Abstract:
An apparatus for welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules includes a welding station having a welding position for welding closed one end of a tube, a rotatable platform having an outside edge, a tube holder positioned adjacent the outside edge, the tube holder movable between a loading position and the welding position, and a feed line positioned in overlying registry with the tube holder at the loading position.

Description:
This application is a continuation of U.S. patent application Ser. No. 09/601,962 filed Oct. 6, 2000 now U.S. Pat. No. 6,474,535, which is a filing under 35 U.S.C. §371 of international patent application number PCT/GB99/03884 filed Nov. 19, 1999. International patent application number PCT/GB99/03884 claimed priority to patent application number 9826526.7 filed in Great Britain on Dec. 2, 1998 and claimed benefit of United States provisional patent application No. 60/109,214 filed Nov. 20, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a welding method and apparatus, and more particularly to a method and apparatus for automated welding of tubes for use in brachytherapy. 
     BACKGROUND OF INVENTION 
     In the treatment of various cancers, and in particular prostate cancer, a process called brachytherapy has proved effective. In brachytherapy, small capsules containing radioactive material are implanted in or near to the tumour. 
     One known form of capsule or canister, commonly used to treat prostate cancer and referred to as a “seed”, is shown in FIG.  7 . The capsule  100  comprises a silver rod  102 , coated with a radioactive isotope of iodine such as I-125, inside a hollow titanium tube  104 . The ends of the tube are welded closed. Resin balls coated with radioactive iodine can be used instead of the silver rod  102 . The completed capsule has a width of approximately 1.0 mm and a length of approximately 4.5 mm. The capsules or seeds can be implanted into a patient individually; alternatively, the capsules can be inserted into medical stitching material or suture, which is then inserted into the prostate and left there. 
     The number of capsules implanted into each patient obviously varies in accordance with the regime of treatment required, but is commonly in the region of 50 to 100. The capsules are normally made by hand, with the welding process used to close the ends of the tube being carried out manually. It will be appreciated that making such a large number of capsules for each patient by hand takes considerable time and expense. 
     SUMMARY OF INVENTION 
     In view of the needs of the art, the present invention provides an apparatus for welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules. The apparatus includes a welding station having a welding position for welding closed one end of a tube, a rotatable platform having an outside edge, and a tube holder positioned adjacent the outside edge. The tube holder is movable between a loading position and the welding position. A feed line is positioned in overlying registry with the tube holder at the loading position. 
     The present invention further provides an apparatus for welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules. This apparatus includes a welding station where one end of the tube is welded closed in a welding operation, a holder support defining a recess, and a holder mounted on the holder support. The holder being in the form of a block slidably mounted within the recess of the holder support. A number of bolts pass through the block and engage the holder support for retaining the block on the holder support. Resilient means act between the block and the holder support such that the block is biassed outwardly by the resilient means. The bolts retain the block on the holder support against the resilient bias and thus serve to define an open condition of the block. The open condition of the block is variable by means of tightening or loosening the bolts. A set screw is provided in the recess to define a point beyond which the bolts may not be tightened. 
     According to a first aspect of the invention, there is provided an automated method of welding closed one end of each of a multiplicity of open-ended tubes suitable for use in the manufacture of brachytherapy capsules, which comprises repeating the steps of loading each tube into a holder, bringing the tube in the holder to a welding station, welding closed one end of the tube, and releasing the welded tube from the holder. 
     With such a method, a large number of tubes can be processed relatively quickly. 
     Preferably, the step of loading each tube into a holder comprises the steps of orientating the tubes into a particular orientation in a feeder, and feeding each tube from the feeder to a holder. Once orientated in the feeder, the tubes can be fed to the holder in a desired orientation. Manual picking up and orientation of the tubes is avoided. 
     As the tubes are for use in making brachytherapy seeds, they will be of a suitable size for brachytherapy implantation, for example, a few millimetres long by about one millimetre wide. The tubes are typically a little longer than the product brachytherapy seed, since it is usual for some of the tube material to be used to form the weld closure. 
