Foundry slip

Abstract

A foundry slip for the manufacture of shell molds, comprising powder particles and a binder, characterized in that it comprises a surfactant. Use of such a foundry slip for the manufacture of a shell mold.

Description

FIELD OF INVENTION

[0001] This presentation concerns the field of foundry work, in particular lost-wax casting processes, and more specifically the slips used in such processes, especially for the manufacture of shell molds for foundry work. TECHNOLOGICAL BACKGROUND

[0002] Lost-wax casting, also known as lost-wax casting, has been known since antiquity. It is particularly well-suited for producing metal parts with complex shapes. For example, lost-wax casting is used to produce turbomachine blades and bladed wheel sectors. In lost-wax casting, the first step is typically the fabrication of a shell mold. This generally involves creating a pattern from a material with a relatively low melting point, such as wax or resin, around which a shell of refractory material is then formed. After the pattern is destroyed—usually by removing the pattern material from inside the shell mold, hence the name—molten metal is poured into the mold to fill the cavity formed by the removed pattern.Once the metal cools and solidifies, the mold can be opened or destroyed to recover a metal piece conforming to the shape of the model.

[0003] To create the shell mold, the wax model is generally dipped in a foundry slip, then coated with sand and dried. These operations can be repeated to form several layers and obtain the desired thickness and mechanical strength for the shell mold.

[0004] In practice, foundry slips are produced in large quantities for use over several months, but their properties degrade over time, impacting the quality of shell mold production. One known method to mitigate this degradation involves regenerating the slip by diluting older slip with more recently produced slip, which partially restores its properties. However, this method results in significant fluctuations in properties, its effects are short-lived, and a substantial portion of the older slip is discarded.

[0005] Alternatively, some additives could be used, but none of these additives provided satisfaction since the improvement of one parameter of the slip was compensated by the unacceptable degradation of another parameter.

[0006] There is therefore a need for a new type of foundry slip, exhibiting increased stability over time. PRESENTATION OF THE INVENTION

[0007] For this purpose, the present presentation concerns a foundry slip for the manufacture of shell molds, comprising powder particles and a binder, characterized in that it comprises a surfactant for stabilizing the covering power.

[0008] A foundry slip is a slip suitable for forming a shell mold into which molten metal will be poured. In particular, unlike a simple slurry, such a slip contains a binder, that is, a compound that ensures cohesion between the powder particles and gives the shell mold its mechanical strength both in its raw state and after sintering. The binder can be inorganic. Examples of binders will be given later. Typically, the powder particles can be sand particles (also known as "flour"), especially refractory particles, generally having a diameter between 1 micrometer and 100 micrometers.

[0009] A surfactant, also called a surface-active agent, is a compound that modifies the surface tension between two surfaces, for example, between two components of a mixture. Surprisingly, the inventor found that adding a particular surfactant to a foundry slip significantly stabilized the slip's covering power—that is, its ability, measured by mass per unit area, to remain on a given surface after quenching and draining. Conversely, the covering power of a prior art slip, without a covering power stabilizing surfactant, tends to increase over time without stabilizing.

[0010] Some surfactants are known as dispersing agents to thin certain suspensions, but for these suspensions, they do not stabilize the covering power due to the absence of a binder. Conversely, in the slip described here, the covering power stabilizing surfactant modifies the interaction between the binder and the powder particles to stabilize the slip's covering power. Generally, compounds previously used as thinning or dispersing agents had no effect on covering power.

[0011] In addition, the surfactant also helps to stabilize the viscosity of the slip.

[0012] Thus, the slip according to this presentation has a composition with key parameters (viscosity, pH, density etc.), including covering power, which are stable over time, which makes it possible to improve the repeatability of the manufacturing process of shell molds and to considerably limit the amount of waste related to the traditional regeneration of slips.

[0013] In some embodiments, the surfactant has a carbon chain comprising at most 4,800 carbon atoms, preferably at most 2,000 carbon atoms, preferably at most 1,000 carbon atoms, preferably again at most 500 carbon atoms, preferably again at most 100 carbon atoms. This prevents the slip from thickening, as the binder molecules could become entangled in an excessively long carbon chain.

[0014] In some embodiments, the surfactant does not contain ammonia ions. Since ammonia ions tend to cause the binder to gel, the use of such a surfactant further stabilizes the slip.

[0015] In some embodiments, the surfactant leaves the pH of the slip unchanged to within ±5%. In other words, the slip's pH is modified by less than ±5% before and after the surfactant is added. This ensures that the slip remains compatible with the other specifications of the shell mold manufacturing process.

[0016] In some embodiments, the surfactant comprises Tiron C6H4Na2O8S2. Preferably, the surfactant is Tiron. Tiron, in addition to satisfying the preceding criteria, is a relatively common molecule, generally used as a complexometric indicator in analytical chemistry to reveal the presence of certain ions, or as a dispersant.

