Vehicle headlamp or the like having a layer which prevents localized heating

Abstract

 A sheet of flexible graphite (10) is disposed on the concave surface (12) of a headlamp reflector or the like, adjacent a source of heat such as light bulb (14) or similar light source. The sheet conducts heat along the surface thereof approximately 30 times faster than through the thickness thereof whereby localized heating of the member in proximity of the source is prevented.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The present invention relates generally to a device having a housing exposed to a source of heat and which is subject to localized heating and more specifically to a device such as a vehicle headlamp in which a layer of material is provide to disperse the heat produced by the light source and prevent localized thermal damage.

Description of the Prior Art

[0002] In a previously proposed arrangement such as shown in Figs. 1 and 2 of the drawings, a vehicle headlamp 1 has been formed of plastic to overcome the production difficulties involved with deep drawing metal sheet, and provided with a base coat, metallic reflective coat and top or finishing coat (which coats are shown as a single relatively thick coat 2 for the sake of illustration only). This arrangement while solving the problems inherent with deep drawing metal sheet, has encountered the problem that when low beam (produced by filament 3) is continuously used for prolonged periods, localized heating of the reflector housing 4 occurs above and slightly forward of the bulb 5. In a specific example wherein a 55W bulb was continuously energized for a prolonged period the temperatures at points A B C D and E were determined. The results are set forth in Table 1. shown below.

[0003] As will be appreciated, the zone in close proximity of point A develops a very high temperature and tends to be thermally deformed, expanded or decomposed to the point of reducing the light reflection efficiency of the device.

[0004] To overcome this problem it has been proposed to form the housing of a plastic having a high thermal resistance (e.g. a plastic which can withstand temperatures in excess of 200 degrees C.) However, as such plastics are inevitably expensive even when used to form only a part of the housing (viz., that part of the housing exposed to excessive heating), it has been proposed to include a heat shield 5 such as shown in Fig. 3. This heat shield 5 can be made of a light metal such as aluminimum but must be disposed at a suitable distance from the actual surface of the reflector housing to prevent heat being conducted directly thereto. Accordingly, an arrangement such as illustrated has been proposed wherein the heat shield 5 is received in slotted extensions 6 which depend from the upper surface 7 of the housing 8. This arrangement while proving reasonably effective has encounted the drawback that injection moulding of a housing having such a complex shape requires a number of operations requiring a complex die arrangement and relatively long period to produce. Moreover, the final product has tended to be heavier than desirable as well as relatively expensive.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a plastic or similar synthetic resin housing, such as a vehicle headlamp housing, which is easy and inexpensive to produce, which is light and which is not subject to thermal damage even upon prolonged exposure to a source of heat.

[0006] The present invention in a broad sense takes the form of a device comprising a member exposed to a source of heat and a layer of material disposed on a surface of said housing in proximity of the heat source, the sheet conducting heat along its surface faster than through its thickness in a manner that localized heating of the housing is prevented.

[0007] In a more specific form, the present invention features a sheet of flexible graphite (a material which exibits anisotropic heat conduction prpperites) which is disposed on the concave surface of a moulded plastic headlamp reflector or the like (having a limited thermal resistance), above a source of heat such as light bulb or similar light source. The sheet conducts heat along the surface thereof faster than through the thickness thereof so that localized heating of the reflector in proximity of the source is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The features and advantages of the arrangement of the present invention will become more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a front elevation of a prior art arrangement discussed briefly in the opening paragraphs of the present specification;

Fig. 2 is a cross sectional view of the arrangement shown in Fig. 1;

Fig. 3 is a front elevation of another prior art arrangement also discussed in the opening paragraphs of the present specification; and

Fig. 4 is a front elevation of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Turning now to Fig. 4 an embodiment of the present invention is shown. In this arrangement a layer or sheet of flexible graphite 10 is disposed on the concave surface 12 of the reflector housing above the light bulb 14. As shown, the graphite layer 10 terminates just before the attachment flange 16 of the housing upon which the lens or transparent cover (not shown) of the arrangement is attached.

[0010] In the disclosed embodiment the basic body 18 of the housing is formed by injection moulding (or the like) a synthetic thermoplastic resin such as 6-nylon, 6-6 nylon, denatured polyphenylene oxide, polycarbonate, polyethylene terephthalate or one of the just mentioned resins reinforced with inorganic materials (e.g. glass fiber). Thermosetting resins such as phenolic and polyester resins may also be used. Materials such as polyethylene and polypropylene for example, are generally less desirable because of lower mechanical strength and heat resistance and poor painting and adhesion characteristics.

[0011] The basic body 18 of the device according to the embodiment shown in Fig. 4 is provided with a base coat or film to seal the surface of the body and prevent the formation of gas during the vacuum evaporation depositing of the reflective metallic second coat. A third top or finishing coat is applied to the reflective surface to finish the article.