     The welding of one end of the tube closed is preferably effected by melting the material of the tube such that the melted material forms the end closure. Preferably, no additional material is required. 
     Closure of the remaining open end of the tube will be effected after the active substance for brachytherapy has been inserted. 
     In general, the tubes to be welded will be formed by being cut from a long length of tubing, and having been so cut they will be randomly orientated. According to the method of the invention, the tubes are orientated in the feeder, so that each tube is ready to be fed to a holder. A preferred method comprises agitating the tubes in order to orientate them in the feeder. 
     Various ways may be used to control feeding of each tube to a holder. For example each tube may be individually picked up by a robot arm and placed in the holder. Preferably, the method comprises arranging the tubes end to end one above another in the feeder, urging a restraining member against a side of the lowermost-but-one tube to prevent it from downward movement, and releasing the lowermost tube from the feeder individually to the holder. The restraining member thus engages the lowermost-but-one tube and in so doing all the tubes above are held in position, allowing the lowermost tube to be individually released. After release, a blocking member is preferably moved into a blocking position below the tubes and the restraining member is retracted to allow the tubes to drop down, the then lowermost tube being stopped by the blocking member. The restraining member can then be advanced against the now lowermost-but-one tube and the feeding cycle is then repeated. 
     In a preferred method, the holder receives a tube, conveys the tube to the welding station, and releases the tube, said holder moving in a closed loop. 
     The provision of a holder which moves in a closed loop facilitates the automation of the welding process. The tube to be welded is supplied to the holder in a particular orientation. The tube is then carried to the welding station where it is welded. The welded tube is released, preferably at an ejection station to which it is carried from the welding station. Since the holder moves in a closed loop, it comes back to its original position, and the procedure can then be repeated. 
     It is possible to use one holder, although the rate of production of such an apparatus is low, as it can only operate on a single tube at a time. Accordingly, it is preferred that a plurality of said holders are provided. This allows greater productivity. 
     The welding of the end of each tube obviously requires high temperatures in order to melt the metal of the tube. It is possible for these high temperatures to cause damage to whatever is holding the tube during the welding operation, particularly if repeated welding operations are carried out. Some form of cooling can be provided to reduce the risk of damage. However, in a preferred method, the holders move through said welding station sequentially, each of the plurality of holders cooling between successive welding operations while the other holders move through the welding station. This ensures that after each of the holders holds a tube while it is being welded, it cools down before it next holds a tube being welded. This reduces the risk of heat damage to the holders. 
     Many possible methods of moving the or each holder through the welding station can be employed. For example, each could be individually driven along a track. 
     However, in a preferred method, the or each holder is mounted at the periphery of a rotatable member. Receiving, welding and releasing stations may conveniently be provided at successive points along the rotational path of the holder. Where a plurality of holders are provided they can all be mounted by the rotatable member, with the advantage of simplicity, as it is then only necessary to provide a single drive means. 
     The holder may for example be a cavity into which the tube can be dropped by the feeder and which restricts movement of the tube adequately during welding. Preferably, however, the holder adopts an open condition when receiving and releasing a tube and a closed condition to grip the tube firmly at least during welding of the tube. This arrangement assists the receiving and releasing steps, whilst properly holding the tube during welding. 
     The holder may be in the closed condition at all times except when receiving and releasing a tube. However, the method preferably comprises causing the holder to move from the open condition to the closed condition when the holder is at the welding station. 
     Thus, means can be provided at the welding station for causing the closing action, rather than such means being provided as part of the holder itself. This can advantageously simplify the design of the holder, which is particularly advantageous where the holder is conveyed from one processing station to another, e.g. by a rotatable member. 
     Viewed from another aspect the invention provides an automated method of welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules, comprising repeating the steps of feeding each individual tube to a holder in an open condition, conveying the holder in the open condition to a welding station, causing the holder to close and grip the tube firmly by means of a holder actuator provided at the welding station, and welding one end of the tube closed. The holder actuator is advantageously not conveyed with the holder but instead is provided at a fixed location at the welding station. 
