EP0527676B1 - Single-use injection nozzle for straw filling machine, especially for artificial insemination of animals and conservation of biological products - Google Patents

Description

The invention relates to a single-use injection nozzle for a filling machine, in biological product, of tubes commonly known as flakes, formed of tubular sections of polymeric material of millimeter diameter internally packed, near an end, d '' a composite stopper comprising, between two porous pads, a volume of gelling powder in contact with an aqueous liquid, the filling being carried out by gripping the flake between two nozzles provided with seals bearing on the end edges, a nozzle d suction connected to a vacuum source and on the flake edge adjacent to the composite plug, said injection nozzle being provided with a flexible conduit intended to be immersed in a balloon containing the biological product, this injection nozzle comprising a rigid tube, hereinafter called the needle, capable of engaging inside the flake and crimped axially in a body which presents two externally reference surfaces respectively defining the centering and the longitudinal position of the injection nozzle with respect to a nozzle support.

Such a nozzle is described in document FR-A-2 651 793.

The so-called glitter tubes in question here have been abundantly described in the art, since patent document FR-A-995 878 of September 20, 1949. Their use has since experienced considerable development, parallel to that of insemination artificial animals, especially cattle.

This description will focus on the insemination of cattle, the practice of which has spread since the beginning and which can be considered typical, although artificial insemination is practiced on equines, sheep, pigs, rabbits, gallinaceous animals, and in aquaculture, this list is not exhaustive.

The filling of the straws is subject to two fundamental requirements: on the one hand, this filling must be carried out under conditions of aseptics usual in veterinary practice, and which allow a reliable identification of the male donor of semen, and on the other hand the manipulations must be determined for large series operations. On this last point, to fix orders of magnitude, a bull's ejaculate can fill up to around 800 flakes, and an artificial insemination center can operate on around sixty bulls per day.

We will quickly go over the harvesting process, quality control of the sperm harvested, and dilution; it should however be specified that the receptacle or balloon from which the sperm will be drawn will contain the volume necessary to fill the batch of glitter (up to 800), the latter having an individual capacity of approximately 1/2 cm3.

We have mentioned above the process of filling the flakes by suction of the diluted semen through the flexible duct, the injection nozzle and the flake body, thanks to the depressurization of the assembly through the nozzle. suction and the composite stopper, which remains permeable to air as long as the powder, included between the two porous pads has not gelled by contact with an aqueous liquid, in this case, the semen dilution liquid. When the straw is filled, the composite stopper closes the end of the straw.

On the machine, the flakes are placed in a hopper, from where a distributor introduces them into calibrated notches equidistant from a conveyor belt, the flakes being perpendicular to the direction of advance of the belt. The carpet advances in steps, comprising, according to the desired rate, one or three notches. At a first work station, nozzles are arranged on movable supports back and forth, transverse to the belt, in equal number, both in suction and in injection, the number of notches per step. The spacing of the supports releases the flakes, and their bringing together ensures the tight contact of the nozzles on the end edges of the flakes. Patent document FR-A-2 651 793 gives relevant information on this filling process.

It should be noted that currently the most efficient machines can operate at a rate reaching 72 cycles per minute, three flakes being filled simultaneously with each cycle, that is to say 216 flakes / minute.

At a second workstation, downstream from the first, the end of the straw is closed, on the side opposite the composite plug, by crushing and welding, in particular using ultrasound, between anvil and sonotrode.

It will be noted that the reliable execution of the weld requires that, over at least the corresponding length, the straw does not contain any liquid. This requires that the injection needle is long enough for the liquid sucked out of the needle to come into contact with the flake wall beyond the area reserved for welding. It will be understood that it is necessary, the outside diameter of the needles being slightly less than the inside diameter of the flakes, that the needles are well coaxial with the flakes, and moreover that the approach stroke of the nozzle supports is precisely adjusted, that the joints between nozzles and slices of straw are well sealed, without the straw being stressed in buckling in a manner dangerous for its integrity, and this, at the rates practiced.

The necessary details led the person skilled in the art to design the nozzles as precision mechanical parts, with a needle rigidly crimped in a metal body which had two reference surfaces for fitting fitted in the nozzle holder, a surface defining the centering. axial, generally a cylindrical bearing surface, and the other defining the longitudinal position, generally an upright flat edge forming a connection between the cylindrical bearing surface and a front gripping part. The seal being constituted by a cylindrical elastomer pad with a central passage through which the needle passed with friction, the pad being in abutment on the front edge of the body.

