METHOD OF MANUFACTURING AN ELECTRIC LAMP, ELECTRIC LAMP OBTAINED BY MEANS OF THIS METHOD AND DEVICE FOR PERFORMING SUCH A METHOD

Abstract: Method of manufacturing an electric lamp having a glass tubular part (101) with a longitudinal axis, which part is sealed by a sealing member in which firstly a constriction with a collar (105) is formed by means of heating at one end of the tubular part whereafter the sealing member is secured in a gas-tight manner. The constriction in the tubular part is provided at some distance from the end whereafter in the same fixture the tubular part is divided by means of fusing at the area of the constriction and the two wall portions located on either side of the constriction are moved apart in the axial direction.

Description: The invention relates to a method of manufacturing an electric lamp having a glass tubular part with a longitudinal axis, which part is sealed by a sealing member, wherein, at one end of the tubular part a constriction with a collar is formed by means of heating, in which subsequently the sealing member is secured in a gas-tight manner. The invention also relates to an electric lamp obtained by means of such a method and to a device for performing such a method. Such a method is generally known, particularly in the manufacture of tubular low-pressure mercury vapour discharge lamps. For securing a sealing member such as a "stem", a constriction is provided in the end of a discharge tube by heating the end of the glass wall at its end until the glass softens and by subsequently giving the glass wall of the end the desired shape by means of a profiling tool so that a collar is obtained. The discharge tube is sealed in a gas-tight manner and is subsequently exhausted by means of the exhaust tube present in the stem as described, for example, in British Patent Specification 1,475,458. A stem is herein understood to mean a glass portion comprising an exhaust tube and electrode pins between which an electrode is arranged. In the known method a tube is cut to the desired length in a first fixture whereafter the constriction with the collar is formed in a second fixture. The use of two different fixtures easily leads to inaccuracies in the shape and dimensions of the end of the tubular part.This is notably unwanted in a bulk manufacturing process because then a reliable sealing by means of the sealing member is obtained to an insufficient extent. The object of the invention is to provide a method of manufacturing an electric lamp having a glass tubular part which is sealed by a sealing member in which a reliable sealing of the tubular part is obtained in a bulk-manufacturing process. According to the invention the method described in the opening paragraph is therefore characterized in that the constriction in the tubular part is provided at some distance from the end, whereafter in the same fixture the tubular part is divided by means of fusing at the area of the constriction and the two wall portions located on either side of the constriction are moved apart in the axial direction. By using only one fixture the specific requirements imposed on the shape and dimensions of the end of the tubular part accommodating the sealing member are complied with to a sufficient extent. A tubular part is obtained with an end whose constriction with the collar is constructed in a reproducible way. A high production rate is obtained by fusing instead of cutting in the same fixture, whilst the dimensions of the constriction and the collar can be determined independently of the tube diameter. An advantageous embodiment of the method in which the tubular part rotates during provision of the constriction and during fusing is characterized in that the rotational speed of the tubular part during fusing is faster than the rotational speed during provision of the constriction. The constriction is preferably provided by firstly softening the glass wall of the tubular part and by subsequently pressing the glass wall inwards by means of a profiling tool that may comprise profiling rollers while the tubular part is rotating. The shape of the constriction is maintained due to a relatively low rotational speed of the tubular part. In fact, the centrifugal force occurring during rotation and being exerted on the glass softened at the area of the constriction should not be too large. During fusing the rotational speed of the tubular part is faster so as to obtain a homogeneous temperature around the tubular part. The molten glass is also pressed outwards (due to the then occurring larger centrifugal force), so that a raised edge is obtained on the outer side of the glass wall. This is important for the further production process of the lamp, notably when providing a suspension of luminescent material during the manufacture of a tubular low-pressure mercury vapour discharge lamp. A practical embodiment of the method is characterized in that the rotational speed of the tubular part during fusing is approximately twice the rotational speed during provision of the constriction. When using this embodiment the said edge at the end of the tubular part has optimum dimensions. The sealing member is then incorporated without any problems. Another embodiment of the method is characterized in that a narrow zone of the glass wall is heated when dividing the tubular part at the area of the constriction and in that simultaneously the portions of the glass wall located on either side of said zone are cooled. By heating a narrow zone whilst dividing the tubular part, in which the directly adjoining glass wall is cooled and the wall portions located on either side of the constriction are moved apart in