[0017] Alternatively or in addition, in some embodiments, the surfactant comprises sodium polyacrylate. Sodium polyacrylate has the generic formula [-CH2-CH(COONa)-]n. Preferably, the surfactant is sodium polyacrylate.

[0018] In some embodiments, the binder is chosen from: ethyl silicate, sodium silicate or colloids including, in particular, colloidal silica, colloidal alumina, colloidal yttria or colloidal zirconia.

[0019] In some embodiments, the surfactant content by mass in the slip is less than 0.1%, preferably less than or equal to 0.05%. A small amount of surfactant is therefore sufficient to stabilize the foundry slip, particularly its covering power. Conversely, too high a quantity of the surfactant used to stabilize covering power can cause excessive variation in covering power. Consequently, the overall composition of the slip remains unchanged. This ensures that the slip remains compatible with the other specifications of the shell mold manufacturing process.

[0020] In some embodiments, the slip is a contact slip configured to come into contact with a wax or equivalent part model. The first slip used, which directly covers the model, is called the contact slip, as opposed to subsequent slips, called reinforcing slips, which cover the preceding layers of the forming shell mold. A contact slip is configured to conform to the shape of the model and not alter it. A contact slip is often kept for longer periods than a reinforcing slip, which is consumed more quickly, hence the increased need for stability in a contact slip.

[0021] In some embodiments, the powder particles comprise at least one compound among alumina, mullite, zircon, zirconia, silica, and mullite-zirconia composites. Mullite refers to silico-aluminous materials.

[0022] This presentation also concerns the use of a foundry slip as previously described for the manufacture of a shell mold. 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 accompanying drawings, in which the single figure is a graph illustrating the evolution of the covering power of different slips over time. DETAILED DESCRIPTION OF IMPLEMENTATION METHODS

[0024] To assess the effects of adding a surfactant to a foundry slip, the inventor first studied a control slip, designated slip A, intended for use as a contact slip in the manufacture of a shell mold. Slip A can have the following composition, expressed as mass percentages: binder (colloidal silica): 29.8%; powder particles (mullite-zircone composite): 70.0%; wetting agent, antifoaming agent and other additives: 0.2%. This mass distribution is given here as an example, it being understood that a variation in mass distribution of between 0.1% and 10% is possible. Slip A has a basic pH and does not contain, even among the aforementioned "other additives," any surfactant that affects covering power.

[0025] Furthermore, as previously stated, the inventor studied a slip C, which was prepared by taking slip A and adding a surfactant to stabilize coverage, in this case Tiron, at a mass concentration of 0.05%, preferably 0.005%. The resulting foundry slip C is therefore also a contact slip. The amount of Tiron can be adjusted by a person skilled in the art according to the initial and target coverage, preferably without exceeding 0.1% by mass. For example, the mass concentration of Tiron can 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 even less than or equal to 0.01%.

[0026] The inventor verified that adding Tiron to the slip had little effect on its pH, specifically a change of ±5% or less. Furthermore, Tiron has a short carbon chain, comprising fewer than one hundred carbon atoms, in this case, six. Tiron does not contain ammonia ions, as it contains no nitrogen at all. Tiron is also a good complexing agent for the chemical elements of the oxides present in slip C and originating from the powder particles; indeed, Tiron has an affinity for these oxides and can interact effectively with them. Moreover, Tiron will be eliminated during the heat treatment of the corresponding shell mold and will not have any detrimental effect on the metal of the casting that will be poured into the shell mold.

[0027] Thus, through its interaction with the oxides and colloidal silica forming the binder, the surfactant, which here is Tiron, ensures good stability of the slip C, in particular its covering power, as we will see with reference to the single figure.

[0028] This figure shows the evolution of the covering power (CP) of four slips as a function of time t. Covering power can be measured in grams per square centimeter (g / cm²) and time in days. To measure the covering power of a slip, a wax model or an object with an equivalent surface finish and a predetermined shape is dipped into the slip for a predetermined time, typically 10 seconds, and then drained for a second predetermined time, typically 120 seconds. The covering power is then calculated as the difference in mass of the model before and after dipping, divided by the surface area of ​​the model. Covering power depends heavily on the composition of the model, the composition of the slip, and the times used in the calculation method; therefore, exact values ​​are not shown in the single figure, only the comparative evolution is representative.

[0029] The four slips compared in the single figure are, on the one hand, slips A and C described previously, whose evolution is represented respectively by curves A and C, and on the other hand, slip B, whose evolution is represented by curve B, and slip D, whose evolution is represented by curve D. Slip B has an initial composition identical to slip A but differs from slip A in that it undergoes regeneration at time points R. Regeneration consists of removing a portion of slip B and diluting the remaining portion in freshly prepared slip. The slip can be diluted in a proportion between 10 and 50%, for example, 20%. This operation is well-known.

