FR2647509A1 - Piston plunger for a metering pump with precompression and improved priming and method for moulding it - Google Patents

Abstract

A metering pump of the prior art, whose precompression function is assured by a differential piston 4 conditioning the opening of the main outlet valve, is provided, in a known way, with a priming assembly constituting a second outlet valve. The latter is designed to open solely when the metering pump is primed, and thus to allow the air initially contained in the pump chamber 23 to be expelled to outside. It consists, for example, of two rings 10, 10' between which there extends a thin flexible wall 20. The priming assembly is therefore retained by abutment between two necks 32, 34 of the outlet channel 33 of the metering pump. The present invention ensures moulding and assembly conditions of the metering pump which are particularly favourable both on the economic front and on that of quality. It makes provision for using two hollow cylinders 300, 310 which engage in one another for the purpose of defining the outlet channel 33 mentioned hereinabove. Each of the two cylinders 300, 310 therefore includes just one of the necks 32, 34 offering the stops for the priming assembly.

Description

 The present invention relates to an improved precompression and priming dosing pump already developed in the prior art with a view to emitting a liquid or pasty product in the form of an aerosol. It relates more specifically to the design of the piston rod that this type of pump includes. Its structure meets the various operating constraints of the pump as well as those of its manufacture by molding. Its molding process is also part of the present invention.

The improved precompression and priming dosing pump of the prior art with which the present piston rod is associated, is described in French patent application No. 88-07337 filed June 2, 1988 by the company STEP (Company
Spraying technique). As its name suggests, the piston rod in question comprises on one side a piston adapted to slide in leaktight manner in the pump body thanks to one or even two peripheral yeasts. This piston extends on the other hand into a rod which extends outside the pump body to offer a grip to the user and allow him to push the piston into the pump body. A central channel passes through the piston and the rod. It is used to put the pump chamber and the outside in communication with a view to the emission of the product. However, this only occurs by opening an outlet valve. In this case, this valve is determined inter alia by the collaboration of a ring housed in the emission channel and a lug protruding from the wall of the latter. For this, the valve is kept in contact with the lug by elastic means also housed in the emission channel downstream of the outlet valve. These means are also supported against a shoulder of the wall of the transmission channel located near its mouth. In other words, the emission channel has two narrowing sections, the shoulder and the lug, while it widens between these two asperities in order to accommodate the ring and the elastic means.

 This configuration is poorly satisfied with a molding of the piston rod in one piece in one piece. In fact, the mold must then have a central finger conforming to the internal shape of the emission channel and therefore admitting a wider intermediate section. When it is forcibly removed from the molded part, this section therefore rubs against the lug and deforms it. A similar stress is then imposed on the lug when the ring and the elastic means are put in place in the transmission channel.

Finally, experience shows that the lug is so damaged that it is no longer able to collaborate effectively with the ring to form the required outlet valve.

 In order to avoid this problem, French patent application no. 88-I 6722 filed on December 19, 1988 by the company VALOIS proposes to produce the piston rod in two parts. One of them keeps a shape identical to the piston rod in one piece described above except that it is amputated from the end of the rod. Thus the mouth of the emission channel and the shoulder are absent from this main part the piston rod. The latter can then be molded without inconvenience to the lug, the mold finger being extracted from the channel by pulling it from the side of the amputated end of the rod. The other part is a end piece intended to replace this end.It is also secured by interlocking within the channel so that it determines a narrowing of section equivalent to the previous shoulder. Thus, before putting it on the piston rod, is it possible to arrange the ring and the elastic means in the channel by introducing them through the amputated end of the main part of the piston rod.

 If this design eliminates the problem of damage to the lug during molding and assembly of the piston rod, the proper functioning of the latter remains dependent on the quality of the connection made between the main part and the mouthpiece. It is essential that the latter is sufficiently solid and the rest despite the aging of the dosing pump to support the thrust transmitted to the nozzle by the elastic means. However, the presence of sealing lips on the main part of the piston rod requires the use of a relatively flexible plastic material to constitute it.

Also, an end piece that is forced into the channel or even snapped into it is likely to dissociate in the long run, fatigue or creep of the material modifying the bonding conditions.

 It is thus that the present invention aims to propose another design of piston rod for the type of metering pump with precompression and improved priming mentioned at the start. The design sought must this time overcome the uncertainty about the solidity of the connections presented by the prior art.