the axial direction, only the glass wall at the area of the heated narrow zone will become increasingly thinner,whilst the ends of the glass wall have a smooth surface after fusing. Unwanted unevennesses are then avoided. In a practical embodiment the zone has a width of at most 5 mm. The invention is preferably used in the manufacture of tubular low-pressure mercury vapour discharge lamps. The invention also relates to a device for performing the method according to the invention, which device is characterized in that it comprises two sets of burners and clamps for clamping the tubular part on either side of the location where the constriction is provided, said clamps being rotatable with respect to the burners and being movable with respect to each other in a direction which corresponds to the longitudinal axis of the said tubular part, one set of burners being present for the purpose of providing the constriction and the other set being present for the purpose of fusing. The device comprises burners for heating the tubular part for the purpose of softening the glass wall so that the constriction can be provided and for the purpose of dividing the tubular part by means of fusing. By clamping the tubular part on either side of the location where the constriction is provided, the tubular part bends less far when a jig is pressed against the glass wall softened at the area of the constriction than when the tubular part is clamped on one side only. Since the tubular part bends to a lesser extent, a constriction of good quality is formed. A preferred embodiment of the device for performing the method according to the invention is characterized in that the burners for fusing are formed as pencil burners whilst furthermore slit-shaped openings are present on either side of each pencil burner, which openings serve to supply a gas stream for cooling the wall portions adjacent the part of the glass wall to be heated. By using a burner having a pointed end in which three slit-shaped openings are present it is possible to heat a narrow zone and to simultaneously cool the wall portions directly adjoining said zone. The central opening is to supply a combustible gas mixture for heating the glass wall. The outer openings are to supply a gas stream ensuring that the glass wall directly adjoining the zone to be heated is cooled. The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings in which Figure 1 shows diagrammatically a tubular low-pressure mercury vapour discharge lamp obtained by means of the method according to the invention; Figure 2 shows diagrammatically the different steps in the production of a tubular low-pressure mercury vapour discharge lamp in accordance with the known method; Figure 3 shows how the tubular part is clamped by using the known method; Figure 4 shows how the constriction with the collar is provided at the end of the tube by means of the known method; Figure 5 shows diagrammatically the different steps in the production of a tubular low-pressure mercury vapour discharge lamp by means of the method according to the invention; Figure 6 shows how the tubular part is clamped by means of the method according to the invention; Figure 7 shows how the constriction is provided in the tube by means of the method according to the invention; Figure 8 shows how the tubular part is divided by means of the method according to the invention and Figure 9 shows how the end is formed by means of the method according to the invention. Figure 1 is a diagrammatic cross-section of a low-pressure mercury vapour discharge lamp having a tubular glass discharge vessel 1 whose inner side is coated with a luminescent layer 2. Stems 3 and 4 are secured in a gas-tight manner to the ends of the tubular glass discharge vessel, which ends are obtained by means of the method according to the invention. The stems have electrodes 5 and 6 between which the discharge is maintained during operation. The lamp contains a small quantity of mercury and a rare gas as a starter gas. In Figure 2 the reference A denotes a long glass tube whose ends are cut off during the first production step of the known method. A tube B of the desired length is then obtained. During the second production step this tube B is subsequently provided with constricted ends with a collar so that a tube of the shape according to C is obtained. During this process the ends of the tube are clamped. This is shown in Figure 3. In this Figure the reference numeral 11 denotes the tube. The clamps are denoted by 12. After the ends have been provided with the constriction and the collar, a luminescent layer is provided in known manner on the inner wall of the tube and the sealing members are secured in a gas-tight manner. The production process step in accordance with the known method of providing the constriction with the collar will be described in greater detail with reference to Figure 4. In this Figure 4 the reference numeral 11 denotes the glass tube which is clamped in the clamps 12. The end of the tube 11 is softened by heating the end. Subsequently the end is brought to the desired shape by means of profiling rollers 13 and a mandril 14. The rollers 13 ensure that the tube is constricted and a collar 15 is obtained because the rollers 13 move in the direction of the mandril 14. The mandril 14 ensures that the tube wall is not pressed too far inwards. The arrow P denotes the direction of rotation of the rollers. In Figure 5 the reference a denotes a tube which is similar to A in Figure 2. During the steps shown in the method according to the invention a tube b is formed during the first production step by means of cutting with a glass cutter. This tube