WO2025083449A1

WO2025083449A1 - Apparatus and method for deriving the viscosity of a molten polymer

Info

Publication number: WO2025083449A1
Application number: PCT/IB2023/060449
Authority: WO
Inventors: Fabrizio Pucci; Fiorenzo Parrinello; Giulia DOMENICHINI; Alberto DI GIACOMO
Original assignee: Sacmi Imola SC
Current assignee: Sacmi Imola SC
Priority date: 2022-10-19
Filing date: 2023-10-17
Publication date: 2025-04-24
Anticipated expiration: 2026-04-17

Abstract

An apparatus (1) for deriving the viscosity of a molten polymer comprises: a duct (10) having an inlet (10I) and an outlet (10U) and traversed by a flow of molten polymeric material; a sensor system (13), operatively in contact with molten polymeric material flowing in the duct (10), to measure values of pressure and, if necessary, temperature, and to generate a measurement signal; a processing unit (14), connected to the sensor system (13) to receive the measurement signal and/or signals and programmed to process the measurement signal to derive estimated values of the viscosity of the molten polymeric material flowing in the duct (10). The sensor system (13) includes an optical fibre defining at least one or more differential pressure sensors and, if necessary, temperature sensors, located inside the duct (10) to be operatively in contact with the molten polymeric material.

Description

DESCRIPTION

APPARATUS AND METHOD FOR DERIVING THE VISCOSITY OF A MOLTEN POLYMER.

Technical field

This invention relates to an apparatus and a method for deriving the viscosity of a molten polymer.

Background art

Viscosity sensors are used in various different industrial contexts to measure or estimate the viscosity of liquids such as, for example, molten plastic, ink, sludge or petroleum.

Typically, with regard to the processing of plastic in particular, an extruder is used to produce a flow of liquid plastic (alternatively, a piston or other means might be used instead); downstream of the extruder there is a processing apparatus, for example, a moulding machine (this situation is typical of beverage lines, for forming caps or parisons) or other machines to perform other processes (such as, for example, the production of plastic film or sections).

The use of sensors in the field of plastic processing is known, for example, from the following patent documents: US5197633A, US20190315048A1 , US5303141A, US20200324311A1 , US10137679B2 and US6492485B1. For example, patent document US5197633A describes the use of proximity sensors, made from optical fibres, to measure the flow rate of the molten plastic.

As regards the measurement of viscosity, several technical solutions are known, for example from patent documents US2019250086, US5708197A, US5197633A, US4425790A, CN104568663A and

CN204439494U. For example, the solution proposed by US4425790A involves measuring a differential measurement of pressure in a mould through which the molten plastic is conveyed. This solution, however, implies constructing an apparatus dedicated to the measurement of viscosity and such an apparatus is complex and expensive because, for example, it requires the use of branching ducts and a dedicated pump.

The need therefore remains for a reliable and mechanically simple system for deriving the viscosity of a fluid flowing in a duct such as, for example, liquid plastic.

Disclosure of the invention

The aim of this invention is to provide an apparatus and a method for deriving the viscosity of a liquid (specifically, but not exclusively, a molten polymer), to overcome the above mentioned disadvantages of the prior art. In particular, the aim of this invention is to provide an apparatus and a method for deriving the viscosity of a liquid (specifically, but not exclusively, a molten polymer), that are reliable and simple, hence inexpensive.

These aims are fully achieved by the apparatus and method of this disclosure, as characterized in the appended claims.

The disclosure relates to an apparatus (or system) for deriving the viscosity of liquid. The concepts and features described in this disclosure could be applicable to deriving viscosity of any liquid, or of liquid or semiliquid substance made to flow in a duct such as, for example, ink, petroleum, sludge or molten plastic. In particular, the concepts of this disclosure apply to the deriving of viscosity of Newtonian fluids and nonNewtonian, pseudoplastic fluids. The rest of this disclosure focuses on the application to molten plastic, that is to say, to the application where the liquid whose viscosity is to be derived is molten plastic, without this disclosure thereby losing in generality (therefore, it is understood that terms such as "molten plastic" or "molten polymeric material" as used in this disclosure can be replaced by the term "liquid").

The molten polymer flows in a duct. In an example embodiment, the duct forms part of the apparatus. The duct has an inlet and an outlet. The apparatus comprises a sensor system. The sensor system includes one or more sensors.

The sensor system is configured to capture a measurement signal, representing one or more quantities that depend on a physical state of the molten polymer flowing in the duct. More specifically, the sensor system measures values of pressure (and, if necessary, also of temperature) of the molten plastic flowing in the duct.

In an example embodiment, the sensor system is operatively in contact with molten polymeric material which flows in the duct. Thus, the sensor system is operatively in contact with molten polymeric material flowing in the duct, in order to capture values of pressure (and, if necessary, of temperature), and the measurement signal generated by the sensor system represents the pressure values captured.