 This is achieved by a piston rod for a precompression and improved priming dosing pump used for the emission of a liquid or pasty product in the form of an aerosol, said piston rod being constituted by a piston adapted to slide so sealed within a pump body in order to isolate a pump chamber therein as well as by a rod extending said piston out of said pump body, a cylindrical channel passing through said piston rod in order to put said pump chamber in communication with the outside when a double outlet valve is opened, at least one ring as well as elastic means being housed in series with an annular clearance in said channel downstream of said double outlet valve between means of support and an annular lug presented by the inner wall of said channel, said double valve being formed by the collaboration, on one side, of said ring and of said lug and, on the other, of said ring with a needle carried by a differential piston contained in said pump chamber, said differential piston being integral with a skirt adapted to fit tightly around and to circulate on a limited stroke along a sleeve communicating internally with a reservoir of said product when return spring more flexible than said elastic means is compressed, said piston rod being formed by a hollow inner cylinder and a hollow outer cylinder, said cylinders being adapted to engage one on the other and their respective inner conduits having a single section narrowing, characterized in that said section narrowing of said inner cylinder is constituted by said annular lug while said section narrowing of said outer cylinder is constituted by said support means.

 More particularly, said inner cylinder comprises a first part of said piston of said piston rod, said first part of said piston having a sealant directed towards said pump chamber for sliding in leaktight manner within said pump body, said outer cylinder comprising a second part of said piston of said piston rod complementary to said first part.

 In certain cases, it is preferable for said cylinders of said piston rod to be provided with connection means making it possible to make said first and second parts of said piston of said piston rod integral with said inner and outer cylinders, respectively. Then, said connecting means of said cylinders of said piston rod advantageously consist of an annular notch formed on the exterior surface of said interior cylinder at the root of said first part of said piston and in an annular boss carried by the interior surface of said cylinder exterior to level of said second part of said piston, said notch and said boss being adapted to snap when said cylinders are engaged one on the other.

 Sometimes, it is recommended that said second part of said piston of said tigepiston comprised by said outer cylinder has a sealing lip directed towards the outside to slide in leaktight manner within said pump body. However, when said dosing pump is provided with an additional intake valve opening as soon as the pistons rise, it is sufficient that said second part of said piston of said piston rod carried by said outer cylinder has a rim outwardly sliding with clearance within said pump body.

According to a preferred form of the present piston rod, said outer cylinder has a throttled end portion of which an inner base is adapted, when engaging said cylinders one on the other, to come opposite said inner conduit of said cylinder interior to constitute said support means of said channel. It is then advantageous that a ring identical to said at least one ring is also housed in said channel, downstream of said elastic means. Said inner base of said end portion of said outer cylinder can therefore have a substantially surface
equal to the section of said rings so that either of these rings can be there
apply.

Advantageously, said elastic means consist of a thin partition
cylindrical integral with said rings and of the same internal diameter as said
rings. In this case, it is preferable that said inner base of said part
terminal of said outer cylinder protrudes into a crown; that when said
cylinders of said piston rod are engaged one on the other, said inner cylinder
extends to the surface of said crown so that a free space exists at
the end of said inner cylinder corresponding to the thickness of which protrudes said
crowned; and that radial channels are formed in said crown to make
communicating said free space with said channel.

Finally, in order to mold the present piston rod, it is advantageous to have recourse
to a mold of said inner cylinder made up of at least two dies, one of said
dies having a finger conforming to said inner conduit of said inner cylinder
up to said lug, demolding is effected by removing said finger from the end of said
conduit opposite to said lug so that said finger does not rub on said lug.

The invention will be better understood on reading the detailed description which follows and
on examining the appended drawings which represent, by way of nonlimiting example, a
embodiment of the present piston rod in context of the metering pump
precompression and improved priming mentioned at the start. In these drawings:
- Figure 1 shows a longitudinal section of a metering pump
improved precompression and priming with this piston rod.
pump is represented in its high rest position,
FIG. 2 shows a detail of the section of FIG. 1,
- Figure 3 shows a longitudinal section of the same dosing pump as the
FIG. 1, but shown in its low priming position,
FIG. 4 shows a detail of the section of FIG. 3,
- Figure 5 shows a detail of a longitudinal section of the same pump
dispenser as Figure 1, but shown in an intermediate position
product emission,
- Figure 6 shows a longitudinal section of a mold suitable for molding a
part of this piston rod,
- Figure 7 shows a longitudinal section of a variant of a metering pump
precompression and improved priming provided with this piston rod. This
variant is shown in its high rest position.

