working copy Nov 2016 - for further info please contact the Authors Study of Moineau-based pumps for the volumetric extrusion of pellets Enrique Canessa, Marco Baruzzo and Carlo Fonda Scientific Fabrication Laboratory (SciFabLab) ICTP - The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy e-mail: scifablab@ictp.it Abstract We report on our on-going research to create a volumetric extruder for pellets or granules of recycled plastic to be used in a RepRap FDM 3D printer for rapid prototyping , by combining a rotor made of a Moineau progress - ing cavity pump coupled with an Auger screw, both contained inside a heated stator. We also introduce an alter - native multi-layer Moineau - based pump – easier to build, implement and clean – for pumping a controlled quan - tity of viscous material in vertical direction. The latter, our screw pump model 131313, consists of a steer Auger portion which increases the pressure inside a layered double-helix stator -like container as a layered heli - cal rotor -like is turned. Keywords : 3D printing, RepRap, fluid dynamics, Moineau 1. Introduction The geometry of a progressing cavity pump, or Moineau pump, is beautiful and simple in theory [1]. It consists of an in - ner (harder) helical rotor and an outer rubber (softer) stator which form a set of small- and fixed-size cavities in be - tween. The rotor displaces viscous material throughout the system by means of the rotation of the cavities. The fluid can flow back or forward and the volumetric flow rate Q is proportional to the rotation rate N in both directions. Progressing cavity pumps have many application areas including food, oil, etc. pumping (see, e.g. , [2]). However, these type of pumps can present big challenges when applied in practice to small dimensions (only few centimetres in length) and at high temperatures (say, higher than 180 Celsius degrees). One novel application for using Moineau-based pumps with such extreme characteristics could found fertile grounds in the rapid production of 3D objects printed directly from thermoplastic pellets or granules of recycled plastic instead than from filament. We discuss here on the use of a Moineau pump combined with an Auger pump (Archimede ’ s screw) as first stage, both 3D printed out of aluminum, for the volumetric extrusion of pellets . In the following, w e also describe in detail our first prototype for a new class of layered screw -like pump. M otivated by the aforementioned implementation constraints, we have sectioned the simplest (2:1) Moineau's progressive cavity pump in several, equally spaced, horizontal layers in or - der to re-design an alternative multi-layer pump system simplified to just a few layers that can be easier to build, assem - ble and clean, and get in practice, a reasonable continuous pumping of viscous material . 2 . Implementation of a thermoplastic extruder using a Moineau pump The first idea to implement a plastic extruder using a Moineau pump was inspired by an article appeared in Issue 3 of “RepRap Magazine” at page 26, entitled “ The road to better paste extrusion ” [3]. The author, Dries Verbruggen, ex - plained the importance of volumetric extrusion and listed, among other solutions, two possible “ positive displacement extruders ” : peristaltic pump and gear pump, comparing them to the non-volumetric Auger pump, but also mentioning briefly the Moineau pump at the very end of the article. After some more literature research, we decided to investigate further on this technology, since a (n+1:n) Moineau is the only pump that can provide a volumetric extrusion and ¬ at the same time ¬ heating and melting the plastic (the cited article [3] was focused on the extrusion of paste, and it did not consider the heating part). With the help of the very detailed mathematical analysis of Moineau pumps found in the article “ The geometry of the Moineau pump ” by Jens Gravensen, and of some additional on-line resources by the same author [1], we were able to create a fully parametric model of a Moineau pump (rotor and stator) with an (almost) arbitrary number of lobes n using the OpenSCAD 3D modelling language [4]. The OpenSCAD files have been submitted to the online repository of 3D models “ Thingiverse" [5] under the Creative Commons - Attribution license. After this first (mostly theoretical) analysis, we started to search for an optimal way to implement the Moineau pump to build a volumetric plastic extruder to be used in a RepRap FDM 3D printer. Usually, the implementation of an standard Moineau pump involve the rotor made of a solid surface (steel coated with chromium), with the stator made of elas - tomer inside a metal tube of complex shape to form the inner cavities that roll around. Our device model is shown in Fig.1. 1