Abstract:
A device for the discharge of a pasty two-component mixture includes a supply container with two chambers, each chamber for containing one component of the two-component mixture and having an outlet stub for connecting to a mixer unit. The mixer unit includes a coupling end having two inlet stubs which form two channels leading into a mixer element, and on an opposite end, an outlet opening. A rotatable holding shell is provided in a substantially surrounding relationship to the two outlet stubs and includes a recess of a shape substantially identical with the cross sectional shape of the coupling end of the mixer housing.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device for discharging a pasty two-component mixture, e.g. a mixture of a dental impression substance and a catalyst for initiating or accelerating the hardening. 
     SUMMARY OF THE INVENTION 
     For applying multi-component substances in mixed form, discharge devices are known which comprise a supply container with at least two chambers for respectively one of the pasty components of the mixture. Assigned to each chamber is an outlet stub for discharging the pasty substance upon application of pressure. To allow discharge of the pasty substances in mixed form, the outlet stubs of the known devices have a mixer unit mounted thereon, comprising a mixer housing including a substantially tubular portion and a coupling end for attachment to a supply container. In the tubular portion of the mixer housing, a mixer element is arranged by which the components, which have been supplied via separate channels, are subjected to a turbulence and mixing effect. Said channels are formed in the coupling end of the mixer housing and extend from the inlet stub of the coupling end to the mixer element. A device of this type is known e.g. from EP-A-0 730 913. 
     Depending on the composition of the pasty components, their mixing will result in the respectively desired chemical reactions. For instance, if one of the two pasty components is a dental impression substance, the adding of a second component initiates and accelerates the polymerisation of this impression substance. Should this second component (also referred to as a catalyst) penetrate, via the mixer unit, from the one chamber into the other chamber of the supply container or should already the outlet stub of the chamber for the dental impression substance by contaminated with the catalyst, the whole device will be rendered useless, which, depending on the extent to which the chambers are filled, will cause a considerable loss of expensive material. Therefore, the two component flows have to remain separated from each other for the longest possible stretch of their moving paths and be brought together only within the tubular portion of the mixer housing. 
     In the known discharge device for pasty two-component mixtures, the mixer housing is detachably locked to the supply container by means of a holding shell. The holding shell is supported for rotation on the mixer housing and is provided with radially outward locking projections formed in the manner of bayonet locks, which can be rotated to move under corresponding grip-over projections of the supply container. 
     The invention further relates to a device for opening a tube bag containing a pasty substance. Said device is further provided to guide the pasty substance out of the tube bag. 
     The pasty substances are offered in rigid containers or cartridges with displaceable bottom walls which are advanced by plunger elements arranged to be advanced by hand or by means of an engine in order to exert pressure on the pasty substances, thus discharging the substances from the cartridges and the cartridges or containers. In recent years, for reducing the packing material, the pasty substances have been increasingly offered in tube bags. For discharging the pasty substances, the tube bags have to be opened, preferably on their end sides. This is preferably performed by means of stationary piercing mandrels or the like cutting elements arranged on the end wall of a holding cap surrounding the end side of the tube bag and comprising a discharge stub. From EP-A-0 787 655, it is known to provide the inner projection of the holding cap with a piercing tube resiliently biased in the direction of the end face of the end of the tube bag. The as of yet known systems for automatic opening of tube bags are in need of improvement in so far as, for opening the tube bag, it is always required to exert pressure onto the tube bag and thus also on the pasty substance in the tube bag. Thus, the process of opening the tube bag is accompanied by the discharge of the pasty substance. 
     As already mentioned above, discharge devices are used intermittently, the period between two successive uses being possibly so long that the mixture in the mixer unit will harden. As a result, the mixer unit has to be removed and replaced by a new one before the discharge of a mixed substance can be continued. Thus, the mixer unit is a disposable component, making it desirable to keep the number of components of this disposable mixer unit as low as possible. 
     It is an object of the invention to provide a device for the discharge of a pasty two-component mixture, comprising a mixer unit with a reduced number of parts. 