 In Figure 1, it is possible to recognize the different components of a metering pump with precompression and improved priming as designed in the prior art. These correspond to six cylindrical parts assembled so that their respective axes of revolution are combined. The resulting common axis is shown here vertically. This arrangement is also used for the following figures with the exception of FIG. 6. Thus the upper part of the sections is situated on the side of the emission of the product while their lower parts are inserted in a tank ( not shown) of product to be issued.

These six fundamental pieces are:
1 / a tower 1 having a base II to adapt to the neck of the product reservoir and to be subjected thereto in leaktight manner by complementary means (also not shown),
2 / a pump body 2 with an upper end 21 snaps into the preceding tower 1 and a bottom 22 of which communicates with the interior of the tank either directly (as shown), or by means of a dip tube fitted on an integral socket of body 2 (not shown). In addition, a sleeve 24 internally extends the bottom 22 of the pump body. The annular space between this sleeve 24 and the pump body 2 then corresponds to the essence of the pump chamber 23 of the dosing pump.

 3J a piston rod 3. According to the present invention, the piston rod 3 is formed by two hollow cylinders 300 and 310 engaged one inside the other. The inner cylinder 300 comprises a piston adapted to slide in leaktight manner within the pump body 2 by means of a sealing lip 36 oriented towards the bottom 22 of the pump body. It extends upwards into a rod pierced with a central channel 33 for the emission of the product. About halfway up the channel 33 projects an annular lug 32 advantageously having the shape of a sealing lip oriented towards the outlet of the channel 33.

 The external cylinder 310 comprises, in this embodiment, also a piston adapted to slide in leaktight manner within the pump body 2 by means of a sealing lip 35 oriented towards the upper end 21 of the pump body. More typically of the present invention, it extends upward into a rod. The latter surrounds the rod of the inner cylinder 300 and is choked above it into a hollow end portion 37 extending from the channel 33.

 As appears better in FIG. 2 of details, a free space 320 is advantageously provided between the end of the rod of the inner cylinder 300 and the throttle of the outer cylinder 310. In addition, a crown 34 protrudes opposite the channel 33 at the base of the terminal part 37 of the rod of the outer cylinder 310. This ring 34 has different radial channels 311 connecting the free space 320 with the channel 33.

 Advantageously, an annular notch 312 is formed at the outer root of the rod of the inner cylinder 300 on the corresponding piston. Likewise, an annular boss is provided at the inner root of the rod of the outer cylinder 310 on the corresponding piston. In this way, the pistons of the two cylinders can be made integral by snap-fastening when their respective rods are engaged one on the other.

 41 a set 100 subsequently called priming set because of the role it plays in the operation of the pump. This assembly 100 consists of two identical rings 10 and 10 'connected to each other by a cylindrical partition 20 of the same axis as the rings. In the absence of external stresses, the latter corresponds for example to a thin hollow cylinder of the same internal diameter as the rings 10 and 10 '. Its small thickness also allows its elastic bending so that it can take the shape of a barrel shown in the drawings.

 In fact, the priming assembly 100 is housed within the channel 33. If a radial clearance is maintained, the assembly 100 is, on the other hand, compressed there axially between the end portion 37 of the rod of the outer cylinder 310 forming the piston rod 3 and the annular lug 32 which both provide support. In FIG. 2 of details, it is in fact better visible how the upper ring 10 ′ rests on the entire surface of its section against the crown 34 of the terminal part 37. The lower ring 10 is applied, for its part, against the lug 32 in a very localized zone 103 which, the elasticity of the parts helping, guarantees a tight contact.

 5 / a second differential piston 4. This first comprises a needle 41 intended to engage in the channel 33 of the first piston. Its top is therefore able to penetrate within the ring 10 already described. This results in a sealed contact at the edge 104 of the ring 10. The differential piston 4 also includes a skirt 42 extending towards the bottom 22 of the pump body 2 so that it is capable of s 'fit around the sleeve 24 integral with this bottom 22. While the outer surface of the skirt 42 is used for guiding inside the pump body 2, its inner surface has a sealing lip 43 below. When the latter comes into contact with the sleeve 24 following the insertion of the pistons 3 and 4, the communication between the reservoir and the pump chamber 23 is interrupted.

