US2948032A - Metal casting molds - Google Patents

Description

United States PatentO 2,948,032 METAL CASTING MOLDS Raymond Renter, Orland Park, 11]., assignor to Nalco Chemical Company, a corporation of Delaware No Drawing. Filed May s, 1958, Ser. No. 732,798

Claims. (Cl. 22-193 This invention is concerned with new and improved ceramic shell molds suitable for producing cast metal ob- I jects. Specifically, it is concerned-with improvements in the casting of metal objects using a lost wax casting technique. More specifically, the invention is directed tonew and improved ceramic shell molds which enable metal objects to be cast without some of the attendant difiiculties heretofore encountered in the various lost wax processes.

carefully packed, set to a hard unitary mass known as an investment. The investment containing the expendable pattern material is fired to melt the pattern from the investment. This leaves a cavity having the desired contour of the particular part to be produced. Molten metal is then poured into the mold cavity, allowed to congeal, and the invested finished mold is then subjected to subsequent treatments whereby the mass of investment material is removed from the finished metal object.

While the process is basically simple, there are many features of the various operations involved which render the investment casting process not entirely satisfactory. Temperatures must be carefully controlled, the ceramic investment materials carefully proportioned, and casting and melt-out temperatures must be kept within certain critical ranges to ensure the production of a suitable cast object,

Recently, there has been developed an improved lost wax casting technique which overcomes many of the objections found in conventional precision investment casting operations. This new and improved process consists in the treatment of the expendable pattern material with a granular ceramic material so that a series of coats or stuccoed layers are built up around the expendable pattern. These layers of stuccoed coats form a ceramic shell around the expendable pattern, and with proper application techniques coupled with the use of suitable ceramic materials and binders, it is possible to produce a thin, strong shell. The ceramic shell thus formed, may be conveniently treated to remove the expendable pattern, which leaves a thin mold capable of having many types of molten metals poured directly into the-mold cavity to produce precision castings of high quality.

The advantages of ceramic shell molding techniques are many and varied when compared to conventional investment casting operations. The ceramic shell molding technique eliminates the preparation assembly of flasks, the tedious preparation of investment materials and the necessity of tamping and packing of investment materials after their mixing. An important advantage, however,

is then placed into a granular ceramic body,which is 2,948,032 Patented Aug. 9, 1960 of the ceramic shell molding technique is that dewaxing of the mold may be accomplished in a relatively short period of time, such as one to five minutes, whereas conventional dewaxing of investment type molds requires many hours. The most important advantage of the ceramic shell mold process resides in the character of the metal grain structure imparted to finished castings.

The ceramic shells may be fired for the purpose of bringing the shell up to the necessary temperature for the pouring of metal within a short period of time, .such as fifteen minutes to one-half hour. Conventionalinvestment molds must be fired for numerous hours to prevent cracking and other imperfections from forming in the molds.

An important characteristic of the ceramic shell process is the type of ceramic material used in preparing the mold. To withstand the dewaxing and metal pouring operations, the finished ceramic shell must be sufliciently porous, yet it must have an extremely low thermal coefiicient of expansion. At the present time, there are only a few materials that have been found to be eminently suited to the production of ceramic shell molds. One of the best ceramics of the prior art is a highly siliceous refractory produced from a high silica glass material known to the art as Vycor glass.

It is well known that ceramic shell molds made from Vycor glass refractories have excellent characteristics. There are, however, some disadvantages which render even this material somewhat unsatisfactory. For instance, it is necessary to use relatively high temperatures, viz., between about 1400" F. to about 1900 F. to insure that the expendable pattern rapidly melts .out without undergoing a rapid change in expansion, which tends to crack or spoil the finished ceramic shell. In foundries equipped with suitable heating equipment, this rapid .melt-out is desirable, but in many cases foundries have melt-out ovens which are not capable of rapidly melting out expendable patterns. It would be extremely beneficial if a new ceramic material were available which had the benefits of the Vycor ceramic, but would enable operators to use a wide range of temperatures in the removal of the expendable pattern from the finished ceramic shell.

Another disadvantage of using the Vycor type ceramic material in the ceramic shell molding process is that it is a relatively expensive product when compared to many of the known ceramic materials used in the conventional precision investment casting operations. It would be a valuable contribution to the art if anew and improved ceramic material were available which could be used in the ceramic shell molding process of the type described above wherein the several disadvantages now ascribed to the use of the Vycor type refractory materials could be overcome.

