HP Nylon Multi-Jet Fusion (MJF) Design Guide: Essential Tips for Optimal 3D Prints

HP Nylon Multi-Jet Fusion (MJF) technology has revolutionized industrial 3D printing by combining speed, precisione, and material efficiency. This advanced powder-based additive manufacturing process creates functional nylon parts with mechanical properties comparable to injection-molded components, but with far greater design freedom. Tuttavia, to fully leverage MJF’s capabilities, designers must understand its unique requirements and constraints. This comprehensive guide will walk you through essential design tips, best practices, and technical specifications to ensure your MJF parts meet performance, quality, and cost targets.

Understanding HP Multi-Jet Fusion Technology

Before diving into design specifics, it’s crucial to grasp how MJF technology works and what makes it different from other 3D printing processes. Unlike selective laser sintering (SLS) that uses lasers to fuse powder particles, MJF employs inkjet nozzles to selectively deposit fusing and detailing agents across a bed of nylon powder. A heating element then passes over the bed, causing the treated areas to melt and fuse together layer by layer.

This approach offers several key advantages:

• Faster print speeds: Continuous heating enables quicker production than laser-based systems
• Superior part consistency: Uniform heat distribution reduces variability across the build platform
• No support structures: Parts can be nested and printed without additional support material
• Eccellenti proprietà meccaniche: MJF parts achieve tensile strengths and impact resistance comparable to injection-molded nylon
• Complex geometry capabilities: Internal channels, Strutture reticolari, and assembled components can be printed in a single build

These benefits make MJF ideal for functional prototypes, low-to-medium volume production runs, custom tooling, and complex assemblies that would be impossible or cost-prohibitive with traditional manufacturing methods.

Size and Volume Considerations

MJF’s build volume and dimensional capabilities define the basic parameters for your design. Understanding these constraints early in the design process prevents costly redesigns later.

Maximum Build Volume

The standard build volume for HP Jet Fusion systems is 380 x 380 x 284 mm, but HP recommends staying within a 356 x 280 x 356 mm working volume for optimal results. This buffer accounts for potential edge effects and ensures consistent part quality across the entire build platform.

For parts exceeding these dimensions, considerare:

• Part splitting: Dividing large components into smaller sections that can be assembled post-printing
• Mechanical joints: Designing interlocking features like dovetail joints or snap fits for assembly
• Adhesive bonding: Creating bonding surfaces optimized for structural adhesives

Minimum Feature Sizes

MJF can produce surprisingly small features, but designers must adhere to minimum size requirements to ensure printability and functionality:

When designing features near these minimums, consider orienting critical features in the XY plane where dimensional accuracy is highest. Vertical features (Asse Z.) typically require slightly larger dimensions to achieve the same level of precision.

Geometry and Structural Design

MJF parts are susceptible to warping and distortion caused by thermal gradients during cooling. Strategic geometry design minimizes these issues while optimizing material usage and part performance.

Wall Thickness Guidelines

Proper wall thickness balances structural integrity with material efficiency and print quality. While MJF can produce walls as thin as 0.3mm for short features in the XY plane, most designs benefit from thicker walls:

• Recommended minimum wall thickness: 1.5mm for general use
• Maximum wall thickness: Avoid walls thicker than 3mm where possible, as excessive thickness can cause:
• Longer cooling times leading to internal stresses
• Surface defects like sink marks
• Increased material usage and cost

For parts requiring thicker sections, incorporate:

• Internal fillets: Smooth transitions between thick and thin sections
• Core-outs: Removing internal material to create hollow structures
• Ribs: Adding reinforcing ribs instead of increasing overall wall thickness

Cantilever Structures

Cantilevered features (overhangs without support) require special consideration:

• For cantilevers with width < 1mm: Maintain an aspect ratio (length/width) < 1
• For cantilevers with width ≥ 1mm: No strict aspect ratio limit, but consider reinforcement
• Recommended reinforcement: Add ribs or fillets at the base of long cantilevers
• Minimum thickness at base: 1mm to prevent stress concentration and failure

Reducing Warpage in Thin/Long Parts

Parts with high aspect ratios (length vs. larghezza) are prone to warping due to uneven cooling. This is particularly problematic for:

• Parts with aspect ratios > 10:1
• Features with abrupt cross-section changes
• Lungo, thin curved segments

To minimize distortion:

1. Increase thickness of long walls to reduce aspect ratio
2. Avoid ridges and ribs on large flat surfaces
3. Smooth transitions between different cross-sections
4. Implement lightweighting with internal lattices or hollow structures
5. Add strategic fillets to distribute stresses evenly

Lightweighting Strategies

MJF excels at producing complex internal structures that reduce weight without sacrificing strength:

Lattice Structures

Replacing solid volumes with lattice patterns offers multiple benefits:

• 30-70% weight reduction
• Improved thermal insulation
• Enhanced shock absorption
• Reduced material usage and cost
• Faster cooling during printing (reducing warpage)