The suction nozzles are no problem; the needle may be short; in addition, the suction nozzles do not come into contact with aseptic parts of the straw, due to the isolation provided by the composite stopper, and moreover, placed in depression relative to the straw, they do not risk inject pollutants into the straw. Simple daily maintenance and serious weekly cleaning are sufficient.

But the situation is quite different for the injection nozzles. The latter, with their flexible conduit, must be rigorously clean and dry at the origin of the filling with sperm of a determined donor male, and must therefore be disassembled after exhaustion of the semen, to be replaced by sterile nozzles for filling a new batch of glitter with sperm from another male donor.

To use the previous examples and define orders of magnitude, in a day, at the rate of 60 bulls and 3 injection nozzles per machine, no less than 180 nozzles will be used each day. After use, the flexible conduits are torn from the connection olives of the nozzles, and put in waste, while the nozzles are stored in alcohol. At the end of the day, the nozzles are cleaned thoroughly, filled with flexible conduits and sterilized to be reused the next day. This presupposes a large flywheel of injection nozzles, handling which is costly in time and which requires careful checks, and always leaves a risk that a nozzle may contain traces of sperm from a previous bull.

In fact, the daily preparation operations of the injection nozzles consume almost as much labor as the actual filling operations.

For a very long time, it appeared desirable to have injection nozzles, provided with a flexible conduit to immerse in the seed flask, which can be discarded after a single use, that is to say the filling of a batch of glitter with diluted sperm from a single donor male.

But the required mechanical precision of the nozzles, as explained above, suggested a prohibitive cost price, that is to say that it would appear to users more profitable to continue the previous errors than to throw away the nozzles after use.

The applicants, with their experience in this area, undertook to re-study, in all its aspects, the problem of single-use injection nozzles. The invention also proposes a disposable injection nozzle for a machine for filling, with biological liquid, so-called commonly flake tubes, formed of tubular sections of polymer material of millimeter diameter internally lined, near one end, of a composite stopper comprising between two porous pads a volume of powder, gelling in contact with an aqueous liquid, the filling being carried out by gripping the straw between two nozzles provided with seals bearing on the end edges, a suction nozzle connected to a vacuum source and bearing on the flake edge adjacent to the composite plug, said injection nozzle being provided with a flexible conduit intended to be immersed in a balloon containing the biological liquid, this injection nozzle comprising a tube rigid, hereinafter called the needle, capable of engaging inside the flake and crimped axially in a body which presents two externally reference surfaces respectively defining the centering and the longitudinal position of the injection nozzle with respect to a nozzle support, this injection nozzle being characterized in that the flexible conduit, consisting of an elastomer of Shore hardness between 41 and 47 and with a reversible elongation capacity of at least 250%, has, with an inner channel diameter of the same order of magnitude as the outer needle diameter, an outer diameter nominal at least 2 times greater than that of the internal channel, and forms, by a straight end section into which the needle is inserted over a length of at least 5 times its external diameter, joint surface for the flake edge away from the composite plug, the flexible conduit being, at least when the nozzle is mounted on the machine, held over at least five times its nominal outside diameter starting from the straight end section, in a tubular hoop made of rigid material of diameter interior adapted to the nominal outside diameter of the conduit, with sufficient tightening to keep the needle sealed, hoop comprising the two reference surfaces of the body.

Thanks to these provisions, the end edge of the flexible conduit constitutes a joint surface for the glitter edge corresponding to the filling end. Already the surfaces in contact with the diluted sperm are reduced inside the flexible duct and the needle. This can be shortened by the length of the conventional joint, reducing the point position error resulting from the error of coaxiality of this needle relative to the body.

