WO2018025150A1

WO2018025150A1 - Optimized apparatus and method for forming coloured plastic objects

Info

Publication number: WO2018025150A1
Application number: PCT/IB2017/054643
Authority: WO
Inventors: Stefano Bergami; Davide BALDISSERRI
Original assignee: Sacmi Cooperativa Meccanici Imola Societa' Cooperativa
Priority date: 2016-08-03
Filing date: 2017-07-31
Publication date: 2018-02-08
Also published as: IT201600081886A1; EP3493976A1; CN109641402A

Abstract

This invention relates to a continuous cycle moulding system (1, 1', 1") for forming coloured plastic objects using at least one colouring liquid, which allows reducing waste of colouring liquid and plastic material when changing or substituting at least one colouring liquid.

Description

DESCRIPTION

OPTIMIZED APPARATUS AND METHOD FOR FORMING COLOURED

PLASTIC OBJECTS Technical field

This invention relates to a continuous cycle moulding system for forming coloured plastic objects using at least one colouring liquid, which allows reducing waste of colouring liquid and plastic material when changing or substituting at least one colouring liquid.

Background art

Systems for moulding or producing plastic objects such as, capsules or caps for example, process or treat at least one plastic material along an operating path. Located along this path are an injection system for injecting at least one colouring liquid, an extruder for melting and simultaneously pressurizing the material, and a moulding machine which receives the flow feeding out of the extruder to give the material the shape and consistency needed to make the plastic objects.

An example of a system of this kind is disclosed in patent document US2004/0197435A1.

When a system like the one described in patent document US2004/0197435A1 is being used and at least one colour of the plastic objects needs to be changed, it is necessary to dispose of the plastic material remaining in the extruder and already at least partly mixed with at least one preceding colouring liquid.

This material cannot be recovered precisely because it is to some extent already mixed with the preceding colouring liquid.

Patent document EP0794222A2 relates to a process for making additives for synthetic filaments in thermoplastic polymer material. However, this solution does not concern the formation of plastic objects by molding. Patent document US3817875A relates to the formation of plastic objects by means of an extrusion mold, in which a dye is inserted at the entrance of a mixer. However, US3817675A does not describe compression molding or injection molding.

Patent document EP1892034A1 describes an extruder to which a plurality of dosing units for a liquid dye is attached.

Disclosure of the invention

This description has for an aim to provide a moulding system for making plastic objects and a moulding method for making plastic objects to overcome this disadvantage of the prior art.

More specifically, the aim of this description is to provide a moulding system for making plastic objects and a moulding method for making plastic objects and which are capable of reducing the amount of plastic material wasted as a result of changing at least one colouring liquid which is mixed with the plastic material to colour the objects.

This aim is fully achieved by the moulding system for making plastic objects and the moulding method for making plastic objects as characterized by what is contained in any combination of one or more of the claims accompanying this application and relating respectively to the system and method.

More specifically, this description concerns a continuous cycle moulding system for making objects from plastic material, defining an operating path for processing the plastic material.

The system comprises an extruder designed to receive plastic material in raw form and to feed out a stream of plastic material in the liquid phase. The system comprises an injection unit (that is, injector assembly) configured to deliver at least one colouring liquid for colouring the plastic material. In one embodiment, the system comprises (at least) a moulding machine designed to receive the stream of liquid plastic material and configured to mould the objects from the plastic material.

The injection unit operates downstream of the extruder to inject the at least one colouring (dyeing) liquid into the stream of liquid plastic material downstream of the extruder.

According to another aspect, the present disclosure concerns a moulding method for making plastic objects, comprising the following steps:

- extruding plastic material in raw form in order to feed out a stream of the plastic material in the liquid phase;

- delivering at least one colouring liquid for colouring the plastic material;

- moulding the plastic objects in a moulding machine designed to receive the stream of liquid plastic material.

The at least one colouring liquid is injected downstream of the extruder. In one embodiment, the at least one molding machine includes an injection molding machine and / or a compression molding machine. The injection molding machine is an alternative machine, where a plurality of objects are printed simultaneously. The compression molding machine is a rotary machine, in which a plurality of molds are positioned on a carousel. !n a possible embodiment (more specifically), the objects produced by the molding machine are capsules (e.g., closures of containers) or preforms, to be blown into containers.

