Vacuum pump and manufacturing process

Abstract

A vacuum pump comprising a stator (2) and two rotors received in at least one pumping chamber of the stator (2), the rotors being configured to rotate in the pumping chamber to drive a gas to be pumped between a suction port (4) and a discharge port (5) of the vacuum pump, characterized in that the stator (2) comprises at least one stator piece obtained by casting iron or steel in a mold receiving at least one sand core comprising a majority of sand selected from one or more of the following sands: artificial globular ceramic sand, chromite sand, bauxite sand, zircon sand, kerphalite sand, olivine sand, the surfaces of the cast stator piece (2) intended to be exposed to the pumped gases being coated. Figure 1

Description

Title of the invention: Vacuum pump and manufacturing method Technical field of the invention

[0001] The invention relates to a vacuum pump, such as a "Roots" or "Claw" type vacuum pump or one of another similar principle. The invention also relates to a method of manufacturing a vacuum pump stator part obtained by casting cast iron or steel. Technical background

[0002] In the semiconductor, flat panel display (FIAT Panel Display), photovoltaic and coating industries, dry vacuum pumps can be used to evacuate corrosive gases such as NF3, C1F3, F2, Cl2, originating in particular from chamber cleaning processes. These corrosive gases can damage the steel or cast iron mechanical components of the vacuum pump, leading to performance losses, or even safety problems due to leaks because oxygen or water vapor can enter the low pressure stages of the vacuum pump and react with the process and cleaning gases or the latter can leak into the atmosphere and therefore towards potential operators.

[0003] One solution is to coat the rotors and the compression chamber of the vacuum pump stator with a nickel-phosphorus coating that has undergone a hardening heat treatment. The nickel-phosphorus coating prevents the formation of corrosion layers in the vacuum pump. The hardening heat treatment of the coating improves the resistance of the vacuum pump to abrasion generated by solid reaction products.

[0004] However, the coated static mechanical components come from foundries using silica sands (also called green sand) whose grains can remain on the surface of the part in high concentration when the foundry skins are not machined, and this despite shot blasting or other technique used for stripping the surfaces. This is particularly the case for surfaces that are difficult for tools to access, such as the transfer channels between the pumping stages.

[0005] These embedded grains of sand and their mineral nature prevent the coating from being properly deposited and adhered well, leaving areas devoid of deposit. However, these areas are exposed to process gases and are therefore subject to their attacks. The corrosive gases attack the substrate in areas with deposit gaps, which then provide access routes under the coating, which crumbles and delaminates, exposing the metal surfaces. The entire casting with corroded portions in the unmachined areas must then be replaced.

[0006] Also, metal particles from these corroded areas, for example transfer channels, can be carried into the pumping chambers where they can scratch the coating.

[0007] In addition, the release of some of these grains of sand during the process of depositing the coating by immersion can generate significant pollution of the various baths, which can be detrimental to the quality of the coating deposited. Summary of the invention

[0008] An aim of the present invention is therefore to propose a vacuum pump which at least partially overcomes the drawbacks of the state of the art.

[0009] To this end, the invention relates to a vacuum pump comprising a stator and two rotors received in at least one pumping chamber of the stator, the rotors being configured to rotate in the pumping chamber to drive a gas to be pumped between a suction orifice and a discharge orifice of the vacuum pump, characterized in that the stator comprises at least one stator part obtained by casting cast iron or steel made in a mold receiving at least one sand core comprising a majority of sand chosen from one or more of the following sands: - artificial globular ceramic sand, - chromite sand, - bauxite sand, - zircon sand, - kerphalite sand, - olivine sand, the surfaces of the cast stator part intended to be exposed to the pumped gases being coated, for example with a nickel alloy-based coating, such as a nickel-phosphorus coating.