The apparatus also includes a processing unit. For example, the processing unit includes a memory and a processor. The processing unit is connected to the sensor system to receive the processing signal and to process it. The processing unit is programmed to derive estimated values of the viscosity of the molten polymeric material flowing in the duct, based on the measurement signal.

The sensor system includes an optical fibre sensor. More specifically, the sensor system includes an optical fibre defining a differential pressure sensor. In an example embodiment, the optical fibre defining the differential pressure sensor is located inside the duct so as to be operatively in contact with the molten polymeric material.

A signal flows inside the optical fibre (i.e., the optical fibre is configured for propagating or transmitting a signal inside the optical fibre itself).

In an example, the optical fibre terminates at an end point. In other words, the optical fibre includes an end point. The end point can be located inside the duct. Preferably, the end point can be (operatively) in contact with the molten polymeric material. Hence, a signal propagates (travels) to the end point of the optical fibre. In particular, the end point constitutes a free end of the optical fibre.

In an embodiment, the optical fibre includes a reflective layer. The reflective layer can be located at the end point. In other words, the reflective layer can be located directly in contact with the end point (i.e., the end point can include a reflective layer). The optical fibre can include a reflective coat (i.e., the reflective coat defines the reflective layer), applied to the optical fibre (in particular, to the end point of the optical fibre). In another example, the reflective layer is located distanced with respect to the end point, preferably inside the duct (i.e., in contact with the molten polymeric material).

Hence, a signal propagates inside the optical fibre to the end point; the reflective layer reflects the signal back inside the optical fibre.

In an example, the end point defines or include the differential pressure sensor.

It is observed that the optical fibre defining (or including) a differential pressure sensor means that the optical fibre includes a waveguide (i.e., a portion of the optical fibre wherein the signal travels) and a sensor (wherein the signal is subjected to an alteration of one or more of its properties). According to one approach, the sensor of the optical fibre includes a sensible (or sensorized) portion, in particular the sensible portion is provided in (along) the waveguide. According to a further approach, the sensor of the optical fibre includes a sensible portion, provided in the end point. In other words, the end point includes a tip (wherein the tip defines the sensor of the optical fibre). In other words, the optical fibre includes a tip provided at the end point of the optical fibre. The tip can have the reflective layer (or coat). The tip can have a sensible layer (or coat).

Hence, the term “differential pressure sensor” can also include the example wherein the optical fibre has a first end point and a second end point. Each of the end point is preferably located inside the duct directly in contact with the molten polymeric material. The optical fibre includes a first waveguide and a second waveguide, so that the signal travels inside the first waveguide to the first and end point and inside the second waveguide to the second end point. The optical fibre can include a first reflective layer and a second reflective layer. In an example, the reflective layers are distanced from the respective end points. In another example, the reflective layers are directly in contact with the respective end points. In this context, in a first approach, the optical fibre has a first and a second sensible portions provided, respectively, in the first waveguide and in the second waveguide. In a second approach, the optical fibre has a first and a second sensible layer provided, respectively, at the first end and at the second end of the optical fibre; in this approach, the optical fibre can include a first tip and a second tip, provided, respectively, at the first and at the second end points. Each of the first tip and second tip have a sensible layer (or coat) and a reflective layer (or coat).

In an example, the sensible layer is made of silicon or titanium dioxide. The sensible layer is configured to change one or more of its properties in response to (a change of) the molten polymeric material, in particular to a change of pressure (and/or temperature) of the molten polymeric material. Hence, generally, with the term “sensible layer” or “sensible portion” it is meant a layer or a portion that is configured to change one or more of its properties in response to a change of properties of the molten polymeric material (for example, to a change of pressure). In an example, the sensible layer can be a deformable layer. Hence, the sensible layer can change its dimension in response to pressure or temperature changes. In an example, the reflective layer is made of gold.

In an embodiment, the tip includes a further reflective layer. The sensible layer can be interposed between the reflective layer and the further reflective layer. The tip can include an adhesive layer. The tip can include a further adhesive layer. The adhesive layers can be made of titanium. One adhesive layer can be located between a reflective layer and the sensible layer. One adhesive layer can be located between the reflective layer and the waveguide (in particular, the end point) of the optical fibre. The optical fibre includes portions, or sections, that define the sensors. In effect, in technologies of essentially known type in the field of optical fibre sensors, these sections or portions of fibre are subjected to modifications in the fibre structure making these portions "sensitive" to a predetermined quantity, in the sense that its optical fibre transmission properties vary as a function of the quantity concerned. For example, if the portion of fibre is sensitive to pressure, thus defining a pressure sensor, that portion of fibre has an optical fibre transmission property that varies as a function of the pressure applied to that portion of the fibre; in another example, if a portion of fibre has an optical fibre transmission property that varies as a function of the temperature applied to that portion of the fibre, that portion of the fibre defines a temperature sensor. Thus, the fibre has a plurality of sensorized stretches; the fibre may also include non-sensorized stretches along which the optical signal is transmitted (in a predetermined manner not dependent on factors or conditions external to the fibre).

