HK1009492A - Ferrule for optical fiber connector and method for production thereof - Google Patents

Description

Ferrule for optical fiber connector and method of manufacturing the same

The present invention relates to a ferrule for an optical fiber connector (hereinafter sometimes referred to as "optical fiber connector") for connecting two adjacent single-type optical fibers and a method for manufacturing the same.

In general, the optical fiber connector shown in fig. 1 is composed of, for example, a plug 100 and a hollow cylindrical sleeve 120 for mating and aligning two such plugs with each other, wherein the plug 100 is connected to an optical fiber cable 110, and the optical fiber cable 110 has an optical fiber inserted and fixed in the plug along the center axis of the plug 100. In contrast to electrical connectors, it is particularly desirable that the fiber optic connector properly align the opposing ends of two optical fibers to be connected.

For this purpose, a ferrule 101 adapted to insert the leading ends of two very thin optical fibers to be connected therein is widely used. Two such ferrules 101 can be placed against each other to connect two optical fibers. It is said that this depends on the process conditions, which include fixing two ferrules 101 each having a leading end of an optical fiber inserted and fixed therein, concentrically one by one in two plugs 100 machined with a predetermined outer diameter; the two plugs 100 are then inserted into a ferrule 120 (from opposite ends of the ferrule, respectively); and, bringing the ferrules 101 into close proximity with each other so as to align the axes of the optical fibers.

As described above, in order to achieve a strict alignment between the axial centerlines of the opposite ends of two optical fibers, the ferrule for an optical fiber joint requires an extremely high accuracy to be secured to the inner diameter of the fine receptacle of the ferrule that substantially accommodates an optical fiber and the outer diameter of the ferrule, and also requires that the axial centerlines of the fine receptacle and the ferrule outer diameter reliably coincide with each other.

Since the conventional ferrule is manufactured by machining a metal or ceramic member, there is room for improvement in mass productivity, i.e., processing expense. In particular, since a ceramic material such as Partially Stabilized Zirconia (PSZ) is subjected to preliminary molding such as powder extrusion or pressure casting, and then subjected to steps such as degreasing, sintering, and machining, the manufacturing process thereof is long and the production cost is inevitably large. Moreover, such brittle and hard materials entail problems such as the need for finishing and the need to obtain a smooth finish by means of surface polishing of their grain size (roughness). To accomplish the task of reducing the expenditure, the idea of injection molding with a carbon fiber reinforced plastic composite material is proposed. To be suitable for such an optical fiber connector on which a ferrule is frequently repeatedly attached and detached, this method may cause problems such as deformation and deterioration due to material properties.

In view of the above problems, an object of the present invention is to provide a method. The method is capable of manufacturing a ferrule having a desired shape with highly satisfactory dimensional accuracy in mass production by a single process by combining a conventional process based on a metal mold casting or molding method with an amorphous alloy exhibiting a glass transition region, and remarkably eliminating or reducing machining such as polishing, thereby providing an inexpensive ferrule exhibiting optimum material characteristics, having highly efficient light transmission properties, and excellent in service life, strength, impact resistance and workability.

To achieve the above object, the present invention provides a ferrule for an optical fiber connector, the ferrule including a receptacle for receiving an optical fiber therein in place, the ferrule being formed of an amorphous alloy having at least one glass transition region, and in particular, the glass transition region having a temperature width of not less than 30K, preferably not less than 60K.

In a preferred embodiment, the ferrule is for an optical fiber connectorCan be formed of an amorphous alloy having a general formula X representing the composition thereof_aM_bAl_cAnd an amorphous phase at least 50% by volume, wherein X in the above formula represents at least one element selected from the group consisting of Zr and Hf, M represents at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and a, b and c represent atomic percentages and satisfy the conditions 25. ltoreq. a.ltoreq.85, 5. ltoreq. b.ltoreq.70 and 0. ltoreq. c.ltoreq.35, respectively.

The present invention also provides several methods of producing such a ferrule for an optical fiber connector as described above.

