US1864365A - Process and apparatus for forming ceramic bodies - Google Patents

Description

June 21, 1932.

T. MONTGOMERY 1,864,365 PROCESS AND APPARATUS FOR FORMING CERAMIC BODIES Filed Sept. 6, 1929 2 Sheets-Sheet l y l 50 7 5 i 52' 3 I I f g-uenfoz fig; f 6274 570%0w w WW??? aftozmq Jun-e 1932- E. T. MONTGOMERY 1,364,365

PROCESS AND APPARATUS FOR FORMING CERAMIC BODIES Filed Sept. 6, 1929 2 Sheets-Sheet 2 Patented June 21, 1932 UNITED STATES PATENT OFFICE EABLE" '1. MONTGOMERY, OF FRANKLIN, OHIO, ASSIGNOR OF ONE-HALF TO A C SPARK PLUG COMPANY, OF FLINT, MICHIGAN, A COMFANY OF MICHIGAN PROCESS AND APPARATUS FOR FORMING CERAMIC BODIES Application filed September 6, 1929. Serial N0. 390,742.

This invention relates to methods and apparatus for forming articles from ceramic material, and particularly for forming such articles as spark plug insulators from a raw ceramic batch.

Such articles are today commonly manufactured by what is known as the wet or plastic process. According to this process the raw materials after being finely pulverized are wet-mixed, forming what is known as slip. The slip is then filter-pressed, that is, forced through cloth filters, this resulting in the separatlon of excess water, which passesthrough the filtering medium while the ceramic material is retained. The material thus collected is subjected to preliminary pugging, rolling or kneading, and is then stored and aged in large storage cellars especially constructed for the purpose. After aging, the raw body is subjected-to a blankforming operation either by plunger or auger machine by which a blank of the desired outside diameter is extruded through a die and, in the case of a spark plug core, with the bore for the central electrode formed by the die, and the blank is cut off to the desired length. The blank thus formed is partially driedto what is called the leather-hard condition, and is then machined to the final shape of the insulator by turning. Instead of turning, the 1 blank may be completely dried and then formed to final shape by the use of an abrasive forming v heel. The blank is then fired and glazed in the usual manner.

This method has a number of obvious disadvantages. It includes .many steps, each of which is an expensive operation. It necessitates the construction of large storage tanks for the aging of the material. In the final forming operation a large amount of scrap is produced which is difficult to handle and very troublesome to return to the raw batch, particularly in the case of the blanks formed y cutting in the dry condition with abrasive forming wheels. In addition to the .expense and difficulty in manufacture, this method likewise produces inherent weakness in the final fired body. The pugging, rolling or kneading operation causes air inclusions which are distributed throughout the mass.

The blank-formin operation causes more or less lamination o the structure, flow surfaces, and segregation of the coarser and finer materials in the body. High dielectric strength and high mechanical strength are essential for spark plug insulators, and these qualities are dependent upon homogeneous physical structure, as well as upon proper chemical composition. The lack of homogeneity' resulting from the laminations, flow this method have a physical structure and dielectric strength unsuitable for spark plug insulators as well as for many other electrical purposes. However the method has the advantages of simplicity and economy.

A third method consists of casting the article in porous molds. Clay casting by methods in use prior to my invention, is necessarily restricted almost entirely to relatively thin and uniform walled articles. By this method porous molds are prepared, usually from such material as plaster. The slip is poured into the mold, and the water passes through the pores of the mold while the ceramic substance in suspension is retained in the mold. The mold is kept full by subsequent additions until the piece has cast solid and filled the mold. Obviously this method of forming the body is slow, tedious and unsatisfactory. While the resulting article has a very desirable physical homogeneity this method of manufacture costs too much to permit its use in the fabrication of low priced articles such'as spark plug cores.

My invention consists in a method of forming ceramic bodies which is much simpler which characterizes cast bodies, it is never-.

theless a relatively rapid process so that the cost of manufacture is comparable with, if not lower than the cost of manufacture by the current wet process.

By my process a very heavy slip with minimum water content is prepared in the usual manner and is forced under very heavy pressure into porous molds. S0 heavy is the pressure, and so slight the water content that within a comparatively short time, from 2 to 5 minutes, in the case of spark plug insulators depending on the pressure used, the mold is filled with a solidceramic body and the water has passed off through the pores of the mold. This is to be contrasted with approximately 30 minutes required for the operation by the old casting process. The pressure may now be relieved, the mold opened up, and the formed body removed. The body will be found to be leather hard, requiring only the usual drying preparatory to firing and glazing.

