Headhsot of a female research student wearing full PPE working in a engineering laboratory

Molten Metal Jetting (MMJ)

Molten Metal Jetting is an emerging metal additive manufacturing process consisting of dispensing and depositing molten metal droplets onto a moving substrate allowing for the building of 3D components.

At the Centre for Additive Manufacturing, we focus on advancing the process, by incorporating automated frameworks and optimising hardware; understanding the fundamental aspects of the process, including the interaction between similar and dissimilar materials; and identifying applications and printing functional parts.

Key contacts

Other AM processes

Our Molten Metal Jetting facility called MetalJet, is a unique system capable of processing high temperature metals up to 2000degC at frequencies of 2kHz, providing the capability to work with a wide range of industrially relevant metals at a fast production rate. These attributes enable us to produce intricate, multi-material, and multi-functional components with high precision.

During the Molten Metal Jetting process, the metal is melted inside the crucible and molten droplets are ejected on-demand from an orifice using a magnetohydrodynamic actuator. The actuation is achieved through the use of permanent magnets in conjunction with tailored electrical pulses, creating a Lorentz force that provides the momentum to push the molten metal from the nozzle in a controlled manner.

The individual ejected droplets are deposited onto a heated substrate that moves beneath the printhead, spreading and solidifying before the next one is deposited, enabling the construction of intricate 3D structures drop-by-drop.

Processes advancements

Amongst recent processes advancements to our MetalJet platform, we have been working on integrating sensing methodologies and developing algorithms for data-driven optimisation. Advancements in the system capabilities include the ability of producing multimaterial parts via deposition of dissimilar materials.

Developing the world’s first multi-metaljetting platform

Our in-house multi-MetalJet platform is the first drop-on-demand multi-metaljetting technique that enables printing functional components from at least two different metals in one printing operation. This is accomplished by coordinating the deposition of dissimilar metallic materials on a substrate such that components with spatially varied compositions can be directly created within the build process. This provides a unique opportunity to 3D print components with diverse properties throughout, while eliminating the need for joining procedures.

The voxel-by-voxel deposition of various metallic microdroplets offers unmatched control for fine-tuning the local properties of 3D parts and sets this technology apart from the existing multi-metal AM techniques. Our expertise in process operation and our understanding of the underlying physics of the process have been transferred from the single-head system to the multi-head system, accelerating our progress. The system is currently fully operational, enabling our current research on the interaction between droplets of various materials at a microscale.

Dr Negar Gilani (front left) and PhD candidate (back right) working on the MetalJet facilities at the Centre for Additive Manufacturing Dr Negar Gilani and PhD candidate carrying out research in the MetalJet facilities at the Centre for Additive Manufacturing

Dr Negar Gilani and PhD candidate carrying out research in the MetalJet facilities at the Centre for Additive Manufacturing

Enabling printing a range of materials

We have achieved significant advancements in the printability of both low- and high-melting-point metals. Tin and indium find widespread usage in electronic applications, with their low melting temperatures (232°C and 157°C) making them suitable for printing on polymer and flexible substrates.

The resulting components demonstrate high structural integrity and electrical conductivity, comparable to the bulk material, thanks to the robust metallurgical inter-droplet bonding. Furthermore, copper and silver (1084°C and 962°C) are distinguished by their unparalleled electrical and thermal conductivities, placing them at the forefront of materials used in electronics and a wide array of other applications.

The strategic selection of both low- and high-temperature metals was instrumental in enabling multimaterial printing. Key tasks included waveform optimisation, ejection temperature adjustments, and compatibility verification with the cartridges.

Indium image here

Boris spider 3D printed in Tin on a copper substrate

Silver overhanging pillars 3D printed on a copper substrate Silver overhang pillars 3D printed on a copper substrate using the Molten Metal Jetting system at the Centre for Additive Manufacturing

Silver overhang pillars 3D printed on a copper substrate using the Molten Metal Jetting system at the Centre for Additive Manufacturing

Copper micro-walls of varying thickness 3D printed on a copper substrate Micro-walls of copper with varying thickness 3D printed on a copper substrate using the Molten Metal Jetting system at the Centre for Additive Manufacturing

Micro-walls of copper with varying thickness 3D printed on a copper substrate using the Molten Metal Jetting system at the Centre for Additive Manufacturing

Understanding the underlying physics of the process

Molten Metal Jetting is a multiphysics process. Our research focusses on understanding the underlying physics of each of step of this process. These steps are grouped in three main stages: droplet formation, droplet in-flight and droplet-susbtrate formation.

