Background art The process of injection-moulding plastic objects, for example, bottle caps or items made of PP or PE, involves filling moulds with melted plastic using different types of injection nozzles. The shape of the used nozzle depends on the type of material to be injected, on its fluidness when in the melted state, and on the operating temperature.

One type of moulding operation consists in intermittently depositing an amount of melted plastic in a moulding cavity; cooling the plastic ; then, removing the moulded object as soon as its structure becomes sufficiently solid to allow safe removal without deformation. In this case, the injection nozzles work intermittently since there is a pause after the melted plastic is injected into the mould. During this pause, the melted material found in the duct of the nozzle could solidify if heating devices are not used to keep the plastic in the melted state. This is particularly true for the part where the plastic flows out of the nozzle : this part is very near to the mould, which is cooled actively; this means that continuous heating must be provided.

To resolve the problem of keeping the plastic in the nozzle in the melted state during the moulding operation, one of the state-of-the-art solutions is to use the so-called torpedo nozzles.

Said nozzles, which have been around for many years, feature an insert, the so- called torpedo, made of a material with a high thermal conductivity. This torpedo is found inside the duct that injects the melted plastic, specifically in the end part of the tip of the nozzle ; thus, near the opening that injects the plastic into the injection chamber. The heating system, which is placed in the nozzles to prevent the plastic from cooling, heats the torpedo that, thanks to its high conductivity, transmits the heat to the melted plastic in the cavity of the tip of the nozzle keeping the plastic in the melted state throughout the injection process.

Another function carried out by the inserts is to mix the plastic by making it flow from a duct with a circular section into a duct with an annular section in the area immediately before the moulding cavity of the object. Unfortunately, this operation can compromise the quality of the finished object due to the visible welding lines on the moulding. To resolve this problem, one of the technical means adopted to improve plastic mixing is to have a number of channels that run through the insert; for example, two or three outlet ducts that communicate with one or more inlet ducts in the insert. The resulting insert looks somewhat like a torpedo, hence the name, and is characterized by an essentially conical tip at the outlet part of the nozzle. The tip of the insert, which is found in the chamber in the tip of the nozzle, helps to maintain the temperature of the plastic at a level that guarantees fluidity.

When using these inserts, care must be taken to design them so that there are no dead zones or obstacles in the feeding duct of the melted plastic. In fact, these could lead to the formation of plastic deposits that could solidify due to localized cooling or due to the use of a particularly viscous material. The resulting partial obstruction could cause moulding defects and/or stoppages in production.

Another problem that arises when using the torpedo nozzle stems from dividing the flow of melted plastic into smaller flows to mix the plastic ; these flows are as many as are the number of ducts in the torpedo. This division can leave marks in the form of streaking. In fact, studies have shown that, in spite of the fact that the flow of plastic is divided at a temperature that enables the plastic sub-flows to flow back together easily after having travelled in the different channels of the insert, streaking is visible in the finished moulding marking the welding point of the plastic flows. The reasons for this phenomenon are not completely understood; anyhow, this phenomenon is undesirable.

Numerous nozzle insert designs have been put forward to improve performance during injection operations. For example, document US-A-5545028 describes a nozzle with an insert with a duct that links the central duct of the body of the nozzle, which feeds the melted plastic, with the chamber of the end part of the tip of the nozzle. In a first version, said duct flows out into a groove found on the

external surface of the insert while, in a second version, it flows out into two grooves that branch off from the duct circumferentially at 180° around the middle part of the distribution portion of the plastic. The groove has a cross section that gradually flares out so that it gradually opens into the end part of the chamber of the nozzle. The purpose of the special design of the surface of the insert is to create a uniform flow of melted plastic flowing axially from the outlet of the channel toward the outlet of the nozzle so that the annular-shaped mass moves homogeneously along the surface of the insert.

Summary of the invention It is an object of this invention to provide nozzles for injection-moulding that resolve the aforementioned problems.

It is a main object of this invention to provide a nozzle that produces a homogenous flow of plastic for the injection operation and eliminates streaking in the finished moulded object.

It is another object of this invention to provide a nozzle that features optimal temperature distribution throughout the feeding duct of the melted plastic in order to maintain the melted mass at an optimal viscosity.

It is an additional object of this invention to provide a nozzle that mixes the plastic in the area immediately before the inlet of the mould chamber.

These objects are achieved by means of a nozzle in accordance with claim 1.

Thanks to the special shape of the nozzle annular area, which is formed by the external surface of the insert and the intemal surface of the chamber of the end part of the nozzle, the melted plastic, after travelling in the ducts that cross the wall of the insert in an essentially radial manner, flows in an axial direction toward the outlet of the nozzle. When moving through this annular area, thanks to the path defined by the grooves that intersect each other, the melted plastic is mixed vigorously forming a homogenous mass. Following said remixing, the plastic mass that is pushed into the moulding cavity does not have the typical streaking caused by rewelding the surfaces of the different flows that flow through each duct that run through the insert in a radial manner.

This invention makes it possible to overcome the defect common to the state-of- the-art torpedo nozzles.

Brief description of drawings Other advantages of the invention will be readily apparent from the more detailed description of a particular version of the invention, given as a nonlimiting example and in conjunction with the following accompanying drawings: - Fig. 1 shows a section view of a nozzle in accordance with the invention - Fig. 2 shows an enlarged side view of an item of the nozzle of Fig. 1.

