The integration of industry-embedded polymer additive manufacturing (AM) assisted investment casting (IVC) presents a viable approach for light weighting parts while offering personalisation in the jewellery manufacturing industry. In specific cases, polymer-AM combined with IVC in silver 925 has demonstrated advantages in terms of fabrication cost, surface quality, and post-processing activities. While it is a relatively new technology compared to investment casting, selective laser melting (SLM) has greater potential to achieve 100% density, control mechanical properties, and push design boundaries, with the potential of reducing environmental impact. The key to successful manufacturing lies in selecting the appropriate tool for the process and designing the piece with that process's design rules in mind.
Objectives
The primary objective of this study was to create custom designed Triply Periodic Minimal Surface (TPMS) rings using precious metal alloys and assess their performance in comparison to traditional manufacturing methods. To achieve this, polymer-AM assisted IVC and SLM were compared for silver 925 builds. This research aimed to demonstrate the viability of TPMS structures for mass customisation and light weighting of precious metal jewellery production.
Implicit Modelling: A Paradigm Shift
Unlike traditional parametric modelling, implicit modelling employs simplified mathematical solutions, which facilitate real-time calculations. This makes it ideal for generative design workflows. One of its key advantages is the ability to replace solid structures with TPMS infills, reducing material consumption and creating lightweight yet robust designs. Three TPMS structures – Diamond, Gyroid, and Split-P unit cells – were employed to create aesthetically pleasing, customisable jewellery designs in nTop. These structures are smooth, interconnected, and exhibit uniform pores, making them well-suited for AM processes like SLM.
Polymer-AM Assisted IVC: Enhancing Durability
For the creation of prototypes, material jet printing was utilised. The investment casting process with a silica-based investment powder and overpressure casting machine proved successful in creating silver cast samples with excellent surface quality. TPMS structures, being naturally self-supporting with no sharp edges, facilitated good material flow for even the most complex geometries.
Silver 925 IVC samples (Courtesy of Gabbi Gucci, Weston Beamor)
SLM: A New Frontier in Precious Metal Manufacturing
SLM brought a paradigm shift to precious metal manufacturing. It operates by fusing gas-atomised powder layer by layer using a fibre laser. While this method offers unique advantages, such as reduced material wastage, it posed challenges in terms of surface roughness and support structure removal. Surface roughness increased significantly compared to traditional casting methods. This increase in roughness required additional post-processing steps to achieve the desired high lustre finish.
EOS M100 SLM Silver 925 ring with support structures (Courtesy of Jade Watts: Cooksongold)
Surface Analysis
A VHX7000 Optical Microscope was employed for surface analysis. For each composite image, a height map was applied, and flattened, to compensate for the curvature of each sample & Six-line positions are measured upon each surface to determine Ra & Rz values. For the IVC sample the Ra values ranged from 1.3μm -2.8μm.
Through SLM, the way the materials are produced changes from investment casting, with increased peaks & troughs. The material has localised fusion, and increased oxidation apparent. For this sample the Ra values ranged from 4.5 μm -12 μm.
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Laura Griffiths
From left: Optical composite images of cast surface and SLM surface (Courtesy of SPECIFIC)
Surface roughness in reflective materials can be attributed to poor absorption, requiring excessive power for fusion, which destabilises the melt pool. Optimising parameters such as speed, power, and laser wavelength can enhance part quality and reduce post-processing time. Eliminating porosity is more achievable with SLM's theoretical 100%- part-density. Factors like beam diameter, thermal conductivity of the material, shorter wavelength laser sources, and optimisation of build, powder bed, and laser parameters are crucial when evaluating system specifications for desired processes.
Conclusion
SLM technology has the potential to leverage three critical factors—design methodology, environmental considerations, and improved mechanical properties—to gain wider acceptance across industries. Optimising SLM parameters, laser wavelength, and material alloy can reduce surface roughness, making SLM-produced parts more comparable to those from investment casting. Innovations in the SLM business can be achieved through material optimisation and process improvements. High-value materials should aim to minimise waste from failed builds, reduce support structures, and efficiently process and recover materials throughout the build and post-processing phases. Investigating processes like electropolishing, dry air polishing, or Plasma electropolishing for future suitability is advisable. Optimising design for additive manufacturing (DfAM), build parameters, laser settings, and material selection can further reduce costs.
