Fundamentals of twin-screw extrusion polymer melting: Common pitfalls and how to avoid them

Abstract

Abstract. The process for compounding engineered polymer formulations is comprised of several unit operations. These typically include, but are not limited to: feedstock introduction, polymer melt-mixing, distributive/dispersive mixing of minerals/fibers, removal of volatiles, and pressurization for discharge. While each unit operation has an impact on process productivity and the quality of the finished product, polymer melt-mixing has a significantly greater impact than the others. First, it consumes 50, 60 or higher percent of the total system energy. Second, it generates the highest radial as well as particle-particle interactive pressure of any unit operation. Third, the negative impact on the process of any design flaws in the melt-mixing configuration is transmitted downstream to all subsequent unit operations. For example, a melt-mixing design that is too intense may degrade the polymer while one that is too weak may result in excessive breakage of glass fiber being fed downstream due to the polymer solidifying on the glass fiber and subsequently being re-melted. Another example of the impact of an incorrect meltmixing configuration would be excessive abrasive wear. Adhesive wear is also possible as well as deformation on both barrel wall and screw elements due to high radial forces. Additionally, non-melting material present during the melt-mixing process could be compacted into "briquettes" by the high radial pressure and would have to be dispersed by subsequent downstream unit operations. Other potential issues associated with a non-optimal melting section are pre-mature and incomplete melting. The former is more of a concern with melting of powder feed stock while the latter is more probable with feed stock comprised of a broad range of particle sizes. However, the consequence of both is to convey unmolten polymer beyond the melting section. While this may not be perceived as a significant issue for most processes, it is an issue if the sole purpose of the process is to uniformly melt the feedstock. This is case for powder to pellet conversion of polyolefins and melt spinning of mono-filament.This paper provides a further discussion of the issues noted above as well as associated examples.

Online optical monitoring of polymer melting in a twin‐screw extruder

“…A polypropylene homopolymer (ELTEXP HV001PF, produced by INEOS Olefins & Polymers Europe) with density of 0.905 g/cm 3 and MFI = 10 g/10 minutes (2.16 kg, 190 C) was used in the experiments. The polymer was supplied in powder form, in order to avoid eventual clogging the multislit die, as explained below.…”

“…Melting is an important process stage, influencing the development of pressure and temperature along the screw axis, as well as accounting for at least 50% of the total energy consumption. [3] Experimental investigations have also shown that immiscible polymer blends and nanocomposites experience major changes of morphology (and of chemical conversion in the case of reactive extrusion) upon melting. [4][5][6] Melting is influenced by the local screw configuration (typically containing restrictive elements), by the physical form and properties of the feedstock and by the operating conditions (feeding rate, screw speed, barrel temperature).…”

New 2.5‐Dtwin screw extruder model for the feed section of a co‐rotating twin screw extruder with comparisons to data

“…Since most heating occurs at the bed surfaces, this energy dissipation can create a surface film of melted polymer to be formed which would likely reduce the efficiency of the melting process unit operation screw configuration. [32] The solid bed is deformed and rearranged as the pellets travel through the nip region between the screws where additional frictional heating can occur at the pellet-pellet contact surfaces.…”

Introduction to Modelling the Correlation Between Grain Sizes of Feed Material and the Structure and Efficiency of the Process of Co-Rotating Twin-Screw Extrusion of Non-Flammable Composites with a Pla Matrix

“…The selection of the appropriate configuration of the plasticising system can therefore be presented as a search for a compromise between the best mixing of components and the lowest possible degradation of components, especially polymers [11,12]. One way to avoid the impact is to crush commercially available granules of polymers to reach smaller grain sizes [1,13].…”