The Importance of Powder Flow in 3D Printing

In this interview, AZoM speaks to Andrew Klein the Director of R&D at the ExOne Company about the importance of powder flow in 3D printing.

Could you give a brief introduction to ExOne and the AM technology it provides?

Founded in 2005, as a spin off from Extrude Hone Corporation, the ExOne Company (ExOne) is a global leader in binder jetting technology. We provide 3D printers, printed products and related services to help companies exploit and integrate 3D printing in their existing manufacturing operations. In 1996, we obtained the license for the 3DP (three-dimensional printing) process developed at MIT (Massachusetts Institute of Technology) for metal and sand parts. Since then we have developed and commercialised systems and materials to optimally meet applications in the automotive, aerospace, heavy industry and energy sectors. Printing with sand, ceramic and metal powders are our core areas of expertise.

Image Credits: ExOne

Can you explain how binder jetting technology works, highlighting applications for which it is particularly useful?

In binder jetting, a liquid binder is used to selectively join powder particles, layer by layer. The process begins with the spreading of a thin layer of powder, the printhead then strategically deposits droplets of binder into the powder bed - just like inkjet printing on paper - the job box subsequently lowers, and another layer of powder is spread. Progressive layering of the powder and binder builds a complete part, with unused powder (around 95%) recycled.

Binder jetting can be used to print a variety of materials including many different metals, sand and ceramics. Sand casts require no further processing, but metal structures are typically cured and sintered, as in metal injection moulding (MIM). Infiltration with a complementary material and/or hot isotactic pressing may also be used to deliver a finished component with the required properties.

Like other AM techniques, binder jetting is particularly cost-effective for lower production volumes. For example, to produce a small, complex component at a rate of around 1,000,000 parts per year, MIM would typically be the least expensive option, but for lower volumes, in the region of 200,000 parts per year or less, the cost of the mould will swing the advantage over to binder jetting. However, as with any part, the exact production volumes will be application dependent.

In terms of unique benefits, binder jetting does not use heat to melt or weld the particles during construction, avoiding the build-up of residual stresses and any subsequent requirement to release them. Furthermore, the developing part is supported by loose powder, negating any requirement to remove the parts from a build plate. Spreading speeds typically outperform those for alternative AM processes, making overall build speed highly competitive. These benefits coupled with the flexibility to print very large objects makes binder jetting the optimal choice for many AM applications.

What types of metal powders do you work with and what properties define their performance?

Historically, 3D printing was used for ‘form and fit’, primarily for prototyping. With the transition into industrial production use, the metal powders used have become subject to much greater scrutiny. We have qualified materials for use in a variety of industries, with our initial focus being stainless steels, and we continue to develop more. Other metals we routinely work with are tool steel, copper, Inconel and tungsten.

Particle size distribution is an important metric for AM metal powders, particularly with respect to sintering performance. Moving to finer powders reduces the temperature and time required to reach a certain sintering density, decreasing the chances of slumping and loss of dimensional integrity. Particle morphology, as quantified by SEM, is also critical as is binder compatibility which we assess by adding a known volume of binder to a powder sample, observing how long it takes to sink in, and then testing curing performance.

However, with these properties alone, it is not possible for us to determine whether a powder will print well. This is crucial because as well as defining robust specifications for qualified materials, we also routinely face the challenge of working with a customer on a new material. We need to be able to answer the question “Will I be able to print with this powder?” Including powder testing alongside our other characterization techniques allows us to do so with confidence.

What powder testing technology do you use and why?

We use an FT4 Powder Rheometer from Freeman Technology which we’ve had for a couple of years now and we use it to assess every powder used. A unique attraction of the FT4 is that it measures dynamic, shear and bulk powder properties, and in fact, metrics of all three types form our routine screen.

In terms of dynamic parameters, we measure basic flowability energy (BFE), stability and flow rate index (FRI); these are all generated by a single testing protocol. BFE is the flow energy measured as the impeller of the instrument travels down through the powder sample. Stability testing involves repeat measurements of BFE under identical conditions – so it provides an indication of how robust the powder is – while FRI is determined by measuring BFE at different impeller speeds. We measure cohesion and wall friction angle using the shear cell functionality of the instrument and the bulk properties of compressibility and permeability.

How is powder testing data used? Can you identify any particular benefits that it has delivered?

By correlating printing performance with powder testing data, we’ve developed a robust specification for powders that will perform well. If we’re considering a new supplier or a customer brings us a new powder to print with, then we can determine with a high degree of confidence whether it will work and if so how to process it, simply from test data. Without the FT4 this just wasn’t possible. These correlations are subject to ongoing refinement, with each new powder adding to the database, but it’s rare for them not to hold. We now have a highly efficient testing system in place that saves us considerable time and effort.

A good illustration of the benefits comes from a project to identify an alternative supply of 316L material. We routinely purchase metal powders to deliver part-printing services and a significantly less expensive 316L powder supply was identified in an effort to reduce costs. The particle size distribution of both powders was essentially the same – Dv50 of 15.8 c.f 16.1µm and comparable differences on Dv10 and Dv90 -  as was the morphology. Printing with the new powder initially produced good results, but in subsequent runs with recycled powder, the quality diminished. We stripped out the machine, started again, but the same thing happened.

Testing with the FT4 revealed that the stability of the two powders was remarkably different. The original powder has a stability of 1.03, meaning that flow properties are essentially unchanged with repeat testing, while that of the new supply is 1.52. The FT4 can detect powders that are insufficiently robust to retain consistent flow properties through the recycling process. Based on this result and other powder testing, a stability value of 1.2 now marks the upper limit of acceptability so powders that will fail in this way are detected prior to processing.

Image Credits: ExOne

What are the key challenges in extending the application of ExOne’s AM technology?

I would say that appreciation of the merits of binder jetting is growing so there is inclination towards its use. We’ve recently added our third qualified stainless steel to our range - 304L, in addition to the previously released 316L and 17-4PH – and we’re working hard to bring in additional materials. The FT4 will play an important role in helping us to do this, while in other projects, we have to ensure the robustness of process performance. A current topic is the impact of humidity. The FT4 is helping us to understand how humidity can impact powder properties and by extension process performance.

Looking ahead, I think that binder jetting has a very bright future. Understanding of 3D printing has matured considerably over recent years. It is now a successful industrial manufacturing technique and there is greater understanding of its benefits, and conversely, where it won’t so easily compete with established industrial processes. By offering customers expert support alongside advanced technology, we can ensure that binder jetting takes its optimal place alongside other manufacturing technologies.

Note: The terms 3D printing and additive manufacturing (AM) are largely used interchangeably in the industry and have been in this article.

About Andrew Klein

Andrew Klein is the Director of Research and Development at ExOne. In this role, he has developed high density sintered metals for ExOne’s printers and leads the company’s metal R&D team, among other efforts.

Andrew studied Mechanical Engineering at Carnegie Mellon University, where he conducted research in robotics with a focus on machine learning. He also received an undergraduate degree in Mechanical Engineering from Bucknell University.