However, it should be noted that, more generally, this disclosure is not necessarily limited to the fact that the machine for producing plastic objects from liquid plastic is a molding machine: it could be any machine for making plastic objects (e.g. semi-worked items) from liguid plastic (for example by blowing or other known technique).

In an embodiment, the system includes an intermediate duct. The intermediate duct is located downstream of the extruder and upstream of the molding machine. The intermediate duct is adapted to receive the flow of liquid plastic material leaving the extruder. The injection unit includes a nozzle having at least one outlet section operably inserted in the intermediate duct to inject the at least one dyeing liquid into the intermediate duct.

The intermediate duct can be placed upstream (or downstream) of a mixer.

The intermediate duct extends along a longitudinal axis. The liquid plastic material passes along the intermediate duct moving in the longitudinal direction.

In an embodiment, the nozzle is movable to vary its orientation with respect to the longitudinal axis of the intermediate duct. Alternatively or in combination, the nozzle is mobile to vary its distance from the longitudinal axis of the intermediate duct (i.e. its insertion depth inside the intermediate duct).

In one embodiment, the system includes a pumping unit, positioned between the extruder and the molding machine.

In an embodiment, the injector assembly is configured to enter the at least one colouring liquid downstream of the pumping group. Alternatively, the injector assembly is configured to enter at least one coloring fluid upstream of the pumping group.

!n one embodiment, the system includes a control unit. The control unit is configured to control the coloring, that is, if is a color control unit.

The control unit is configured to receive (at least) an input signal, representative of a mixing quality between said liquid plastic material and said at least one coloring liquid.

As for the detection (or calculation) of the input parameter, the system may include a monitor assembly for monitoring the plastic material contained in the extruder to verify its degree of plasticizing (for example by detecting solid grain concentration). In one embodiment, the monitor assembly includes a plurality of optical sensors arranged along the extruder, spaced along a direction in which the extruder is elongate (substantially the axis of the extruder auger). As for the input (or calculation) of the input parameter, an example is provided by the patent application WO2018/181381A1 of the same applicant, which is herein incorporated by reference.

In one embodiment, the control unit is configured to generate a control signal, depending on the input signal.

In one embodiment, the control unit is connected to the injector assembly to control it, via the control signal (therefore, depending on the input signal).

In an embodiment, the injector assembly comprises an auxiliary pump for pumping said at least one coloring liquid into the flow of liquid plastic material, !n this case, in a possible embodiment, the control unit is programmed (configured) to control the auxiliary pump; that is, the control unit is connected to the auxiliary pump to control it via the control signal. In an embodiment, the control unit is programmed (configured) to control the nozzle, via the control signal (depending on the input signal); in one embodiment, the control unit is programmed (configured) to control the nozzle, its orientation or its depth of insertion within the (intermediate) duct, wherein liquid plastic material flows through the duct and wherein the nozzle is operatively inserted in the duct.

Brief description of drawings

These features will become more apparent from the following description of respective example embodiments of the system and method provided purely as non-limiting examples, with reference to the accompanying drawings, in which:

- Figure 1 schematically illustrates a first possible embodiment of a system according to this description;

- Figure 1 a is a transverse cross section of a part of the embodiment of the system shown in Figure 1 or in Figure 2;

- Figure 2 schematically illustrates a second possible embodiment of a system according to this description; - Figure 3 schematically illustrates a third possible embodiment of a system according to this description.

Detailed description of preferred embodiments of the invention

Figure 1 schematically illustrates a first possible embodiment of a continuous cycle moulding system for making objects from plastic material according to this description and denoted by the numeral 1 . The system 1 defines an operating path for processing the plastic material.

The system comprises an extruder 2 designed to receive plastic material in raw form and to feed out a stream of plastic material in the liquid phase. In the drawings, the material in the raw form is denoted by the letter G. The drawings also show a possible infeed duct 12 adapted to transfer the material G from a possible infeed hopper T to the extruder 2.

The extruder 2 may, for example, comprise a rotary feed screw.

The system 1 comprises an injection unit 3 configured to deliver at least one colouring liquid for colouring the plastic material.

