Designing for Precision in Additive Manufacturing

HP Multi Jet Fusion (MJF) has revolutionized 3D printing, making it not just a prototyping tool, but a full-scale production solution. With its high speed, fine detail, and excellent material properties, it’s an ideal choice for engineering-grade parts. But if you want MJF parts to fit together properly or include durable threads, you need to design for the unique quirks of the process.

Unlike CNC machining, which cuts parts to precise dimensions, MJF builds components layer by layer using thermoplastic powder and fusing agents. This introduces variables like material shrinkage, powder spread, and thermal expansion, all of which can impact how parts fit together or how well threads hold up under repeated use.

If you’re designing for MJF, understanding these factors upfront can mean the difference between a perfect fit and a frustrating assembly process. Let’s break down what you need to know.

Getting the Right Fit in MJF 3D Printing

Fit is everything when designing parts that need to connect, slide together, or lock into place. In machining, tolerances can be extremely tight—often in the IT6-IT8 range. With MJF, you’re looking at IT10-IT12, which is good enough for most engineering applications but not as precise as machined components.

Because MJF parts shrink slightly as they cool, you have to account for that in your design. If two parts need to slide together smoothly, you’ll need to leave enough clearance. If they need to be pressed together and stay put, you might need a slight interference fit.

There are three common types of fit to consider:

• Clearance fit: This allows for easy movement between parts, such as a hinge or sliding mechanism. In MJF, adding extra clearance is critical because powder residue and slight warping can make tight fits difficult to assemble.
• Interference fit: This creates a tight, secure connection where one part is slightly larger than the hole or slot it fits into. Due to MJF’s shrinkage factor, you may need to experiment with sizing to ensure a snug fit without excessive force.
• Transition fit: This sits between clearance and interference, where parts may require light force or temperature changes to assemble.

Since MJF doesn’t always produce perfectly uniform surfaces, a little post-processing—like sanding or reaming holes to final size—can improve the consistency of fit.

Designing Threads in MJF: What Works and What Doesn’t

Threads are one of the trickiest features to 3D print, and MJF is no exception. Unlike CNC machining, where threads are cut with precision, MJF builds them layer by layer, which can introduce inconsistencies, weak edges, and powder-filled gaps if not designed correctly.

If you’re printing small, fine threads—anything below about M6 (6mm in diameter)—you may run into issues with poor resolution and structural weakness. Larger threads tend to print better, but even then, they may not hold up under repeated use unless properly reinforced.

The best way to ensure strong, functional threads in MJF parts is to carefully choose the right approach:

• Directly printed threads: Best for low-load applications, but require extra clearance and cleanup to ensure smooth engagement.
• Machined threads: The most precise and durable option. MJF parts can be printed with pilot holes, then tapped or cut using a die post-printing.
• Self-tapping screws: A simple way to create a thread without extra machining, but best for applications where the thread won’t be engaged and disengaged frequently.
• Threaded inserts: The most durable solution, using metal inserts that are either press-fit, heat-installed, or ultrasonically welded into the part.

For functional, long-lasting threads, threaded inserts are almost always the best option. They provide high strength and reusability, making them ideal for assembly-intensive applications.

Post-Processing for Better Fit and Thread Performance

Because MJF leaves a fine layer of powder on the part surface, cleaning up the part after printing is key to getting good fit and thread function. Powder residue can get stuck in threaded features, holes, and mating surfaces, making assembly harder than expected.

Methods like air blasting, ultrasonic cleaning, or light sandblasting help remove loose powder and improve part quality. If additional strength is needed, applying coatings, lubricants, or dry film treatments can reduce friction and improve thread durability.

If cosmetic appearance is important, dyeing is a better option than painting since paint buildup can interfere with precise fit and thread engagement.

Why Fit and Threads Matter in Production-Grade 3D Printing

The difference between a well-designed 3D-printed part and a frustrating one often comes down to fit and thread quality. Whether you’re designing for snap-fit assemblies, threaded fasteners, or sliding mechanisms, taking the time to optimize dimensions, tolerances, and post-processing methods can significantly improve part performance and longevity.

HP Multi Jet Fusion offers incredible speed, strength, and scalability, but understanding the nuances of fit and thread design is essential for achieving high-quality results.

If you’re looking to take advantage of MJF’s full potential for precision parts, having an experienced manufacturing partner can make all the difference. At RapidMade, we specialize in 3D printing for production, prototyping, and custom part solutions.

For expert advice on designing for fit, tolerances, and threads in HP Multi Jet Fusion, trust the 3D printing service at RapidMade to see how we can help bring your designs to life!