The following HP Multi-Jet Part Design Guide has been assembled to help users of the MJF process for the manufacturing of their parts better understand how to go about designing parts to harness all the advantages while minimizing the limitations of the MJF process. For more information on the Hewlett-Packard (HP) MJF 3D Printing process check out our process page or our how it works page for this technology.
Before sending a job to a 3D printer, the model to be printed needs to be tessellated. That means that its geometry needs to be converted into triangles, which are used by the printer to create layers. It is very important to pay attention to this step: if not done correctly, it can cause problems such as inaccuracy or slow processing. Standard formats in the additive manufacturing industry include 3MF (with more information about the model) and STL. A normal file size for a model is about 1–30 MB, but the size depends on the type of software that created it, the number of triangles, the number and level of details, and so on. When exporting to STL in a CAD package, you are often required to introduce some parameters such as angle tolerance and deviation chord height. These parameters define the resolution and file size of the part. The following tips will help you to export with the best surface to file size ratio.
Too many triangles are difficult to process and, when a certain size is reached, the extra triangles do not provide any further accuracy. For this reason, an excess of triangles could increase processing time for no benefit. Triangulation of a surface causes faceting of the 3D model. When the parameters described in the last section are used to output an STL model these issues are eliminated. Also, specialized 3D printing software like Magics can be used to eliminate unneeded triangles from an STL file without effecting its accuracy or quality.
There are some specifications to bear in mind in order to avoid issues in parts, and to achieve the best quality. The minimum printable features in planes X, Y, and Z are as follows:
Multi Jet Fusion technology allows you to print letters and drawings with a very high resolution and definition. For the best possible output, any text, number, or drawing included in a part is recommended to have at least .05″ [1.27mm] of depth or height. The best orientation for embossing letters is to place them upside down in the build chamber, while for debossing letters it is better to place them face up to achieve the best resolution.
The general rule of thumb for the MJF process is +/- .0075″ [0.2mm] up to 3.93″ (100mm) and +/- 0.2% above that value. For a much more detailed breakdown of 3D printed part accuracy beyond what is detailed in this HP Multi-Jet Part Design Guide please see our page dedicated to this topic.
We have included 3 of the most popular examples / options of the internal structures that can be used with MJF parts in this HP Multi-Jet Part Design Guide:
Multi Jet Fusion allows you to print topology-optimized, generative designs or even small lattice structures. This kind of design helps to reduce the weight of the part and the quantity of material used, which not only reduces the cost of the part but also helps to reduce the operating cost in applications that are very weight-sensitive.
Parts can be printed hollow, this greatly recuses the amount of heat that builds up in the part during the printing process allowing for more accurate parts and also allows them to be lower cost (less raw nylon and fusing agent are used). One down side to consider is that due to the parts being hollow they can be a bit weaker. If the parts printed are hollow, drain holes can to be added to the design to remove the unsintered material. The minimum recommended diameter of these drain holes is .25″ [6.35mm] and a minimum of 2 of them is recommended to allow for compressed air depowdering. We can also leave powder trapped in the parts, this helps save cost on complicated depowdering / plugging of parts but if the part needs to be dyed black this is not recommended as it causes the parts to float. At F3DP we utilize a specialized software called Magics that allows us to very easily and quickly hollow and add drain holes to parts as needed. We offer this for free to our customers and can work with you to make sure you original design intent is maintained.
This option is a combination of the benefits found in hollowing a part combined with the benefits of a latus structure. It keeps the part very strong but cuts out a large volume of material saving on both cost and loss of accuracy due to high heat during the build. The only concession that needs to be made to a parts design is that one end of the honeycombed region needs to be left open so the cells can be depowdered. If this is not a option please consult with F3DP directly on your part as there may be other alternative designs we can propose to you. At F3DP we utilize a specialized software called Magics that allows us to very easily and quickly honeycomb parts as needed. We offer this for free to our customers and can work with you to make sure you original design intent is maintained. The following table will give you basic dimensions that should be used for designing honeycomb features should you choose to design them yourself:
Sometimes a pair of printed parts need to fit together for the final application. In these cases, you are recommended to have gaps of at least .015″ [0.4mm] (±.008″ [0.2 mm] of tolerance of each part) between the interface areas that should fit together, in order to ensure correct assembly.
Assembly parts that are printed together should have a minimum clearance of 0.7 mm. Parts with very thick walls above 50 mm should have a higher gap in order to ensure proper performance.
Parts larger than the maximum build size can be printed with Multi Jet Fusion by splitting them into different parts. They can then be joined together by gluing, welding, or by pin inserts. If you plan to glue parts together, you are recommended to include interlocking features such as those shown in the pictures below: as a guide to position the parts, to help them to bond together, and to facilitate the gluing process. Remember to leave an additional space of .004″ [0.1mm] – .008″ [0.2mm] between parts for the glue, in addition to the minimum spacing between parts printed as assemblies (see above). At F3DP we utilize a specialized software called Magics that allows us to very easily and quickly split parts to make glue joints as needed. We offer this for free to our customers and can work with you to make sure you original design intent is maintained.
We recommend using Bob Smith Industries Maxi-Cure™ / Insta-Set™ glue for assembling HP MJF parts.