The injection unit 3 operates downstream of the extruder 2 to inject the at least one colouring liquid into the stream of liquid plastic material downstream of the extruder 2.

In an example embodiment of the system 1 , the injection unit 3 comprises a nozzle 4. The nozzle has at least one outlet section 4a. The outlet section 4a is positioned in such a way that the nozzle 4 injects or delivers the colouring liquid into the stream of liquid plastic material previously fed out of the extruder 2.

In Figure 1 a, the stream of liquid plastic material is denoted by the reference letter F.

In an example embodiment of the system 1 , the injection unit 3 comprises at least one auxiliary pump 5 for pumping the at least one colouring liquid through the nozzle 4.

The auxiliary pump 5 pumps the colouring liquid through the nozzle 4 and thus through the outlet section 4a of the nozzle 4. In an example embodiment, the system 1 comprises an intermediate duct 6 designed to receive the stream of liquid plastic material feeding out of the extruder 2.

In an example embodiment of the system 1 , as shown in Figure 1 a, the outlet section 4a of the nozzle 4 is operatively inserted in the intermediate duct 6 in such a way that the nozzle 4 injects or delivers the colouring liquid into the intermediate duct 6.

In an example embodiment of the system 1 , the intermediate duct 6 is elongate and/or extends along a longitudinal axis H, shown only in Figure 1 a. Figure 1 a shows a cross section of the intermediate duct 6, orthogonal to the longitudinal axis H.

In an example embodiment of the system 1 , the nozzle 4 is oriented radially relative to the intermediate duct 6 and, thus, relative to the longitudinal axis H of the intermediate duct 6.

In an example embodiment of the system 1 , the outlet section 4a of the nozzle 4 defines an outfeed direction of the at least one colouring liquid.

The outfeed direction is preferably directed from the outlet section 4a towards the longitudinal axis H of the intermediate duct 6.

In an example embodiment of the system 1 , the outfeed direction defined by the outlet section 4a of the nozzle 4 is directed radially relative to the longitudinal axis H of the intermediate duct 6.

In an example embodiment of the system 1 , the nozzle 4 is movable relative to the intermediate duct 6 between at least a first position, where the outlet section 4a is at a first distance from the longitudinal axis H, and at least a second position, where the outlet section 4a is at a second distance from the longitudinal axis H, different from the first distance.

In an example embodiment of the system 1 , the nozzle 4, in order to move from the first position to the second position and vice versa, is movable radially relative to the longitudinal axis H of the intermediate duct 6.

In an example embodiment of the system 1 , the nozzle 4 is movable to vary its orientation relative to the longitudinal axis H of the intermediate duct 6.

In an example embodiment of the system 1 , the nozzle 4 is movable to vary the orientation and/or inclination of the outfeed direction of the at least one colouring liquid from the outlet section 4a relative to the longitudinal axis H of the intermediate duct 6.

Thus, the nozzle 4 might be movable to vary its orientation relative to the longitudinal axis H of the intermediate 6 and to vary the distance between the outlet section 4a and the longitudinal axis H.

In an example embodiment of the system 1 , the injection unit 3 comprises a movement system configured to make the nozzle 4 move at least from the first position to the second position and vice versa. There might also be more than two such positions, in which case the movement system is configured to make the nozzle 4 move from any of these positions to any of the other positions.

The movement system may be configured to vary the orientation of the nozzle 4.

In the latter case, the movement system is thus configured to vary the orientation and/or inclination of the outfeed direction of the at least one colouring liquid from the outlet section 4a relative to the longitudinal axis H of the intermediate duct 6.

The movement system may thus be configured both to vary the orientation of the nozzle 4 relative to the longitudinal axis H of the intermediate duct 6 and to vary the distance between the outlet section 4a of the nozzle 4 and the longitudinal axis H of the intermediate duct 6.

The movement system might, for example, comprise a telescopic coupling between two parts of the nozzle 4.