[0030] Slip D has an initial composition identical to slip C, except for the mass proportion of Tiron which is 0.1%.

[0031] The four foundry slips A, B, C, and D were kept agitated throughout the measurements. The covering power of the slips must remain between a lower limit (Min) and an upper limit (Max), illustrated in the single figure, to meet the desired technical specifications. The range between the Min and Max limits can be approximately 5 to 10% of the target covering power.

[0032] As represented by the long dashed curve A, the covering power of slip A increases continuously over time, exceeding the upper limit Max and never falling below it. This slip, whose behavior conforms to the prior art, is unsatisfactory from the point of view of covering power.

[0033] As represented by curve B in bold, regularly regenerated slip B exhibits a covering power that remains mostly within the desired Min-Max range. However, even apart from the processing and pollution constraints imposed by regeneration, its covering power shows significant fluctuations that impact the characteristics of the contact layer of the shell mold, and consequently, the surface quality of the part cast in said mold.

[0034] As represented by the short-dashed curve C, slip C, containing a surfactant as previously described, exhibits relatively stable coverage. The small variations observed are due to measurement deviation and / or the addition of water to compensate for losses through gradual evaporation of the water contained in the colloidal silica. Neither Tiron nor any other agents were added during the tests after the initial addition of Tiron to slip C.

[0035] As represented by the thin line curve D, slip D, comprising a covering power stabilizing surfactant in a quantity greater than or equal to 0.1% by mass, has a covering power lower than the minimum limit Min, therefore too low compared to the slip specifications.

[0036] Furthermore, it was observed that Tiron also acted as a dispersing agent, thinning the slip and improving the tempering of the models during mold making. This allows for better slip coverage of enclosed or less accessible areas.

[0037] As shown in the single figure, slip C, containing a surfactant, specifically Tiron, exhibits a significantly increased lifespan thanks to the stabilization of its covering power. Adding a surfactant to a foundry slip is inexpensive and easy to implement. Such a foundry slip therefore allows, at a lower cost, for better control of the manufacturing parameters of shell molds, reduced process costs, decreased industrial waste, and simplified slip use.

[0038] Other surfactants besides Tiron could be used to stabilize a foundry slip, for example sodium polyacrylate, with the generic formula [-CH2-CH(COONa)-]n.

[0039] Instead of colloidal silica, the slip could include another binder, for example chosen from: ethyl silicate, sodium silicate or colloids including, in particular, colloidal alumina, colloidal yttria or colloidal zirconia.

[0040] Instead of or in addition to the mullite-zircone composite, the slip could include other powder particles, notably chosen from alumina, mullite, silica, zircon, zirconia, all silico-aluminous based materials and their mixtures.

[0041] According to one variant, instead of including Tiron in the initial composition of slip C, it is possible to add it during the use of the slip.

[0042] Foundry slip C can be used to make a shell mold. For this purpose, a pattern, typically made of wax, can be dipped into slip C, then drained, sandblasted, and dried. These operations can then be repeated, preferably with another slip acting as a reinforcing slip.

[0043] Although the present invention has been described with reference to specific embodiments, modifications may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various embodiments illustrated / mentioned may be combined in additional embodiments. Therefore, the description and drawings should be considered illustrative rather than restrictive.

Claims

1. Foundry slip for the manufacture of shell molds, comprising powder particles including at least one compound from alumina, mullite, zircon, zirconia, mullite-zirconia composites, a silico-aluminous material and mixtures thereof, and a binder selected from ethyl silicate, sodium silicate or colloids including, in particular, colloidal silica, colloidal alumina, colloidal yttria or colloidal zirconia, the foundry slip comprising Tiron C6H4Na2O8S2 and / or sodium polyacrylate as a surfactant for stabilizing coverage, the slip having the following composition, expressed as mass percentages with a possible variation between 0.1% and 10%: - binder: 29.8%; - powder particles: 70.0%; - Surfactant for stabilizing coverage: non-zero content less than 0.1%; - Wetting agent, anti-foaming agent and other additives: 0.2%.

2. Foundry slip according to claim 1, wherein the surfactant has a carbon chain comprising at most four thousand eight hundred carbon atoms.

3. Foundry slip according to claim 1 or 2, wherein the surfactant leaves the pH of the slip unchanged to within 5%.

4. Foundry slip according to any one of claims 1 to 3, wherein the surfactant is Tiron C6H4Na2O8S2.

5. Foundry slip according to any one of claims 1 to 3, wherein the surfactant is sodium polyacrylate.

6. Foundry slip according to any one of claims 1 to 5, wherein the mass content of the surfactant in the slip is less than or equal to 0.05%.

7. Foundry slip according to any one of claims 1 to 6, the slip being a contact slip configured to come into contact with a pattern of part.

8. Use of a foundry slip according to any one of claims 1 to 7 for the manufacture of a shell mold.

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