In addition, it might seem a priori that the insertion of the needle into the inner channel of a highly flexible elastomeric duct could not ensure the mechanical precision of position and orientation of the needle, because of the flexibility of the conduit, and that the use of the terminal cross-section of the conduit as the joint surface could not ensure the sealing of the joint in the absence of positive maintenance of the conduit against the push of the flake edge. However, the Applicants have discovered that, since the material of which the flexible conduit was made had an almost perfect elastomer behavior, with reversible deformations without variation in volume creating tensions normal to the deformation surfaces, that the stresses created in the wall mass by repercussion of the forces applied to the boundary surfaces of the duct were very substantially distributed in revolution around the axis of the internal duct, and the thickness of the duct wall being large enough to regularize this distribution of revolution, it was possible to center the needle in a tubular hoop with a precision of mechanical quality, provided, moreover, that the needle is inserted into the internal channel over at least five times its external diameter, that the hooping is established over a length of at least five times the external diameter of the conduit, and that 'it is not induced asymmetric efforts.

In addition, due to the mode of deformation of the elastomer constituting the conduit, any effort directed from the outside to depress a surface of the conduit results in a tendency to expansion of the neighboring parts. Thus, pressing the glitter end edge on the straight end section of the duct tends to force the outer wall surface of the duct on the hoop, and the surface of the inner channel on the needle, the recoil of the conduit in the hoop being prevented by the friction of the elastomer in the hoop, friction which increases with the radial expansion of the conduit. The joint surface has an excellent reversible elasticity.

It will be noted that the practical equality of the outside needle and inside diameter of the duct channel, in the absence of the hoop, allows easy insertion of the needle.

Preferably the needle protrudes from the straight end section of the flexible conduit of sufficient length to provide at the end of the straw where the needle enters, an unfilled area available for closing the straw and the wall welding. There remains, between the liquid in the straw and the weld, an air bubble which acts as a buffer to prevent rupture of the straw during the freezing and thawing of the contents of the straw, the preservation of which is, as usual, ensured. by storage in liquid nitrogen.

Preferably also the needle, at its free end spaced from the flexible conduit, has a bevel cut, which facilitates introduction into the straw.

The outside diameter of the needle will preferably be around 2.0 and 1.4 mm, depending on whether the corresponding flakes are said to be normal, with an outside diameter of 3 mm, or fine, with an outside diameter of 2 mm.

The nominal outside diameter, that is to say in the absence of constraints, of the flexible conduit will preferably be 4.5 mm.

The preferred elastomer will be a biologically compatible block copolymer of styrene and butadiene.

The preferred material of the needle is a polyester copolymer called PETG.

It is preferred that the flexible conduit, at its end remote from the needle, comprises a section of rigid tube inserted into the inner channel by one end, the other end being cut at a bevel. This arrangement, known per se, allows the diluted semen contained in the balloon to be used up.

In a preferred arrangement, the hoop is permanently placed on the flexible duct. It can in this case be made of a molded polymer material, and formed, either by overmolding, or by assembling two half shells assembled along a plane passing through the axis.

• la figure 1 représente, schématiquement, une position de remplissage de paillette équipée d'une buse d'injection selon l'invention ;
• la figure 2 est une coupe de la buse d'injection proprement dite ;
• la figure 3 est une vue en bout, par l'arrière, de la buse de la figure 2 ;
• la figure 4 est une vue analogue à celle de la figure 2, mais avec suppression de la frette ;
• la figure 5 est une vue latérale d'une buse, munie d'une frette à demeure ;
• la figure 6 est une vue analogue à la figure 5, avec une frette en deux coquilles collées ;
• la figure 7 est une coupe éclatée prise dans le plan VII - VII de la figure 6.

Secondary characteristics and advantages of the invention will emerge from the description which follows, by way of example, with reference to the appended drawings in which:

• Figure 1 shows, schematically, a straw filling position equipped with an injection nozzle according to the invention;
• Figure 2 is a section of the injection nozzle itself;
• Figure 3 is an end view, from the rear, of the nozzle of Figure 2;
• Figure 4 is a view similar to that of Figure 2, but with elimination of the hoop;
• Figure 5 is a side view of a nozzle, provided with a permanent hoop;
• Figure 6 is a view similar to Figure 5, with a hoop in two glued shells;
• FIG. 7 is an exploded section taken in plane VII - VII of FIG. 6.