A customer requested to know how well our standard glue joint would hold up to repeated shock loads. We devised this simple test to prove out the strength of a HP MJF glue joint as well as do a metal vs Nylon wear test at the same time. Here are the results of that testing we are including in this HP Multi-Jet Part Design Guide:
One of the most useful abilities of the HP MJF technology is its ability to print threads directly into a part. This saves the need to use a insert like a Helicoil or heat set insert. It also eliminates the need for doing secondary tapping operations after the part is printed. In our experience with production parts the use of a printed in thread can save days worth of extra shop time spent tapping holes.
When designing long (over 6″ [152mm]) and twisting (more than one 90 deg turn) passages for things like compressed air or vacuum it is recommended to keep the minimum diameter of the passage to .1875″ if at all possible. To remove material from narrow ducts, consider designing and printing a strip or a chain through the duct. When the parts have been printed, you can pull out the chain to dislodge most of the material. Any remaining material can be removed by the normal cleaning process. If duct work is small in diameter or complex in routing its recommended to consult with F3DP on its design to incorporate more advanced best practices in to the design.
Multi Jet Fusion parts are made of thermoplastic materials and can be re-melted and re-formed once printed. Heat-staking and ultrasonic installed inserts are recommended for thermoplastic materials; however, pressin, self-threading and expansion inserts may also be used in some applications. The following information is being provided in this HP Multi-Jet Part Design Guide to help you with selecting the proper insert for your application:
The hole diameter is very important to achieve the desired strength, oversized holes will result in a reduction of the joint strength and undersized holes can potentially crack the part. Multi Jet Fusion parts have a tolerance of +/- .0075″ [0.2mm] in small features and this is usually above the supplier specifications. For this reason, it is important to select a type of insert that is compliant with hole deviations and a lower performance should also be expected.
Traditionally a boss diameter is two times the external diameter of the insert for inserts under .23″ [6 mm], a .11″ [3 mm] wall thickness applies for all larger inserts. Exceptions include applications incorporating supported bosses, reinforced materials, and heat installation. Special consideration should be given to cold press installations where stress will be increased and may require larger boss diameters.
It is important that the insert bears the load, and not the plastic, to avoid jacking the insert out. The mating component hole should be smaller than the face of the insert but still allow the connecting threaded fastener sufficient space to function normally. The mating component must also withstand the stress generated by the clamping force. In instances where the mating component will also be plastic, the use of a secondary insert or collar should be considered to avoid creep.
The recommended insert for Multi Jet Fusion parts depends on the type of installation and the geometry of the insert. This is a summary of the main types of insert available for plastic parts:
Snap fits provide simple and economical way to assemble parts, often saving time in assembly. This is possible with a protruding feature on one part, which flexes during assembly into a groove or slot in the mating part. When fully seated the protruding feature returns to its initial position.
The cantilever snap-fit is the most commonly used type of snap-fit. It consists of a cantilever beam with an overhang at the end. In this type of snap-fit there is a direct relationship between the robustness of the assembly and the strength of the snap-fit.
When it is not possible to design a cantilever snap-fit without compromising the robustness of the assembly and the strength of the snap-fit due to material or geometrical constraints, an L-shaped snap-fit can be an alternative. Adding a groove to the base
of the snap-fit increases its flexibility while reducing the strain on the beam, compared with a cantilever snap-fit.
The U-shaped snap-fit is another alternative to the cantilever snap-fit when it is necessary to increase the snap-fit flexibility within a reduced space. This U-shaped alternative is extremely flexible, and thus easier to remove. This type of snap-fit is usually used in cases where the parts need to be pulled apart repeatedly or when two parts don’t require a lot of force to stay in position (e.g., in a battery compartment lid).
The annular snap-fit is an assembly method usually used between two cylindrical or ring-shaped parts or between two rotationally symmetric parts, where the deformation required to assemble or disassemble the snap-fit is made in a 360º direction at the same time.
With this assembly method, one part is designed with an undercut and the other is designed with a mating lip. The joint occurs through the interference between both parts during the assembly operation.
The torsional snap-fit is an assembly method where the flexible point is in a torsional bar instead of the self–snap-fit body. When the torsional bar is pushed down, it turns slightly and opens the joint.
The minimum recommended thickness at the base of the cantilever is 1mm (0.04in).
The minimum overhang depth (t) should be at least 1mm (0.04in)
It is recommended to add a radius at the base of the cantilever to avoid sharp corners and reduce the stress concentration. This radius should be at least half of the thickness (h) of the base of the cantilever.
It is recommended to avoid sharp edges at the end of the snap-fit overhang, adding a small chamfer to prevent breaking during the assembly operation.
As mentioned previously, the snap-fit overhang usually has a gentle chamfer to facilitate the assembly operation. The inclination of this chamfer angle (α) directly affects the mating force (P). If the angle (α) is reduced, the mating force (P) will also reduce. The
recommended assembly angle value should be between 35º and 40º.
The way the overhang is designed determines whether the snap-fit can be disassembled and reassembled several times. The disassembly angle (β) affects the ease of joint disassembly. For example, a 90º angle (β) can never be disassembled. However,
a snap-fit with a disassembly angle (β) equal to the assembly angle (α) will need the same mating force (P) for both operations.
When designing a snap-fit, there must be a gap between the protruding feature and the groove to ensure a proper performance, even including the worst tolerance case as shown in the following figure:
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