Apparatus for injection moulding of plastic materials

Apparatus for injection moulding of plastic material includes a mould having at least one gate towards a moulding cavity and at least one injector cooperating with the gate. The gate is formed by a gate insert screwed and arranged resting on a centring seat with conical surface of the mould.

FIELD OF THE INVENTION

The present invention refers to apparatus for injection moulding of plastic materials comprising a mould having at least one gate towards a moulding cavity and at least one injector cooperating with the gate. The injector typically including a nozzle provided with a nozzle terminal defining a path for the flow of plastic material towards the gate.

STATE OF THE ART

A conventional injection moulding apparatus is schematically represented—in partial section—inFIG. 1of the attached drawings: it comprises a hot runner or distributor1for the fluid plastic material and one or more injectors2,3for the introduction of pressurised plastic material into the cavity of a mould, whose plate or die is indicated with4, through respective gates5.

Each injector2,3comprises a tubular nozzle7provided with a nozzle end, generally indicated with8, which defines a fluid path for the injected plastic material. In the case ofFIG. 1, the injectors2,3are of the so-called “shutter” type: each one of them is provided with a valve pin9provided with a shutter terminal10that can be axially displaced, by means of an electrical or fluid actuator11, between a receded position and an advanced position for opening and closing the nozzle terminal8.

The nozzle terminal8may have different configurations, represented in detail by way of example inFIG. 2relative to the injector2ofFIG. 1, and inFIG. 3relative to the injector3ofFIG. 1. In both cases, the nozzle terminal8comprises an inner tubular element or tip12and an outer annular element or ring nut13.

In the case of the nozzle terminal8ofFIG. 2, the tip12extends axially with the distal end12athereof beyond the distal end13aof the ring nut13, projecting into the gate5. Thus, the gate mark (represented in plan view underFIG. 2) which remains on the surface of the moulded article reveals very small dimensions, exclusively according to the diameter of the shutter terminal10of the valve pin9, this being particularly advantageous when it comes to moulding aesthetic components, in particular transparent or semi-transparent, and even more so in the case of motor vehicle lamps which require very high quality standards.

This solution, so-called with “outer” ring nut i.e. one not directly projecting into the moulding cavity, requires an accurate operation of the mould die4, at the height of the distal ends of the tip12and ring nut13, due to the constant variation of the radial overall dimension thereof as well as in particular the area of the gate5which receives the shutter terminal10of the valve pin9in the advanced closing position thereof. This implies the extension of the moulding apparatus production, procurement and commissioning times. Furthermore, the machining difficulty often leads to concentricity errors between the shutter terminal10of the valve pin9and the gate5of the die4, or the entire die insert if the gate5is not provided directly in the die4of the mould but rather in a hollow insert, indicated with14, applied in a corresponding seat of the die4. This entails a higher wear rate on the two components or even the breaking of the die or die insert, with serious economic damage for the mould.

The solution represented inFIG. 3, in which the nozzle terminal8is of the so-called “through” ring nut13, i.e. whose distal end13aaxially extends beyond the distal end12aof the tip12and it is directly projected into the moulding cavity, was proposed in an attempt to overcome such drawbacks. In this case, the gate5with which the shutter terminal10of the valve pin9cooperates is formed in the area of the distal end13aof the ring nut13, hence enabling considerable simplification in the operation of the die4, hence reducing times and risk of error. However, as represented in plan view in the lower part ofFIG. 3, to the gate mark corresponding to the shutter terminal10there is also added that of the distal end13aof the ring nut13, with dimensions that are much greater and actually unacceptable when it comes to moulding transparent aesthetic components and in particular motor vehicle lamp lenses.

The solution described in document US-2017/0100866 in the name of the Applicant, herein represented inFIG. 4, was proposed with the aim of overcoming these drawbacks.

This solution provides for an injector7wherein the tip12of the nozzle terminal8has a distal end12aconfigured so as to be coupled with the gate5, cooperate with the shutter terminal10of the valve pin9and have—in cross-section—an overall thickness2A equal to or smaller than that of the cross-section B of the shutter terminal10of the valve pin9. In this case, the gate5is formed in the die insert14, inserted into the seat of a mould plate as mentioned.

The characteristic according to which the distal end12aof the tip12is extended (so to say), i.e. it is such to be able to extend in the gate5of the mould to project into the moulding cavity, offers the advantage of considerably simplifying the operation of the die insert4of the mould.