In an example embodiment of the system 1 , the injection unit 3 comprises an inlet duct 7. In this case, the nozzle 4 belongs to or is integral with or linked to the inlet duct 7, or is built into the inlet duct 7. The auxiliary pump 5 is, in this case, designed to pump the at least one colouring liquid through the inlet duct 7. In an example embodiment of the system 1 , the inlet duct 7 is in fluid communication with the intermediate duct 6 so that the nozzle 4 itself is in fluid communication with the intermediate duct 6.

In an example embodiment of the system 1 , the inlet duct 7 is in fluid communication with the intermediate duct 6 in such a way that the outlet section 4a of the nozzle 4 is operatively inserted in the intermediate duct 6, so that the nozzle 4 injects or delivers the at least one colouring liquid into the intermediate duct 6.

In an example embodiment of the system 1 , the nozzle 4, in order to move from the first position to the second position and vice versa, is movable along at least one portion of the inlet duct 7.

In Figure 1 a, the nozzle 4 is shown at different example positions: one represented by a solid line and two by dashed lines. These positions are associated with respective different distances of the outlet section 4a of the nozzle 4 from the longitudinal axis H of the intermediate duct 6.

The system 1 comprises at least one moulding machine 8 designed to receive the stream of liquid plastic material and configured to mould the plastic objects.

The moulding machine 8 might, for example, be a rotary compression moulding machine. Alternatively, the moulding machine 8 might be an injection moulding machine.

The system 1 might also comprise more than one moulding machine, which might even be of mutually different types.

In an example embodiment of the system 1 , the system comprises at least one pumping unit 9 operatively interposed between the extruder 2 and the moulding machine 8.

The injection unit 3 is configured to inject the at least one colouring liquid downstream of the pumping unit 9, as shown in Figure 1 , or upstream of the pumping unit 9, as shown in Figure 2.

Figure 2 shows a second possible embodiment of a system according to this description, labelled 1 ', and which might differ from the embodiment of Figure 1 even only in the position, along the operating path, of the point where the at least one colouring liquid is injected.

In the embodiment of Figure 1 , the injection unit 3 is configured in such a way that the outlet section 4a of the nozzle 4 is operatively inserted in a portion of the intermediate duct 6 situated downstream of the pumping unit 9.

In the embodiment of Figure 2, the injection unit 3 is configured in such a way that the outlet section 4a of the nozzle 4 is operatively inserted in a portion of the intermediate duct 6 situated upstream of the pumping unit 9. The pumping unit 9 is positioned to operate in the intermediate duct 6 or between at least two portions of the intermediate duct 6.

The pumping unit 9 may comprise at least one main pump 10, which thus constitutes at least part of the pumping unit 9 itself.

The main pump 10 of the pumping unit 9 may, for example, be a gear pump.

In an example embodiment of the system 1 , the system comprises a controller, which may be defined "colouring controller".

The colouring controller comprises a control unit which may be defined "colouring control unit".

In Figure 1 , the colouring control unit is part of the block labelled C. The block labelled C, as will become clearer as this description continues, represents an electronic assembly comprising both the colouring control unit and a line control unit, which is described further on.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control the flow rate generated by the auxiliary pump 5, hence the speed of the auxiliary pump 5.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control at least one operating parameter of the extruder 2.

The colouring controller may be configured in such a way that the colouring control unit can receive at least a first input signal. This input signal, for example the one indicated by the arrow X, may represent and/or indicate the quality of mixing the plastic material with the at least one colouring liquid.

The colouring controller may be configured in such a way that the colouring control unit can receive at least a second input signal, not shown in the drawings.

The second input signal may represent and/or indicate the flow rate generated by the main pump 10 of the pumping unit 9, hence the speed of the main pump 10 of the pumping unit 9.

The colouring controller may be configured in such a way that the colouring control unit receives only one between the first and the second input signal, or both of the signals.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control the auxiliary pump 5 by means of at least a first drive signal, indicated by the arrow U1 .

The colouring controller may be configured in such a way that at least a first input signal U1 is a function of at least one between the first input signal and the second input signal.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control the extruder 2 by means of at least a second drive signal, indicated by the arrow U2.

The colouring controller may be configured in such a way that at least a second input signal U2 is a function of at least one between the first input signal and the second input signal.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control both the auxiliary pump 5 and the extruder 2 by means of the respective first and second drive signals U1 and U2.