One of the methods is characterized by the following steps: providing a vessel for melting and holding an alloy material capable of producing an amorphous alloy having a glass transition region, preferably an amorphous alloy having the general formula; providing a metal mold having a cavity in the shape of a product to be manufactured; connecting a hole formed, for example, in a lower portion or an upper portion of the container to a gate of the metal mold, for example, by disposing the metal mold below or above the container; applying pressure on the alloy melt so that a predetermined amount of the melt passes through the hole at the lower or upper portion of the container and is injected into the interior of the metal mold; and solidifying the melt in the metal mold at a cooling rate of not less than 10K (kalvin scale)/second to form a substantially amorphous phase product.

Another method is characterized by the following steps: heating an amorphous material formed of an alloy represented by the above general formula to a temperature of a super-cooled liquid region; inserting the obtained hot amorphous material into a container maintained at the same temperature; connecting a metal mold having a cavity in the shape of the product to be manufactured to the container; a predetermined amount of the super-cooled liquid alloy is charged into the metal mold by stagnation of the super-cooled liquid to form a hoop.

Other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. Wherein the drawings are as follows:

FIG. 1 is a schematic front view, partially broken away, of an example of a conventional fiber optic splice;

FIG. 2 is an enlarged schematic cross-sectional view illustrating one embodiment of the ferrule of the present invention;

FIG. 3 is a partially schematic cross-sectional view showing an example of an apparatus used in the method of the present invention; and

figure 4 is an enlarged schematic cross-sectional view showing another embodiment of the ferrule of the present invention.

After the present inventors have made studies on amorphous alloys, they have continuously revealed various alloys exhibiting glass transition characteristics and exhibited their material properties. Among the other alloys mentioned above, the Zr-TM-Al and Hf-TM-Al (TM: transition metal) amorphous alloys having a wide gap between the glass transition temperature (Tg) and the crystallization temperature (Tx) exhibit high strength and high corrosion resistance, have a wide (not less than 30K) supercooled liquid range (glass transition range) Δ Tx-Tg, and have an extremely wide (not less than 60K) supercooled liquid range in the case of the Zr-TM-Al amorphous alloys. In the above temperature range, these amorphous alloys exhibit their very satisfactory workability even at low stress of less than several tens MPa due to their viscous flow. They are characterized in that a very easy and stable manufacture of amorphous granular material can be achieved even with a cooling rate of the order of tens of K/S according to a casting method. The above amorphous alloys of Zr-TM-Al and Hf-TM-Al are disclosed in U.S. Pat. No. 5,032,196, issued on 16.7.1991 and invented by Masumoto et Al, which is incorporated herein by reference. After further investigating the search materials for use with these alloys, the inventors determined that: these alloys produce amorphous material from the molten state by metal cross-casting and by a stagnation molding process also using the glass transition range, and allow the shape and size of the cavity of a metal mold to be reproduced very accurately, and the ferrule is suitable for use in an optical fiber joint, in view of the fact that the physical properties of these alloys are positive factors. This invention accomplishes this result.

Various amorphous alloys of Zr-TM-Al and Hf-TM-Al to be used in the present invention have a wide range of Δ Tx in spite of the large number of alloy compositions and determination methods. Zr₆₀Al₁₅Co_2.5Ni_7.5Cu₁₅(Tg: 625K and Tx: 768K) for example has an extremely wide Δ Tx of 116K. It also has a satisfactory oxidation resistance, making it difficult to oxidize even when it is heated in air to a high temperature such as Tg. The Vickers hardness of this alloy is 460(DPN) at temperatures from room temperature to around Tg, while its tensile strength is 1600MPa and flexural strength is up to 3000 MPa. The alloy has a coefficient of thermal expansion as small as 1X 10 at a temperature from room temperature to around Tg^-5The Young's modulus is 91GPa, and the elastic limit in the compressed state exceeds (4-5)%. In addition, the alloy has high toughness, so that the pendulum impact value is (6-7) J/cm²Within the range. Although this alloy exhibits its very high strength characteristics as described above, its stress is as low as around 10MPa when it is heated to the ground transition range. Therefore, the alloy is characterized by easy processing and low stress on the fine parts during manufacturing, resulting in high precision of the parts with complex shapes. In addition, this alloy is characterized by an extremely high surface flatness when manufactured into shaped (deformed) articles, due to the property of the so-called glass (in amorphous) material, whereas the phenomenon of a step formation when slip bands appear on the surface of a crystalline alloy during deformation thereof is substantially impossible.