By my process the article is homogeneous and entirely free from the lamination, flow surfaces and segregation of the coarser and finer materials in the body which characteri'zes present-day wet process porcelains. In addition, due to the use of a porous die, the casting takes place concentrically, thus building up a solid homogeneous structure from the outside surface to the center, which is ideal from an electrical standpoint. Obviously, also the employment of a heavy slip and of extremely high pressure insures a denser structure and a higher degree of homogeneity than is obtainable by ordinary pouring methods;

In contrast to the dry or dust pressed process and the prior casting process, with my process any number of different wall or section thicknesses can be either successively or simultaneously cast within the limit of wall thickness which a given casting slip will build upon the given porous mold being used. This is accomplished by supplying cast ng slip under high pressure to any point where extra-thickness is required until that section has cast solid.

In developing my process a number of problems were encountered. In the casting of substantially solid bodies, such as spark plug cores, I found it essential to feed the slip to the mold through a number of very fine apertures rather than through one large aperture as might at first be thought suflicient. I found that the pressure resulted in solidification of the material not only'in the mold but also in the aperture and-in the adjacent portion of the feed line, forming 9.

sprue. Upon removing the mold from the feed nozzle, the sprue is necessarily broken ofl. In the case of large feed apertures the breaking off of the sprue often resulted in cracking the article, rendering it useless. Likewise the hardened sprue upon passing into the mold next applied to the nozzle, was in the form of a large solid lump which if not actually damaging the mold, at least prevented the flow of slip to all parts of the mold. I overcame this difficulty by providing my feed nozzle with a number of very fine apertures. Upon removing the casting the very fine sprues formed in the fine apertures were sheared off, causing no damage to the cast bodies. Upon casting the next body, the sprues passed on into the die cavity, and be cause of their smallness, although partly solidified, caused no difficulty but formed a homogeneous part of the article.

Another difliculty encountered arose from the uneven cross-section of spark plug insulators. .With molds of equal porosity throughout, it was found that the portion of the mold of small diameter adjacent the nozzle plugged up before the mold was entirely filled. To overcome this difficulty I conceived and successfully employed the idea of using molds having a greater degree of porosity at the end remote from the feed nozzle than at the end adjacent the nozzle. Instead of this, if preferred, a hollow spindle may be used with outlets at the enlarged section such as the shoulder section of the spark plug cores.

I also found it essential to design my apparatus so that no valves would be interposed in the feed line, since any such valves when subject to the abrasive action of the slip would remain serviceable but a very short time. This necessitated laying out my apparatus so that flow of slip would stop immediately upon cutting off the molding pressure.

My process is also distinguished from prior suggestions by the solution of the problems briefly dealt with above as well as others that will be referred to hereinafter. The overcoming of these obstacles makes it possible for me to obtain by a rapid casting operation spark plug insulators and other shapes having a physical structure far superior to any previously made. All this will be dealt with in detail in the courseof the following description.

In the drawings:

Fig. 1 is a side elevation of one form ofmy apparatus.

Fig. 2 is a top plan view of the mold of Fig. 1. Fig. 3 is a vertical section through the mold taken along the parting line.

v Fig. 4 is a section on line 4-4 of Fig. 3. Fig. 5 is a view of the eccentric locking pin 0! the mold. sections.

Fig. 6 is a horizontal section through the air pressure control valve.

Fig. 7 is a horizontal section through one half of the mold taken on line 77 of Fig. 3.

Fig. 8 is a view corresponding to Fig. 3 but showing a modified form of mold and of feed nozzle to supply slip to the mold.

Fig. 9 shows a slight variation of the form shown in Fig. 8.

The mold shown in the drawings is formed of two halves 1 of porous material, each half formed with a depression or recess 2 of proper shape to produce the desired article. These half molds are mounted in metal boxes 39 fitting in metal shells 3 provided with hinge members 4-5 connected by pin 6 and supported on a pedestal 7 attached to a suitable support 8. The boxes and shells are preferably open at the back as shown at 49, F ig. 7 to permit escape of the water that has passed through the mold. Each shell has a semi-circular cap 9 secured by screws 10. The two shells have ears l2 and 13 provided with longitudinal holes 14 which are preferably elliptical in cross-section. A locking pin 16, having cam or eccentric lugs 17 may be dropped into these holes, as indicated in Fig. 3, and when turned by means of the lever 18, these cam lugs draw the halves of the shell together. Pins 20 serve ashandles to open the mold after the locking pin has been removed.