 The inner surface of the skirt 42 is further provided with a recess 45 serving as a stop against the sleeve 24 and thus determining the low position of the differential piston 4 (see FIG. 3). Finally, between its needle 41 and its skirt 42, the differential piston 4 has a step 44 directed upwards. In addition to its role in the operation of the differential piston 4, the step 44 provides a stop for the piston rod 3 in the low position.

 6 / a return spring 5 disposed between the differential piston 4 and the bottom 22 of the pump body 2. This spring 5 is more flexible than the flexible partition 20.

 These various parts collaborate differently depending on whether the pump is primed or whether it emits the product. In the first case, in fact, the pump chamber 23 contains only air, a very compressible fluid. The user can then press the end part 37 of the piston rod 3 and lower it completely down to the low position of FIG. 3. The differential piston 4 is thus found completely blocked at the bottom 22 of the pump body 2 while that the piston of the cylinder 300 inside the rod 3 rests against the step 44. Consequently, as is more visible in FIG. 4 of details, the top of the needle 41 pushes the ring 10 upwards.

It follows that the contact 103 between this ring 10 and the lug 32 of the transmission channel 33 is broken. Now there is a passage between the pump chamber 23 and the exterior via, first of all, the gap between the periphery of the ring 10 and the wall of the channel 33 and, then, the channels 311 of the crown 34. This mechanism therefore allows the air initially contained in this chamber 23 to be removed.

 Note that due to the relative elasticity of the external cylinder 310, the compression of the user in this case also has the effect of deforming this cylinder 310 in a barrel making the free space 320 disappear between this last and the cylinder 300 interior. This phenomenon, combined with that described above, results in increased flexion of the partition 20 which strengthens the tightness of the needle contact 41 ring loden 104.

When the user stops pressing on the piston rod 3, the partition 20 immediately seeks to return to its initial shape. It follows the very rapid restoration of contact 103 between the ring 10 and the lug 32 so that the pump chamber 23 is completely isolated from the outside. As it is also isolated from the reservoir by virtue of the lips 43 for sealing the skirt 42 circulating on the sleeve 24,
Increasing its volume as the return spring relaxes, causes the development of a depression within it. In this embodiment, the vacuum can reach a substantial value thanks to the presence in particular of the sealing lip of the cylinder 310 outside of the piston rod 3 which prevents any infiltration of air. At the same time, an effort of suction urges this lip 35 and effectively opposes the separation of the two cylinders 300 and 310 forming the piston rod 3 that the assembly 100 would otherwise tend to cause. This strengthens their connection at the snap 312 and, in some cases, can even make this last point unnecessary.

 As soon as the pistons have returned to their high position in FIGS. 1 and 2, the passage offered between the skirt 42 and the sleeve 24 allows filling of the pump chamber 23 with the aid of the product sucked in from the reservoir by vacuum. The barrel deformation of the cylinder 310 outside of the piston rod 3 has for its part long since disappeared, the assembly 100 having pushed the crown 34 from the first moments of the ascent of the pistons.

 A new actuation of the piston rod 3 this time results in the compression of the product trapped in the pump chamber 23. In fact, the more flexible return spring 5 is always stressed in the first place. And while with the depression of the pistons 3 and 4, its length decreases, the volume of the pump chamber 23 drops. The sealing lip 43 of the skirt 42 circulating on the sleeve 24 as well as the sealing lip 36 of the cylinder 300 inside the piston rod 3 moreover guarantee the insulation of the chamber 23 so that the pressure of the product trapped within it increases. This then causes the differential piston 4 to be withdrawn by means of the pressure which is exerted on the step 44 and is opposed to the return force of the spring 5.

The ring 10 therefore remains all the better maintained at 103 against the lug 32 that the outer cylinder 310 tends to deform in a barrel and to compress the priming assembly 100. Rather, it is the contact 104 with the needle 41 to break as shown in FIG. 5. The product under pressure then fuses between the needle 41 and the ring 10 to reach the outside via the central holes of the ring 10, of the partition 20, of the ring 10 'and finally of the terminal part 37 of the exterior cylinder 310.