[0010] The use of these sands does not prevent the castings produced from having sand grains on the skin of the parts. However, these sands are much more resistant to the corrosive gases used in the cleaning processes of semiconductor manufacturing processes, photovoltaic panels, flat screens and deposits, than the silica sand (or green sand) of the prior art. A better bond between the sand grain inclusions on the surface and the coating is also observed with these types of sand. This better compatibility engulfs the sand grains in the coating, which makes it possible to reduce the formation of gaps at the edges of the sand grains so that it is more difficult for corrosive gases to infiltrate under the surrounding coating. Consequently, the sand inclusions in the uncoated stator casting skins exposed to corrosive gases have better resistance chemical and mechanical resistance to these gases. Castings are therefore more resistant to corrosion, including unmachined surfaces. Furthermore, the process is simple to carry out and reasonably costly.

[0011] The vacuum pump may further comprise one or more of the features described below, taken alone or in combination.

[0012] The coating has, for example, a thickness of between 5 pm and 50 pm, such as 25 pm.

[0013] The vacuum pump comprises, for example, at least two pumping stages, each defining a pumping chamber, the pumping chambers of the successive pumping stages being connected in series by at least one respective transfer channel of the stator.

[0014] The stator parts produced by casting are, for example, complementary half-shells, assembled together to form the pumping chambers of the at least two pumping stages and the transfer channels between the pumping chambers.

[0015] According to another example, the stator parts produced by casting are sliced ​​stator elements, assembling axially and each comprising at least one separating partition and at least one portion of transfer channel.

[0016] The invention also relates to a method for manufacturing a vacuum pump stator part obtained by casting cast iron or steel as defined above, characterized in that the stator part is produced in a mold receiving at least one sand core comprising a majority of sand chosen from one or more of the following sands: artificial globular ceramic sand, chromite sand, bauxite sand, zircon sand, kerphalite sand, olivine sand, the surfaces of the stator part produced by casting intended to be exposed to the pumped gases being coated, for example with a nickel alloy-based coating such as a nickel-phosphorus coating.

[0017] The coating may further be heat treated to have a hardness greater than 700Hv.

[0018] According to an exemplary embodiment, the artificial globular ceramic sand cores are produced by 3D printing.

[0019] According to an exemplary embodiment, surfaces of the cast stator part, in particular pumping chambers, positioning and holding elements and sealing surfaces, can be machined before being coated. Brief description of the figures

[0020] Other advantages and characteristics will appear on reading the description of the invention, as well as the attached drawings in which:

[0021] [Fig.l] [Fig.l] shows a perspective view of stator parts of a stator in the disassembled state of a vacuum pump according to a first exemplary embodiment.

[0022] [Fig.2] [Fig.2] shows a model plate, four cores and a lower mold for the manufacture of stator parts of two stators such as that of [Fig.l].

[0023] [Fig.3] [Fig.3] shows a perspective view of rotors and elements of a stator with the disassembled state of a vacuum pump according to a second exemplary embodiment.

[0024] [Fig.4] [Fig.4] shows a perspective view of a stator part of the stator of the [Fig.3],

[0025] In these figures, identical elements have the same reference numbers. Detailed description

[0026] The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Single features of different embodiments may also be combined or interchanged to provide other embodiments, without departing from the scope of the invention, as defined by the claims.

[0027] A primary vacuum pump is defined as a volumetric vacuum pump, which is configured to, using two rotor shafts, suck in, transfer, and then discharge the gas to be pumped at atmospheric pressure. The rotor shafts are driven in rotation by a motor of the primary vacuum pump.

[0028] “Upstream” means an element that is placed before another in relation to the direction of circulation of the gas to be pumped. Conversely, “downstream” means an element placed after another in relation to the direction of circulation of the gas to be pumped.

[0029] The axial direction is defined as the longitudinal direction of the pump in which the axes of the rotor shafts extend.

[0030] The vacuum pump comprises a stator 2 forming at least one pumping chamber of a pumping stage T1-T6 ([Fig.l]).

[0031] According to an exemplary embodiment, the vacuum pump comprises at least two pumping stages T1-T6 each defining a pumping chamber, the pumping stages T1-T6 being connected in series between a suction port 4 and a discharge port 5, such as between two and ten pumping stages (six in the illustrative example). This vacuum pump is for example a primary vacuum pump.

