US2795022A

US2795022A - Method of making moulds

Info

Publication number: US2795022A
Application number: US347829A
Authority: US
Inventors: Shaw Noel
Original assignee: Shaw Process Dev Corp
Current assignee: Shaw Process Dev Corp
Priority date: 1953-04-09
Filing date: 1953-04-09
Publication date: 1957-06-11
Anticipated expiration: 1974-06-11

Description

METHOD OF MAKING MOULDS Noel Shaw, Ivy Lea, Haydon Bridge, England, assignor, by mesne assignments, to Shaw Process Development Corp., Port Washington, N. Y.

No Drawing. Application April 9, 1953, Serial No. 347,829

6 Claims. (Cl. 22-192) This invention broadly relates to the process of what is known as precision casting of metals and metal alloys, and particularly pertains to the method of making moulds and to moulds effected by that method and which are especially adapted for the production of high precision castings from ferrous and non-ferrous metals or materials which cannot be conveniently die cast, as disclosed in copending British patent application numbered 8,556, dated April 3, 1952, for Improvements in or Relating to Moulds for Metal Casting.

Precision casting, with the exclusion of die casting, is a relatively new art, and therefore it seems proper to evaluate the same for the purpose of facilitating a comparison between the prior art and the present invention.

Of the known methods presently used the following are the most noteworthy:

, (a) Investment casting, the so-called lost wax method, wherein a wax pattern is placed within a refractory mould, whereupon the wax is melted and the metal to be cast is poured into the mould;

(b) The mercury pattern process where frozen mercury is employed as a pattern similar to the lost wax method;

The shell method of casting which comprises the production of shell-like moulds of sand bonded with a thermo-setting resin; and

(d) The method of providing refractory moulds by covering a pattern with a refractory paste-like mixture and permitting that mixture to set, whereupon the pattern is removed by opening the mould and the mould material is then subjected to a slow drying and a careful heating or baking process before being used for pouring the metal into the mould.

The lost wax method is applicable for casting of relatively small volume and weight. The mercury process allows making castings of larger sizes, and the shell mould process allows the casting of still larger pieces.

The method of making refractory moulds and slowly drying and heating or baking the mould material is most suitable for making fairly large castings but possesses definite drawbacks and disadvantages. Since that process approaches closest the present disclosure, the same shall be more fully described. I

It provides the making of refractory moulds for casting metals by mixing comminuted refractory material into a paste or slurry with an organic silicate and sufiicient water to hydrolyse the silicate, and to which mixture is added a gelling accelerator which may be of acid or basic origin such as hydrochloric acid, lime or magnesia, respectively. From the resulting semi-liquid paste or slurry the mould is formed by intimately applying the paste to the pattern, whereupon the mould material is allowed to set, such setting being effected by the induced hydrolysis and the gelling of the silicate. When thus set the mould is stripped or opened and the pattern is removed. The opened mould parts are now subjected to a lengthy drying-out process and to a careful heating procedure with the employment of gradually rising temperatures in order United States Patent C 7 2,795,022 Patented June 11, 1957 ice to avoid thermal shocks of any kind which could cause the mould material to crack.

This procedure has been followed since it was generally assumed that the removal of volatiles, such as alcohol formed by the hydrolysis, and the water vapor would cause disruption or distoration of the set mould unless such removal of volatiles and water vapor is carried out carefully in a slow drying and careful baking procedure.

The aforesaid cautious treatment of the said mould requires considerable space for the storage of moulds during drying and consumes a relatively long time, and in addition considerable furnace space is lost by the lengthy period of slow baking or firing.

In spite of the aforesaid precautions it has been found that moulds produced by that method often crack or become bodily distorted due to shrinkage, thereby causing undesirable dimensional changes in the mould.

From the foregoing it becomes very clear that the afore-described heretofore used method of producing moulds is not reliable and therefore is not applicable for continuous and precise foundry production.

In the preparation of the mould paste such as described above, various organic silicates have been employed, but on account of its cheapness and availability ethyl orthosilicate has been mostly used. In the present invention it is contemplated to employ ethyl orthosilicate, although it will become obvious that any other alkyl silicate which yields an alcohol on hydrolysis and which alcohol is sufficiently volatile to burn when ignited, can be readily substituted; any of such usable silicates shall be hereinafter termed a low alkyl silicate.

As refractory material for the type of moulds indicated there have been used a variety of refractory factors such as silica, alumina, zirconin, zirconia, and like refractory oxides and silicates, the choice of the refractory material being determined by the casting temperature of the metal to be cast.

Contrary to the heretofore used procedure of slow drying and careful firing of set moulds, the principal object of the present invention involves the method of rapidly removing volatiles from the mould parts after the organic silicate binder has set by hydrolysis and gelling.

