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On-Machine Measurement, Monitoring and Control
Published in Richard Leach, Simone Carmignato, Precision Metal Additive Manufacturing, 2020
Bianca Maria Colosimo, Marco Grasso
Dimensional deviations in PBF can be classified into (i) shrinkage and oversize effects, (ii) warping and curling, (iii) dross formation at down-facing surfaces, (iv) super-elevated edges and (v) other in-plane geometrical distortions. Regarding the size of the part, shrinkage has been reported by different authors (Thomas 2009, Sharratt 2015), even though the opposite effect (i.e. parts that are systematically larger than the nominal) may occur in practice, depending on dimensional compensation settings applied to the job file. Warping is mainly due to the heat dissipation mechanisms and to thermal stresses (Sharratt 2015). The so-called curling phenomenon is one type of warping effect caused by non-uniform thermal expansion and part contraction, usually associated with an uneven shrinkage between the top and the bottom of overhanging areas (Gibson et al. 2010, Mousa 2016). A combination of shrinking and warping effects yield curved profiles of down-facing surfaces intended to be flat. In the presence of the down-facing surfaces, dross formation and defective contours are known to be caused by a lack of, or improper design of, supporting structures.
Additive Manufacturing of Polymers
Published in Amit Bandyopadhyay, Susmita Bose, Additive Manufacturing, 2019
Amit Bandyopadhyay, Kellen D. Traxel, Caitlin Koski, Susmita Bose
Critical parameters that govern the properties and characteristics of these parts are the layer-by-layer deposition orientation relative to the build plate, layer thickness, air gap distance between raster-paths, width of deposition, and extrusion head temperature, among others [14]. In general, parts with larger layer thicknesses (300–400 µm) lead to lower tolerancing capability than with lower layer thickness (10–50 µm), particularly in places of changing cross section from layer to layer [14]. In addition, for acrylonitrile butadiene styrene (a common FDM thermoplastic), lower reciprocating wear performance has been reported for larger layer thicknesses due to the debris, cracks, and fracture that can be caused in the microstructure [15]. Elsewhere, polylactic acid fatigue properties have been shown to be strongly affected with the build direction, namely, alternating 45° layers showed the strongest fatigue life, indicating that anisotropy is a challenge with this process [16]. Because of the heat input using this process, warping can be another by-product of large layer thickness and must be accounted for with the optimization of baseplate heating and extrusion temperature to minimize thermal gradients.
Seasonal temperature distribution in rigid pavements
Published in Andreas Loizos, Imad L. Al-Qadi, A. (Tom) Scarpas, Bearing Capacity of Roads, Railways and Airfields, 2017
K. Bayraktarova, L. Eberhardsteiner, R. Blab
In the following, results from the simulation of the temperature distribution for different meteorological stations and periods of time are presented. The evaluation method was developed with data from meteorological station in Gänserndorf and for the period 2004–2009. Figure 4 presents a typical temperature distribution obtained from the numerical simulations. The curves represent temperature profiles through a concrete slab with thickness of 25 cm for various times of the day. A strong inward curvature of the temperature profile indicates high temperature gradients. Figure 5 compares the difference between the night and daily profiles. It should be noted that the temperature gradients are positive during the day and negative during the night. Temperature gradients can be derived from the computed temperature distributions and used further as input for the estimation of resulting warping stresses.
Uniaxial tensile testing standardization for the qualification of fiber reinforced plastics for fused filament fabrication
Published in Mechanics of Advanced Materials and Structures, 2021
William H. Ferrell, Jason Clement, Stephanie TerMaath
As fan speed increases beyond 25%, rapid reduction in the ultimate tensile load and the increased onset of brittle failure is evident, suggesting reduced interlayer adhesion. The longer a polymer remains above the Tg, the more time the polymer chains will have to reptate into a thermodynamically favorable state. Increasing fan speed decreases the amount of time polymer chains spend above the Tg and thus inhibits the amount of reptation between layers. Additionally, specimens fabricated at increased fan speeds showed significant signs of warping and reduced number of successful prints and tests, as demonstrated in Figure 10. Warping is the physical manifestation of uneven cooling rates, where internal cooling rates significantly lag external cooling rates [69, 72]. At fan speeds of 50% and above, roughly only a quarter of prints resulted in valid tests with failures in the gauge length. As all specimens fabricated at the higher speeds exhibited some specimen warping, the validity of all results is questionable given the induced bending moments during testing.