Powder Bed Fusion and Binder Jetting are two of the most common classes of metal 3D Printing technology. Each one provides unique advantages and considerations as it relates to meeting the needs of different applications. Powder Bed Fusion has grabbed headlines in the industry for the longest, but binder jetting's emergence has grabbed its fair share of headlines as well. Could either process be the solution for one of your applications? Here we provide a quick tale of the tape, comparing these two popular metal 3D Printing process families.
Powder Bed Fusion
Powder Bed Fusion involves the use of a focused energy source - commonly an infrared laser or electron beam - to selectively melt layers of metal powder. This process results in highly dense parts that provide strengths typically surpassing cast parts (and occasionally forged parts).
As its name would suggest, Binder Jetting involves the targeted jetting of a binding agent to hold particles of powdered material together. This process takes place layer-by-layer to produces a "green part," basically a fragile matrix of metal held together with adhesive. This part can then be used for non-stress applications or more commonly undergoes post-processing steps, most notably sintering.
How accurate are these processes? How reliably do they deliver that accuracy? Or said in the most straightforward engineering terms, which tolerances can they hold?
As Low As .001"
Powder Bed Fusion technologies are very accurate, with some technologies capable of achieving tolerances as tight at .001" (25 microns). It should be noted that metal Powder Bed Fusion is generally expected to meet tolerances of .005" +/- .002" per inch, and certain powder bed technologies like Electron Beam Melting have global tolerances that are looser than that. Generally speaking, Powder Bed Fusion is more accurate than Full Sinter Binder Jetting.
Binder Jetting is a relatively accurate 3D Printing technology, but much of its accuracy depends on what level of post-processing you have in mind for your print. Since we are specifically looking at metal 3D Printing, then for the purposes of this discussion we are weighing Full Sinter Binder Jetting and Infiltrated Binder Jetting. Full sinter involves the creation of a green part, but then sintering that part in an oven once the shape is set — which results in roughly 20% shrinkage and creates a challenge in delivering tight tolerances. This is not as noticeable with certain types of geometries and smaller parts. Additionally, this can be improved through repeated manufacturing. Nonetheless, its default tolerances are higher. Infiltrated Binder Jetting doesn't experience the same degree of shrinkage because the metal matrix is filled with another lower-melting-temperature metal instead of allowing the matrix to sinter down on itself. Still, the nod generally goes to powder bed when we talk about accuracy.
How expensive are these processes? Both relative to each other and to more traditional metal manufacturing processes like CNC, Casting, or Metal Injection Molding?
Cost Effective for Intricate Designs
Powder Bed Fusion prints are generally pretty expensive. The underlying energy source - an expensive laser or electron beam - is quite expensive. The powder commonly required for these machines is also more refined than that required for binder jetting, also driving up cost. Generally speaking, Powder Bed Fusion becomes the most cost effective solution when a part has been designed with the process in mind. This means the designer has eliminated unnecessary mass and structured the part to be self-supporting throughout the build.
Lower Cost Inputs = Lower Cost Outputs
By jetting binding agent rather than accurately tracing a path, Binder Jetting can effectively arrive at a geometry much faster than Powder Bed Fusion. While this analogy oversimplifies things (especially for Electron Beam Melting), Binder Jetting is a paint brush to Powder Bed Fusion's pen or pencil. This approach gives Binder Jetting particular usefulness for chunkier parts that still have a good bit of complexity. Augmenting this reduction in machine time, Binder Jetting leans on lower cost solutions to create geometries. Glue and an oven are decidedly cheaper than a fiber laser or electron beam. Additionally, Binder Jetting doesn't require the same fine granulated powders that powder bed systems do — this means cost savings as well.
Which materials can each technology process? Is there a broader range of materials available to one process or the other? How do material options compare to traditional manufacturing technologies?
Selective Due to Economics
Nearly every metal can be used in Powder Bed Fusion systems. Certain powder bed systems are better at processing high temperature materials whereas others are more conducive to lower-melting-temperature materials. Generally driving this is the extent to which the powder bed chamber is heated during manufacturing. Lower temperature chambers can be more prone to cracking and internal stress when processing high temperature materials. More than capability is actually economics when it comes to commonly available materials for Powder Bed Fusion. Materials commonly associated with less critical applications (e.g., iron) are not as prevalent as higher value metals like Titanium, Aluminum, Stainless Steel, Copper, and Nickel Inconel superalloys (e.g., 625, 718).
Limited Due To Nascency
Because the first step of Binder Jetting's process simply involves binding particles of metal together with glue, it offers tremendous material flexibility, in theory.1 Materials such as sand, gypsum, metal, and plastic can be bound with the Binder Jetting process. With regards to the next step — sintering — some limitations emerge. The extreme temperatures required for melting titanium, for instance, are challenging within a sinter furnace. Stainless Steel is the most common material used in the Full Sinter Binder Jetting process, in part because its behavior with regards to shrinkage in the sintering step is perhaps best understood.
Will the process deliver me the density I need for repeated cycles and fatigue?
99.5% and Up
Powder Bed Fusion delivers dense parts. Straight off the machine, density is typically north of 99.5%. This is significantly above the density of cast parts, which typically run around 98% dense. This is one key consideration in how PBF parts can deliver better-than-cast material properties. Additionally, parts can undergo a Hot Isostatic Press (HIP) post-processing step to bring density within a hair of 100%.
The Binder Jetting process generally cannot deliver the same density as Powder Bed Fusion parts. Quite simply, while the sintering process can create density typically on par with cast parts, it does not achieve complete density. This level of density is appropriate for many applications. However, for certain tasks with high cycle time fatigue concerns, this may be a limiting factor. Again, Hot Isostatic Press (HIP) as a post-process step can be used to improve overall density.
Does one process provide unique advantages when it comes to available geometries? Do each operate with the same design constraints?
Master of Complex Lattices
When it comes to design, Powder Bed Fusion offers a great deal of design freedom. Relative to traditional manufacturing processes, powder bed is capable of processing extraordinarily complex designs just as fast - actually faster - than standard blocks of material. Especially with laser systems, very fine features can be achieved. We generally don't recommend features smaller than 1mm, although we commonly can resolve them. In general, this allows for extraordinarily complex lattice structures to eliminate weight while retaining strength. Notable to consider is that Powder Bed Fusion systems generally require the inclusion of supports, made from the same material as the part, to prevent the metal from warping under the rapid heating and cooling that occurs during the process. Certain powder bed platforms operate with a heated chamber which reduces the need for such supports, but that can create other design considerations. Either way, you should try to design your part to be self-supporting. Just imagine if your part was a skyscraper — would any aspect of it need scaffolding to be built? This is critical, because parts designed without supports in mind can drive the price of a part up more than 50%.
Support Needs Reduced, But Sintering Must Be Considered
The Binder Jetting process does not have the same support restrictions as powder bed systems because there isn't a thermal component to creating the green part. With that being said, the post-processing step can be problematic for fine features, as the green part shrinks in size by 15-20%. This can also come with the threat of internal stresses. As a result, binder jetting parts are generally limited to sizes smaller than a fist for cost-effective printing and sintering.
The Bottom Line
It's really a tie. Depending on the part you've designed, the quantities you seek, the material you desire, and the performance you require, either process might carry the day. The good news is that 3Diligent offers both of these 3D printing technologies (any many more!), and our experts can help you design to take advantage of either process.