According to the embodiment chosen and represented in FIG. 1, a straw 1, according to the usual terminology used in this description, has been put in place at a filling position of an automatic machine for filling straws with diluted animal sperm. This straw 1 consists, in the usual way of a section of approximately 130 mm long and with an external diameter of 3 mm, with a wall thickness of approximately 2/10 of mm, in a transparent thermoplastic polymer material (this glitter, with a practical capacity of 1/2 cm3 is said to be normal). A first end section 12 is provided for filling. The straw 1 ends at its other end with a wafer 13, and comprises, inside and near the wafer 13, a composite stopper 11 conventionally formed from a cylinder of a powder 11a, capable of gelling on contact of an aqueous liquid, taken between two porous pads 11b and 11c. For filling, the straw 1 is taken between two nozzles, a suction nozzle 2, and an injection nozzle 3, both mounted on respective nozzle supports 20 and 30, movable parallel to the length of the straw 1, to disengage away from the latter after filling and, in approximation, make tight contact by the edges 12 and 13 on the joint surfaces 34a and 23.

The suction nozzle 2 as a whole, is conventional and consists of a body of revolution 21 pierced with a central bore, and where is crimped, in axial position, a needle 22 formed here of a thin stainless steel tube cut at a bevel at its end, which penetrates axially in the tube 10 of the straw 1, on the side of the composite plug 11. A seal 23, formed of a section of tube with a thick wall of elastomer, is force-fitted onto the needle 22 until it is pressed on the front edge of the body 21. At the rear, the body 21 is shaped as a nozzle, on which is engaged an elastomer tube 24, connected to a source of vacuum consisting of a vacuum pump. The tube 24 is, not shown, caught between two jaws capable of tightening to crush the tube 24 and form a valve.

The injection nozzle 3 as a whole, according to the invention, is held in a nozzle support 30, movable back and forth as has been said above.

The details of the nozzle 3 will be seen, if necessary, in FIGS. 2 and 3. This nozzle has a needle 33, taken in an extruded tube of PETG polyester copolymer, marketed by Eastman under the name of KODAR PETG 6763, with an outside diameter of 1.7 mm and a wall thickness of 0.1 mm. This needle, terminated at a bevel 33a at its free end, has a total length of 25 mm, and is inserted, over a length of 10 mm, in a flexible elastomeric conduit 34, which has an inner channel diameter 34a of about 1.8 mm, and a nominal outside diameter of 4.5 mm. By nominal outside diameter is meant the outside diameter of the conduit when the latter is not deformed by external forces.

The elastomer chosen here is a block copolymer of styrene and butadiene, with a Shore hardness of 44, and capable of a reversible elongation of at least 250%. This polymer is marketed under the brand name of Kraton by the Polymer Division of the Shell Chemical Company, it is intended for pharmaceutical, medical and food packaging applications.

The flexible conduit 34 is inserted into a hoop 31, 32, the internal diameter of which is less than the nominal diameter of the conduit 34, in a tightening ratio such that the needle 33 will be kept watertight. For example, with a flexible conduit 34 with a nominal diameter 4.5 mm, an internal channel diameter 34c ′ of 1.8 mm, and an external needle diameter of 2, 0 mm, the inner diameter of the hoop will be 4.0 mm. The hoop has a first part 31, anterior, of external shape here hexagonal, which extends towards the front (side of the needle 33) flush with the terminal straight section 34a of the conduit 34, which forms a seal for section 12 of the straw 1 (this hexagonal shape was chosen to facilitate gripping and handling, but is not necessary). The rear part 32 has a cylindrical outer surface, which forms a reference for centering the needle 33 relative to the support 30 and thus to the glitter tube 10. The rear part 32 is connected to the front part 31 by a flat radial shoulder. 32a, which forms a reference surface for its longitudinal position, abutting on the front face of the support 30.

Beyond the rear part 32 of the hoop, the conduit 34 passes between jaws 38a and 38b provided for obliterating the inner channel 34c of the conduit 34 by crushing the wall by bringing these jaws together, and thus acting as a valve.

At the end 34b of the flexible conduit 34, spaced from the needle, is inserted a rigid section of tube 35 of the same kind as the straw tube 10, the free end 35a of which, cut at a bevel, is immersed in a biological product, here diluted semen 17 of bull, contained in a balloon 36. The use of a rigid tube 35 allows the end 35a to bear on the bottom, to aspirate all the contents of the balloon.