     According to the invention, to achieve the above object, there is proposed a device for the discharge of a pasty two-component mixture comprising: 
     a supply container with two chambers for respectively one pasty component of the mixture, the supply container being provided with a respective outlet stub for each chamber, 
     a mixer unit comprising a tubular mixer housing with a mixer element arranged therein, the mixer housing comprising a coupling end to be coupled to the supply container and having a two inlet stubs and two channels for the pasty components of the mixture leading to the mixer element, and comprising, on its opposite end, an outlet opening for the mixture, the inlet stubs being adapted for insertion into the outlet stubs or vice versa, and 
     a holding shell for the coupling end of the mixer housing arranged for rotation on the supply container, the holding shell surrounding the two outlet stubs and comprising a recess of a shape at least in parts substantially identical with the cross sectional shape of the coupling end of the mixer housing, the holding shell being rotatable between a receiving rotational position in which the recess is oriented corresponding to the orientation of the coupling end of the mixer housing in the state where the inlet stubs are connected to the outlet stubs of the supply container, and 
     a locking rotational position in which at least a part of the recess edge of the holding shell is in engagement over parts of the coupling end of the mixer housing. 
     In the inventive discharge device, the holding shell holding the mixer unit is arranged not on the mixer housing but on the supply container. On this supply container, the holding shell is supported for rotation about its longitudinal axis. The holding shell surrounds the two outlet stubs of the supply container and is provided, on its front end facing away from the supply container, with a recess which in the first rotational position of the holding shell (receiving rotational position) allows the mixer housing to be mounted onto the outlet stubs of the supply container, while, in a second rotational position (locking rotational position) the edge of the holding shell delimiting the recess has been moved over partial regions of the coupling end, e.g. over radially projecting flange portions of the coupling end of the mixer housing, thus locking the mixer housing to the supply container. Therefore, the holding shell is not a component of the mixer unit anymore and thus will be preserved for the whole period during which the discharge device is used for application of the two-component mixture. 
     In an advantageous embodiment of the invention, the coupling end is provided with two diametrically opposite and radially extending projections, while the holding shell is formed with a recess of a shape identical with the cross sectional shape of the coupling end at the level of the projections of the latter. Particularly, the coupling end of the mixer housing is of a substantially oval shape in the region of the radial projections. The recess of the holding shell, when in its receiving rotational position, is arranged relative to the outlet stubs of the supply container in the same manner as the inlet stubs of the mixer housing relative to the outer projections. Thus, the coupling end of the mixer housing can be moved through the recess of the holding shell until the inlet stubs of the mixer housing have been received by the outlet stubs of the supply container, or vice versa. In this position, the recess edge of the holding shell is arranged above the outer projections on the coupling end of the mixer housing so that, by rotating the holding shell into the locking rotational position, regions of the recess edge engage the outer projection from above. Suitably, the holding shell is in both of these rotational positions secured against unintended further rotation or reverse rotation, which is preferably accomplished by a corresponding locking effect. For the handling of the holding shell, it is further suitable to allow rotation of the holding shell only between its receiving rotational position and its locking rotational position. In this regard, it would be convenient to realize the limitation of the rotational movement by corresponding stopper elements on the holding shell and the supply container. 
     For this purpose, the invention proposes a device for opening a tube bag containing a pasty substance, which device does not necessarily have to be realized in combination with the above described mixer holding device although such a combination is preferable, and which comprises 
     a receiving cap for receiving an end side of the tube bag, which end side comprises an end face, the receiving cap comprising an end wall forming an abutment plane for the end face of the tube bag, and an edge extending from the end wall, 
     an opening formed in the end wall and surrounded by a stub projecting from the end wall, and 
     a piercing tube guided in the stub for axial displacement. 
     In this device, it is provided according to the invention that the piercing tube is manually displaceable from a retracted position in which the piercing tube does not extend beyond the abutment plane into an advanced position in which in the piercing tube extends beyond the abutment plane. 
     The inventive device is provided with a piercing tube which is guided to be axially displaced in the discharge stub of a receiving cap. In the opened state of the tube bag, the pasty substance will be discharged through this stub to be applied directly, or, in case of a two-component material, to be mixed with another component of the pasty substance. The discharge stub is arranged to radially project from the end wall of the receiving cap, with a continuously surrounding edge projecting from the end wall in a direction opposite to the discharge stub. Thus, the receiving cap surrounds one of the two end sides of the tube bag whose end face abuts the end wall of the receiving cap on an abutment face formed by the latter. The piercing tube of the inventive device is manually displaceable from a retracted position to an advanced position. This displacement can be performed either directly by hand in that the piercing tube is manually shifted within the stub, or indirectly by manually moving an operating element which in turn via an abutment face is in abutment on the piercing tube and thus, when being moved itself, will pull along the piercing tube. In both cases, the piercing tube can be displaced from the retracted position in which it does not protrude beyond the abutment plane of the receiving cap, formed by the end wall, into the advanced position in which is projects beyond the abutment plane and thus, with the receiving cap receiving the tube bag, is immersed into the end face of the latter and opens the tube bag. The piercing tube can be arranged in the stub ex factory and will thus be located in its retracted position. By way of alternative thereto, it can be provided that the piercing tube is inserted into the stub by the user. In both cases, it is suitable if the piercing tube in its retracted position is secured against unintended movement from its retracted position by a locking effect or the like. This is advantageous to the user to whom, under the tactile aspect, the retracted position will be perceivable as a defined position of the piercing tube. 