 The emission continues in this way until the differential piston 4 abuts against the sleeve 24 at the step 45. The pump chamber 23 which therefore ceases to decrease in volume, sees its pressure drop. The latter soon becomes unable to further oppose the return spring 5 so that the differential piston 4 goes up and that the contact 104 of its needle 41 with the ring 10 is restored. The user noting that the product is no longer emitted, releases his effort again and the pump chamber 23 is not long in filling with product as at the end of priming, thus preparing the dose for subsequent actuation .

 The piston rod 3 which has just been described lends itself to molding without disadvantage for the quality of the lug 32. For example, FIG. 6 very schematically shows how its internal cylinder 300 could be produced. It would suffice to use a two-part mold. A first part 62 will have in hollow the imprint of the external surface of the cylinder 300 and, projecting in the center of the latter, a first finger 63 matching the internal shape of the channel 33 downstream of the lug 32. A second part 61 of the mold would, in turn, have a second finger 64 matching the interior shape of the channel 33 upstream of the lug 32. By pulling part 62 to the right of Figure 6 and part 61 to the left, the cylinder 300 find naked without the pin 32 does not suffer from friction.

 It is understood that a comparable molding system can be used to produce the cylinder 310 outside the piston rod. Before its assembly with the inner cylinder 300, the priming assembly 100 is engaged in the latter downstream of the lug.

For this purpose, the assembly 100 is advantageously reversible. This is the case of the assembly 100 described so far which comprises two identical rings 10 and 10 'while only the ring 10 in contact with the lug is useful for the proper functioning of the pump. Then, assembly is completed by engaging the outer cylinder 310 on the inner cylinder 300 and causing them to snap, if necessary.

 The variant described in the preceding pages involves two sealing lips carried by the piston rod 3. It has been seen that each of them is active in a different phase of the operation of the pump: the lip 36 of the inner cylinder 300 participating to the isolation of the pump chamber 23 during the compression of its contents and the lip 35 of the outer cylinder 310 guaranteeing the development of a sotable vacuum in the chamber 23. However, the metering pump of the prior art is sometimes arranged in such a way that, when the pistions rise, its chamber 23 is the site of a lesser depression. To this end, French patent 2,558,214 filed in 1984 by the company VALOIS provides for example one or more bores 46 in the differential piston 4. These are provided with thin lips 47 projecting from the step 44 and acting as a valve. anti-return. Thus, their presence does not in any way modify the phase of pressurization of the pump chamber 23. On the other hand, during its increase in volume, the bores 46 practically immediately place the chamber 23 in communication with the reservoir, and this well before the skirt 42 leaves the sleeve 24.

 With this type of dosing pump of the prior art, the sealing lip 35 of the outer cylinder 310 is no longer useful. It is then preferable to eliminate it in order to reduce the friction of the piston rod 3 on the pump body 2 and thus authorize the use of a more flexible return spring 5. Instead, a simple annular rim 35 can be provided. If its diameter is less than that of the interior of the pump body 2, the flange 35 must remain suitable for contact with the tower 1 in the pump rest position. This contact is in fact useful for the stop of the piston rod 3. I1 may also be necessary to isolate the tank from the outside when an air vent 12 is otherwise provided. It is understood, however, that the molding of the rim 35 with the external cylinder 310 does not present any particular difficulty. The constitution of the piston rod 3 in two parts even allows easy adaptation to this second type of pump-deseuse since the inner cylinder 300 and the priming assembly 100 remain, for their part, unchanged. Their respective molds therefore apply to all of the dosing pumps mentioned here. Note, however, that in this variant, the snap-fastening 312 or any other connection means is essential in order to keep the two cylinders 300 and 310 integral, especially during the raising of the pistons to which no suction is now no longer opposed.

 Other variants of the present piston rod will not fail to appear to those skilled in the art. However, these can only use technical equivalents of the crown 34, the lug 32, the snap 312 ... and must remain within the framework of a constitution of the piston rod in two cylinders engaged one in the other, each of them providing a means of support for the priming assembly 100 according to the following claims.

