Metal founding – Process – Shaping a forming surface
Reexamination Certificate
2000-10-03
2002-09-17
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping a forming surface
C164S516000, C164S517000
Reexamination Certificate
active
06450243
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improvements in and relating to investment casting. In particular, the invention relates to a method of investment casting which involves the successive application of one or more coats of a refractory slurry to an expendable pattern, so as to build up a shell. The present invention further provides a refractory slurry for use in the method of the invention, and a kit of ingredients for putting the method of the invention into effect.
BACKGROUND TO THE INVENTION
The process of investment casting, otherwise known as the lost wax process, is well known and widely used. Typically, the process involves dipping a wax model into a slurry comprising a binder and a refractory material, so as to coat the model with a layer of slurry; applying a stucco coating of dry refractory to the surface of the layer; allowing the resulting stuccoed slurry layer to dry; and applying further stuccoed slurry layers as appropriate to create a shell mould around the wax model having a suitable thickness. After thorough drying, the wax model is eliminated from the shell mould, and the mould is fired.
Environmental considerations dictate that the binder used in the investment casting process should be water-based, rather than alcohol-based. Customarily, the binder used comprises an aqueous colloidal silica sol. When combined with a suitable refractory material into a slurry, aqueous silica sots are capable of gelling and drying to form a green shape having an acceptable degree of green strength. However, where an unmodified aqueous silica sol is used, the time taken for this process is disadvantageously long. A single stuccoed slurry layer, applied to a wax model in the course of investment casting, may take between 3-8 hours to dry. Where the model comprises recessed parts or other complex configurations, the drying time may be increased to 24 hours or more. During production of a shell mould having several stuccoed layers, this time must be multiplied by the number of coats applied. Typically, 4-8 coats are required in order to build a shell of acceptable thickness, thereby bringing the total production time to the order of between 12 hours and several days.
Considerable effort has therefore been devoted in the prior art towards accelerating the coat drying time. Thus, for example, EP-A-0638379 discloses that the addition to a colloidal silica sol binder of an elastomeric polymer, such as styrene butadiene, results in a significant reduction in the drying time and an improvement in green strength. The addition of soluble organic polymers to a colloidal silica sol binder for use in investment casting is also disclosed in U.S. Pat. No. 4,996,084. Soluble organic polymers however readily “wet out”, and it has been found that the green strength of a shell mould comprising such polymers is temporarily reduced by the permeation of steam through the mould, for example during the elimination of the wax model from the mould. Moreover, soluble organic polymers are expensive, and their use in investment casting may significantly increase the cost of this process.
OBJECTS OF THE INVENTION
It remains therefore a desirable object to provide an alternative means for reducing the time required to build a shell mould of suitable thickness during the process of investment casting.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, therefore, there is provided a method of investment casting, comprising the steps of mixing a binder, a refractory material, and a quantity of water-insoluble organic fibres to form a slurry; coating an expendable pattern with a coat of said slurry; and drying said coat or allowing said coat to dry to form a shell.
Optionally, a plurality of coats may be applied successively to said expendable pattern, each coat being partly or wholly dried or allowed to dry prior to the application of the next coat. Typically, between 2-10 coats, more preferably 3-8 coats, still more preferably 3, 4 or 5 coats, are successively applied to said pattern. In some embodiments, said expendable pattern is precoated in accordance with known conventional methods with a coat of slurry comprising no water-insoluble organic fibres, prior to the application in accordance with the present invention of one or more coats of fibre-modified slurry.
According to a further aspect of the present invention, there is provided a slurry comprising a binder, a refractory material and a quantity of water-insoluble organic fibres, which slurry is adapted for use in the method of the present invention.
According to yet another aspect of the present invention, there is provided a kit adapted for putting the method of the present invention into effect, said kit comprising a quantity of a binder, a quantity of a refractory material, and a quantity of water-insoluble organic fibres; said binder, refractory material and fibres being adapted to be mixed to form a slurry in accordance with the present invention. Said refractory material may be packaged and/or supplied separately from the other ingredients of the kit.
Surprisingly, the present inventors have found that a refractory slurry comprising a quantity of water-insoluble organic fibres is capable of forming significantly thicker coats around dipped objects than are slurries of the types known in the prior art. An increase in coat thickness obviously implies a concomitant decrease in the number of dipping cycles required to build a mould of sufficient thickness, and hence a significant reduction in the rate of refractory mould production. Coats of fibre-modified slurry are subject to a comparable drying time in comparison with the products of the prior art, and have been found to possess a comparable green strength.
It has furthermore been found that the efficacy of elastomer-modified binders of the kind disclosed in EP-A-0638379 and U.S. Pat. No. 4,996,084 is largely destroyed by exposure of the binders to low temperatures (0° C. and below). In contrast to the elastomer-modified compositions of the prior art, however, fibre-modified refractory slurries in accordance with the present invention have been found to be compatible with many types of antifreezes. This will therefore make possible the addition of antifreeze to binders intended for use in the fibre-modified slurries of the present invention, hence facilitating winter transport and storage of such binders.
DETAILS OF THE INVENTION
Advantageously, said fibres are dispersed in said binder prior to the addition of said refractory material. This will promote the formation of a smooth and stable slurry. However, said refractory material may alternatively be added to said binder prior to the addition of said fibres.
Optionally, said expendable pattern may be coated with said slurry by means of pouring said slurry over the pattern. More preferably, however, said pattern may be coated by means of dipping the pattern into a receptacle containing said slurry. Conveniently, a plurality of patterns, which may for example be held on a “tree”, may be dipped simultaneously into said receptacle, thereby enabling the simultaneous production of a plurality of shell moulds.
Advantageously, a plurality of coats of slurry may be applied successively to said expendable pattern. In accordance with usual practice, each coat of slurry may be stuccoed with a dry refractory material such as an aluminosilicate such as Molochite® (available from English China Clay), or mullite (available from Cermatco), or zircon, or fused silica, prior to the application of the next coat. Additionally, or in the alternative, one or more of the coats, typically one or more of the outermost coats, may be stuccoed with small polystyrene beads. This will serve to improve the insulating properties of said outermost coats. Preferably, each coat of slurry may be completely covered with a layer of said dry refractory material or said small polystyrene beads prior to the application of the next coat.
Advantageously, said method may further include the step of eliminating said expendable pattern from sai
Duffey Daniel James
Shaw Richard Dudley
Lin Kuang Y.
Nixon & Vanderhye PC
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