The slip is prepared in the manner usual in the wet process of making ceramic bodies, by finely pulverizing and thoroughly mixing the constituents, with the addition of water, but with the water content held to very low limits. In the case of slip prepared for use in the manufacture of spark plug cores, the material is preferably sufliciently thick so that if no pressure is applied very little flow will take place.

The slip may be forced into the mold by any means preferred so long as adequate pressure is applied to it to produce a homogeneous body within a practicable time. I have pre ferred to employ air pressure to force the slip into the die and accomplish this in the following manner. The slip is placed in a container 22 having a removable cap 23 and a pipe 24 connects to this container and to a source of air under pressure. When the valve plug 25 is in the position shown in Fig. 6 in the valve body 26, air will flow to the container and force out the slip through the pipe 27 to the mold. But when this plug is turned ninety degrees to the left, the air in the container escapes through the opening 28 until it is no longer above that of the atmosphere. I have indicated at 50 a pressure regulator interposedbetween valve 25 and container 22 to insure uniformity in pressure applied to the slip. The level of the slip in the container should be below the discharge end of the pipe 2? to avoid leakage by gravity.

It is obvious, of course, that the nozzle through which slip is fed to the die must make a tight joint with the die so that the air pressure will be effective to force the slip into the mold. When the mold is of relatively soft material such as plaster, I may fit the nozzle to the mold as shown in Fig. 3. Here the nozzle 30 is shown screwed onto the pipe 27 and is held from turning by the jamb nut 31. The bottoms 32 of the shells 3 are provided with complementary recesses which aocurately fit the nozzle. A packing collar 33, preferably of rubber, surrounds the nozzle and seals the oint between the upper end of the nozzle and the porous molds.

I call particular attention to the design of the nozz.e 30. This nozzle is formed with a number of small passages 34 through which the liquid slip may enter the mold cavity. It is Very important that these passages be of small cross-section. Whether the passages be large or small the effect of the application of pressure to the slip is to force the slip into the die and to not only solidify the material in the die cavity but also to partially solidify the material in the feed apertures and in the adjacent portions of the supply pipe 27, formting a. sprue or sprues. When the casting is completed and the body is removed the sprues are broken off and where large feed openings are employed the breaking off of the large sprue often results in cracking of the body. With the large sprue also difficulty is encountered in the clogging of the partially solidified material in the nozzle and in the plugging of the mold cavity by the large lump of solidified material thus formed, preventing complete filling of thedie and at times injuring the die in case it is made of plaster or other relatively soft material. The relatively fine sprues produced by the use of my small apertures are readily broken off upon removing the cast article from the nozzle. These sprues are preferably sheared from the casting simply by turning it a few degrees before lifting it off the nozzle. The material left in these small openings does not get materially harder than the slip and in no way interferes with the continuous operation of the 115 process. When the next shot is made, this sprue material passes on into the cavity and becomes an integral part of the casting.

I have shown the nozzle 30 provided with a spindle 35 extending throughout the length of the mold cavity and having its upper end received in brass insert 41 fitted into the mold and adapted to relieve the relatively soft material of the mold from wear as a result of contact with the spindle in casting. In the case of s ark plug porcelains this spindle forms the Ore for receiving the usual center wire.

My improved process is carried out as follows:

With the container 22 filled with slip and the die closed about the nozzle and locked in place b rotation of the eccentric locking pin 16, va ve 25 is manipulated to apply air pressure to the slip in the container. This pressure should be in the neighborhood of' from 80 to 200 lbs. per sq. in. in the case of batches such as are commonly employed. The slip is forced through the small apertures 34 into the die cavity and the continued application of air pressure causes the water of the slip to pass outwardly through the pores of the die leaving the ceramic substance deposited on the walls of the mold cavity. This deposition continues until the cavity is filled with a solid mass of deposited material whereupon the air pressure is released, the mold is opened, and the cast article removed, the article first being given a slight twisting movement to break off the sprues formed at the apertures 34. Using pressures such as above indicated, the time required for casting will be from 2 to 5 minutes. As each workman can attend to a number of molds, and as the molds can be designed in multiple, this makes it readily possible for the workman to attain a high hourly production of formed bodies.

The resulting product will be found to be in the condition termed leather hard, and can be readily cut to remove the fin produced at the joint between the mold sections without danger of breakage. The body is ready to be dried, glazed and fired by either the one fire or two fire process.

