Claims
- 1. A green mould for lost pattern casting with a shell, said shell comprising a graphite coating and a hydrocarbon binder, the binder incorporating a polyol with at least three alcohol functions, said polyol being stable to decomposition and melting at temperatures of 200.degree. C. and below to preserve the binding capability of said binder.
- 2. A mold according to claim 1, wherein said shell comprises multiple layers of graphite and binder.
- 3. A mould according to claim 1 or 2, wherein the binder comprises pentaerythritol.
- 4. A mould according to claim 1 or 2, wherein the polyol represents approximately 6 to 12% by weight of the shell.
- 5. A mould according to claim 1 or 2, wherein the graphite represents approximately 76 to 84% by weight of the shell and the binder correspondingly represents 24 to 16% by weight of the shell and contains, besides the polyol, a polyvinyl binder.
- 6. A mould according to claim 5, wherein the coating is an internal coating coated withan external mineral coating of colloidal silica, colloidal allumina or aluminosiliceous colloids.
- 7. A mould according to claim 6, wherein the external mineral coating represents approximately 4 to 6% of the weight of the shell constituted by the internal coating and the external coating.
- 8. A mould according to claim 1 or 2, wherein the shell is surrounded by a block comprising 40 to 50% by weight graphite, 15 to 25% by weight of a refractory mineral compound at 1800.degree. C. and 30 to 40% by weight of a mineral binder based on colloidal silica, colloidal alumina and/or aluminosiliceous colloids.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 83 20019 |
Dec 1983 |
FRX |
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Parent Case Info
This application is a continuation of application Ser. No. 675,637, filed Nov. 28, 1984, now abandoned.
The present invention relates to a metal moulding process using a lost pattern, the metals being in particular titanium and its alloys, as well as to moulds for performing this process and to processes for the production of said moulds.
The casting of molten titanium and its alloys in a mould is subJect to a major difficulty, namely the reactivity of the molten metal relative to all known refractory materials at the very high temperature necessary for casting the metal of 1800.degree.to 2000.degree. C.
At such temperatures, the molten titanium reduces all oxides and refractory compounds, namely alumina, glucina, rutile, zirconia, zirconium, etc. The reaction leads to oxygen being absorbed by the metal and to an unacceptable modification of the mechanical characteristics of the castings produced. These reactions between the titanium and the oxides increase when the metal is melted and cast under a vacuum of at least 10.sup.-1 Torr, which facilitates the decomposition of the materials constituting the mould.
One of the only refractory materials which can be resisted by molten titanium at 2000.degree. C. without any violent reaction is graphite or the various forms of carbon. However, contact must be relatively short, otherwise the carbon slowly dissolves into the liquid metal, leading to an increase in the hardness of the castings, which may become unacceptable or require machining.
It is also possible to produce machined graphite casting moulds, but this solution rapidly becomes excessively costly if the castings to be reproduced have a complex shape. It is also possible to fix or shrink onto a permanent pattern graphite mixtures in the form of powders or agglomerated granules with thermosetting or setting casting resins using a catalyst in a procedure similar to that of conventional sand casting. These moulds are generally baked in vacuo and introduced into the melting oven under a vacuum in order to receive the molten metal.
Althorgh they make it possible to obtain sound and relatively accurate castings, these processes do not make it possible to produce complex castings with close dimensional tolerances characterizing lost pattern casting processes with metals and alloys, such as steels or alloys of aluminium.
Processes are also known in which a graphite shell is produced in the form of successive layers around a pattern or an assembly of patterns made from wax, this procedure being well known in lost pattern casting. For the titanium, the ceramic materials generally used are replaced by graphite materials in powder and granule form, whilst the mineral ceramic binders are replaced by purely organic binders.
The production of such shells with a thickness of a few centimetres leads to considerable difficulties, because the organic binders do not rapidly attain the hardness of mineral binders. Moreover, they decompose at 150.degree.to 200.degree. C. and the thus produced shell can be subject to deformation and cracking on removing the wax and on baking in vacuo. Thus, the castings do not always have the desired appearance and reproduction and the number of rejects involved can be high.
The present invention is directed at a mould for lost pattern casting, specifically a shell and a block associated therewith and which obviates the disadvantages referred to hereinbefore.
