Misconceptions of Metal Additive Manufacturing – Multiple Lasers
When talking to customers about our metal 3D printers, they often say to me, “Their printer has more lasers than yours. So their printer will be more productive than yours”. On the face of it, the statement seems like it makes perfect sense. More the lasers, faster will be the melting of powder and so the print job will complete faster. Correct? Well, not necessarily.
Let’s take an example of a quad laser system. Imagine four lasers melting metal powder in their respective quadrants.
Melting results in remnants which need to be carried away by the gas flowing through the build chamber. This means that if the gas is flowing say from front to back then the debris will come in the way back two lasers. The effective laser power will drop on that section of the powder bed and you will not get the accurate melting you desire leading to all kinds of problems like porosity. To take care of this problem engineers need to come up with innovative methods of gas flow. Some work better than others. While it is true that multiple lasers can melt metal powder faster, it is more important to ensure that the metal powder is melted properly. Gas flow plays a very crucial role here.
Also, it’s important to ensure that while solving one problem you are not introducing another one. Using multiple lasers to print a single part may result in joins and seams, internal as well as external. This is not good specially for aerospace parts where seams reduce the fatigue resistance.
Our DMP Flex 350 has a build area of 275 x 275 mm and has just one laser. Mainly because we believe one laser is enough. However, our DMP Factory 500 has a build area of 500 x 500 mm and we realized that one laser would not be able to do the job quickly enough. But instead of doing the most obvious thing and having a quad laser system we decided to opt for three lasers, all in a straight horizontal line.
The left and right lasers can reach more than half of the platform from their respective sides. Whereas, the center laser can reach the entire 500 x 500 mm build area. So when we build really large parts, the left and right lasers work on the infills of the part and the center laser works on the boundaries of the part. This way we get higher productivity with two lasers working simultaneously as well as excellent surface quality with the center laser doing the boundaries. Moreover, since the lasers are in a straight line, instead of four quadrants, we don’t have have the problem where debris caused by the front lasers interfere with the back lasers. We can benefit from a smooth laminar flow of gas from front to back as the lasers melt powder from back to front. We don’t need to mess with complicated gas flow systems and hope that they work. Our inline three laser system with unidirectional laminar gas flow is simple, elegant and it works.
If we had opted for a quad laser configuration, maybe we would have been able to build many small parts spread across the platform quickly. But large parts would pose a serious challenge because all four lasers would be required to print the part and it would result in joins and seams, both internal as well as external. I have come across customers of multiple laser metal printers who fill just one quadrant or one half of their platform while building parts because they want to the printer to use just one laser for greater accuracy and consistency. This defeats the whole purpose of opting for a multiple laser system.
Speaking of increasing productivity in metal additive manufacturing, there are many ways to skin a cat. Having multiple lasers is just one of them. People don’t think about it much, but recoating time can take up a significant amount of build time. In most metal printers, the laser doesn’t start firing unless the recoater has reached the end and has stopped moving. This is because minor vibrations caused by recoater movement can mess with the laser optics and result in a beam that is off by a few microns here and there. This problem is eliminated in our DMP Flex 350 and Factory 500 systems because they modular in nature. They have a removable print module which not only contains the build plate and all the powder, but also the recoater blade and its mechanism. The movement of the recoater blade doesn’t affect the laser optics in the rest of the printer. So as soon as the recoater blade moves out of the build area the laser starts firing. As the laser is firing the blade continues to move to the end then comes back getting fresh powder with it and parks itself close to the build area waiting for the laser to stop firing. This will save maybe 3 to 4 seconds per layer. But add this over thousands of layers and you could end up saving an hour or two of build time. I intentionally brought up the seemingly uninteresting topic of recoating to show you how optimizing small things can add up to significant time and hence cost savings. Take a look at this video and notice the motion of the recoater while the laser is firing.
Another example is the heated build plate, something that’s allegedly required to successfully build large bulky parts. I consciously used the word “allegedly” because our DMP Flex 350 and Factory 500 do not use a heated build plate. We have built parts as heavy as 250 kgs on the DMP Factory 500 without a heated build plate. I bring this up because when you have a heated build plate you need to wait for it to gradually heat up and then wait for it to gradually cool down. This can add hours to build time and is often ignored by customers when they do their productivity calculations.
When calculating build time, I always urge customers to start the clock when they close the door to start the build and stop the clock when they open the door to remove the part. Because the time in between is the actual build time. Just adding more lasers to a printer doesn’t necessarily mean that you will have a more productive system. In fact, it could be detrimental to what you are trying to achieve if the gas flow isn’t properly managed and joins and seams get introduced into the part.
This brings me back to the point I made in the first part of this series about certification. Your final goal should be to produce a part that is certified as per the target standards and requirements. For that to happen, it should not matter where the part is placed on the platform – bang in the center printed by four lasers, on an edge printed by two lasers or in a corner printed by a single laser. It should not matter whether the part is located downstream of the gas flow direction or upstream. A system certified to build high quality parts should not leave anything to chance or depend on the skill and experience of the person preparing the build. A certified workflow should build certified parts. Always. Period.
I strongly believe that while multiple lasers contribute to higher productivity, they should be considered as an option only after all other factors have been optimized to the maximum. To drive my point in, I would like to mention a feature in 3DXpert called 3D Zoning which allows the user to set different parameters for different sections of the same part. We were working with an aerospace customer on a large part which had four large cones.
The size and geometry of the part dictated that each cone would be built by multiple lasers of our DMP Factory 500. Of course, the boundaries would be built using the center laser which would result in a smooth outer surface. But the customer didn’t want joins on the inside of the part as well. That posed an interesting challenge. We wanted to use multiple lasers to build the part quickly. But we also didn’t want joins and seams on the surface as well as the inside of the part.
The solution to this interesting problem was provided by 3D Zoning, a feature unique to 3DXpert which can be used to program a printing strategy whereby each cone would be built using a single laser only from start to finish. That way we used 3 lasers to build 4 cones. We got an excellent surface finish as well as consistent interior micro-structures. Of course, the part took a longer time to build as compared to if we had let the software decide on a printing strategy which optimized all three lasers. But in this case, inner part micro-structures and outer surface finish were of paramount importance to the customer. Productivity came later. There is no point printing parts fast if they are going to be rejected by quality assurance or worse, fail in the field.
So before selecting a multi-laser solution just for productivity sake, it will be helpful to understand exactly what kind of parts you will be able to build consistently with dependable repeatability, what you are going to lose and where you will need to compromise.
If you found this article interesting, you may want to click here to read other articles in this blog series.