Generally, an amorphous alloy begins to crystallize and remains therein for a long period of time when heated to its glass transition range. In contrast, the alloy having such a wide Δ Tx range as described above possesses a stable amorphous phase and, when held at a suitably selected temperature in the Δ Tx range, does not produce any crystallization in about two hours. Therefore, the user of the alloy does not have to worry about crystallization during the standard forming process.

The alloy shows this characteristic without any retention during the transition from the molten state to the solid state. Generally, the manufacture of an amorphous alloy requires rapid cooling. In contrast, this alloy enables the easy manufacture of a granular material that is a single amorphous phase from a melt by effective cooling at a rate of about 10K/S. The solid granular material thus formed has a very smooth surface. The alloy has transferability so that even a scratch of about several micrometers on a surface of a metal mold due to polishing can be accurately reproduced.

Therefore, when the present alloy is used as the material for the ferrule, it is only required that the surface of the metal mold used to form the ferrule be adjusted to meet the desired surface quality requirements for the ferrule, since the resulting ferrule will faithfully replicate the surface quality of the metal mold. Therefore, in the conventional metal mold casting method or molding method, the work for adjusting the size and surface roughness of the mold can be omitted or reduced by using the alloy.

The alloy features include lower hardness, high tensile strength, high flexural strength, lower Young's modulus, high elastic limit, high impact resistance, smooth surface and high precision of casting or molding. This makes the alloy suitable for use as a ferrule material for optical fiber connectors. These characteristics may even allow such alloys to be molded in mass production using conventional methods.

The advantages resulting from the use of such an alloy for the manufacture of the ferrule will be described in more detail below.

The first advantage is that high-precision shaped articles can be produced in a mass production manner. The diameter of the receptacle of the ferrule in which an optical fiber is directly received is required to be as close as possible to substantially the diameter of the optical fiber. Previously ceramic shaped articles made by injection casting, degreasing and sintering were not able to meet the dimensional accuracy and surface quality of a ferrule. It is therefore customary to form a shaped article of a size which permits machining, then to machine it to its inside diameter by wire lapping and finally to machine it to its outside diameter by a complicated polishing process. In the present invention, the use of a precisely formed metal mold in the casting and the viscous molding (glass molding) also enables mass production of molded articles without the need for a finish polishing process or only an additional simple finishing treatment. The metal mold is very effective in producing a molded article that meets the requirements for roundness of the cross section of the small insertion hole and finish machining of the inner surface of the hole. To ensure a tight fit of a glass fiber, the PC finish often employed at the leading end of a ferrule, so that the leading end has a spherical convex surface, is no longer necessary. It is sufficient to perform a final polishing after the optical fiber is put in place. Thus, the long machining processes using metal and ceramic materials can be largely dispensed with. The same indicia apply to the outer diameter of the ferrule and are effective for consistency between the axial line of the outer diameter of the ferrule and the axial line of its small socket.

A second advantage resides in the mechanical properties of the ferrule, such as strength and toughness. Since ferrules for an optical fiber connector are repeatedly and frequently attached and detached, the abutting portions of the leading ends thereof must not be too hard, and have no scratches or cracks. The hardness, strength and toughness of the alloy are sufficient to prevent the above-mentioned defects.

The above-mentioned amorphous alloy having the above-mentioned characteristics can be advantageously used as materials for ferrules and other components of optical fiber splices and precision parts of micromachines and ferrules.

Now, the ferrule of the present invention will be described below with reference to the accompanying drawings.

Figure 2 shows a preferred embodiment of the ferrule of the present invention. This ferrule 1 has an insertion through hole 2, the through hole 2 having a small-diameter straight hole portion 3 formed along its axial center line for inserting an optical fiber end portion therein, a large-diameter portion 5 for inserting a shielded optical fiber portion therein, and a tapered diameter portion 4 disposed between the small-diameter portion 3 and the large-diameter portion 5 and expanding from the small-diameter portion 3 toward the large-diameter portion 5.

In such a ferrule 1, it is required that the length of the small diameter portion 3 of the through hole should not be less than 1 mm to insert an optical fiber end, and the total length of the small diameter portion 3 and the tapered diameter portion 4 should not be less than 6 mm.