     The holder may have a floor portion for preventing downward movement of the tube during conveyance. However, this is not necessary and the method preferably comprises sliding the tube on a slide plate during conveyance of the holder. It has been found, contrary to expectations, that the lower ends of the tubes are not damaged as a result of sliding along the slide plate. 
     Thus, as long as the tubes are of uniform lengths, the top end of the tube will always be the same height above the slide plate. Provided the welder is also fixed in position, the distance between the upper end of the tube and the welding unit will remain constant. This can ensure that the tube is in the correct position for welding. Thus it is strongly preferred that the multiplicity of tubes for use in the preferred embodiment of the present invention are of uniform length. 
     It is possible for the welded tubes (also referred to as “cans”) to be removed from the holders by being picked out, either by hand or by providing a further piece of apparatus to carry out this step. However, it is preferred that the slide plate be provided with a cut-out region, allowing cans to fall past the slide plate. The cans are then removed from the holders by the force of gravity. A preferred method therefore comprises conveying the welded tube to a release position where the slide plate is provided with a cutout region, allowing the can to fall through the cut-out region. 
     The holder can take a number of forms, as long as the tube is held correctly. For example, the tube could be held in an inflatable collar, which is normally deflated and can be inflated when it is necessary to hold the tube tightly. However, it is preferred that said holder is in the form of a pair of jaws which are biassed to an open position and can be selectively closed. This allows the tube to be inserted into the holder when they are in their open position, where the spacing between the jaws is relatively wide, and so facilitates insertion of the tube. This also allows for release of the tube by opening the jaws. 
     It is possible for the jaws themselves to be provided with means allowing them to be closed tightly around the tube. For example, a solenoid or a pneumatic cylinder could have its ends attached to the halves of the jaw, and would then be able to open or close the jaws e.g. at the welding station. However, as described above, a holder actuator is preferably provided at the welding station to close the jaws. This simplifies the construction of the jaws, and, where a plurality of sets of jaws are provided, involves the provision of just one holder actuator, rather than one for each set of jaws. 
     In a preferred method, the holder is in the form of a block slidably mounted in a recess in a holder support, said block being biassed outwardly by resilient means, e.g. compression springs, retained between said block and said holder support. This arrangement has the advantage that the holder can be closed tightly by pushing the block against the resilient bias, and this can be done by means such as a solenoid or an air cylinder. 
     Preferably, the block is retained on the holder support by bolts passing through the block and engaging in the holder support, the block being biassed outwardly by the resilient means, and the bolts retaining the block on the holder support against the resilient bias and thus serving to define the open condition of the block, which can be varied by tightening or loosening the bolts. When the block is in its open position, it is biassed away from the recess, and so the distance between the block and the recess is a maximum. This is the position where the tube will preferably be loosely held in the holder. The bolts can be tightened or loosened to change this spacing, for example to accommodate thicker or thinner tubes, which may be needed to make different types of capsule. 
     The spacing between the block and the recess is important, as it must be large enough to allow the tube to easily enter the holder, but sufficiently small to ensure that the tube is held as required. In particular, the tube must be held in such a way as to ensure that when the holder is closed tightly around it, it is in the correct position for welding. The spacing can be set by inserting a gauge member of known thickness into the gap and tightening the bolts until the jaws just contact the gauge member, but this is an awkward procedure and does not guarantee that the bolts will not be over tightened. Accordingly, it is preferred that a set screw is provided in said recess, said set screw defining a point beyond which said bolts may not be tightened. This ensures that the spacing of the block and the recess is set correctly. 
     Provision of the set screw to ensure correct spacing of the block and the recess is also considered to be of inventive merit, and so according to a further aspect of the invention there is provided an automated method of welding closed one end of each of a multiplicity of open-ended tubes suitable for use as brachytherapy capsules, comprising repeating the steps of receiving a tube in a holder mounted on a holder support, conveying the tube past a welding station where one end of the tube is welded closed in a welding operation, and releasing the welded tube, said holder being in the form of a block slidably mounted in a recess on the holder support and retained on said holder support by bolts passing through said block and engaging in said holder support, said block being biassed outwardly by resilient means acting between said block and said holder support, said bolts retaining said block on the holder support against the resilient bias and thus serving to define an open condition of the block, said open condition of said block being variable by means of tightening or loosening said bolts, and wherein a set screw is provided in said recess to define a point beyond which said bolts may not be tightened. 