Another object of this invention is the provision of making mould material by the preferred employment of acid gelling accelerators, as against basic accelerators.

A still further object of this invention is the provision of a relatively simple and inexpensive method of making peculiarly crazed or micro-cracked moulds of high porosity, which latter however will not allow cast metal to fill the crack formation but which facilitates the ready escape of gases during pouring.

The foregoing and numerous other objects and additional advantages of the present invention will become more fully understood from the following description.

The method of the present invention is based upon the new finding that if the casting mould is stripped after it sets by gelling and thereafter is immediately fired while free air circulation about all surfaces of the mould is provided, the disadvantages encountered with the aforedescribed known method of slow drying and careful baking the mould are completely obviated.

The general procedure in accordance with the present method consists of producing a refractory mould for metal casting by mixing comminuted refractory material with a suitable binder, by adding thereto a desired, heretofore defined lower alkyl silicate and an aqueous acid or gelling accelerator, all of which ingredients are mixed to a semi-fluid homogenous paste consistency. Now the pattern for the desired casting is covered by the paste to produce a mould, which latter is allowed to set by gelling. When set, the mould is stripped or opened and the pattern is. removed and the set mould portions are now placed upon a supporting surface which will permit free air circulation about all surfaces of the mould parts, whereupon the alcohol or any other burnable volatiles formed by the hydrolysis of the binder are ignited as they escape from all surfaces of the mould. It is to be noted that such ignition of the volatiles takes place immediately after the mould is set by gelling and that the ignition and burning of the volatiles occurs evenly at all parts of the mould surfaces due to the free air circulation about them. Such ignition or burning of the volatiles is preferably effected by the simultaneous application of flames to all surfaces of the mould, including inner portions thereof, so that all of the escaping volatiles continue to burn by themselves.

When burning of the volatiles ceases, the mould is assembled and the metal is poured at once, although in some cases, particularly where the casting includes thin sections requiring the metal to be kept in its molten state for a relatively long time period, or when the casting metals have a high melting point, it is desirable to submit the mould to further heating in a suitable furnace, such as of the muffie type, in which case the mould, after firing, can be placed directly into the furnace at its full temperature for the casting.

In describing the present method more specifically, the refractory material used is formed into a paste or slurry with, for example, ethyl silicate and an aqueous acid, such as hydrochloric acid, by pouring the ingredients into suitable containers, as is well known in the art (see for example Prange, U. S. Patent No. 1,909,008). After the formation of the mould about the pattern, the mould material sets by gelling of the binder and the mould is stripped, the mould parts are at once placed upon a suitable grid to provide air circulation about all surfaces of the mould parts, whereupon the mould parts are rapidly fired in that the volatiles are ignited by the simultaneous application of intense flames to all surfaces of the mould parts. The burning of the formed and escaping volatiles is allowed to continue preferably under a hood to remove the fumes, and when the burning is seen to have ceased, the mould parts can be at once assembled and the metal at once poured. When the metal is congealed, the mould material is removed by cracking it loose from the cast.

Due to the rapid firing of the mould by flash-burning of the volatiles, as above described, a mould structure of a specific character is obtained which is highly advantageous to successful casting. The rapid loss of the volatiles from the set mould material effects a peculiar type of internal shrinkage characterized by an internal micro-cracking of the mould material which occurs without causing the slightest external dimensional changes of the. mould. Thus the mould material attains a state of minute, uniform porosity. Due to the internal microcracking of the mould material, interlacing minute cracks or crazes are produced which preclude shrinkage of the mould material during its solidification. Therefore, if, by chance, an external shrinkage should occurs, such shrinkage is very negligible. The rapid increase of the temperature to which the mould material is subjected has the effect of accelerating the action of internal microcracking, which latter is very minute but very uniform and produces uniform granulation of the mould material.

There are definite reasons for the necessity of firing the mould before pouring the casting, one of them being the rapid driving off of all traces of volatiles. If ignition or firing is not effected rapidly and almost immediately upon the solidification of the mould material, the mould material will disintegrate or at least become seriously weakened. Ignition therefore serves the dual purpose of driving off the volatiles and to provide the required granulitic structure of the mould material.

In consequence of the aforesaid micro-cracking of the mold material, the latter attains a state of remarkable permeability which. facilitates the ready escape of gases from the mould interior during casting, thereby obviating the necessity of constructing special gas vents used in foundry practice. Moreover it has been found that the mould produced according to the present method is highly resistant to thermal shock.