     The advantage of the inventive opening device resides in that the process of opening the tube bag is separated from the process of discharging the pasty substance from the tube bag. Thus, the tube bag will not be subjected to an increased pressure when being opened, preventing an undesired escape of pasty substance from the tube bag when the latter is being opened. This facilitates the handling of the tube bags. 
     The inventive device is useful both for receiving caps for a single tube bags and for combined receiving caps for a plurality of tube bags, e.g. two tube bags. In the latter case, the receiving cap comprises a plurality of discharge stubs respectively provided with one piercing tube. These piercing tubes can be connected to each other to be driven in common into the respective tube bags. Of course, it can also be provided that the piercing tubes are not connected to each other so that each tube bag can be “tapped” separately. 
     It is particularly convenient if the piercing tube in its retracted position projects beyond the discharge end facing away from the end wall of the receiving cap. Notably, in this case, the axial displacement of the piercing tube can be effected by a mere movement into the stub. Once the piercing tube has been completely moved into the stub, the advanced position has been reached and the piercing tube has been driven into the tube bag. 
     Suitably, it is provided that the piercing tube in its advanced position is secured against undesired return movement in the direction of the retracted position, and in its retracted position is secured against undesired movements into the advanced position. As already mentioned above, this can be realized by locking the piercing tube in or on the stub. As an alternative thereto, the piercing tube, which in its retracted position projects from the discharge end of the stub, can be protected by a removable protective cap which can be pushed over the stub and fixed to the stub. This protective cap serves for protection during shipment and, prior to insertion of the piercing tube, is removed, e.g. unscrewed, in order to expose the piercing tube. An alternative to the protective cap resides in a spacer arranged between the end of the piercing tube projecting from the stub, and the discharge end of the stub. Particularly, this spacer is formed as a resilient clip surrounding the piercing tube. This resilient clip can be laterally withdrawn from the piercing tube so that the latter can then be axially displaced. The spacer is supported between an outer projection on the piercing tube and the discharge end of the stub. 
     If the piercing tube is moved not directly manually but indirectly manually via an operating element, this operating element can be utilized as a safety means for protecting the piercing tube from undesired movements from the retracted position into the advanced position. The operating element is suitably provided in the manner of a shell element arranged on the stub and displaceable axially along the stub. This shell element can be locked, in two axial positions corresponding to the retracted position and the advanced position of the piercing tube, to the stub and respectively the receiving cap against undesired axial movements. Advantageously, the shell element is formed as a screw element comprising an inner thread portion adapted for engagement with an outer thread portion on the stub. By means of these threaded portions, the shell element can be axially displaced for driving the piercing tube into the tube bag. Both the shell element and the stub are provided, on both sides of their threaded portions, with regions which are free of threads. As long as the two threaded portions are not in threaded engagement with each other, a rotation of the shell element will not lead to an axial displacement. Besides, the threaded portions, while not in mutual threaded engagement, preclude a linear axial displacement of the shell element. Thus, the latter is secured against undesired axial movements before and after the threaded engagement of the two threaded portions. 