     Since the holder will hold the tube while it is being welded, it is necessary for the holder to be formed from a material which has sufficient heat-resistance. It is preferred that the or each holder is formed from Elkonite. This material, an alloy of tungsten and copper, has good heat-resistance, but is easier to work than tungsten itself. 
     It is also preferred that the holder support, e.g. rotatable member, is formed from Elkonite. 
     The invention also extends to apparatus for carrying out the methods for welding closed one end of each of a multiplicity of tubes for use as brachytherapy capsules as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will now be provided by way of example only and with reference to the accompanying drawings, in which: 
         FIG. 1  is an overall perspective view of the preferred embodiment of the automated tube welder; 
         FIG. 2  is an enlarged perspective view of the feed line and the escapement assembly; 
         FIG. 3  is a perspective view of the escapement assembly; 
         FIG. 4  is a perspective view of the turret; 
         FIG. 5  is an enlarged exploded perspective view of the turret; 
         FIG. 6  is a perspective view of the table plate and slide plate; and 
         FIG. 7  is a cross-sectional view of a brachytherapy capsule or seed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a general perspective view of the preferred embodiment of the automated tube welder  10 . The titanium tubes which are used to form the capsules have a width of around 1.0 mm and a length of around 6.5 mm. The tubes arrive in a bowl  22  of a vibrating hopper. The vibration of the hopper serves to orientate the tubes, and they pass into a gravity feed line  30 . The tubes pass through to the lower end of the gravity feed line, where they are stopped by an escapement assembly  40 . The escapement assembly  40  allows only one tube through at a time, and retains the remaining tubes in a vertical stack in the feed line. The tube released by the escapement assembly falls into a jaw  52  of a generally circular turret assembly  50 . The turret  50  has four circumferentially equispaced jaws  52 , each of which can hold one tube, and the turret  50  is rotated to move the tube sequentially through a number of operating stations. The first station is the one where the tube is received by the jaws. The turret is then rotated by 90° twice, to bring the tube to a welding station  80  incorporating a welding unit  84 . A sensor  82  is located at or near the welding station, and as the turret rotates the sensor detects the presence or absence of a tube sticking up from the turret. The welding unit is only operated if the presence of the tube has been detected by the sensor assembly; if a tube is detected, then the upper end of the tube is welded closed by the welding unit. Once the welding process is started, it follows a preset welding program, which ends with a signal being sent back indicating that the welding process is complete. If the welding program is started and the weld sequence is not completed, the process will terminate with a weld error signal. If, on the other hand, no tube is detected, then the welding unit is not activated. The turret  50  is then rotated by a further 90° to bring the welded tube or can to an ejection station  90 , where it is ejected. The operation of the automated tube welder  10  is controlled by a control unit, which in the preferred embodiment is an Allen Bradley PLC 5/11 programmable logic controller, using RS Logix software from Allen Bradley. 
     The various parts of the tube welder will now be described in greater detail. 
     The bowl  22  of the vibrating hopper  20  receives the tubes, and delivers them lengthwise down an exit chute  24 . The width of the chute  24  is such that only one tube can pass down the chute at a time. This is achieved by having the diameter of the chute slightly greater than the diameter of the tubes (which is approximately 1 mm) but substantially less than the length of the tubes (which is around 6.5 mm), so that an in-line arrangement of the tubes results. Hoppers of this type are available from Service Engineering Inc., and the hopper will not be described further. 
     The upper end of the gravity feed line  30  is attached to the lower end of the exit chute  24 , and the tubes leave the exit chute  24  and enter the feed line  30  under gravity. The width of the teed line  30  is similar to that of the chute  24 , so that only one tube can pass down the feed line at any one time. The tubes thus form a vertical column in the feed line, stacked end to end. 
