Accuracy vs. Resolution in 3D Printing: Understanding the Difference

While EnvisionTEC is known for its surface finish, outstanding surface finish does not necessarily translate to dimensional accuracy.

A lot of words are commonly thrown around when talking about 3D printers and their capabilities. Two of those words are accuracy and resolution, which are often treated as interchangeable – but they’re really not. There’s a difference, and it’s important to understand what that difference is if you’re going to find the right 3D printer for your specific application – especially regarding applications like dental 3D printing, where precision is of the utmost importance.

EnvisionTEC recently published a white paper entitled “Understanding 3D Printer Accuracy: Cutting Through the Smoke and Mirrors,” which will be circulated at the ADA 2017 show in Atlanta later this month. You can also read the full paper here.

3DPrint.com recently had a chance to talk to Chris Kabot, Dental Applications Specialist at EnvisionTEC, about what accuracy means and why it’s so important in dental 3D printing in particular.

“Accuracy is paramount when 3D printing dental applications,” Kabot tells us. “Now different applications require different accuracy parameters but overall Z resolution and XY resolution do not equal dimensional accuracy, which is what is commonly misunderstood in the dental industry. The most important printer spec that is related to accuracy is the XY resolution, typically the finer the XY resolution the more accurate the parts are.

“For orthodontic applications we suggest using a machine with an XY resolution finer than 100 Microns and for dental applications finer than 75 Microns. When it comes to dimensional accuracy, which is what really matters, we want at least 80% of the data points to be within 100 Microns, for restorative dental applications we expect at least 80% of the data points to be within 50 microns. The American Board of Orthodontics is the first dental organization to actually publish some standards on accuracy requirements and they state that ortho models must have 99.9% of the data points to be accurate within 200 Microns, so at EnvisionTEC we actually have a higher internal standard than the ABO suggests!”

Chris Kabot

3D printing is capable of achieving better accuracy than other technologies, as it’s not constrained by things like tool size or complex angles as milling is. In milling, the XY resolution of the machine equals the size of the bit being used, and that bit cannot be smaller than 200 microns. By contrast, EnvisionTEC’s Vida 3D printer, which is the company’s most popular dental printer, has an XY resolution of 73 microns.

“The dimensional accuracy of a part is determined by many factors such as XY resolution, how the software cuts up the image, how the machine screws move the platform, how the firmware controls the projector, etc.,” Kabot continues. “At EnvisionTEC we have a lot of IP on our ERM (enhanced resolution module) which is why our printers yield some of the most accurate parts in the dental industry and is why the VIDA consistently wins accuracy analyses carried out by 3^rd party organizations. Part of our IP also has to do with pixel shifting and gray scale manipulation which allows us to print at a sub-pixel level giving our parts the surface quality and accuracy that is needed for highly precise dental parts.

“Not all DLP printers are created equal and other competing machines are normally printing with 2 shades of gray (black & white), at EnvisionTEC our Gray scale manipulation allows us to capture over 200 shades of gray which is why our parts tend to be more accurate than competing DLP units.”

This dental model shown left was 3D printed horizontally on the EnvisionTEC Vida 3D printer and has scan accuracy against the original digital model of 96.3% at 100 µm layer thickness. This dental model at right was printed on a low-priced SLA printer and has scan accuracy against the original model of 69.8%; that means the model is out of tolerance by 30+%.

Accuracy, defined in the simplest way, is how closely the 3D printed part compares to the design file. To truly test accuracy, a 3D printed part must be 3D scanned, and then the scanned file needs to be compared to the original design file. Resolution, meanwhile, is “the smallest output unit the machine can meet,” Kabot tells us. In DLP 3D printing, the XY resolution is related to the pixel size, while in SLA printing the XY resolution is related to the spot size of the laser. Accuracy and resolution are both important, so how do you know you’re getting the best of both when shopping for a 3D printer?

These bite guards were printed in EnvisionTEC’s clear E-Guard material nested together on a slight slant as a strategy to more accurately print this clear material. Print orientation can affect final print accuracy, especially if the resin color is clear.

“The proof is in the pudding right? I always suggest to any potential customer to get a sample part printed,” says Kabot. “Design the file yourself as you normally would and send that file to manufacturers to get a benchmark produced. If they don’t want to print the part for you they probably aren’t being 100% honest on the capability of the machine. In dental 3D printing we are truly in the Wild Wild West where regulations are thin and the market is consistently berated by marketing that always has an angle to it. Something I do that really proves our accuracy is to send an accuracy analysis with the part to the customer so they can not only test the printed part but see how it matches up to the designed file!”

The technicalities of accuracy and resolution are complex, but EnvisionTEC does a great job of breaking them down in the white paper, which you can read here. Copies will also be available at ADA 2017 in EnvisionTEC’s booth (#753) next to the CAD CAM stage.

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