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Purpose Recent work has demonstrated the possibility of selectively sintering polymer powders with radio frequency (RF) radiation as a means of rapid, volumetric additive manufacturing. Although RF radiation can be used as a volumetric energy source, non-uniform heating resulting from the sample geometry and electrode configuration can lead to adverse effects in RF-treated samples. This paper aims to address these heating uniformity issues by implementing a computational design strategy for doped polymer powder beds to improve the RF heating uniformity. Design/methodology/approach Two approaches for improving the RF heating uniformity are presented with the goal of developing an RF-assisted additive manufacturing process. Both techniques use COMSOL Multiphysics® to predict the temperature rise during simulated RF exposure for different geometries. The effectiveness of each approach is evaluated by calculating the uniformity index, which provides an objective metric for comparing the heating uniformity between simulations. The first method implements an iterative heuristic tuning strategy to functionally grade the electrical conductivity within the sample. The second method involves reorienting the electrodes during the heating stage such that the electric field is applied in two directions. Findings Both approaches are shown to improve the heating uniformity and predicted part geometry for several test cases when applied independently. However, the greatest improvement in heating uniformity is demonstrated by combining the approaches and using multiple electrode orientations while functionally grading the samples. Originality/value This work presents an innovative approach for overcoming RF heating uniformity issues to improve the resulting part geometry in an RF-assisted, volumetric additive manufacturing method.

Purpose Additive manufacturing (AM) of thermoplastic polymers for powder bed fusion processes typically requires each layer to be fused before the next can be deposited. The purpose of this paper is to present a volumetric AM method in the form of deeply penetrating radio frequency (RF) radiation to improve the speed of the process and the mechanical properties of the polymer parts. Design/methodology/approach The focus of this study was to demonstrate the volumetric fusion of composite mixtures containing polyamide (nylon) 12 and graphite powders using RF radiation as the sole energy source to establish the feasibility of a volumetric AM process for thermoplastic polymers. Impedance spectroscopy was used to measure the dielectric properties of the mixtures as a function of increasing graphite content and identify the percolation limit. The mixtures were then tested in a parallel plate electrode chamber connected to an RF generator to measure the heating effectiveness of different graphite concentrations. During the experiments, the surface temperature of the doped mixtures was monitored. Findings Nylon 12 mixtures containing between 10% and 60% graphite by weight were created, and the loss tangent reached a maximum of 35%. Selective RF heating was shown through the formation of fused composite parts within the powder beds. Originality/value The feasibility of a novel volumetric AM process for thermoplastic polymers was demonstrated in this study, in which RF radiation was used to achieve fusion in graphite-doped nylon powders.

One of the most promising features of additive manufacturing is ability to reduce part count and assembly by printing complex shapes. This feature is further highlighted by utilizing multi-material printing in which stiff and compliant materials can be deposited by the same system. Air-acoustic speakers are traditionally comprised of several assembled components with different material properties including a stiff enclosure, stiff diaphragm, and compliant suspension. In this presentation, we will describe a modular acoustic speaker design. The design consists of fully printed structural components (which are threaded together, and assembled with a voice coil and magnet). The basic design as well as printing process will be described and acoustic measurements reported. A range of printed designs, which can be interchanged among each-other will be presented, and future design ideas which take advantage of the printing process explored.

Design for additive manufacturing (DFAM) guidelines are important for helping designers avoid iterations and leverage the design freedoms afforded by additive manufacturing (AM). Comprehensive design guidelines should incorporate a variety of features of interest to designers, and given the wide variety of AM processes and their associated capabilities and limitations, those guidelines may need to be process- or even machine-specific. One way to generate detailed DFAM guidelines is to implement a metrology study focused on a strategically designed test part. This paper describes how quantitative design guidelines are compiled for a polymer selective laser sintering (SLS) process via a metrology study. As part of the metrology study, a test part is designed to focus specifically on geometric resolution and accuracy of the polymer SLS process. The test part is compact, allowing it to be easily inserted into existing SLS builds and therefore eliminating the need for dedicated metrology builds. To build a statistical foundation upon which design guidelines can be compiled, multiple copies of the test part are fabricated within existing commercial builds in a factorial study with materials, build orientations, and locations within the build chamber as control factors. Design guidelines are established by measuring and analyzing the as-built test parts. The guidelines are summarized in this paper and documented in a publicly accessible, online web tool.