It therefore is an object of the invention to provide a new and improved ceramic shell mold useful in the production of cast metal objects using a lost wax process.

Another object is to provide a ceramic shell mold which has an extremely low thermal coeflicient of expansion and from which expendable pattern materials may be removed, using a wide range of temperatures.

An important object of the invention is to provide a ceramic shell mold which may be rapidly produced, using a minimum number of coats to build up the various stuccoed layers that compose the shell mold. Other objects will appear hereinafter.

In accordance with the invention, it has been found that a suitable ceramic shell mold useful in producing cast metal objects may be effected by using, as the ceramic ingredient in making said mold, a granular fused silica having a silica content of not less than 97% silica,

- as SiO and a thermal coefiicient of expansion not greater 3 than about 6 10- cm./cm./ C. Expressed in another form, the ceramic shell molds of this invention comprise a plurality of siliceous coats bonded by a suitable binding agent, with these siliceous coats being composed of a granular fused silica of the type thus described.

In a preferred form, the fused silicas chosen will have the highest SiO content with the lowest thermal coefficients of expansion. Thus, a fused silica having a silica content of 97% and a coeflicient of expansion from about 5.56 10 cm./cm./ C. is less desirable than a fused silica having a silica content of about 99.8% and a thermal coefficient of expansion no greater than from about 4--5 10 cm./cm-./ C. A typical silica of the type useful in the practice of the invention has the following typical analysis.

A material of this type has a thermal coefiicient of expansion of about 10-" cm./cm./ C. Silica products of this type are readily prepared by grinding very pure fused silica glasses which are well known to the art.

An important aspect of the invention resides in the use of an aqueous colloidal silica binder which has a relatively high silica content in relation to the other ingredients found in this binder. Such binding materials may be prepared having silica contents, expressed as SiO between 12% and 48% and contain relatively small amounts of inorganic metallic oxides or salts in relation to the silica present.

The colloidal silica sol binders which are most useful in the practice of the invention contain relatively large amounts of silica, such that the SiO content is between about 18% to about 48% by weight, with excellent results being obtained with silica sol binders containing between 30% and 35% by weight of silica.

Aqueous colloidal silica sols of the type used as binders in the practice of the invention are available commercially from several sources and are most usually prepared using the techniques in accordance with the teachings of Bird, U.S. Patent 2,244,325. This patent shows that aqueous collodial silica sols of high purity may be prepared by passing a dilute solution of an aqueous alkali metal silicate solution in contact with a cationic exchange resin in the hydrogen form. The sols produced by the Bird patent are relatively dilute, but may be con centrated using known evaporation techniques such as, for example, those described in Bechtold et a1. U.S. 2,574,902, and Parma, U.S. 2,601,235.

While the Bird method for preparing aqueous colloidal silica sol is preferred, it will be understood that any of the known methods for preparing aqueous colloidal silica sols may be used to produce colloidal binders suitable for adaptation in the practice of the present invention. A complete summary of the various methods and techniques for producing aqueous colloidal silica sols is set forth in the Specification of Bechtold et al., U.S. 2,574,902. The

Example I Commercial sodium silicate was diluted with Chicago tap water to produce a sodium silicate solution having present therein about 4.5% SiO The weight ratio of Na O:SiO was about 1:32, with a specific gravity of about 1.050. This diluted sodium silicate was passed through a column of hydrogen form sulfonated polystyrene divmylbenzene 'copolymer cation exchanger of I the type disclosed in U.S. Patent 2,366,007. The efliuent contained about 3.5% SiO had a pH of 3.5 and a conductivity of about 400 to 800 micromhos. To this silicic acid sol effluent was added an amount of 26 Baum ammonium hydroxide sufficient to adjust'the pH of the acid sol to about 9.0.

Example II The procedure used in Example I was the same except that the solution of the starting sodium silicate contained about 10% SiO The finished sol had an SiO concentration of about 7% and a specific gravity of about 1.050. Ammonium hydroxide was added to the sol so that the final pH was about 10.5.

