WO2010006641A1

WO2010006641A1 - Device for electrostatically coating a work piece

Info

Publication number: WO2010006641A1
Application number: PCT/EP2008/059214
Authority: WO
Inventors: Alessandro Mattozzi
Original assignee: Abb Research Ltd
Priority date: 2008-07-15
Filing date: 2008-07-15
Publication date: 2010-01-21

Abstract

A device for electrostatically coating a work piece (2), the device includes an atomizer (1) for atomizing a liquid containing abrasive elements, wherein the atomizer (1) comprises an atomizing member (3) adapted to atomize the liquid into a mist (4), and a distribution unit (7), which is provided with one or more outlets (5) for the liquid, for distributing the liquid to the atomizing member (3), characterized in that at least a part of the atomizer (1) is provided with a coating (10) of pure diamond like carbon (DLC) adapted to protect the atomizer (1) from abrasive wear caused by the abrasive elements in the liquid.

Description

DEVICE FOR ELECTROSTATICALLY COATING A WORK PIECE

FIELD OF THE INVENTION

The present invention relates to a device for electrostatically coating a work piece. The device is used for applying various types of liquid based coatings on a work piece in an automated painting process. For example, the device is used in the automotive industry for applying car bodies with a coating, such as paint, varnish, and etcetera. The device includes an atomizer for atomizing a liquid containing abrasive elements, wherein the atomizer comprises an atomizing member adapted to atomize the liquid into a mist, and a distribution unit, which is provided with one or more outlets for the liquid, for distributing the liquid to the atomizing member.

PRIOR ART

In applications comprising different types of devices for electro- statically coating a work piece, the atomizer and the work piece has opposite electric potential. The function of the distribution unit is to output an adequate amount of liquid through one or more of the outlets, to accelerate the liquid and to distribute the accelerated liquid on the atomizing member so that the liquid is transformed into a mist, which is attracted to the surface of the work piece by means of the potential difference between the atomizer and the work piece. The acceleration of the liquid can for example be done by rotating the distribution unit, while the atomizing member is in a static position or by rotating the whole of the atomizer. The atomizing member comprises a serrated or grooved surface adapted to separate the liquid into a mist of small liquid particle, a so called atomization or bell-cup. A problem with such devices is that the liquid contains abrasive elements, such as metallic fillers or other abrasive particles, which results in wear of surfaces in contact with the liquid and hence a reduced life time of the device.

US6409104 discloses a bell atomizer comprising a wear resistant silicon-doped amorphous carbon coating, which limits the influence of the abrasive particles in the liquid, applied on the surface of the atomizer.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved device for electrostatically coating a work piece.

This aim is obtained by a device as defined by claim 1 .

Such a device is characterized in that at least a part of the at- omizer is provided with a coating of pure diamond like carbon (DLC) adapted to protect the atomizer from abrasive wear caused by the abrasive elements in the liquid.

A pure diamond like carbon is an amorphous carbon material with properties similar to natural diamonds, such as mechanical properties like high hardness and electrical properties like high work function. The hardness of the DLC coating is higher than 1000 HV, preferably higher than 2000 HV, and most preferably higher than 4000 HV. The work function is the minimum energy needed to remove an electron from the surface of a material to a point immediately outside the surface of the material. Due to the hardness of the coating, the coating has a high wear resistance against the abrasive elements in the liquid. The pure DLC coating has an improved hardness in comparison to a DLC coating doped with silicon. Thus, the pure DLC coating has an improved resistance against abrasive wear, which increases the life time of the device, without significantly increasing the weight of the device.

The pure DLC coating further improves the wettability between the coating and the liquid in comparison to a DLC coating doped with silicon. The term "wettability" refers to the contact angle between a droplet of the liquid and the surface of the coating. The lower the contact angle is between a droplet and the surface, the higher the wettability. A higher wettability improves the dis- tribution of the liquid to the atomizing member. An improved wettability of the surface of the distribution unit results in the ability to accelerate a larger amount of liquid, which is distributed on the atomizing member by the outlet. Moreover, a higher wettability improves the atomization function of the atomizing member so that the liquid more easily will be separated by the serrated surface of the atomizing member into a mist of liquid particles. Hence, an improved wettability of the atomizer results in an improved ability for the atomizer to atomize the liquid.