A composite material system comprised of a monostable negative stiffness (NS) structure within a polymer matrix was designed, fabricated, and experimentally evaluated. The monostable negative stiffness (NS) structure was designed using a combination of analytical and numerical models and manufactured in stainless steel. The NS structure was arranged in parallel with different polymer matrices to experimentally evaluate the effects of the matrix properties on the overall stiffness and energy dissipation of the composite NS-matrix system when loaded in uniaxial compression. A strong influence of the matrix properties on the stiffness and energy absorption capacity of the composite system was observed. Unlike conventional composites for which there is a natural tradeoff between stiffness and energy absorption capacity, the composite NS-matrix system enhanced stiffness while simultaneously improving energy absorption relative to a neat matrix, but only when the stiffness of the matrix was carefully matched to the stiffness of the NS structure.

Additive manufacturing (AM) is a smart manufacturing technique that fabricates components directly from 3-D models by selectively joining materials. This technology is revolutionizing the way products are created by reducing turnaround times, increasing viable component complexity, and enabling economic low-volume and mass-customized production. As this technology gains traction as a means of producing end-use components, focus has been placed on improving quality and consistency to ensure components can perform and their traditionally manufactured counterparts. This chapter will focus on selective laser sintering (SLS), an industrial AM technique for producing polymer components. Design guidelines will be presented that, when followed, improve the probability of creating high-quality components that match the design intent. SLS failure modes and process control will also be discussed to provide an understanding of how to properly build components. Incorporating the design guidelines and understanding machine control methodologies allow users to create high-quality components consistently.

Additive manufacturing enables the fabrication of metallic architected materials for structural applications. However, variability in the manufacturing process can contribute to various types of defects, which lead to mechanical properties that are much worse than predicted. Architected materials comprised of many small struts further exacerbate the process variability. The geometry and mechanical properties of individual struts differ from the bulk material, with the extent of the disparity related to the size and orientation of the struts with respect to the build platform. Small features are also more susceptible to process defects including porosity and surface roughness. Postprocessing operations such as hot isostatic pressing seek to alleviate some of these defects with minor success. This manufacturing uncertainty complicates the prediction of lattice structure performance, sometimes requiring computationally expensive stochastic finite element models informed by CT scanning to obtain accurate results. In this chapter, the effective mechanical properties of individual lattice struts are evaluated experimentally with a high-throughput tensile testing procedure. Then, these effective mechanical properties are incorporated into finite element models of the entire lattice structure. Results indicate that this approach yields accurate predictions of lattice structure properties without requiring CT scanning or computationally expensive stochastic finite element analysis.

Additive manufacturing (AM) has many potential industrial applications because highly complex parts can be fabricated with little or no tooling cost. One barrier to widespread use of AM, however, is that many designers lack detailed information about the capabilities and limitations of each process. To compile statistical design guidelines, comprehensive, statistically meaningful metrology studies need to be performed on AM technologies. In this paper, a test part is designed to evaluate the accuracy and resolution of the polymer powder bed fusion (PBF) or selective laser sintering process for a wide variety of features. The unique construction of this test part allows it to maximize feature density while maintaining a small build volume. As a result, it can easily fit into most existing selective laser sintering builds, without requiring dedicated builds, thereby facilitating the repetitive fabrication necessary for building statistical databases of design allowables. By inserting the part into existing builds, it is also possible to monitor geometric accuracy and resolution on a build- and machine-specific basis in much the same way that tensile bars are inserted to monitor structural properties. This paper describes the test part and its features along with a brief description of the measurements performed on it and a representative sample of the types of geometric data derived from it.