[0032] The vacuum pump further comprises two rotors configured to rotate synchronously in opposite directions in the pumping chambers of the pumping stages T1-T6 so that the rotors drive a gas to be pumped between the suction port 4 and the discharge port 5.

[0033] The rotors have, for example, lobes with identical profiles, for example of the “Roots” type with two lobes (see [Fig. 3]) or more or of the “Claw” type or of another similar principle of volumetric vacuum pump. The shafts carrying the rotors are driven by a motor located, for example, at one end of the vacuum pump.

[0034] Each pumping stage T1-T6 is formed by a pumping chamber receiving two mating rotors, the pumping chambers comprising a respective inlet and outlet. During rotation, the gas sucked from the inlet is trapped in the volume generated by the rotors and the stator 2, then is driven by the rotors towards the next stage.

[0035] The pumping chambers of the successive pumping stages T1-T6 are connected in series one after the other by at least one respective transfer channel 6a, 6b, 7a, 7b connecting the outlet of the preceding pumping stage T1-T6 to the inlet of the following pumping stage T1-T6. The vacuum pump may further comprise two transfer channels 6a, 6b, 7a, 7b configured to connect in parallel, on either side of a pumping chamber, an outlet of the pumping chamber of a preceding pumping stage to an inlet of a following pumping stage. In this example, there are two transfer channels 6a, 6b, 7a, 7b per pumping chamber to the five inter-stages connecting the six pumping chambers together.

[0036] The inlet of the first pumping stage T1 communicates with the suction port 4 of the vacuum pump. The outlet of the last pumping stage T6 communicates with the discharge port 5. The axial dimensions of the rotors and the pumping chambers are for example equal or decreasing with the pumping stages, the pumping stage T1 located on the side of the suction port 4 receiving the rotors of larger axial dimension.

[0037] These vacuum pumps are called “dry” because in operation, the rotors rotate inside the stator 2 without any mechanical contact between them or with the stator 2, which makes it possible not to use oil in the pumping stages T1-T6.

[0038] The stator 2 comprises at least one stator part obtained by casting cast iron or steel made in a mold receiving at least one sand core. More particularly and as illustrated in [Fig.2], the mold comprises an upper mold 13 and a lower mold (not shown) between which is received at least one sand core 15, 16. The sand core 15, 16 corresponds to the hollow parts of the part from the casting.

[0039] The sand core 15, 16 comprises a majority of sand chosen from one or more of the following sands: artificial globular ceramic sand, chromite sand, bauxite sand, zircon sand, kerphalite sand, olivine sand, the surfaces of the stator part 2 produced by casting intended to be exposed to the pumped gases being coated.

[0040] By “majority” we mean a larger proportion in mass.

[0041] The term “sand” means a granular solid material formed of small particles in particular of mineral origin and notably less than 2mm. The grains of sand are agglomerated into nuclei for example by processes using different types of resins with polymerization by thermochemical catalysis.

[0042] The use of these sands does not prevent the castings produced from having grains of sand on the skin of the parts. However, these sands are much more resistant to the corrosive gases used in the cleaning processes of semiconductor manufacturing processes, photovoltaic panels, flat screens and deposits, than the silica sand (or green sand) of the prior art. A better bond between the inclusions of grains of sand on the surface and the coating is also observed with these types of sand. This better compatibility engulfs the grains of sand in the coating, which makes it possible to reduce the formation of gaps at the edges of the grains of sand so that it is more difficult for corrosive gases to infiltrate under the surrounding coating. Consequently, the sand inclusions in the uncoated stator casting skins exposed to corrosive gases have better chemical and mechanical resistance to these gases.Castings are therefore more resistant to corrosion, including unmachined surfaces. Furthermore, the process is simple to carry out and reasonably costly.

