WO2008040943A2

WO2008040943A2 - Improvements relating to the measurement of viscosity

Info

Publication number: WO2008040943A2
Application number: PCT/GB2007/003691
Authority: WO
Inventors: Stephen Thompson; Marion Mcafee
Original assignee: The Queen's University Of Belfast
Priority date: 2006-10-02
Filing date: 2007-09-28
Publication date: 2008-04-10
Also published as: WO2008040943A3

Abstract

A method of determining the viscosity of an extrudate material in an extrusion system, comprising the steps of (i) acquiring measurements (T, N) of one or more process conditions of the extrusion system, (ii) acquiring indications of throughput of the extrudate material through the extrusion system, (iii) acquiring pressure measurements (Pm) of the extrudate material at one or more locations of the extrusion system, (iv) using a first model (40) to generate an estimated viscosity (η) of the extrudate material using data relating to the material and one of the measurements of the or each process condition, (v) using a second model (42) to generate an estimated pressure (Pe) of the extrudate material using data relating to the extrusion system geometry, the estimated viscosity (η) of the extrudate material and one of the indications of the extrudate material throughput, (vi) comparing the estimated pressure (Pe) of the extrudate material with a pressure measurement (Pm) of the extrudate material to generate a pressure difference value (Pdif), (vii) using the pressure difference value (Pdif) to generate a viscosity correction value, (viii) feeding the viscosity correction value into the first model (40) and using the viscosity correction value in the generation of a next estimated viscosity of the extrudate material, and (ix) repeating steps (iii) to (viii) one or more times, each repetition using another of the measurements of the or each process condition and another of the indications of the extrudate material throughput, wherein the generated pressure difference values are driven towards a minimum value.

Description

Improvements relating to the Measurement of Viscosity

The invention relates to improvements relating to the measurement of viscosity, particularly the viscosity of a material in an extrusion system.

Plastic products are growing in popularity on a global scale, and extrusion is a fundamental stage in the manufacture of 70% of such products. Extrusion is therefore a very important industrial process, but one which faces many technical challenges. For example, controlling the quality of an extrudate material, and hence the final product, presents various problems. The materials required to be extruded can be highly variable and unpredictable in nature, and selection of appropriate operating conditions of an extrusion system for each material, to obtain a desired extrudate material quality is a complex task. This results in large amounts of energy and material being wasted during long set-up times, and by the not in-frequent operation of the extrusion system using non-optimum operating conditions.

Monitoring of the quality of the extrudate material during the extrusion process is therefore desirable, to be able to reduce set-up times and improve operation of the extrusion system. The viscosity of the extrudate material is recognised as a key indicator of quality. However, in-process viscosity measurement to a required standard has proved difficult to achieve. Typically, viscosity sensors estimate, or predict, viscosity by measuring processing conditions of an extrusion system, and applying these measurements to some formula specific to the material being extruded by the system. Such viscosity sensor systems are inherently open-loop, and therefore suffer from all the problems associated with open-loop systems, i.e. they are intolerant to any changes in the extrudate material or processing conditions. Improvements in viscosity measurement are therefore desirable.

According to a first aspect of the invention there is provided a method of determining the viscosity of an extrudate material in an extrusion system, comprising the steps of

(i) acquiring measurements of one or more process conditions of the extrusion system,

(ii) acquiring indications of throughput of the extrudate material through the extrusion system,

(iii) acquiring pressure measurements of the extrudate material at one or more locations of the extrusion system,

(iv) using a first model to generate an estimated viscosity of the extrudate material using data relating to the material and one of the measurements of the or each process condition,

(v) using a second model to generate an estimated pressure of the extrudate material using data relating to the extrusion system geometry, the estimated viscosity of the extrudate material and one of the indications of the extrudate material throughput, (vi) comparing the estimated pressure of the extrudate material with a pressure measurement of the extrudate material to generate a pressure difference value,

(vii) using the pressure difference value to generate a viscosity correction value, (viii) feeding the viscosity correction value into the first model and using the viscosity correction value in the generation of a next estimated viscosity of the extrudate material, and

(ix) repeating steps (iii) to (viii) one or more times, each repetition using another of the measurements of the or each process condition and another of the indications of the extrudate material throughput, wherein the generated pressure difference values are driven towards a minimum value.

