Patented Mar. 4, 1941 4 PATENT OFFICE REFRACTORY MOLD MATERIAL FOR CAST LIETAL PRODUCTS Cornell Joel Grossman, Millburn, N. J
5 Claims.
This invention relates to refractory materials for cast metal products and more particularly to refractory mixtures adapted for the forming of molds for casting molten metal therein and more specifically to refractory mold material adapted for use in the manufacture of cast metallic dentures.
One of.the objects of the present invention is to provide a refractory mixture adapted to be cast while in the plastic state about a form or pattern defining the desired cast metal product and which in the as cast and as fired condition is dense and strong especially at the surface defined by the pattern and substantially free from surface irregularities and of loose. material r more fully hereinafter disclosed.
In accordance with these objects, I have discovered that a mixture of fine and coarse silica, preferably one consisting of particles of a size passing about 20 mesh with from 20-40% thereof passing about 200 mesh is the most satisfactory refractory for the manufacture of molds for cast metallic dentures. The difficulty in the use of this refractory is in the bonding of this material at low temperatures and at high temperatures, or in the as cast and pre-fired" condition respectively.
In the forming of molds for east metallic dentures, a pattern is first formed of wax or similar material of the size, shape and configuration of the metal denture desired. The mold material is then formed into a plastic mass and cast about the pattern preferably by vibrating the same there around and is backed up with suitable refractory backing material such as plaster of Paris. Theimolded refractory then is dried and is slowly heated to a temperature approximately 1500 F. to thoroughly consolidate the same and then is allowed to cool slowly to a lower temperature known as the pre-heat temperature desired for casting the molten metal in the mold. The molten metal then is cast, preferably by centrifugal casting means, into the pre-fired and preheated mold.
In the forming of cast metallic dentures it is highly desirable that the mold material employed in addition to surface density and smoothness shall evidence substantially no shrinkage or expansion within the temperature range atmospheric to about 1500'- F. and also that the binding agent employed for strength in the as cast" No Drawing. Application July 17, 1940, Serial N0.-345,981
and as fired condition shall be substantially free from carbon and sulfur. I
'I have discovered that by mixing magnesium trisilicate with the silica and by employing an acid phosphate solution in the forming of the plastic solution in the forming of the plastic refractory mass to mold around the pattern, I may obtain a relatively quick setting bond for the silica in the as cast condition of adequate strength for the purpose which on firing retains its strength up to the maximum firing temperatures usually employed, namely about 1500 F. The resultant fired refractory product meets all the exacting requirements for molds for east metallic dentures, particularly the non-expanding and non-shrinking requirement.
The exact reaction involved in the use of these materials is not clearly apparent. The magnesium trl-silicate appears to react initially while in the plastic or moistened condition with the acid phosphate to form complex magnesium-silico-phosphates which serve as a low temperature bonding agent. It is possible that the phosphoric acid reacts also, at least superficially, with the silica refractory to form silico phosphates. On firing the silico-phosphates appear to retain the original bond even at temperatures materially above 1500 F.
The silica refractory employed is preferably prefired or pre-shrunk silica of approximately the maximum density obtainable, for example electrically fused silica. Natural flint silica, however, has proven satisfactory for the present invention.
The magnesium tri-silicate is preferably freshly precipitated'and unfired silicate of the maximum chemical reactivity towards phosphoric acid, although various admixtures of pre-fired and freshly precipitated silicate may be employed without departure from the present invention, as one skilled in the art may perceive. In place of magnesium tri-silicate or in partial substitution therefor other silicate compounds reactive with phosphoric acid to form refractory silico-phosphate compounds, may be employed without departure from the present invention, as one skilled in the art also will perceive.
The acid phosphate solution employed may be .varied widely with respect to composition and the desired strength and density in refractory in the "as cast condition. The chemical reactivity of these acid phosphate compounds may be increased, if desired, by additions of an acid to the water solution of the same, preferably phosphoric or hydrochloric acid. To obtain the desired high temperature bond, the metal component of the acid phosphate preferably is one pre-fired condition by additions of magnesium oxide to the refractory mixture of silica and magnesium tri-silicate. Preferably such oxide should be freshly precipitated oxide to obtain its maximum chemical reactivity. The oxide appears to react with the phosphoric acid or acid phosphate compounds to form acid magnesium phosphates which evidence excellent bonding properties at least superficially on the silica refractory augmenting that obtained by the siilco-phosphate compounds formed by the reaction of the trisilicate and acid phosphate.
