Replacing Interior Solid Build Material with Air


p/n 33-D206 Rev A


Note: For the investment casting industry, “part” in this paper refers to what is normally referred to as the wax pattern.


This document contains information on how to change solid build material in the interior of a part into a combination of build material and air, a technique that may be used to reduce part weight and material use in printed parts.  The resultant parts have a solid outer "skin" and a porous, sponge-like interior. Care must be taken to avoid over-weakening your parts and compromising the strength of gated locations. Extra care should also be taken when manually removing support wax from thin non-solid parts to avoid accidentally breaking them.  Shrinkage factors (applied in 3D Sprint) may also need to be different than with solid parts.

No special software or add-ins are required, this feature is a standard toolset group in 3D Sprint called Infill. Infill consists of three sub-commands (Offset, Lattice and Vent Drain), which allow for the easy assignment of specified lattice structure to the volume inside a shell. Lattice structure generation occurs inside the printer rather than at the time of infill assignment within 3D Sprint.

The Infill feature can have great benefits for material use and weight reduction of printed parts.  Some parts will benefit more than others do, depending on part size and cross-sectional thickness. The shell thickness default is 0.08” (2mm), but is adjustable down to a minimum of 0.04” (1mm).  Material usage savings range from 10-40%, with a 25% savings being typical.

The greatest material use and weight reduction benefit happens when you reduce wall thickness as thin as possible.  However, when applying an Infill to a part in 3D Sprint you will want to be sure the part will maintain enough rigidity to resist deformation and breakage through any additional manufacturing processes. The axis of greatest strength is in the (as printed) vertical direction, with less strength in the (as printed) horizontal directions.

While additional Infill lattice structures are available to use, Cellular Structure (CS) is the only lattice to use in the ProJet MJP 2500 IC product. The CS infill is an approximately 50 percent porous structure of build material. The other lattice options available (Grid, Star, and Jacks) are a combination of build and support material with little total material usage reduction and the potential for part quality issues.

The cross-section of a part with the Cellular Structure Infill printed on a ProJet MJP 2500 IC product is shown below. This picture shows both the porous build structure inside the part shell (green) and a section of the porous support structure for an overhanging feature of the part.

1b.pngIn addition to material usage and weight reduction benefits, use of Infill can also benefit the Print Quality artifact known as build-support separation shown below, which can sometime occur on large flat parts.  Like the artifact itself, the benefit of Infill will be highly geometry dependent, but the default/recommended Cellular Structure infill has been shown to dramatically reduce the artifact in some cases.



Note:  It should be noted that internally porous parts can have different behavior in post-processing, since they can be buoyant (depending on the amount of infill).  This can be beneficial if the dominant volume of support material to be removed normally faces up when placed in the bottom of the bath, but the part floats pointing down when infilled.  However, floating parts can also move and damage each other without a restraining device.  Please consult the  Post Processing Troubleshooting and Post Processing Best Practices for more information regarding floating parts during post processing.


To apply an Infill:

  1. Import your design into the 3D Sprint Print window.
  2. Select the “Infill > Offset” command. The preset infill lattice type is applied to the offset core that is created in this step (should default to Cellular Structure).  The shell thickness “Offset” is set at this step. 0.08” (2mm) is the default/recommended shell thickness, but can be changed depending on the part and/or later process requirements (minimum = 1mm).
  3. To change the lattice type (not recommended) or to apply a lattice to an existing core, select “Infill > Lattice” command and apply the new lattice or set it as a preset.
  4. The "Infill > Vent Drain" command is not intended for use on the ProJet MJP 2500 IC 



Interior shells can be easily created with the Offset command under the Infill toolset. The preset lattice settings will be applied to the core of the offset part. The lattice presets can be changed under the Lattice command. In this window, distance refers to the wall thickness of the shell you are creating.


To apply an inward offset:

  1. Select Infill > Offset
  2. Select a part or parts.
  3. Enter an offset distance.
  4. Click Set


Under the Parts List a new group will be created containing the part and infill.



Wall Thickness Reduction - Wall thickness is set by the distance value when applying an inward offset and has the greatest effect on cost and weight reduction of your part. We recommend avoiding wall thicknesses less than 0.08” (2mm).  Exceptions include small parts or parts where weight reduction outweighs all risks.  Large parts and/or parts being joined  may need thicker wall thicknesses to maintain structural integrity through later processes.  Thinner shell thicknesses are also less robust to any jetting abnormality and may result in top surface grooves.

In addition, parts printed with an incorporated gate may need thicker walls for attachment of the gate.  Joints and/or gate attachment locations should be placed on the (as printed) top or bottom for increased strength compared to the sides (due to the structure of the infill), if possible.  Good gate attachment success has been experienced with 0.20” (5mm) shells for parts weighing less than 0.5 pounds (225 grams).




The Lattice command can be used to apply a selected infill geometry to an inner offset or to an entire part.

The default/recommended structure available for the ProJet MJP 2500 IC in 3D Sprint is Cellular Structure (CS), which is a porous structure, made with the build material.  Three additional structures available in 3D Sprint, in order of increasing ratio of build to support material, are Grid < Jacks < Star (at same cell size, thickness). Auto Support and Solid Build are also available and offer a support infill or solid part respectively.  Due to part deformation, it is not recommended to use any of the infill lattice options other than Cellular Structure in the ProJet MJP 2500 IC.

Note: When a lattice type is selected, a preview of the lattice can be seen in the lattice command window showing the Fill Ratio (% of volume filled with build material).  For lattice options other than Cellular Support, the remaining percentage of the volume is filled with support material.


Vent Drain

The Vent Drain feature is not intended for use in the ProJet MJP 2500 IC.  Holes to the interior of the porous structure will allow large amounts of cleaning fluid to be absorbed during post-processing, which will be very difficult to completely remove afterwards.