The pure DLC coating further results in a higher work function of the coating in comparison to a DLC coating doped with silicon, which improves the resistance to corona discharge effects during operation of the device. Thus, a high work function resists that electrons are leaving the surface of the atomizer. Hence, a high work function assures that the electric potential between the atomizer and the work piece is maintained. The term "corona discharge" relates to an electric discharge accompanied by ionization of the surrounding atmosphere. When the corona discharge appears, it will induce wear to the surface of the device. Thus, an improved resistance to corona discharge of the device will improve the life time of the device.

The term "pure diamond like carbon (DLC)" refers to a DLC carbon based coating, which has a level of impurities which is less that 5%, preferably less than 1 %. According to one embodiment of the invention, the coating is provided to at least a part of the distribution unit surrounding the outlets.

The area surrounding the outlets is mostly affected by the abrasive particles in the liquid. The abrasive particles wear the surface of the distribution unit, which influences the ability of the distribution unit to accelerate the liquid and to evenly distribute the liquid on the atomizing member. Hence, the function of the atomizer is reduced by the abrasive particles in the liquid. By protecting the area surrounding the outlets with the coating, the operative time of the atomizer is improved.

According to one embodiment of the invention, the atomizer has a body, which is made of a metallic material, wherein the coating is provided on the body of the atomizer.

The metallic body of the atomizer is adapted for adhesive bonding between the coating and the body of the atomizer. Hence, the body of the atomizer has properties, such as hardness, elastic and mechanical properties etcetera, which are adapted for adhesive bonding between the coating and the body of the atomizer. Thus, tensions that are introduced, such as tensions due to elastic and thermal differences between the coating and the body, are kept on sufficiently low levels for the operation of the device. Thereby, it is assured that the coating remains intact on the atomizer during operation of the device.

According to one embodiment of the invention, the atomizer has a body, which is made of a polymeric material, wherein the coating is provided on the body of the atomizer. Preferably, the polymeric material is selected from the group of Polyacetal, Polyphtalamide, Polyphenylsulphate, Polyesteretherketones, or mixture thereof. With the use of a polymeric material the material cost and the manufacturing cost of the atomizer is reduced. Moreover, the design of the atomizer has a lower weight in comparison to an atomizer with a body made of a metallic material. Thereby, the energy use of movable parts, such as the distribution unit, is reduced. The polymeric material is adapted for adhesive bonding between the coating and the body of the atomizer.

According to one embodiment of the invention, the device com- prises an intermediate layer between the body and the coating, wherein the intermediate layer is adapted to create an adhesive bonding between the body and the coating.

The intermediate layer has the function of creating a stronger bonding between the coating and the body of the atomizer than would be possible to obtain without the intermediate layer. Thereby, the integrity of the coating and the function of the coating are improved. The intermediate layer functions as a transition between the properties, such as hardness, elastic and me- chanical properties etcetera, of the coating and the body. Hence, tensions that are introduced, such as tensions due to elastic and thermal differences between the coating and the body, are kept on sufficiently low levels for the operation of the device. Thereby, it is assured that the coating remains intact on the atomizer during operation of the device. By means of the intermediate layer the coating is bonded to various materials of the body with different properties.

Preferably, the intermediate layer (12) is selected from the group of Al, Cu, Ni, Sn, Au, Ti and Fe, or mixture thereof.

Preferably, the intermediate layer (12) has a thickness of 0.5 - 10 μm. By using above presented thicknesses of the intermediate layer a transition between the properties of the coating and the body is created, which is adapted to create a strong adhe- sive bonding of the intermediate layer to the body and the coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 a shows an example of the invention comprising the device including an atomizer.

Fig. 1 b shows an example of a cross section along the line A-A of the atomizer in Figure 1 a.

Fig. 2 shows an example of the invention comprising an atomizer positioned on a painting robot apparatus.

Fig. 3a shows an example of the invention comprising an atom- izer.

Fig. 3b shows an example of a cross section of the atomizer shown in Figure 3a.