Additions of zirconium or of titanium double silicates such as the zirconium magnesium double silicate appear to materially increase the pre-fired strength of the mold material and to give added surface density and coherence to the same.
From the above disclosure it is believed apparent that the present invention may be wideto 25% phosphoric acid, to obtain a fast setting mold material which on firing will evidence excellent strength and density'for most purposes. As an illustration, a mixture consisting of 90% electrically fusedsilica with about 80% of the grains passing 20 mesh but retained on 200 mesh and about 20% of the grains passing 200 mesh, with about 10% magnesium tri-silicate as a fine powder (each dry weight) when intimately mixed together in the dry state and then formed into a plastic mass with a 10% phosphoric acid solution will set to a relatively hard and dense mass within a few minutes. The mold when fired at 1500" F. for several hours will be found to produce accurately dimensioned castings of the pattern employed with any of the metals commonly employed in the forming of metallic dentures.
By lowering the tri-silicate content of this mixture to about 5% the setting time of the mass will be lengthened to about hour; by increasing the phosphoric acid strength to 20% the mold strength will be materially increased and the setting time decreased; by substituting an acid phosphate, such as Al or Mg acid phosphate in part or in whole for the phosphoric acid disclosed, I may obtain improved mold strength and density in the as cast condition without materialshortening of the setting time, or material alteration of the strength and density of the same in the as firedcondition.
Alternatively, by incorporating magnesium oxide or similar refractory metal oxides reactive with phosphoric acid and phosphates into the refractory mixture in addition to the silica and tri-silicate compounds specified above, in amounts ranging from small but effective amounts" up to about 5%, the normal mold strength andij; ensity, particular surface density in the as fired condition may be beneficiated. Acid soluble refractory metal hydroxides are substantial equivalents for the oxide.
As a further improvement, particularly with respect to the pre-fired strength and density of the mold, I may add to the refractory mixture above given from small but effective amounts up to about 30%.-,(dry weight) of zirconium or titanium double silicate'compounds, such as the zirconium magnesium silicate compound, ground to pass about 200 mesh, preferably replacing equivalent amounts of the' fine silica content specified. Such additions materially increase the pre-fired strength and density of the mold and are particularly desirable to improve the surface strength and density.
Wherein in the above specific embodiment I have specified a mixture consisting of coarse silica and 20% fine silica, it is not to be construed'that I am limited thereby as anymixture of coarse and fine silica wherein the fine silica passing 200 mesh is within the range 20-40% and the coarse silica size is below about 20 mesh is within the scope of the present invention.
Having hereinabove described the present in vention generically and specifically and given the'specific embodiment thereof with modifications thereof, it is believed apparent that the present invention may be widely varied without essential departure therefrom and all such modifications and departures are contemplated as may fall within the scope of the following claims:
What I claim is:
1. A refractory mixture for use with phosphoric acid or a similar binder to form molds for casting molten metal therein, said mixture compris-- ing at least 60% of a mixture of fine and coarse silicaand magnesium tri-silicate, the latter constituent being presentin amount ranging from small buteifective amounts up to about 20%.
' 2. A refractory mixture for use with phosphoric acid or a similar binder to form a mold for cast metallic dentures, said mixture comprising of about 80% silica passing about 20 mesh and 5 retained on about 200 mesh, about 20% silica.
passing about 200 mesh and about 10% mag- I nesium tri-silicate.
3. A refractory mixture for use with phosphoric acid 'or asimilar binder to form a mold for easting molten metal therein, said mixture comprising at least 60% of a mixture of fine and coarse silica and magnesium tri-silicate.
4. The method of forming a refractory mold for cast metallic dentures which comprises forming a mixture comprising at least 60% of silica I,
and magnesium tri-silicate, forming the mixture into a plastic mass with phosphoric acid or similar binder and flowing the plastic mass over. the surface of a pattern in a pluralityof coat-' ings to the depth and thickness desired, allowing the coatings to set, drying the coatings and then