Thus, in the solution according to this disclosure, the optical fibre has a first and a second (sensorized) portion, spaced from each other and each defining a pressure sensor; that way, the optical fibre defines a differential pressure sensor because it allows the processing unit to derive the difference in pressure between the first and the second portion of fibre.

Thanks to the differential pressure sensor, the apparatus knows the difference between two zones of the duct which are spaced from each other.

Starting from this differential pressure, the processing unit uses mathematical formulas known in the sector of rheology, to derive an estimated value (both apparent and real, or apparent for all the liquid and real also for the Newtonian and pseudoplastic fluids) for the molten polymeric material flowing in the duct.

The sensor system also includes a device to generate an optical signal that travels along the fibre. The sensor system is also configured to detect and analyse a distortion or alteration of the optical signal fed to the fibre, such distortion or alteration being produced by the sensorized parts of the fibre according to the quantities which the sensorized parts are sensitive to.

This approach makes it possible, in a particularly simple and inexpensive way, to estimate the viscosity of the molten polymeric material flowing in the duct. This approach also makes it possible to measure the differential pressure to a particularly good degree of precision (in the order of mbars). In an example embodiment, the optical fibre has a first end outside of the duct (connected to an optical signal generator) and another end positioned inside the duct. In this example embodiment, therefore, the optical fibre has an active stretch, positioned inside the duct and in contact with the molten plastic (the active stretch includes the sensorized portions), and an initial stretch, positioned upstream of the active stretch, outside the duct, hence not in contact with the molten plastic. In this example, the fibre defines an "interrupted" optical circuit in which a part of the optical signal is reflected back to be received and processed by the sensor system.

In a possible example, the active portions of the optical fibre are made using Bragg grating for measuring pressure and/or temperature. A "parallel" bundle of fibres, preferably held together in a single jacket, may also be used.

The sensor system may also include a temperature sensor for measuring the temperature of the molten polymeric material flowing in the duct. The temperature sensor, too, comprises an optical fibre sensor. Preferably, the same optical fibre that defines the differential pressure sensor also defines the temperature sensor. For example, the optical fibre may include a third portion that is sensitive to temperature. For example, the third (temperature-sensitive) portion may be interposed, along the length of the optical fibre, between the first and the second (pressure-sensitive) portion. Thus, the temperature sensor is also located inside the duct so as to be operatively in contact with the molten polymeric material. The presence of the temperature sensor in the same optical fibre that defines the pressure sensors has the advantage of making the apparatus even more simple and unobtrusive.

In a possible example embodiment, the active stretch of the optical fibre (the stretch located inside the duct) may include a plurality of sensorized zones to define a corresponding plurality of temperature sensors. Preferably, the temperature sensors are positioned at different distances along the longitudinal axis of the duct. This feature advantageously allows deriving information representing a temperature profile inside the duct.

Thus, the processing unit is programmed to derive apparent viscosity values for the molten polymeric material flowing in the duct, based on the measurement signal. Preferably, the processing unit is also programmed to apply Rabinowitsch or Weissenberg-Rabinowitsch corrections to the apparent viscosity values in order to derive corresponding real viscosity values. These corrections are implemented using formulas of essentially known type in the field of rheology. The processing unit is programmed to also derive values of shear rate for the molten polymer flowing in the duct. This feature allows estimating the viscosity even more precisely.

Thus, the first and the second stretch of the fibre, defining a first and a second temperature sensor, are spaced from each other and located at two different points of the duct; preferably, they are spaced along the longitudinal direction of the duct, corresponding to the direction of flow of the molten plastic along the duct.

In an example embodiment, the duct has two stetches with different cross section sizes; that is to say, it has a first stretch, having a first cross section size (for the flow of the plastic material), and a second stretch, having a second cross section size (for the flow of the plastic material), the second cross section size being smaller than the first cross section size.

In this case, by way of an example, the first sensorized stretch of the optical fibre is positioned in the first stretch of the duct and the second sensorized stretch of the optical fibre is positioned in the second stretch of the duct. That way, the fibre defines a first and a second differential sensor, where the first differential sensor of the optical fibre (defined by the first sensorized stretch of the optical fibre) is positioned in the first stretch of the duct and the second differential sensor of the optical fibre (defined by the second sensorized stretch of the optical fibre) is positioned in the second stretch of the duct. In this case, the optical fibre defines two distinct differential sensors, positioned in two stretches of duct with different cross section sizes. The cross section sizes and the ducts are known to the sensor system.

The presence of pressure sensors positioned in stretches of duct with different cross section sizes allows deriving a parameter (what is known as the “power-law index”, n) necessary to estimate the viscosity without the need for variations in the flow rate of the molten plastic inside the duct. This is particularly useful when the apparatus is applied directly in plants where the flow rate of the plastic cannot be varied at will, for example because it is controlled by an extruder on the basis of the production requirements of the industrial plant.