     In addition, the inner surfaces of the chute and the feed line are formed to be as smooth as possible. This reduces the friction between the tubes and the chute and feed line, and so reduces the chance of the tubes jamming during feeding. 
     At the lower end of the feed line  30  is the escapement assembly  40 , which ensures that the tubes leave the feed line one by one. The escapement assembly includes a plate  42 , which normally blocks the passage of tubes from the feed line  30 , such that the lowermost tube rests on the plate  42 . The escapement assembly also includes a resilient member  44 , which normally urges the tube just above the lowermost tube against the side of the feed line  30 , with sufficient force to prevent this tube from moving downwards when the tube below it moves, but not so much force that the tube will be damaged. 
     It will be appreciated that the walls of the tubes are very thin, so as to minimize any attenuation of the radiation from the radioisotope such as radioactive iodine, and it is preferred that the walls have a thickness of around 0.05 mm. Excessive force can damage the tubes, by crushing the walls towards each other, and this is obviously undesirable. For this reason, the resilient member is preferably in the form of a spring which is easily compressed. 
     The plate  42  and the resilient member  44  are moved by air cylinders, controlled by the control unit. 
     In operation, the escapement assembly  40  will normally have the plate  42  positioned so as to block the passage of tubes from the end of the feed line  30 . The stack of tubes will thus be supported on the plate. The resilient member  44  then presses the lowermost-but-one tube against the side of the feed line  30 , to prevent it from moving. The plate  42  is then moved to allow the lowermost tube to fall. It is possible for the plate to be formed with an opening which is brought into and out of registration with the opening at the bottom of the feed line, but it is preferred for the entire plate to be moved out of the way, to ensure that the lowermost tube is reliably dropped. 
     When the plate  42  is moved to allow the lowermost tube to drop from the feed line, it will be appreciated that the weight of all of the other tubes in the stack is supported by the tube which is pressed against the wall of the feed line  30  by the resilient member  44 . However, since each tube has a very low mass (of the order of 10 mg) the total mass which must be supported in this way is relatively small. 
     The plate  42  is then returned to its original position, and the resilient member  44  is withdrawn. This allows the entire stack of tubes to drop down, so that they are again supported on the plate  42 . Each time this cycle is repeated, one tube is allowed to drop from the bottom of the feed line  30 . The operation of the escapement assembly  40  is controlled by the control unit. In addition, an interlock in the escapement  40  prevents both the plate  42  and the resilient member  44  allowing the tubes to pass at the same time. 
     Directly below the plate  42  of the escapement assembly  40 , there is a funnel member  32 . The funnel has a passage therethrough to allow a tube to pass down it. The purpose of the funnel is to ensure that the tube which is allowed out of the feed line  30  by the escapement assembly  40  is directed accurately, rather than simply allowed to fall. This is important, as if the tube were to simply be dropped from a height above the jaws  52  of the turret assembly  50 , it could tumble out of its vertical orientation and as a result may not be properly received by the jaws  52 . 
     In addition, a sensor unit  46  detects the presence of each tube as it passes through the escapement assembly  40 . If the sensor fails to detect any tubes passing through the escapement assembly for a certain period of time during the operation of the apparatus; then an error message is generated by the control unit, and the user is informed of the location of the problem. The most likely causes for the error message being generated are that the tubes have jammed somewhere in the feed line  30  or that there are no tubes remaining in the hopper  20 . In either case, it is necessary for the operator of the apparatus to intervene. 
     As mentioned above, each tube that drops from the feed line  30  falls into a set of jaws  52  mounted on the turret assembly  50 . There are four such sets of jaws  52 , equispaced around the circumference of the turret  50 . The turret itself is formed from a flat generally circular plate  54 , rotatably mounted and driven by an indexing motor in such a way that it rotates in steps of 90°. The plate is made of a tungsten/copper alloy called Elkonite, which has a higher melting point than titanium. This is vital because a defined region of each titanium tube has to be at least partially melted during the welding process to form the weld, which involves relatively high temperatures. It is possible to form the turret from tungsten, rather than an alloy thereof, but tungsten is brittle and difficult to work, and thus forming the turret from tungsten presents a number of difficulties. In addition, Elkonite has the desirable property of heating up slowly but cooling quickly. 