b is longer than the tube B of Figure 2. During the second production step the constrictions are provided at some distance from the respective ends (tube c). The tube is then divided at the area of the constriction and the loose ends then obtained are removed so that tube d is obtained. This is effected in the same fixture. The shape and length of tube d correspond to those of tube C of Figure 2. In order to ensure that the tube is not warped when the constriction is provided by means of a profiling tool, the tube is firstly clamped on either side of the location where the constriction is to be provided. This is shown in Figure 6. In this Figure 6 the tube 21 (which has the shape of tube b) is clamped in the clamp-pairs 22 and 23 on either side of the locations where the constrictions are to be provided. Figure 7 shows in detail how the constriction is provided by means of a profiling tool, that comprises a profiling roller 24. The tube 21 is clamped in the clamps 22 and 23. The arrows P indicate in which direction the tube with the clamps rotates. The tube wall is softened at the area where the constriction is to be provided. The constriction 25 is formed by pressing profiling tool 24 on the softened glass wall. Figure 8 shows in greater detail how the tube is subsequently divided into tube part 101 and part 102. The clamps 103 and 104 correspond to the clamps 22 and 23 of Figure 7. Here again the arrows P indicate the direction of rotation of the tube with the clamps. By dividing the tube at the area of the constriction and by removing part 102, tube 101 is obtained which is provided with a constriction with a collar 105. The tube is divided by means of fusing. The burner which is used for this purpose has three openings 106, 107 and 108. The opening 106 is provided via supply duct 109 with a combistable gas mixture for heating the glass wall. The openings 107 and 108 are provided via the supply ducts 110 and 111 with a cooling gas stream. This cooling gas stream is used to cool the glass wall directly adjoining the zone to be heated. During fusing the part 102 must be pulled away from the tube. This is indicated by the arrow Q to the left. Since the tube is rotated during fusing, the molten glass will be pressed slightly outwards. This is shown in Figure 9. An outwardly raised edge 203 is present at the end on the tube 201, which is provided with constriction with a I collar 202. When providing the suspension of luminescent material (which is often effected when the tube assumes a vertical position) liquid is prevented from coagulating against an inwardly raised edge. This is also a drawback when providing the sealing member, the so-called stem, at a later stage. In a practical embodiment of a tubular low-pressure mercury vapour discharge lamp the length between the constriction at the two ends is 1200 mm. The thickness of the glass wall at the constriction is approximately 2 mm. The internal diameter at the area of the collar is 23 mm. Elsewhere in the lamp the internal diameter is 26 mm. The height of the collar is approximately 6 mm.

Claims: 1. A method of manufacturing an electric lamp having a glass tubular part with a longitudinal axis, which part is sealed by a sealing member, wherein, at one end of the tubular part a constriction with a collar is formed by means of heating, in which subsequently the sealing member is secured in a gas-tight manner, characterized in that the constriction in the tubular part is provided at some distance from the end, whereafter in the same fixture the tubular part is divided by means of fusing at the area of the constriction and the two wall portions located on either side of the constriction are moved apart in the axial direction. 2. A method as claimed in Claim 1, in which the tubular part rotates during provision of the constriction and during fusing, characterized in that the rotational speed of the tubular part during fusing is faster than the rotational speed during provision of the constriction. 3. A method as claimed in Claim 2, characterized in that the rotational speed of the tubular part during fusing is approximately twice the rotational speed during provision of the constriction. 4. A method as claimed in Claim 1, 2 or 3, characterized in that a narrow zone of the glass wall is heated when dividing the tubular part at the area of the constriction and in that simultaneously the portions of the glass wall located on either side of said zone are cooled. 5. An electric lamp manufactured by means of the method as claimed in Claim 1, 2, 3 or 4. 6. An electric lamp as claimed in Claim 5, characterized in that the lamp is a tubular low-pressure mercury vapour discharge lamp. 7. A device for performing the method as claimed in Claim 1, 2, 3 or 4, characterized in that the device comprises two sets of burners and clamps for clamping the tubular part on either side of the location where the constriction is provided, said clamps being rotatable with respect to the burners and being movable with respect to each other in a direction which corresponds to the longitudinal axis of the said tubular part, one set of burners being present for the purpose of providing the constriction and the other set being present for the purpose of fusing. 8. A device as claimed in Claim 7, characterized in that the burners for fusing are formed as pencil burners, whilst furthermore slit-shaped openings are present on either side of each pencil burner, which openings serve to supply a gas stream for cooling the wall portions adjacent the part of the glass wall to be heated.

Inventors: ROMBERG, HENDRIK
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J9
IPC codes: H01J9, C03B23