Example III A portion of the sol of Example I was placed in an evaporating kettle and heated until ammonia and steam vapors began to come ofi. At this point, a small amount of permanent alkali (KOH) was added and fresh ammonia stabilized sol wasadded to maintain the evaporation volume constant. Throughout the process, the pH was never allowed to go below 8.5. This was accomplished by-continually adding gaseous ammonia during the heating process. The constant evaporation was continued with constant checks being maintained to keep the pH always above 8.5, and was continued until specific gravity of the sol reached 1.20 at 68 F. When this specific gravity had been obtained, an amount of potassium hydroxide was added to give the finished sol a pH of 9.0. This sol had an SiO concentration of 30%.

Example IV Another sol was produced by using the method shown in Example III. In this instance, however, the concentration process was continued until the sol had an SiO concentration of about 48% An extremely useful type of aqueous colloidal silica sol binder for use in the practice of this invention are those aqueous silica sols which have such typical properties as an SiO content of from 15.2 to 16.5% by weight, a conductivity of about 2200 to 2400 micromhos, a pH from about 8.5 and a specific gravity of about 1.10 to about 1.111 at 68 F., a methyl orange alkalinity of about 107 to 117 grams per gallon, expressed as calcium carbonate in a relative viscosity at 77 Ref 1.5 to 3.0 centipoises. This characterization describes a relatively narrow range of colloidal silica sols which have extremely high gel strengths when used to bind ceramic materials and are disclosed in my pending application Serial No. 456,627, now U.S. 2,856,302, filed September 16, 1954. Any of the silica sols disclosed in the specifications in the teaching of this application are deemed to be within the scope of this invention. For purposes of illustrating how sols of this type are prepared, the following is given by Way of example.

' Example V A silica sol was prepared by passing an aqueous sodium silicate solution having a specific gravity of 1.045 at 68 F. and containing 4.0% SiO through a column of a hydrogen form polystyrene divinylbenzene sulfonic acid cation'exchange resin (Nalcite HCR) and the efliuent having a specific gravity of 1.026 at 68 R, an SiO content of 4.5 a pH of 3.7, and a conductivity of-800 micromhos, had ammonium hydroxide added thereto until the pH was adjusted to 9.1. Eighty-five gallons of ammonium hydroxide adjusted silica sol produced from the ion exchange operation were placed in a gallon steam kettle and the temperature was raised until the liquid began toboil. At this point, additional ammonium hydroxide adjustedion exchanger efiluent solwas added to maintain the volume in the kettle constant and enough potassium hydroxide was added as was indicated from previous experience to have a pH of 8.5 at the end of the concentration. The evaporation concentration .process continued-for 8 hours. During the first few hoursit was necessary to add ammonium hydroxide to the boiling sol to maintain the pH at 9. Near the end of the process the pH was gradually allowed to fall to be 8.5 at the end. The total amount of potassium hydroxide added during the process was equal to 9.2 grams per gallon of produced sol. Throughout the process described, specific gravities were constantly checked and the highest point ever reached was 1.115. After the concentration stopped, by

volume of Chicago tap water was added to adjust the gravity to 1.111. The finished sol had the following For a more complete understanding of the techniques used in the practice of the invention, the following general description of the preparation of the ceramic shell molds using the low thermal coefficient of expansion silica materials is presented.

The expendable pattern material is usually composed of a parafiin wax, although it may be composed of other known expendable pattern materials, such as polystyrene and other thermoplastic resinous substances. It is placed into a ceramic slip which comprises an aqueous-colloidal silica sol vehicle having suspended therein the ganu-lar fused silica refractory material.

To produce a suitable ceramic slip using the aqueous colloidal silica sols, and the high silica, low thermal'coefiicient of expansion silica refractory, it is desirable that the particle size of the silica refractory does not exceed 100 microns nor should it be less than 0.1 micron. The

amount of aqueous colloidal silica sol or vehicle used in relationship to the siliceous refractory may be varied over relatively wide range although usually from to 50% by weight of the aqueous silica sol material may be used and from 50% to 80% by Weight of the siliceous refractory. The weight ratio of collodial silica vehicle to ceramic material usually should be maintained within the weight ratio of 1:1 to 1:3.