     The turret  50  is rotatably positioned above a table plate  70 , shown in FIG.  6 . It will be seen from this Figure that the central region of the table plate  70  over which the turret  50  rotates is removed, to allow the shaft of the indexing motor to pass through the table plate  70  and be attached to the turret  50  in order to drive it. In addition, the part of the table plate over which the turret  50  rotates is formed as a recess which accommodates a slide plate  72 . The slide plate is formed of a material which has good wear resistance and good sliding qualities, and in the preferred embodiment is formed of aluminium bronze. The slide plate and the portion of the table plate  70  which it overlies are continuous around most of their circumferential extent, but have a cut-out region  74  in one portion. The purpose of this cut-out region will be described later. 
     Each set of jaws  52  comprises a block  56  which is accommodated in a recess  58  in the plate  54 . As best shown in  FIGS. 4 and 5 , two sets of jaws  52  are positioned at the ends of a first diameter of the plate  54 , and the other two sets are mounted at the ends of the diameter normal to the first diameter. Each block  56  is mounted on guide pins  60  so as to be slidable in the recess  58  towards and away from the centre of the plate along the diameter. The block  56  is held in place by two bolts  62  which extend through the block parallel to the diameter of the plate, and engage in holes formed in the plate. Springs  64  surround the bolts  62  and are positioned between the recess  58  and the block  56  to bias the block outwardly until the block comes into contact with the heads of the bolts. 
     The end of the block  56  which faces the recess  58 , and the end of the recess  58  which faces the block  56 , are spaced from each other, and the space between the end of the block and the end of the recess defines the region into which the tube falls. In order to adjust the size of the region into which the tube falls, the bolts  62  can be tightened against the springs  64  (to reduce the size of the region) or loosened, so that the springs  64  push the block outwardly (to increase the size of the region). 
     During the setting-up of the turret  50 , the bolts  62  are tightened to set the width of the jaws  52 . This can be done by putting a gauge member of known thickness into the jaws and tightening the jaws until they just contact the gauge. However, this is an awkward and inaccurate procedure. To ensure that the jaw spacing is set correctly, a set screw  66  is provided in the recess  58  to ensure that the bolts  62  cannot be tightened beyond a set position. However, the springs  64  and guide pins  60  are arranged so that the block  56  can still slide into the recess  58  beyond this point, to reduce the width of the jaws. 
     The ends of the block  56  and the recess  58  are both formed with a semicylindrical groove  68 . The radius of the groove  68  is very slightly less than the radius of the tube. When the bolts  62  have been tightened as far as they can, the distance between the ends of the block  56  and the recess  58  is set to be around 0.5 mm, which is less than the thickness of a tube. However, a tube can be easily accommodated between the semicylindrical grooves  68 , and indeed there is substantial play between the grooves  68  and the walls of the tube. 
     Rotation of the turret  50  brings the tube held in the jaws  52  firstly to a welding station  80 , located 180° away from the escapement  40 , and then to an ejection station  90  90° further on. 
     It will be noted that there is no station positioned at 90° from the escapement assembly. However, if the process requires, a station could be put here to carry out an optional step useful to prepare the tube for welding. For example, an air blast could be used to blow any debris away from the end of the tube. 
     The welding station  80  comprises a sensor  82 , a welding unit  84 , and a clamping air cylinder  86 . The welding unit  84  comprises a plasma arc welding torch, in which argon is used as an inert gas. Supply of current and argon gas to the welding torch is regulated by the welder power supply. The position of the welding head  88  of the welding unit can be adjusted in all three dimensions to ensure that it is properly aligned with the upper end of the tube. 
