METHOD AND APPARATUS FOR FORMING COATING

OVER INNER WALL OF TUBE Field of the Invention

[0001] The present invention relates to a method and apparatus for forming a coating. In particular, the present invention relates to a method and apparatus for forming a coating over an inner wall of a tube.

Background of the Invention

[0002] In the prior art, there are many methods for forming a coating over the inner wall of an object such as a lamp tube and a bulb. For example, in a conventional method for forming a phosphor coating over the inner wall of a glass lamp tube, a suspension formed of phosphor powder, binder, water, and so on is filled into the glass lamp tube. The suspension filled into the glass lamp tube is removed by the gravity within the glass lamp tube, so that a phosphor coating is formed over the inner wall of the glass lamp tube. Then, the glass lamp tube with the phosphor coating formed over the inner wall is dried by heated air. Then, the dried phosphor coating is baked at a temperature of 500-600 ⁰ C, where the binder is removed by oxidation, to avoid reduction in light output of the glass lamp tube due to the presence of the binder, and the glass lamp tube with the phosphor coating without the binder formed over the inner wall is thus obtained, which can be used as a fluorescent lamp tube. The glass lamp tube described above may be a straight tube or a spiral tube.

[0003] In a conventional method for forming a coating over the inner wall of an ellipsoidal bulb of a High Intensity Discharge (HID) lamp, a powder mixture containing phosphor is charged by an external charger. Then, the charged powder mixture is blown into the bulb through a nozzle with a potential. Since an electric field is established between the nozzle and the bulb by a high voltage of 30-50 kV, the charged powder mixture is driven by the electric field onto the inner wall of the bulb to form a coating.

[0004] In the case of coating a long narrow tube using such an electrostatic coating method, however, it may be difficult to maintain a homogenous charge on the tube to form a homogeneous coating. [0005] Compared to the prior art, a method and apparatus for forming a coating with a desired homogeneousness over the inner wall of a tube in a fast, cost effective and high-quality way are needed.

Summary of the Invention

[0006] Embodiments of the present invention provide a method and apparatus for forming a coating with a desired homogeneousness over the inner wall of a tube in a fast, cost effective and high-quality way.

[0007] According to an aspect of the present invention, there is provided a method for forming a coating over an inner wall of a tube, including: winding a tube onto a mold to form the tube into a desired shape, when the mold is heated by a flame; and blowing a coating material with a first potential into the tube from at least one end of the tube, wherein the mold and the flame have a second potential and a third potential, respectively, which are different from the first potential, so that a coating is formed over the inner wall of the tube with the coating material by utilizing a potential difference between the first potential and the second and third potentials.

[0008] According to another aspect of the present invention, there is provided an apparatus for forming a coating over an inner wall of a tube, including: a mold, for forming a tube into a desired shape; a flame generating device, configured to generate a flame for heating the mold; and a blowing device, configured to blow a coating material with a first potential into the tube from at least one end of the tube, wherein the mold has a second potential which is different from the first potential, and the flame generating device is further configured to enable the flame to have a third potential which is different from the first potential, so that a coating can be formed over the inner wall of the tube with the coating material by utilizing a potential difference between the first

potential and the second and third potentials.

[0009] According to another aspect of the present invention, there is provided a tube having at least one coating formed through the method according to the present invention. [0010] According to another aspect of the present invention, there is provided a lamp, including a tube having at least one coating formed through the method according to the present invention, wherein the tube is used as a lamp tube.

[0011] Compared to the conventional method of forming a coating by filling a suspension in the prior art, with the solutions according to the embodiments of the present invention, a homogenous coating can be formed because the gravity has no influence. In addition, the cost of the coating process can be saved because of elimination of the process steps including wet coating, drying and baking. Moreover, the cost of the coating process can also be saved because of elimination of the binder and the water. Further, since no binder is used, when the tube is to be used as a fluorescent lamp tube, the light output is improved because no residual binder can be contained in the coating if the baking is insufficient. [0012] In addition, compared to the conventional method of electrostatic coating in the prior art, with the solutions according to the embodiments of the present invention, the area in which it is necessary to apply an electric field can be shrinked because the electrostatic coating is performed within a wound tube, so that it is easy to maintain a homogeneous electric field to form a homogeneous coating over the inner wall of the tube. In addition, because of the electrical conductivity of the flame, a homogeneous potential can be achieved around the outside of the tube to form a homogeneous potential difference, so that the coating material can form a homogeneous coating over the inner wall of the tube. Moreover, since the mold also has a potential and the potential of the mold may be the same as the potential of the flame, a more homogeneous potential difference can be formed around the tube, so that the coating material can form a more homogeneous coating over the inner wall of the tube. Further, by setting different

potentials of the flame and the mold as required, a coating with a desired homogeneousness can be formed over the inner wall of the tube .