The ingredients are mixed using good agitation and:

Ingredients: Percent by weight A. An aqueous colloidal silica sol having an SiO content of from 12% to 48% by weight B. A granular fused silica having a thermal coeflicient of expansion of not more than about 6 10-7 cm./cm./ C. and a silica content, expressed as SiO of at least 97% by weight 60-80 C. A compatible wetting agent .0l.5

While any compatible wettingagent may be used which is compatible with the silica sol, the non-ionic type, such as, for example, amyl alcohol reacted with 5 moles of ethylene oxide, are preferable. Particularly good formrulations have been prepared using the triethanol amine salt of dodecylbenzene sulfonic acid. In some cases it is necessary to use minor amounts of a silicone antifoam to prevent foaming which sometimes occurs.

The expendable pattern is dipped into the ceramic slip v 6 to produce a thin coating of the slip on the pattern. It is then removed from the slip and coated with the siliceous refractory material to produce a stuccoed finish or coat. The coat is then allowed to dry and the process continued until suflicient number of layers have been built up to produce a shell which is of sufilcient strength and density to enable the molten metal to be poured.

The stucco coat may be dusted, sprayed, or sprinkled on the slip-treated expendable pattern using simple, me-

chanical means.

To produce satisfactory molds, particularly where ferrous metals are concerned, it is desirable that the stucicoing ceramic material have the particle size within the range from about 50 to 2000 microns, and, of course, should be of the same siliceous refractory material as that contained in the slip.

It is desirable that the first coat of ceramic placed on the expendable pattern produces a very smooth surface. To achieve this effect, it is beneficial to use a slip having suspended therein very, fine particle size ceramic material as well as using a fine particle size stucco coating. Larger particle size slips and stucco coatings may be used for the second and third or subsequent coats without affecting the smooth inner surface of the mold. For instance, the prime coats might be composed of a fused silica having a particle size not in excess of 75 microns in both the slip and stucco coats and the later coats might use fused silica having particle sizes in excess of microns. In using this procedure, as well as the general procedures outlined above, it is beneficial to have a finer particled size fused silica in the slip than the fused silica used as the stucco coat.

There is no special criteria as to the drying times or cyclesbetween coats, but a rule to follow in judging the drying time between coats is that a coat must be sulficiently dry so that when the next coat is applied, the previous one will not slough off. This rule requires that the first coat be thoroughly dried and the final assembly of coats be thoroughly dried before dewaxing but only a fraction of the moisture need be removed from the intermediate coats before applying the next.

In order to enable a plurality of coats to be rapidly applied to an expendable pattern, it has been found that the slip-coated and stuccoed pattern may be sprayed with a minor amount of the substance referred to herein as a gel accelerator whereby the collodial silica binder may rapidly form a relatively rigid gel network, which binds the ceramic slip and the stuccoed coat together in a relatively short period of time.

Thus, a slip-coated stuccoed pattern may be treated with such a substance such as a low molecular weight, water soluble alcohol such as methanol, or ethanol, and the like, as well as dilute aqueous solutions of a certain inorganic electrolyte such as magnesium sulphate, sodium fluoride, and other alkali or alkaline earth salts whereby a rapid and uniform controlled gelation may be obtained in a relatively short period of time. Ammonia gas may also be used where facilities permit such an operation to be conducted. By using this gel acceleration technique, it is possible to set the several coats used in building up the ceramic shell mold within a period of time as short as five minutes.

After a sutficient number of refractory coats have been built upon the expendable pattern material and have been dried, the next step is the melt-out of the expandable pattern material, which in the case of this invention, may be performed using temperatures ranging from between 700 F. and 2000 F. Where suitable equipment is available, it is preferred to conduct the melt-out operation at a temperature ranging from about 1400" F. through 1700 F. Since relatively elevated temperatures are employed in the melt-out operation, it is expedient that a suitable furnace be used in this operation which allows the rapid escape of the molten expendable pattern material since high temperatures tend to cause a rapid burning of the expendable pattern, particularly in the case where the expendable pattern material is wax or polystyrene.

A'wide range of melt-out temperatures may be used to dewax shell molds made of the siliceous ceramic material of this invention. Standard equipment now used in the precision investment casting industry may be employed in making ceramic shell molds. The ceramic materials currently being used to produce ceramic shell molds, due to their higher thermal coefiicients of expansion, must be carefully melted out at relatively high temperatures to overcome the effects of the high thermal coeflicient of expansion of the expendable pattern material. When a low temperature melt-out was used in prior art ceramic shell molds, it was discovered that serious cracking and disfiguration of the shell occurred, thus rendering it not suitable for the casting of metal objects.