     The sensor assembly  82  detects the presence or absence of a tube in the jaws  52  of the turret  50 . An optical detection system is used, and a signal from the sensor  52  is passed to the control unit. If the sensor detects a tube, then the welding unit  84  welds the upper end of the tube closed. If no tube is detected, then the welder is not operated. Of course, the sensor  82  could be positioned at the previous (empty) station if desired. 
     The clamping air cylinder  86  serves to close the jaws  52  of the turret  50 . The piston of the clamping air cylinder  86  can be operated to move horizontally towards the centre of the turret when a set of jaws  52  are positioned at the welding station  80 . When the piston moves in this manner, it applies a force to the block  56 , pushing it into the recess  58 . This clamps the tube between the groove  68  in the block  56  and the groove  68  in the recess  58 , and thus ensures that the tube is accurately positioned for welding. It also ensures that there is good contact between the tube and the jaws, which is necessary for successful welding. The operation of the clamping air cylinder  86  is controlled by the control unit. 
     It will be appreciated that since the faces of the block and recess come together after the block has travelled a very short distance into the recess, the block has limited travel. As a result, it is extremely unlikely that the tube will be crushed or otherwise damaged by the jaws while it is being held for welding. 
     A quarter of a revolution further on is the ejection station  90 . This station is positioned adjacent the parts of the table plate  70  and slide plate  72  which have the cut-out region  74 . As the turret  50  rotates, the welded tube or can is brought to a position above the cut-out region  74 . Normally, since the can is only held loosely in the jaws  52 , it will fall from the jaws  52  under the influence of gravity, and will pass through the cut-out region  74  and down a delivery chute  92  into a collection bin (not shown) 
     On occasion, the can does not fall from the jaws  52  under gravity when it is positioned above the cut-out region  74 . For this reason, an ejection unit is provided, comprising a vertical air cylinder  94 , with the piston of the air cylinder biassed into its uppermost position by means of a spring. An ejector block  96  is attached to the lower end of the piston. When air is supplied to the cylinder  94 , the piston is pushed downwardly, and the lower surface of the ejector block  96  contacts the upper end of the can. This suffices to push the can downwardly out of the jaws  52  into the delivery chute  92 , and ensures that the cans are reliably ejected. Supply of air to the cylinder  94  is regulated by the control unit. 
     Further, although not shown in the diagrams, the welder is entirely enclosed by a plastic safety cage. The safety cage reduces the risk of operator injury (whether through welder burns or through trapping a finger or similar between moving parts). Further, the safety cage allows excess argon to build up near the welder to improve welding. In addition, the safety cage protects the welding area from external air currents, and reduces the amount of airborne contaminants near the welder. 
     The safety cage is provided with doors, to allow access to the welder when necessary (for example, when charging the hopper  20  with tubes). The doors have switches, which pass a signal to the control unit indicating whether the doors are closed or not. For automatic operation of the welder, the doors must be closed. 
     The operation of the automated tube welder will now be described. Tubes are placed into the bowl  22  of the vibratory hopper  20 , and are delivered down the exit chute  24  into the gravity feed line  30 . The tubes form a vertical stack, with the lowermost tube resting on the plate  42  of the escapement assembly  40 . The turret  50  is positioned such that a set of jaws  52  is aligned beneath the bottom of the funnel  32  of the gravity feed line  30 , to receive a tube therefrom, and is held stationary in that position. The block  56  of the jaws  52  is biassed outwardly by the springs  64 , and so the space between the grooves  68  in the block  56  and the recess  58  is somewhat larger than the cross-section of the tube. This facilitates entry of the tube into the jaws  52 . 
     The resilient member  44  of the escapement assembly  40  then urges the lowermost-but-one tube in the stack against the side of the feed line  30 . The plate  42  of the escapement assembly  40  is then moved to allow the lowermost tube in the stack to fall through the funnel  32  into the jaws  52 . The tube released in this way falls under gravity into the gap between the grooves  68  in the block  56  and the recess  58 , until its lower end comes into contact with the slide plate  72  beneath the turret  50 . The length of the tube is slightly greater than the thickness of the turret, and so the upper end of the tube stands proud of the turret. 
