CASTING SLURRY

Abstract

A casting slurry for the manufacture of shell molds, including powder particles and a binder, further including a surfactant. Use of such a casting slurry for the manufacture of a shell mold.

Claims

1. A casting slurry for manufacturing shell molds, comprising powder particles, a binder and a hiding power stabilizing surfactant, the mass content of said surfactant in the slurry being less than 0.1%.

2. The casting slurry as claimed in claim 1, wherein the surfactant has a carbon chain comprising at most four thousand eight hundred carbon atoms.

3. The casting slurry as claimed in claim 1, wherein the surfactant leaves the pH of the slurry unchanged to within 5%.

4. The casting slurry as claimed in claim 1, wherein the surfactant is Tiron C.sub.6H.sub.4Na.sub.2O.sub.8S.sub.2.

5. The casting slurry as claimed in claim 1, wherein the surfactant is sodium polyacrylate.

6. The casting slurry as claimed in claim 1, wherein the binder is selected from: ethyl silicate, sodium silicate or colloids.

7. The casting slurry as claimed in claim 1, wherein the mass content of the surfactant in the slurry is less than or equal to 0.05%.

8. The casting slurry as claimed in claim 1, the slurry being a contact slurry configured to come in contact with a part pattern.

9. The casting slurry as claimed in claim 1, wherein the powder particles comprise at least one of alumina, mullite, zirconia, mullite-zirconia composites.

10. A method of manufacturing a shell mold, comprising using a casting slurry as claimed in claim 1.

11. The casting slurry as claimed in claim 6, wherein the colloids include colloidal silica, colloidal alumina, colloidal yttria or colloidal zirconia.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention and its advantages will be better understood upon reading the following detailed description of embodiments of the invention given by way of non-limiting examples. This description refers to the appended drawings, in which the single FIGURE is a graph illustrating the change in the hiding power of different slurries as a function of time.

DETAILED DESCRIPTION OF EMBODIMENTS

[0024] In order to assess the addition of a surfactant to a casting slurry, the inventor first studied a control slurry, denoted slurry A, intended to be used as a contact slurry for the manufacture of a shell mold. Slurry A may have the following composition, expressed in percentages by mass: [0025] binder (colloidal silica): 29.8%; [0026] powder particles (mullite-zirconia composite): 70.0%; [0027] wetting agent, anti-foaming agent and other additives: 0.2%.
This mass distribution is given here by way of example, with the understanding that a variation of the mass distribution between 0.1% and 10% is possible. Slurry A has a basic pH value and does not comprise, even among the above-mentioned other additives, any surfactant having an effect on the hiding power.

[0028] In addition, as mentioned above, the inventor studied a slurry C, which was prepared by taking slurry A and adding a hiding power stabilizing surfactant, in this case Tiron, at a mass content of 0.05%, preferentially 0.005%. The resulting casting slurry C is therefore also a contact slurry. The amount of Tiron can be adjusted by the skilled person according to the initial hiding power and the desired hiding power, preferably not exceeding 0.1% by mass. For example, the mass content of Tiron may be less than or equal to 0.08%, preferably less than or equal to 0.05%, preferably less than or equal to 0.02%, and preferably still less than or equal to 0.01%.

[0029] The inventor verified that the addition of Tiron to the slurry hardly changed its pH, i.e. by a value less than or equal to plus or minus 5%. In addition, Tiron has a short carbon chain, comprising less than one hundred carbon atoms, in this case six carbon atoms. Tiron does not comprise ammonia ions as it does not comprise nitrogen at all. Tiron is also a good complexing agent for the chemical elements of the oxides present in slurry C and coming from the powder particles; in fact, Tiron has affinities with these oxides and can effectively interact with them. Furthermore, Tiron will be eliminated during the heat treatment of the corresponding shell mold and has no harmful effect on the metal of the part to be cast in the shell mold.

[0030] Thus, by its interaction with the oxides and colloidal silica forming the binder, the surfactant, here Tiron, ensures good stability of slurry C, particularly its hiding power, as will be seen in reference to the single FIGURE.