[0043] According to a first embodiment, the sand core 15, 16 comprises a majority of artificial globular ceramic sand. It is for example entirely made of artificial globular ceramic sand. This sand comprises a majority of alumina and silica, and in particular more alumina (A12O3) than silica. It comprises more than 50% alumina and traces of iron oxides, i.e. more than 1% iron oxide.

[0044] According to a first example of composition, the artificial globular ceramic sand (marketed under the brand cerabeads ®), contains between 60% and 62% of Al2O3, between 35% and 38% of SiO2, and between 1% and 2% of Fe3O4.

[0045] According to a second example of composition, the artificial globular ceramic sand (marketed under the brand name CBS2®), contains between 66% and 68% of Al2O3, between 14% and 16% of SiO2 and between 2% and 4% of Fe2O3.

[0046] Artificial globular ceramic sand contains traces of iron oxide which can facilitate the embedding of the coating around the sand grain inclusions. This sand also has the advantage of being able to be used for the manufacture of sand cores by 3D printing.

[0047] According to a second embodiment, the sand core comprises a majority of chromite sand. It is for example entirely made of chromite sand. This sand comprises a majority of chromium oxide (Cr2O3), iron oxide (Fe2O3), alumina (A12O3) and magnesium oxide (MgO). It contains more than 40% chromium oxide, more than 20% iron oxide, more than 10% alumina and more than 5% magnesium oxide. Chromite sand contains iron oxide which can help to facilitate the embedding of the coating around the sand grain inclusions.

[0048] According to an example of composition, the chromite sand comprises between 44% and 47% of Cr2O3, between 24% and 28% of Fe2O3, between 14% and 16% of Al2O3 and between 9% and 11% of MgO.

[0049] According to a third embodiment, the sand core comprises a majority of bauxite sand. For example, it is made entirely of bauxite sand. This sand comprises more alumina (A12O3) than silica. It comprises more than 40% alumina and traces of iron oxides, i.e. more than 1% iron oxide.

[0050] According to an example of composition, the bauxite sand contains 48% to 61% A12O3, 2% to 23% Fe2O3, 1% to 7% SiO2, 2% to 4% TiO2) depending on whether it is of karstic or lateritic type.

[0051] Bauxite sand contains iron oxide which can help facilitate the embedding of the coating around the sand grain inclusions.

[0052] According to a fourth embodiment, the sand core comprises a majority of zircon sand. It is for example made entirely of zircon sand. This sand comprises a majority of zirconium oxide (ZrO2) and silica, and in particular more zirconium oxide (ZrO2) than silica. It comprises more than 50% zirconium oxide.

[0053] According to an example of composition, the zircon sand contains between 65% and 67% of ZrO2, between 31% and 33% of SiO2, between 1% and 3% of U (uranium) and / or thorium and / or oxidized hafnium.

[0054] According to a fifth embodiment, the sand core comprises a majority of kerphalite sand. It is for example made entirely of kerphalite sand. This sand comprises a majority of alumina (A12O3) and silica, and in particular more alumina (A12O3) than silica. It comprises more than 50% alumina and traces of oxides, i.e. more than 1% oxides.

[0055] According to an example of composition, kerphalite sand contains between 50% and 61% Al2O3, between 36% and 37% SiO2 and traces of oxides of iron, manganese, titanium, copper, magnesium and potassium. This sand becomes mullite when heated.

[0056] According to a sixth embodiment, the sand core comprises a majority of olivine sand. It is for example made entirely of olivine sand.

[0057] According to an example of composition, the olivine sand contains silica and magnesium oxide in equal parts, for example 49% SiO2, 49% MgO and 2% FeO.

[0058] Surfaces of the stator part 2 from the casting, in particular the chambers of pumping of stages T1-T6, positioning and holding elements and sealing surfaces, can be machined to ensure precision of production.

[0059] Then, the surfaces of the stator part 2 produced by casting which are machined or not and which are intended to be exposed to the pumped gases are coated. The coating makes it possible to modify the surface of the cast iron in particular so that it has better resistance to corrosion, abrasion and so that it facilitates the transport of powders.