     The plate  42  of the escapement assembly  40  is then returned to its original position, and the resilient member  44  releases the tube which it was holding, which then drops down to rest on the plate  42 . 
     The turret is then rotated by 90°. The tube is carried along with the jaws  52 , and its lower end slides on the slide plate  72 . The weight of the tube is sufficiently small for the lower end of the tube to avoid being damaged as it slides on the slide plate  72 . As there is no station positioned 90° from the escapement, no operations are carried out on the tube at this stage in the process. In the meantime, while the turret  50  is stationary, the escapement assembly  40  is operated again, to drop a tube into the second set of jaws. 
     The turret  50  is again rotated by 90°. The first tube is brought to the welding station  80 , where its presence is detected by the sensor  82 . The second tube is moved to the empty station. 
     A signal that a tube is present is passed to the control unit, which then prepares the welding unit  84  to make a weld. At the welding station  80 , the air cylinder  86  is activated by the control unit to clamp the first tube firmly in position in the jaws  52 , and while it is held in this way its upper end is welded closed. Following welding, the pressure in the air cylinder  86  is released, and the jaws  52  spring back to their previous position. Of course, if no tube is present in the jaws  52  (as will occur in the stages before the first tube reaches the welding station  80 ) then the sensor  82  passes a signal to this effect to the control unit, and the welder is not activated. 
     It will be appreciated that during the welding of the upper end of the tube, it is important for the turret  50  to be held stationary, to ensure that the tube remains beneath the welder. During this stationary period, the escapement assembly  40  is operated again to drop a tube into the third set of jaws  52 . 
     The turret is again rotated by 90°. The first tube, by now welded closed at one end, is brought to the ejection station  90 , where it falls into the delivery chute  92 , either under its own weight or after being tapped by the ejector block  96 . The second tube is brought to the welding station  80 , where its presence is detected, the jaws  52  are closed by the air cylinder  86  and the end of the second tube is welded closed. The third tube is carried along by the jaws  52  but nothing is done to it. The escapement assembly  40  is again operated while the turret  50  is stationary during the welding process to drop a tube into the fourth set of jaws  52 . 
     The turret is again rotated by 90°. This brings the second tube to the ejection station  90 , the third tube to the welding station  80 , and the fourth tube to the empty station. The escapement assembly  40  is operated again to drop a tube into the original set of jaws  52 , which are now empty. 
     The process then continues, with each tube being dropped into a set of jaws  52  and conveyed through the welding station  80 , where one end of the tube is welded  30  closed, to the ejection station  90 , where it is ejected. The apparatus thus continuously and automatically welds closed one end of each of the titanium tubes. 
     The time taken to complete a weld is the critical time in the cycle, which determines how quickly the welder can operate. At present, completion of a weld takes approximately three seconds, and rotation of the turret by 90° takes roughly two seconds. Each 90° step delivers a welded tube to the ejection station  90 , and the production rate is around  600  welded tubes or cans per hour. 
     It will be noted that, although the welding unit  84  is operated in each cycle, each set of jaws  52  only holds the tube being welded every fourth cycle. This allows the material forming the turret  50  to be less heat-resistant than would be necessary if it were always the same part holding the tube being welded. In particular, it allows the turret to be formed from a tungsten alloy rather than tungsten itself. 
     The welded titanium tubes or cans produced by this process are received in a collection bin. They are subjected, to a visual inspection, and those that are acceptable are filled with the appropriate radioisotope such as silver rods or resin balls coated with radioactive iodine. The capsules then have their other end welded closed. At present, the inspection, insertion and welding steps are carried out manually. However, it will be appreciated that even if these steps are carried out manually, having the first end of the tube welded closed automatically significantly reduces the amount of work and time involved in the production of the capsules. In addition, the automated process produces more consistent results than the manual welding of cans. 
     It is apparent that many modifications and variations of the invention as hereinabove set forth may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of example only, and the invention is limited only by the terms of the appended claims.