The deposited filament, from which the 3d printed specimens are comprised (sometimes referred to as rasters, fibres, roads, tracks or extrudates in other studies). Extruded-filament geometry:The cross-sectional geometry of the extruded filament.EFW: Extruded-filament width F: Filament direction (sometimes referred to as longitudinal direction in other studies -parallel to the print bed)Interface: The region of joining between two extruded filaments. Interlayer bonding:The interfacial bonding between layers (extruded filaments). LH: Layer heightLoad-bearing area: The cross-sectional area of the specimen, which bears mechanical load. Specific load-bearing capacity:The maximum load capacity of specimens normalised by the weight of the unit length of the specimen gauge. Z: Z-direction (normal-to-filament direction and normal to the print bed).

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Do extrusion temperature, printing speed and layer time affect mechanical performance of interlayer bonds in material extrusion additive manufacturing (MEAM)? The question is one of the main challenges in 3D printing of polymers. This paper aims to analyse the independent effect of printing parameters on interlayer bonding in material extrusion additive manufacturing (MEAM). In previous research, printing parameters were unavoidably inter-related, such as printing speed and layer cooling time. Here, original specimen designs allow the effects to be studied independently for the first time, to provide new understanding of the effects of a wide range of thermal factors on mechanical properties of 3D-printed polylactide (PLA). The experimental approach used direct GCode design to manufacture specially designed single-filament-thick specimens for tensile testing to measure mechanical and thermal properties normal to the interface between layers. In total, five different extrusion temperatures (a range of 60°C), five different printing speeds (a 16-fold change in the magnitude) and four different layer times (an 8-fold change) were independently studied. The results demonstrate interlayer bond strength to be equivalent to that of the bulk material within experimental scatter. This study provides strong evidence about the crucial role of microscale geometry for apparent interlayer bond strength relative to the role of thermal factors. By designing specimens specifically for the MEAM process, this study clearly demonstrates bulk-material strength can be achieved for interlayer bonds in MEAM even when printing parameters change several-fold. Widespread industrial and academic efforts to improve interlayer bonding should be refocused to study extrusion geometry -the primary cause of anisotropy in MEAM.