[0031] This FIGURE shows the change in the hiding power HP of four slurries as a function of time t. Coverage can be measured in grams per square centimeter (g/cm.sup.2) and time in days. To measure the hiding power of a slurry, a wax pattern or an object having an equivalent surface state having a predetermined shape is dipped into said slurry for a first predetermined time, typically 10 seconds, and then drained for a second predetermined time, typically 120 seconds. The hiding power is then calculated as the difference in mass of the pattern before and after dipping, relative to the surface of the pattern. The hiding power is highly dependent on the composition of the pattern, the composition of the slurry and the times used in the calculation method, which is why the exact values have not been shown in the single FIGURE, only the comparative change being representative.

[0032] The four slurries compared on the single FIGURE are on the one hand the slurries A and C described above and whose change is represented respectively by curves A and C, and on the other hand a slurry B whose change is represented by curve B and a slurry D whose change is represented by curve D. Slurry B has an initial composition identical to slurry A but differs from slurry A in that it undergoes regeneration at times R. Regeneration consists in removing part of slurry B and diluting the remaining part in a freshly prepared slurry. The slurry can be diluted in a proportion between 10 and 50%, for example 20%. Such an operation is known per se.

[0033] Slurry D has an initial composition identical to slurry C, except for the mass proportion of Tiron which is 0.1%.

[0034] The four casting slurries A, B, C, D were kept stirred throughout the measurements. The hiding power of the slurries must remain between a lower limit Min and an upper limit Max, shown in the single FIGURE, to meet the desired technical specifications. The amplitude of the interval between the Min and Max limits may be about 5 to 10% of the target hiding power.

[0035] As represented by the long-dashed curve A, slurry A sees its hiding power increase continuously over time, until it exceeds the upper limit Max and never falls below it again. This slurry, which behaves in accordance with the prior art, is not satisfactory from the point of view of hiding power.

[0036] As represented by the bold-line curve B, slurry B, regularly regenerated, has a hiding power that remains mostly in the desired Min-Max range. However, even apart from the processing and pollution constraints imposed by regeneration, its hiding power exhibits significant fluctuations which affect the characteristics of the contact layer of the shell mold and, consequently, the surface quality of the part cast in said mold.

[0037] As represented by the short-dotted curve C, slurry C, comprising a surfactant as indicated above, has a relatively stable hiding power, the small variations observed being due to the deviation of the measurement and/or to the addition of water to compensate for the losses by progressive evaporation of the water contained in the colloidal silica. Neither Tiron nor other agents were added during the tests, after the initial addition of Tiron to slurry C.

[0038] As represented by the fine-line curve D, slurry D, comprising a hiding power stabilizing surfactant in an amount greater than or equal to 0.1% by mass, has a hiding power below the minimum limit Min, thus too low in relation to the specifications of the slurry.

[0039] In addition, it was found that Tiron also had an influence as a dispersing agent, making the slurry more fluid and improving the dipping of the patterns during the manufacture of the molds. This improves the slurry coverage of enclosed or less accessible areas.

[0040] As can be seen from the single FIGURE, slurry C, comprising a surfactant and more particularly Tiron, has a considerably longer service life thanks to the stabilization of its hiding power. Adding a surfactant to a casting slurry is inexpensive and simple to process. This type of casting slurry therefore makes it possible, at lower cost, to better control the manufacturing parameters of shell molds, process costs, to reduce industrial waste and to simplify the use of the slurries.

[0041] Surfactants other than Tiron could be used to stabilize a casting slurry, for example sodium polyacrylate of the generic formula [CH2-CH(COONa)-]n.

[0042] Instead of colloidal silica, the slurry could comprise another binder, for example selected from: ethyl silicate, soda silicate or colloids comprising, in particular, colloidal alumina, colloidal yttria or colloidal zirconia.

[0043] Instead of or in addition to the mullite-zirconia composite, the slurry could comprise other powder particles selected from alumina, mullite, silica, zircon, zirconia, all alumino-silicate-based materials and mixtures thereof.

[0044] According to one alternative, instead of including Tiron in the initial composition of slurry C, it is possible to add it during the use of the slurry.

[0045] Casting slurry C can be used to make a shell mold. For this purpose, a pattern of the part, typically made of wax, can be dipped in casting slurry C and then drained, covered with sand and dried. These operations can be repeated afterwards, preferably with another slurry acting as a reinforcing slurry.

[0046] Although the present invention has been described with reference to specific embodiment examples, modifications may be made to these examples without going beyond the general scope of the invention as defined by the claims. In particular, individual features of the different embodiments illustrated/mentioned may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.