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 a similar principle. The invention also relates to a method for manufacturing a vacuum pump stator component obtained by casting iron or steel. Technical background

[0002] In the semiconductor, flat panel display (FPV), photovoltaic, and coating industries, dry vacuum pumps can be used to evacuate corrosive gases such as NF3, C1F3, F2, and Cl2, particularly those originating from chamber cleaning processes. These corrosive gases can damage the steel or cast iron mechanical components of the vacuum pump, leading to performance losses and even safety problems due to leaks. Oxygen or water vapor can enter the low-pressure stages of the vacuum pump and react with the process and cleaning gases, or these gases can leak into the atmosphere and thus potentially harm operators.

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

[0004] However, the coated static mechanical components are produced by foundries using silica sand (also called green sand), the grains of which can remain on the surface of the part in high concentrations when the casting skins are not machined, despite shot blasting or other surface cleaning techniques. This is particularly true for surfaces that are difficult to access with tools, such as transfer channels between pumping stages.

[0005] These embedded sand grains and their mineral nature prevent the coating from being properly deposited and from adhering properly, leaving areas without coating. These areas are, however, exposed to process gases and are therefore subject to their attack. The corrosive gases attack the substrate in the areas with coating gaps, which then create access points under the coating, causing it to flake off, delaminate, and expose the metal surfaces. The entire casting, with corroded portions in the unmachined areas, must then be replaced.

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

[0007] Furthermore, the release of some of these sand grains during the coating immersion deposition process can generate significant contamination of the various baths, which can be detrimental to the quality of the deposited coating. Summary of the invention

[0008] One object of the present invention is therefore to propose a vacuum pump which at least partially resolves the drawbacks of the prior 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 port and a discharge port of the vacuum pump, characterized in that the stator comprises at least one stator part 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 part intended to be exposed to 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 from exhibiting sand grains on their surface. However, these sands are much more resistant to the corrosive gases used in cleaning processes for manufacturing semiconductors, photovoltaic panels, flat panel displays, and coatings than prior art silica sand (or green sand). A better bond is also observed between the surface sand grain inclusions and the coating with these types of sand. This improved compatibility embeds the sand grains in the coating, reducing the formation of voids at the grain boundaries and thus making it more difficult for corrosive gases to penetrate beneath the coating. Consequently, the sand inclusions in the uncoated stator casting skins, when exposed to corrosive gases, exhibit greater resistance. chemical and mechanical resistance to these gases. Castings are therefore more resistant to corrosion, including unmachined surfaces. Furthermore, the process is simple to implement and reasonably priced.

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

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

[0013] The vacuum pump includes 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, which assemble 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, assembled 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 iron or steel as defined above, characterized in that the stator part is 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.

[0017] The coating can also be heat-treated to exhibit a hardness greater than 700Hv.

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

[0019] According to one 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 features will become apparent upon reading the description of the invention, as well as the accompanying drawings in which:

[0021] [Fig.1] Fig.1 shows a perspective view of stator parts of a stator in the disassembled state of a vacuum pump according to a first embodiment.

[0022] [Fig.2] Fig.2 shows a pattern 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 the disassembled state of a vacuum pump according to a second embodiment example.

[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 bear 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. Simple features of different embodiments can 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 positive displacement vacuum pump configured to draw in, transfer, and then discharge the gas to be pumped at atmospheric pressure using two rotor shafts. The rotor shafts are driven in rotation by a primary vacuum pump motor.

[0028] The term "upstream" refers to an element that is placed before another with respect to the direction of flow of the gas to be pumped. Conversely, the term "downstream" refers to an element placed after another with respect to the direction of flow 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 includes a stator 2 forming at least one pumping chamber of a pumping stage T1-T6 ([Fig.1]).

[0031] According to one 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 that 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 may, for example, have lobes with identical profiles, for example, two-lobe "Roots" type (see [Fig. 3]) or more, or "Claw" type, or of another similar positive displacement vacuum pump principle. 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 coupled rotors, the pumping chambers comprising a respective inlet and outlet. During rotation, the gas drawn in from the inlet is trapped in the volume generated by the rotors and the stator 2, and is then carried by the rotors to the next stage.

