In this post, we would like to take a tour of the existing 3D metal printing methods. The idea is to identify as much working solution as possible. Knowing their pros and cons will enable us to identify the best candidate technology for our system.
Let start with "metal like" printing. This method consist of using a mix of traditional 3D printing polymer which serves as a binder for metal particles. One example is the Alumide material offered by i.materialise. The idea is to have the look and sometimes the feel of a metallic object while keeping some plastic properties, like flexibility and the relatively easy and low cost process. Since the metals particles are not fused together the light will be reflected in many directions making the object will look like a dull metal piece. With enough metal, the object can get some properties like high density, thermal capacity and conductivity. This can gives the piece a slight cold and heavy metal feel. The printing process used can be Selective laser sintering, where the thermoplastic binder is melted by a laser but not the metal, Fused deposition modeling, where the melted binder and solid metal particles are extruded from a filament. Although no examples where found, Stereolithography might also be a potential process used, but if it is possible at all, the layers would probably have to be kept very thin due to the presence of metal particles reflecting and scattering the curing light.
Fused metal deposition
Like it's plastic counter part, fused metal deposition is the process of extruding molten metal from a hot nozzle. The source of solid metal can be from a spool of wire or a syringe of microbeads. This process usually use low melting temperature metals and eutectic alloys. Eutectics are mix of two metals or more that can have a lower melting point than the separated pure metals. Since most metals have a rather sharp solid <-> liquid transition, the control of the behavior of the deposited metal can be tricky and the metal should be cooled rapidly to stay in place. Compared to other process, this is rather simple because the printed part does not require further processing. However the choice of metals is limited and most low melting temperature alloys are rather soft metals.
InkJet metal binding
The inkjet process (see it here and here) starts with a bed of metal powder. Layer by layer, a liquid binder is printed, by an industrial inkjet head, on successive layers of powder. After completion, the solvent in the binder is evaporated by air drying or a low temperature cooking. The remains holds the metal particles together. The part, now in the so called green state, is still fragile, but strong enough to allow for the loose powder removal. The parts are then heated to a high temperature to be sintered or impregnated by a lower melting point alloy.
In the case of sintering, the metal powder is heated just below its melting point. The temperature is enough to allow for metal atoms migration on the surface of the particles. Because of the surface tension forces, this metal is more likely to migrate towards the contact point between the particles, fusing them together. Since the starting part is not fully dense, the resulting part remains porous and shrinking of the part can occur.
In the case of metal impregnation, the space between the particles is filled by a lower melting point metal. The capillary forces, very strong in small gaps, are responsible for the metal absorption. In everyday life, this is the equivalent of a sponge in a layer of water. The result is an almost void free part. Since the original particles are not melted, the part usually remains the same size throughout the process.
The basic process of printing is relatively simple, but the post printing process require time and precision.
Direct metal laser sintering (DMLS)
This process also starts with a metal powder bed. But this time, instead of binding the powder with a liquid, each layer of powder is directly molten by a laser beam onto the other layer. This result in a fully solid part, straight from the printer. Because the laser power can be adjusted and there is no other materials involved, many metals can be printed this way. However, high power laser doesn't come cheap and are very dangerous in household application.
Electron beam melting (EBM)
This process resemble DMLS, but this time an electron beam is used to melt the metal. Electron from an electron gun are given kinetic energy by passing in a strong electric field. Upon impact with the metal particles, this energy is transfered to those particles as heat. Because electron wouldn't go far in air, this process is made under vacuum. The vacuum also allows the melting of metals that would otherwise react with the oxygen in the air when melted.
Casting with 3D printed sand mold
This process is very similar to the InkJet metal binding, except that, this time, fine sand is used instead of metal and the printed part is the negative of the desired part. Once the sand mold is printed, the metal can be cast. This method allows the used of any castable metal. However some post processing is required to remove the sand and the channels used for casting and cooling shrinkage.
Casting with lost wax or sand mold made from 3D printed original parts
The idea is to simply print the desired part out of plastic or wax, make a sand mold out of it and cast it. However, the possible geometry with sand casting are limited to allow for the removal of the original part. Lost wax casting can be used instead, however, the burning of the plastic part can be a very dirty step. Although they can be very cheap, those methods require a lot of post printing processing and many equipment.
Here end our tour of the existing 3D metal printing technology. If you know of other or want to add some information, please share with us in the comments.