In an example embodiment, the duct has a longitudinal stretch, oriented along a longitudinal axis, and the active stretch (or at least part of it, specifically the end part of it) of the optical fibre is oriented parallel to the longitudinal axis; preferably, the active stretch (or the part of it) is aligned with the longitudinal axis of the duct.

In an example embodiment, the apparatus comprises a guiding element to guide the fibre into the duct. The guiding element has an external portion, positioned outside the duct, and an internal portion extending into the interior of the duct. The guiding element internally defines a passage having an inlet, formed in the external portion of the guiding element, and an outlet, formed in the internal portion of the guiding element. The initial stretch of the optical fibre is housed inside the passage and is positioned upstream of the active stretch (in the direction of flow of the molten plastic inside the duct); the active stretch is positioned downstream of the outlet of the passage. In a possible example, the passage in the internal portion of the guiding element defines a curve so that the outlet of the passage is oriented parallel to the longitudinal axis; preferably, the outlet of the passage is aligned with the longitudinal axis.

The optical fibre has a core and a jacket. The core may be made, for example, from glass, and the jacket from stainless steel (this allows the fibre to resist medium to high temperatures. The jacket may include a ceramic layer, interposed between the outer layer (of stainless steel) and the core (of glass), to prevent possible mechanical problems or damage due to the different thermal properties of steel and glass.

This disclosure also provides a method for deriving the viscosity of a molten polymer (or of a liquid in general).

The method comprises a step of conveying the molten polymeric material through a duct having an inlet and an outlet.

The method also comprises a step of generating a measurement signal, representing values of pressure, via a sensor system. Preferably, the sensor system is operatively in contact with molten polymeric material which flows in the duct.

The method comprises a step of processing the measurement signal to derive estimated values of the viscosity of the molten polymeric material flowing in the duct.

The method comprises capturing the measurement signal through a sensorized optical fibre, defining a differential pressure sensor, located inside the duct so it is in contact with the molten polymeric material.

In an example, the optical fibre terminates at an end point, preferably located inside the duct. Preferably, the end point can be (operatively) in contact with the molten polymeric material.

The method can include a step of propagating of a signal by the optical fibre to the end point. In an embodiment, the optical fibre includes a reflective layer. The reflective layer can be located at the end point (directly in contact with the end point) or distanced with respect to the end point, preferably inside the duct (i.e., in contact with the molten polymeric material). The method can include a step of reflection of the signal by the reflective layer back inside the optical fibre.

In an example, the end point defines the differential pressure sensor (i.e. the end point includes the differential pressure sensor).

According to one approach, the sensor of the optical fibre includes a sensible (sensorized) portion, in particular the sensible portion is provided in a waveguide of the optical fibre. According to a further approach, the optical fibre, in particular the end point, includes a tip (wherein the tip defines the sensor of the optical fibre). In other words, the optical fibre includes a tip provided at the end point of the optical fibre. The tip can have the reflective layer (or coat). The tip can have a sensible layer (or coat).

In an example, the sensible layer is made of silicon or titanium dioxide. The sensible layer is configured to change one or more of its properties in response to a change of pressure (and/or temperature) of the molten polymeric material. Hence, the method can include a step of alteration of the properties of the sensible layer by the molten polymeric material. For example, the method can include a step of deformation of the sensible layer, for example in response to pressure or temperature changes of the molten polymeric material.

In an example, the reflective layer is made of gold. In an embodiment, the tip includes a further reflective layer. The sensible layer can be interposed between the reflective layer and the further reflective layer. The tip can include an adhesive layer. The tip can include a further adhesive layer. The adhesive layers can be made of titanium. One adhesive layer can be located between a reflective layer and the sensible layer. One adhesive layer can be located between the reflective layer and the waveguide (in particular, the end point) of the optical fibre. Processing comprises applying Rabinowitsch or Weissenberg- Rabinowitsch corrections to the apparent viscosity values for molten polymeric material flowing in the duct, in order to derive corresponding real viscosity values.

Also measured are values of temperature of the molten plastic flowing in the duct; preferably, the measurement signal captured also represents the temperature because the optical fibre also defines one or more temperature sensors.

The optical fibre has a (free) end positioned inside the duct and immersed in the flow of molten plastic; preferably, the flow of the molten plastic has the effect of disposing the stretch of optical fibre, which is positioned inside the duct in contact with the molten plastic, in such a way as to make it parallel to the longitudinal direction of the flow of the molten plastic itself.

In an example, two sensorized stretches of fibre are positioned in stretches of the duct having different cross section sizes for the molten plastic, defining corresponding differential pressure sensors; the purpose of this, during processing, is to calculate the “power-law index” even in the presence of molten plastic flowing at a constant rate (stationary). Each sensorized stretch of fibre may define a differential sensor, that is to say, each sensorized stretch may in turn comprise a pair of sensors defining a differential sensor for measuring a differential pressure; the two sensorized stretches of fibre may be located in stretches of duct having different cross section sizes for the flow of plastic. In this example, therefore, there are four sensors, that is to say, two pairs of sensorized fibre stretches, corresponding to two differential sensors.