The colouring controller may be configured in such a way that the colouring control unit can drive and/or control only the auxiliary pump 5, by means of the first drive signal U1 , or only the extruder 2 by means of the second drive signal U2.

As regards controlling and/or driving the main pump 10, it should be borne in mind that the system 1 defines (or is included in) a production line which may comprise other machines besides the ones described in this document.

The production line may comprise a controller of its own, which may constitute a line controller. In addition, or alternatively, the system might include one (or more) machine controllers; for example, there might be a controller for the extruder. The machine controller (of the extruder for example) may be subordinated to a line controller or may coincide with it. The line or machine controller comprises a (line or machine) control unit of its own.

In one embodiment, the colouring controller may be separate (even physically, in the sense of hardware) from the line or machine controller. In one embodiment, the colouring controller is built into (physically, in the same hardware as) the line or machine controller.

In the embodiment illustrated in Figure 1 , the colouring controller is built into the line controller.

Whatever the case, in at least one embodiment, the colouring controller is, in functional terms, distinct and independent (at least with regard to software) from the line or machine controller.

In the embodiment illustrated in Figure 1 , the line control unit and the colouring control unit are built into a single electronic assembly, represented by the block labelled C. In a different embodiment of the local controller and of the line controller, on the other hand, the line control unit and the colouring control unit are distinct, even physically, and are connected to respective distinct and separate electronic assemblies.

The colouring controller may be configured in such a way that the line control unit can drive and/or control the flow rate generated by the main pump 10 of the pumping unit 9, hence the speed of the main pump 10 of the pumping unit 9. The speed of the main pump 10 of the pumping unit 9 may be a function, for example, of the speed of the line as a whole. The speed of the main pump 10 may depend on the speeds and/or flow rates of other pumps which perform the same function as the main pump 10. The label U3 in Figure 1 denotes at least one drive signal by which the line control unit controls and/or drives the main pump 10 of the pumping unit 9. The colouring control unit and the line control unit might also be configured to mutually interact, for example to exchange signals representing and/or indicating corresponding parameters. In an example embodiment, the colouring control unit operates as a function of a parameter representing the operation of the main pump 10 (for example, flow rate).

The considerations set out above in connection with the colouring controller and the line controller apply to any possible implementation of the system according to this description and hence, for example, to a first possible embodiment of the system labelled 1 in Figure 1 , and/or a second possible embodiment of the system labelled V in Figure 2 and/or a third possible embodiment of the system labelled 1 " in Figure 3.

In the embodiment of Figure 3, the injection unit 3 comprises a second auxiliary pump 5'.

Thus, the auxiliary pump labelled 5 in Figure 3 may be considered as a "first" auxiliary pump 5.

In the embodiment of Figure 3, the injection unit 3 also comprises a second inlet duct 7'.

Thus, the inlet duct labelled 7 in Figure 3 may be considered as a "first" inlet duct 7.

In the embodiment of Figure 3, the injection unit 3 might also comprise a second nozzle, not illustrated, having a respective outlet section operatively directed towards the inside of the intermediate duct 6. There might also be a second movement system to vary the distance between this outlet section and the longitudinal axis of the intermediate duct 6, and/or to vary the orientation of the second nozzle relative to the longitudinal axis of the intermediate duct 6. The features and functions of the second auxiliary pump 5' of the injection unit 3, of the second inlet duct 7' of the injection unit 3 and of the second nozzle of the injection unit 3, may be equivalent to the corresponding features and functions of the first auxiliary pump 5 of the injection unit 3, of the first inlet duct 7 of the injection unit 3 and of the first nozzle 4 of the injection unit 3, respectively.

The second auxiliary pump 5' may pump at least one colouring liquid which may be the same as or different from the one pumped by the first auxiliary pump 5.

The injection point of at least one colouring liquid pumped by the first auxiliary pump 5 may be upstream or downstream of the main pump 10 of the pumping unit 9.

The injection point of at least one colouring liquid pumped by the second auxiliary pump 5' may be upstream or downstream of the main pump 10 of the pumping unit 9.

The solid line labelled 7 in Figure 3 indicates the position of the first inlet duct in the case where the injection point of at least one colouring liquid pumped by the first auxiliary pump 5 is upstream of the pumping unit 9 (main pump 10).