Driving the generated pressure difference values towards a minimum value, will have the result of driving the estimated viscosities towards a true value of the viscosity of the extrudate material in the extrusion system. Even though the viscosity of the extrudate material may be changing, the method used to estimate the viscosity adjusts, so that the estimated viscosities are always driven towards the true value of the viscosity of the material. This will be true at any given point in time.

The method may further comprise using an estimated viscosity generated by the first model as an indication of the viscosity of the extrudate material.

The method may further comprise outputting an estimated viscosity of the extrudate material, for example, to a user of the extrusion system. The method may further comprise repeatedly outputting an estimated viscosity of the extrudate material, for example, to a user of the extrusion system.

Acquiring the measurements of the one or more process conditions of the extrusion system may comprise continuously acquiring the measurements of the one or more process conditions. Such measurements may be acquired using one or more analogue measuring devices.

Acquiring the measurements of the one or more process conditions of the extrusion system may comprise acquiring a measurement of the or each process condition at a plurality of sample times. This is referred to as sampling the or each process condition. Such measurements may be acquired using one or more digital measuring devices. The measurements of the one or more process conditions may comprise temperature measurements of the extrusion system at one or more locations thereof. The temperature measurements may comprise temperature measurements from an extrusion barrel of the extrusion system at the one or more locations of the barrel. The measurements of the one or more process conditions may comprise indications of the speed of rotation of at least one screw of the extrusion system. The indications of the speed of rotation of the at least one screw of the extrusion system may be generated by acquiring measurements of a voltage or current input to the at least one screw and using the voltage or current measurements to determine the speed of rotation of the at least one screw.

Acquiring the indications of the extrudate material throughput may comprise continuously acquiring the indications of the extrudate material throughput. Such indications may be acquired using an analogue measuring device.

Acquiring the indications of the extrudate material throughput may comprise acquiring an indication of the extrudate material throughput at a plurality of sample times. This is referred to as sampling the throughput. Such indications may be acquired using a digital measuring device.

The indications of the extrudate material throughput may comprise measurements of the rate at which the extrudate material is fed through the extrusion system. The measurements of the rate may be acquired using a feedrate sensor. The indications of the extrudate material throughput may comprise indications of the speed of rotation of at least one screw of the extrusion system. The indications of the speed of rotation of the at least one screw of the extrusion system may be generated by acquiring measurements of a voltage or current input to the at least one screw and using the voltage or current measurements to determine the speed of rotation of the at least one screw.

Acquiring the measurements of the pressure of the extrudate material at one or more locations of the extrusion system may comprise continuously acquiring the measurements of the pressure. Such measurements may be acquired using an analogue measuring device.

Acquiring the measurements of the pressure of the extrudate material at one or more locations of the extrusion system may comprise acquiring a measurement of the pressure at the or each location at a plurality of sample times. This is referred to as sampling the pressure. Such measurements may be acquired using a digital measuring device.

The measurements of the pressure of the extrudate material at one or more locations of the extrusion system may comprise measurements from a pressure sensor placed in an end part of an extrusion barrel of the extrusion system. The end part of the barrel may be adjacent a die of the extrusion system.

The method may further comprise acquiring measurements of power used by at least one screw of the extrusion system.

Acquiring the measurements of the power of the at least one screw may comprise continuously acquiring the measurements of the power. Such measurements may be acquired using an analogue measuring device.

Acquiring the measurements of the power of the at least one screw may comprise acquiring a measurement of the power at a plurality of sample times. This is referred to as sampling the power. Such measurements may be acquired using a digital measuring device.

The method may further comprise acquiring measurements of torque of at least one screw of the extrusion system.

Acquiring the measurements of the torque of the at least one screw may comprise continuously acquiring the measurements of the torque. Such measurements may be acquired using an analogue measuring device.

Acquiring the measurements of the torque of the at least one screw may comprise acquiring a measurement of the torque at a plurality of sample times. This is referred to as sampling the power. Such measurements may be acquired using a digital measuring device.

Using the first model to generate an estimated viscosity of the extrudate material using one of the measurements of the or each process condition may comprise using a temperature measurement and a screw speed measurement to estimate the viscosity of the extrudate material. Using the first model to generate an estimated viscosity of the extrudate material may comprise using a knowledge of one or more properties of the extrudate material.