[0012] The anisotropic heat conductive layer which characterizes the present invention, in the disclosed embodiment is formed by grinding natural graphite, kish, thermally decomposed graphite or the like, having a highly layered crystalline structure, treating the resulting powder in a powerful oxiding medium such as concentrated sulphuric acid/ permanganate solution and subsequently heating the resulting product to about 800 to 1000 degrees C. whereby suphur oxide gases (SO_x) are generated which expand the particles to 10 to 200 times their original size. The resulting slug-like particles are then formed into a sheet of flexible graphite by calender rolling or compression shaping techniques.

[0013] It has been found that calender rolling is better suited to mass production requirements but is not suited to the formation of a article having a density above 1.6gm/cm³. Thus, it is preferred to first calender roll, the expanded graphite particles and then press the resulting sheet into the desired shape. In the case that the expanded particles are directly compressed the density of the resulting product obtained is quite low and the material becomes difficult to handle during mass production.

[0014] The above described product is commercially available under the trade names of NICA film (produced by the NIPPON CARBON CO. LTD.), GRATFOIL (produced by the UNION CARBIDE CORP.) and BALCAFOIL (produced by the NIPPON BALCA CO. LTD).

[0015] A sheet of flexible graphite of the nature described above contains no organic matter and therefore exhibits good stability at elevated temperatures in air and further exhibits a heat conduction anistropy wherein heat is conducted along the suface of the sheet faster then though the thickness thereof. For example, a sheet having a density of 1.5 gm/cm³ exhibits a surface heat condution of 120 Kcal/m.hr.degree C, which is approximately that exhibited by aluminium, but a heat conduction through the thickness thereof of only approx=mately 4 Kcal/m.hr.degree C or 1/30 that of the surface conduction.

[0016] For practical considerations it has been found that it is appropriate to use a flexible graphite sheet having a density within a range of 0.7 - 1.9gm/cm³ and a thickness within the range of 0.05 - 1.0mm. That is to say, it has been found that a sheet having a density lower than 0.7 gm/cm³ lacks tensile strength and that a sheet having a density above 1.ggm/cm³ is difficult and expensive to produce. Further, a sheet having a thickness of less than 0.05mm lacks mechanical strength and is readily damaged during production while a sheet having a thickness in excess of 1.0mm is difficult and expensive to produce.

[0017] During production of the headlamp reflector which embodies the present invention, it is necessary to shape the graphite sheet so as to have a contour which corresponds to that part of the base body on which it is to be disposed. To achive this two methods are possible. One is to compress the expanded graphite particles using a mould having a shape which corresponds in contour to that of the site which which the sheet is to be attached. The second is to first form the sheet by calender rolling and then place the thus formed sheet in the same mould. In the case that the contour of the surface to which the sheet is to be attached is complex, a sheet having a density of 0.2 - 0.5gm/cm³ which is pressed using a rubber pressing method (preferably using hydraulic pressure) has been found suitable.

[0018] The graphite sheet may be disposed directly on the basic body and bonded in position using an adhesive having good heat and adhesive properties with respect to both of the base body material and the graphite sheet. In this connection adhesives of the epoxy or phenol group have been found suitable. By way of example, PLYOPHEN TD-735 (phenolic type) or PLYOPHEN LA-1159 (epoxy type), both produced by the DAINIPPON INK AND CHEMICALS INC., may be used in this connection. Alternatively, the base coat may be first applied to the base body and the graphite sheet set into the paint film while it is still fluid after which the surface of the graphite sheet may be coated.

[0019] As previously mentioned the base coat is provided not only to provide a smooth surface on which to apply the reflective layer but to suppress generation of gas upon exposure to a vacuum. The base coat may take the form of a conventional urethane, polyester or melamine- alkyd type paint. The selection of the paint of course must be made taking into account its heat resistance and adhesion to both of the base body and the graphite sheet. The paint may be applied using a flow-coater if desired and in a manner to provide a thickness of 10 - 20 .

[0020] The reflective film may be applied using a vapour deposition technique. Either of a resistor heating vapour depositing technique or a sputtering deposition techique may be used. In terms of luster, cost and application speed, the resistor heating method is deemed advantageous, however in the case that the metal to be applied is chromium or stainless steel the sputtering technique is preferred. The thickness of this layer should be from 500 - 3000 Å. The final or top coat which may be a urethane or acryl melamine varnish, by way of example, should be formed on top of the reflective layer in a manner to have a thickness of 5 to 15_u.

TEST

[0021] In order to demonstrate the merit 'of the present invention 4 groups (of 2) reflector base bodies were produced via injection moulding as follows:

Group I - two reflector base bodies R-1, R-2 formed of 6-nylon produced by the TOYOBO CO., under the trade name of T-42202 (referred hereinafter as NY);

GROUP II - two base bodies R-3, R-4 formed of polyethylene terphthalate containing 60wt % resol type glass fiber (which will be referred to as PET hereinafter);

GROUP III - two base bodies R-5, R-6 formed of phenolic resin produced by the TEIGIN CO., under the trade name M-111 and which will be referred to hereinafter as PH; and

GROUP IV - two base bodies R-7, R-8 formed of an unsaturated polyester resin manufactured by the MITSUI TOATSU KAGAKU CO., under the trade name of Ester BMC and which will be referred to=as UP hereinafer.

