While there has been plenty of buzz around the HP Multi-Jet technology, little has been written about its nuances and design considerations. Here are key points to know to create optimal designs.

–Chuck Alexander, director of product management, Stratasys Direct Manufacturing

HP Multi Jet Fusion technology is the latest 3D printing innovation to make headlines. It will continue to strengthen 3D printing’s place in manufacturing. And it offers new possibilities for low-volume production parts and functional prototyping due to its breakthrough speeds and fine feature resolution.

Multi Jet Fusion functions like other 3D printing processes by building parts layer-by-layer, but it adds in infrared heating alongside fusing and detailing agents to build high-strength, nearly-isotropic parts. While there has been plenty of buzz around this breakthrough technology, little has been written about the nuances and design considerations. It’s still a fairly new technology in the additive manufacturing industry and a baby when compared to veteran technologies like stereolithography and fused deposition modeling (FDM).

Thus, Stratasys Direct Manufacturing spent months studying and fine-tuning the Multi Jet Fusion process before launching it to understand the technology and what can impact the mechanical performance of a design.

How Multi Jet Fusion works

It’s important to understand how Multi Jet Fusion works in order to design for the process. It begins with a layer of powdered material applied from top-to-bottom on the build platform. The machine applies droplets of fusing and detailing agents along with thermal energy across the powdered material from left to right to fuse the layer. At the end of the scans, supply bins refill the recoater with fresh material. After finishing each layer, the build platform retracts and the material recoater begins again. After the print is finished, the build unit with the material and parts are rolled onto a processing station for cooling and powder excavation. Leftover powder is recycled for use in future builds.

Multi Jet Fusion parts get their high strength and surface quality from the fusing, detailing, and transforming agents unique to the process. The fusing agent is selectively printed where particles will be fused together, whereas the detailing agent is selectively printed to reduce or amplify the fusing agent.    Transforming agents regulate the interaction of the fusing and detailing agents with each other and the material to control part attributes at the voxel level. For the HP printers, a voxel is an individually addressable volume element; designers can control the properties of each voxel in an additive build. The attributes that can be controlled at this level include:

–Dimensional accuracy and detail

–Surface roughness, texture and friction coefficient

–Tensile strength, elasticity, hardness and other material properties

–Electrical and thermal conductivity

This distinct process results in many unique benefits, including:

–Highly detailed features – Multi Jet Fusion can produce a 0.02 in. fine feature resolution. This opens up the opportunity to create small intricacies and complex design features, such as embossed text, small holes and living hinges.

–Low cost batch manufacturing – Multi Jet Fusion is one of the fastest 3D printing systems on the market. The speed coupled with its large build envelope makes Multi Jet Fusion suitable for high-volume batches of small parts to achieve low per-unit pricing. This capability is especially beneficial when working with an additive manufacturing service bureau that can batch multiple orders.

Design considerations

As with any manufacturing process, Multi Jet Fusion has its own set of limitations and considerations to keep in mind when designing for the build process. Stratasys Direct took months to validate Multi Jet Fusion to understand the necessary quality systems and material handling procedures, as well as to verify mechanical properties and dimensional performance criteria to ensure repeatability and consistency when meeting specifications. The team learned how to properly design for the process to optimize parts for speed and accuracy. In many ways designing for Multi Jet Fusion is similar to designing for other powder bed fusion processes, however the process is distinctly unique in the following aspects:

–Fine feature resolution – Multi Jet Fusion parts have a fine feature resolution of 0.02 in. Anything smaller will print, but it may not be fully dense or meet specified material properties.

–Materials – Today, HP 3D High Reusability PA 12 is the only material available, but additional materials are in development, like PA 11.

–Color – Most Multi Jet Fusion parts are built in a shade of black or grey due to the black fusing agent. However, parts can be painted or texturized with color and there are new machines that can build in full color with transforming agents.

–Surface finish – The average surface finish of Multi Jet Fusion parts is 125 to 250 microinches RA. Surfaces can also be hand-sanded or tumbled for a smoother finish.

–Part size – The build envelope for the Multi Jet Fusion machine is 16 in. x 12 in. x 16 in. Stratasys Direct recommends a maximum part size of 14.96 in. x 11.25 in. x 14.96 in. to add a buffer around parts for the printing agents.

Beyond the differences between Multi Jet Fusion technology and other powder bed fusion processes, there are three main design and mechanical limitations to keep in mind when designing for the process.

–Wall thickness – Nylons, like any thermoplastic, shrink as they solidify. Very thick walls can accumulate heat and cause spot shrinkage in dense areas with an accumulation of material, resulting in geometric deformations. Therefore, walls should be at least 0.02 in. to 0.12 in. (0.5 to 3.0 mm). Thinner walls are possible, but may contain inaccuracies and deformation due to non-uniform in-process shrinkage. For parts with high aspect ratio, Stratasys Direct recommends to either increase the wall thickness or add ribs or fillets to reinforce the part.

–Orientation – Parts built with Multi Jet Fusion have higher isotropic characteristics than any other 3D printing technology, meaning they’re nearly as strong in the Z orientation as they are in the XY orientation. However, there are a few orientation adjustments to make when the application requires a part with a high level of detail, accuracy or strength. If your application requires a smooth surface finish, place visible features in the XY plane rather than the Z direction to avoid a stair-stepping effect. Also, place parts face down toward the build platform for a smoother surface finish on that side. Lastly, position pins and clips horizontally whenever possible.

–Dimensional accuracy and minimum feature size – Typical tolerances for Multi Jet Fusion parts are ± 0.010 in. (0.25 mm) or ± 0.001 in./in. (0.025 mm/25.4 mm), whichever is greater. Stratasys Direct has achieved tighter tolerances, but it depends on the overall dimensions and design. The minimum practical feature size for Multi Jet Fusion is also 0.02 in. (0.5 mm), including hole diameters, shaft diameters and feature clearances. The minimum printable font size for embossed or debossed lettering is 6-point.

In addition, there are special considerations for specific design features, such as bosses, holes, inserts, joints, living hinges, ribs, gussets, fillets, bulkheads and snap latches.

Multi Jet Fusion’s ability to deliver repeatable, isotropic mechanical properties, combined with its speed and low cost per unit make it a versatile technology for a variety of functional applications. The first step in taking advantage of this remarkable technology is learning how to design for it.

Stratasys Direct Manufacturing
www.stratasysdirect.com