3D printing tolerances are acceptable variations in size for overall part dimensions and individual features. They’re expressed as plus or minus (±) values in millimeters or inches and vary by the type of 3D printing technology that’s used to produce the part. These tolerances are also a function of variables such as material shrinkage, layer thickness, minimum feature size, and build size.
Although ISO-286 includes additive manufacturing techniques, a lack of standards surrounding 3D printing tolerances complicates the work of the designer who needs accurate and precise parts, especially at the microscale. Unlike macroscale 3D printing for larger parts, microscale 3D printing produces parts with overall dimensions of less than 1 millimeter (mm). Where do accuracy and precision fit in?
Microscale Accuracy and Precision
Accuracy refers to the ability to achieve the designer’s desired dimensions. For example, an extremely accurate macroscale 3D printer produces a 10 mm pillar at 10 mm – and not at 5 mm or 15 mm. Precision refers to how close measurements of the same item are to each other. If the 3D printer produces three pillars that are supposed to be 10 mm at 10 mm, 11 mm, and 12 mm, then the printer is not very precise.
In additive manufacturing, the ability to achieve accuracy and precision is a function of factors that include:
- Data resolution
- Material changes
- Slicing sequences
- Layer height or thickness
3D printing tolerances are also especially important. If the tolerances are “loose”, the ability to achieve accurate and precise dimensions is limited. If the tolerances are “tight”, designers can expect greater accuracy and precision. Yet project success is not just a function of an individual piece of equipment. It’s also a matter of which 3D printing technology is used.
To illustrate this point, let’s compare the 3D printing tolerances of standard stereolithography (SLA), material jetting (MJ), digital light processing (DLP), and projection micro stereolithography (PµSL). As you read the following sections, remember that 1 mm equals 1000 microns (µm).
Standard Stereolithography (SLA) Tolerances
Standard SLA equipment uses a galvanometer, an instrument for detecting and measuring small electric currents. Galvanometer systems are fine for some sub-millimeter tolerances, but they can’t achieve accuracy and precision in just tens of microns (µm). Today, the tightest dimensional tolerances that standard SLA printers can achieve is ± 100 µm (0.1 mm).
Material Jetting (MJ) Tolerances
3D printers that use MJ technology have a print bar or gantry that moves across the build surface. This gantry is relatively heavy and moves both back and forth. Because of the combination of mass and acceleration/deceleration, it’s difficult for MJ 3D printers to maintain controlled positional accuracy. Typically, these systems support dimensional tolerances in the range of ± 300 µm (0.3 mm).
Digital Light Processing (DLP) Tolerances
Three main factors determine the accuracy and precision of DLP systems: the resolution of the projector, the optical character of the projection lens, and the mechanical movement of the Z stage. Because the entire cross-section of the part is projected at once, the projector’s energy consistency also affects part tolerances. Today, most DLP systems can achieve overall tolerances of ± 300 µm (0.3 mm)
Projection Micro Stereolithography (PµSL) Tolerances
3D printers that use PµSL technology use liner encoders, mechanical design, calibration, an optical system, and software to achieve tighter tolerances than standard SLA, MJ, or DLP. Boston Microfabrication (BMF), the world leader in micro-precision 3D printing using PµSL technology, can achieve printing tolerances of ± 10 µm (0.01 mm) to around ± 25 µm (0.025 mm), depending on the type of machine.