Analytical and Numerical Models of Thermoplastics: A Review Aimed to Pellet Extrusion-Based Additive Manufacturing

Recent developments in additive manufacturing have moved towards a new trend in material extrusion processes (ISO/ASTM 52910:2018), dealing with the direct extrusion of thermoplastic and composite material from pellets. This growing interest is driven by the reduction of costs, environmental impact, energy consumption, and the possibility to increase the range of printable materials. Pellet additive manufacturing (PAM) can cover the same applications as fused filament fabrication (FFF), and in addition, can lead to scale towards larger workspaces that cannot be covered by FFF, due to the limited diameters of standard filaments. In the first case, the process is known as micro- or mini-extrusion (MiE) in the literature, in the second case the expression big area additive manufacturing (BAAM) is very common. Several models are available in literature regarding filament extrusion, while there is a lack of modeling of the extrusion dynamics in PAM. Physical and chemical phenomena involved in PAM have high overlap with those characterizing injection molding (IM). Therefore, a systematic study of IM literature can lead to a selection of the most promising models for PAM, both for lower (MiE) and larger (BAAM) extruder dimensions. The models concerning the IM process have been reviewed with this aim: the extraction of information useful for the development of codes able to predict thermo-fluid dynamics performances of PAM extruders.

Functional morphology of the mouthparts and alimentary tract of the slipper lobster Thenus orientalis (Decapoda : Scyllaridae)

The mouthparts and proventriculus of Thenus orientalis Lund are adapted to ingest soft flesh, which is consistent with the diet of this and other scyllarids. The crista dentata are reduced, with food transfer into the oesophagus facilitated by large stout setae on the second and third maxillipeds. The mandibles exert little force and most food maceration is effected by the gastric mill. Ingestion is aided by mucus secreted by rosette glands in the paragnaths and membranous lobe, as well as expansion of four longitudinal folds in the oesophageal wall. The cardiac stomach has considerable food storage capacity by extension of its membranous walls, reduced ossicles and simplified ventral filtration channels. The filtering ability of the pyloric filter press is consistent with other macrophagous decapods. The dorsal caecum above the pyloric stomach has an absorptive columnar epithelium that contains acid mucin granules and protein. Muscular walls and longitudinal folds in the hindgut facilitate faecal pellet extrusion.

Faecal firing in a skipper caterpillar is pressure-driven

Many leaf-rolling caterpillars have a rigid anal comb attached to the lower surface of the anal plate (or shield) situated above the anus. This comb is widely assumed to be a lever used to 'flick' away frass pellets. An alternative mechanism to explain pellet discharge is proposed on the basis of observations on the caterpillar of the skipper Calpodes ethlius. The model proposes that the underside of the anal plate serves as a blood-pressure-driven surface for the ejection of faecal pellets. Rather than acting as a lever, the anal comb serves as a latch to prevent the premature distortion of the lower wall of the anal plate until the anal haemocoel compartment is fully pressurized. The anal comb is swung into position during pellet extrusion by retractor muscles attached at its base and held in place by a catch formed by a blood-swollen torus of everted rectal wall. As the caterpillar raises the blood pressure in its anal compartment by contracting its anal prolegs, the comb eventually slips over the toral catch. This causes the underside of the anal plate to move rapidly backwards as the blood pressure is released, projecting the pellet resting against it through the air. Simulation suggests that a local blood pressure of at least 10 kPa (75 mmHg) would be required to accelerate the lower surface of the anal plate outwards at a rate fast enough to discharge a 10 mg pellet at an observed mean velocity of 1.3 m s-1.