According to an aspect, this disclosure provides a plant for continuous processing of plastic materials. The plant represents an implementation of one of the possible applications of the apparatus according to this disclosure.

The plant comprises an extruder, configured to receive raw plastic as input and to produce as output a flow of molten plastic material, a processing machine configured to perform a process on the plastic in a continuous cycle (for example, a compression or injection moulding machine, which may be a rotary or reciprocating machine); and a connecting duct for feeding to the processing machine the molten plastic feeding out from the extruder.

The plant also comprises an apparatus for deriving the viscosity of a molten polymer and including one or more of the features described in this document. In an example embodiment, the duct of the apparatus (inside which the sensorized optical fibre is located) is the connecting duct (or is formed from a part thereof).

In an example, the plant comprises a controller, configured to adjust one or more process parameters correlated with the activity of the extruder. The controller may be connected to the processing unit to receive the estimated values of viscosity of the molten polymeric material and may be programmed to adjust the one or more process parameters according to the estimated values of viscosity of the molten polymeric material.

The process parameters may comprise one or more parameters selected among the following: temperature, mixture of input materials (virgin and/or recycled), additives, screw speed.

The purpose of this feedback on the process parameters such as temperature, mixture of input materials (virgin and/or recycled), additives, screw speed is to maintain and/or reach a setting of the measured variable, that is to say, the viscosity. The controller can therefore adjust the one or more process parameters to maintain and/or reach a certain value of the measured viscosity.

According to another aspect, this disclosure provides a method for continuous processing of plastic materials.

The method comprises a step of extruding raw plastic to produce a flow of molten plastic material.

The method also comprises a step of continuously processing (in a processing machine) the molten plastic material from the extruder. The method also comprises a step of deriving the viscosity of the molten polymer using the method according to one or more of the features set out in this description.

Preferably, the duct in which the optical fibre is located is part of a connecting duct used to feed the molten plastic feeding out from the extruder to the processing machine.

In an embodiment, the method comprises a step of performing feedback control on one or more process parameters, according to the estimated values of the viscosity of the molten polymeric material, where the process parameters may be correlated with the activity of the extruder.

Brief description of drawings

These and other features will become more apparent from the following description of a preferred embodiment, illustrated by way of non-limiting example in the accompanying drawings, in which:

- Figure 1 shows a processing plant according to one or more aspects of this disclosure;

- Figures 2A and 2B show a cross section of an optical fibre according to one or more aspects of this disclosure;

- Figures 3A, 3B show an apparatus according to one or more aspects of this disclosure;

- Figures 4A-4C show schematically a portion of an optical fibre, according to one or more aspects of this disclosure.

Detailed description of preferred embodiments of the invention

The numeral 1 in the accompanying drawings denotes an apparatus for deriving the viscosity of a molten polymer.

The apparatus 1 comprises a duct 10 having an inlet 101 and an outlet 10U. The duct 10 is traversed by molten polymeric material flowing from the inlet 101 to the outlet 10U. The duct 10 includes an inside wall 10A; the inside wall 10A defines an interior I of the duct 10, in which the molten polymeric material flows. The duct 10, specifically the interior I of the duct 10, has a longitudinal stretch oriented along a longitudinal axis X.

The duct 10 has an inlet portion 110, defining the inlet 101 of the duct. At the inlet portion 101, the interior I of the duct 10 has a first diameter or cross section size D1 , defined perpendicularly to the longitudinal direction. The duct 10 has a first portion 111 ; at the first portion 111 , the interior I of the duct 10 has a first length L1 in the longitudinal direction and, preferably, a diameter or cross section size equal to D1. The apparatus 1 comprises a guiding element 12, located in the first portion 111 of the duct 10. In an example, the guiding element 12 has an external portion 12A, located outside the duct 10 (that is, located externally with respect to the interior I of the duct 10), and an internal portion 12B extending into the interior I of the duct 10. In another example, the guiding element 12 comprises only an external portion 12A.

The duct 10 includes a second portion 112, where the interior I of the duct 10 has a length L2 in the longitudinal direction. The duct 10 includes a third portion 113, where the interior I of the duct 10 has a length L3 in the longitudinal direction and a diameter or cross section size D2, defined perpendicularly to the longitudinal direction. In an example, the diameter D2 is between 5 mm and 100 mm. In an example, L3 is greater than 10 mm.

