Method for manufacturing small tubular lamps

Abstract: Disclosed is a method for manufacturing small tubular lamps. In manufacturing a small tubular lamp, a moving amount of a tube stock (1) is adjusted according to a wall thickness thereof to make the stock-guide amount uniform, and internal pressure during the stock-guide step is also adjusted so as to have a suitable dimension of the outside diameter thereof, whereby the working amount in the succeeding step is reduced to decrease an error in dimensional difference, particularly in wall thickness and internal volume.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for manufacturing small tubular lamps used for discharge lamps such as small metal halide lamps. Description of the Prior Art In manufacturing a small tubular lamp, for example, as shown in Figs. 4 and 5, a starting tube a formed of glass, quartz, etc. is set on a glass lathe or a working device provided with a function similar to that of the glass lathe. At least one of left and right collets (bearings) is moved by a predetermined amount toward the center, while heating the starting tube a by a burner b in a rotating state, whereby padding is gathered on a predetermined portion of the starting tube a . Then, the starting tube a is fully heated and softened, and thereafter the predetermined portion of the starting tube a is covered with a mold c for blow molding. A predetermined pressure is applied into the starting tube a to inflate and suitably shape the starting tube a . In metal vapor discharge lamps such as metal halide lamps, vapor pressure of a luminous material sealed into a luminous tube greatly influences on output, light color, etc. Therefore, it is desirable to minimize unevenness between individuals. Because of this, it is demanded in the process of manufacturing a luminous tube that dimensions of various parts, volumes, etc. need to have reproducibility with high precision. Particularly, in a metal halide lamp whose comsuption power is low (specifically, 100 W or less), severe improvement in precision is demanded with the trend of smaller types. The aforementioned demand is not absolutely fulfilled by the aforesaid conventional manufacturing method. Despite the fact that for example, a demand for a headlamp for an automobile or the like of which comsuption power is low is anticipated, yield is poor and cost is high, thus posing a problem in that they cannot be sufficiently supplied to the market. The US-A-3 897 233 depicts a process for forming a quartz arc tube in which the vitreous tubing is heated to plasticity and necked down by cone-sections to a short neck at two places close together. The US-A-4 801 323 discloses a method for forming a tubular discharge vessel for a compact low pressure discharge lamp by heating and binding a straight glass tube into a U-shape with a plurality of straight sections. The wall thickness is adjusted at the tubular sections where the bends are formed, by first heating a central section, moving sections lateral to the central section toward one another and thereby compressing the heated central section and thickening its wall. It is an object of the present invention to provide a method for manufacturing small tubular lamps capable of molding small tubular lamps with high working precision and with low cost. SUMMARY OF THE INVENTION For solving the aforementioned problems, the present invention provides a method for manufacturing small tubular lamps as defined by claim 1. With the above-described manufacturing method, precision in the manufacture process is improved to solve the aforesaid problem. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an inert gas piping system and control circuit showing one embodiment of a method for manufacturing a small tubular lamp according to the present invention; Fig. 2 is a front view in an enlarged scale showing principal parts of the embodiment shown in Fig. 1; Fig. 3 is a side view in an enlarged scale showing principal parts shown in Fig. 1; Fig. 4 illustrates a conventional step of forming a preform; and Fig. 5 illustrates a conventional molding step. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described hereinafter by way of an embodiment shown in the drawings. In Fig. 1, reference numeral 1 designates a starting tube. Opposite ends of the starting tube (a glass tube, a quartz tube, etc.) 1 are airtightly held by collects 2A and 2B. The collets 2A and 2B can be moved by a suitable distance in an axial direction of the glass tube 1 by pulse motors 3A and 3B. A burner 4 is arranged in the middle between the collets 2A and 2B. In Fig. 1, the solid line indicates piping of inert gas, and the dotted line indicates an electric wiring. The inert gas flows through a needle valve 5 and a flow meter 6 and is branched into a direction of the glass tube 1 and a direction of a bypass 7. A valve 8A and a coupler 9A are interposed between a branch point (inlet of the bypass 7) and one end of the glass tube 1. The inert gas is once branched and passes through respective flowpassages, after which the gases are again joined together. The gases are discharged passing through a variable valve 10 with an actuator. A fine differential pressure gauge 11 is provided at the aforesaid branch point. The fine differential pressure gauge 11 measures a pressure difference between the atmospheric pressure and inert gas line, and the measured signal is supplied to a PID control unit 12. The PID control unit 12 drives the variable valve 10 according to a difference between a preset pressure and an actual pressure and is operated so as to maintain the pressure of the inert gas line constant. The pulse motors 3A and 3B are driven by drivers 13A and 13B to which is supplied an output pulse of a pulse oscillator 14. The pulse oscillator 14 is controlled by a microcomputer 15. In Fig. 1, reference numeral 20 designates a roller. The roller 20 is provided for the molding step applied to the starting tube 1, the structure of the roller 20 being shown in detail in Fig. 2 and Fig. 3. Above the starting tube 1 is rotatably supported, by an arm 21, a disk-like roller 20 formed of carbon or the like and formed at the peripheral edge with a peripheral groove the form of which corresponds with the outside form of the finished tubular lamp as an object. The arm 21 is rotated about a fulcrum provided in a column 26 according to a position of a cam 22 driven by a bowl screw 23 to displace the roller 20 in the radial direction of the starting tube 1. A position detection bar 25 is provided on the arm 21, and a position sensor 24 is actuated when the position detection bar 25 reaches a predetermined position to notify that state. The method for manufacturing a small tubular lamp according to the present invention will be described hereinafter in accordance with sequential steps. (1) Premeasured inside diameter and outside diameter of the glass tube 1 are inputted into a microcomputer (micon) 15. (2) The microcomputer 15 calculates an adequate preform amount value and an adequate pressure value in the glass tube 1 from a sectional area value of the glass tube 1. (3) Upon receipt of command from the micon 15, the PID control unit 12 manipulates the variable valve 10 to stabilize the inert gas line at computed excess pressure in the range of 2 to 6 mmHg. (4) After the glass tube 1 has been molten by the burner 4, the pulse oscillator 14 is actuated in accordance with the command of the micon 15, and the computed number of pulse signals are sent to the drivers 13A and 13B to drive the pulse motors 3A and 3B. Thereby, the collets 2A and 2B are moved by the calculated distance in the direction of the center, whereby the preforming step is executed and next, the molding step proceeds. (5) A predetermined portion of the starting tube 1 is again heated and molten, and the cam 22 is driven to move down the roller 20, which is placed in pressure contact with the starting tube 1 by its own weight. (6) A position of the roller 20 is detected by the position detection bar 25, and the step of (5) above is repeated till the roller 20 reaches a predetermined position. (7) Upon termination of the aforesaid steps, a portion in contact with the roller 20 is re-heated to remove distortion. The function and effects of the method for manufacturing small tubular lamps using the above-described steps according to the present invention will be described hereinafter. (1) First, the preform amount is adjusted by the preform step according to a sectional area of the starting tube 1, whereby a volume of glass material of the portion to be formed as a tubular lamp can be maintained constant by the succeeding step. (2) At that time, an internal pressure of the starting tube 1 is set to an adequate excess pressure within the range of 266.644 to 799.937 Pa (2 to 6 mmHg), whereby the outside diameter of the center portion of the preform portion can be maintained substantially constant substantially equal to the outside diameter of the tubular lamp to be completed by the succeeding step. (3) Furthermore, the outside diameter of the center portion to be prepared is substantially made to be equal to the outside diameter of the final product as described above, whereby the molding step by the roller 20 is to have its object principally for arranging the shape. The working amount decreases, and therefore the lowering of the accuracy in the molding step can be reduced. By the functions of the above paragraphs (1) to (3), the excellent dimensional accuracy and tubular lamps excellent in standardization of shapes are obtained. Table 1 shows specific measured values of a center outside diameter, a center inside diameter and an internal volume of tubular lamps which are extremely small as tubular lamps for discharge called a double end type having a center outside diameter of 6 mm and a length of tube of 7.5 mm except a seal portion manufactured by the method according to the present invention. A dimension difference in the center outside diameter is 0.03 mm, and particularly, a dimension difference even in the center inside diameter is 0.07 mm which has been heretofore considered to be difficult in management. Uniformity with such high precison can be realized. As a result, an internal volume which is an integrated value of the respective dimensional accuracy is also 1.8 x 10⁻³ cc. Thus, the dimensions of various portions are formed into uniform with extremely high accuracy. It is therefore readily assumed that a discharge lamp, for example, such as a metal halide lamp which employs a tubular lamp produced by use of the present invention also has a high uniformity. Materials No. Center outside diameter (mm) Center inside diameter (mm) Internal volume (x10⁻³cc) 1 6.01 2.99 29.7 2 6.03 3.02 30.5 3 6.00 2.99 29.4 4 6.03 3.06 31.2 5 6.02 3.02 30.2 As described above, according to the present invention, the preform amount is adjusted according to a sectional area of a starting tube, and in addition, suitable internal pressure is applied during the preforming step so that the outside diameter may have a predetermined dimension, whereby the working amount in the succeeding working step can be extremely reduced to render possible molding a tubular lamp with high accuracy, and unevenness of the lamp characteristics can be reduced. Accordingly, small metal halide lamps can be used even for uses which have been heretofore difficult to be applied. Moreover, the preform position, preform amount, pressure difference between inside and outside of a starting tube, etc. can be controlled by a microcomputer to thereby freely mold any dimension and shape. Furthermore, the dimensional allowance of the starting tube is improved, and selection of materials becomes unnecessary. Tubular lamps can be produced with low cost.

