Designing with Shells and Infill

Designing with Shells and Infill

Best Practice

p/n 33-D276 Rev A

The MJP melt away support technology allows a part to be hollowed and filled with a complex lattice pattern. These type of patterns are difficult to create in CAD but are easily done with the Shells and Lattice Infill feature built into the 3D Sprint software that comes with the printer. The lattice structures require internal supports for feature creation, but with MJP these are simply melted and drained out. Additionally, the implementation by 3D Systems requires no additional mesh triangle or memory overhead and slices quickly with no added time. The Shells and Lattice infill function offered in 3D Sprint works on the imported 3D file and is able to quickly and easily hollow out a solid part into a user selectable shell thickness and infill the part within the shell with a given lattice pattern and density. The tool also adds vent holes to allow the melted support to be removed. This novel and efficient implementation allows structures to be created down to the voxel level which would be impossible with traditional methods.

This capability can reduce cost and/or weight of printed parts typically with no impact on the visual or dimensional accuracy of the part. The combination of the thin wall and sparse lattice structure results in a lightweight but rigid part with high strength-to-weight. Additionally, the support material costs less than the build material and so using the shells and infill capability in 3D Sprint allows for a substantial part cost reduction.
The exact amount saved is dependent on the shell thickness and the percent acrylate used in the infill pattern, and is typically in the range of 40-70% weight reduction and 20-35% cost reduction.
The Shells and Lattice Infill capability was introduced in 3D Sprint 2.7 and is very simple to use. The workflow is very easy and involves only a few steps.

1. Import your design into 3D Sprint.
2. Select Infill -> Offset, and input your desired wall thickness.
   Wall Thickness: the thickness of the part’s shell.
3. If you would like to change the pattern type, select Infill -> Lattice and apply the new pattern.

     a. Pattern Type: The unit geometry of the pattern.
     b. Cell Size: The size of the cell patterned throughout the core of your part. For example, if using the “star” pattern, this is the size of each individual star in the lattice pattern.
     c. Thickness: The diameter of each strut within the patterned cell.
     d. The Fill Ratio is the percent of build material/air that will exist in the core of your final part.

4. Select Infill -> Vent Drain to add vent and drain holes to the part. This allows removal of the internal support material, reducing weight and improving part quality. Typically, two or more small holes are required to let the melted support material drain.

The different patterns available combined with the various cell sizes and rod thicknesses allows for many different patterns to be generated.

While the tool is simple, and the outcome robust and precise, it can often be difficult for a user to identify the exact shell thickness and type and sizing of the infill pattern for a given application. Typically, this will require some iterative testing by the user based on their application. There are some basic guidelines that can help and a few diagnostic parts were created by 3D Systems that can be printed and used as a visual tool in the pattern type and selection. In terms of the shell, it is recommended to avoid any thickness less than 0.5 mm (0.0197 in). Thinner walls are allowed by the software and can work, but may not fill completely for some geometries or might be sensitive to cracking at edges and/or in areas with sharp corners which creates stress concentrations. Anything thicker than 0.5mm should work well. Of course, if the thickness is set too thick there will be no infill created because the entire part will be shell. Similarly, if the infill rod is set to thick, or the cell size set to small, the pattern created will simply be a solid build material infill which is no different than a solid part without any infill. Additionally, there are infill patterns which entrap the support material within the structure making it impossible to drain out. This will achieve a cost reduction, but will be incapable of reducing weight. In the software, the infill portion is colored based on the type of pattern and the density for visualization purposes.
This allows the user to see the infill areas relative to the shell prior to printing and will help in selecting and/or validating the correct parameters for a given application.

Both the infill and the shell can be individually turned on and off by selecting the blue “eye” button to the left of the respective part in the part list dialog box.