Using the second model to generate an estimated pressure of the extrudate material using one of the indications of the extrudate material throughput may comprise using a measurement of power used by at least one screw of the extrusion system. Using the second model to generate an estimated pressure of the extrudate material using one of the indications of the extrudate material throughput may comprise using a measurement of torque of at least one screw of the extrusion system. Using the pressure difference value to generate a viscosity correction value may comprise performing integration and gain functions on the pressure difference value.

According to a second aspect of the invention there is provided a viscosity sensing system, adapted for use in an extrusion system, the apparatus comprising at least one extrusion process condition measurement apparatus which acquires measurements of a process condition of the extrusion system, at least one extrudate material throughput determination device which acquires indications of the throughput of an extrudate material through the extrusion system, at least one pressure sensor which acquires pressure measurements of the extrudate material at one or more locations of the extrusion system, at least one processor which uses a first model to generate an estimated viscosity of the extrudate material using data relating to the extrudate material and a measurement of the or each process condition, uses a second model to generate an estimated pressure of the extrudate material using data relating to the extrusion system geometry, the estimated viscosity of the extrudate material, and an indication of the extrudate material throughput, uses a comparator model to compare the estimated pressure of the extrudate material with a pressure measurement of the extrudate material and generates a pressure difference value, uses a pressure/viscosity conversion model to generate a viscosity correction value using the pressure difference value, feeds the viscosity correction value into the first model and uses the viscosity correction value in the generation of a next estimated viscosity of the extrudate material. The viscosity sensing system may further comprise an output module which outputs the estimated viscosity of the extrudate material.

The at least one extrusion process condition measurement apparatus may continuously acquire measurements of a process condition. The at least one extrusion process condition measurement apparatus may acquire a measurement of a process condition at a plurality of sample times. The at least one extrusion process condition measurement apparatus may comprise one or more temperature sensors. The at least one extrusion process condition measurement apparatus may comprise one or more screw speed determination devices which determine the speed of rotation of at least one screw of the extrusion system. The or each screw speed determination device may measure the voltage or current input to the at least one screw and use the voltage or current measurements to determine the speed of rotation of the at least one screw.

The at least one extrudate material throughput determination device may continuously acquire indications of the throughput. The at least one extrudate material throughput determination device may acquire an indication of the throughput at a plurality of sample times. The at least one extrudate material throughput determination device may comprise a feedrate sensor. The feedrate sensor may measure the rate at which the extrudate material is fed through an extrusion barrel of the extrusion system. The at least one extrudate material throughput device may comprise a screw speed determination device.

The viscosity sensing system may further comprise at least one screw power measurement device which may measure the power being used by at least one screw of the extrusion system. The at least one screw power measurement device may continuously measure the power. The at least one screw power measurement device may measure the power at a plurality of sample times. The viscosity sensing system may further comprise at least one screw torque measurement device which may measure the torque of at least one screw of the extrusion system. The at least one screw torque measurement device may continuously measure the torque. The at least one screw torque measurement device may measure the torque at a plurality of sample times.

According to a third aspect of the invention there is provided a computer storage medium comprising a computer program which when executed on a processor causes the processor to carry out the method of the first aspect of the invention.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of an extrusion system comprising a viscosity sensing system according to the invention, and

Figure 2 is a block diagram representing a method of determining the viscosity of an extrudate material according to the invention.

Figure 1 shows an extrusion system 1 comprising an extruder barrel 3, a rotatable screw 5 mounted within the barrel 3, a die 7 attached to a first end of the barrel 3, a hopper 9 connected to a second, opposite end of the barrel 3, and heaters 13 mounted around the barrel 3. A material to be extruded is placed in the hopper 9, and introduced from there into the extruder barrel 3. The screw 5 is rotated which forces the extrudate material down the length of the barrel 3. The extrudate material in the barrel 3 is heated by the heaters 13, causing it to change to a molten state. The extrudate material exits the extrusion barrel 3 into the die 7. This comprises one or more orifices, through which the extrudate material is extruded, to form an extrusion product. Such extrusion products include various profile shapes, various tubes, films, sheets or fibres.

A viscosity sensing system 20 is used in conjunction with the extrusion system 1. The viscosity sensing system 20 measures a plurality of process conditions of the extrusion process of the extrusion system 1. In this embodiment, the process conditions which are measured are temperature from the extruder barrel 3 and speed of rotation of the screw 5 of the extrusion system 1. It will be appreciated, however, that other process conditions could be measured. The viscosity sensing system 20 comprises first, second and third temperature sensors, 23, 24, 25, which are placed at spaced locations in the extruder barrel 3 of the extrusion system 1 as shown, and used to measure temperature settings T from the barrel 3. The viscosity sensing system 20 further comprises a screw speed determination device 26, which measures the voltage input to the screw 5 and uses the voltage measurements to determine the speed of rotation N of the screw 5.