[0035] The pumping chambers of successive pumping stages T1-T6 are connected in series by at least one transfer channel 6a, 6b, 7a, 7b, respectively, 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 include 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 at the five interstages 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 pumping chambers are, for example, equal or decreasing with the pumping stages, with the pumping stage T1 located on the side of the suction port 4 receiving the rotors with the largest 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 iron or steel 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 at least one sand core 15, 16 is received. The sand core 15, 16 corresponds to the hollow parts of the cast part.

[0039] The sand core 15, 16 comprises 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 part 2 intended to be exposed to pumped gases being coated.

[0040] The term “majority” means a larger proportion by mass.

[0041] The term "sand" means a granular solid material made up of small particles in particular of mineral origin and especially less than 2mm. The sand grains 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 from exhibiting sand grains on their surface. However, these sands are much more resistant to the corrosive gases used in cleaning processes for manufacturing semiconductors, photovoltaic panels, flat panel displays, and coatings than prior art silica sand (or green sand). A better bond is also observed between the surface sand grain inclusions and the coating with these types of sand. This improved compatibility embeds the sand grains in the coating, reducing the formation of voids at the grain boundaries and thus making it more difficult for corrosive gases to penetrate beneath the coating. Consequently, the sand inclusions in the uncoated stator castings exposed to corrosive gases exhibit better chemical and mechanical resistance to these gases.Cast parts are therefore more resistant to corrosion, including unmachined surfaces. Furthermore, the process is simple to implement and reasonably priced.

[0043] According to a first embodiment, the sand core 15, 16 comprises a majority of artificial globular ceramic sand. For example, it is made entirely of artificial globular ceramic sand. This sand comprises a majority of alumina and silica, and in particular more alumina (Al₂O₃) than silica. It contains more than 50% alumina and traces of iron oxides, i.e., more than 1% iron oxide.

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

[0045] According to a second example of composition, artificial globular ceramic sand (marketed under the brand name CBS2®) contains between 66% and 68% Al2O3, between 14% and 16% SiO2 and between 2% and 4% 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 usable for manufacturing sand cores by 3D printing.

[0047] According to a second embodiment, the sand core comprises a majority of chromite sand. For example, it is made entirely of chromite sand. This sand comprises a majority of chromium oxide (Cr2O3), iron oxide (Fe2O3), and alumina. (Al₂O₃) 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 facilitate the embedding of the coating around the inclusions in the sand grains.

[0048] According to one example of composition, chromite sand comprises between 44% and 47% Cr2O3, between 24% and 28% Fe2O3, between 14% and 16% Al2O3 and between 9% and 11% 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 contains more alumina (Al₂O₃) than silica. It contains more than 40% alumina and traces of iron oxides, i.e., more than 1% iron oxide.

[0050] According to an example of composition, bauxite sand contains 48% to 61% Al2O3, 2% to 23% Fe2O3, 1% to 7% SiO2, 2% to 4% TiO2) depending on whether it is karst or teritic type.

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

[0052] According to a fourth embodiment, the sand core comprises a majority of zircon sand. For example, it is 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 contains more than 50% zirconium oxide.

[0053] According to one example of composition, 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 hafnium oxides.

[0054] According to a fifth embodiment, the sand core comprises a majority of kerphalite sand. For example, it is made entirely of kerphalite sand. This sand comprises a majority of alumina (Al₂O₃) and silica, and in particular more alumina (Al₂O₃) than silica. It contains more than 50% alumina and traces of oxides, i.e., more than 1% oxides.

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

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

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

[0058] Surfaces of the cast stator part 2, in particular chambers of Pumping of the T1-T6 stages, the positioning and holding elements and the sealing surfaces, can be reworked by machining to ensure the accuracy of their manufacture.

[0059] Then, the surfaces of the cast stator part 2, whether machined or not, and intended to be exposed to pumped gases, are coated. The coating modifies the surface of the cast iron, notably to improve its resistance to corrosion and abrasion, and to facilitate 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 5pm, such as between 5pm and 50pm, such as 25pm.

