Overview – LMJ is a process whereby metal feedstock is melted in a crucible. It is then ejected as a stream of small droplets of metal onto the build surface, progressively building up the shape of the desired part.

Pros - A cost competitive method of manufacture that has the potential to build parts quickly

Cons – Currently limited to Aluminium, equipment can have issues in reliably dispensing metal droplets over long prints. Inter layer fusion is problematic if the temperature differential between the droplet and the part is not kept constant. If only a heated build bed is used, as the part gets taller, the difference in temperature of the droplet and part becomes larger and larger until there is poor fusion between the droplet and part.

Assessment – Although the process is interesting, it is not mature enough to consider using.

Overview – A continuous welding process, be that by laser or arc. The feedstock is typically wire although powder can be used. The systems can be frame mounted or robot mounted to allow for complex shapes to be produced.

Pros – Using wire feedstock is very cost effective and DED systems can build up material quickly. A wide range of materials can be used allowing for process flexibility.

Cons – Equipment can command a high price, in excess of £1/4m. The thermal nature of the process means that temperature control within the part must be managed. There is a strong potential for part warping and internal stresses to build. It is debatable whether the typical, as printed, surface finish is suitable to be left visible in the end part. Parts require post machining once printed e.g. cutting from the build plate with a wire EDM, supports cut from the part and surfaces machined to tolerance. The cost of this post processing equipment and ongoing labour costs add up. The process is not very suitable to make small components with thin walls or internal features.

Assessment – When considering the cost of the equipment, the productivity of the machine and the cost of post processing to reach an acceptable surface finish, the business case for adoption of DED is currently challenging.

Overview – A method whereby a polymer filament doped with metal powder is used in a heated printer head to extrude beads of material layer by layer to build a part. The, as printed, parts are called green parts. These then need to be “de-bound”, which means to remove the polymer binder from the part. In this process, there is a significant proportion of polymer binder which must be removed by dissolving the polymer in a caustic bath. This creates what is known as a “brown part”. This then needs to be sintered in a furnace to get a solid metal.

Pros – the cost of equipment is lower than all other metal AM technologies. Consumer grade printers can be used to print the metal filament. Productivity can be enhanced by having more low-cost printers. There are a variety of furnaces available on the market to sinter the brown parts. Parts can be printed as “full density” or “partial density” meaning that a continuous outer skin can be maintained but with less infill material. This is useful for mass reduction without compromising the external form.

Cons – Parts need to be de-bound in caustic chemicals. This adds process time and uses hazardous chemicals. There is a limit to the section thickness that can realistically be de-bound, typically 10mm. Any thicker than this and it will take days to de-bind, slowing production. For example, parts need to be regularly weighed to see whether they have lost the required material before concluding the de-binding step. Parts can also be weakened and crack as a result. The metal filaments are quite costly impacting piece part affordability. In the sintering process, the high binder content can shorten the maintenance intervals for the furnace. There is also the potential for geometry to warp in sintering due to gravity acting on the metal when it is in its weakest state.

Assessment – The process limitations in maximum section thickness is a significant limitation for Rheinmetall UK’s use cases. The high material costs reduce the cost effectiveness of the process. The need for a chemical de-binding step adds time and therefore cost whilst also reducing process safety.

Overview – A sintered metal technique that uses polymer Selective Laser Sintering (SLS) machines to fuse a polymer coated metal powder feedstock. The SLS machine prints as normal and fuses the polymer together encapsulating the metal. After printing, the green part is then chemically de-bound to remove the polymer and then thermally sintered in a furnace to solidify the metal.

Pros – The cost of SLS machines has dropped significantly and this process enables existing machinery to be used for metal part production (subject to purchasing de-binding and sintering equipment or subcontracting the processes). A good selection of metal feedstocks are available since the polymer doesn’t interact with the metal.

Cons – Part wall thickness is limited due to the de-binding stage and part size is limited due to the mechanics of sintering. The time to de-bind can be lengthy. Parts with any sort of overhang will require supporting in sintering. This can either be done with modelling support structure into the original CAD and then cutting this away after sintering or printing a separate part setter and then hand assembling the setters and part on the sinter tray. The setter and part can be kept separate from one another by using a ceramic spray. Material porosity can be higher than processes like Direct Metal Laser Sintering (DMLS)

Assessment – As with FFF, the process limitations in maximum section thickness is a significant limitation for our use cases. Similarly, the need for chemical de-binding adds time and cost. The Additional labour times for assembling setters and parts would add to overall production costs and process variability.

Overview – Another sintered metal approach but this time a wax mould is 3D printed layer by layer. At the end of each layer, a metal paste is spread to fill the mould. The water in the paste is dried via a vacuum heating method so as not to melt the wax and then the process is repeated. Upon completion of printing, the wax is removed and the parts liberated. They then need to be thermally sintered to solidify the metal. The machines are designed as a carousel where each stage performs a specific function. Although this makes the machines larger, it greatly increases the productivity enabling multiple batches to be manufactured at the same time.

Pros – if parts can be tightly packed then the mold jet process can be very cost effective and highly productive. Feedstock prices are very competitive and can even be mixed on site. Used wax can be downcycled to reduce the impact of waste. Surface quality of parts should be exceptional with the process and there is an innovative in-process quality assurance function using image analysis and defect repair process which should add confidence to the quality of the final part.

Cons – The printer equipment is expensive, upwards of £300k for the smaller printer, over £1/2m for the larger (note a furnace is also required). It also requires a fair amount of factory floor space. The production time for each part is fairly slow taking about 6 minutes per layer. If the parts have an open form factor and cannot be nested then they can become more expensive to manufacture. As with Cold Metal Fusion, if parts have overhanging sections, a setter will be required in the furnace to avoid warping. These need to be printed separately and pieced together by hand before sintering.

Assessment – MoldJet technology is an interesting mass manufacture method for smaller components. It requires a significant upfront investment which makes adoption of the technology more challenging unless throughput can be achieved on day 1.