A study about weak intralayer bonding in extrusion-based additive manufacturing due to resumed extrusion during filling

The strength of a printed polymeric component obtained by a material extrusion additive manufacturing technology is highly affected by the bonding quality between the deposited filaments (beads). Filament bonding is usually associated with material temperature and can be observed between layers (interlayer) and in the same layer (intralayer). The latter is not much explored in the literature and is rarely studied when the material extrusion must be resumed to complete a layer filling. A non-continuous tool path is usually found in filling complex part geometries with a raster strategy, where the area to be filled is divided into more than one continuous raster segment (CRS). The contact between the filaments of two adjacent CRSs (at 100 % density), defined as resumed contact (RC) for simplicity, can be affected by a weak bonding effect. This work aimed to experimentally study this effect by varying the time taken to resume printing, causing the temperature to drop at the RC, and measuring the influence on bonding strength. The bead width at the resumed extrusion was also analyzed. The results show that the influence of time taken to resume printing (contact temperature) on bonding strength was not significant for the part geometry tested and printer used. Notwithstanding, material failure under tensile load always occurred in the RC region. The results also showed some bead width variation due to under extrusion at the start of the extrusion, which can reduce the contact area. Therefore, the weak intralayer bonding at the RC is a fact, and it is affected, among others, by a combination of cold bonding and any contact area reduction due to under extrusion at the start of the extrusion. This issue must be carefully considered when printing an end-use part where the material strength is paramount.