The duct 10 includes a fourth portion 114, where the interior I of the duct 10 has a length L4 in the longitudinal direction. The duct 10 includes a fifth portion 115, where the interior I of the duct 10 has a length L5 in the longitudinal direction and a diameter or cross section size D3, defined perpendicularly to the longitudinal direction. Preferably, L5 is greater than 10 mm. The fourth portion 114 defines a portion of duct 10 (specifically, of the interior I of the duct 10) which is tapered (that is, conical in shape), that is to say, at the fourth portion 114, the cross section of the duct 10 the longitudinal axis X towards the fifth portion

At the inlet portion 110, at the first portion 111 , at the second portion 112 and at the third portion 113, the duct 10 defines a first stretch of the interior I of the duct 10 having a first cross section size, equal to the diameter or cross section size D2, for the flow of plastic material. At the fifth portion 115, the duct 10 defines a second stretch of the interior I of the duct 10 having a second cross section size, equal to the diameter or cross section size D3, for the flow of plastic material. More specifically, the cross section size of the first stretch is greater than the cross section size of the second stretch.

The duct 10 includes a sixth portion 116, where the interior I of the duct 10 has a length L6 defined in the longitudinal direction.

The duct 10, specifically the interior I of the duct 10, has a transverse stretch oriented along a transverse axis Y. The duct 10 includes a seventh portion 117, where the interior I of the duct 10 has a length L7 defined in the transverse direction. The seventh portion 117 includes an outlet portion 118, defining the outlet 10U of the duct 10. At the outlet portion 10U, the interior I of the duct 10 has a fourth diameter or cross section size D4, defined perpendicularly to the transverse direction.

The apparatus 1 comprises a sensor system 13. The sensor system 13 includes an optical fibre, comprising a core 13A, a cladding 13B and an outer jacket 13C. For temperatures between 5°C and 150°C, the core 13A and the cladding 13B may be made from glass (for example, silica) or plastic (for example, PMMA) and the outer jacket 13C may be made from plastic (for example, PE, PVCF, which makes for greater flexibility or fluororesins, which make for greater chemical resistance) or from stainless steel (to promote the transfer of heat to the optical fibre, without affecting its sensing properties, and to protect the optical fibre against handling, shocks and bending). For temperatures between 150°C and 700°C, the core 13A and the cladding 13B may be made from glass (for example, silica) or plastic (for example, PMMA) and the outer jacket 13C from stainless steel. For temperatures of approximately 700°C and higher, the core 13A may be made from a single sapphire crystal (having a glass transition temperature T_g of 2030°C) and the outer jacket 13C may be made from metal (stainless steel for temperatures up to 900°C, or special alloys resistant to high temperatures for temperatures above 900°C). The optical fibre may comprise an intermediate jacket 13C, between the outer jacket 13C and the cladding 13B, for example, made from ceramic material, to reconcile the thermal properties of the outer jacket 13C with the glass of the core 13A.

The optical fibre is partly located inside the interior I of the duct 10. More specifically, the optical fibre has a first portion (or first stretch) which is sensorized and which defines a first sensor 131 and a second portion (or second stretch) which is sensorized and which defines a second sensor 132; the first and second portions of the optical fibre together define an optical fibre sensor. The first sensor 131 and the second sensor 132 are operatively in contact with molten polymeric material flowing in the interior I of the duct 10 to measure values of pressure.

The first sensor 131 and the second sensor 132 each define a differential sensor; each sensor therefore includes a pair of sensors for measuring differential values. The first sensor 131 and the second sensor 132 are positioned along an active stretch of the optical fibre, where the active stretch is positioned in the interior I of the duct 10, in contact with the molten plastic. The optical fibre comprises an initial stretch positioned upstream of the active stretch, on the outside of the duct 10 and not in contact with the molten plastic.

In an embodiment, the guiding element 12 internally defines a passage having an inlet, formed in the external portion 12A of the guiding element, and an outlet, formed in the internal portion 12B of the guiding element 12; the initial stretch of the optical fibre is housed in the passage and is positioned upstream of the active stretch, whilst the active stretch of the optical fibre is positioned downstream of the outlet of the passage in the guiding element 12. The active stretch of the optical fibre is at least 10 mm long.

The first sensorized portion or sensorized stretch is positioned in the first stretch of the interior I of the duct 10 and the second sensorized portion or sensorized stretch is positioned in the second stretch of the interior I of the duct 10, hence the first sensor 131 and the second sensor 132 are positioned in stretches of the duct 10 having different cross section sizes, in the first stretch of the duct 10 and in the second stretch of the duct 10, respectively.

In an example, the passage in the internal portion 12B of the guiding element 12 defines a curve so that the outlet of the passage is oriented parallel to the longitudinal axis X.

The first sensor 131 and the second sensor 132 each define a differential pressure sensor; hence, the first sensor 131 measures a differential pressure value in the first stretch of the interior I of the duct 10, and the second sensor 132 measures a differential pressure value in the second stretch of the interior I of the duct 10.

In a possible embodiment, the first sensorized portion (the first sensor 131 ) and the second sensorized portion (the second sensor 132) may be configured to measure the temperature of the molten polymeric material flowing in the duct 10.