An important feature of the invention resides in the use of an expendable pattern material that has a relatively low thermal coefiicient of expansion which enables the processes of the invention to be carried out over even a wider range of temperatures than those that can be used with conventional expandable pattern materials. Low coelficient of expansion, expendable pattern materials have been recently introduced to the art. They may be described as oxidized, low molecular weight polyolefins having molecular weights between about 500 and 2000. Of the several low molecular weight, oxidized polyolefinic materials available for use in ceramic shell molding operations, the most preferred are the low molecular Weight oxidized polyethylenes. These low molecular weight oxidized polyethylenes come in several grades and are unique in that they may be emulsified in hydrocarbon oils and contain as part of their molecular configuration, a plurality of polar groupings such as keto or aldehyde groups which seem to contribute greatly to their relatively low thermal coefiicient of expansion. When these low molecular weight polyolefins are used as expendable pattern materials, it is possible to conduct melt-out operations at temperatures only a few degrees higher than the melting point of the particular polyolefin used. As a general rule, the higher molecular weight po-lyolefins will have higher melting points than the lower molecular weight polyolefins. All of these materials may be used to good advantage in the practice of the invention.

When the low molecular weight polyolefins are used as expendable pattern materials, it is possible to blend with the high siliceous refractories of the type used with this invention, other refractories such as mullite, kyanite, sillirnanite, and the several well known silica flours or sands which are often used in the precision casting in dustry. Such blends while tending to increase the thermal coefiicient of expansion of the finished ceramic shell, have the advantage of somewhat lessening the cost of the finished mold, and when a relatively simple pattern is to be used, the dilution of the preferred siliceous refractory is not too great a factor. It should be noted, however, that in the preferred form of the invention, the ceramic shell mold is composed entirely of the preferred siliceous refractory materials.

Example VI The following example illustrates the practices of the inventionand are presented here for purpose of illustration only.

The following materials were used in preparing ceramic shell molds:

Ingredient A.-Fused silica having an SiO content of 97.3% by weight and a thermal coefiicient of expansion of about l0- cm./cm./C. ground so that the largest particle present was no greater than 75 microns.

Ingredient B.-Same as ingredient A except that the smallest particle size was no smaller than 95 microns.

to five liters. the prepared slip and was stuccoed with ingredient B.

Ingredient C.Same as ingredient A except that the largest particle size was no greater than microns. Ingredient D -same as ingredient A except that the smallest particle size was no smaller than 3.00 microns. Ingredient E.The colloidal silica sol of Example III. Prime contra-2500 milliliters of ingredient B were placed in a 6 liter steel beaker. 13.5 pounds of ingredient A were addedusing good mechanical stirring which was then followed by 15 milliliters of 50% by weight solution of triethanolamine salt of dodecyl benzene sulfonic acid and four drops of a commercial silicone antifoam.

The total volume of the slip had now been increased A clean wax pattern was then dipped into The wax pattern, so treated was then dried for three hours at room temperature. The dried, stuccoed pat- .term was then dipped into a solution of ingredient E and thence again into the slip, .after which it was stuccoed and allowed to dry.

After the initial two coats had been prepared, another slip was prepared by adding 10 pounds of ingredient C to a 6-liter steel beaker containing 2600 ml. of ingredient E. In this slip coat, the wetting agent was omitted.

The wax pattern containing 2 stuccoed coats was immersed into this new slip and was then stuccoed with ingredient D. This procedure was repeated two more times. The finished, treated pattern was then allowed to dry for 24 hours.

The dried, coated pattern was placed in a melt-out oven, the temperature of which was 1980" F. The wax melted out within a period of a few minutes. The mold was held at this temperature for 15 minutes longer to decarburize the mold.. The mold was then removed and cooled and it was determined that no cracks or fissures occurred during the melt-out and decarburizing steps.

The mold was later reheated to a temperature of 1500 F. when it was then removed from the furnace, and No. 302 stainless steel was poured into the mold and allowed to 0001. Several light taps with a wooden mallet re moved the mold from the metal, after the cooling had taken place. An inspection of the cast metal object showed it to be satisfactory in all respects.