On the other hand, the low wall thickness A of the distal end12aof the tip12enables equally considerably reducing the dimensions of the gate mark visible on the moulded article, which will be slightly larger than the imprint of the shutter terminal of the valve pin and basically similar thereto.

However, the particularly thin and extended configuration of the distal end12aof the tip12entails considerable dilatation in the axial direction, upon the variation of the moulding conditions.

This negatively affects the aesthetic quality of the moulded component, in that the distal end12awill tend to retract or extend axially.

In the first case there will be a non-perfect closing coupling between the distal end12aof the tip12and the shutter terminal10of the valve pin9, with ensuing formation of burrs in proximity of the gate5.

In the second case instead, the distal end12aof the tip will extend beyond the gate5into the moulding cavity, thus leaving a much more marked, evident and aesthetically unacceptable gate mark (some sort of coining) on the moulded piece.

FIGS. 5 and 6schematically show the state of the art regarding injectors of the “free-flow” type. Contrary to what has been described up to now, the use of a valve pin that can be displaced axially by means of an actuator is not provided for. The nozzle terminal8aprovides for shutting off the flow of the injected plastic material at the gate5by simply acting on the temperatures, in particular by cooling the plastic material at the end of the injection to solidify and stop the flow.

Similarly to the cases ofFIGS. 1-4, the nozzle terminal8acomprises an inner element or tip12and an outer annular element or ring nut13. In particular in the cases ofFIGS. 5 and 6, the tip12is of the “torpedo” type, i.e. characterised by a conical-shaped distal section which ends with the vertex12athereof in proximity of the gate5. The communication between the flow channel of the injector7and the gate5occurs through one or more inclined holes30obtained in the tip12.

InFIG. 5, the ring nut13has a configuration similar to that ofFIG. 2. As a matter of fact, it is “external”, i.e. it does not project into moulding cavity directly. As previously mentioned, this configuration requires accurate machining of the die insert14at the height of the distal ends of the tip12and ring nut13, thus entailing an extension of the times required for producing, procuring and commissioning the moulding apparatus. Besides this, the die insert14is directly subjected to wear due to the flow of plastic material which flows during the numerous moulding cycles that are usually provided for. The wear is particularly accelerated when the plastic material is loaded with reinforcement material (e.g. glass fibre) with the aim of increasing the mechanical resistance of the moulded component. The wear of the die insert14, in particular in the gate5area, is unacceptable in the long term due to the aesthetic quality of the moulded component. Thus, there arises the need to replace the entire die insert14(or the die4itself, if the insert14is not provided for) with ensuing increase of production times and costs as well as long shut down times.

The solution represented inFIG. 6, in which—similarly to the case ofFIG. 3—the ring nut13is of the “through” type i.e. its distal end13aaxially extends beyond the distal end12aof the tip12and it is directly projected into the moulding cavity, was proposed in an attempt to overcome such drawbacks. In this case, the gate5is formed in the area of the distal end13aof the ring nut13, hence enabling considerable simplification when it comes to machining the die insert14. However, as schematised in plan view in the lower part ofFIG. 6, the gate mark of the distal end13aof the ring nut13has unacceptable dimensions from an aesthetic point of view if compared with the mark visible inFIG. 5.

US 2003/235638 discloses an injection moulding apparatus having a gate insert resting upon a centring seat of the mould having a conical surface. Similar arrangements are also disclosed in JP H07186203 and JP H05309695.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the drawbacks of the aforementioned prior art solutions, so as to guarantee the desired aesthetic quality of the moulded details as well as lesser times and costs required for producing, procuring and commissioning the moulding apparatus.

This object is attained according to the invention by means of injection moulding apparatus of the type defined above, whose peculiar feature consists of that the gate insert has threading which is screwed with a corresponding threading of the mould.

The gate insert can be fitted within a hollow die insert applied in a mould plate and formed with the inner threading and the centring seat, or instead said inner threading and said centring seat for the gate insert can be directly formed in the mould plate.

The gate insert has conveniently a base formed centrally with an annular axial appendage coupled with the aforementioned centring seat and having a conical wall with thickness tapered towards the moulding cavity. Such axial appendage advantageously keeps the base of the gate insert spaced from the mould surface.

The invention considerably advantageously applies to injection moulding apparatus provided with multiple injectors controlled according to sequential or cascade cycles, or also in multi-cavity injection moulding characterised by a high number of moulding cycles with low times.