Fig. 4 shows another example the invention comprising the device including an atomizer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 a shows a device for electrostatically coating a work piece 2 including an atomizer 1 for atomizing a liquid. The function of the atomizer 1 is to atomize a liquid, that is, the liquid is separated into a mist 4 of small liquid particle which is to be de- posit on the surface of the work piece 2. The atomizer 1 comprises an atomizing member 3 adapted to atomize the liquid into a mist 4. The atomizing member 3 comprises a serrated surface with grooves adapted to separate the liquid into a mist 4 of small liquid particle, that is, the liquid becomes atomized. The atomizer 1 further comprises a distribution unit 7 provided with one or more outlets 5 for the liquid, which output the liquid to be atomized. The liquid contains abrasive elements, such as metallic fillers or other abrasive particles, which has the ability to wear a surface in contact with the liquid that has a lower hardness than the abrasive elements. The distribution unit 7 has the function to accelerate the liquid and to distribute the liquid on the atomizing member 3. The distribution unit 7 accelerates the liquid by means of rotation, so that the liquid gets distributed on the atomizing member 3. The liquid is in contact with the distribution unit 7 and flows by means of centrifugation on the surface of the distribution unit 7 from the outlets 5 of the distribution unit 7 to the atomizing member 3. As the liquid gets in contact with the atomizing member 3, the liquid is pressed through the serrated surface so that the liquid gets atomized and is released into the surrounding immediately outside the atomizer 1 . The atomized liquid is thereafter drawn to the work piece 2 by means of an electrostatic induced force.

The atomizer 1 is provided with a coating 10 of pure diamond like carbon (DLC) adapted to protect the atomizer 1 from abra- sive wear caused by the abrasive elements in the liquid. The coating 10 can be applied with different types of deposition methods, such as a Physical Vapor Deposition (PVD) process. For example, the PVD method comprises using a vacuum chamber, wherein the device is positioned in the chamber. Carbon ions with enough kinetic energy to form an amorphous and dense diamond like structure hit the surface of the device. The carbon ions are generated and accelerated by means of a discharge between a graphite cathode and an anode. The coating 10 is a pure DLC coating 10, which means that the amount of impurities in the coating 10 is low. For example, the pure DLC coating 10 has a level of impurities which is less that 5%, pref- erably less than 1 %. X-ray diffuse scattering (XDS) measurements of the DLC coating 10 has shown that the average impurity level is less than 1 % and local maximum of impurity level less is than 5%. Thus, the DLC coating is technically pure under XDS analysis. The coating 10 is classified as a tetrahedral amorphous carbon coating 10 (ta-C coating 10), which has the properties of being approximately hydrogen free. The coating 10 is characterized by high hardness and high wear resistance. The hardness of the coating 10 is typically around 4000 HV. The coating 10 can be applied with different thicknesses depending on the size of the particles of the abrasive elements. A typical value of the thickness of the coating 10 is 2 μm. The coating 10 can be applied to the surface of the device so that the parts of the device that are in contact with the liquid are protected from abrasive wear from the abrasive elements of the liquid. For example, the coating 10 is applied on various components of the device, such as all surfaces of the atomizer 1 , distributing member 7, atomizing member 3, the grooves of the atomizing member 3, the outlets 5, pumps for the liquid 26, liquid passage 32, liquid reservoir 36, and etcetera. The wear resistance of the coating 10 can further protect the device from other types of abrasive wear, such as during maintenance operation when the atomizer 1 is brush off and cleaned or during a collision of the surface of the atomizer 1 with an external object. Thus, the coat- ing is adapted to protect the device from various types of wear or contact interaction.

The distribution unit 7 and the atomizing member 3 are the parts of the atomizer 1 that are subjected to the largest exposure to the liquid and thereby also the abrasive elements in the liquid. The part of the distribution unit 7 surrounding the outlet 5, which has the function of accelerating the liquid, is particularly exposed to the abrasive elements. The acceleration of the liquid results in a large contact between the liquid and the surface of the distribution unit 7, which increases the abrasive wear induced by the abrasive elements. The coating 10 is adapted to protect the atomizer 1 from the influence of the abrasive elements so that the life time of the atomizer 1 is improved.

For the efficiency of the atomizer 1 , it is important that the sur- face of the distribution unit 7 has high ability to wet the surface of the distribution unit 7. An improved wettability of the surface results in the ability to accelerate a larger amount of liquid, which is feed on to the atomizing member 3 by the outlet 5.

Hence, an improved wettability of the surface of the distribution unit 7 results in an improved ability for the atomizer 1 to atomize the liquid. The coating 10 has an improved wettability, which thus improves the function of the atomizer 1 .

For the operation and the life time of the atomizer 1 , it is impor- tant that the surface of the atomizer 1 has a high work function. A high work function of the atomizer 1 assures that the electric potential between the atomizer 1 and the work piece 2 is maintained. A high work function also reduces the risk of corona discharge, which wears the surface of the atomizer 1 . The coating 10 has an improved work function and thus a higher resistance to corona discharge, which improves the life time of the atomizer 1 .

