Selective Powder Deposition is a 3D printing process, which you can use to print metal, glass, ceramic, and composite materials.
There are 2 main variants: infilling and sintering.
With infilling you get a 100% solid object, and with sintering you get a porous object.
The process is shown in detail in the
best metal 3D printer video,
but if you prefer to read text, here it is:
First, the printer selectively deposits build and support powders into a crucible. When it finishes filling the whole crucible, you add some infill material, and to create a reducing atmosphere you add some coke, close the lid, and bake it in a kiln. The temperature in the kiln should be above the melting temperature of the infill, but below the melting temperature of the powders. When the infill melts, it soaks the build powder. But it doesn't soak the support powder, because the support powder is not wettable by the infill. When it cools down, remove it from the kiln, and clean it with a wire brush.
The 2nd variant, sintering - is even simpler - you just don't add any infill, and bake it at the temperature sufficient to sinter the build powder. Note that when you use sintering, the object will shrink and get distorted. By changing the baking temperature, you can influence the amount of shrinkage, distortion and porosity, but it will never be 100% solid. This variant is useful if you want to obtain a porous object, to make a filter, for example. But if you want a 100% solid object, with no shrinkage, no distortion, and no porosity - use the 1st variant - the infill.
In theory, SPD can be used to print almost any material. Actually, the printer itself is material agnostic, and can pour any powder that flows through a small hole. But when it comes to baking, some materials are easier than others.
3D Printing process: Selective Powder Deposition (SPD)
Build volume of Model C: about 279x274x110 mm
Build volume of Model G: about 610x610x310 mm
Pourer diameter: 0.9 mm and 1.9 mm
Layer height: 0.1 to 1 mm (user configurable in GUI)
Min width of a detail: one pourer diameter
Min height of a detail: one layer height
You can use SPD to 3D print metals, glass, ceramics, and metal-ceramic composites. The printer itself is material agnostic, and can pour any powder that flows through a small hole. There 2 main methods: infilling and sintering.
1. Infilling produces a solid object, and can work with any material combinations where the infill material has lower melting temperature than the powder.
2. Sintering produces a porous object. Also, the object shrinks when it's sintered. The shrinkage and porosity depends on the powder material, powder size and the baking temperature. This method can be used with a powder of pretty much any material, or a mixture of different materials.
Different materials require different baking temperatures and atmospheres. For example, glass can be baked in an oxidizing atmosphere, such as air. Metals require a reducing atmosphere to prevent oxidation. Most commonly used reducing atmospheres are hydrogen and carbon monoxide. Hydrogen is supplied from a bottle or a generator. Carbon monoxide is produced by placing a piece of coke into the crucible.
Non-reactive metals, such as iron, copper, nickel, tin, lead, bismuth, antimony, molybdenum, cobalt, tungsten, palladium, cadmium, silver, gold, and platinum are easy to reduce with carbon monoxide or hydrogen. Reactive metals, such as chromium, are difficult to reduce and require a very dry hydrogen. To see which metals can be reduced by which atmosphere and at which temperature, look at the Ellingham diagram or interactive Ellingham diagram.
For glass and glass-ceramic composites - a recycled glass can be used, which is very cheap.
When using the infill method - there is no shrinkage because the build powder preserves its shape and size. There is a tiny distortion due to uneven thermal expansion of different powders. But overall, the size and shape are well preserved.
When using the sintering - there is shrinkage and consequently shape distortion. Shrinkage and porosity can be controlled by choosing baking temperature and powder size. This method is useful for printing metal foam and porous glass.
SPD itself doesn't reduce the strength in any way. The microstructure of the printed parts is similar to the cast ones. The strength of a printed object is determined by its material composition, baking atmosphere, and cooling rate.
Printing time depends on many factors. Most significant one is the surface area of the object. Most of the prints that we have done so far took between 2 and 24 hours.
That depends. For high precision applications you would need to machine the print to obtain the desired tolerances. For some other applications the prints might be used as is.
For the infill method, the baking temperature must be above the melting temperature of the infill, but below the melting temperature of the build powder.
For the sintering method, the baking temperature is chosen to balance the desired porosity and the desired strength.
The hold time should be sufficient for the heat to get to the middle of the crucible and melt the infill or sinter the powder. For a small crucible 2 hours is usually enough.
Kiln or furnace type depends on the material you want to print. It should be able to provide the required temperature and atmosphere. For example, to melt copper infill metal, your kiln should be able to go above the melting temperature of copper, which is 1084°C, so most pottery kilns would work. A kiln with a programmable digital controller is preferred. A new pottery kiln might cost you about $1000. A used one you might find for a few hundred dollars on Craig's List, if you look for a while. Hydrogen furnaces are more expensive, but also more versatile.
You can buy the consumables from 3rd parties. In general, for a powder to flow well, its particles should be spherical, and have size at least 5-10 times smaller than the hole. So, for example, for a 2 mm hole, the particles should be smaller than 200 or 400 microns. And for a 1 mm hole, the particles should be smaller than 100 or 150 microns, depending on the material and humidity. Powders smaller than 40 microns are not recommended, because they might cake and clog the pourer, especially for hygroscopic powders at high humidity. Also, powders smaller than 20 microns might get airborne, which isn't healthy. So, the ideal particle size is about 100 microns for the 0.9 mm hole, and 200 microns for the 1.9 mm hole. The powders and the infill metal shouldn't have too many impurities. For example, reactive metals, especially in the powder, might oxidize and prevent infilling. Crucibles for baking with coke should be non-porous and have a tight lid. Crucibles for baking with a hydrogen-argon atmosphere can be porous and don't need any lid.
Most of the things you can buy from atomwell.com
Infill metals you can buy from MetalShipper, RotoMetals, and eBay.
Stainless steel crucibles, sand, and iron powder from TriDPrinting.com.