PSI - Issue 42

Manuel Sardinha et al. / Procedia Structural Integrity 42 (2022) 1274–1281 M. Sardinha et al. / Structural Integrity Procedia 00 (2022) 000–000

1275

2

One of the most widely used AM technologies is fused filament fabrication (FFF). In this process, an extrusion head system is commonly attached to a carriage, where a thermoplastic filament material is heated to a semi-molten state and extruded in a raster configuration. Heat is dissipated by conduction and forced convection, and the reduction in temperature caused by these processes causes the material to quickly solidify onto the surrounding filaments, with the potential to cause a non-uniform cooling process [6]. The factors that most influence the homogeneity of the cooling during the build process include the existence of di ff erent filling regions, thickness variations, and geometric asymmetry or curvatures of the part [7]. Re-melting and rapid cooling may aggravate non-uniform thermal gradients, potentiating residual stresses and, therefore, part distortions. Warping (dw) is a distortion in which the geometry of the printed part does not follow the intended shape [8]. Curling is one of the most common warping scenarios caused by the part’s various elongations and residual stresses, resulting in upward bending [8, 9]. In FFF, this phenomenon is highly associated with the lack of adhesion between the first layer and the build platform [3]. Figure 1(a) illustrates the warping (the curling type) deformation of an FFF part, as well as the thermal forces that promote localized layered geometrical shrinkage. The curling e ff ect occurs most prominently in the component’s edge regions. In this work, the term ’warping’ is used to refer to the curling phenomena, since this is the only type of warping e ff ect under evaluation, and the most common in FFF parts.

Fig. 1: Illustrations of production and thermal processing during the build of a FFF part: (a) Warping deformation and related thermal forces; (b) Scheme of the top layer of a FFF part being ironed.

Warping e ff ect strongly depends on the extruded material, and its thermal expansion coe ffi cient, and linearly in creases with material shrinkage rate [10]. As can be seen in Table 1, warping increases with the length of the stacking section, layer thickness, porosity, printing speed and infill density. In opposition, warping tends to decrease with the increment of cooling time, the chamber temperature, and the bed and the printing temperatures. Since thermal stresses cause warping, good adhesion to the build platform helps to decrease it. This study is focused on Acrylonitrile butadi ene styrene (ABS) since its potential to produce warped parts is significantly more severe than in other common FFF materials such as Polylactic acid (PLA). Up to less than 3% in PLA in comparison with up to 34.53% in ABS [11].

Table 1: Summary of the influence of printing parameters on warping. 1 Warping decreases until a certain point.

Parameters

If Warping Source

Layer thickness Extrusion Width

[12, 13, 14]

↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑

↑ ↓

[12] [12] [10]

↑ ↓

Time of Cooling Cycles Chamber Temperature Printing Temperature

↓ 1 ↓ 1 ↓ 1 ↑ ↓ ↑ ↑ ↑

[7, 13, 15]

Bed Temperature

[3, 16]

Build Platform Contact Area ↑ Number of layers

[10, 12, 13]

[10, 13]

Infill Density Printing Speed

[17, 18, 19]

[10, 12]

90” / 0” or 0” / 0” or 90” / 90” -

[10, 17, 20]