In tomographic reconstruction, the mathematics of generating a 3 dimensional object by illuminating a vat from many directions with 2 dimensional patterns, in order to be perfect, require access to negative light intensities¹. While negative intensity is, of course, not possible, we emulate negative intensity by using a photo initiator system where one component that is inert in the ground state is excited by one wavelength of the incoming light to an exited state that functions as a polymerization inhibitor. We call this use pseudo-negative illumination². In principle, this idea can be extended to more than two wavelengths but the options are limited by the finite width of the absorption that lead to excitation. Furthermore, the multi wavelength principle can also be used to generate spacial property contrast in the printed objects³ ⁴.

We give examples of such a photo initiator/photo inhibitor system. We show how this concept allows us to approach speed, precision and design freedom at the same time. Here we combine polyspectral tomographic reconstruction with pseudo-negative illumination to resolve the trilemma. We used the same units on a digital micro mirror device to modulate two colors of illumination simultaneously to control the chemical stability of the binary photo inhibition. The methodology enables a typical 4K projector to 3D-print up to 8.9 billion voxels in a Ø30×50 mm vial at two-digit-µm-resolution in minutes, with prolonged process window and geometrically high fidelity greyscale printing.

Volumetric 3D printing is revolutionizing the manufacturing landscape by enabling the creation of complex, functional structures within seconds, with resolutions as fine as 20 µm. Unlike traditional layer-by-layer techniques, this approach builds entire three-dimensional objects in one step, offering unparalleled design freedom and drastically reducing production times. Recent advancements, as demonstrated in our publications in Advanced Materials (2023)[1], Materials Horizons (2024)[2], and Progress in Polymer Science (2023)[3], underscore its transformative potential in applications ranging from patient-specific implants to precision-engineered devices.

A critical challenge in volumetric 3D printing lies in managing light interaction with the entire volume of the photoresist. To ensure precise fabrication, the photoresist must withstand significant irradiation without undesired crosslinking in non-target regions. Radical inhibition is crucial in this context, particularly for thiol-ene systems, which inherently resist oxygen inhibition. TEMPO, a well-known radical inhibitor, has shown promise in achieving this balance, yet its exact role in volumetric 3D printing remains insufficiently understood.

To address this gap, a comprehensive framework is presented here to study the role of the widely reported radical inhibitor TEMPO in the volumetric 3D printability of a thiol-ene photocrosslinkable photoresist composed of triallyl isocyanurate and pentaerythritol tetrakis(3-mercaptopropionate).[4] Through photorheological measurements, kinetic modeling, FTIR spectroscopy, and validation via volumetric 3D printing, the relationship between inhibitor concentrations and volumetric 3D printing is elucidated. This work provides a robust methodology for predicting optimal printing conditions of photoresists for tomographic volumetric 3D printing, obviating the need for extensive trial-and-error.

Selective Laser Sintering (SLS), also known as powder bed fusion, is a 3D printing technique that uses laser irradiation to fuse layers of thermoplastic or thermoplastic-elastomer powders in a heated chamber [1]. Despite its potential for producing high-quality end-use parts, the adoption of SLS has been slow. Polyamide 12 (PA 12) dominates ~80 % of the SLS market, and the availability of alternatives, especially high melting thermoplastics (Tm of 220-260 °C), is limited. To promote the use of new materials in SLS, a reliable and efficient procedure is needed [2, 3].

This presentation introduces the key material requirements for SLS, focusing on melt-processability and powder flow. It covers both bulk (melting/crystallization behavior, melt rheology) and powder properties (particle shape and flowability). Additionally, a new high melting point SLS formulation based on aliphatic polyketones is presented as a case study for the optimization of thermal and powder characteristics (size, shape, flow) in relation to SLS processing conditions [4]. A benchtop CO2 laser printer is used to evaluate the process window, with a focus on limited material usage (~50 g per layer). The microstructure, tensile properties, and fatigue behavior of printed parts are compared with commercial references: PA 12 (Tm ~ 185 °C) and PA 6 (Tm ~ 220 °C). This work provides guidance for the screening and development of new materials for SLS.

3D printing is increasingly accessible, but prints often fail between layers. Improving the inter-layer strength is therefore crucial, yet current characterization methods primarily revolve around the printing of tensile bars and subsequently relate the tensile properties with adhesion [1]. An issue is the averaging of the adhesive strength and the introduction of printing defects. T-peel tests are used [2,3] but mimic standardized tests for tapes. A distinctive approach to exclude voids is to print two filament strands side-by-side using fused deposition modeling to create a single interface (see Figure 1), which can be peeled apart. The sample is pulled simultaneously in opposite directions, thereby keeping the crack-opening point relatively stable, and allowing the usage of X-ray scattering techniques. Isotactic polypropylene (iPP) is used to study the effect of processing parameters and the influence of molecular weight. Higher molecular weight iPP results in more shear at the interphase when printing. As a result, iPP forms β-phase instead of α-phase, see Figure 1. β-iPP is more ductile than α-iPP [4], resulting in a fracture energy that is four times higher. While lower molecular weight materials are favored generally for diffusion, higher molecular weight iPP improves interfacial mechanical properties. Furthermore, increasing the printing speed amplifies the shear, resulting in relatively more β-phase at the interphase, and thus a higher fracture energy. The characterization method has been shown to compare materials under different printing conditions relatively and can be combined with microscopy and spectroscopy techniques to investigate the link between microstructure and processing parameters.

