  Citation: Gaspar-Cunha, A.; Covas, J.A.; Sikora, J. Optimization of Polymer Processing: A Review (Part I—Extrusion). Materials 2022, 15, 384. https://doi.org/10.3390/ma15010384 Academic Editor: Andrea Sorrentino Received: 23 November 2021 Accepted: 13 December 2021 Published: 5 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Optimization of Polymer Processing: A Review (Part I—Extrusion) António Gaspar-Cunha 1, * , José A. Covas 1 and Janusz Sikora 2 1 Institute of Polymers and Composites, University of Minho, Campus de Azurém, 4804-533 Guimarães, Portugal; jcovas@dep.uminho.pt 2 Department of Technology and Polymer Processing, Faculty of Mechanical Engineering, Lublin University of Technology, Nadbystrzycka 36, 20-618 Lublin, Poland; janusz.sikora@pollub.pl * Correspondence: agc@dep.uminho.pt Abstract: Given the global economic and societal importance of the polymer industry, the continuous search for improvements in the various processing techniques is of practical primordial importance. This review evaluates the application of optimization methodologies to the main polymer processing operations. The most important characteristics related to the usage of optimization techniques, such as the nature of the objective function, the type of optimization algorithm, the modelling approach used to evaluate the solutions, and the parameters to optimize, are discussed. The aim is to identify the most important features of an optimization system for polymer processing problems and define the best procedure for each particular practical situation. For this purpose, the state of the art of the optimization methodologies usually employed is first presented, followed by an extensive review of the literature dealing with the major processing techniques, the discussion being completed by considering both the characteristics identified and the available optimization methodologies. This first part of the review focuses on extrusion, namely single and twin-screw extruders, extrusion dies, and calibrators. It is concluded that there is a set of methodologies that can be confidently applied in polymer processing with a very good performance and without the need of demanding computation requirements. Keywords: polymer processing; single screw; twin screw; injection moulding; blow moulding; thermoforming; optimization; artificial intelligence 1. Introduction Polymer processing is an important industrial activity that converts raw materials, such as polymers, polymer compounds, polymer blends, composites, and nanocom- posites, into useful products mostly for applications in packaging, building and con- struction, mobility, electrical and electronics, medical, agriculture, household, leisure, and sports. For example, in 2019, more than 55,000 European companies (plastics raw materials producers, plastics converters, recyclers and machinery manufacturers in the EU28 Member States) employed over 1.5 million people, and converted 50.7 Mt of plas- tics [1]. A progressively more sustainable and better performing range of polymer sys- tems, together with increasingly more efficient and intelligent extrusion, injection mould- ing, blow moulding, and thermoforming—the most important processing techniques for thermoplastics–are paramount to create or improve products with more advanced performances and functionalities. Thermoplastics processing typically involves three functional steps: plasticization of a solid polymer (usually supplied in pellet form), flow and shaping of the melt, and cooling. Thus, an understanding of polymer processing requires a good knowledge of heat transfer, melt rheology, fluid mechanics, and morphology development, among others. In the case of reactive extrusion, plasticization is combined with chemical reactions (polymer synthesis and/or modification) into a single process. In their seminal book on polymer processing, Materials 2022, 15, 384. https://doi.org/10.3390/ma15010384 https://www.mdpi.com/journal/materials