Claims: A method for manufacturing small tubular lamps comprising the steps of forming a preform portion by heating a predetermined portion of a starting tube (1) to its softening point, said starting tube being made of vitreous material and having inner and outer diameters, while rotating the starting tube (1) about the axis of the tube and moving one end of the starting tube (1) toward the other thereby upsetting the tube in the heated portion to form a preform portion, and forming said preform portion into a predetermined shape by re-heating said preform portion up to and forming it at its softening point by pressing, characterized in that in said preform forming step the distance over which said one end of this starting tube is moved toward the other one is adjusted according to the sectional area of said starting tube (1), which is calculated from the premeasured inner and outer diameters of the tube, so that the glass volume of the preform portion is constant over sequential preforms and an internal pressure higher than suitable atmospheric pressure in a range of 266.644 to 799.932 Pa (2 to 6 mmHg) is applied into said starting tube (1) to maintain the outside diameter of the center of said preform portion substantially constant, and said pressing step is carried out by placing a roller (20) movable in the radial direction of said starting tube (1) in pressure contact with said preform portion which is heated to its softening point, said roller being formed by a disc having a peripheral groove in its peripheral edge, which groove corresponds with the final form of the tubular lamp and rotating said starting tube (1) till said roller (20) reaches a predetermined position. A method according to claim 1, wherein said starting tube comprises a glass tube or a quartz tube.

Inventors: MUTO MASAAKI, SHIMADA HIROHARU, YAGUCHI YASUHISA, MUTO, MASAAKI, SHIMADA, HIROHARU, YAGUCHI, YASUHISA
Applicants: STANLEY ELECTRIC CO LTD, STANLEY ELECTRIC CO., LTD.
Application year: 1991
Main classification: C03B23
IPC codes: C03B23, H01J9