Improved curing oven

Abstract: An improved curing oven is disclosed. The present invention 10 provides an improved curing oven which cures materials with electromagnetic radiation in the ultraviolet region. The invention consists of a surface 12 lined with a layer of material 14 effective to radiate ultraviolet radiation in response to the application of infrared radiation. In a specific illustrative implementation, the invention 10 includes a second surface 18 overlying the first surface 12 providing a passageway therebetween. The oven temperature is set and maintained by a conventional temperature controller 20. The temperature controller 20 measures the oven temperature and controls a valve 22 which adjusts the gas pressure from a gas supply 24 to a set of oven burners 26. Air is supplied through inlet ports 28 included in the second surface 18.

Description: The present invention relates to curing ovens, and more specifically to paint curing ovens. Many automobile paint curing processes involve the baking of a painted object at high temperatures for long periods of time in conventional gas ovens. Cross-linking or curing of the paint is typically achieved by exposing the paint to infrared or heat radiation emitted by a gas flame. It is generally known in the art that ultraviolet radiation is more effective in curing paint at a lower temperature than that which would be used for infrared radiation. For this application, ultraviolet light provides the necessary cross-linking at lower temperatures with shorter processing times. Cross-linking of the paint is not achieved in an efficient manner in conventional gas ovens which rely on infrared radiation to cure the paint. For this reason, there is a need in the art for an improved curing oven which achieves cross-linking of the paint at lower temperatures. US-A-4798960 (Keller et al.) describes an apparatus for the curing of UV-curable inks immediately after printing in a rotary printing press. The apparatus comprises a UV-radiating lamp and a reflector. The reflector is coated with multiple layers of hafnium dioxide and silicon dioxide to form an interference filter such that the reflector reflects UV radiation but absorbs the longer wavelength radiation such as infrared emitted by the UV-radiating lamp. In this way the ratio of UV to infrared radiation striking the UV-curable inks is increased, and undesirable heating of the UV-curable inks and of the printing substrate is minimised. US-A-4755673 (Pollack et al.) describes selective thermal radiators for frequency conversion of incident radiation through the Welsbach effect. For example, a Welsbach material screen is used to convert incident IR radiation into visible radiation, permitting visual observation of IR radiation and facilitating control and monitoring of IR equipment such as IR lasers. The Welsbach material may also be used as a dynamic IR target which converts visible radiation into a high resolution IR source pattern. The Welsbach material may alternatively be used as a temperature stable material for converting solar radiation into heat. The present invention provides an improved curing oven for curing materials inside the curing oven with electromagnetic radiation in the ultraviolet region. The improved curing oven according to the present invention comprises at least one surface coated with a layer of material effective to radiate ultraviolet radiation towards the materials inside the curing oven in response to the application of infrared radiation; and a heating means for applying infrared radiation to the said layer of material. Preferably the said layer of material includes a lining of Welsbach material. Preferably the heating means comprises a gas flame or electric lamps. In preferred embodiments the curing oven comprises a first plurality of the said coated surfaces, a second plurality of surfaces overlying the first plurality of said surfaces and providing a passageway therebetween, said second plurality of surfaces including at least one inlet port for intaking air; and means for injection of fuel into the passageway to provide a flame therein for applying the infrared radiation to the said layer of material. Preferably, a temperature controller sets and maintains the temperature inside the oven. The temperature controller measures the oven temperature and controls a valve which adjusts the gas pressure from a gas supply to a set of oven burners. Air is supplied through the inlet ports included in the second surface. Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. Fig. 1 shows a cross sectional view of an illustrative implementation of a curing oven constructed in accordance with the teachings of the present invention shown in operative relationship to a painted object. Fig. 2 shows the electromagnetic spectrum, including wavelength, frequency, and energy per photon on a logarithmic scale. Referring to Figure 1, the curing oven 10 consists of a surface 12 coated with a layer of Welsbach material 14 effective to radiate ultraviolet radiation in response to the application of infrared radiation. The Welsbach material 14 can be painted on to the surface 12 to a thickness of approximately 5-10µm. The Welsbach material (a selective radiator) is characterized by a wavelength-dependent emissivity such that the emissivity is high in the short wavelength (visible or ultra violet) region and low in the long wavelength (infrared) region. When this material is coated over a surface, the emission spectrum changes. Because of the low emissivity in the long wavelength region, most energy in infrared cannot radiate out. Instead, this energy is forced to radiate in the short wavelength region. Examples of the Welsbach material are ThO₂, MgO, ZrO₂ with 10% CaO, and Al₂O₃. The surface 12 is constructed to encompass a painted object 16 to provide even exposure of radiation to all surfaces of the painted object 16. The invention 10 includes a second surface 18 overlying the first surface 12 providing a passageway therebetween. The oven temperature is set and maintained by a conventional temperature controller 20. The temperature controller 20 measures the oven temperature and controls a valve 22 which adjusts the gas pressure from a gas supply 24 to a set of oven burners 26. Air is supplied through inlet ports 28 included in the second surface 18. The construction of various natural gas burners is well known. For example, see page 471-473, Van Nostrand's Scientific Encyclopedia, Sixth Edition (1983). As shown in Fig. 2, ultraviolet and visible radiation have shorter wavelengths than infrared or heat radiation. The wavelength λ of the electromagnetic radiation is related to the frequency v by: v = c/λ where c is the speed of light. The energy E of each photon can be then be calculated from equation [2] below: E = hv where h is Planck's constant. Higher frequency radiation consists of higher energy photons which break molecules more efficiently. Increasing ultraviolet and visible radiation produce cross-links in the paint more quickly than that of infrared or heat radiation alone, thus reducing curing times. Since Welsbach material has a property of converting infrared radiation into radiation rich in visible and ultraviolet radiation, proper application of this material can boost ultraviolet radiation without increasing input power. The invention is not limited to use of gas ovens. The invention can be applied to ovens utilizing lamps or any other oven designs using infrared radiation. Further, the invention is not limited to the curing of paint. It can be applied to curing ovens used for curing plastics, bonding, and other materials requiring oven curing.

