Apparatus and Method for Producing an Object By Means of Additive Manufacturing

An apparatus and a method for producing an object by means of additive manufacturing, the apparatus comprising: - a process chamber arranged for receiving in a build space of the process chamber a bath of material arranged for producing the object; - a ultrasound source element arranged for emitting a beam of focused ultrasound energy in the build space for processing a selective part of the material of the bath of material for producing the object; - a control unit, communicatively coupled to the ultrasound source element, arranged for controlling the ultrasound source element such that a frequency and/or amplitude of the beam of focused ultrasound energy is set at a predetermined frequency and/or predetermined amplitude taking into account a characteristic of material of the bath of material in a focus spot of the beam of focused ultrasound energy and/or such that a frequency and/or an amplitude of the beam of focused ultrasound energy is/are set at a predetermined value, preferably the predetermined frequency and/or the predetermined amplitude, taking into account a focus distance of the beam of focused ultrasound energy

Apparatus and Method for Producing an Object By Means of Additive Manufacturing

An apparatus and a method for producing an object by means of additive manufacturing, the apparatus comprising: - a process chamber arranged for receiving in a build space of the process chamber a bath of material arranged for producing the object; - a ultrasound source element arranged for emitting a beam of focused ultrasound energy in the build space for processing a selective part of the material of the bath of material for producing the object; - a control unit, communicatively coupled to the ultrasound source element, arranged for controlling the ultrasound source element such that a frequency and/or amplitude of the beam of focused ultrasound energy is set at a predetermined frequency and/or predetermined amplitude taking into account a characteristic of material of the bath of material in a focus spot of the beam of focused ultrasound energy and/or such that a frequency and/or an amplitude of the beam of focused ultrasound energy is/are set at a predetermined value, preferably the predetermined frequency and/or the predetermined amplitude, taking into account a focus distance of the beam of focused ultrasound energy

Fiber-induced crystallization in elongational flows

Morphology development at the fiber/matrix interphase in fiber-reinforced isotactic polypropylene composites is a widely studied topic. While the application of shear flow may strongly enhance the nucleation density around the fiber, little is known about the influence of fibers on the crystallization of polypropylene subjected to an extensional flow. In this work, the flow around a single glass fiber upon uniaxial elongation of the melt is examined using X-ray scattering and diffraction techniques and compared to the response measured for the neat matrix. A comparison between a neat and compatibilized matrix is made given the strong influence of the addition of an adhesion modifier on the bulk crystallization kinetics of polypropylene. The flow is applied using an in-house-built filament stretching extensional rheometer, which, due to its midfilament control scheme, allows for in situ X-ray experiments. Combined small-angle X-ray scattering/wide-angle X-ray diffraction patterns are acquired during the flow and subsequent crystallization step. Postcrystallization area scans of the filament show that the introduction of a single glass fiber gives rise to the development of β-phase crystals, particularly in the area around the fiber ends, and in contrast to what is observed for the matrix materials alone, where solely α-phase is found. Surprisingly enough, the addition of a single fiber (0.00045 vol %) alters the crystallizing polymorph in almost the entire filament. However, the addition of the adhesion modifier hinders the formation of β-phase crystals around the fiber due to an acceleration of the bulk crystallization kinetics. Finite element simulations provide insight into the flow field around the fiber during stretching and demonstrate that the flow is no longer uniaxial extension, but dominated by shear, even though the volumetric amount of fiber as compared to the matrix is negligible. These findings explain the experimental observation of substantial β-phase formation after the introduction of a single fiber, while this is not observed in the matrix material. Worth noting, the formation of β-phase polypropylene depends not only on the presence and the strength of the flow but predominantly on the type of flow, i.e., shear as opposed to elongation

A numerical study of extensional flow-induced crystallization in filament stretching rheometry

A finite element model is presented to describe the flow, resulting stresses and crystallization in a filament stretching extensional rheometer (FiSER). This model incorporates nonlinear viscoelasticity, nonisothermal processes due to heat release originating from crystallization and viscous dissipation as well as the effect of crystallization on the rheological behavior. To apply a uniaxial extension with constant extension rate, the FiSER plate speed is continuously adjusted via a radius-based controller. The onset of crystallization during filament stretching is investigated in detail. Even before crystallization starts, the rheology of the material can change due to the effects of flow-induced nucleation on the relaxation times. Both nucleation and structure formation are found to be strongly dependent on temperature, strain rate and sample aspect ratio. The latter dependence is caused by a clear distribution of crystallinity over the radius of the filament, which is a result of the nonhomogeneous flow history in the FiSER. Therefore, this numerical model opens the possibility to a priori determine sample geometries resulting in a homogeneous crystallinity or to account for the nonhomogeneity

A numerical study of extensional flow-induced crystallization in filament stretching rheometry

A finite element model is presented to describe the flow, resulting stresses and crystallization in a filament stretching extensional rheometer (FiSER). This model incorporates nonlinear viscoelasticity, nonisothermal processes due to heat release originating from crystallization and viscous dissipation as well as the effect of crystallization on the rheological behavior. To apply a uniaxial extension with constant extension rate, the FiSER plate speed is continuously adjusted via a radius-based controller. The onset of crystallization during filament stretching is investigated in detail. Even before crystallization starts, the rheology of the material can change due to the effects of flow-induced nucleation on the relaxation times. Both nucleation and structure formation are found to be strongly dependent on temperature, strain rate and sample aspect ratio. The latter dependence is caused by a clear distribution of crystallinity over the radius of the filament, which is a result of the nonhomogeneous flow history in the FiSER. Therefore, this numerical model opens the possibility to a priori determine sample geometries resulting in a homogeneous crystallinity or to account for the nonhomogeneity.</p