What level of dimensional accuracy can MJF parts achieve?
From a manufacturing and quality engineering perspective, the dimensional accuracy achievable with Multi Jet Fusion (MJF) is among the best for polymer powder bed fusion processes, but it is subject to a well-defined and predictable variability influenced by part geometry, material, and build orientation. As a general rule, MJF parts can consistently achieve a dimensional accuracy of ± 0.3% with a lower limit of ± 0.2 mm (± 0.008").
Typical Dimensional Tolerance
This means that for a feature on an MJF part:
A 100 mm dimension would be expected to fall within 99.7 mm and 100.3 mm.
A small 10 mm feature would be held to a tighter absolute tolerance of 9.8 mm to 10.2 mm (respecting the ±0.2 mm lower limit).
This level of accuracy is sufficient for a vast range of functional applications, including enclosures, housings, jigs, fixtures, and many end-use parts in industrial equipment and consumer products.
Key Factors Influencing MJF Accuracy
Several intrinsic aspects of the MJF process dictate its accuracy:
Uniform Thermal Curling: Unlike processes that use a point laser, MJF's area-wide fusing with detailing agents minimizes differential thermal stresses. This results in less curling and warping, leading to more predictable and uniform shrinkage, which is the primary driver of dimensional deviation.
Isotropic Shrinkage: MJF parts shrink relatively uniformly in the X, Y, and Z axes after cooling. This isotropic behavior makes dimensional compensation in the CAD model more straightforward and reliable compared to more anisotropic processes like FDM.
Powder Particle Size: The fine polyamide powder used allows for the creation of sharp features and thin walls, contributing to the potential for high accuracy.
Practical Guidelines and Design Considerations
To achieve the best possible accuracy, design and manufacturing practices are crucial:
Critical Features: For features requiring tighter tolerances than the standard ±0.3% (e.g., press-fit holes, mating interfaces), CNC Machining is often employed as a secondary operation to bring those specific features into final specification. This hybrid approach is common for high-performance applications.
Holes and Shafts: Small holes tend to close up slightly due to powder sintering, while small pins or shafts may become slightly larger. Designing with slight compensations or planning for a post-process drilling or reaming operation is recommended for critical diameters.
Wall Thickness: Very thin walls (below 1 mm) may be challenging to produce accurately and consistently. Maintaining robust wall thicknesses ensures the part can withstand the powder removal process and exhibits predictable shrinkage.
Surface Finish: The as-printed surface has a slightly grainy, matte finish. While this doesn't significantly affect the dimensional envelope, a smooth surface for sealing or bearing applications can be achieved through tumbling or vibratory finishing, with a negligible impact on critical dimensions.
Comparison with Other Processes
MJF offers a compelling balance of accuracy, speed, and cost:
Vs. FDM: MJF is significantly more accurate and does not suffer from anisotropic (direction-dependent) weaknesses or visible layer lines.
Vs. SLA: While SLA can achieve higher accuracy and a smoother surface finish, its parts are often more brittle and not as well-suited for functional, durable applications as MJF's nylon parts.
Vs. SLS: MJF and SLS are comparable in accuracy, but MJF often has an edge in producing more consistent mechanical properties and a better surface finish straight from the machine due to its detailing agent.
In summary, MJF provides a high level of dimensional accuracy suitable for most functional prototyping and end-use production applications. By understanding and designing for its characteristic shrinkage and limitations, engineers can reliably produce parts that meet stringent fit and function requirements, resorting to secondary machining only for the most critical of features.