DETAILED DESCRIPTION OF THE INVENTION

In order to overcome the drawbacks of the previously mentioned prior art solutions, the invention proposes a solution that is conceptually different and capable of guaranteeing the desired aesthetic quality of the moulded components as well as lesser times and costs required for producing, procuring and commissioning the injection moulding apparatus. Basically, the element of the invention consists of an insert, referred to as gate insert, positioned in proximity of the gate in a conventional gate insert, where present, or directly in a seat of the mould die or plate.

Initially referring toFIGS. 7 and 8, in which parts identical or similar to those described previously with reference toFIGS. 1-6are indicated using the same reference numbers, this gate insert is indicated with15and it is screwed into the die insert14resting—at the lower part—on a seat16—with conical surface—of such die insert14in proximity of the gate5.

In detail, and as represented inFIGS. 10 to 12, the gate insert15consists of a metal hollow body having a circular base17with an axial perimeter wall18formed with an outer threading19and provided—at the edge thereof—with notches29for engaging a manoeuvring tool (not illustrated). Axially departing from the base17is an annular central appendage20projecting on the opposite side of the wall18and delimiting, at the free end thereof, the gate5.

As better observable inFIG. 8, the central appendage20is generally conical shaped, with an inner wall21and an outer wall22having respective conical surfaces with different conicity, in a manner such that the central appendage20is tapered towards the mould cavity. The conicity of the outer wall22is complementary to that of the conical seat16of the die insert14.

In a first preferred embodiment, regarding the “shutter-type” injection and visible inFIGS. 7-12, the gate insert15cooperates with the shutter terminal10of the valve pin9to open or close the gate5. In particular, the distal end of the inner conical surface21has a cylindrical terminal portion28, which actually forms the gate5, whose radial thickness is considerably smaller than the transversal dimension of the cylindrical shutter terminal10of the valve pin9which—with such cylindrical portion28—sealingly cooperates in the advanced position for closing the valve pin9in which the gate5is closed.

Thus, the cylindrical terminal portion28of the gate insert15, which can also be substantially of the sharp edge type, has an aesthetically irrelevant overall thickness (e.g. about 0.2 mm) if compared with the diameter of the shutter terminal10(e.g. about 2.0 mm).

The outer threading19of the wall18of the gate insert15is engaged with a corresponding inner threading24formed in the wall of a recess25of the die insert14. In the fully screwed condition, obtained by means of the previously mentioned manoeuvring tool, the outer conical surface22of the central appendage20rests on the conical seat16of the die insert14so that the base wall17of the gate insert15is kept spaced from the recess bottom25thus creating an axial gap26.

According to a preferred configuration, the gate insert15has an inner geometry that is complementary to that of the shutter terminal10of the valve pin9not just at the gate, but also upstream with respect thereto. To this end, at a short distance from the shutter terminal10, the valve pin9conveniently has a portion with conical surface23which, in proximity of the advanced closing position, rests against a part of the inner conical surface21of the central appendage20of the gate insert15. The coupling between the conical and cylindrical surfaces of the valve pin9and of the central appendage20of the gate insert15attains an accurate self-centring of the shutter terminal10with respect to the gate5when closing the valve pin9, thus ensuring a neat and clean closing of the gate5, typical of cylindrical shutter systems. As a matter of fact, with respect to conical shutter systems, the plastic residue between the shutter terminal10and the wall of the gate5, which entails inaccurate closure and ensuing defects on the moulded piece, does not occur any more.

According to an advantageous characteristic, the gate insert15is made of a material that is harder and more resistant to wear with respect to the die insert14(or to the mould plate4), it being the only element directly cooperating with the valve pin9. Hence, the die insert14may be made of less costly material, with clear advantages for the customer.

According to a further solution, when the injection apparatus is operating, the displacement speed of the valve pin9from the opening position to the closing position thereof, controlled electronically for example, can be conveniently reduced in the final stage of the closing movement. Thus, the centring can be carried out in an even more controlled manner, thus limiting the wear of the components and any risks of breaking the valve pin9besides the gate5. Thus, this solution enables using less expensive materials for the gate insert15, and in cascade fashion for the die insert14(where present).

However, should wear problems arise during the operation of the moulding apparatus, it would still be sufficient to replace the gate insert15(to be carried out by the supplier of the hot runner injection system) only and thus not the entire die insert14, usually provided by the customer with high costs and production times considering the required machining operations. Thus, this would substantially not affect the customer, save for commissioning times.