Although all steps are part of the same process, to be able to understand the underlying physics, each of them have been researched separately, allowing us to achieve a better understanding of the overarching behaviour.

Schematic of Molten Metal Jetting multiphysics process.

Metaljetting multiphysics process

Droplet formation

Our research has been focussing on understanding the underlying physics of the Molten Metal Jetting process through an experimental approach combined with analytical analysis, which has allowed us to gain insights into the mechanism of microdroplet formation. This understanding facilitated the optimisation of the actuation waveform, enabling the printing of several materials with improved precision.

Droplet in-flight

Computational and analytical modelling of molten drop cooling during flight, validated by experiments, facilitated the development of an effective printing strategy to maintain drops in their liquid state upon impact. Additionally, these models predicted the droplet temperature upon deposition onto the substrate, serving as an input for subsequent models.

Droplet-substrate interaction

Numerical and computational models have been developed to precisely simulate the spreading and solidification of droplets upon impact on the substrate, ultimately enabling the prediction of the final morphology of droplets. This information facilitates the optimisation of printing parameters to achieve fully dense structures.

Morphology of individual molten Tin droplets at 670 degC deposited onto a) copper b) tin and c) zinc substrates at 30 degC.

Morphology of individual molten Tin droplets at 670 degC deposited onto copper, tin and c) zinc substrates at 30 degC as part of Molten metaljetting research at the Centre for Additive Manufa

The computational models developed to date have been instrumental in elucidating the critical elements necessary to achieve sufficient bonding at interfaces, as well as identifying the various modes of bonding. This information has been used to optimise process parameters, thereby facilitating the manufacturing of 3D structures with high structural integrity.

Advanced characterisation techniques, combined with computational analysis, shed light on the microstructure of MetalJet manufactured components, revealing the influence of thermal conditions on developing diverse microstructures. This provides exciting avenues for tailored microstructure design using this technology.

Through computational analysis, validated by experiments, the physical mechanism of residual stress generation during the process was revealed. Armed with this understanding, efforts were made to identify solutions to rectify these issues through virtual experiments, hence providing a basis for a more targeted experimental investigation.

Cross-section displaying inter-droplet and droplet-substrate interfaces for copper droplets and alumina substrate.

Interface of an individual copper droplet with alumina substrate captured by peeling and mounting droplets on a C tape.

Interface of an individual copper droplet with aluminium nitrate substrate captured by peeling and mounting droplets on a C tape.

Interface of an individual copper droplet with copper substrate captured by peeling and mounting droplets on a C tape.

Printing functional components

We investigated the feasibility of directly printing molten metal droplets using the MetalJet platform for the production of customised 2D and 3D functional components.

MetalJet presents a distinct advantage over other technologies for creating such functional objects due to its direct fabrication process routes of high-temperature metallic materials, eliminating the need for pre- and post-processing. This contrasts with systems using inks loaded with metal nanoparticles, which require post-processing steps to evaporate the ink, adversely affecting performance.

3D electronic circuit printed on the MetalJet system in microscale precision.

3D electronic circuit printed with microscale precision using a bespoke Molten Metal Jetting platform, MetalJet as part of research at the Centre for Additive Manufacturing

Bridge structures 3D printed in Tin on copper substrate using the MetalJet system.

Bridge structures 3D printed in Tin on copper substrate using the Molten Metal Jetting system at the Centre for Additive Manufacturing

Antenna printed with microscale precision on the MetalJet system using Tin on a flexible substrate. Antenna 3D printed using Tin on a flexible substrate with microscale precision using a bespoke molten metal jetting platform MetalJet as part of research at the Centre for Additive Manufacturing

Antenna 3D printed using Tin on a flexible substrate with microscale precision using a bespoke molten metal jetting platform MetalJet as part of research at the Centre for Additive Manufacturing

Crucially, MetalJet surpasses the limitations of other technologies in 2D structures, enabling the creation of 3D structures and expanding the spectrum of design options and their efficiency. As a proof of concept, we conceptualised, fabricated, and evaluated a MetalJet-printed antenna. We have demonstrated that with the superior properties obtained, the technology offers significant opportunities in the realm of electronics and beyond.