     Embodiments of the invention will be described in greater detail hereunder with reference to the Figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a discharge device for pasty two-component mixtures (particularly dental impression substances with catalyst), with the mixer unit shown in the state briefly before being coupled to the supply container, 
     FIG. 2 is a plan view of the front end of the supply container to which the mixer unit is fastened, wherein the holding shell, arranged on this end of the supply container, is arranged in its receiving rotational position, 
     FIG. 3 is a lateral view, partially in section, of the discharge device with the mixer unit coupled thereto, 
     FIG. 4 is a sectional view, taken along the line IV—IV of FIG. 3, of the holding shell in its locking rotational position, 
     FIGS. 5 to  7  are views of a further embodiment of a device for the simultaneous opening of two tube bags, with the device shown in different operational states, 
     FIGS. 8 and 9 are views of a third embodiment of an opening device for two tube bags, with the device again shown in different operational states, and 
     FIGS. 10 and 11 are views of an alternative embodiment of the device according to FIGS.  8  and  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a partially sectional lateral view of a discharge device  110  for pasty two-component mixtures. The discharge device  110  comprises a rigid supply container  112  including two substantially cylindrical chambers  114  arranged to receive respectively one pasty component. On the front end  116 , the supply container  112  comprises an end wall  118  with two outlet stubs  120  projecting therefrom, each of them corresponding to one of the chambers  114 . Arranged around the outlet stubs  120  is a receiving collar or a holding shell  122  arranged for rotation on the end wall  118  of supply container  112 . On its front end  124  projecting beyond outlet stub  120  and facing away from end wall  118 , the holding shell  122  is formed with a recess  126  provided as an opening surrounded on all sides by a recess edge  128 . 
     Into the holding shell  122 , there is inserted the coupling end  130  of a mixer unit  132  comprising a mixer housing  134  with a tubular portion  136 . The tubular portion  136  has a mixer element  138  arranged therein. Tubular portion  136  has an outlet end  140  while the coupling end  130  comprises two inlet stubs  142  with separate channels  144  extending therefrom to mixer element  138 . 
     As particularly evident from the plan view according to FIG. 2, the opening  1276  is of a substantially oval shape. The coupling end  130  of mixer housing  134  is shaped corresponding to this opening shape. Thus, with the holding shell  122  oriented according to FIG. 2, the mixer unit  132  can be moved by its coupling end  130  through the opening  126  until the inlet stubs  142  have been received by the outlet stubs  120 . By subsequent rotation, the opening edge  128  of holding shell  122  is caused to grip over the two diametrically opposite and radially extending shoulder faces  146  in the transition region between the tubular portion  134  and the flange-like coupling end  130  of mixer unit  132 . This locking position is shown in FIGS. 3 and 4. In this turned position of holding shell  122 , the latter will thus have its opening edge  128  engage over the coupling end  130  of mixer unit  132  and will in this manner hold the mixer unit  132  in the operating state connected to supply container  112 . 
     In the presently described discharge device, only the mixer unit  132  without the holding shell  122  has to be designed as a disposable article. This reduces the volume of disposable material for the mixer unit  132 , which has a favourable effect on the environmental friendliness and, besides, on the production costs. 
     From FIGS. 2 and 4, it is further evident that the holding shell  122  in its two rotational end positions which in this embodiment are displaced by 90° relative to each other, is secured against respectively a further rotation. This is realized by a radially extending stopper element  148  on the holding shell  122 , designed to abut against a respective counterpart element  150  arranged on end wall  118 . The two counterpart elements  150  are arranged diametrically opposite each other and engage over an annular flange  152  of holding shell  122  so that, in addition to their function as rotation limiting elements, they provide for a rotatable support of holding shell  122  on supply container  112 . 
     By lending a suitable shape to the opening  126  of holding shell  122 , it is possible to encode the interface between mixer unit  132  and supply container  112  so that mixer unit  132  can be mounted only in case of a specific orientation of mixer unit  132  and holding shell  122 . This coding can also be realized by additional engaging projection and recess elements on mixer unit  132  and supply container  112 . For instance, the outlet stubs  120  and thus also the inlet stubs  142  can have different diameters or shapes. 
     The mixer interface has been described above as used in connection with a rigid supply container with a static mixer but is useful also for dynamic mixers provided with a rotatably driven mixer shaft. Further, the coupling of the mixer can be used also in a pressure container for pasty material accommodated in tube bags. Systems of this type require special opening mechanisms for opening the tube bags. Such a tube bag opening system will be described hereunder. 