- I have so far made no reference to the perforated metal member 37 shown embedded in the mold in Figs. 3 and 7. The purpose of this member is to reduce the porosity of the mold at the end adjacent the supply nozzle 30. Without this,'particularly in the case of articles such as spark plugs having portions of enlarged section remote from the feed nozzle, the ceramic substance will solid-- ify in the end of the mold adjacent the nozzle and it will be found impossible to fill the upper portion of the mold, and particularly the portion of enlarged cross-section. This is because the water in leaving the ceramic substance will follow the path of lowest resistance, which is the portion of the mold of least cross-section and closest to the nozzle, and will completely plug up this portion of the mold beforethe enlarged section has become filled- The result isfaulty castings. The employment of a perforated insert is but one method of reducing the porosity of a ortion of the mold to overcome this diflicu ty. If desired, material of less porosity may be employed for one portion of the mold than the other- Or a portion of the mold may be treated with oils or other fluids to reduceits porosity.

As an alternative method of insuring complete filling of the die cavity, I may resort to .all kinds of ceramic mixes.

8 I have shown the spindle 35' formed with a counter bore at its lower end communicating with the supply nozzle. At its upper end and at the enlarged portion of the mold cavity the counter bored portion is drilled as at 42. The slip is forced through these apertures into the mold upon application of pressure. The roximity of these apertures to the enlarge ortion of the cavity insures that this portion as Wellas the reduced end portions will be completely filled with ceramic material.

In Fig. 9 I have shown a slightly modified form of spindle 35". The enlarged bore portion '44 of this spindle is drilled at the shoulder portion 46 as at 48 to direct the slip into the adjacent portion of the plug of enlarged section.

Figs. 8 and 9 likewise indicate an alternative type of seal for the feed nozzle. Here the rubber gasket 33 is dispensed with and accurate metal to metal contact of nozzle 30 and metal box 39 of the mold is relied upon to prevent the escape of slip.

The process and apparatus as described in this specification is susceptible of wide variation in practice. The process is applicable to It has a special utility in the forming of bodies which are too -short, in order words, not plastic enough to be formed by the usual plastic process.

My process thus makes it possible to form bodies from materials having especially desirable properties which it has not heretofore been possible to employ because of lack of means of giving them shape. The water content of the slip may be varied as is found best with particular mixes, and for particular shapes. However, it is essential that casting be accomplished within a relatively short time and for this purpose a thick slip and heavy pressure are necessary.

Pressure may be applied to the slip by various means. Power driven plungers may be employedto force the slip out of the reservoir. Pumping, however, will not be found satisfactory as the valves cannot withstand the abrasive action of the slip. Hand pumping'is entirely inadequate as sufficient pressure cannot thus be obtained. Employment of air pressure is much superior to other methods as the pressure can easily be kept constant producing castings of uniform density which will all respond in the same manner to the subsequent drying and firing treatment. This is especially important in working with ceramic materials where slight changes in the 'raw' body produce striking changes in-the fired product.

' The molds may be made of any preferred material. I have produced successful castings with plain paster molds, with plaster molds faced with fabrics such as muslin; linen and silk or with very fine mesh wire cloth.

the expedients shown in Figs. 8 and 9. InFig. I have had success .with molds made of fired 1 clay and various porous ceramic bodies including porous alundum. I have also made successful castings with porous graphobronze molds. In the case of the last named particularly it is possible to accurately control the porosity in the various portions of the mold, and in constructing them of this material, the porosity adjacent the feed nozzle was kept at a lower value than in other portions of the mold. I have had particular success with plaster. molds. They never clog and always release perfectly throughout the life of the mold. When dry they cast and release very rapidly, as fast as the cycle of closing, casting, unlocking and opening can be completed. As they gradually get wet from the water absorbed from the slip, casting time increases and after a certain length of time have to be changed to keep up with the speed of the operation. However, I have successfully run one set of plaster molds continuously for eight hours without change. In using plaster molds I have found it desirable to leave the molds open at the back so that the excess moisture can drain out of the mold. This is illustrated in Fig. 7. When thus arranged a state of equilibrium is reached where the moisture draining out becomes equal to that forced from the slip into the mold in the casting operation. OW- ing to the softness of the plaster molds Ihave found it desirable to line the mold cavity with harder porous material, such as the wire cloth indicated diagrammatically at 40 in Figs. 8 and 9. The metal facing is formed by means of suitable dies, is anchored in place and the plaster is then cast about it to form the mold.

I have described the necessity of variation in porosity in different portions of the mold to take care of bodies of varying cross section. Except where reduction in porosity is required for such reasons, the porosity should be as great as the material used in the mold will permit without sacrifice of the requisite mechanical strength.