Another advantage arising from the cone-cone contact between the valve pin9and gate insert15consists of the possibility of dissipating the heat of the shutter towards the die insert14, typically provided with paths27for the coolant fluid. Being surrounded—during operation—by plastic material at a high temperature, the valve pin9tends to be particularly hot even at the gate5level. This entails that the plastic material actually sticks to the shutter terminal10of the valve pin9. Once through with closing, the moulded piece clearly cannot be detached from the shutter terminal10, with ensuing aesthetically unacceptable defects. The indirect contact (through the gate insert15) between the valve pin9and the cooled die insert14, enables to dissipate the heat of the valve pin9and thus overcome the problems related to sticking.

Besides between the valve pin9and the gate5, the gate insert15is also designated for carrying out the centring between the injector2, as a whole, and the gate5. In particular, the ring nut13guarantees this centring by resting against the axial wall18of the gate insert15, suitably machined with tolerance.

Thus, all the elements (ring nut13, injector2, valve pin9, gate insert5) are mutually centred using the same component, i.e. the gate15. Thus, all the user of the injection apparatus needs to do is provide the conical seat16of the die insert14with due tolerances, besides just threading24, to guarantee the centring of the gate insert15(and thus all elements) with respect to the gate5.

With respect to the state of the art, and in particular in case of an outer ring nut13, the machining of the die insert14by the customer is considerably simpler, quicker and less expensive, given that all that is required is to provide the conical seat22with the due tolerances and just threading24.

According to a further advantageous characteristic, the seat25obtained in the die insert14is configured for housing different gate inserts15in terms of axial extension, maintaining the conical seat16and the threading24unaltered.FIG. 9shows an example of a different configuration in which the base17of the gate insert15has a greater thickness and the wall18projects further from the recess25towards the injector3.

Thus, the same die insert14may be combined with longer or shorter injectors by simply installing a gate insert15with lesser or greater axial extension, without having to replace or machine the die insert14again.

Furthermore, it should be observed that given that there is no contact between the base17of the gate insert15and the recess25of the die insert14, the presence of the aforementioned axial gap26enables eliminating the chances of error when performing the cone-cone centring between the gate insert15and the die insert14.

According to an alternative embodiment, shown inFIG. 13, the gate insert15cooperates with a nozzle terminal8areferred to as of the “free-flow type”, i.e. without the valve pin9and the relative shutter terminal10.

Similarly, to what has been described in the case ofFIG. 5, the tip12is of the “torpedo” type, i.e. characterised by a conical-shaped distal section which ends with the vertex12athereof in proximity of the gate5. The communication between the flow channel of the nozzle7and the gate5occurs through one or more inclined holes30obtained in the tip12. The ring nut13is external, i.e. it does not project into moulding cavity directly.

With the aim of overcoming problems arising from the accurate machining of the die insert14, the wear as a function of the same and any unacceptability of the gate mark (in case of the “through” ring nut13ofFIG. 6), there is provided for the use of the gate insert15in a manner similar to the cases described above.

The insert15delimits the gate5but leaves a small and thus aesthetically acceptable mark, as shown in plan view in the lower part of page13. The machining required for the die insert14is limited to the conical seat22and the threading24.

Furthermore, only the gate insert15is subjected to direct wear during operation due to the flow of plastic material (possibly loaded, e.g. with glass fibre) in proximity of the gate5. Thus, according to an advantageous embodiment, the insert15may be made of material that is highly hard and resistant to wear. The die insert14may in turn conveniently be made of material that is less hard and resistant to wear, thus entailing considerable saving on costs for the customer. However, in case of excessive wear of the gate insert15, replacement thereof will suffice, without affecting the die insert14in any manner whatsoever.

Lastly, all other advantages, solutions and characteristics described above regarding the preferred embodiment shall apply to the “free-flow” embodiment, obviously except for those regarding the presence of the valve pin9and the relative shutter terminal10.

Obviously, without prejudice to the principle of the invention, the construction details and the embodiments may widely vary with respect to what has been described and illustrated, without departing from the scope of protection of the present invention as defined in the claims that follow. Thus, as pointed out, the presence of the die insert14is not strictly necessary, and the gate insert15will—in this case—be directly applied in a suitable seat of the mould plate or die4, configured similarly to the recess25.