The rapid removal of the volatiles through application of intense flames of heat immediately after setting of the mould material takes place produces what may be termed as freezing of the mould dimensions, that is a so-tospeak instantaneous setting of the mould to the required dimensions. Furthermore, the peculiar structure of the mould material with its minute micro-cracks or crazing is the result of the rapid thermal shock to which the mould material is subjected by the application of intense flame or heat immediately. following the setting of the mould material.

Summarizing, the most essential feature of the present disclosure is the use in the mould paste mixture of refractory material with an alkyl. silicate and an aqueous acid gelling agent or accelerator, and the rapid, uniform and intense firing of the mould made from the above ingredients immediately after the mould material has set by gelling, that is after the setting of the mould material is completed, and the elfectuating of such firing of the mould so that free air circulation is provided at all surface portions of the mould, and wherein the rapid intense firing of the mould constitutes a severe thermal shock to the mould, thereby effecting the peculiar internally porous structure of the mould material resulting from the rapid burning at intense heat of the volatiles escaping from the set mould, and which internally porous structure thus produced causes the dimensional freezing of the mould, and wherein the thus produced mould is rendered what may be termed immune to subsequent severe thermal shocks.

While specific ingredients are given in the foregoing as well as specific steps prescribed in the production of the mould, it is readily apparent that other materials equal in their co-action to those given may be readily employed, and by the same token the method steps may undergo changes and modifications which may become necessary in the production of moulds for specific purposes. For the above reasons the foregoing specific disclosures are not intended to limit the scope of the present invention except as defined in the annexed claims.

What is claimed as new is:

1. Method of producing refractory moulds which comprises preparing a slurry comprising comminuted refractory materials and a binder of a liquid lower alkyl silicate, water and a gelling accelerator, pouring said slurry over a pattern, allowing the slurry to gel, immediately separating the gelled mass of the slurry from the pattern, and immediately thereafter igniting the surfaces of the gelled mass and allowing the intense flames resulting from said ignition to burn until the fiammables are consumed, thereby freezing the mould dimensions and obtaining a mould having a porous structure of microcracks which will not allow cast metal to fill the crack formation but which will facilitate the ready escape of gases during pouring.

2. Method in accordance with claim 1, wherein the alkyl silicate is ethyl silicate.

3. Method of producing refractory moulds which comprises preparing a slurry comprising comminuted refractory materials and a binder of a liquid lower alkyl silicate, water and a gelling accelerator, pouring said slurry over a pattern, allowing the slurry to gel, immediately separating the gelled mass of the slurry from the pattern, immediately placing the separated gelled mass upon a supporting surface which permits free air circulation about all surfaces thereof, immediately igniting the surfaces of the gelled mass and allowing the flames resulting from said ignition to burn until the flammables are consumed, thereby freezing the mould dimensions and obtaining a mould having a porous structure of microcracks which will not allow cast metal to fill the crack formation but which will facilitate the ready escape of gases during pouring.

4. Method in accordance with claim 3 wherein alkyl silicate is ethyl silicate.

5. A mould as produced in accordance with the method of claim 4.

6. A mould as produced in accordance with the method of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,909,008 Prange May 16, 1933 2,027,932 Ray Jan. 14, 1936 2,388,299 Thielemann Nov. 6, 1945 2,491,096 Feagin D66. 13, 1949 2,496,170 Mann Jan. 31, 1950 FOREIGN PATENTS 183,874 Austria Nov. 25, 1955 OTHER REFERENCES The Iron Age, Nov. 9, 1944, pages 52-5 8. The Iron Age, Feb. 8, 1945, pages 52-57.

Claims

1. A METHOD OF PRODUCING REFRACTORY MOULDS WHICH COMPRISES PREPARING A SLURRY COMPRISING COMMINUTED REFRACTORY MATERIALS AND A BINDER OF A LIQUID LOWER ALKYL SILICATE, WATER AND A GELLING ACCELERATOR, POURING SAID SLURRY OVER A PATTERN, ALLOWING THE SLURRY TO GEL, IMMEDIATELY SEPARATING THE GELLED MASS OF THE SLURRY FROM THE PATTERN, AND IMMEDIATELY THEREAFTER IGNITING THE SURFACES OF THE GELLED MASS AND ALLOWING THE INTENSE FLAMES, RESULTING FROM SAID IGNITION TO BURN UNTIL THE FLAMMABLES ARE CONSUMED, THEREBY FREEZING THE MOULD DIMENSIONS AND OBTAINING A MOULD HAVING A POROUS STRUCTURE OF MICROCRACKS WHICH WILL NOT ALLOW CAST METAL TO FILL THE CRACK FORMATION BUT WHICH WILL FACILITATE THE READY ESCAPE OF GASES DURING POURING.