Multi-material 3D printing is the key to fabricating innovative components across diverse fields, including biomedical, automotive/aerospace, and electronics.1 Especially electronics can benefit since it can support recycling by producing easily dismantlable compounds, highlighting its potential for sustainable manufacturing. Therefore, the "Design for Disassembly" concept has already been employed in the creation of such components through multi-material 3D printing in vat photopolymerization.2 However, when two dissimilar materials are used, the adhesion of the interface layers is crucial to the (thermo)mechanical properties of the component.2 The binding properties at the interface of the materials, whether created via covalent bonds or entanglements, are determined by the functional groups present in their respective formulations. The entanglements can result from an interpenetrating polymer network (IPN) state, which is formed through the intermingling of the two formulations during the printing process. Covalent bonds can only form if both formulations contain functional groups that can polymerise with each other. Here we introduce a (meth)acrylate-system as material A, an allyl-thiol system as material B, and an acrylate-thiol system as material C, with the objective of examining the interface of 3D-printed components on a microscopic scale. The findings of the (thermo)mechanical tests of the multi-material compounds are compared directly to those of the single-material prints. The process under discussion facilitates the comparison of theoretical reachable and measured properties. Moreover, the intermingling of the two materials at the interface was tracked via SEM imaging and EDX measurements, with two commercially available photoinitiators being investigated for this purpose.

Polyetherketoneketone (PEKK) is a high-performance thermoplastic with potential for lightweight, energy-efficient thermal management applications in electric motor housings and avionics power electronics. However, its low intrinsic thermal conductivity (~0.2 W/mK) limits its effectiveness in heat dissipation. This study explores additive manufacturing and boron nitride (BN) dispersion to enhance thermal conductivity using BN surface modification which helped to improve the compatibility between BN and PEKK. PEKK + unsized BN (70:30 wt.%) composite samples were prepared using a micro-compounder and subsequently processed via pellet-based 3D printing at 380 °C. The incorporation of BN resulted in a ~300% improvement in thermal conductivity, reaching approximately 0.8 W/mK. However, scanning electron microscopy (SEM) revealed BN agglomeration and filament porosity particularly in between printed layers, which hinder further improvements. To address this, BN was surface-functionalized to reduce agglomeration and enhance compatibility with PEKK, leading to greater thermal stability and conductivity compared to pure PEKK and unsized BN composites. Rheological studies optimized printing parameters via minimising interlayer porosity caused by differential cooling rates between PEKK and its composites. Micro-computed tomography (µ-CT) confirmed reduced porosity and improved BN distribution. The combined effects of BN functionalization and process optimization resulted in a more homogeneous micro-structure and improved thermal conductivity. These findings highlight the critical role of filler-matrix compatibility in tailoring processing and thermal properties. This study demonstrates that PEKK-BN composites can serve as viable lightweight alternatives to metals in aerospace and electronic applications, contributing to enhanced energy efficiency and as a result more sustainable material use.

Metamaterials are engineered periodic, sometimes aperiodic, structures that enable the manipulation of acoustic and elastic waves at subwavelength scales. A key feature being the creation of frequency bandgaps, ranges of frequencies, in which wave propagation is either severely attenuated or entirely inhibited [1]. Polymer metamaterials, in particular, offer enhanced control over wave propagation, but the influence of viscous losses in them remains inadequately studied, especially for those produced via additive manufacturing [2,3]. This study presents a characterization approach for the viscoelastic properties of 3D-printed polymer metamaterials and evaluates their effects on wave dynamics [4]. Dynamic mechanical thermal analysis (DMTA) and tensile testing are used to characterize acrylonitrile butadiene styrene (ABS), a polymer commonly used in fused deposition modeling (FDM). The frequency dependence of storage and loss moduli is incorporated into mechanical models to simulate the dynamic response of viscoelastic metamaterials. Numerical transmission data obtained from finite element modeling are experimentally validated using non-contact optical laser vibrometry. The experimental results reveal bandgaps at approximately 19–21 kHz and 40–44 kHz, which align with predictions from a viscoelastic numerical model while purely elastic model predicts lower frequencies. The findings demonstrate the importance of accurately accounting for viscoelastic effects in the design of polymer metamaterials for ultrasonic applications, providing essential insights for future work in optimizing the dynamic response of 3D-printed metamaterials.