Brief Descriptions of the Drawings [0013] Those skilled in the art will have a better understanding of the present invention through the following description of the embodiments thereof taken with reference to the accompanying drawings, in which

[0014] Fig.l shows a flow chart of a method for forming a coating over an inner wall of a tube according to an embodiment of the present invention;

[0015] Fig.2 shows an apparatus for forming a coating over an inner wall of a tube according to an embodiment of the present invention, with the tube wound around a mold; [0016] Fig.3 shows a flow chart of a method for forming a coating over an inner wall of a tube according to another embodiment of the present invention; and

[0017] Fig.4 shows a flow chart of a conventional process for fabricating a spiral glass tube. Detailed Descriptions of the Embodiments

[0018] Hereinafter the description will be taken in connection with a process for fabricating a spiral glass tube for the purpose of illustrating the embodiments of the present invention. However, those skilled in the art should understand that the tube according to the embodiments of the present invention is not limited to a spiral glass tube, and may be a tube of other suitable materials and shapes.

[0019] Fig.4 shows a flow chart of a conventional process for fabricating a spiral glass tube. [0020] As shown in Fig.4, at step 401, a glass tube is heated in a furnace to its softening point.

[0021] At step 403, the heated glass tube is straightened by an air

pressure .

[0022] At step 405, the straightened glass tube is moved onto a rotating screw mold which is heated by a gas flame, so that the glass tube is wound around the screw mold to form a spiral glass tube. In another case, the screw mold may be made static, and the glass tube itself may be rotated in a suitable manner to wind the glass tube around the screw mold to form a spiral glass tube.

[0023] At step 407, an air pressure is applied into the glass tube to enable the glass tube to establish a correct spiral form on the screw mold.

[0024] At step 409, the glass tube having a correct spiral form is taken off from the screw mold to obtain a spiral glass tube.

[0025] Fig.l shows a flow chart of a method for forming a coating over an inner wall of a tube according to an embodiment of the present invention.

[0026] As shown in Fig.l, optionally at step 101, a glass tube is heated in a preheating device to its softening point. As described above, the material of the tube is not limited to glass, and may be other suitable materials. In certain cases, step 101 may be omitted.

[0027] Optionally at step 103, the heated glass tube is straightened by a gas pressure. In certain cases, step 103 may also be omitted.

[0028] At step 105, the straightened glass tube is moved onto a rotating mold which is heated by a flame, to form a spiral glass tube. As described above, the tube is not limited to be formed into a spiral shape, and may be formed into other suitable shapes, for example, a circular shape, a U shape, and so on.

[0029] In the case that steps 101 and 103 are omitted for example, step 105 may be simplified to a step in which the glass tube is directly moved onto a rotating mold which is heated by a flame, so that the glass tube is wound around the mold to form a spiral glass tube.

[0030] Alternatively, the mold may be made static, and the glass tube

itself may be rotated in a suitable manner to wind the glass tube around the mold to form a spiral glass tube.

[0031] The mold can be applied a suitable potential, preferably a ground potential. In addition, the flame may have a potential which is the same as that of the mold, preferably a ground potential.

[0032] Optionally at step 107, a gas pressure is applied into the tube from at least one end of the tube, to enable the tube to establish a correct form on the mold. The gas as used may be air or nitrogen. In certain cases, step 107 may be omitted. [0033] At step 109, a coating material having a suitable potential is blown into the tube from at least one end of the tube to form a coating over the inner wall of the tube. The coating material may include for example phosphor powder. Preferably, the potential of the coating material may be lower than the potentials of the mold and the flame, for example -30 to -50 kV.

[0034] Because of the electrical conductivity of the flame, a homogeneous potential can be achieved around the outside of the tube. Therefore, a homogeneous potential difference can be formed around the tube by the flame having a homogeneous potential and the coating material having a potential, so that the coating material can form a homogeneous coating over the inner wall of the tube .

[0035] In addition, since the mold also has a potential and the potential may be the same as the potential of the flame, a more homogeneous potential difference can be formed around the tube, so that the coating material can form a more homogeneous coating over the inner wall of the tube.

[0036] In addition, the coating material may be blown into the tube through the gas pressure applied at step 107. [0037] Optionally at step 111, the tube with a coating formed over the inner wall is taken off from the mold. In the case that the coating material includes phosphor powder, the tube thus obtained can be used as a fluorescent lamp tube.

[0038] Fig.2 shows an apparatus for forming a coating over an inner wall of a tube according to an embodiment of the present invention, with the tube wound around a mold.

