Sandblasting media and methods for surface treatment with such sandblasting media

Description

[0001] The invention relates to the cleaning and treatment of the external surfaces of a part with a sandblasting media (which could also be called "medium" for the singular mark), the part being usefully a part such as an aeronautical part. A cleaning and / or surface treatment method is therefore concerned, which proposes projecting such a sandblasting media under pressure onto the surface to be treated. The present invention is particularly suitable for cleaning and treating parts obtained by an additive manufacturing process.

[0002] Parts obtained by an additive manufacturing process have a high roughness with an arithmetic roughness of the profile (Ra) ranging from 5µm to 50µm. A first factor which contributes to this roughness is associated with the stacking in successive layers specific to the technique of the additive manufacturing process which creates a step effect like the part illustrated on the figure 1 The presence of this roughness can generate functional and mechanical problems in a mechanical assembly. Another factor that contributes to roughness is the presence of powder grains that are agglomerated on the surface of the part. These powder grains agglomerated on the surface can be released during the operation of the part and can damage the mechanical system associated with this part, such as for example an oil circuit. These grains can also be released during handling by an operator and can present a health and environmental risk. Finally, for parts obtained by additive manufacturing, it is necessary to remove the residual powder that would not have been fused during manufacturing. This step is carried out with a brush and a vacuum cleaner but which turns out to be insufficient.Furthermore, in some cases, when this step has been omitted and when the part has been subjected to heat treatment, it is then impossible to remove the powder which has sintered to the part under the thermal effect.

[0003] To fully exploit the performance of a part obtained by additive manufacturing, it is therefore imperative to eliminate surface roughness and remove residual powder grains present on the surface. To do this, it is known to use a sandblasting technique based on corundum particles to reduce roughness. This technique consists of projecting corundum particles under pressure onto the surface to be treated of the part. This technique reduces roughness but presents a significant risk of contamination for the part. Indeed, corundum particles can be embedded on the surface of the part when the corundum particles impact with the surface of the part. This encrustation rate is around 5% of the volume of projected corundum particles.The presence of these corundum particles on the surface of the part has the same harmful effects as the powder grains, and can have an impact on the mechanical properties of the part.

[0004] Another solution is to project pressurized dry ice particles onto the surface of a part to remove powder grains present on the surface of the part before heat treatment. The term "dry ice" refers to carbon dioxide CO2 when it is in solid form. It has the property of vaporizing directly without melting, passing from the solid state to the gaseous state, without passing through the liquid state. This technique is similar to sandblasting and allows roughness to be reduced or powder grains to be removed thanks to the hardness of the dry ice particles. The advantage of dry ice over conventional sandblasting techniques is that it sublimates after impact under the effect of the thermal energy produced by the impact. The dry ice particles transform into gas and evaporate into the atmosphere immediately.Therefore, unlike conventional sandblasting where the projected corundum particles can become embedded in the surface of the part, dry ice particles do not present a risk of contamination and leave the part clean. The hardness of dry ice allows the powder grains to be detached from the surface of the part and the residual powder to be removed without leaving any residue on the surface of the treated part.

[0005] However, sandblasting based on the use of dry ice particles is not effective enough to remove residual powder when the part to be treated has already undergone heat treatment. In fact, the residual powder has agglomerated on the surface of the part under the thermal effect by a sintering phenomenon. It is then no longer possible to remove it with sandblasting based solely on dry ice.

[0006] The invention therefore relates to a new sandblasting media which makes it possible both to eliminate roughness and to remove powder grains and residual powder present on the surface of a part obtained by additive manufacturing or any other technique. In particular, the proposed new sandblasting media is effective in removing residual powder even when the part has previously undergone heat treatment. The new sandblasting media therefore has increased cleaning efficiency while preserving the integrity of the part, unlike the sandblasting media of the prior art.