[0060] The coating is for example a nickel alloy-based coating, such as a nickel-phosphorus coating, which comprises for example between 9 and 14% phosphorus by mass, such as between 10% and 13% phosphorus.

[0061] The coating has, for example, a thickness greater than or equal to 5 pm, such as between 5 pm and 50 pm, such as 25 pm.

[0062] The coating is for example deposited by a technique of immersing the stator part.

[0063] The coating may be heat treated with a heating step at a treatment temperature above 250°C for a treatment time of more than one hour, to have a hardness greater than 700Hv, such as between 800Hv and 1000Hv.

[0064] According to a first exemplary embodiment illustrated in [Fig.l], the stator 2 comprises at least one first and at least one second complementary half-shell 8, 9. The half-shells 8, 9 are for example closed at their axial ends by a first and a second end piece of the stator 2. The end pieces form for example supports for the bearings of the rotor shafts, in particular for the bearings and can also form all or part of certain pumping chambers, in particular those of small axial thickness.

[0065] The half-shells 8, 9 are assembled together along an assembly surface 11 to form the pumping chambers of the at least two pumping stages T1-T6. The assembly surface 11 is for example a flat assembly surface, passing for example through a median plane of the vacuum pump. The flat assembly surface 11 contains for example the axes of the rotor shafts. This flat assembly surface 11 may be strictly flat or may have for example complementary relief shapes or grooves for sealing joint spars between the half-shells.

[0066] The successive pumping chambers are separated from each other by separating partitions 12b which are perpendicular to the longitudinal direction defined by the axial direction of the rotor shafts. Orifices are provided in the separating partitions 12b and in the end pieces for the passage of the rotor shafts. The pumping chambers, the partition walls 12b and the transfer channels 6a, 6b, 7a, 7b are partly formed in the first half-shell 8 and partly in the second half-shell 9. Each half-shell 8, 9 therefore comprises at least one half-partition 12b and one half-channel assembling with one half-partition 12b and one half-channel of the other half-shell to form a partition wall between two successive pumping chambers and transfer channels 6a, 6b, 7a, 7b between the pumping chambers. The half-partitions can be mounted by assembly in a cradle of said half-shell or be made in one piece with the cradle.

[0067] The stator parts 2 produced by casting may be the half-shells 8, 9, in particular the half-shells 8, 9 each comprising at least one half-partition 12b and / or at least one half-channel.

[0068] For this purpose, for example, an upper mold 13 and a lower mold (not shown) are provided between which at least one sand core 15 is placed for the first half-shell 8 and / or at least one sand core 16 for the second half-shell 9 (here two cores 15, 16 of each). For example, a model plate 14 is used to generate the upper mold 13 by imprint. A similar method using a model plate can be used to generate the lower mold.

[0069] The sand cores 15, 16 have shapes corresponding to the hollow parts of the half-shells 8, 9, namely the pumping chambers and the transfer channels 6a, 6b, 7a, 7b.

[0070] Then, the cast iron or liquid steel is poured between the upper 13 and lower molds fixed against each other to form, after cooling, shakeout and removal of the sand cores (clearing) then de-massaging, desanding and finishing, the half-shells 8, 9.

[0071] Surfaces of the half-shells 8, 9 produced from casting and accessible to the tools, in particular pumping chambers, positioning and holding elements and sealing surfaces, such as the assembly surfaces 11 of the half-shells 8, 9, can be machined.

[0072] Then, the surfaces of the half-shells 8, 9 which are intended to be exposed to the pumped gases, in particular all or part of the machined surfaces on the one hand, and the non-machined surfaces (because they are not accessible) on the other hand, such as the surfaces of the transfer channels 6a, 6b, 7a, 7b, are coated for example with a nickel alloy-based coating, such as a nickel-phosphorus coating. The coating can then be heat-treated as described above.