For effective lattice design:

• Maintain minimum gap sizes of 1mm between lattice struts for powder removal
• Use uniform cell sizes for consistent mechanical properties
• Consider gyroid or honeycomb patterns for optimal strength-to-weight ratio

Hollow Structures

Enclosing hollow spaces within solid walls:

• Reduces material consumption
• Lowers part weight
• Improves print stability by reducing thermal mass
• Requires minimum 2mm wall thickness for structural integrity
• Must include drain holes (minimum 5mm diameter) for powder removal, ideally placed on opposite faces

Topology Optimization

Using software tools to algorithmically distribute material only where needed:

• Creates organic, efficient shapes based on load requirements
• Typically reduces weight by 40-60%
• Maintains structural performance where it matters most
• Requires careful consideration of print orientation and support needs

Dimensional Accuracy and Tolerances

Understanding MJF’s dimensional capabilities ensures parts fit together properly and meet functional requirements without excessive post-processing.

General Tolerance Guidelines

HP MJF technology achieves impressive accuracy, with typical tolerances of:

• ±0.2mm for parts up to 100mm in size
• 0.2% of dimension for parts larger than 100mm
• These tolerances apply after standard sandblasting post-processing

For reference, this places MJF in the IT Grade 13 range according to ISO 286 standards, comparable to injection molding for many applications.

Assembly Tolerances

When designing mating parts, account for these tolerance ranges by incorporating appropriate clearances:

• Minimum clearance for assembled parts: 0.4mm total (0.2mm per part)
• Co-printed assemblies (printed together in one build): Minimum 0.7mm clearance
• Parti in movimento: Increase clearance to 0.7mm minimum to prevent binding
• Parts with walls >30mm thick: Additional 0.3mm clearance recommended

Achieving Tighter Tolerances

For features requiring tighter tolerances than standard MJF capabilities:

1. Design features slightly oversized to allow post-processing
2. Identify critical dimensions that require machining after printing
3. Include pre-machining features like pilot holes to guide machining operations
4. Considerare threaded inserts instead of printed threads for precision connections

Surface Finish and Aesthetics

MJF produces parts with a naturally matte, slightly grainy surface finish (in genere 125-250 microinches RA). While this is sufficient for many functional applications, designers can influence surface quality through strategic design choices.

Layer Line Visibility

Like all layer-based 3D printing processes, MJF parts exhibit some layer lines (often called “stair-stepping”). With MJF’s 80-micron layer height, these lines are less noticeable than in FDM printing but still require consideration:

• Optimize angles: Surfaces facing upward should maintain angles >20° from horizontal to minimize visible stepping
• Downward-facing surfaces: These typically show fewer layer lines and can be used for cosmetic features
• Avoid shallow angles: Surfaces with angles <5-10° from horizontal will show more prominent layer lines

Embossed and Engraved Details

MJF can produce high-resolution text and graphics with proper design:

• Minimum depth/height: 1mm for all embossed or engraved details
• Optimal orientation: Embossed features achieve best resolution when oriented upside down
• Engraving orientation: Engraved details should face upward during printing
• Font selection: Use sans-serif fonts with thick strokes for best readability
• Avoid fine details: Lines thinner than 0.5mm may not resolve clearly

Post-Processing Considerations

Design with post-processing in mind if enhanced surface finish is required:

• Sabbiatura: Standard for removing loose powder, can also smooth surfaces
• Vapor smoothing: Creates glossy surfaces but may reduce dimensional accuracy
• Pittura: Requires proper surface preparation; design for easy masking if needed
• Tintura: MJF parts accept dye well, especially black; design for uniform color penetration

Assembly Design for MJF

One of MJF’s greatest strengths is its ability to produce functional assemblies, either as co-printed components or separate parts designed to fit together perfectly.

Co-Printed Assemblies

Printing multiple components in a single build with pre-designed clearances allows for immediate functionality:

• Minimum clearance: 0.7mm between moving parts
• Increased clearance needed for:
• Parts with large contact surfaces
• Components with thick cross-sections (>3mm)
• Assemblies requiring frequent movement

Snap-Fit Connections

Snap fits create strong, reusable assemblies without additional fasteners:

Design Guidelines

• Cantilever snap fits: Most common type, consisting of a flexible beam with an overhang
• Minimum beam thickness at base: 1mm
• Minimum overhang depth: 1mm
• Recommended radius at base: 0.5 x beam thickness to reduce stress concentration
• Assembly angle: 35-40° for easy insertion
• Disassembly angle: Match assembly angle for reversible connections
• L-shaped snap fits: Provide more flexibility than cantilevers in limited space
• Include groove at base to increase flexibility
• Reduce strain compared to straight cantilevers
• U-shaped snap fits: Highest flexibility option
• Ideal for parts requiring frequent disassembly
• Occupies more space but requires less insertion force