If the length L1 of the small diameter portion of the ferrule for inserting an optical fiber end therein is less than 1 mm, it is impossible to easily achieve parallelism between the small diameter portion 3 and the outer surface of the ferrule, and the connection of two optical fibers by abutting the end faces of the two ferrules against each other is liable to be mismatched. Therefore, the length L1 must be not less than 1 mm.

The method for connecting an optical fiber to the ferrule 1 without breaking the optical fiber is to peel off a leading end portion of a shielded optical fiber (striping) in a length equal to a small diameter portion 3 into which the optical fiber is inserted, preferably in an angular direction corresponding to an inclination angle theta of a tapered diameter portion 4 of the ferrule, process an adhesive on a leading end portion of the shielded optical fiber, and finally insert the end into a through hole 2 of the ferrule 1. In addition, the work of inserting the optical fiber into the ferrule is stable in itself, and the adhesion strength between the optical fiber and the ferrule is sufficient. At this time, when the inside diameter of the large diameter part 5 of the ferrule 1 for inserting a shielded optical fiber therein is slightly larger than the outside diameter of the shielded optical fiber, by inserting the shielded optical fiber stripped off by the skin into the through hole 2 of the ferrule 1, since the tapered diameter part 4 is provided between the small diameter part 3 for inserting an optical fiber end and the large diameter part 5 for inserting a shielded optical fiber, the adhesive applied on the leading end part of the shielded optical fiber can be completely penetrated until the leading end part of the small diameter part 3 for inserting an optical fiber end passes through the intermediate medium of the tapered diameter part 4. As a result, the end of the optical fiber and the leading end of the shielded optical fiber are adhered to the through hole 2 of the ferrule 1 with outstanding strength. After optionally bringing the end face of the ferrule to which the optical fiber is to be connected into abutment with the end face of the other ferrule of the optical fiber which has been subjected to PC polishing, the former ferrule is mated with a plug as shown in fig. 1 and can then be used to connect the two optical fibers.

One example of a method of manufacturing the ferrule of the present invention will be described below with reference to fig. 3.

In fig. 3, reference numeral 11 denotes a vessel which can be used to melt a melt prepared by making an alloy material such as one of the above-described amorphous alloys and containing the alloy material therein. A pair of split metal molds 21 and 22 are provided below the container 11 to collectively form a cavity 24 in the shape of a product as produced. Any of the known methods such as, for example, high frequency induction heating and resistance heating, may be used to heat the vessel 11. The cavity 24 is defined by a pair of metal molds 21 and 22 and a mandrel 23. The shape of the mandrel 23 is formed of three parts, i.e., a small diameter part 23a, a large diameter part 23c and a tapered diameter part 23b connecting the two with each other in between, and is identical to the shape of the insertion hole 2 of the above-described ferrule 1. The length of the small diameter portion 23a of the mandrel 23 is short as compared with that of the equivalent portion in the conventional concept. Therefore, the mandrel 23 is easy to manufacture, has high strength, and has a long service life. In addition, because of the high strength of the mandrel, high molding pressures can be used for the cavity, thereby allowing the formation of a ferrule having high density and dimensional accuracy.

To realize the processing of the ferrule, a step is performed in which an elongated hole 12 formed on the bottom of a container 11 is first combined with a gate 26 formed by a pair of metal molds 21 and 22 together, while a melt 30 in the container 11 is pressurized by an inert gas medium, so that, for example, a predetermined amount of molten alloy is taken into the elongated hole 12 in the bottom of the container 11, passes through an annular flow passage 25 of the metal mold combination and is injected into a cavity 24, and the molten alloy is solidified at a cooling rate of preferably not less than 10K/S.

In addition to the alloy casting method described above, extrusion molding is also a ready method for the manufacture of ferrules. Since the above-mentioned amorphous alloy has a large supercooled liquid region Δ Tx, a hoop having a predetermined shape can be obtained by heating the amorphous alloy material to a temperature in the supercooled region, inserting the alloy material into a container kept at the same temperature, attaching the container to metal molds 21 and 22 provided with a cavity having the shape of the article to be manufactured as shown in fig. 3, pressing a predetermined amount of the heated alloy into the cavity by stagnation of the supercooled liquid, and finally molding the alloy.