     FIGS. 5 to  7  illustrate a first embodiment of a device  10  for opening two tube bags. The device  10  comprises a receiving cap  12  provided with an end wall  14  and an edge  16  projecting therefrom. Formed in the end wall  14  are two openings  18 , 20  in whose region a stub  22  projects from end wall  14  in a direction opposite to edge  16 . The stub  22  is formed with two channels  24 , 26  starting from the openings  18 , 20  and extending to the discharge end  28  of stub  22  facing away from end wall  14 . Between the two openings  18 , 20 , end wall  14  comprises a partition wall  30  projecting from end wall  14  in the direction of edge  16 . In this manner, two receiving chambers for two tube bags  32 , 34  are provided, each of them containing a pasty mass  36 , 38 . The end sides  40 , 42  of the two tube bags  32 , 34  are surrounded by the receiving cap  12 , or the edge  16  and the partition wall  30 , with the end faces  44 , 46  of the tube bags  32 , 34  abutting the end wall  14  of the receiving cap along an abutment plane  47 . The tube bags  32 , 34  can be connected to receiving cap  12 , which is performed e.g. by bonding. 
     Each channel  24 , 26  has a piercing tube  48 , 50  arranged therein which has a bevelled and thus pointed piercing tube end  52 , 54 . The two piercing tube tubes  48 , 50  are arranged for axial displacement in the channels  24 , 26  and in their retracted position according to FIG. 5 project beyond the discharge end  28  of stub  22 . On their ends  56 , 58  facing away from the piercing tube ends  52 , 54 , the two piercing tube tubes  48 , 50  are connected to each other as illustrated at  60 . First, the piercing tube tubes  48 , 50  are provided to open the tube bags  32 , 34  by penetrating into the end faces  44 , 46 , and, second, they shall allow the pasty substances  36 , 38  to pass for discharging them in the opened state of the tube bags  32 , 34 . 
     Arranged in stub  22  is a screw cap  62  serving as an operating element for axial displacement of the piercing tube tubes  48 , 50 . The screw cap  62  is substantially cylindrical and is provided, on one end  64  facing towards the receiving cap  12 , with an inner thread portion  66 . On the other end  66 , the screw cap  62  is provided with an inner shoulder  70  abutting a surrounding radial flange of the two piercing tube tubes  48 , 50 . Between the inner shoulder  70  and the inner thread portion  66 , the screw cap  62  is formed smooth and without projections on its inner side. 
     In the central region along its radial extension, stub  22  comprises an outer thread portion  74  adapted to be brought into threaded engagement with the inner thread portion  66  of screw cap  62 . Above and below the outer thread portion  74  of stub  22 , the latter comprises smooth, projection-free regions of a similar or slightly larger axial extension as compared to the axial extension of the inner thread portion  66 . At its front end  28  facing away from receiving cap  12 , stub  22  is provided with an external securement projection  76 . 
     In the production process, the screw cap  62  is shifted via the securement projection  76  onto the stub  22  until the inner thread portion  66  is located between the securement projection  76  and the outer thread portion  74  of stub  22  (see FIG.  5 ). In this situation, the piercing tubes  48 , 50 , which have been inserted into the channels  24 , 26  before the screw cap  62 , are arranged in their retracted position in which their piercing tube tips  52 , 54  do not project beyond the abutment plane  47 . The piercing tube mandrels  52 , 54  will remain in this retracted position also when the screw cap  62  is subjected to a force in the axial direction since the inner thread and outer thread portions  66 , 74  prevent movement of cap  62 . Only when rotating the screw cap  62  while simultaneously applying slight axial pressure, the screw cap  62  is moved in the axial direction since the two thread portions  66 , 74  are brought into or are in threaded engagement (see FIG.  6 ). By this axial movement of screw cap  62 , also the piercing mandrels  48 , 50  are axially advanced so that their piercing tips  52 , 54  will pierce into the end faces  44 , 46  of the tube bags  32 , 34 . The piercing mandrels  48 , 50  are completely in their advanced position in which their piercing tips  52 , 54  extend beyond the abutment plane  47  when the two thread portions  66 , 74  have again been brought out of their mutual engagement by further rotation of the screw cap  62  (see FIG.  7 ). In this situation, an axial displacement of the screw cap  62  is in turn prevented by the mutually blocking thread portions  66 , 74  so that the piercing mandrels  48 , 50  are secured in their advanced position. 