For filling a straw 1, the latter being held in a leaktight manner by its edges 12, 13 bearing on the joint surfaces 34a and 23, the suction nozzle 2 is placed in communication with the vacuum source ( the vacuum pump) through the non-crushed elastomer tube 24. Depression is transmitted, successively through the needle 22, the composite stopper 11 (the powder 11a of which is not gelled) the tube 10 of the straw, the needle 33, the internal channel 34c of the flexible conduit 34 (the jaws 38a and 38b being separated) and the rigid tube section 35. In response, the diluted sperm 37 rises through the internal channel 34c of the flexible conduit 34, until it spurts through the beveled end 33a of the needle 33, to fill the straw 1. When the diluted sperm comes into contact with the composite plug 11, it passes through the porous pad 11c to wet the powder 11a which gels and becomes impermeable. The filled straw, the tube 24 and the flexible conduit 34 are obliterated by tightening the valve jaws (38a, 38b for the conduit 34) and the supports 20 and 30 move apart to release the straw 1, which will be transferred to a post of welding. There, it will be clamped between the anvil and the sonotrode of an ultrasonic welder, at its end close to its edge 12, in an area spared from filling due to the length of penetration of the needle 33 into the straw 1.

It will be appreciated that, although the clearance between the interior of glitter 1 and needle 33 is very reduced, and the protruding length of needle 33 is relatively large, in order to provide an unfilled zone for welding, the coaxiality between needle 33 and hoop 31 , 32 can be maintained with precision, despite the intrinsic flexibility of the flexible conduit 34. This precise maintenance of the coaxiality can be attributed to the fact that, between the needle 33 and the hoop 31, 32, the elastomer, which does not change in volume by deforming, can only be deformed in axial shear, strictly of revolution. But this is only true if we can neglect the effects of longitudinal transitions, which means that the lengths along the axis must be large compared to the diameters. The ratio of five between length and diameter is representative of the transition between long and short insertions and hooping.

When the dose of diluted semen, corresponding to an ejaculate of a determined male, all that has been in contact with the sperm must be removed, and, according to the invention thrown in the waste since the goal is to, remove all cleaning and sterilization of parts likely to retain traces of sperm.

However, the hoop 31, 32, unlike the bodies of injection nozzles of the prior art, was not in contact with sperm. It is therefore possible to use reusable hoops, and to discard, as suggested in FIG. 4, only the flexible conduit 34 with its needle 33, and to insert a needle assembly 33, conduit 34 taken out sterile from a package, in a reusable fret.

But it seems preferable, insofar as it is possible to execute frets at low cost, to constitute the complete nozzle from the start, and to throw away the whole assembly after a single use.

As shown in FIG. 5 where the references of the corresponding elements are increased by 50, the nozzle constitutes a one-piece assembly of the needle 83, of the flexible conduit 84, and of a hoop 81, 82 tightened on the flexible conduit 84, for example by overmolding with a two-component resin.

It is also possible, as shown in FIGS. 6 and 7 where the references of the corresponding elements are increased by 100, by injection molding shells 131a, 132a and 131b, 132b, of identical shapes and capable of being joined along a plane passing through the axis of the hoop. In parts 131a and 131b, there will have been housings 138a, 138b, and complementary pins 139a, 139b, to ensure the axial and longitudinal coincidences of the shells. As can be seen in FIG. 7, the flexible conduit 134 is trapped between the shells 131a, 132a and 131b, 132b, the opposite faces of which have been glued, for example using a cyanoacrylic adhesive.

It will be appreciated that the complete nozzles thus produced can be manufactured at low cost, to be delivered ready to use in sterile packages, and will be discarded after use at no significant cost, even if no action was taken. not take into account the costs of cleaning the injection nozzles of the state of the art.

Of course, the invention is not limited to the examples described, but embraces all the variant embodiments thereof within the scope of the claims.

It is, for example, obvious that it is possible to transpose the injection nozzles for normal flakes of 3 mm outside into nozzles for fine flakes, of 2 mm outside and with a capacity of approximately 1/4 cm3. For such fine flakes, the outside diameter of the needle must be at most 1.4 mm, in order to penetrate safely into the fine flakes.

However, the dimensions of the duct, that is 4.5 mm, and externally and 1.8 internally can be kept, to reduce the number of models of parts when this is not imposed by the dimensions of glitter; but the inner diameter of the hoop will be reduced to 3.6 mm.

Furthermore, the inner channel diameter 34c must be kept within relatively narrow limits. Indeed, the volume of the internal channel, which is a dead volume, increases with the square of the diameter. On the other hand, the viscosity of the diluted sperm imposes a minimum diameter of the channel to obtain a pressure drop in the channel compatible with the source of vacuum, and the desired filling speed.