Figure 4 shows another embodiment of the present invention ferrule. In this embodiment, the ferrule 1a has a small diameter portion 3a therein for inserting an end of an optical fiber and a tapered diameter portion 4a formed at an end portion of the small diameter portion 3a along an axial center line thereof. A flange 6 having a large diameter portion 5a formed along the axial centerline of the ferrule for inserting a shielded optical fiber portion therein is integrally connected to the ferrule 1 a. Specifically, the terminal end portion of the ferrule 1a having the tapered diameter portion 4a therein is tightly fixed by interference fit or adhesion to a hole portion 7 in the leading end portion of the flange 6 so that it is two-piece assembled. As a result, the insertion hole 2a is formed along the axis by the medium of the tapered diameter portion 4a engaging the small diameter portion 3a of the ferrule 1a and the large diameter portion 5a of the flange 6 with each other. The embodiment described above in which the collar is integrally joined to the flange also has the advantages of the invention described above.

The present invention will be described more specifically below by way of several working examples, and the effects of the present invention will be confirmed. Example 1:

production of the metals of the components involved Zr shown in the following Table by melting₆₀Al₁₅Co_2.5Ni_7.5Cu₁₅A plurality of alloys of (a). Each alloy was placed in a quartz crucible and melted thoroughly by high-frequency induction heating. At 2 kg force/cm²The melt was injected into a copper mold having a cylindrical cavity with a diameter of 2 mm and a length of 30 mm through an elongated hole formed in the lower portion of the crucible under a gas pressure of (1), and then kept at room temperature to obtain a cylindrical section sample for measuring mechanical properties thereof. The results are shown in the following table

Alloys used	Tensile Strength (MPa)	Flexural Strength (MPa)	α10-5K (Room temperature-Tg)	E(GPa)	Hardness Hv	Tg(K)	Tx(K)
								Zr67Cu33Zr65Al7.5Cu27.5Zr65Al7.5Ni10Cu17.5Zr59Al15Co2.5Ni7.5Cu15	1,8801,4501,4801,590	3,5202,7102,7702,970	0.80.80.91.0	99939291	540420430460	603622630652	669732736768

As is clear from the table: the bending strength of the obtained amorphous alloy material is obviously higher than the value (about 1000MPa) of the partially stabilized zirconia which is used as the hoop material, and the Young modulus of the amorphous alloy material is about half of that of the zirconia, and the hardness value of the amorphous alloy material is about one third of that of the zirconia. Indicating that these alloy materials have the characteristics necessary for use as hoop materials. Example 2:

zr prepared by melting each of the relevant component metals in advance after using a WC-containing hard metal produced by die-casting as shown in FIG. 3₆₀Al₁₅Co_2.5Ni_7.5Cu₁₅The alloy may be molded to form a ferrule according to the procedure of example 1. The dimensions of the metal mold, which are set in advance for the finish machining of the ferrule, should be such that the finish machining or polishing process is reduced and the entire inner surface of the metal mold is machined to have a mirror finish. The surface of the leading end of the ferrule having the insertion hole into which the optical fiber is inserted has a convex spherical shape with a radius of curvature of 20 mm and is mirror finished. After casting, the gated joint was removed by precision cutting.

The article thus formed appeared specular over the entire surface. The overall length of the ferrule was 10.5 mm, and the length (L) of the small-diameter portion inserted into the end of the optical fiber₁) Is 4 mm. The outer diameter of the ferrule, the inner diameter of the receptacle into which the optical fiber is inserted, the concentricity of these two diameters, the roughness of the outer and inner bore surfaces of the ferrule, and the cylindricity of the ferrule, which is of particular interest for its accuracy, all fall within the tolerance range of the relevant dimensions, i.e. 0.1 to 0.5 μ. Example 3:

a steel die as shown in fig. 3 was connected to a metal extruder and a ferrule was produced by extruding the same alloy as used in example 2. For the extrusion, amorphous billets of 25 mm diameter and 40 mm length prepared separately by casting were used. The billets were preheated to 730K, and the entrance portion and the molding portion of the container and the metal mold of the extruder were similarly preheated to 730K. The hot billet sections are inserted into an extruder vessel and then sprayed into a metal mold. The metal mold is cooled. The molded article is then removed from the mold, removed from its inlet portion, and inspected. It can be found that: the appearance, dimensional accuracy, surface roughness, and the like of the molded article were almost equal to those of the cuff obtained from example 2.

