||In the polymer extrusion process the product quality like mechanical, optical, electrical properties and homogeneity etc. can be achieved by controlling the melt temperature, melt pressure or viscosity within a narrow fluctuation range. In the earlier studies there are many literatures in connection with the extrusion quality and related quality controls; i.e. temperature control, pressure control and viscosity control. In each of the control strategies, it is believed that the most effective to maintain product quality utilising viscosity control, because a polymer viscosity closely correlates with its composition and molecular distribution, and hence the characteristic of the material.|
In the viscosity control strategy, viscosity is an induced variable calculated from either the (1) flow rate and pressure drop with in-line rheometer or (2) melt temperature, screw speed (or pressure), geometrical dimensions of extruder, and extrusion material constants without in-line rheometer; the former method may interfere the output rate while the latter one does not.
On the demand of using viscosity-measuring instruments as sensors to control the quality of the products, we developed an empirical off-line viscosity model, which is used to derive the extrusion viscosity models in the control process without in-line rheometer. The off-line viscosity model is proved more accuracy than other previous suggested models, such as WLF and Andrade’s equations, to fit the experimental data. Polypropylene (PP) was used in this study to test the effectiveness of the extrusion viscosity models. Comparing the calculated results, it was found that the viscosity characteristics obtained by the extrusion viscosity models are in agreement with those obtained by using an in-line rheometer. Both methods can be used to obtain the viscosity in the polymer extrusion process.
The objective of this study is to develop extrusion viscosity models together with collected data from several experimental tests and template rule-base to build a Multi-Input Multi-Output (MIMO) fuzzy logic closed-loop controller for the plastics extrusion control. The objective of this controller is to eliminate process variations and to produce the polymer of consistent quality. The fuzzy logic is provided for designing the MIMO closed-loop controller because it is suitable for applying to the polymer extrusion process control with such advantages as handling complex problems like non-linear, time varying behaviour and poor quality measurements happened in the extrusion process, etc. The experimental pre-tests include (1) investigation of the relationship between melt temperature and barrel setting temperatures (2) investigation of the relationship between melt pressure and screw speed and (3) building the relation equation between measured viscosity, melt temperature and speed for the in-line rheometer, etc.
In order to test the effectiveness of the MIMO FLC, an off-line simulation program is developed, and the closed-loop tests are performed on the extruder. The test results prove that the designed MIMO FLC can effectively control the quality of products.