In our previous article we covered considerations on the overall approach for designing for the process depending on the flexibility the designer has with the part. In this article, we’re honing in specifically on designing for BJP considerations by highlighting some of the critical process steps which form design constraints.
Binder Jet Processing (BJP) has been in the news a lot in 2020. BJP has tremendous potential, and we would like to spend some time describing the design space. You could say we are bound and determined to help people design for BJP. We often hear people comparing it to Powder Bed Fusion, because it does look similar from a printing perspective. It uses powder. It creates a bed. At this point however they differ; where the energy on PBF would melt the powders to densify them, BJP jets a binder, or glue, to precisely bind the powder particles together. They are still separate particles until much later when they get densified. When designing for BJP, Design for Additive Manufacturing (DfAM) and Modify for Additive Manufacturing (MfAM) are still critical and take on a meaning of their own. It is the P, or the processing, that is significant in BJP.
Part Printing Process
BJP can refer to a few different methods, but we’re primarily going to review single metal BJP as opposed to sand and infiltrated variants which have been around for years. In the single metal version, the P for processing is a series of important steps. These steps control how and if you get a dense part. As BJP is a “cold” process where the printing does not melt the particles, materials that are not conventionally weldable, or have residual stress issues or high melting temperatures can be considered. This process flexibility on the front end has to be balanced with the densification process on the back end. For example, aluminum is traditionally difficult to sinter, because it reacts with oxygen and forms a protective Al2O3 layer. This layer is insoluble in aluminum and will also act as a barrier to sintering. Other alloys like steel do not have such thermochemistry issues and are readily sinterable. For these alloys, the binder simply needs a path to get out. Egress path forms one of the most significant design constraints. In addition, the material being processed and the binder have to be compatible, otherwise they can react or leave behind contaminants.
Figure 1 outlines the basic processing steps involved in BJP. For brevity, we’ll emphasize the post print steps (in green) as the printing process is detailed here.
Curing of the binder is necessary to develop sufficient mechanical strength in the bound metal part for handling, in most of the BJP processes. Curing is typically done as a batch process wherein the entire build box is fit into an oven and heated to 150-260°C (300-500°F) for a couple of hours. The temperature allows the polymer in the binder to crosslink increasing its strength, and therefore the strength of the printed part. Figure 2 shows a Computed Tomography (CT) scan of a BJP sample after the curing step highlighting the powder particles (gray) and cured binder (black).
Often overlooked, the build box then needs to have the excess powder removed. Having done this a few times, it is an oddly soothing process to vacuum, sweep and brush away the excess, non-bound particles for re-use. It is a little like being Indiana Jones without the snakes – and you kind of know where your treasure is. The binder, if not strong, creates anxiety in this step, because the parts are quite weak and can be easily broken.
DfAM and MfAM consideration: Part detail resolution is an important MfAM consideration when placing the parts in the build layout. The DfAM considerations will perhaps dominate, but MfAM will dictate success or failure in survival of the part in this process.
There are several important subtopics to sintering: Setters and Trays, Furnace Environment and Control, De-Binding and Sintering. Table 1 highlights just a few of the topics and impacts.
Table 1: Sintering Steps and Impacts
|Setters and Trays
|Furnace Environment and Control
|Temperature and sintering process dependent
|Atmosphere, temperature ramp control
|Chemical interactions and thermal control
|Final de-bind and densification
|Slumping, part distortion or material interaction
|Excessive time in the furnace to achieve density or de-bind
|Trapped binder could be a defect
|Failure to achieve density requirements or excessive sintering time
|DfAM or MfAM
|DfAM and MfAM
|DfAM and MfAM
|DfAM and MfAM
Furnaces Environments and Control
Choice of atmosphere in the furnace and thermal control is closely related to the binder employed and the material being processed. There is no one single solution to sintering. The atmosphere in the furnace could be a reducing gas, flowing hydrogen, or a vacuum. Control of the furnace thermal profile is important to achieve good results given variable ramp up and hold schemes to “burn out” the binder.
DfAM and MfAM consideration: Part section thickness and transitions from thick to thin can drive failures and limit the ability for the volatile elements that result from binder breakdown to migrate to the surface and exit the part.
In addition to the temperature and environment, be mindful the part will be necessarily shrinking to densify. This shrinkage is influenced by gravity, so some tooling may be required to control slumping. It is also necessary to consider that great care was taken to put the binder in the right place and cured. Now, the binder needs to be removed as the particles transition from being held together by the binder, to being held together by powder particle to powder particle bonding.
Sintering is fairly simple in concept. Where two particles contact, atoms can begin to move from one to the other, and, with time, multiple particles coalesce in 3 stages: 1) Neck growth, 2) Intermediate sintering and 3) Final sintering. Table 2 shows the main considerations, impacts, and DfAM/MfAM influence.
Table 2: 3 Stages of Sintering
|Stage 1: Neck Growth
|Stage 2: Intermediate
|Stage 3: Final
|Packing density / disruption by the binder impact
|Packing density and part densification
|Gas egress from part
|Particle contact initiates sintering
|Higher density speeds sintering, part densification leads to shrinkage
|Decrease in part density
|DfAM or MfAM
|DfAM and MfAM
All sintering stages are important, but the packing density of powders in the printed part is important in Stage 3 as a higher density produces more rapid sintering. The powder bed density can be impacted by the binder impacting the surface. The bulk of densification occurs during this stage which drives the entire body to shrink up to 15-20% by volume. This shrinkage results in distortion of the part, because thick features shrink more than thin features. Abrupt changes in section thickness can result in surface cracks due to the differences in shrinkage. Large openings and overhangs can distort due to gravity during this stage of sintering, and the drag of the shrinking part on the tray can cause the bottom of the part to shrink less than the top. Watch this space for a future article on BJP and sintering.
DfAM and MfAM consideration: Sintering is mostly influenced by MfAM, but the basic design of the part is also important. The DfAM consideration will be driven by allowing binder egress, whereas MfAM will be critical on how to enable the part to survive not only the printing process, but mainly the cure through sintering process.
When designing for any process, you have to appreciate all of the processes involved. We highlighted some of the critical process and design driven constraints, because we are down with BJP. It is not as simple as “it’s 100X faster than PBF”. PBF and BJP operate in different design spaces and have their own constraints. Being aware of these constraints up front can help you decide if your part requirements can be met and if BJP is the right process. Perhaps BJP will enable you to succeed where other processes did not. The choice of material, the section size, and the furnace conditions are all important considerations. Process economics is also a critical consideration, because, as we detailed before, printing speed may be irrelevant in the speed of the supply chain and ultimately, the cost of the part. If you cannot meet your business requirements, you may be singing the wrong tune.
Feature image courtesy of ExOne.
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