Material Considerations

HP’s MJF materials, particularly PA 12, offer good elongation at break (in genere 10-15%), making them suitable for snap fits. To ensure durability:

• Keep maximum strain below 5% for repeated use
• Use larger radii to distribute stress
• Consider tapered beams for more uniform strain distribution

Threaded Connections

While MJF can print threads directly, they have limitations:

• Minimum recommended thread size: M6 (or ¼ inch) for reliable printed threads
• Smaller threads: Use self-tapping screws or threaded inserts instead
• Thread design tips:
• Use less restrictive tolerances for internal threads
• Employ tighter tolerances for external threads
• Add 0.2-0.4mm gap between mating threads
• Remove sharp edges with 0.1mm minimum radius

Threaded Inserts

For applications requiring strong, reusable threads:

• Recommended insert types: Heat-staking or ultrasonic inserts work best with MJF nylon
• Hole diameter: Follow manufacturer recommendations, typically with +0.1mm tolerance for MJF parts
• Boss design: Diameter should be 2x insert diameter for inserts <6mm
• Spessore murale minimo: 3mm for inserts larger than 6mm
• Hole depth: 1.5x insert length to ensure proper installation

Adhesive Bonding

When joining larger components:

• Design for bonding area: Increase surface area with textures or grooves
• Alignment features: Include locators like pins and holes for precise positioning
• Bond line clearance: Add 0.1-0.2mm gap for adhesive coverage
• Joint design:
• Use jigsaw or dove-tail joints for thickness <1.7mm
• Implement overlap joints for thickness >1.7mm
• Avoid straight butt joints which offer minimal strength

Part Orientation Strategies

Strategic orientation of parts within the build chamber significantly impacts quality, forza, and surface finish.

Key Orientation Considerations

• Precisione: Features requiring tight tolerances should be placed in the XY plane
• Forza: Mechanical properties are slightly higher in the XY plane than Z direction
• Finitura superficiale: Downward-facing surfaces generally have smoother finishes
• Deformazione: Long flat surfaces should be aligned horizontally to minimize distortion
• Load paths: Orient parts so primary loads align with the XY plane for maximum strength
• Post-elaborazione: Position hard-to-clean features for easy powder removal

Orientation for Specific Features

• Snap fits: Orient cantilever beams in XY plane for best dimensional accuracy
• Threads: Place critical threads horizontally where possible
• Cosmetic surfaces: Position visible surfaces facing downward
• Load-bearing features: Align with XY plane for optimal strength
• Thin walls: Orient vertically to reduce warping

File Preparation and Export Guidelines

Proper file preparation ensures your design translates accurately to the printing process.

Recommended File Formats

While STL files are universally compatible, HP recommends 3MF format for MJF printing due to:

• Smaller file sizes for equivalent detail
• Better preservation of design intent
• Support for additional data like materials and colors
• Reduced likelihood of mesh errors

Tessellation Parameters

When exporting from CAD software:

• Deviation chord height: 0.05mm for optimal surface accuracy
• Angle tolerance: 1 degree between adjacent triangles
• Avoid excessive triangles: More than necessary increases file size without benefit
• Minimum triangle size: 0.05mm to prevent processing issues

Common File Errors to Avoid

• Non-manifold geometry: Ensure all edges are shared by exactly two faces
• Holes and gaps: Repair before printing to prevent incomplete fusion
• Overlapping triangles: Causes confusion in slicing software
• Inverted normals: Can lead to missing surfaces in the final print
• Insufficient detail: Too few triangles create faceted surfaces

Software tools like Materialise Magics, Autodesk Netfabb, or HP SmartStream 3D Build Manager can help identify and repair these issues before printing.

La prospettiva della tecnologia Yigu

Alla tecnologia Yigu, we recognize HP MJF as a transformative 3D printing technology for functional parts production. The key to successful MJF implementation lies in designing specifically for the process—leveraging its strengths in complex geometries while accounting for its unique constraints. By integrating lightweighting strategies, optimizing part orientation, and following assembly design best practices, manufacturers can unlock MJF’s full potential for cost-effective, high-quality production. We recommend early collaboration between design and manufacturing teams to maximize the benefits of this innovative technology.

Domande frequenti

Q1: What is the minimum wall thickness for MJF parts?

A1: For short walls in the XY plane, 0.3mm is possible, but 1.5mm is recommended for most applications. Vertical walls (Z direction) need minimum 0.5mm thickness, with 1.5mm preferred for structural integrity.

Q2: Can MJF parts be used for functional applications requiring strength?

A2: SÌ, MJF parts made from PA 12 offer excellent mechanical properties comparable to injection-molded nylon, with tensile strengths around 48 MPa and elongation at break of 10-15%, making them suitable for many functional applications.

Q3: How do I design parts that require tight tolerances beyond MJF’s capabilities?

A3: Design critical features slightly oversized to allow post-processing like machining. Include pilot holes to guide machining operations, and consider using threaded inserts instead of relying on printed threads for precision connections.