[0007] Certainly, in DE 10 2012 017906 there is proposed a method for cleaning and treating external surfaces of a part with a sandblasting medium comprising first ice particles and second particles having a hardness of between 2000HV and 2500HV, said second particles being embedded on the surface and in the volume of said first particles, said method comprising a step in which a flow of sandblasting medium is generated and projected towards an area of ​​the surface to be treated of the part.

[0008] But the problem remains: how to prevent the second particles from becoming embedded in the surface of the part to be treated.

[0009] One solution, proposed by the invention, is to provide that the projection speed of the particles is adjusted so as to avoid separation between the second particles and the first particles upon impact of the first particles against the surface of the part.

[0010] This differs from what has been proposed so far, where dry ice was used exclusively.

[0011] In the invention, thanks to the presence of the first ice particles which form a protective barrier, the second abrasive particles can no longer be directly embedded on the surface of the part. The abrasive particles are then released following the sublimation of the dry ice or the melting of the ordinary ice, leaving the part clean.

[0012] Thus, unlike conventional sandblasting media where abrasive particles can pollute the surface of the part by becoming embedded in the surface of the part after sandblasting, the new media leaves a clean surface, free of any solid element specific to the sandblasting media, making it possible to obtain a cleaned and treated surface without residue. The invention can be advantageously supplemented by the following features, taken individually or in any of their technically possible combinations: the first particles are water ice particles having a temperature between -10°C and -20°C, the first particles are dry ice particles having a temperature between -60°C and -80°C, the proportion of the second particles is between 20% and 40% by volume relative to the first ice particles, preferably between 20% and 30%, the second particles are corundum particles, the first particles have a diameter between 1mm and 50mm, preferably between 20mm and 30mm, the second particles have a diameter between 0.01mm and 0.5mm, preferably between 0.1mm and 0.2mm.

[0013] According to one embodiment of the invention, the projection speed is between 10m / s and 290m / s, preferably between 100m / s and 150m / s.

[0014] Another aspect of the invention relates to a method of manufacturing a sandblasting media as defined above, comprising the following steps: providing liquid carbon dioxide; subjecting the liquid carbon dioxide to expansion at atmospheric pressure to form dry ice snow; sprinkling second particles onto the dry ice snow; mixing the assembly to obtain a first mixture; compressing the first mixture to form a dry ice solid with the corundum particles embedded on the surface and in the volume of said solid; extruding said solid through a plate so as to obtain a cylinder; cutting said cylinder to obtain first dry ice particles with desired dimensions.

[0015] The invention can be advantageously supplemented by the following characteristics, taken individually or in any of their technically possible combinations: the quantity of the second particles introduced into the dry ice snow is between 20% and 40% by volume relative to the first ice particles, preferably between 20% and 30%, the second particles are corundum particles. Brief description of the drawings

[0016] Other characteristics, details and advantages of the invention will appear on reading the detailed description below, and on analyzing the attached drawings, in which: [ Fig. 1 ] schematically shows a sandblasting media according to an embodiment of the invention used in a method of surface treatment of a part obtained by additive manufacturing; [ Fig. 2 ] shows the state of the treated surface of the part with the first particles of dry ice embedded following treatment; [ Fig. 3 ] shows the condition of the treated surface of the part once the first dry ice particles have melted. Description of the embodiments

[0017] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0018] There figure 1 schematically illustrates a new sandblasting media 10 according to an embodiment of the invention. The media is projected in a flow of compressed air by a suitable device 2 such as a gun towards a surface to be treated of a rough part 1 obtained for example by additive manufacturing.

[0019] The rough part 1 is made up of a succession of layers 4 forming steps. In addition, powders 3 are agglomerated on the surface of the steps.

[0020] The media 10 comprises a plurality of first ice particles 11 and second abrasive particles 12 which are embedded on the surface of the first particles and in the volume of the latter.