According to the present invention, as described above, a ferrule satisfying the dimensional accuracy and surface quality requirements required for an optical communication joint can be manufactured efficiently and at low cost by using a metal mold casting method or a molding method and using a plurality of amorphous alloys having a wide transition region, such as Zr-TM-Al and Hf-TM-Al amorphous alloys. In addition, since the amorphous alloy used in the present invention is excellent in strength, toughness and corrosion resistance, the ferrules made of the amorphous alloy can be used for a long period of time without suffering abrasion, deformation, peeling or other similar disadvantages.

Although certain specific embodiments and examples of processes are disclosed herein, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments and examples are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (8)

1. A ferrule for an optical fibre connector provided with a receptacle into which an optical fibre is inserted into position, the ferrule being formed from an amorphous alloy having at least one glass transition region.

2. The ferrule of claim 1 wherein the glass transition region has a temperature range of not less than 30K.

3. The ferrule of claim 1 wherein the glass transition region has a temperature range of not less than 60K.

4. The ferrule of claim 1 formed of an amorphous alloy having a composition represented by the formula and containing an amorphous phase in a volume ratio of at least 50%, the formula being:

X_aM_bAl_c

wherein X represents at least one element selected from the group consisting of Zr and Hf, M represents at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and a, b and c represent atomic percentages and satisfy the conditions 25. ltoreq. a.ltoreq.85, 5. ltoreq. b.ltoreq.70 and 0. ltoreq. c.ltoreq.35, respectively.

5. The ferrule as claimed in claims 1 to 4, which is provided along its axial center line with a small diameter portion into which an optical fiber end is inserted, a large diameter portion into which a shield optical fiber end is inserted, and a tapered diameter portion connecting the two portions, the length of the small diameter portion being not less than 1 mm, and the sum of the length of the small diameter portion and the length of the tapered diameter portion being not less than 6 mm.

6. A method of manufacturing a ferrule for an optical fiber splice, comprising the steps of:

providing a vessel containing a melt of said alloy material and having an opening therein for receiving a molten material;

providing a metal mold provided with a gate and a cavity having the shape of a product to be manufactured;

connecting the hole formed on the container to a gate of the metal mold;

applying pressure to said melt in the container to cause a predetermined amount of said melt to be fed into said mold through the orifice of said container to inject said melt into said cavity;

solidifying the melt in the metal mold at a cooling rate of not less than 10K/sec to obtain a product formed substantially in an amorphous phase.

7. The ferrule of claim 6, wherein the alloy material is formed of an amorphous alloy having a composition represented by a formula and containing an amorphous phase at a volume ratio of at least 50%, the formula being:

X_aM_bAl_c

wherein X represents at least one element selected from the group consisting of Zr and Hf, M represents at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and a, b and c represent atomic percentages and satisfy the conditions 25. ltoreq. a.ltoreq.85, 5. ltoreq. b.ltoreq.70 and 0. ltoreq. c.ltoreq.35, respectively.

8. A method of manufacturing a ferrule for an optical fiber splice, comprising the steps of:

heating an amorphous material to a super-cooled liquid region temperature, the amorphous material being formed of an alloy; the alloy composition is represented by a general formula and contains an amorphous phase of at least 50% by volume, the general formula being:

X_aM_bAl_c

wherein X represents at least one element selected from the group consisting of Zr and Hf, M represents at least one element selected from the group consisting of Mn, Fe, Co, Ni and Cu, and a, b and c represent atomic percentages and satisfy the conditions 25. ltoreq. a.ltoreq.85, 5. ltoreq. b.ltoreq.70 and 0. ltoreq. c.ltoreq.35, respectively;

inserting the obtained hot amorphous material into a container maintained at the same temperature;

attaching a metal mold having a cavity in the shape of a product to be manufactured to the container;

a predetermined amount of said alloy is introduced under pressure into said metal mold by means of stagnation of said super-cooled liquid to form a hoop.