The viscosity sensing system 20 comprises an extrudate material throughput determination device. In this embodiment, this comprises the screw speed determination device 26, i.e. the screw speed measurements are used to determine indications of the throughput of the extrudate material through the extrusion system 1. It will be appreciated, however, that the extrudate material throughput determination device could comprise a feedrate sensor, which is separate from the screw speed determination device 26. The viscosity sensing system 20 comprises a screw power/torque measurement device 28. The screw power/torque measurement device 28 is connected to a motor of the screw 5 of the extrusion system 1 , and measures the electrical power being used by the screw 5, and uses this to determine the power/torque E of the screw 5.

The viscosity sensing system 20 further comprises a pressure sensor 30, placed at the end of the extruder barrel 3 of the extrusion system 1 , adjacent the die 7. The pressure sensor 30 is used to measure the pressure Pm of the extrudate material in this end part of the barrel 3.

The temperature sensors 23, 24, 25, the screw speed determination device 26, the screw power/torque measurement device 28, and the pressure sensor 30 of the viscosity sensing system 20 are each configured in this embodiment of the invention, to acquire a measurement of the temperature etc. at a plurality of sample times, whilst the extrusion system 1 is running. The measurements are acquired at a sample rate of 1 Hz. It will be appreciated, however, that different sample rates could be used, or continuous measurements of the temperature etc. could be taken. All of the measurements as described above, are measurements which are commonly made for extrusion systems.

The viscosity sensing system 20 further comprises a computer 32. The temperature sensors 23, 24, 25, the screw speed determination device 26, the screw power/torque measurement device 28, and the pressure sensor 30, are each connected to the computer 32. Measurements of the temperature, screw speed, screw power/torque and pressure are passed to the computer 32. The computer 32 uses the measurements to determine the viscosity of the extrudate material in the extrusion system 1. Figure 2 illustrates a block diagram representing a method used by the computer 32 to determine the viscosity of the extrudate material of the extrusion system 1. Measurements of the process conditions, i.e. measurements of the temperature T and measurements of the screw speed N, at a plurality of sample times, are fed into a first model 40, which is a viscosity estimation model. The first model 40 uses data relating to the extrudate material comprising a knowledge of the composition of the extrudate material, and the temperature and screw speed measurements to estimate the viscosity η of the extrudate material, using the following digital model:

77 _{t) = 2.7 *10 ³ + 0.27 η _it__{9 )} - 1.3x10 ³ _N °-2f, + 3.9 x10 ⁵ T 3r,-₂ ⁵ ₎

+ 4.8 x10 ⁵ T 3 -^1_.-5

40 )

where N is the screw speed, T3 is the temperature measurement at one location in the extruder barrel of the extrusion system, and the subscript t indicates a current sample time and the subscript (t-n) indicates a measurement taken at a sample time n samples from t.

This viscosity estimation model has been developed using a combination of theory, a knowledge of the extrudate material and experiment. Knowledge of the extrudate material may be inferred from extruder measurements and/or from laboratory tests and/or from data given a supplier of the extrudate material. It will be appreciated that there are a number of models which may be used to estimate the viscosity of the extrudate material. The digital model detailed above is given as an example.

Once an estimate of the viscosity has been determined, the method comprises passing the estimated viscosity η to a second model 42, which is a pressure estimation model. The second model 42 also receives measurements of the screw speed N and measurements of the screw power/torque E. This model uses the estimated viscosity and the screw speed and screw power measurements to determine an estimate of the pressure Pe of the extrudate material in the extrusion barrel of the extrusion system 1 , using the following model:

P ₁ = 0.49 + 0.22 p _t_₂ + 2.0 *10 ^~5 E ^f² - N l? - η °:f + 3.9 xlO -⁵ E If² - N l? - ηll⁶

where, again, the subscript t indicates a current sample time and the subscript (t-n) indicates a measurement taken at a sample time n samples from t.

Again, this model has been developed using a combination of theory and experiment. It will be appreciated that there are a number of models which may be used to estimate the pressure of the extrudate material.