Claims (13)

1. Single-use injection nozzle (3) for a machine for filling, with biological products, tubes usually called straws (1), formed by tubular sections (10) of polymer material with a diameter of a few millimetres provided internally, near one end, with a composite stopper (11) comprising, between two porous plugs (11b, 11c), a volume of powder (11a) which forms a gel in contact with an aqueous liquid, the filling being effected by gripping the straw between two nozzles (2, 3) fitted with seals (23, 34a) bearing on the end sections (12, 13), a suction nozzle (2) connected to a source of reduced pressure and bearing on the section of straw (13) near the composite stopper (11), the said injection nozzle (3) being fitted with a flexible tube (34) intended to be dipped into a flask (36) containing the biological product (37), this injection nozzle (3) comprising a rigid tube (33), hereinafter called a needle, suitable for insertion into the straw (1) and crimped axially in a body which has two external reference surfaces (32, 32a) defining respectively the centring and longitudinal position of the injection nozzle relative to a nozzle support (30), this injection nozzle (3) being characterised in that the flexible tube (34), made from an elastomer having a Shore hardness between 41 and 47 and a reversible elongation capacity of at least 250%, has, with an internal channel diameter (34c) of the same order of magnitude as the external diameter of the needle (33), a nominal external diameter at least twice that of the internal channel (34c), and forms, by a right cross-section at the end (34a) where the needle (33) is inserted over a length of at least five times its external diameter, a seal bearing surface for the section (12) of straw remote from the composite stopper (11), the flexible tube (34) being gripped, at least when the nozzle (3) is mounted on the machine, over a distance equal to at least five times its nominal external diameter starting from the right cross-section at the end (34a), in a tubular hoop (31, 32) of rigid material with an internal diameter matched to the nominal external diameter of the tube (34) with sufficient gripping force for a sealed retention of the needle (33), the hoop incorporating the two reference surfaces (32, 32a) of the body.
2. Injection nozzle according to Claim 1, characterised in that the needle (33) projects from the right cross-section at the end (34a) of the flexible tube (34) over a sufficient length to form, at the end of the straw (1) where the needle enters, an unfilled region available for closing the straw (1) by welding the wall (10).
3. Injection nozzle according to one of Claims 1 and 2, characterised in that the needle (33) has, at a free end (33a), a bevel cut projecting from the flexible tube.
4. Injection nozzle according to any one of Claims 1 to 3, characterised in that the needle has an external diameter of approximately 2.0 mm or approximately 1.4 mm depending on whether it is intended respectively either for straws with an external diameter of 3 mm, called standard straws, or for straws with an external diameter of 2 mm, called fine straws.
5. Injection nozzle according to any one of Claims 1 to 4, characterised in that the nominal diameter of the flexible tube (34) is 4.5 mm.
6. Injection nozzle according to any one of Claims 1 to 5, characterised in that the elastomer forming the flexible tube (34) is a block copolymer of biologically compatible styrene and butadiene.
7. Injection nozzle according to any one of Claims 1 to 6, characterised in that the needle (33) is held in an extruded tube of a PETG-type polyester copolymer.
8. Injection nozzle according to any one of Claims 1 to 7, characterised in that the flexible tube (34) at its end (34b) remote from the needle, has a section of rigid tube (35), inserted into its internal channel (34c) by one end, and bevel cut at its other end (35a).
9. Injection nozzle according to any one of Claims 1 to 8, characterised in that the hoop (31) comprises two parts longitudinally, a front part (31) shaped to facilitate gripping with a first overall diameter, and an externally cylindrical rear part (32) of diameter less than the overall diameter of the first part (31) and being connected to this first part through a radial shoulder (32a) perpendicular to the axis of the hoop (31, 32).
10. Injection nozzle according to any one of Claims 1 to 9, characterised in that the hoop (81, 82) is permanently mounted on the flexible tube.
11. Injection nozzle according to Claim 10, characterised in that the hoop (81, 82) is made from a moulded polymer material.
12. Injection nozzle according to Claim 11, characterised in that the hoop is moulded by casting.
13. Injection nozzle according to Claim 11, characterised in that the hoop is formed in two shells (131a, 132a and 131b, 132b) fitted together over a plane passing through the axis.

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