Difference Between Accuracy and Precision in 3D Printing
When discussing 3D printing, we often hear the terms accuracy, precision, and tolerance grouped when they all have different distinct definitions. Before we dig deep into accuracy and 3D printing, it will help to understand those differences and how they apply.
Accuracy
When defining accuracy in 3D printing, it is how close the final measurements are to the true value. The true value equals the dimensions you designed in CAD. You are essentially looking at the final product and then looking back at how closely that product is printed compared to the digital design.
Precision
Most often confused with accuracy, precision also deals with measurement, but it is how consistent the printing is throughout the entire process. So, even if the accuracy is off and the printed model does not match your CAD measurements, the precision could be relatively high because it was consistent throughout the process.
Tolerance
The tolerances in 3D printing aren’t necessarily a specific measurement mistake, but more so the amount of deviation in accuracy and precision that is acceptable in the design. So, each project will have tolerances, and some prints may deviate slightly from designed dimensions, but it does not impact the design in the long run.
What Impacts 3D Printing Accuracy?
While both the accuracy and precision are important, it is far easier to be more precise than more accurate. This is partially because accuracy comes down to a culmination of factors before, during, and after printing.
Machine Accuracy
The final dimensional accuracy is largely determined by the accuracy of the machine in general. Yes, some types of 3D printers are more accurate than others. In general, the most accurate 3D printers will be material jetting.
Since it doesn’t use heat in the printing process, shrinking and warping will not occur, eliminating a large part of the inaccuracy risks.
Although material jetting tends to have the highest dimensional accuracy overall, that doesn’t mean other 3D printing processes are not accurate. For instance, SLA has a very high dimensional accuracy when it comes to parts less than 1000 cubic centimeters.
SLS also tends to have a high accuracy rate in printing for parts greater than 1000 cubic centimeters. Lastly, FDM one of the most cost-effective and, therefore, more widely used printing machines. It has lower limits but is still relatively accurate for smaller, less complicated designs.
Design
The type of design you use and how well you design the CAD object can also impact the final dimensional accuracy. If you have measurements off in the CAD design or do not use proper support systems, then accuracy is bound to be off because the machine prints, the object will not support itself and is at risk of collapse.
The size of the object you design will also impact the accuracy. Smaller objects tend to have a much higher final dimensional accuracy than larger objects. This is because they’re easier for the machine to print, and there is less room for error.
Materials Used
Not all materials will interact with each other or the machines in the same way. So, it is important to have a basic understanding of which type of material is best for the type of printing you plan to do. Beyond that, standard SLA resin tends to consistently have high dimensional accuracy compared to other materials like flexible SLA resin.
Heat Emitted
As mentioned earlier, heat can cause some distinct dimensional inaccuracies while printing. If the machine runs at high heat to functionally print an object, that object is then subject to the possibility of warping or even shrinking in some sections. These effects will wildly throw off the final measurements in comparison to the original CAD design.
Larger objects and larger expanses of material are more susceptible to warping and shrinking and flat areas or unsupported parts of the structure.
Post-Processing
Similar to design flaws, post-processing is determined by the person running the printing project. In most cases, post-processing won’t impact accuracy too much, but depending on the materials and object, improper cooling can cause objects to change shape and throw off measurements.
Accurate Types of 3D Printing
Material Jetting
As mentioned before, material jetting is the most accurate type of 3D printing currently available. This is because material jetting has a dimensional tolerance of ± 0.1% and a lower limit of ± 0.05 mm.
Since heat isn’t used in the printing process, there is a lower likelihood of warming or shrinking. However, objects have still been known to warp when exposed to heat or humidity during the post-processing step. Additionally, material jetting is unable to make very durable products in general, and it struggles to print thin walls or detailed features. These restrictions make the application a bit more limited, but very accurate nonetheless.
SLA
Stereolithography (SLA) printing is a very smooth and accurate 3D printing process. When using an SLA design, all parts are made from cured resin, and the dimensional tolerance is ± 0.15% and a lower limit of ± 0.01 mm.
The most inaccuracies occur post-processing because the resin in SLA printing takes more time to harden completely. If printing larger or unsupported objects, you’re likely to experience warping of some kind.
SLS
Selective Laser Sintering (SLS) is another relatively accurate 3D printing process and is most often used for intricate geometric designs. The dimensional tolerance is ± 0.3% and a lower limit of ± 0.3 mm.
SLS printing differs from the other types because it uses lasers to sinter layers of powder together to make an object. While it is more accurate than the high heat process of FDM, there is still a high chance that the layers of the object created with SLS printing will cool at varying rates, especially when printing large objects.
FDM
Fused Deposition Modeling (FDM) is probably the most common type of 3D printing. It is easy to develop prototypes using and has a dimensional tolerance of ± 0.15% and a lower limit of ± 0.2 mm.
It varies in terms of accuracy because it uses thermoplastics through a heated nozzle. All objects are printed one layer at a time, and because of this, layers tend to cool at different rates. This increases the likelihood of warping and shrinkage during the printing process but during post-processing.
Achieve Higher Dimensional Accuracy
Part of achieving high dimensional accuracy when 3D printing, no matter the machine you choose, is about getting used to the processes. Like most things, 3D printing design takes practice, and then beyond that, learning the systems takes a specific set of knowledge and skills to master. However, by being aware of where and why accuracy or precision mistakes happen most often in printing, you can easily avoid them throughout the process.