The model detailed above is given as an example. It may be noted that the pressure estimation model used in this embodiment of the method is specific for the geometry of the extrusion system 1 , and provided that this does not change (for example a change of screw), the structure of this model will not need to be changed when different extrudate materials are used in the extrusion system 1.

The method then comprises passing the estimate of the pressure, Pe, to a comparator model 44. Pressure measurements, Pm, acquired from the pressure sensor 30 of the viscosity sensing system 20 are also passed to the comparator model 44. This model compares the estimated pressure with the measured pressure, and determines a pressure difference value, Pdif. The pressure difference value Pdif is passed to a pressure/viscosity conversion model 46, which comprises an integrator/gain device, and uses the pressure difference value to calculate a viscosity correction value. The viscosity correction value is then fed to the viscosity estimation model 40. The model 40 uses the viscosity correction value, and temperature and screw speed measurements, to generate a new estimate of the viscosity. This is passed to the pressure estimation model 42, which uses the new estimated viscosity and screw speed and screw power measurements, to generate a new estimate of the pressure of the extrudate material. This is passed to the comparator model 44, which compares the new pressure estimate to the measured pressure, and determines a new pressure difference value. This is passed to the pressure/viscosity conversion model 46, which calculates a new viscosity correction value. This is fed to the viscosity estimation model 40, and the process is then repeated.

Using the invention, the viscosity of the extrudate material of the extrusion system 1 can be predicted to within 1 % accuracy.

It can be seen that the invention enables measurements of a number of properties of an extrusion system to be acquired at a plurality of sample times, repetitive estimation of the viscosity of the material being extruded by the system, and repetitive correction of the estimated viscosity as necessary. At least some of the steps of the method are repeated to drive the output of the comparator model (the difference in estimated and measured pressure signals) to a minimum value of zero. This has the result of driving the viscosity estimated by the viscosity estimation model towards a true value of the viscosity of the extrudate material of the extrusion system. This is the case over all sample periods. The estimated viscosity of the extrudate material may be repeatedly output from the viscosity estimation model to a user of the extrusion system. The method of the invention is able to correct the estimated viscosity in response to changes or disturbances in the operation of the extrusion system or the extrudate material, and repeatedly provide an accurate reflection of the viscosity of the extrudate material.

As an example of the adaptive ability of the method, if the extrusion system 1 experiences a disturbance comprising a change in the material being extruded, this will not be detected by the viscosity estimation model 40. This model receives measurements of the temperature and screw speed, which measurements are not immediately affected by such a disturbance. The estimated viscosity will therefore be inaccurate. However, the pressure estimation model 42 receives measurements of the screw speed, i.e. the feedrate of the extrudate material and measurements of the screw power. These measurements will be affected by this disturbance. The pressure estimation model 42 uses these 'affected' measurements and the 'incorrect' estimated viscosity in the estimation of the pressure of the extrudate material. This estimated pressure will not match the measured pressure, which will result in a pressure difference value not equal to zero. The viscosity correction value calculated using this pressure difference value is fed to the viscosity estimation model 40, and used to correct the estimated viscosity. Thus the method can respond to a disturbance in one or more properties of the extrudate material.

Claims

1. A method of determining the viscosity of an extrudate material in an extrusion system, comprising the steps of (i) acquiring measurements of one or more process conditions of the extrusion system,

(v) using a second model to generate an estimated pressure of the extrudate material using data relating to the extrusion system geometry, the estimated viscosity of the extrudate material and one of the indications of the extrudate material throughput,

(vi) comparing the estimated pressure of the extrudate material with a pressure measurement of the extrudate material to generate a pressure difference value,

(vii) using the pressure difference value to generate a viscosity correction value,

(viii) feeding the viscosity correction value into the first model and using the viscosity correction value in the generation of a next estimated viscosity of the extrudate material, and

2. A method according to claim 1 , which further comprises using an estimated viscosity generated by the first model as an indication of the viscosity of the extrudate material.

3. A method according to claim 1 or claim 2, which further comprises outputting an estimated viscosity of the extrudate material.

4. A method according to any preceding claim, in which acquiring the measurements of the one or more process conditions of the extrusion system comprises continuously acquiring the measurements of the one or more process conditions.

5. A method according to any of claims 1 to 3, in which acquiring the measurements of the one or more process conditions of the extrusion system comprises acquiring a measurement of the or each process condition at a plurality of sample times.