Claims (12)

Claims:  1.- Piston rod for metering pump with precompression and improved priming used for the emission of a liquid or pasty product in aerosol form, said tigepiston (3) being constituted by a piston adapted to slide tightly within a pump body (2) in order to isolate a pump chamber (23) therein as well as by a rod extending said piston out of said pump body (2), a cylindrical channel (33) passing through said rod - piston (3) in order to put said pump chamber (23) in communication with the outside when opening a double outlet valve (103, 104), at least one ring (10) as well as elastic means (20) being housed in serine with an annular clearance in said channel (33) downstream of said double outlet valve (103, 104) between support means (34) and an annular lug (32) presented by the inner wall of said channel (33), said double valve (103, 104) being formed by the collaboration, on one side, of said ring (1 0) and said lug (32) and, on the other, said ring (10) with a needle (41) carried by a differential piston (4) contained in said pump chamber (23), said differential piston (4) being integral with a skirt (42) adapted to fit tightly around and to circulate on a limited stroke along a sleeve (24) communicating internally with a reservoir of said product when a return spring (5) more flexible than said elastic means (20) is compressed, said piston rod (3) being formed by a hollow inner cylinder (300) and a hollow outer cylinder (310), said cylinders (300, 310) being adapted to engaging one on the other and their respective interior conduits having a single section narrowing, characterized in that said section narrowing of said interior cylinder (300) is constituted by said annular lug (32) while said section narrowing outer cylinder (310) is co set up by said support means (34).  2. A piston rod according to claim 1, characterized in that said inner cylinder (300) comprises a first part of said piston of said piston rod (3), said first part of said piston having a sealing lip (36). directed towards said pump chamber (23) for sliding in leaktight manner within said pump body (2), said external cylinder (310) comprising a second part of said piston of said piston rod (3) complementary to said first part.  3.- piston rod according to claim 2, characterized in that said cylinders (300, 310) of said piston rod (3) are provided with connecting means (312) for making integral said first and second parts of said piston of said tigepiston (3) respectively comprised by said inner (300) and outer (310) cylinders.  4. A piston rod according to claim 3, characterized in that said connecting means (312) of said cylinders (300, 310) of said piston rod (3) consist of an annular notch formed on the outer surface of said cylinder ( 300) inside the root of said first part of said piston and in an annular boss carried by the inside surface of said cylinder (310) outside at said second part of said piston, said notch and said boss being adapted to snap when said cylinders (300, 310) are engaged one on the other.  5.- piston rod according to any one of claims 2 to 4, characterized in that said second part of said piston of said piston rod (3) formed by said outer cylinder (310) has a sealing lip (35) directed outward to slide in leaktight manner within said pump body (2).  6. A piston rod according to claim 3 or claim 4, said pump-metering device being provided with a valve (47) for complementary admission opening from the start of the raising of the pistons (3, 4), characterized in that that said second part of said piston of said piston rod (3) formed by said outer cylinder (310) has a flange (35) extending outwardly sliding with play within said pump body (2).  7. A piston rod according to any one of the preceding claims, characterized in that said external cylinder (310) has a choked end portion (37), an internal base of which is adapted, during the engagement of said cylinders (300, 310) one on the other, coming opposite said inner conduit of said inner cylinder (300) to constitute said support means (34) of said channel (33).  8. A piston rod according to claim 7, characterized in that a ring (10 ') identical to said at least one ring (10) is also housed in said channel (33), downstream of said elastic means (20).  9. A piston rod according to claim 8, characterized in that said inner base of said end portion (37) of said outer cylinder (310) has a surface substantially equal to the section of said rings (10, 10 ') so that either of said rings (10, 10 ') can be applied thereto.  10.- piston rod according to claim 9, characterized in that said elastic means (20) consist of a thin cylindrical partition integral with said rings (10, 10 ') and of the same internal diameter as said rings (10, 10') .  11. A piston rod according to claim 10, characterized in that said interior base of said end portion (37) of said exterior cylinder (310) projects into a crown; in that, when said cylinders (300, 310) of said piston rod (3) are engaged one on the other, said inner cylinder (300) extends to the surface of said crown so that 'A free space (320) exists at the end of said inner cylinder (300) corresponding to the thickness of which protrudes from said crown; and in that radial channels (311) are provided in said ring to communicate said free space (320) with said channel (33).  12. A method of molding a piston rod (3) according to any one of the preceding claims, characterized in that the mold of said inner cylinder (300) consists of at least two dies (61, 62), one of said dies (61, 62) having a finger (63) conforming to said inner conduit of said inner cylinder (300) up to said lug (32), demolding is effected by removing said finger (63) through the end of said conduit opposite to said lug (32) so that said finger (63) does not rub against said lug (32).