The dashed line labelled 7 in Figure 3 indicates the position of the first inlet duct in the case where the injection point of at least one colouring liquid pumped by the first auxiliary pump 5 is downstream of the pumping unit 9 (main pump 10).

The solid line labelled T in Figure 3 indicates the position of the second inlet duct in the case where the injection point of at least one colouring liquid pumped by the second auxiliary pump 5' is upstream of the pumping unit 9 (main pump 10).

The dashed line labelled 7' in Figure 3 indicates the position of the second inlet duct in the case where the injection point of at least one colouring liquid pumped by the second auxiliary pump 5' is downstream of the pumping unit 9 (main pump 10). In an example embodiment of the system 1 , the system comprises a mixer 1 1 operatively interposed between the extruder 2 and the moulding machine 8.

In an example embodiment of the system 1 , the intermediate duct 6 places the extruder 2 in fluid dynamic communication with the mixer 1 1 .

The mixer 1 1 is adapted to mix the at least one colouring liquid fed in by the first auxiliary pump 5 and/or by the second auxiliary pump 5' with the stream of liquid plastic material fed out of the extruder 2.

For example, the mixer 1 1 may be static or dynamic.

As stated above, the system 1 may comprise a local controller comprising a local control unit C and configured in such a way that the local control unit C receives at least the first input signal X.

The first input signal X indicates and/or represents a mixing quality relative to a point on the operating path situated downstream of the mixer 1 1 .

In an example embodiment of the system 1 , the input signal may represent a quality of mixing the plastic material with the colouring liquid injected by the first auxiliary pump 5 and/or a quality of mixing the plastic material with the colouring liquid injected by the second auxiliary pump 5'.

Figures 1 -3 also show a possible outfeed duct 13 adapted to transfer the stream of liquid plastic material, mixed with the at least one colouring liquid, from the mixer 1 1 to the moulding machine 9.

In the latter case, the input signal X thus preferably indicates a quality of mixing the at least one colouring liquid with the stream of liquid plastic material at a point situated in the outfeed duct 13.

A moulding method for making plastic objects according to this description comprises the following steps:

- delivering at least one colouring liquid for colouring the plastic material; - moulding the plastic objects in a moulding machine 8 designed to receive the stream of liquid plastic material. The at least one colouring liquid is injected downstream of the extruder 2. An example embodiment of the method further comprises a step of pressurizing the stream of liquid plastic material by means of at least one pumping unit 9.

The at least one colouring liquid may be injected downstream of the extruder 2 and upstream of the pumping unit 9, or downstream of the pumping unit 9.

An example embodiment of the method further comprises a step of mixing the stream of pressurized liquid plastic material with the at least one colouring liquid, by means of a mixer 1 1 .

The at least one colouring liquid may be injected downstream of the pumping unit 9 and upstream of the mixer 1 1 , as shown in the respective embodiments of Figure 1 and Figure 3.

In an example embodiment, the at least one colouring liquid is injected into at least one intermediate duct 6 by means of a nozzle 4 having an outlet section 4a which is inserted in the intermediate duct 6.

The at least one intermediate duct 6 is interposed between the extruder 2 and the moulding machine 8 or between the extruder 2 and the mixer 1 1 if the mixer 1 1 is present.

The method may comprise a step of adjusting the position of the outlet section 4a of the nozzle 4 relative to the at least one intermediate duct 6.

This adjustment step may comprise a step of adjusting the distance between the outlet section 4a of the nozzle 4 and the longitudinal axis H of the intermediate duct 6.

The adjustment step may comprise a step of adjusting the orientation and/or inclination of the outlet section 4a of the nozzle 4 relative to the longitudinal axis H of the intermediate duct 6.

The adjustment step may, for example, also be performed by means of the local controller C during the operation of the system 1 .

The pumping unit 9 might also comprise at least one further main pump

10, for example to reduce the flow rate of the single pump. In the case of gear pumps, for example, this would allow reducing flow rate pulsation and pressure on account of the reduced tooth size of each main pump 10.

