Cristobalitic shell-mould for casting, the products and the proc

Details Cristobalitic shell-mould for casting, the products and the proc Cristobalitic shell-mould for casting, the products and the proc

1987-11-16

1989-07-04

Godici, Nicholas P.

Metal founding

Process

Shaping a forming surface

106 383, 106 389, 164361, 164516, 164529, B22C 100, B22C 904

Patent

active

048441481

DESCRIPTION:

BRIEF SUMMARY
The present invention concerns a cristobalitic shell-mould for casting, more particularly for precision casting; it also concerns the products and the process used for preparing the said mould.
When casting certain articles, especially for example articles with controlled solidification, the shell-moulds used are subjected to significant stress in some extremely severe conditions (temperatures, thermal shocks). Now it is known that it is not possible to improve in particular the mechanical properties of these shell-moulds by increasing the thickness of their wall, beyond a thickness of the order of 8 mm, without entailing serious risk of deterioration of the metallurgical qualities of the articles. One has therefore looked to obtain these desired improvements of the mechanical properties by modifying the composition and/or the structure of the material forming the shell-mould.
Among the materials best adapted for forming shell-moulds usable at high temperatures the use of a mixture of silica and zircon has already been described. The process for making shell-moulds with the said material consists of a first step of immersing a wax model in a muc composed of fine particles of refractory material (zircon) in a binder (colloidal silica) and of a second step of applying particles of refractory material on the coating of wet mud thus obtained; these steps are repeated after drying and a shell mould having the desired thickness (4 to 8 mm) is obtained.
There has already been described in another connection casting cores in which there has been introduced, beside the silica blend, finely ground zircon, a silicone resin forming a binder, and possibly a lubricant and a catalyst, from 2.5 to 3.5 % by weight of cristobalite. These casting cores contribute, after having been coated with wax, to the realisation of shell-moulds such as described above. It is clear that the casting cores thus described possess interesting mechanical properties due to the presence of cristobalite, but the problems which are posed for good operation of casting cores are different from those which are posed in the case of shell-moulds.
Finally, there has already been described, for various uses, the process of transformation, at elevated temperatures, of silica into cristobalite under the influence of various mineral catalysts (alkali or alkaline earth salts for example) but it is also known that the said cristobalite takes two crystalline forms, an .alpha. form stable at low temperature (below about 250.degree. C.) and a .alpha. form stable at high temperature, the said forms having different densities.
Taking all this information into account, the present invention is related to the idea of seeing if it were not possible, with a view to improving their mechanical properties at high temperatures, to make shell-moulds comprising cristobalite. The success of this idea was by no means obvious since the density variations between the two types of cristobalite would risk destroying or weakening the said moulds.
It was found:
on one hand, that in order to benefit from the improvement of the properties not to use a mineral catalyst for silica transformation, (alkali or alkaline earth salts for example)
on the other hand, that in order to avoid the harmful effects of the transformations of the .alpha. and .alpha. forms of cristobalite, it was desirable to control and limit the quantity of cristobalite present in the shell-mould when the latter is subjected to temperature variations in the region where the change in the crystalline form of cristobalite operates.
The present invention therefore firstly concerns an aqueous bath (or a mud) enabling, by the dipping of a wax, the achievement of a coat of material giving rise to a shell-mould, the said bath comprising, by weight, from 20 to 40% of colloidal silica, from 20 to 40% of finely ground zircon, from 15 to 25% of silica sand and being characterized in that it contains from 15 to 35% of finely ground fused silica and from 1 to 5% of finely ground cristobalite.
It is to be remembered that
a finely

REFERENCES:
patent: 2211133 (1940-08-01), Krosta
patent: 3249972 (1966-05-01), Watts
patent: 3540519 (1970-11-01), Yates

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