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Design Guide: MJF 3D Printing

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Multi Jet Fusion (MJF) is an additive manufacturing (AM) technology comparable to Selective Laser Sintering (SLS). It is capable of printing highly complex industrial parts and prototypes with a high level of precision and with consistent isotropic mechanical properties.

To help achieve better part quality and dimensional accuracy, it is important that your 3D model closely follows a set of geometric recommendations. This article offers an overview and considerations of the MJF process, best design practices, and weight-saving and cost-reduction design tips and tricks.

 

MJF 3D printing design guide

MJF 3D Printing Process

MJF uses an inkjet array similar to that of a color 2D printer to selectively fuse a bed of powdered material (mainly nylon) into a solid layer. The process repeats layer by layer until the final part is complete. Like SLS, MultiJet Fusion belongs to the powder bed fusion 3D printing category; hence, it does not need any support structures, allowing for greater design freedom.

 

MJF Process Considerations 

  • Shrinkage and Warping: MJF components are susceptible to shrinkage and warping due to the high temperature experienced during printing.
  • Surface Considerations: MJF prints naturally have rough surfaces. With overly thick designs, there is a higher likelihood that a part will improperly cure and result in an uneven pockmarked surface known as “elephant skin.”
  • Powder Removal: Excess powder on or inside the part have to be accounted for and cleaned off. Escape holes are needed to remove the unsintered powder from the inner sections of the component.

 

Characteristics of MJF 3D Printing

Maximum Build Size 380 x 285 x 380mm
Resolution ±0.3mm
Dimensional Accuracy ±0.3% (with a lower limit of ±0.3 mm)
Materials PA11, PA12
Surface Structure Rough
Support Not required

MJF Design Guidelines

Wall Thickness

MJF walls that are designed too thin are at high risk of warping. We recommend a wall thickness of at least 1mm. Avoid overly thick walls, as they can lead to an uneven pockmarked surface.

 

Designing unsupported walls for 3D printing

Assembly Parts

Adequate clearances must be designed to prevent the assembly from fusing together into a single solid part. To avoid this, we recommend leaving a minimum gap of 0.9 mm between mating parts.

Flat Surfaces

Large, flat plains are susceptible to warping. To avoid this, ribs can be added to crucial features, however, this may not always solve the problem. Hence, it is recommended to such features whenever possible. Consider replacing solid-volume areas with lattice structures or designing short planes with sufficient part thickness (at least 0.3mm).

Lattice Structures

Adequate clearance is needed in lattice structure designs to enable easy removal of residual material inside the part. The recommended minimum gap size in a lattice structure is 1 mm.

Embossed Features

Embossed features must be designed using a minimum height to ensure visibility and to prevent the detail from wearing off during post-processing. We recommend a minimum height of 0.5mm. To ensure such details come out nicely, make them larger than the indicated.

 

Engraved Features

With MJF, text and engraved details are at risk of closing up and won’t appear visible if not designed with a minimum depth and width. We recommend a minimum depth of 1mm and width of 1mm.

 

How to design engraved text and details

Summary of MJF Best Design Practices

Wall Thickness At least 1mm thick; avoid overly thick walls
Assembly Parts Minimum gap of 0.9mm
Flat Surfaces At least 0.3mm thick; avoid large, flat plains
Lattice Structures Minimum gap of 1mm
Embossed Features Minimum height of 0.5mm
Engraved Features Minimum depth of 1mm; minimum width of 1mm

MJF Cost Reduction Tips

It is worth noting that SLS is often a better option than MJF for more solid parts; however, for lower density parts, MJF generally costs less. This section outlines weight-saving solutions/designs that can help reduce the overall cost of your MJF part.

 

  • Hollow Out Your Part — Hollowing out the center of your designs can help reduce the amount of material used and overall weight. Don’t forget to add escape holes to allow for powder removal.
  • Replace Solid Parts With Lattice StructuresWell-designed lattice structures can help reduce the weight of your part significantly whilst retaining structural integrity. Remember to leave adequate gap space.

Getting Your Parts Printed

Use this 3D Printing Design Guidelines when designing your MJF parts for machining, then export your 3D CAD files in an STL format. Once ready, upload your file here to get a quote.

If you are unsure if MJF is the right solution for your project, contact us at info@hlhrapid.com. Our team of 3D printing experts is always available to provide on-hand support and recommendations on the most suitable process.

 

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