The goal of the Jewellery Industry Innovation Centre (JIIC) and its parent institution, the Birmingham School of Jewellery, UK, is to encourage its students to develop, design and produce computer aided design (CAD) examples of jewellery products to challenge, prove, and democratise the processes and materials required for the application of precious metal direct metal laser melting (DMLM) technology into the production facilities of small and medium-sized enterprises (SMEs) within the UK jewellery manufacturing sector.
After a piece of jewellery has been designed and before it can be manufactured using DMLM a small number of core activities need to be both clearly understood and to take place:
- Pre-processing (preparing pieces for manufacture). This essentially refers to all front-end software-related activities including the design process.
- Processing (manufacturing jewellery items using DMLM). This refers to the actual manufacturing process using DMLM technologies.
- Post-processing (manual and automated finishing and polishing processes). This refers to the post-DMLM manufacture processing stages up to the point where an article is ready for sale.
If a jewellery designer understands, even on a fairly superficial level, what is involved in each of these core activities then they will be able to better design jewellery that not only takes advantage of the geometric freedoms that DMLM offers but also can be suitably post-processed to an acceptable quality of finish.
Finishing techniques for DMLM printed jewellery
Mechanical, or mass finishing, techniques are often found to be the best for this stage because the DMLM of jewellery will probably prove to be most commercially effective when used to produce geometrically complex designs. These designs by their very nature will present many unique and product specific challenges when ready to be polished and finished. Mass finishing technologies are based on the correct application of media flow pressure and speed to the jewellery item to be polished.
Generally, the higher the pressure exerted by the media on the jewellery, and the faster the media flows across the jewellery parts, the faster the desired finishing results can be achieved. But this flow has to be either uniform or directed, depending where the polishing is required. Centrifugal disc finishing is an industrial mass finishing process adapted for the surface treatment of jewellery. The process is carried out in a cylindrical container, which is open at the top, while the bottom consists of a turntable-like disc separated from the container wall by a microscopically small gap. During operation, the work pieces and the grinding or polishing media in which they are immersed rotate at a high speed, creating a toroidal abrasive flow of the media; the relative difference in speed of the components and media produces the polishing effect.
The contact between the jewellery pieces and the medium generates a very intense finishing effect which is up to 20 times more efficient than can be achieved with conventional systems like vibratory finishers. A process refined by precious consortium partner Finishing Techniques Ltd is the ‘stream finishing’ process (sometimes referred to as immersion polishing). This is a fairly new concept to jewellery polishing and features short processing times because the medium is compressed against the wall of a large spinning bowl (centrifugal disc finishing) and the parts are held and rotated in this flow by use of a rotating fixture similar to an electric drill chuck on an extended shaft. Because the rotating head is fixed but with an adjustable angle of attack when immersed into the bowl, it can be easily automated and has shown excellent reliability and repeatability. The use of small, light media can produce an excellent finish, the finishing energy coming from the relative speed of both the jewellery part and the medium. A final polish by hand completes the process.
Frank Cooper is the Technical Manager at Jewellery Industry Innovation Centre, his knowledge and experience in additive manufacturing for the jewellery industry is unrivalled. This column is an adapted extract from a full research article entitled “Sintering and additive manufacturing: ‘‘additive manufacturing and the new paradigm for the jewellery manufacturer’’ from Springer’s ‘Progress in Additive Manufacturing’ journal.