I finally had some time to get back to this:
Now we can talk about powder printers.
Powder based 3D printers are a mature technology that has been around for more than 30 years and are used for both metals and polymers. Let’s stick to polymers for now.
Powder printers are sold under different names and slightly different technology brands but at the core they are mostly the same. The generic name for powder printing is PBF (powder bed fusion) but you will also see: SLS, HSS, MJF, and SAF depending on the brand.
The PBF process has these steps:
1] Lower the Z stage into the heated powder chamber 1 layer. Then use a roller to pack a single layer of powder onto the Z stage.
2]. Selectively sinter the areas of the powder surface that are the sections of the desired parts. (There are typically many parts being built in the powder bed at the same time since this process is inefficient to run at 1 part at a time.)
Repeat steps 1 and 2 until all of the parts are completed resulting in a chamber filled with packed powder with the parts buried inside.
Then:
3] Slowly cool the powder chamber with the powder cake either in the printer or in an external cooling system.
4] Post process by extracting the parts from the powder cake and clean them up with compressed air and grit blasting. A well packed powder bed will be ~20% parts and the rest is unused powder.
The main difference between different PBF printers is how to selectively sinter only the part of the powder surface that is desired.
The traditional process known as SLS uses a high power laser to rapidly scan the powder surface where the part sintering is desired. The laser is fixed and the beam rapidly scans the desired areas for sintering using mirrors.
A newer alternative known as HSS uses a 2 step process.
Step 1: ink jet print an IR absorber on the areas of the powder surface to sinter.
Step 2: pass a strong IR lamp of over the entire bed.
The differential of IR absorption rates between the jetted and non jetted regions will sinter only the regions jetted with IR absorber. The IR absorbers can be opaque (black) or clear and the process needs to be well controlled to get the desired results. HP uses a “detailing agent” (mostly water) jetted on the powder adjacent to the IR absorbers which stops that powder from sintering. The HSS patents have been licensed by more than one company and is sold as MJF or SAF. The process is the same on both of these with minor differences in implementation.
An important difference between PBF and FDM is the materials that they use. The PBF process requires semi-crystalline polymers for reasons I’ll skip for now and almost all PBF printing is Nylon. Most of the advantages and limitations of PBF vs FDM are largely related to the materials that they use. These technologies are complementary and suitable for different purposes.
The pros of PBF apply to all of PBF printers :
- High resolution
- consistent strength in X,Y,Z
- Self supporting- no support structures needed
- High through-put
- No support structures
- Parts are usually printed by experienced professionals
Powder printed parts are high resolution, have Z strength (and strain) matched better to X and Y. The process has high through put as long as the powder bed is densely packed with parts. A well packed chamber is about 20% parts. The rest is unused powder.
These are expensive printers with expensive installation at more than $200,000 per printer. That is a benefit to us because these expensive installations are typically run by skilled users at service bureaus. The results tend to be more professional than just buying parts from a guy with a printer and some filament. They know the details about how and where to pack for best consistency and accuracy for different size parts.
The nylon powder can be neat or fiber filled. The fibers will increase stiffness and strength in x-Y at a sacrifice of some toughness. In general the powders come from only a few vendors and are much more consistent and known than internet FDM filament.
Since the parts are built in a packed powder keg no supports are needed and the only support removal needed is to remove the un-sintered powder. This is usually easy unless it is trapped inside the geometry with no way out.
The Cons also apply to all of the printers:
- Limited materials choice
- Limited color choices
- Shrink, distortion, and accuracy.
- Long time to part
- Part variation based on chamber placement
- Layer orientation may change in subsequent orders if not specified.
- Powder waste, reuse, and consistency
Due to the process requirements nearly all PBF parts are nylon or fiber filled nylon. There is some polypropylene (PP) available but the properties aren’t amazingly different. Even though limited materials is a drawback, these are great materials. (Hidden auto interior parts are molded out of PP and auto intake manifold parts are nearly all molded glass filled nylon and are happy under the hood. Note that the auto intake parts are carefully designed with ribs and reinforcements to work in this application Do Not feel comfortable printing FF forward parts unless you have measured the temperature and understand the loads the parts must endure AT PEAK TEMPERATURE!)
Laser based PBF parts are nearly all white and the ink jet based ones are mostly charcoal grey so no real color choices. If using the white parts there are dying systems to create colors but this is expensive and niche. Other than dying there aren’t really color options.
Because PBF prints an entire, full powder chamber we never print just one part. Combine this with cooling times plus post processing and the TTP (time to part) is usually 20 hours or more vs printing a single, small FDM part. Even though TTP can be high there is high thruput since many parts are printed at the same time. This makes PBF an attractive option for higher volumes of parts but when ordering from a service bureau it might take a few days to get your parts depending on when they can fit them into a build.
The biggest downside of PBF printing is part accuracy and distortion. The high shrink rates of the semi-crystallin materials and thermal gradients as the powder cake cools means that larger or flatter parts will distort. Dimensional accuracy also rapidly declines as parts get bigger. We typically think of PBF being good for parts about the size of your hand. Also, distortion can vary depending on the placement and orientation of the part in the powder cake. A skilled operator will pack parts carefully but there will be variations. Most parts are sliced while the printer is printing so there is also no guarantee that the parts will be printed in the same orientation unless you specify this with your vendor. FDM parts usually have a clear optimal orientation to minimize time or supports but that is not true with PBF parts.
While the powder itself tends to be quite consistent from major vendors there is the problem of powder reuse. A well packed printer will be 20% parts and 80% scrap powder. The question is how much of the scrap powder can be reused? There are various claims from various manufacturers but one should be skeptical. Most shops blend a mix of virgin and sifted, used powder in some ratio. Low cost vendors like Shapeways used nearly all used powder. Many aerospace companies spec all new or carefully controlled ratios. Most professional shops will do a decent balancing act but be careful. Ask for data if you can.
Summary
PBF is a great technology for manufacturing parts that are strong, have high detail, and cost effective. There are a few flavors of the technology (SLS, SAF, MJF, etc) but the advantages and disadvantages are largely the same. While the material choices are limited, the materials are capable. Due to shrink, distortion, cost, and consistency PBF technology is typically used for smaller parts. We typically think size of your hand or smaller but there are exceptions. Colors are limited but for many applications we dont care.
This technology is complementary with FDM which has other tradeoffs and neither one will displace the other.
There are some newer, lower cost PBF options on the market for around $50k (Formlabs). These are not quite up to the same standard as their more expensive cousins.
My apologies for the long post. This is a rich subject and I have done my best to summarize but that also means leaving out some details. Feel free to ask questions.