     Now, as indicated in FIG. 7, it is possible to connect to the piercing mandrel  48 , 50  a dynamic or static mixer  77  for mixing the pasty material  36 , 38  passing through the piercing mandrels  48 , 50 . The mixer  77  is held on the screw cap  62 , notably in the same manner in which the mixer unit  132  is held by means of the holding shell  122  to the supply container  112  according to FIGS. 1 to  4 . Thus, the screw cap  62  fulfils a dual function, in that, starting from FIG. 6, it is first used for the advancing of the piercing mandrels  48 , 50  into the tube bags  32 , 34  (FIGS. 7 and 8) and then, by corresponding rotational positioning, will bring about the “correct orientation” of its opening  75  relative to the coupling end  73  of mixer  77  so that this coupling end  73  can be inserted into the screw cap  62  from above. Subsequently, the screw cap  62  is rotated to allow for the fixation of the mixer  77  in the manner of a bayonet lock as indicated in greater detail in FIGS. 1 to  4 . These rotational movements of the screw cap  62  are performed in the position according to FIG. 7 in which the screw cap  62  is free of the threaded engagement with the stub  22 . 
     The arresting of the screw cap  62  in its receiving rotational position and the locking rotational position, and the limiting of its rotation, as described in connection with the holding shell  122  according to the embodiment of FIGS. 1 to  4 , can be realized by locking the screw cap  62  to the receiving cap  12 . Since the axial distance of the screw cap  62  from the receiving cap  12  when screwing the latter onto stub  22  for advancing the piercing mandrels  48 , 50 , is different from the position according to FIG. 7, the locking and the limiting of movement can be provided by such a design that locking projections and recesses required for this purpose as well as stoppers will cooperate only when the thread portions  66 , 74  according to FIG. 7 are out of engagement. 
     FIGS. 8 and 9 show an alternative embodiment of the opening device  10 ′. In as far as the individual parts of this device  10 ′ correspond to those according to FIGS. 5 to  6  or are identical in function, such parts are provided with identical reference numerals with prime symbols. In the device  10 ′, a separate receiving cap  12 ′ with an end wall  14 ′ and a surrounding projecting edge  16 ′ is provided for each tube bag  32 ′, 34 ′. Each receiving cap  12 ′ comprises a stub  22 ′ formed with a channel  24 ′ or  26 ′ extending therethrough. These two channels  24 ′, 26 ′ in turn have to piercing tubes  48 ′, 50 ′ arranged therein which in the retracted position according to FIG. 8 project beyond the ends  28 ′ of the stubs  22 ′. The piercing tubes  48 ′, 50 ′ are connected to each other at  60 ′ on their ends  56 ′, 58 ′ facing away from receiving cap  12 ′. For securing the piercing tubes  48 ′, 50 ′ against undesired movements from their retracted positions according to FIG. 8, the device  10 ′ comprises a protective cap  78  which is detachably connected to stub  22 ′; in FIG. 8, this condition is realized by corresponding grip-over and grip-under projections  80 , 82  on the stub  22 ′ and the cap  78 , respectively. By rotating the cap  78 , these projections  80 , 82  can be released from their mutual engagement so that the cap  78  can be removed. By manual pressure on the ends  56 ′, 58 ′ and respectively the connection  60 ′ of the two piercing tubes  48 ′, 50 ′, the latter can be axially moved in the channels  24 ′, 26 ′ beyond the abutment plane  47 ′ and into the tube bags  32 ′, 34 ′ (see FIG.  9 ). Subsequently, the mixer is mounted onto the piercing tubes  48 ′, 50 ′ while the mixer is secured on the stubs  22 ′ by corresponding securement elements. 
     FIGS. 10 and 11 show a third embodiment of a device  10 ″ which is designed as an alternative to the device  10 ′. The difference to the device  10 ′ resides in the securement of the piercing tubes  48 ′, 50 ′ against undesired movements from the retracted position into the advanced position. In the variant according to FIGS. 10 and 11, use is made of a spacer  84  formed as a resilient clip, externally surrounding the piercing tubes  48 ′, 50 ′ projecting from the ends  28 ″ of the stubs  22 ″ and thus being arranged between the surrounding outer flange  72 ″ of the piercing tubes  48 ″, 50 ″ and the end  28 ″ of the stubs  22 ″ (see FIG.  11 ). The clip  84  is formed as a two-legged bracket whose two legs  86  have their mutually opposite longitudinal edges  88  abut on the flange  72 ″ on the one hand and on the ends  28 ″ of the stubs  22 ″ on the other hand. By lateral withdrawal of the clip  84 , the piercing tubes  48 ″, 50 ″ are released and thus can be axially advanced by pressing them down. 
     Although a preferred embodiment of the invention has been specifically illustrated and described herein, it is to be understood that minor variations may be made in the apparatus without departing from the spirit and scope of the invention, as defined the appended claims.