3D Printing is Not a Cheat Code

Engineers and designers are always interested in new technologies that can help them produce parts that fit their function. When a new technology arrives on the scene, there are often years of trial and error approaches as guiding principles are developed. Over time, certain manufacturing technologies have nearly dogmatic principles known as best practices. These practices are taught in academia and reinforced in real world applications.

For example, with CNC machining we typically look at internal holes using hole depth-to-diameter of no more than 8 without special tooling requirements. This means that a 0.25” hole really should not be more than 8X its diameter deep (2”). Even techniques like making corner radii a common size, and larger than the aforementioned hole ratio, will significantly reduce cost and increase the reproducibility of your part.

Similarly, for other tried-and-true manufacturing technology you see common rules that, when followed, significantly decrease the chances of your design needing modification for manufacturability. Injection mold has its draft angles and even wall thickness requirements. Sheet metal has guidelines for bend radius to material gauge of >1:1, as well as bending best practices like keeping the bends in one direction whenever possible or keeping a minimum distance from a hole feature to the bend.

With the onset of 3D printing, referred to as additive manufacturing, there has been a Wild West mentality of “shoot first, ask questions later” on printed parts. New users of the technology are sometimes surprised at the part’s surface, unexpected dimensions outside of tolerances, or critical features that are too fragile to function. This does not always lead to the desired outcome of the designer and can leave a bad impression of the technology.

Additive manufactured parts are grown from the bottom-up versus cut down from stock, so their surface roughness depends the material used. For example, a selective laser sintered nylon piece is grown in a heated powder bed of nylon by a laser etching the cross section of a part before a new layer of material is deposited. As the laser fuses the powder the edges gain a slightly grainy texture, like a sugar cube. Other processes, like fused deposition modeling, use a heated filament to zig-zag back and forth building a part from bottom to the top. Because the base process is this filament you see more significant layer effects, especially on gradual curves where the layer-by-layer splits look like stair steps.

If a cosmetic face is the top requirement for the part as designed then additive manufacturing may not be the best fit. Getting an injection-mold-like surface finish takes significant manual finishing or machining. Some processes do have better finishes than others though. If a smooth matte finish is acceptable, UV-cured photopolymer processes like stereolithography, polyjet, HP multi-jet fusion, or Carbon DLS could be a great alternative. These materials are smoother because their base-material is a liquid resin which shows less layering than materials like thermoplastics. What additive can do well is create the shape of the part in some good engineering-grade materials.

A best practice for a designer working on a component in additive manufacturing would be to offset the CAD model to predict the outcome of the grown part. Depending on the process, features can be very dependent on other part characteristics. For example, in laser sintering you can see small gaps and holes close up. Sometimes standard sized holes are drilled open in post processing but “if the hole is irregular, or has a keyway, the general recommendation is to offset the interior surface by 0.006”. This means that a keyway will be 0.012” wider (0.006” per side) but when printed the hole will be near nominal. General tolerances can be applied as well. For example, here are some common process tolerances:

• Selective Laser Sintering (SLS): +/- 0.005” or +/- 0.002” per inch, whichever is greater.
• Fused Deposition Modeling (FDM): +/- 0.004” or +/- 0.002” per inch, whichever is greater.
• Polyjet: +/- 0.004”, or +/- 0.001” per inch, whichever is greater.
• Metal laser sintering (DMLS, SLM): +/- 0.001” - 0.006” is typical for positive features, and +/- 0.004” - 0.006” for negative features such as holes.

Build size in additive can also play a factor on quality. Growing to the maximum machine size can lead to warped parts. Thicker geometries, large broad parts, and parts with uneven wall thicknesses may deviate due to thermal shrinkage and stress. This can look like warping or twisting. Even wall thicknesses and similar design guidelines as injection molded parts are encouraged.

Some processes that are great for building large parts may have issues on smaller details. FDM works well for mechanical geometries, larger parts, and parts which may not require ultra-fine features. Very narrow pegs and lips may not resolve due to the bead thickness deposited. Minimum feature size, including text features, to be at least 0.035”.

Some additive manufacturing processes, such as metal laser sintering, require support structures. These are disposable grown materials which are designed to hold an overhanging surface to a flat build plate. For metal parts, these could be fine metal tines or a solid mass of metal. Because these features are removed prior to delivering to the engineer they may have an inconsistent look to the rest of the piece. From a design standpoint, critical features should be designed in a way where they can grow with no or minimum support needs. This will give a much more predictable outcome to the print.

Orientation may play a role on strength of a part. For design considerations, small feature like pegs and clips should be planar, meaning they can all be oriented as close as possible to one direction. This allows the machine setup operator to orient the part so those features get the best orientation for strength. Features built horizontally will be stronger than features built vertically in most processes. Designing parts that will be vertical to a thicker cross section, or giving supporting ribs can also compensate for orientation differences.