[0062] The coating is for example deposited by a stator part immersion technique.

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

[0064] According to a first embodiment illustrated in [Fig. 1], the stator 2 comprises at least one first and at least one second complementary half-shells 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 rotor shaft bearings, in particular for the bearings, and can also form all or part of certain pumping chambers, in particular those with a small axial thickness.

[0065] The half-shells 8, 9 are assembled with each other 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 planar assembly surface, passing, for example, through a median plane of the vacuum pump. The planar assembly surface 11 contains, for example, the axes of the rotor shafts. This planar assembly surface 11 may be perfectly flat or may, for example, have complementary relief shapes or grooves for sealing strips between the half-shells.

[0066] The successive pumping chambers are separated from each other by partition walls 12b which are perpendicular to the longitudinal direction defined by the axial direction of the rotor shafts. Openings are provided in the partition walls 12b and in the end pieces for the passage of the rotor shafts. The Pumping chambers, partition walls 12b, and 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-wall 12b and one half-channel assembling with a half-wall 12b and a half-channel from 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-walls can be assembled within a cradle of said half-shell or be formed as a single piece with the cradle.

[0067] The stator parts 2 produced by casting can 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, 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 and 16 of each). For example, a pattern plate 14 is used to generate the upper mold 13 by impression. A similar process using a pattern 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 molten iron or steel is poured between the upper 13 and lower molds fixed against each other to form, after cooling, deburring and removal of the sand cores (deburring) then deburring, desanding and finishing, the half-shells 8, 9.

[0071] Surfaces of the half-shells 8, 9 produced by casting and accessible to 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 that are intended to be exposed to the pumped gases, in particular all or part of the machined surfaces on the one hand, and the unmachined (because inaccessible) surfaces 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 stator elements in slices 17, 18, 19, 20 that are axially assembled. Each stator element in slices 17, 18, 19, 20 is axially assembled (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 forms the pumping chambers of the pumping stages that receive the rotors 3. The stator elements in sections 17, 18, 19, and 20 each have at least one transverse partition 21. Holes are provided in the partition 21 for the passage of the rotor shafts 3.

[0074] The cast stator parts 2 can be the sliced ​​stator elements 17, 18, 19, 20, each comprising at least one separating partition 21 and at least one portion of a transfer channel 6, 7. For example, there are 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 cast stator parts can be machined.

[0076] Then, the surfaces of the stator parts 2 that 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 unmachined 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

Demands

1. A method for manufacturing a vacuum pump stator component obtained by casting iron or steel, the 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 component is made in a mold receiving at least one sand core (15, 16) 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 component (2) intended to be exposed to the pumped gases being coated with an alloy-based coating nickel such as a nickel-phosphorus coating.

2. A manufacturing process according to the preceding claim, characterized in that the coating is heat-treated to exhibit a hardness greater than 700Hv.

3. A manufacturing method according to any one of the preceding claims, characterized in that the artificial globular ceramic sand cores (15, 16) are made by 3D printing.

4. A manufacturing method according to any one of the preceding claims, characterized in that surfaces of the cast stator part (2), in particular pumping chambers, positioning and holding elements and sealing surfaces, are machined before being coated.

5. A manufacturing method according to any one of the preceding claims, characterized in that the coating has a thickness between 5pm and 50pm, such as 25pm.

6. A manufacturing method according to any one of the preceding claims, characterized in that the vacuum pump 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 transfer channel (6a, 6b, 7a, 7b) respective of the stator (2).

7. A manufacturing method according to claim 6, characterized in that the cast stator parts are complementary half-shells (8, 9), which assemble together to form the pumping chambers of at least two pumping stages (T1-T6) and the transfer channels (6a, 6b, 7a, 7b) between the pumping chambers.

8. A manufacturing method according to claim 6, characterized in that the stator parts produced by casting are sliced ​​stator elements (17, 18, 19, 20), axially assembled and each comprising at least one separating partition (21) and at least one portion of a transfer channel (6, 7).

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