[0021] According to one embodiment of the invention, the first ice particles are dry ice particles obtained from liquid CO2. The dry ice particles sublime upon contact with the surface of the part and evaporate in the form of gas, leaving no residue.

[0022] According to one embodiment of the invention, the dry ice particles are in the form of rods, called pellets, having a length of between 1 mm and 60 mm and a diameter of between 1 mm and 50 mm, preferably between 20 mm and 30 mm. They can also be in an ellipsoidal shape as in the example illustrated in the figure 1 .

[0023] Advantageously, the first particles of dry ice have a temperature between -60°C and -80°C.

[0024] Alternatively, the first ice particles may also be water ice particles obtained from frozen water. The ice particles are projected onto the surface of the part to be treated and melt under the thermal effect upon contact with the surface and the resulting liquid evaporates leaving no residue. The first particles are water ice particles which have a temperature between -10°C and -20°C

[0025] It is known to project dry ice particles onto a surface to be treated in a part to perform cryogenic cleaning. The effectiveness of surface treatment and cleaning is based on the combination of three effects: mechanical effect due to the kinetic energy of the dry ice particles at the time of impact on the part; thermal effect due to the temperature of the particles, the residues become fragile and shrink; blast effect created by the sublimation of the dry ice which causes the residue to detach. However, a treatment based solely on these dry ice particles is not effective in reducing roughness and / or removing powders which are agglomerated on the surface of a part which has previously been subjected to heat treatment. The authors of the present invention have found a way to solve this problem, thanks to a new sandblasting medium which combines the dry ice particles with abrasive particles having a hardness between 2000HV and 2500HV which are embedded on the surface and in the volume of the dry ice particles.

[0026] This new media makes it possible to combine the effects of the dry ice particles mentioned above and the abrasive power of the second particles, to reduce roughness and eliminate agglomerated powders.

[0027] Furthermore, thanks to the presence of dry ice particles which form a protective barrier, the abrasive particles can no longer be directly embedded on the surface of the part. They are carried by the dry ice particles which are embedded on the surface of the treated part following sandblasting as illustrated in figure 2 The abrasive particles are then released following the sublimation of the dry ice or the melting of ordinary ice, leaving the part clean.

[0028] Thus, unlike conventional sandblasting media where abrasive particles can pollute the surface of the part by embedding themselves on the surface of the part after sandblasting, the new media leaves a clean surface as illustrated in figure 3 , free of any solid element specific to the sandblasting media, allowing a cleaned and treated surface to be obtained without residue.

[0029] According to the invention, the second abrasive particles preferably have a hardness of between 2000HV and 2500HV.

[0030] They have a diameter between 0.01mm and 0.5mm, preferably between 0.1mm and 0.2mm.

[0031] Preferably, the second particles comprise corundum particles. Advantageously, the proportion of the second particles of the second particles is between 20% and 40% by volume relative to the first ice particles, preferably between 20% and 30%.

[0032] The method of manufacturing the novel media comprising first dry ice particles and second abrasive particles embedded on the surface and in the volume of the first particles will now be described.

[0033] The first step in the process is to obtain CO2 snow by expanding liquid carbon dioxide contained in a chamber. Pressurized liquid CO2 is introduced into a chamber. The pressure inside the chamber is equal to or close to atmospheric pressure. The flow of pressurized liquid CO2 expands inside the chamber, which is accompanied by a drop in temperature to form solid CO2 snow.

[0034] In a second step, corundum particles or other abrasive particles with a hardness between 2000HV and 2500HV are sprinkled on the dry ice snow and mixed with the CO2 snow to obtain a first mixture.

[0035] In a third step, this first mixture is then introduced into compacting and extrusion means to form a compacted solid mixture of CO2 with corundum particles embedded in the solid CO2.

[0036] In a fourth step, the compacted solid is then pressed through an extruded plate to form cylinders which are subdivided into dry ice rods or pellets with the corundum particles embedded on the surface and within the volume of the dry ice pellets.