Claims: A curing oven (10) for curing materials inside said curing oven by means of electromagnetic radiation in the ultraviolet region, said curing oven (10) comprising: at least one surface (12) coated with a layer of material (14) effective to radiate ultraviolet radiation towards said materials in response to the application of infrared radiation; and a heating means (26) for applying infrared radiation to the said layer of material (14). A curing oven (10) according to claim 1 wherein the layer of material (14) includes a lining of Welsbach material. A curing oven (10) according to claim 1 or 2 wherein the heating means (26) comprises a gas flame. A curing oven (10) according to claim 1 or 2 wherein the heating means (26) comprises electric lamps. A curing oven according to any preceding claim wherein the curing oven (10) comprises: a first plurality of said surfaces (12); a second plurality of surfaces (18) overlying the first plurality of said surfaces (12) and providing a passageway therebetween, said second plurality of surfaces (18) including at least one inlet port (28) for intaking air; and means for injection of fuel into the passageway to provide a flame therein for applying the infrared radiation to the said layer of material (14). A curing oven according to claim 5 wherein the means for injection of fuel comprises a supply of natural gas (24). A method of curing paint by means of a curing oven (10) according to any preceding claim, the method comprising the steps of: applying infrared radiation to a layer of material (14) effective to radiate ultraviolet radiation in response to the application of infrared radiation; and exposing the paint to the ultraviolet radiation radiated by the said layer of material (14). A method according to claim 7 wherein the infrared radiation is applied by a gas flame. A method according to claim 7 or 8 wherein the infrared radiation is applied by an electric lamp.

Inventors: CHANG DAVID B, DRUMMOND JAMES E, POLLACK SLAVA A, SHIH I-FU, CHANG, DAVID B., DRUMMOND, JAMES E., POLLACK, SLAVA A., SHIH, I-FU
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1991
Main classification: F26B3
IPC codes: B05D3, B05C9, F26B3