Similar satisfactory castings were made from aluminum, beryllium, copper, brass and magnesium.

Example VII Example VI was repeated except that ,the expendable pattern was composed of an oxidized polyethylene having molecular weight of 2000. The melt-out step was conducted at 300 F. After decarburizing at a higher temperature and cooling, an inspection showed the mold to be perfect.

Example VIII The technique of Example VI was repeated except that the wax was melted out at 700 F., and molten aluminum was-poured into the mold while the mold temperature was 72 F. The resulting casting was satisfactory.

Thus, the invention herein described is capable of reducing the time necessary to form the casting mold in comparison with known methods, gives better metal grain surfaces of the finished castings, and offers a wider range of temperatures which may be used in the removal of the expendable pattern from the finished ceramic mold. These improvements, along with others enumerated in the foregoing description, enable the production of excellent castings from various metals in an efficient manner.

The invention is hereby claimed as follows:

1. The process of producing a ceramic shell mold by a lost wax process which comprises coating an expendable pattern with a ceramic slip comprising an aqueous colloidal silica sol vehicle having suspended therein a granular, fused silicahaving a thermal coefficient of expansion of not more than about 6x10" cm./cm./ C. and a silica content, expressed as SiO of at least 97% by weight, stuccoing the treated pattern with a granular, fused silica having a thermal coefficient of expansion of not more than 6 10- cm./cm./ C. and a silica content, expressed as SiO of at least 97% by weight, drying the stuccoed, coat, forming a plurality of superposed similar coatings on said pattern, and removing the expendable pattern from the ceramic shell, by heating said coated pattern to a temperature of at least 700 F. for a period of time sufficient to remove the expendable pattern from the ceramic shell.

2. The process of claim 1 where the expendable pattern in composed primarily of wax, the aqueous colloidal silica sol has an SiO content of from 12% to 48% by weight and the wax is melted from the ceramic shell at a temperature within the range of from 700 F to 2000 F.

3. The process of claim 1 where the expendable pattern material is an oxidized polyolefin having an molecular weight within the range of from 500 to 2000.

4. The process of producing a ceramic shell mold by a lost wax process which comprises coating an expendable pattern with a ceramic slip comprising an aqueous colloidal silica sol while having suspended therein a granular fused silica having a thermal coeflicient of expansion of not more than about 6 10 cm./cm./ C., a silica content, expressed as SiO of at least 97% by weight, and an average particle size within the range of 0.1 to not more than 100 microns in diameter, stuccoing the treated pattern with a granular, fused silica having a thermal coefiicient of expansion of not more than 6x10 cm./cm./ C., a silica content, expressed as SiO of at least 97% by weight, and an average particle size within the range of 50 to 2,000 microns in diameter, drying the stuccoed coat, forming a plurality of superimposed similar coatings on said pattern, and

10 removing the expendable pattern from the ceramic shell by subjecting the shell to a temperature between 700 F. and 2,000" F., for a period of time suflicient to remove the expendable pattern therefrom.

5. The process of claim 4 where the expendable pattern is composed primarily of wax, and the aqueous colloidal silica sol has an S10 content of from 12% to 48% by Weight and has a silica to metal oxide ratio, expressed as siO zNa O, of not less than 50:1.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Searle: Refractory Materials, pub. 1950 (3rd edition), Charles Grifiin & Co., London (pp. 704-705).

Iler: Colloid Chemistry of Silica and the Silicates, pub. 1955 by Cornell Univ. (page Norton: Refractories, pub. 1931 by McGraw-Hill, New York (pages 35 and 40).

Materials and Methods, February 1956 (pages 177, 178,

Foundry, Casting Stainless Steel in Shell Molds, September 1952 (pages 108 and 110).

Foundry, August 1955 (pages 140, 142, 143).

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2,948,032 August 9, 1960 Raymond Renter It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 44, {column 5, lines 41 and 52, and column 6, line 46, for "collodial", each occurrence, read colloidal column 9, line 7, after "stuccoed" strike out the comma; line 9, after "shell" strike out the comma; line 14, for "in" read is line 20, for "an", second occurrence, read a column 10, line 3, after "2,000 F, strike out the comma Signed and sealed this 9th day of May 1961o (SEAL) Attest:

ERNEST W, SWIDER DAVID L, LADD Attesting Officer Commissioner of Patents