Each sensor is made in the form of a fibre Bragg grating (FBG), that is to say, strips of material having different refraction indices alternating with each other. The sensor system 13 (or each of the sensors 131 and 132) measures pressure values and generates measurement signals. The apparatus 1 comprises a processing unit 14, connected to the sensor system 13 to receive the measurement signals and programmed to process the measurement signal to derive estimated values of the viscosity of the molten polymeric material flowing in the duct 10 (that is, in the interior I).

Using formulas of essentially known type in the field of rheology, the processing unit 14 derives from the measurement signal of the first sensor 131 and the second sensor 132 (hence from differential pressure values captured by the first sensor 131 and the second sensor 132) values of apparent viscosity and apparent shear rate for the molten polymeric material flowing in the duct 10; the processing unit applies Rabinowitsch or Weissenberg-Rabinowitsch corrections to obtain real values of viscosity and/or shear rate of the molten polymer. The data processed by the processing unit 14 can be used to vary the parameters of an extruder 2 located upstream of the apparatus 1 (the temperature, for example) and/or to vary the type of material fed to the extruder 2.

The apparatus 1 preferably forms part of a processing plant L for continuous processing of plastic materials. The plant L comprises an extruder 2. The extruder 2 includes an inlet or hopper 21 to receive raw plastic as input, and an outlet 22 to feed out a flow of molten plastic material. The plant L comprises a connecting duct, connecting the outlet 22 of the extruder 2 to a processing machine 3 which also forms part of the plant L. The connecting duct feeds the molten plastic feeding out from the extruder 2 to the processing machine 3.

The apparatus 1 is interposed between the extruder 2 and the processing machine 3; more specifically, the duct 10 of the apparatus 1 defines the connecting duct, so that the inlet 101 of the duct 10 is connected to the outlet 22 of the extruder 2 to receive the molten plastic. The outlet 10U of the duct 10 is also connected to the processing machine 3.

The optical fibre terminates at an end point 133, located inside the duct 10, so to be in contact with the molten polymeric material. The optical fibre includes a reflective layer 136 that can be located directly in contact with the end point 133 or distanced with respect to the end point 133.

In an example, the optical fibre includes a waveguide 134 and a tip 135 provided at the end point 133 of the optical fibre. The tip 135 has a reflective layer (or coat) 136 and a sensible layer (or coat) 137. The sensible layer 137 is configured to change its dimension in response to a change of pressure and/or temperature of the molten polymeric material. In particular, the sensible layer 137 (as well as the reflective layer 136) is directly in contact with the molten polymeric material. The sensible layer

137 and the reflective layer 136 are provided as coating of the end point of the optical fibre. In an embodiment, the tip 135 includes a further reflective layer 136’, so that the sensible layer 137 is located between the reflective layer 136 and the further reflective layer 136’. The tip 135 can include an adhesive layer

138 and a further adhesive layer 138’. The adhesive layers 138 and 138’ are configured for making adhering the reflective layer 136 and further reflective layer 136’. In particular, the adhesive layer 138 is located between the reflective layer 136 and the sensible layer 137 and the further adhesive layer 138’ is located between the further reflective layer 136’ and the end point 133 of the optical fibre.

Claims

1. An apparatus (1 ) for deriving the viscosity of a molten polymer, comprising:

- a duct (10) having an inlet (101) and an outlet (10U) and traversed by a flow of molten polymeric material;

- a sensor system (13), operatively in contact with molten polymeric material flowing in the duct (10), to measure values of pressure and to generate a measurement signal;

- a processing unit (14), connected to the sensor system (13) to receive the measurement signal and programmed to process the measurement signal to derive estimated values of the viscosity of the molten polymeric material flowing in the duct (10), characterized in that the sensor system (13) includes an optical fibre defining a differential pressure sensor located inside the duct (10) to be operatively in contact with the molten polymeric material.

2. The apparatus (1 ) according to claim 1 , wherein the optical fibre terminates at an end point (133), the end point (133) being located inside the duct (10) to be in contact with the molten polymeric material.

3. The apparatus (1 ) according to claim 2, wherein the optical fibre includes a reflective layer (136) directly in contact with the end point (133).

4. The apparatus (1 ) according to one or more of the preceding claims, wherein the end point (133) defines the differential pressure sensor.

5. The apparatus (1 ) according to one or more of the preceding claims, wherein the optical fibre also defines a temperature sensor located in the interior (I) of the duct (10) to be operatively in contact with the molten polymeric material.

6. The apparatus (1 ) according to one or more of the preceding claims, wherein the processing unit (14) is programmed to derive apparent viscosity values for molten polymeric material flowing in the duct (10), based on the measurement signal, and is also programmed to apply Rabinowitsch or Weissenberg-Rabinowitsch corrections to the apparent viscosity values in order to derive corresponding real viscosity values.

7. The apparatus (1 ) according to claim 6, wherein the processing unit (14) is programmed to also derive values of shear rate for the molten polymer flowing in the duct (10).

