If you own a 3D production system or have considered 3D printing as a solution, it is important to understand which is more important for your application: dimensional accuracy and repeatability or resolution. In this blog article, we discuss why layer thickness is listed as microns in 3D printing and why that matters. First, let's look at the definitions of accuracy, repeatability and resolution:
- Accuracy refers to how closely a manufacturing system’s output conforms to a tolerance within a specified dimensional range.
- Repeatability captures a system’s ability to produce consistent output, time after time.
- Resolution refers to the smallest unit the system can reproduce.
Dimensional accuracy is a very important factor many people take into consideration when choosing an additive solution that works best for them. This specification will impact factors such as how well small or fine features look and how accurate critical surfaces will be. Typically, this is measured in microns or micrometers. A micron is equal to 0.001mm or 0.000039in. For comparison, a human hair is anywhere from 20 – 200 microns in diameter and the longest human chromosome is 20 microns long. So, when the question is posed of "Do microns really matter?" in some case, you're really splitting hairs.
How do These Measurements Relate to Additive Manufacturing?
Dimensional accuracy is dependent on the system producing the part and the overall size of the part. Stratasys offers 3D printers that build parts using different technologies such as Fused Deposition Modeling (FDM), PolyJet and Stereolitography. FDM machines fuse layers of thermoplastic material through an extruder. Meanwhile, PolyJet machines build parts from layers of photopolymers that are cured by UV light (like the process used by an inkjet printer). Stereolitography combines high resolution and fine build layers with a generous build capacity capable of producing highly detailed parts, prototypes and casting patterns in large scale.
Fused Deposition Modeling (FDM) is a process in which semi-liquefied plastic filament is extruded on an X/Y plane on a preprogrammed route. These machines work by cutting parts up into layers and printing each one before raising the bed to print the next one.
Typically, these layers or slices are measured in inches, not microns (1 inch = 25,400 microns) so a difference of a few microns probably won't make much of an impact on quality or accuracy. But that doesn’t tell the whole story. Stratasys machines, for example, have rated dimensional accuracy anywhere from 0.005" to 0.008" (127 – 204 microns). This means that for every inch of your part, you can expect to be held to this accuracy. The machines in the F123 series, for example, can print with an accuracy of 200 mm (.008 in), or +/- .002 mm/mm (.002 in/in), whichever is greater.
Unfortunately, the smaller or finer you get with FDM parts, the less accurate they will become. This is because there is only so much an extruded piece of plastic with a diameter of anywhere from 0.005" - 0.020" that is rapidly expanding and shrinking can do to fit into tight spaces. A good rule of thumb is to have your smallest feature be at least two times as big as your layer height. But for super small features (Example, a 0.0050" hole) FDM may not be a good fit, and there are other technologies better suited for those types of features.
PolyJet technology dispenses droplets of a UV (Ultraviolet light) curable resin down onto a build plate layer by layer. This process is similar to how your standard inkjet printer works. It creates much finer layer heights, down to 14 microns. That’s thinner than the average human hair! So, if realism, full-color mixing or fine features are what you are after, choosing a PolyJet 3D printer makes a lot of sense. In this scenario, the difference of 10 or 20 microns may create a big impact because the layers are so fine. This could cause features to not show up well or for color to not mix properly.
Newest additions to the Stratasys PolyJet family include the J850 and the J826 which have the capacity to print with up to seven materials at a time. The J850 can print down to under 100 mm–±100; above 100 mm– ±200 or ± 0.06% of part length, whichever is greater. The J826 has the capacity to print down to under 100 mm – ±100μ; above100 mm – ±200μ.
Stereolithography (typically referred to as SL or SLA) is a process where a bath of resin is curd layer by layer with a laser or other light-emitting device like a screen or projector. This technology can produce single material parts with a high degree of accuracy and finish.
Due to this high degree of accuracy, a change of a 10-15 microns cold have adverse effects on the part. But for larger parts where the tolerances are more forgiving, those changes won’t make much of a difference. It is also important to make sure these machines are calibrated for the specific resin you are using and to make sure the resin bath is clean and clear. If those steps are not followed, there can be problems with large and small parts.
So, Do Microns Matter in 3D Printing?
Sort of. It really depends on what your end goal is with your part. If you are making large jigs or fixtures that are going to hold specific parts for painting, then a few microns of accuracy are not going to affect that. If you are making small-scale models with a lot of fine features, then a change of even a few microns can show up on your parts. But by designing your part for the additive process, you can mitigate a lot of problems with accuracy and fine feature resolution. At TriMech, we offer an entire DFAM (Design for Additive Manufacturing) course designed to help users design parts that are more efficient the first time by utilizing design principles that are only possible with additive technologies.
Now that you are more familiar with microns and accuracy when 3D printing with different technologies, you might be wondering which could be the best fit for your needs? Don't worry, we can help! Watch our on-demand webinar to learn about the differences between PolyJet and FDM technologies.