The coating 10 can be applied on most materials that are com- patible with a vacuum environment, for example metallic materials such as Al, Ti, Fe and etcetera, and polymeric materials such as Polyacetal, Polyphtalamide, Polyphenylsulphate, PoIy- esteretherketones and etcetera. The coating 10 can also be applied to various ceramic materials.

A surface treatment can be applied on the body 14 of the atomizer 1 prior to the coating 10 is applied on the body 14. The surface treatment improves the adhesive bonding between the coating 10 and the body 14. The treatment comprises cleaning the surface of the body 14 of the atomizer 1 and treatment such as ion milling or ion etching. Thereafter the body 14 is placed in a vacuum chamber where the plasma vapor deposition process of the coating 10 is applied.

An intermediate layer 12 can be applied on the body 14 of the atomizer 1 prior to the coating 10 is applied on the body 14. The intermediate layer 12 further improves the adhesive bonding between the coating 10 and the body 14 of the atomizer 1 . The intermediate layer 12 consists of various materials. For example the intermediate layer 12 can consist of a metallic material such as Al, Cu, Ni, Sn, Au, Ti and Fe. The intermediate layer 12 is applied with various types of deposition methods such as elec- troless plating. The thickness of the intermediate layer 12 is dependent on the material of the body 14 of the atomizer 1 and the thickness of the coating 10 so that an optimal adhesive bonding is achieved between the coating 10 and the body 14. A typical thickness of the intermediate layer 12 is 0.5 - 10 μm.

The liquid is provided with a charged element so that the liquid generates an electrostatic induced force when the charged ele- ment is exposed to an electric field. The atomizer 1 and the work piece 2 are given different electric potentials by means of a voltage unit 20, which creates an electric field directed from the atomizer 1 to the work piece 2 or vice versa. As the liquid is exposed to the electric field, the electrostatic induced force draws the atomized liquid to the work piece 2 so that the work piece 2 becomes coated with the liquid. The direction of the electric field is selected based on the sign of the charge element, that is, if the charge element is positive or negative charged, so that charged element and thus also the liquid will be drawn to the surface of the work piece 2.

The device comprises a drive unit 22 adapted to supply the atomizer 1 with energy for the operation of the atomizer 1 , such as for the rotation of the distribution unit 7. The device further comprises a paint supply source 24 for the liquid, which is con- nected to the atomizer 1 . The liquid is supplied to the atomizer 1 by means of various types of pumps and valves 26.

Figure 1 b shows an example of a cross section along the line A- A of the atomizer 1 in Figure 1 a. The distribution unit 7 and the atomizing member 3 comprise a body 14. The bodies of the distribution unit 7 and the atomizing member 3 can be made of the same material or of different materials. For example, the distribution unit 7 is made of a polymeric material such as Polyacetal, and the atomizing member 3 is made of a metallic material such as Al. Both the distribution unit 7 and the atomizing member 3 has been applied with a coating 10 of pure diamond like carbon (DLC) adapted to protect the atomizer 1 from abrasive wear caused by the abrasive elements in the liquid. The coating 10 is adapted for the contact between the liquid, wherein the liquid wets the coating 10. The liquid is accelerated and distributed by means of centrifugation from the distribution unit 7 to the atomizing member 3. The coating 10 is further adapted for the potential difference between the atomizer 1 and the work piece 2 so that the coating 10 has a high resistance against corona discharge effects. The atomizer 1 also comprises an intermediate layer 12 positioned between the coating 10 and the body 14 of the atomizer 1 . The intermediate layer 12 is adapted for adhesive bonding between the coating 10 and the body 14. The in- termediate layer 12 improves the bonding between the coating 10 and the body 14. Thereby, the integrity of the coating and the function of the coating are improved.

Figure 2 shows an example an atomizer 1 positioned on a paint- ing robot apparatus 100. The painting robot apparatus 100 comprises a first arm 102 and a second arm 104. By means of the painting robot apparatus 100 and the atomizer 1 , various objects comprising complex geometries can be applied with paint.

Figure 3a shows an example of an atomizer 1 comprising a pure DLC coating 10. The atomizer 1 comprises a distribution unit 7 and an atomizing member 3. The distribution unit 7 is provided with a plurality of outlets 5 for a liquid. In the shown example, the atomizer 1 has two types of outlets, paint passages 5a and solvent passages 5b. The solvent is a liquid, which is used for washing the atomizer 1 by means of spreading solvent via the solvent passages 5b so that the surface of the atomizer 1 becomes clean from residual paint. Further, the atomizing member 3 comprises various types of grooves 3a. The function of the grooves 3a is to separate the liquid into a mist 4 of liquid parti- cles.