The molds may obviously have shapes as various as the articles to be produced in them. They may in some cases be made in more than two parts, if preferred. In the case of some special shapes it would be necessary to use auxiliary supply tubes built into the mold, the tubes being similar in design to the tubes 35 and 35 shown in Figs. 8 and 9.

I am aware that it has heretofore been proposed to form hollow bodies by pouring slip into porous molds, and then apply to the slip a slight degree of pressure, say a few pounds per square inch, by means of a hand pump, to accelerate the casting operation. With such an arrangement there is no appreciable reduction in molding time as compared with that required for ordinary poured castings not only because of the low pressure but because a thin slip must be used where pumping is resorted to. My invention is distinguished from such proposals in that I employ a heavy slip and a pressure in the neighborhood of from to 200 lbs. per square inch depending upon the character of the slip a homogeneous slip of such high viscosity as i to be substantially lacking in flow characteristics, applying heavy pressure of substantially unvarying value to the surface of the slip thereby causing the slip to flow and forcing it in a fine stream or streams into a closed porous mold and continuing the application of said pressure until the mold is completely filled with a compact body of ceramic material and the major portion of the water content has passed through the walls of the mold, cutting off the pressure, the body from the mold.

2. The method of producing ceramic castings which consists in forcing slip in a fine and removing stream under heavy pressure into the die cavity in a closed porous mold, continuing the operation until the cavity is completely filled with a compact ceramic body, cutting off the pressure, removing the body and at the same time breaking off the fine sprue formed at the entrance to the die cavity, and repeating the operation, the sprue material retained in the supply line thereupon entering the mold and forming part of the body next produced.

3. The method of producing ceramic castings which consists in forcing slip in fine streams under heavy pressure'into the die cavity in a closed porous mold until the mold is completely filled with a compact ceramic body, cutting off the pressure, and removing the body, thereby breaking off the fine sprues formed at the point of discharge ofthe slip into the die cavity.

4. The method of producing ceramic castings which consists in forcing slip in fine streams under heavy pressure into the die cavity in a closed porous mold .until the mold cavity is completely filled with a' compact ceramic body, cutting off the pressure, removing the body and at the same time shearing off the fine sprues flush with the entrance to the die cavity. I 5. The combination of a closed porous mold having a die cavity therein, a source of slip under heavy pressure, a conduit for leading slip from the source to the mold, a nozzle for the conduit adjacent the entrance to the die cavity, said nozzle being provided with one or more fine passages through which slip is fed to the 'mold cavity. 6. A closed porous mold for use in making ceramic castings having less porosity adjacent the inlet end than at pdrtions more re mote therefrom to insure solid castings.

7. A closed porous mold having portions of different cross-section, the portion of enlarged cross-section having greater porosity than the portion of reduced cross-section to 5 insure solid castings.

8. A porous mold for .use in the manufacture of ceramic castin s havin an insert of reduced porosity em edded 1n a portion thereof and between the die cavity and the outer wall of the mold for reducing the -fiow of excess water therethrough from the die cavity throughout said portion:

9. A. porous plaster mold for use in the manufacture of ceramic castings having the usual die cavity, and a foraminous metal liner for said cavity.

10. In apparatus for producing ceramic castings, the combination of a closed porous mold having a portion of enlarged cross section, a source of supply of slip under pressure, and a hollow spindle communicating with the source of supply and extending into the die cavity in the mold to a point adjacent the portion of enlarged cross-section, the Walls of the spindle being perforated adjacent said point to introduce slip into the cavity at the point of largest cross-section.

11. In apparatus for making ceramic castings, the combination of a mold holder hav-' ing relatively movable parts, means for clamping the parts together in molding position, a porous mold fitted in each of the parts of the holder, said molds together providing a die cavity, said holders being apertured to permit escape of excess water forced through the pores of the mold.

12. The method of producing ceramic castings of substantially unvarying physical characteristics which consists in preparing a homogeneous slip of such high viscosity as to be substantially lacking in flow characteristics, applying heavy pressure of the order of approximately 80 to 200 pounds per square inch to the surface of the slip, thereby producing flow'of the slip and forcing it in a fine stream or streams into a closed porous mold, and containing the application of such pressure until the mold is completely filled with a compact body of ceramic material and the major portion of the water content has passed through the walls of the mold, cutting off the pressure, and removing the body from the mold.

In testimony whereof I aflix my signature.

EARLE T. MONTGOMERY.

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