Accuracy, precision and tolerance in 3D printing

The fact that a 3D printer has "high resolution" among its specifications does not mean that all 3D printed parts will be accurate and precise. Understanding the meaning of accuracy, precision and tolerance is essential to achieve a good result in any 3D printing. Below we detail what each of these terms means in the context of 3D printing.

Accuracy

We understand exactly how close a measurement is to its true value. If we set an example as a target, a shot will be more accurate the closer it is to the center of the target. In 3D printing, the true value will be the projected dimensions in the CAD design. Therefore, the accuracy of a 3D printed piece will be greater the more it resembles its digital design.

Precision

Accuracy measures the repeatability of a measurement. Following the example of the target, the shots could always be impacting near the same point (which would be accurate), although that point might not be the center of the target. In the context of 3D printing, this translates into reliability to produce the expected results in each print. In the field of engineering, when comparing different 3D printing materials, the term "precision" is used to refer to the ability of a material to print very complex geometries.

Image 1: Example of differences between accuracy and precision. Source: Formlabs

Tolerance

Tolerance defines how accurate it needs to be in a given 3D print. The tolerance is defined by the user and will depend on each specific application. A mechanical assembly component, for example, will require stricter tolerances than 3D printing of a plastic case.

Returning to the example of the target, in the example on the left the shots are very close to each other and we can define them as accurate, while in the example on the right they are further away from each other and we could say that they are not accurate. Now, if we define as tolerance an acceptable range of precision a distance of 3 rings, then the shots would be within the specifications.

Image 2: Tolerance is the acceptable range of accuracy. Source: Formlabs

Accuracy and precision in 3D printing

When choosing a 3D printer, it is very important to identify specific needs. For this, it is key to understand the concepts of accuracy and precision.

An inaccurate and accurate 3D printer may be the best option for certain applications. For example, a low-cost FDM 3D printer will produce less accurate parts, but for educational use where students are learning about 3D printing, it may not be important that the dimensions of the printed part match exactly those of the CAD design. However, it will be important to be confident that the 3D printer will work consistently as expected, within the tolerances the user needs, to achieve a successful experience.

On the contrary, for applications in the industry, a 3D printer that guarantees accuracy and good accuracy with very strict tolerances is surely necessary.

There are four major factors that determine the accuracy and precision of a 3D printer:

1. 3D printing technology

3D printing is an additive process where parts are built in layers, and each layer presents a risk of inaccuracy. In addition, the process by which layers are formed affects the level of accuracy (repeatability) of the layers. In a 3D FDM print the layers are extruded by a nozzle (nozzle) that lacks the ability to achieve complex details, while in 3D stereolithography (SLA) printing, the liquid resin material is cured with a high precision laser to form each layer, being able to achieve much finer and more reliable details when it comes to repeatedly achieving high quality results.

Image 3: Difference between piece printed in FDM (left) and SLA (right). Source: Formlabs

The specifications of the 3D printer alone do not represent the final dimensional accuracy. A common error is the description of the XY resolution as dimensional accuracy. For digital light processing (DLP) printers, the XY resolution is the projected pixel size. Many 3D printer systems use this projected pixel size or XY resolution as the overall accuracy figure (for example, taking a projected pixel size of 75 microns and stating that the machine's accuracy is ± 75 microns). However, these data have no implications on the accuracy of a printed piece. There are many sources of error that can have an impact on accuracy, from components to calibration, as well as the following factors that we will analyze (materials and post-processing). In short, the best way to evaluate a 3D printer is to inspect real pieces against theoretical ones.

2. Materials

The properties of the material used for 3D printing can also affect the probability of deformation of a print. As we indicated earlier, everything depends on the needs of each application. In dental 3D printing, accuracy with respect to 3D design will be essential, so there are specific products for these applications that guarantee a very accurate result. On the contrary, for 3D printing of a non-functional prototype, where the objective may simply be to have a general idea of the physical product, accuracy will be less important.

3. Post-processed

Normally, 3D printed parts with resin require a post-printing curing process, in which the part can contract. This must be taken into account in the design, considering said contraction to ensure that the resulting part after the curing process is dimensionally accurate with respect to the original CAD design.

4. Ecosystem

Achieving a successful 3D printing requires paying attention not only to the 3D printing itself, but to the whole process as a whole. The design, the laminating software, the materials, the temperature, the post-processing tools… everything influences the final result.