How Selective Powder Deposition works

Overview

Selective Powder Deposition (SPD) is a new 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 usually get a porous object. The best metal 3D printer video helps visualizing it.

Variant 1 - SPD with infill

First, the 3D printer selectively deposits build and support powders into a crucible. When it finishes filling the whole crucible, you add some infill material, 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 chosen such that it's not wettable by the infill.

To prevent metal oxidation, the atmosphere in the crucible should be reducing. The easiest way to create a reducing atmosphere is to place some carbon (metallurgical coke) or hydro-carbon (plastic) in the crucible. But some metals require a controlled atmosphere furnace with hydrogen and argon.

After the infilling, if you cool down the crucible quickly - you'll get a composite material, because the atoms of the build powder and the infill didn't have enough time to mix. But if you hold it at high temperature for a while - the atoms of the build powder and the infill will diffuse and mix with each other, so you'll get a uniform alloy. The time required for diffusion depends on the metals and temperature. For example, at 790°C, copper+tin can mix uniformly in 2 hours and you'll get bronze. But copper and tungsten, even at 1090°C, won't alloy much even in a day, so you'll get a composite.

When it cools down, remove it from the kiln, and clean it with a wire brush or sand blaster.

Variant 2 - SPD with sintering

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 sintering causes shrinkage and warping. By changing the baking temperature, you can influence the amount of shrinkage, warping and porosity. With sintering it's hard to achieve 100% density with pure metals (those that have a single melting point), but it's easy with glass, and some metal alloys, though it requires a vacuum furnace.

Materials

In theory, SPD can be used to 3D 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. For the details see Which materials can be 3D printed with SPD

Advantages

Parts are 100% solid.

AFAIK, Selective Powder Deposition is the only method that can produce 100% solid metal 3D prints.

Laser printers can't do that, because they melt metal one pixel at a time, and it solidifies one pixel at a time. During the solidification metal shrinks and breaks away from the neighboring pixels, causing voids and warping.

No warping or internal stress

AFAIK, SPD with infill is the only metal 3D printing method that doesn't cause any warping or internal stress. This is because the printed object is warmed up and cooled down all together at the same time.

In contrast, laser printers use localized melting, which causes localized shrinkage, and thus warping. And the processes that rely on sintering always have some warping due to uneven shrinkage.

Versatile - can 3D print many different materials, even glass

SPD can be used to 3D print many different materials. AFAIK, SPD with sintering is the only way to 3D print glass.

Comparison of metal 3D printing methods (2022)

Cost Internal stress Shrinkage Porosity Strength Resolution Shape limitation Max size
Laser, SLS, SLM, EBM, DMLS high high1 none low-med med-high high med med
MIG welding med high1 none low-med med-high low med huge
FDM or Binder Jet + sintering in air low none high2,3 low-med2,3 low-med2,3 med med small
FDM or Binder Jet + sintering in argon high none high2,3 low-med2,3 med-high2,3 med med small
iro3d SPD with infill low none none none high med low large
iro3d SPD with sintering for ceramics and pure metals low none low2,4 high2,4 low2,4 med low large
iro3d SPD with sintering for glass and some alloys low none med-high2,5 none-high2,5 low-high2,5 med low large

1Localized heating and cooling cause internal stress, which can cause warping, so additional support structures are needed for most shapes.

2With sintering, shrinkage and porosity can be traded for one another - reducing the particle size or increasing the temperature or time - results in lower porosity and higher strength, but higher shrinkage. And vice versa.

3With FDM and Binder Jet sintering, the powder particle size must be relatively small, so the shrinkage is usually high, but porosity can be low and strength medium-high. Sintering in argon rather than air - increases the strength, but also increases the price.

4With SPD, the powder particle size must be relatively large, so when the powder is non-viscous the shrinkage is usually low, but porosity is high and strength is low.

5Glass (and some alloys) become viscous at high temperatures, so sintering them naturally results in low porosity. And when using a vacuum furnace - 0% porosity and full strength can be achieved. This is very useful for glass - it makes it transparent.

Products and specifications

iro3d logo

Model-C

3D Printing process: Selective Powder Deposition (SPD)
Build volume: 279 x 274 x 110 mm
Printer size: 690 x 530 x 610 mm
Shipping box size: 770 x 620 x 440 mm
Shipping weight: 24 kg
Power: 12V DC or 100-240V AC, 40W
Pourer diameters: 0.9 mm and 1.9 mm
Min layer height: 0.1 mm
Min width of a detail: one pourer diameter
Min height of a detail: one layer height
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iro3d logo

Model-G

3D Printing process: Selective Powder Deposition (SPD)
Build volume: 610 x 610 x 310 mm
Printer size: 1320 x 1010 x 1230 mm
Shipping box size: TBD
Shipping weight: TBD
Power: 24V DC or 100-240V AC, 60W
Pourer diameters: 0.9 mm, 1.9 mm and 3.9 mm
Min layer height: 0.1 mm
Min width of a detail: one pourer diameter
Min height of a detail: one layer height
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