[0039] As shown in Fig. 2, an apparatus 201 for forming a coating over an inner wall of a tube may include a mold 203, a flame generating device 205 and a blowing device 207.

[0040] The mold 203 can be used as a mold to form a tube with a desired shape, for example, a spiral shape, a circular shape, a U shape, and so on. The mold 203 can be applied a suitable potential, preferably a ground potential. Preferably, the mold 203 may be rotatabe to wind a tube around the mold 203 to form a tube with a desired shape. Alternatively, the mold 203 may be fixed. Here the tube itself may be rotated in a suitable manner to be wound around the mold 203 to form a tube with a desired shape. [0041] The flame generating device 205 can generate a flame for heating the mold 203. The flame generating device 205 can also enable the flame to have a potential that is the same as that of the mold 203, preferably a ground potential.

[0042] The blowing device 207 can apply a gas pressure into the tube from at least one end of the tube, to enable the tube to establish a correct form on the mold 203. The gas as used may be air or nitrogen . The blowing device 207 may also blow a coating material having a suitable potential into the tube from at least one end of the tube. Preferably, the potential of the coating material may be lower than the potentials of the flame and the mold 203, for example -30 to -50 kV. The coating material may include phosphor powder. In addition, the blowing device 207 may blow the coating material into the tube through the gas pressure applied into the tube as described above . [0043] Because of the electrical conductivity of the flame, a homogeneous potential can be achieved around the outside of the tube. Therefore, a homogeneous potential difference can be formed around the tube by the flame having a homogeneous potential and the coating material having a potential, so that the coating material can form a homogeneous coating over the inner wall of the

tube .

[0044] In addition, since the mold 203 also has a potential and the potential may be the same as the potential of the flame, a more homogeneous potential difference can be formed around the tube, so that the coating material can form a more homogeneous coating over the inner wall of the tube.

[0045] The apparatus 201 may also have a preheating device (not shown) , for preheating the tube to a softening temperature of the tube. The tube may be made of glass. The blowing device 207 may straighten the preheated glass tube through a gas pressure.

[0046] In addition, according to another embodiment of the present invention, a tube having at least one coating formed over the inner wall through the method according to the present invention can be used as a fluorescent lamp tube. Here the at least one coating may include a coating including phosphor . The tube may be made of glass .

In addition, in the case of a glass lamp tube for example, the at least one coating may further include a protective coating formed before the coating including phosphor is formed, so that mercury

(Hg) used in the lamp tube may be prevented from migrating into the glass wall.

[0047] In addition, according to another embodiment of the present invention, the tube having at least one coating formed through the method according to the present invention can be included in a lamp and used as a lamp tube. [0048] While the present invention has been described in connection with some embodiments thereof with reference to the accompanying drawings, it is to be appreciated that various modifications and variations can be made by those skilled in the art upon reading the disclosure of the present invention, and such modifications and variations is to be considered falling into the scope of the invention as defined in the following claims.

[0049] For example, in the method for forming a coating over an inner wall of a tube according to the embodiments of the present invention, the order in which the steps are performed is not limited by the

order in which the steps are described. Instead, the order in which the steps are performed may be adjusted as required. For example, step 107 may be performed simultaneously with step 109. Step 105 may also be performed simultaneously with step 107 and/or step 109. Fig.3 shows a flow chart of a method for forming a coating over an inner wall of a tube according to another embodiment of the present invention, which is different from Fig.l in that steps 105, 107 and 109 are performed simultaneously. In addition, the steps of the method according to the embodiments of the present invention may be added, cancelled or modified as required and need not be completely identical to those described above. For example, in some embodiments, step 109 may be repeated to form more than one coating. In some other embodiments, steps 101 and 103 may be omitted and step 105 may be simplified. In some other embodiments, step 107 may also be omitted.

[0050] In addition, the potentials of the coating material, the flame and the mold are not limited to those described above. For example, in some embodiments according to the present invention, the potential of the flame may be set to be different from that of the mold, so that the established electric field may be shaped as required to form a desired coating. In this regard, an optimal coating may be homogeneous with more or less constant coating thickness or heterogeneous with a desired gradient of the coating thickness. For example, in the case that the potential of the mold is a ground potential, the potential of the flame may be a positive potential, so that less coating material can be contained in the coating formed over the inner wall of the tube on the side of the flame than that on the side of the mold. As a result of this, the light output of a lamp tube with such a coating formed over the inner wall could be optimized by having more reflections on the mold side of the inner wall and more transmissions on the flame side of the inner wall. In some embodiments according to the present invention, the potential of the flame may be set to be different from that of the mold by 20 to 30% of the potential difference between the coating material and the flame or the mold.