8. The apparatus (1 ) according to one or more of the preceding claims, wherein the duct (10) has a first stretch, having a first cross section size (D2) for the flow of the plastic material, and a second stretch, having a second cross section size (D3) for the flow of the plastic material, the second cross section size (D3) being smaller than the first cross section size (D2), and wherein the optical fibre includes a first sensorized stretch, positioned in the first stretch of the duct (10), and a second sensorized stretch, positioned in the second stretch of the duct (10).

9. The apparatus (1 ) according to one or more of the preceding claims, wherein the duct (10) has a longitudinal stretch, oriented along a longitudinal axis (X), and wherein the optical fibre has an active stretch, located inside the duct (10) and oriented parallel to the longitudinal axis (X).

10. The apparatus (1 ) according to claim 9, comprising a guiding element (12), having an external portion (12A), located outside the duct (10), and an internal portion (12B) extending into the interior of the duct (10), the guiding element (12) internally defining a passage having an inlet formed in its external portion (12A), and an outlet formed in its internal portion (12B), wherein the optical fibre has an initial stretch, housed in the passage and positioned upstream of the active stretch, the active stretch being located downstream of the outlet of the passage.

11. The apparatus (1 ) according to claim 10, wherein the passage in the internal portion (12B) of the guiding element (12) defines a curve so that the outlet of the passage is oriented parallel to the longitudinal axis (X).

12. The apparatus (1 ) according to one or more of claims 9 to 11 , wherein the active stretch of the optical fibre is at least 10 mm long.

13. The apparatus (1 ) according to one or more of the preceding claims, wherein the optical fibre has a core (13A) made from glass, and an outer jacket (13C) made from stainless steel.

14. The apparatus (1 ) according to claim 13, wherein the optical fibre also includes an intermediate jacket (13C) made from ceramic material.

15. A plant (L) for continuous processing of plastic materials, comprising:

- an extruder (2), configured to receive raw plastic as input and to produce as output a flow of molten plastic material;

- a processing machine (3) configured to perform a process on the plastic in a continuous cycle;

- a connecting duct for feeding the molten plastic feeding out from the extruder (2) to the processing machine (3);

- an apparatus (1 ) for deriving the viscosity of a molten polymer, according to one or more of the preceding claims, wherein the duct (10) of the apparatus (1 ) is formed from a part of the connecting duct.

16. The plant (L) according to claim 15, comprising a controller configured to adjust one or more process parameters correlated with the activity of the extruder (2), wherein the controller is connected to the processing unit (14) to receive the estimated values of the viscosity of the molten polymeric material and is programmed to adjust the one or more process parameters according to the estimated values of viscosity of the molten polymeric material.

17. A method for deriving the viscosity of a molten polymer, comprising the following steps:

- conveying molten polymeric material through a duct (10) having an inlet (101) and an outlet (10U);

- via a sensor system (13) operatively in contact with the molten polymeric material flowing in the duct (10), generating a measurement signal representing values of pressure;

- processing the measurement signal to derive estimated values of the viscosity of the molten polymeric material flowing in the duct (10), characterized in that the sensor system (13) includes an optical fibre defining a differential pressure sensor located in the interior (I) of the duct (10) to be in contact with the molten polymeric material.

18. The method according to claim 17, wherein the optical fibre terminates at an end point (133), the end point (133) being located inside the duct (10) to be in contact with the molten polymeric material.

19. The apparatus (1) according to claim 18, wherein the optical fibre includes a reflective layer (136) directly in contact with the end point.

20. The apparatus (1 ) according to one or more of claims 17 to 19, wherein the end point (133) defines the differential pressure sensor.

21. The method according to one or more of claims 17 to 20, wherein, first, the step of processing derives apparent viscosity values for the molten polymeric material flowing in the duct (10), based on the measurement signal, after which Rabinowitsch or Weissenberg-Rabinowitsch corrections are applied to the apparent viscosity values in order to derive corresponding real viscosity values.

22. The method according to one or more of claims 17 to 21 , wherein the duct (10) has a first stretch, having a first cross section size (D2) for the flow of the plastic material, and a second stretch, having a second cross section size (D3) for the flow of the plastic material, the second cross section size (D3) being smaller than the first cross section size (D2), and wherein the optical fibre includes a first sensorized stretch, positioned in the first stretch of the duct (10) to capture the measurement signal, and a second sensorized stretch, positioned in the second stretch of the duct (10) to capture a further measurement signal, the further measurement signal being processed together with the measurement signal.

23. A method for continuous processing of plastic materials, comprising the following steps:

- in an extruder (2), extruding raw plastic to produce a flow of molten plastic material;

- in a processing machine (3), continuously processing the molten plastic material from the extruder; - deriving a viscosity of the molten polymer using the method according to one or more of claims 17 to 22, wherein the duct (10) in which the optical fibre is located is part of a connecting duct used to feed the molten plastic feeding out from the extruder (2) to the processing machine (3).

24. The method according to claim 23, comprising a step of performing feedback control on one or more process parameters correlated with the activity of the extruder (2), according to the estimated values of the viscosity of the molten polymeric material.