[0022] To one of each of the above mentioned groups (R-1, R-3, R-5, R-7) a sheet of flexible graphite produced by the NIPPON CARBON CO. LTD., under the name of NICA film FL-100 having a density of 1.0 gm/cm³ and a thickness of 0.2mm was bonded using a phenolic adhesive. To the other of each of the groups (R-2, R-4, R-6, R-8) a layer of flexible graphite manufactured by the company mentioned immediately above under the trade name of NICA film FL-200 having a density of 1.5 gm/cm and a thickness of 0.2mm was similarly bonded.

[0023] Upon adequate curing of the bond each base body was washed with acetone and subsequently spray painted with EXP1436A, EXP1436B and EXP1436C (manufactured by the FUJIKURA KASEI CO. LTD.) mixed in the weight ratio of 100:20:25, in a manner to form a paint layer having a thickness of approximately 20_u. The base bodies were then fired for 1Hr. at 170 degrees C. Next, an aluminium reflective film having a thickness of approximately 700 A was vacuum deposited on each using a resistor evaporation technique. Finally, the top coat consisting of EXP 1434 (a single component type urethane varnish) and SL8395 thinner (FUJIKURA KASEI CO. LTD) mixed in a weight ratio of 100:50, was sprayed on to form a film approximately 10_u thick which as fired for 30 minutes at 70 degrees C. Alternatively, a top coat of AL-3 (an acryl melamine type varnish produced by NIPPON OIL AND FATS CO., LTD.) and SL8395 thinner (FUJIKURA KASEI CO., LTD.) may be used.

[0024] For the sake of comparison four further reflector bodies C-1 to C-4 were produced in a manner identical to the units R-1, R-3, R-5 and R-7, described above with the exception of the ommission of the flexible graphite sheets.

[0025] All twelve of the units were then exposed to the low beam produced by 55W bulbs for 30 hours in a dead calm 20 degree C atmosphere. During this period the temperatures at portions corresponding to A, B, C on the concave reflective surface and the corresponding portions A',B',C' on the convex external surface of the reflector were measured. The results of this test are set forth in Table 2.

[0026] As will be appreciated from Tables 2 and 3 the peak temperatures developed in close proximity of the bulb were reduced while the temperatures of adjacent zones increased due to the anisotropic heat conducting characteristics of the flexible graphite layer. As shown the reflectors provided with the inventive layer underwent no deterioration demostrating clearly the utility of the invention.

[0027] The present invention is not limited to headlight reflectors and may find application in other devices and apparatus wherein localized heating is a problem.

Claims

1. A device comprising:

a member having a limited thermal resistance and which is subject to localized heating due to exposure to a source of heat; and

a layer of material disposed on a surface of said member in proximity to said source, said layer conducting heat along its surface faster than it conducts heat through its thickness so as to prevent localized heating of said member.

2. A device as claimed in claim 1, wherein said layer is formed of graphite.

3. A device as claimed in claim 1, wherein said layer is a sheet of flexible graphite formed by treating powdered graphite in an oxidizing solution and subsequently heating same to induce expansion thereof.

4. A device as claimed in claim 3, wherein said sheet of flexible graphite has a density of 0.7 - 1.9 gm/cm³ and a thickness of 0.05mm

5. A device as claimed in claim 1, wherein said source of heat is a light source and said member is a moulded plastic light reflector having a concave surface, said layer being formed on said concave surface.

6. A device as claimed in claim 5, wherein said moulded plastic light reflector is formed from a synthetic resin selected from among the group comprising: 6-nylon, 6-6 nylon, denatured polyphenylene oxide, polycarbonate and polyethylene terephthalate.

7. A device as claimed in claim 5, wherein said member is made of plastic, said member further comprising a film formed on said concave surface thereof.

8. A device as claimed in claim 7, wherein said film is formed over the top of said flexible graphite layer so as to conceal same.

9. A device as claimed in claim 7, wherein said flexible graphite layer is set into said film and retained in place thereby.

10. A device as claimed in claim 5, wherein said sheet is disposed on the sarface of said film.

11. A method of preventing localized heating in a member having a limited thermal resistance and which is subject to localized heating due to exposure to a source of heat, comprising the steps of:

disposing a layer on a surface of said member in proximity of said source; and

conducting heat along the surface of said layer faster than conducting heat through the thickness of said layer.

12. A method as claimed in claim 11, further comprising the step of forming said layer of graphite.

13. A method as claimed in claim 11, further comprising the step of forming said layer of flexible graphite formed by treating powdered graphite in an oxidizing solution and subsequently heating same to induce expansion thereof.

14. A method as claimed in claim 13, further comprising the step of arranging said flexible graphite to take the form of a sheet having a density of 0.7 - 1.9 gm/cm³ and a thickness of 0.05 - 1.00mm

Drawing