the leather, and of the sewing thread, notwithstanding the fact that the casting was made of'grayiron cast in the bonded sand mold. From the foregoing it will 'be seen that the invention makes possible the formation of accurate and smooth molds of a wide variety of sizes and design of high permeability (for example, exceeding A. F. A. 50) made up of refractory particles, and a thermo-setting binder and formed over a one-piece wasteable pattern even though it may have undercuts such that a rigid pattern cannot be withdrawn as such from the mold.

I claim:

1. The process of casting moltenmaterials which comprises making of a-tough resiliently flexible material a moving theset pattern from said pattern mold, forming on the surfaces of individual grains of sand a thin dry non-adhesive coat of binder which is adapted-'to-securely bond contacting grains at atemperature below that at which the plaster pattern is weakened, to retain the bond above said temperature and to decompose at ametal-casting temperature, applying said coated sand to the surface of said pattern, finer particles thereof being in contact with'said surfaceto give fidelity of surface reproduction andcoarser particles'backing said surface layer-to give desired structural strength with required permeability, heating the pattern with said appliedsand until the binder on the sand is bonded at the-contact-points,-and heating further until the plaster of thepattern is substantially dehydrated, whereby it is weakened for disintegration and removal, disintegrating the pattern and removing-itfrom the resulting bonded sand mold, assembling the resulting bonded sand mold with other mold parts to form a mold, casting molten material therein at a temperature at which its sensible heat raises the mold,to a substantial depth from its surface, above the decomposition temperature of the set binder; and, after solidification of "the casting, removing the at least partly decomposed casting mold therefrom.

2. The method defined in claim l'wherein the finer coated sand, in a dry free flowing-state, isapplied to the pattern in excess while-the surface of the pattern is wet so as to hold a thin layer of said sand'thereon and an excess fine sand'is poured off and'the-coarser coated sand, in a like dry free-flowing state, is then applied onto the thus held layer of fine sand.

3. A process as defined in claim 1 in which the-amount of binder applied to the sand grains is SllfilClBHlf 'l0 form a strong bond when in contact with a similar coat on adjacent grains, but insufficient to give as strong abond with a plaster surface which is otherwise free from adhesive material.

4. A process as defined in claim 1 'in which an auxiliary binder is used which is adapted to set at a lower temperature and to hold stability of form and dimension during setting of the principal binder.

'5. A process as defined in claim 1 and which further comprises applying a pigment to the surface of-the pattern, before application of the coated sand thereto,

whereby to keep the binder-supplied with the sand from adhesive contact with said pattern and to give fine fidelity conformity to the pattern surface.

6. A process as defined i-nclaim l and which further comprises lacquering the surface of the plaster pattern Without substantial penetration of the lacquer into the pattern,'rthereby :to keep the binder supplied with the sand from adhesive contact with said pattern.

7. A process as defined in claimd and which further comprises-applying to the surface of' the pattern, before application of the coated sand thereto, a liquid non-sol vent of the binder material to keep the binder supplied with the sand from bonding to said pattern. I 1

8. A process of making molds for casting which com prises making a pattern of a hydrated calcium sulfateplaster material which is strong and 'dimensionally stable 7 at room temperature, whereby it forms an effective support for the mold, but is made readily disintegrable -by heat of the hereinafter specified baking operation,

thoroughly soaking the pattern with cwate'r, then coatingthe water-soaked surface of the pattern with a parting: material which forms a barrier film adhering to the-wet pattern but more strongly held by the said bonding resin,

applying a relatively thin coat of refractory granulesr provided with a thermosetting bonding resin distributed over the contacting surfaces of said granules in amount less than sufficient to fill the interstices between saidgranules, baking the pattern and .the mold togetherat.

a resin-setting temperature at which the pattern becomes disintegrable, carrying the baking to a temperature at which the calcium sulfate is transformed into a'friable anhydrite, whereby the pattern becomes :disintegrable and the plaster can easily be removed from-.the mold and wasted without damage to the mold surface, and disintegrating and removing the pattern from the resulting mold.

9. The process of makingmolds for casting which coinprises forming a plaster pattern which initially-is a rigid cohesive body of'substantial green strength, driving into said pattern-preformed metal parts with the portions. thereof which are tobe embedded in the casting-'projecting into the pattern andother portions projecting therefrom, applying on said pattern and said inserts, in intimate surface contact throughout, a mixture, of sand and a binder, bonding the granules of sand to each other and to said insert by said binden 'disintegratingandremoving the pattern material, and casting molten metal into theresulting cavity. a

10. The process of casting moltenmaterials which comprises making a pattern mold with surface corresponding to the desired surface of the casting mold, moldingo'n' said surface a-settable plaster slurry of such composition that when set it will form a substantially rigid pattern which is strong and dimensionally stable at room temperaturebut which becomes'disintegrable when baked at the setting temperature of the resin hereinafter referred 'to; removing the set pattern from said pattern mold, form'- ing'on the surfaces of individual grains of sand athin 1 dry non-adhesive coat of binder which is adapted 'to securely bond contacting grains at'a temperature below that at which: the plaster pattern is weakened, to retain" the bond above said temperature to decompose Vat ametalcasting temperature, applying said coated sand to'the surfaceof said pattern, heating'the pattern With said 'applied sand until the binder on the sand is bonded iat the contact points and heating further until the plaster of the pattern is substantially dehydrated, whereby 'it is weakened for disintegration and removal, disintegrating the pattern and removing it from the resulting bonded sand mold,i-assembling the resulting'bonded sandurrold with other mold'parts to form amold, casting molten material therein at a temperature atwhich 'its'sensible heat raises the mold, to a substantial depth from it s surface, above the decomposition temperature of the;set

binder; and, after solidification of the casting, removing the at least partly decomposed casting mold therefromi' References Citedin' the file of this 1 patent UNITED STATES PATENTS 1,758,095 Wallace .May 13,, 1930 (Other references on following page) 17 UNITED STATES PATENTS Emery et a1 Feb. 27, 1934 Robie May 21, 1940 Gunsaulus et a1 Nov. 18, 1941 Bean May 30, 1944 Thielemann Nov. 6, 1945 Slatis Sept. 24, 1946 Meyer June 10, 1947 Feagin et a1. May 18, 1948 Mayer Nov. 30, 1948 Moore et a1 Aug. 16, 1949 Grube Dec. 13, 1949 Fiat Final Report 1168, 6 pages.

18 Nies et a1 Jan. 10, 1950 Bodger June 6, 1950 Weston Aug. 8, 1950 Valyi Sept. 5, 1950 Koonce Apr. 24, 1951 Duesbmy Sept. 18, 1951 FOREIGN PATENTS Great Britain Feb. 18, 1947 OTHER REFERENCES (Copy in 22/ 188B.)

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