[0073] According to a second embodiment shown in Figures 3 and 4, the stator 2 comprises slice stator elements 17, 18, 19, 20 which assemble axially. Each slice stator element 17, 18, 19, 20 assembles axially (i.e. in a direction parallel to the axis of the shafts) with another stator element or with one of the two end pieces of the stator 2, to form the pumping chambers of the pumping stages receiving the rotors 3. The stator elements in slices 17, 18, 19, 20 each comprise at least one transverse separating partition 21. Holes are provided in the separating partition 21 for the passage of the rotor shafts 3.

[0074] The stator parts 2 produced by casting may be the slice stator elements 17, 18, 19, 20, each comprising at least one separating partition 21 and at least one portion of transfer channel 6, 7. There are for example two transfer channels 6, 7, arranged partly laterally, on either side of each pumping chamber ([Fig.4]).

[0075] The pumping chambers and the portions of the transfer channels 6, 7 then accessible from the stator parts obtained by casting can be machined.

[0076] Then, the surfaces of the stator parts 2 which are intended to be exposed to the pumped gases, in particular all or part of the machined surfaces on the one hand, and in particular the machined surfaces of the pumping chambers, the positioning and holding elements and the sealing surfaces, and the surfaces of the transfer channels 6, 7, machined or not, as well as the non-machined surfaces, are coated, for example with a nickel-based coating, such as a nickel-phosphorus coating. The coating can then be heat-treated as described above.

Claims

Claims

1. Vacuum pump comprising a stator (2) and two rotors (3) received in at least one pumping chamber of the stator (2), the rotors (3) being configured to rotate in the pumping chamber to drive a gas to be pumped between a suction port (4) and a discharge port (5) of the vacuum pump, characterized in that the stator (2) comprises at least one stator part obtained by casting of cast iron or steel made in a mold receiving at least one sand core (15, 16) comprising a majority of sand chosen from one or more of the following sands: - artificial globular ceramic sand, - chromite sand, - bauxite sand, - zircon sand, - kerphalite sand, - olivine sand, the surfaces of the stator part produced by casting intended to be exposed to the pumped gases being coated.

2. A vacuum pump according to claim 1, characterized in that the coating is a nickel alloy-based coating, such as a nickel-phosphorus coating.

3. Vacuum pump according to one of the preceding claims, characterized in that the coating has a thickness of between 5 pm and 50 pm, such as 25 pm.

4. Vacuum pump according to one of the preceding claims, characterized in that it comprises at least two pumping stages (T1-T6) each defining a pumping chamber, the pumping chambers of the successive pumping stages (T1-T6) being connected in series by at least one respective transfer channel (6a, 6b, 7a, 7b) of the stator (2).

5. Vacuum pump according to claim 4, characterized in that the stator parts produced by casting are complementary half-shells (8, 9), assembling together to form the pumping chambers of the at least two pumping stages (T1-T6) and the transfer channels (6a, 6b, 7a, 7b) between the pumping chambers.

6. Vacuum pump according to claim 4, characterized in that the stator parts produced by casting are slice stator elements (17, 18, 19, 20), assembling axially and each comprising at least at least one partition wall (21) and at least one portion of transfer channel (6, 7).

7. Method for manufacturing a vacuum pump stator part obtained by casting cast iron or steel according to one of the preceding claims, characterized in that the stator part is produced in a mold receiving at least one sand core (15, 16) comprising a majority of sand chosen from one or more of the following sands: - artificial globular ceramic sand, - chromite sand, - bauxite sand, - zircon sand, - kerphalite sand, - olivine sand, the surfaces of the stator part (2) produced by casting intended to be exposed to the pumped gases being coated, for example with a nickel alloy-based coating such as a nickel-phosphorus coating.

8. Manufacturing method according to the preceding claim, characterized in that the coating is heat treated to have a hardness greater than 700Hv.

9. Manufacturing method according to one of claims 7 or 8, characterized in that the sand cores (15, 16) made of artificial globular ceramic sand are produced by 3D printing.

10. Manufacturing method according to one of claims 7 to 9, characterized in that surfaces of the stator part (2) from the foundry, in particular the pumping chambers, the positioning and holding elements and the sealing surfaces, are machined before being coated.

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