[0037] An example of a method for surface treatment of a part using a sandblasting media of the present invention will now be described, assuming that the first particles are dry ice particles.

[0038] It comprises (regardless of the type of first particles) a step in which a flow of blasting media is generated and projected towards an area of ​​the surface to be treated of the part. Under the effect of compressed air, the first particles or pellets of dry ice are accelerated so as to reach a predetermined speed. This speed is adjusted so as to avoid separation between the second particles and the first particles during the projection phase and during the impact of the first particles against the surface of the part. The projection speed is between 10m / s and 290m / s, preferably between 100m / s and 150m / s. Tests are carried out to determine the optimal speed.

[0039] Before starting the operation of treating and cleaning the surface of a part, parameters must be predefined: the dimensions of the first particles of dry ice and the second particles, the projection speed at the outlet of the nozzle of the projection device, the projection pressure, the projection flow rate.

[0040] The projection flow rate is between 10kg / h and 100kg / h.

[0041] If the first particles are dry ice particles, thanks to this high speed and a very low temperature between -60°C and -80°C, the impurities freeze and cracks appear. The pellets enter this crack, burst and thus detach the deposits. This cryogenic cleaning effect is combined with the abrasive action exerted by the abrasive particles embedded in the dry ice pellets to remove the sintered powders on the surface of the heat-treated part.

[0042] The combined action of dry ice pellets and abrasive particles removes agglomerated and sintered powders from the surface of the part that has been subjected to heat treatment, while leaving the part clean after the operation. Industrial application

[0043] The invention may be applied in particular to the field of cleaning and treatment of parts, in particular parts obtained by additive manufacturing to remove roughness and powder residues specific to additive manufacturing.

Claims

1. Method for cleaning and treating external surfaces of a part (1) with a blasting medium (10) comprising first particles of ice (11) and second particles (12) having a hardness between 2000 and 2500HV, said second particles being embedded in the surface and in the volume of said first particles, said method comprising a step in which a flow of blasting medium is generated and projected towards an area of the surface of the part to be treated, characterized in that the projection speed of the particles is adjusted to avoid separation between the second particles and the first particles on impact of the first particles against the surface of the part.

2. Method according to claim 1, wherein the first particles are water ice particles having a temperature between -10°C and -20°C.

3. Method according to claim 1, wherein the first particles are dry ice particles having a temperature between 60°C and -80°C.

4. Method according to any one of claims 1 to 3, wherein the proportion of the second particles is between 20 and 40% by volume with respect to the first particles of ice, preferably between 20 and 30%.

5. Method according to any one of claims 1 to 4, wherein the second particles are corundum particles.

6. Method according to any one of claims 1 to 5, wherein the first particles have a diameter between 1mm and 50mm, preferably between 20mm and 30mm.

7. Method according to any one of claims 1 to 6, wherein the first particles have a diameter between 0.01mm and 0.5mm, preferably between 0.1 and 0.2mm.

8. Method according to any one of claims 1 to 7, wherein said projection speed is between 10m / s and 290m / s.

9. Method according to any one of claims 1 to 7, wherein said projection speed is between 100m / s and 150 m / s.

10. Method for manufacturing the blasting medium used in the method according to any one of claims 1 to 9, comprising the steps of: - supplying liquid carbon dioxide; - subjecting the liquid carbon dioxide to expansion to atmospheric pressure to form dry ice snow; - sprinkling second particles on the dry ice snow, with a quantity of second particles introduced into the dry ice snow between 20 and 40% by volume with respect to the first particles of ice; - mixing the whole to obtain a first mixture; - compressing the first mixture to form a dry ice solid with the second particles embedded in the surface and in the volume of said solid; - extruding said solid through a plate so as to obtain a cylinder; and - cutting said cylinder to obtain first dry ice particles with desired dimensions.

11. Method according to claim 10, wherein the second particles are corundum particles.