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Written by Perry Cain
The majority of FDM 3D printed parts are not printed solid. Printing solid parts requires high amounts of material and long print time resulting in high costs. To optimize the printing process most parts are printed with solid shells and filled with infill. Shells and infill play an important role on the quality, appearance and function of FDM printed parts.
This article will discuss the difference between shells and infill and how shells and infill can be employed to optimize a design.
A standard FDM print can be broken down into 4 sections. The parameters of these sections can each be altered to optimize a design:
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Shells are the number of layers on the outside of a print. For FDM shells are always the first areas to be printed per layer. Several shell related design considerations for FDM printing are:
Most shells are printed with a thickness of 2 nozzle diameters (usually this corresponds to 0.8mm).
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Like most wooden doors are not solid but have a low density core, FDM prints are typically printed with a low density infill. Most FDM slicer programs will by default print parts with a 18% - 20% infill which is perfectly adequate for the majority of 3D printing applications. This also allows for faster and more affordable prints.
The strength of a design is directly related to infill percentage. A part with 50% infill compared to 25% is typically 25% stronger while a shift from 50% to 75% increases part strength by around 10%.
Understanding the application of a final printed part allows a designer to specify the optimal infill percentage. A prototype where form is important can be printed with very low infill saving significantly on cost and time whereas a bracket that will experience loading will need a higher infill percentage. As mentioned above, the standard 20% that most printers use as a default should be acceptable for most application and any deviations from this should be discussed with your 3D printing service provider.
Infill also plays a critical role in how extended/protruding features are connected to a model. Snap-fit connections are a good example of this. The base of a snap-fit connection is often a weak point. With a low infill density (20%) the cantilever is much more likely to break as the short extruded clip is only connected to the body of print by a small cross-sectional area. Increasing infill percentage means that more of the body of the print is connected to the cantilever increasing the strength of this connection.
Where a 3D printed part is going to be drilled or screwed infill percentage becomes a very important factor. Consider a print with low infill that is going to be drilled and then screwed to another surface. Often the drill holes will go through the top and bottom layers of the print and miss the infill of the model creating a very weak connection. For these applications, a higher infill is desirable (50% minimum).
Connections that utilize clearance holes and bolts are better suited to parts with a low infill percentage. The shells, walls, and infill offer good compressive strength providing better anchoring of the part.
For a standard print, infill is simply printed as an angled hatch or a honeycomb shape. The four most common infill shapes are:
Infill geometry | Description |
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Rectangular - Standard infill pattern for FDM prints. Has strength in all directions and is reasonably fast to print. Requires the printer to do the least amount of bridging across the infill pattern. |
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Triangular or diagonal - Used when strength is needed in the direction of the walls. Triangles take a little longer to print. |
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Wiggle - Allows the model to be soft, to twist, or to compress. Can be a good choice particularly with a soft rubbery material or softer nylon. |
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Honeycomb - Popular infill. It is quick to print and is very strong, providing strength in all directions. |
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