## 1 Introduction  
The use of in-line rheological measurement methods during the processing of thermoplastic materials has been the subject of scientific research for a long time [1; 2; 3]. The rheological characteristics of molten polymers, which can be measured by conventional off-line techniques [4; 5], are usually not the same as the properties of these materials in the actual process, such as extrusion or injection molding [6; 7]. The main problem here is the high shear forces to which the polymers are subjected during the flow. With many standard devices used to determine viscosity, such as rotational and capillary rheometers, it is difficult to achieve actual process conditions [8; 9]. The subject of research concerning attempts to reflect the shear conditions prevailing during extrusion was the subject of many studies; in this case, most of the works are based on the analysis of measurement results on capillary rheometers [10; 11]. A more complicated issue is the attempt to carry out similar measurements during injection molding, which results from much higher shear forces [12; 13; 14]. However, attempts to simulate the conditions of technological processes in off-line measurements will never allow reflection of the influence of the entire process on the properties of the material. It is difficult to take into account the residence time of the material, changes in the geometry of processing tools, or other conditions specific to a given type of manufacturing technique during a standardized test. The demand for in-line measurements is particularly important during the manufacturing of thermal/shear-sensitive materials since the rheological characteristic of the raw material supplied to the technological process strongly differs from the properties of the samples taken after the extrusion/injection step. In the case of plastics processing, the need for measurements may turn out to be useful in controlling the properties of multi-component compositions, such as polyvinyl chloride (PVC) blends, or processes based on materials with unstable properties, such as recycled plastics. The PET foil extrusion process discussed in this article is an example of using post-consumer materials. Despite advanced purification processes, the material charge may turn out to be less valuable due to its origin. The most common form of recycled PET is shredded flakes. This type of material might consist of bottles or thermoformed containers, which means that a large share of the input material might be already been processed several times. Each subsequent processing significantly affects the key rheological parameter of PET, which is the intrinsic viscosity (IV). Although for cast film extrusion, the tolerance of the IV range is relatively wide, usually from 0.7 to 1.0 dL/g, while for PET bottles is 0.78-0.85 dL/g [15; 16; 17], the changes in viscosity during a large-scale process may affect numerous functional properties of the foil, such as strength, thickness, transparency. Among the many methods of improving the viscosity of PET, the techniques of reactive extrusion are currently gaining popularity, where a polymer material with low viscosity is subjected to a chain extension reaction in the melt phase on an extruder. It allows for significant cost savings compared to methods such as solid-state condensation [18]. In the case of reactive extrusion of thermoplastic polyesters, numerous chemical compounds turn out to be effective [19; 20; 21; 22], but due to the high sensitivity of the reactions to thermal conditions, this process often requires optimization. For the presented study, the main reactive compound was pyromellitic dianhydride (PMDA), the effectiveness of which has been confirmed for many processing techniques [23; 24; 25; 26]. The in-line viscosimeter (VIS system) that was used for the conducted study was working together with the gear pump that creates a defined volumetric polymer flow from the main melt channel of the extrusion system to a slit capillary which has a defined geometry.