Figure 3b shows an example of a cross section of the atomizer 1 shown in Figure 3a. The atomizer 1 is attached on a rotational shaft 30, which is adapted to rotate the atomizer 1 . The rota- tional shaft 30 comprises a feed tube 34 with a liquid passage 32, wherein paint is feed to the atomizer 1 . The atomizer 1 comprises a distribution unit 7 and an atomizing member 3. The surface of the atomizer 1 has a coating of pure diamond like carbon (DLC) 10 adapted to protect the atomizer 1 from abrasive wear. The atomizer 1 has a liquid reservoir 36 in which excessive paint is accumulated. The atomizer 1 is configured to be rotated so that the liquid is distributed to the atomizing member 3. The distribution unit 7 comprises a hub 37 and a rim 38. In the centre of the hub 37 are solvent passages 5b provided. In the outer part of the hub 37 are paint passages 5a provided. The atomizing member 3 is provided in the outer periphery of the distribution member 7. The atomizing member 3 comprises grooves 3a and a paint releasing edge 3b. The paint releasing edge 3b releases the atomized paint into the surrounding of the atomizer 1 . The atomizer 1 is connected to a high voltage power generator through the rotational shaft 30. Under normal condition high voltage direct current (DC) in the order of 10 000 to 100 000 V is provided to the atomizer 1 during painting operation. The atomizer 1 is connected to a paint supply source 24 (not shown in the figure). The atomizer 1 atomizes the supplied paint by means of centrifugal force. Figure 4 shows an example the invention comprising the device including an atomizer 1 . The atomizer 1 is mounted in a housing 40, which partly surrounds the atomizer 1 . The atomizer 1 is at- tached to a rotational shaft 30, which has the function of providing rotation to the atomizer 1 . Within the rotational shaft 30 is a feed tube 34 provided, which feeds liquid to the atomizer 1 . Under normal condition the atomizer 1 is rotated in an interval of 10 000 to 100 000 rotations per minute. The rotation of the rota- tional shaft 30 and the atomizer 1 is induced by a motor 42, for example an electric motor or an air motor. The rotational shaft 30 is connected to a bearing 44, such as a static air bearing. The atomized liquid mist 4 can be shaped by means of shaping air 48.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the body 14 of the atomizer 1 can consist of various materials that fulfill the function of the atom- izer 1 . Different part of the device can have different thickness of the coating 10. The intermediate layer 12 can be applied to all surfaces having the coating 10 or only to the areas where additional adhesive bonding between the coating 10 and the body 14 is needed.

Claims

1 . A device for electrostatically coating a work piece (2), the device includes an atomizer (1 ) for atomizing a liquid contain- ing abrasive elements, wherein the atomizer (1 ) comprises an atomizing member (3) adapted to atomize the liquid into a mist (4), and a distribution unit (7), which is provided with one or more outlets (5) for the liquid, for distributing the liquid to the atomizing member (3), characterized in that at least a part of the atomizer (1 ) is provided with a coating (10) of pure diamond like carbon (DLC) adapted to protect the atomizer (1 ) from abrasive wear caused by the abrasive elements in the liquid.

2. A device according to claim 1 , characterized in that the coating (10) is provided to at least a part of the distribution unit (7) surrounding the outlets (5).

3. A device according to claim 1 and 2, characterized in that the atomizer (1 ) has a body (14), which is made of a metallic material, wherein the coating (10) is provided on the body (14) of the atomizer.

4. A device according to claim 1 and 2, characterized in that the atomizer (1 ) has a body (14), which is made of a polymeric material, wherein the coating (10) is provided on the body (14) of the atomizer.

5. A device according to claim 4, characterized in that the de- vice comprises an intermediate layer (12) between the body

(14) and the coating (10), wherein the intermediate layer (12) is adapted to create an adhesive bonding between the body (14) and the coating (10).

6. A device according to claim 4, characterized in that the polymeric material is selected from the group of Polyacetal, Polyphtalamide, Polyphenylsulphate, Polyesteretherketones, or mixture thereof.

7. A device according to claim 5, characterized in that the in- termediate layer (12) consists mainly of a metallic material.

8. A device according to claim 5 and 7, characterized in that the intermediate layer (12) is selected from the group of Al, Cu, Ni, Sn, Au, Ti and Fe, or mixture thereof.

9. A device according to claim 5, characterized in that the intermediate layer (12) has a thickness of 0.5 - 10 μm.