Understanding DPI in binder jetting
One of the biggest misunderstandings in the field of binder jetting has to do with a seemingly simple indicator: dpi, or dots per inch.
As the name suggests, DPI is an actual measurement that accurately represents how many ink droplets can be produced per inch of the printhead.
The assumption that higher DPI values are equivalent to higher final part accuracy and surface resolution is a common misconception in the field of 3D printing - and this view is almost as wrong as that in the field of polymer printing: thinner layers automatically equal higher quality. (this is not the case!) in both cases, there are a number of other considerations.
So let's talk about DPI. For an interesting discussion on this topic, we contacted exone technology researcher Dan brunermer, a world leading expert in binder jetting. He has been engaged in this work since 2001. He developed an early metal binder jetting system R2, which is still in production today.
"Well, the first problem is powder," Dan explained. "If you don't have powder to keep the authenticity of DPI, it's useless."
Reason 1: Powder
Unlike printing on paper, the printhead in the binder jet system prints on powdery materials: metal, sand or other ceramics. As Dan explained, these powders are not all perfect small circles of the same size.
Microscopically, a pile of powder has a series of characteristics, including round or spherical Non spherical (including angular and polyhedral particles). More importantly, the powder particles are not all the same size. The powder is distributed, usually D50, in which half of the powder diameter is higher than a certain value and the other half is lower than a certain value. For D50 of 22 microns, half of the particles are lower than this value and the other half is higher than this value.
More importantly, when you put a bag of powder into the machine, the powder will arrange or squeeze in by itself. This way is like a pile of marbles of different sizes in a glass, and the small marbles squeeze into the space between the big marbles. To be exact, this is a mixture.
The purpose of the printhead in binder jetting sounds simple: drop enough binder on these particles with enough energy to completely wet or saturate them in the X, y and Z directions, so that they can accurately combine adjacent particles where needed.
At the micro level, this means that the binder must hit the mixed particle bag, basically enclosing each particle separately with mucus, so that it will stick to the adjacent particles. Depending on the powder you want to print, your printhead may also need to handle a range of binder viscosities. Some powders prefer thinner binders, while others prefer thicker ones.
Now, do some math. 1200 DPI printhead is one of the most frequently used printhead in industrial printheads. Each droplet produced by the printhead is equivalent to 21.17 microns, about half the cross-section of human hair.
So, let's assume that you use a 1200 DPI printhead to print the binder onto a metal powder bed with a D50 of 30 microns (standard processed additive manufacturing metal powder). Can the binder maintain a tolerance of 1200 DPI when touching the powder particles?
No. That's because half of the individual particles themselves are actually larger than the droplets sprayed on them. In this case, brunemer explained, "you're actually wasting DPI. No matter how high the precision of DPI is, it's useless."
Reason 2: layer thickness
OK, let's take another example and assume that the powder size is not a problem for our 1200 DPI printer. Imagine that we use a powder with a D50 of 9 microns, with a maximum particle size of about 14 microns, which is much lower than the binder droplet size of 21.7 microns. Now, we want to print a common layer thickness: 50 microns. Will we maintain 1200 DPI accuracy on the 3d printer?
In X and y, the answer is tentative. (more on that later). However, in the Z direction, DPI will not be maintained - because a layer of 50 microns is equivalent to 508 DPI. To maintain 1200 DPI in the vertical direction, you need a layer thickness of 21.7 microns or less, which will double the number of layers to print.
Eventually, this will also reduce your working speed by more than 50%, because the printhead must run more than twice to reach the same height as the 50 micron layer.
If you want to print at a reasonable speed and layer thickness, Dan explained, "you certainly won't choose a voxel of 1200 DPI."
Reason 3: powder compaction
OK, now let's assume that we have the right size of powder and layer thickness to maintain the authenticity of 1200 DPI.
Now, you must ensure that each powder layer is consistent throughout the powder bed and that the compaction level is high. If you are not familiar with whether it is easy to print with D50 powder of about 9 microns, I can give an example. It is like making a cake. When you pour out the flour, it will get together. When you try to spread it out, it forms a cloud.
It requires a highly engineered powder management system with high precision and control to distribute, pave and compact your ultrafine powder to maintain any DPI.
This is essential to maintain DPI in the X, y, and Z directions regardless of powder size.
If your powder layer after layer cannot be consistent and tight, you may be like printing 1200 DPI of binder on a piece of paper with holes, because these small particles can not gather tightly to absorb the binder.
"If your powder is inconsistent and not dense, the binder will penetrate into the gap and the resolution will be lost," brunemer explained.
What's worse, the final shape will not be very dense and intact in microstructure, and will not actually burn well into metal parts. "Compaction is an important predictor of sintering success," Dan said. "High green density of binder sprayed parts means less shrinkage, less deformation and tighter tolerances."
Reason 4: nozzle position control
Sometimes, too high accuracy can be a challenge. This is why it is easier to shoot a target with a shotgun than a rifle. (shotgun pursues a breadth, not a precision)
If you have an ultra precision 1200 DPI printhead, you must ensure that each droplet accurately hits the X and Y positions in the digital bitmap target. At this time, software, mechanical control and their interaction are very important.
If you have a high DPI printhead and a substandard system to deliver your 21.7 micron ultra-fine ink droplets to the exact X and Y positions in the print bed, you can't actually achieve your goal. At this point, smaller ink droplets actually pose a greater challenge to the control system because they have less space for fault tolerance.
As Dan said, "if I don't have a drive accurate to within 20 microns, 1200 DPI doesn't matter. If I can't find the location of all 1200 drops of ink correctly, 1200 DPI doesn't matter."
Reason 5: ink drop quality
Now, let's assume that we have the right powder, layer height and powder management system, and accurately align the printhead. Everything is ready to output and receive 1200 DPI ink drops so that DPI can be maintained?
Did you know that not all 1200 DPI printheads can provide 21.7 micron droplets of the same quality? Droplets can be transported horizontally in different shapes and energy or velocity.
"Strangely," Dan explained, "we found in our research that imperfect droplets can produce better parts. This is called binder jetting, not binder falling, right?"
"If you carefully observe the working principle of the nozzle, it sprays some fine streams with good quality and energy to really penetrate into the particles. If the droplet is too slow or too small or has too little energy, it will only bounce off the powder surface. In fact, we try to make our ink droplets more like jets than droplets."
These five reasons are just a few reasons why DPI can not represent the quality of binder jet 3D printing. We haven't even talked about how to maintain DPI in the final metal sintering stage.
So what is the best way to determine the quality of the binder injection system?
"The final answer is to have a complete system that takes all these variables into account to build, design and optimize," Dan said. "The real key is not any specific number, but to understand how the whole system works and how the whole process works together, so that you can really get high-resolution and high repeatability parts."
In order to really compare the accuracy and surface finish of the binder injection system, the best way is to print your parts with a competitive system and compare the benchmark.