Claims

1 . A continuous cycle moulding system (1 , 1 ', 1 ") for making objects from plastic material, defining an operating path for processing the plastic material and comprising:

- an extruder (2) designed to receive plastic material in raw form (G) and to feed out a stream (F) of plastic material in the liquid phase;

- an injection unit (3) configured to deliver at least one colouring liquid for colouring the plastic material;

- at least one moulding machine (8) designed to receive the stream (F) of liquid plastic material and configured to mould the objects from the plastic material,

characterized in that the injection unit (3) operates downstream of the extruder (2) to inject the at least one colouring liquid into the stream (F) of liquid plastic material downstream of the extruder (2).

2. The system (1 , V, 1 ") according to claim 1 , comprising a mixer (1 1 ) operatively interposed between the extruder (2) and the moulding machine (8), wherein the injection unit (3) is configured to inject the at least one colouring liquid upstream of the mixer (1 1 ).

3. The system (1 , V, 1 ") according to either of the preceding claims, wherein the at least one moulding machine (8) is a rotary compression moulding machine (8).

4. The system (1 , V, 1 ") according to any one of the preceding claims, wherein the at least one moulding machine (8) is an injection moulding machine (8).

5. The system (1 , V, 1 ") according to any one of the preceding claims, comprising an intermediate duct (6) designed to receive the stream (F) of liquid plastic material feeding out of the extruder (2), wherein the injection unit (3) comprises a nozzle (4) having at least one outlet section (4a) operatively inserted in the intermediate duct (6) to inject the at least one colouring liquid into the intermediate duct (6).

6. The system (1 , V, 1 ") according to claim 5, wherein the intermediate duct (6) extends along a longitudinal axis (H) and wherein the nozzle (4) is movable relative to the intermediate duct (6) at least between a first position, where the outlet section (4a) is at a first distance from the longitudinal axis (H), and a second position, where the outlet section (4a) is at a second distance from the longitudinal axis (H), different from the first distance.

7. The system (1 , V, 1 ") according to claim 5 or 6, wherein the intermediate duct (6) is elongate along a longitudinal axis (H) and wherein the nozzle (4) is oriented radially relative to the intermediate duct (6).

8. The system (1 , 1 ', 1 ") according to any one of claims 5 to 7, wherein the nozzle (4) is movable to vary its orientation relative to a longitudinal axis (H) of the intermediate duct (6).

9. The system (1 , V, 1 ") according to any one of the preceding claims, wherein the injection unit (3) comprises an auxiliary pump (5) for pumping the at least one colouring liquid into the stream (F) of liquid plastic material.

10. The system (1 , V, 1 ") according to claim 9, comprising a colouring control unit (C) configured to receive at least one input signal (X) representing a quality of mixing the plastic material with the at least one colouring liquid, and connected to the auxiliary pump (5) to generate and transmit control signals of the auxiliary pump (5) as a function of the at least one input signal (X).

1 1 . The system (1 , V, 1 ") according to any one of the preceding claims, comprising at least one pumping unit (9) positioned between the extruder (2) and the moulding machine (8), wherein the injection unit (3) is configured to inject the at least one colouring liquid downstream or upstream of the pumping unit (9).

12. A moulding method for making objects from plastic material, comprising the following steps:

- extruding plastic material in raw form (G) in order to feed out a stream (F) of plastic material in the liquid phase; - delivering at least one colouring liquid for colouring the plastic material;

- moulding the plastic objects in a moulding machine (8) designed to receive the stream of liquid plastic material,

characterized in that the at least one colouring liquid is injected downstream of the extruder (2) into the stream (F) of plastic material in liquid phase.

13. The method according to claim 12, further comprising the following steps:

- pressurizing the stream of liquid plastic material by means of at least one pumping unit (9);

- mixing the stream of pressurized liquid plastic material by means of a mixer (1 1 ),

wherein the at least one colouring liquid is injected after pressurizing and before mixing.

14. The method according to claim 12 or 13, wherein the at least one colouring liquid is injected into at least one intermediate duct (6) by means of a nozzle (4) having an outlet section (4a) which is inserted in the intermediate duct (6), wherein the method comprises a step of adjusting the position of the outlet section (4a) of the nozzle (4) relative to the intermediate duct (6).