Have a peek under the hood of your powder bed fusion machine and here’s what you get: infra-red laser(s), powder reservoirs and overflows, a spreading mechanism, some form of atmosphere control in the build chamber and a software handling digital files and controlling the process sequence.
Fundamentally, there are no processing differences between suppliers' offers!
For the price of a house, or a luxury car, you get a machine that will be responsible for the reputational credibility of your company and the commercial viability of your production. Yet, it rarely comes with a builders’ or after sale guarantee.
In the next 3 articles, we’ll review the points you will want to keep in mind before you invest in AM technology. Hardware, ease of use, maintenance and end-products quality are a few topics you’ll want to discuss in detail with the suppliers. In the first part, let’s have a look at the hardware itself and what you should pay attention to.
Machine robustness and reliability, technology maturity
Machine reliability, maturity of the equipment
A good clue is in the maturity of the product: how long has it been out? Are current customers happy with it? And could you talk to them? And don’t confuse the official launch date of the unit with the actual release date of the usable product. Products might only just be launched as an PR coup during an exposition, in order to ‘keep up with the Joneses’ of AM. But is it actually for sale?! Does the delivery date keep being pushed back? Be aware that lapse of time between launch date and sale delivery of a robust product might unfortunately extend up to a couple of years… And if it's a new product: can you try before you buy? More dedicated centres mushroom around he world where you can do just that.
Processing parameters sets available
Powder usage efficiency
This aspect will be detailed in the 2nd part dedicated to ‘use and maintenance’.
The maximum build height will also be affected by the thickness of the build substrate. Thick (~30/50mm) platforms are used to prevent warping during build when large, chunky components are melted. Standards platform are usually ~15/25mm thick depending on the machines suppliers. Keep that in mind when you prepare the digital files and choose the best orientation, and calculate the numbers of parts you can build in one run.
Resolution vs build speed
The fine features of your products portfolio will dictate the resolution you may need. The minimal resolution of PBF machines is directly derived from the laser track width in the XY plane. As a rule of thumb, theoretical lasers spot sizes oscillate in the 50/500um range depending on manufacturers. Hence, actual laser weld width, which vary with processing parameters, will scale from ~70/900um diameters upwards.
PBF technology relies on successively melting layers upon layers. Each layer is a thin cross-section of the part derived from the original CAD data. In the physical world, each layer must have a ﬁnite thickness and the resulting part is an approximation of the original data. Thin layer thickness values tend to minimise staircase effect and results in a ﬁnal part very similar to the original. The layer thickness also determines factors like the accuracy of the ﬁnal part plus its material and mechanical properties. It also determines build speed and how much post-processing is required. The smaller the layer thickness, the lower the surface roughness value. Inversely, the larger the z-step, the faster the build rate.
Note: a few manufacturers use high precision positioning scales, recoating mechanisms or elevators. But what’s the point of a 0.010um Z-step resolution (=layer thickness) if the best surface roughness achievable varies between 5 and 50um…
Atmosphere control and condensation
One or two sensors are usually directly positioned in the build chamber. It may be advisable to enquire the sensing accuracy of the O2 sensors and what the control loop consists of. Do they record the O2 levels or help actively control the build environment? Does the machine come with spares readily available when one packs in? These sensors are susceptible to being coated/dusted by fine condensate that may be generated during process. In that respect, you may want to enquire about a safe cleaning procedure and frequency.
Condensation can occur during processing. It is usually noted on the inner walls of the build chamber. Obviously, this is to be avoided as the presence of H2O in the chamber could contaminate the material and ruin the properties of your components. Have a chat with your supplier to assess if they’ve noticed this and how they’ve fixed the issue.
Processing at high temperature
Commonly, electron beam PBF setups can reach build platform temperatures of up to 800C whereas laser based setups usually reach up to a maximum of 170/200C, yet more suppliers are working towards providing higher-temperature options. It’s worth asking how high the build platform temperature rises. If you don’t need this option now, would it be possible to upgrade at a later stage?
: Alexandros Beiker Kair, Konstantinos Sofos, Additive Manufacturing and Production of Metallic Parts in Automotive Industry, 2014