6. A method according to any preceding claim, in which the measurements of the one or more process conditions comprise temperature measurements of the extrusion system at one or more locations thereof.

7. A method according to any preceding claim, in which the measurements of the one or more process conditions may comprise indications of the speed of rotation of at least one screw of the extrusion system.

8. A method according to any preceding claim, in which acquiring the indications of the extrudate material throughput comprises continuously acquiring the indications of the extrudate material throughput.

9. A method according to any of claim 1 to 7, in which acquiring the indications of the extrudate material throughput comprises acquiring an indication of the extrudate material throughput at a plurality of sample times.

10. A method according to any preceding claim, in which the indications of the extrudate material throughput comprise measurements of the rate at which the extrudate material is fed through the extrusion system.

11. A method according to any preceding claim, in which the indications of the extrudate material throughput comprise indications of the speed of rotation of at least one screw of the extrusion system.

12. A method according to any preceding claim, in which acquiring the measurements of the pressure of the extrudate material at one or more locations of the extrusion system comprises continuously acquiring the measurements of the pressure.

13. A method according to any of claims 1 to 11 , in which acquiring the measurements of the pressure of the extrudate material at one or more locations of the extrusion system comprises acquiring a measurement of the pressure at the or each location at a plurality of sample times.

14. A method according to any preceding claim, in which the measurements of the pressure of the extrudate material at one or more locations of the extrusion system comprise measurements from a pressure sensor placed in an end part of an extrusion barrel of the extrusion system.

15. A method according to any preceding claim, which further comprises acquiring measurements of power used by at least one screw of the extrusion system.

16. A method according to claim 15, in which acquiring the measurements of the power of the at least one screw comprises continuously acquiring the measurements of the power.

17. A method according to claim 15, in which acquiring the measurements of the power of the at least one screw comprises acquiring a measurement of the power at a plurality of sample times.

18. A method according to any preceding claim, which further comprises acquiring measurements of torque of at least one screw of the extrusion system.

19. A method according to claim 18 in which acquiring the measurements of the torque of the at least one screw comprises continuously acquiring the measurements of the torque.

20. A method according to claim 19, in which acquiring the measurements of the torque of the at least one screw comprises acquiring a measurement of the torque at a plurality of sample times. This is referred to as sampling the power.

21. A method according to any preceding claim, in which using the first model to generate an estimated viscosity of the extrudate material using one of the measurements of the or each process condition comprises using a temperature measurement and a screw speed measurement to estimate the viscosity of the extrudate material.

22. A method according to any preceding claim, in which using the first model to generate an estimated viscosity of the extrudate material comprises using a knowledge of one or more properties of the extrudate material.

23. A method according to any preceding claim, in which using the second model to generate an estimated pressure of the extrudate material using one of the indications of the extrudate material throughput comprises using a measurement of power used by at least one screw of the extrusion system.

24. A method according to any preceding claim, in which using the second model to generate an estimated pressure of the extrudate material using one of the indications of the extrudate material throughput comprises using a measurement of torque of at least one screw of the extrusion system.

25. A method according to any preceding claim, in which using the pressure difference value to generate a viscosity correction value comprises performing integration and gain functions on the pressure difference value.

26. A viscosity sensing system, adapted for use in an extrusion system, the apparatus comprising at least one extrusion process condition measurement apparatus which acquires measurements of a process condition of the extrusion system, at least one extrudate material throughput determination device which acquires indications of the throughput of an extrudate material through the extrusion system, at least one pressure sensor which acquires pressure measurements of the extrudate material at one or more locations of the extrusion system, at least one processor which uses a first model to generate an estimated viscosity of the extrudate material using data relating to the extrudate material and a measurement of the or each process condition, uses a second model to generate an estimated pressure of the extrudate material using data relating to the extrusion system geometry, the estimated viscosity of the extrudate material, and an indication of the extrudate material throughput, uses a comparator model to compare the estimated pressure of the extrudate material with a pressure measurement of the extrudate material and generates a pressure difference value, uses a pressure/viscosity conversion model to generate a viscosity correction value using the pressure difference value, feeds the viscosity correction value into the first model and uses the viscosity correction value in the generation of a next estimated viscosity of the extrudate material.

27. A computer storage medium comprising a computer program which when executed on a processor causes the processor to carry out the method according to any of claims 1 to 25.