Detailed description of the invention Figure 1 shows a nozzle, hereinafter also referred to as reference 1, inserted into a nozzle-holding plate 2. In the top part of the nozzle-holding plate 2, as shown in the figure, there is a mouth 3 for feeding the melted plastic to a moulding cavity not shown in the figures. The nozzle 1 comprises a body 4 having an axial duct for feeding the melted plastic coming from a known feeding duct 5 not shown in the figure. The body 4 of the nozzle includes known fastening means for securing it to the nozzle-holding plate 2.

The nozzle 1 comprises a sleeve 6 that surrpunds the body 4 and incorporates electric resistances or other suitable heating devices, not shown in detail, for heating the nozzle and the ducts in which the melted plastic flows. Some electric wires 7 are shown schematically in the bottom part of the Figure.

An insulating sleeve 8 envelops the body of the nozzle 1 in order to minimize heat dispersion toward the nozzle-holding plate that maintains an operating temperature lower than the one of the nozzle 1 since it is in contact with the mould-holding plate not shown in the figure, which is force cooled during moulding. A ring nut 9 is used to fasten the heating 6 and insulating 8 sleeves and the thermocouples that control the heating sleeves 6.

An insert 10 is placed at the extremity of the body 4 of the nozzle 1. This insert fits into a cylindrical housing machined in the body 4 and is coaxial with the duct 5. A ring nut or threaded ring 11 fastens solidarily the insert 10 to the nozzle body 4.

The insert 10, the so-called torpedo, is shown in detail in Figure 2. Its shape is provided substantially with a cylindrical part and an ogival part. The cylindrical part is subdivided into three parts. The first segment 12 is cylindrical and fits

entirely into the housing machined in the body 4 of the nozzle. The striking surface 21 at the end of this segment 12 is shaped like a circular crown and is coupled with the ring nut or ring 11 in order to fasten the insert 10 to the body of the nozzle. A second segment 13 of the insert has an outer cylindrical surface with a diameter section equal to the inside diameter of the inside hole of the ring nut 11. Finally, a third segment 14 of the insert has an outside cylindrical or conical surface with a diameter section smaller than the diameter of the inside hole of the ring nut 11. When the nozzle is in the assembled position, the difference between the outside diameter of the third segment 14 of the insert and the inside diameter of the ring nut 11 forms an annular cavity 21 that opens up directly into the end chamber 20 of the nozzle 1. Alternatively, this segment of the insert can have a conical shape with a very small top angle so that the annular cavity widens as it approaches the tip of the insert.

The segment 15 at the end of the insert has an ogival or conical shape that ends in a tip.

The insert 10 features an axial central hole 16 that goes through the second segment 12 and splits into four ducts 17,17', 17", 17"', which have axes arranged orthogonally or slanting with respect to axis A of the insert 10. The three ducts 17,17', 17", 17"'link the axial hole 16 with the external surface of the insert, which they flow out onto. The number of ducts can vary, but is at least two. The outlet holes of the ducts 17,17', 17", 17"'communicate with the grooves 18 that cover the entire external surface of the insert 10 and have a spiral shape; furthermore, some are right-hand spirals and some are left-hand spirals. Preferably, each hole comes out onto a point of intersection of two grooves of opposite direction. Some geometric figures 19 are produced in basso-rilievo on the external surface of the insert due to the intersection of the different spiral grooves 18 and the fact that the spirals move in opposite directions. The grooves should preferably, but not necessarily, have the same tilting angle as the path with respect to axis A. The geometric figures should essentially, but not necessarily, be rhombus shaped.

The grooves 18 can be suitably shaped so that their cross section features a decreasing area as they move away from the outlet hole of the ducts 17,17',

17", 17"'. This can be accomplished by keeping constant the diameter of the cylindrical segment 14 and decreasing the depth of the groove or, alternatively, keeping constant the depth of the groove and shaping the external surface of the segment 14 conically. It is also possible to create grooves with a combination of these solutions.

What follows is a description of the operation of the nozzle of the invention. The plastic is heated to the moulding temperature by means of known heating devices and fed under pressure through the duct 5 by means of known feeding means not shown in the figures. The plastic flows through the ducts 17,17', 17", 17"'and flows in the grooves 18 in the second segment 13 of the insert. Since in this segment the outside diameter of the insert is equal to the inside diameter of the ring nut 11, the spiral grooves 18 form ducts that intertwine forcing the melted plastic to flow in a crossed way, mixing the plastic in the process. When the plastic reaches the third segment 14 of the insert that forms an annular cavity inside the ring nut 11, this leaves the grooves and fills said cavity. Hence, in this segment there are two types of movement: the spiral movement of the plastic that follows the grooves 18, and the axial movement of the plastic that comes out of the grooves and fills the annular area 21. The mixed plastic flows into the end chamber 20 from where it is injected into the moulding cavity. This combination of different types of movement mixes the plastic vigorously after flowing through the ducts 17,17', 17", 17"'.

In this way, the drawback of the known torpedo nozzles is resolved, doing away with the streaking marking the rewelding surfaces after flowing through the insert.