The shell comprises graphite and a hydrocarbon binder. According to the invention, the binder comprises a polyol having at least three alcohol functions. This shell has a relatively good titanium resistance after baking.
Among the polyols which can be used in the composition of the binder are triols, such as glycerine and hexols such as sorbitol. Preference is given to tetrols. Preference is given to polyols with a quaternary carbon atom, whose carbon-containing coking residue is high on baking. By far the most preferred polyol is pentaerythritol, whose melting point of 250.degree. C. is relatively high, so that it has a good resistance on removing the wax, which is insoluble in water, so that it does not increase the viscosity of the slip used in the preparation of the shell and which is insoluble in alcohol, so that the shell is not modified by successive dipping processes and during the production of the block. Generally, the polyol represents approximately 6-12%, to 6 to 8% of the weight of the shell.
Preferably, the graphite is a mixture of powders with different grain sizes variable between 0 and 1000 microns and represents 76 to 84% by weight of the shell. Correspondingly, the binder represents 24 to 16% by weight of the shell and, apart from the polyol, contains a polyvinyl binder which optionally is an acrylic binder, as well as optionally adjuvants.
According to a preferred variant, the shell comprises an inner layer having the constitution referred to hereinbefore, coated with an outer mineral layer of colloidal silica, colloidal alumina or aluminosiliceous colloids.
For example, use is made of colloidal silica in an aqueous medium or silicic acid in an alcoholic medium. This external mineral coating acts as an attachment coating to the block, whose binder is siliceous or aluminosiliceous. The external mineral coating generally represents approximately 4 to 6% of the total weight of the shell.
The shell is generally approximately 0.8 to 3 mm thick and is preferably 1 to 2 mm thick. Below 0.8 mm it is not able to fulfil as adequately its function of ensuring an inert contact with the titanium and of not being porous. Above 3 mm, cracking can appear during the baking of the mould.
The shell, whose main function is to ensure an inert contact, has a very inadequate mechanical strength for containing the molten metal to be poured. It is virtually impossible to increase the thickness of the shell, on the one hand because the latter would split on baking and on the other hand because it would deform, so that the mould cavity would lose its geometrical integrity. It is for this reason that a block is formed round the shell.
Apart from its mechanical reinforcing function around the shell, the block must also absorb the heat from the molten metal as quickly as possible, so as to solidify and cool the latter as quickly as possible. This limits the dissolving of the carbon in the metal, which improves the mechanical characteristics of the moulded titanium casting.
In order to increase the thermal diffusivity, the block is largely constituted by graphite (40 to 50% by weight). In order to obtain an adequate mechanical strength after baking, the block also contains 15 to 25% of refractory mineral substances, such as aluminium oxide, zirconium oxide or zirconium silicate, chosen from among the mineral compounds which are refractory at 1800.degree. C. and above and which have the best thermal diffusivity. In order to further improve the latter characteristic, as well as the mechanical strength, the constituents of the block have a grain size between 0 and 3 mm and are intimately mixed in such a way that the final specific gravity exceeds 1.6. In order to bond the graphite and oxide grains or refractory compounds, use is made of a bonding agent which does not disappear during the vacuum baking of the mould at 1000.degree. C. The baking prior to casting is necessary in order to eliminate any gaseous phase, which would otherwise appear at the time of casting the molten metal. Use is made of a mineral binder chosen from among the colloidal silica, colloidal alumina and aluminosiliceous colloids. The invention is also directed at a process for producing a mould according to the invention consisting of dipping a lost wax casting in a slip containing graphite and a polyol having at least three alcohol functions, drying the graphite layer, dipping the pattern coated with the graphite coating in a mineral slip containing a solution of colloidal silica, colloidal alumina or aluminosiliceous colloids, drying the pattern coated with the graphite and mineral coatings and pouring a slip containing 40 to 50% by weight of graphite, 15 to 25% by weight of refractory mineral compound at 1800.degree. C. and 30 to 40% by weight of a mineral binder based on colloidal silica or colloidal alumina or aluminosiliceous colloids around the pattern coated with the graphite and mineral coatings.
US Referenced Citations (5)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 49-45825 |
May 1974 |
JPX |
Continuations (1)
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Number |
Date |
Country |
| Parent |
675637 |
Nov 1984 |
|
Patent Grant
4700768