It can be difficult to select the correct infill type and wall/cell sizes for a given application. To further assist in the selection of the correct infill type and wall and cell sizes, a sample of the cell is shown on the build plate next to the part. This helps the user visualize the final structure that will be created. This small cell can be seen in the lower center of the images shown above. Of course, the calculated “Fill Ratio” provided by 3D Sprint is also very valuable for sizing. For example, a lower fill ratio (lower amount of build material) will result in weaker parts that are less expensive to print. Higher fill ratio corresponds to more build material and will be heavier, stronger and more expensive. For additional help, 3D Systems created a set of diagnostic parts that the user can print to further facilitate the pattern selection. There are a total of 9x parts, 3x each for each of the three infill types (Jacks, 3D Grid and Star). There are a total of 36 different infills represented on each parts for a total of 108 (3x36) patterns per lattice type. While this might seem like a lot, the printer has amazing fidelity capability and is able to create microscopic structures requiring a loupe for visualization up to very large scale structures for engineering and aeronautic applications, for example. Additionally, some patterns overlap sufficiently such that the support is trapped within an acrylate shell within the structure. This entrapped support can be used for purely visual aesthetic purposes, for cost reduction, or to create surface texture (as the support is able to be removed from the surface, but not internally). Of the three parts in the diagnostic, one is for smaller patterns, a second for medium size patterns, and a third for the largest patterns. The patterns increase monotonically in both cell size and thickness horizontally and vertically for easy identification and selection. The parts are available for download. Pictures with respective cell size, thickness size, and resulting build/air infill ratio are shown here and can typically help the user select the appropriate pattern for a given application even without printing our the physical part.

It is possible to create infill patterns so small, that a “new” material can be formed that does not melt
during the post processing. For example, a clear material can be turned opaque white

There are a few customer uses for this type of pattern. This capability can be used to create a visual model
with a few different shades. For example, to make shaft a different color compared to a gear for visualization.
It is possible to create 3-5 visually different shades with these small structures. A second use is to create a
better aesthetic for internal entrapped structures or marking. The mix of build and support create a more
uniform entrapped structure compared to support only. Entrapped support can sometimes have a visual
mottle pattern due to entrapped air and material shrinkage. Adding the mixture of build and support creates
a smooth white structure without any defects or mottle.
It is not necessary to create a shell in order to use the lattice functionality. The part can be defined in
total as a lattice and the resulting entire part will be converted to a lattice structure all the way out to the
edge of the part. This has many aesthetic applications as well as functional capabilities. For example, to
create functional capillary fluidic systems where surface tension of the fluid drives the flow due to the very
small cell size.

Often metal inserts, thread forming or cutting screws or dowel pins are used for assembly purposes in parts and fixtures. Typically, the user would add a simple cut hole into the part for these purposes. However, the shells and lattice infill capability will add a wall thickness to this hole in the part with thickness driven by the shell thickness. If a thin shell thickness is selected it may not provide sufficient holding strength or the increased hoop stress from the thin shell may result in a failure.

If a different wall thickness is desired for these type of fasteners, the user should create an assembly of parts or a single part that contains separate bodies. Both these workflows allow the user to select only certain sections for shells and infill, while leaving other sections solid.

The 3D Sprint tool is simple, flexible, and powerful, but for some specific needs CAD can offer a better infill/pattern solution. For example, to create a stiff, but light tube (say, for a structural member of an airfoil or a model rocket body) one might create the tube walls with smaller tubes packed in a circle. Alternatively, one might add a structure of internal ribs within the tube or create a tube with a thick wall and then cut features out to create a lattice infill pattern.

These type of patterns create high strength to weight, particularly in bending along their long axis. This is true because the stiffness of a beam member in bending is created at the outer most fibers of the structure where the stress is the highest (there is no stress at the centerline of the part because there is no stretching at the centerline). In addition to high bending strength to weight, this structure also maximizes the tensile and compressive strength in the axial direction of the tube. Using the shells and lattice infill tool would not achieve similar results. All such internal structures done in CAD or with the 3D Sprint tool are enabled by the MJP wax melt-away supports. Each designer should consider the appropriate geometry for his or her respective application. The following are some examples for reference.