3D Printing the Mold

3D printers belong to the class of fused deposition modelling (FDM) devices. They build parts layer-by-layer by melting model or support polymer wire inside a hot fuse which is then extruded onto a platform and cured on the fly. Our system uses acrylonitrile-butadiene-styrene  (ABS) co-polymer for the model and polylactic acid (PLA) as support material, but similar systems with different materials will work just as well to make the mold. The only caveat is that the model material must be dissolvable in order to remove the cast actuator once it is set. More details are given in the Mold Removal section.

The printer software calculates the tool path from a triangulated surface file of the CAD model (usually an .STL file). The 3D printer cannot print material on air, so support structures are used to build a printing platform and to fill any voids. The model material layers then build the actual part on top of the support material. The support structures can now be removed, leaving only the model behind. We used the following printers to make this actuator mold:

CAD File conversion

First, the CAD part-file has to be exported into .stl (File → Export → STL). This file format delivers the necessary triangulation for the 3D printing software. In the red square, important exportation parameters are highlighted. The lower this value is, the larger the file size will be, but also the better the input resolution of the mold. We suggest to use a triangle resolution and an adjacency tolerance of 10-3 at least.  

Click here for larger image

Set-up 3D Printer

The .stl-file now can be loaded into the 3D printing software (File → Open STL). Choose the layer resolution to be 0.254 mm and set the model interior to “Sparse – Low density”. We want the mold to be easily dissolvable in later steps, so having a low density will allow solvent to penetrate the mold easier. Then, click on the “Orientation”-tab (upper left).

Click here for larger image

Depending on the positioning of the model part, more or less support material is needed. Therefore, the part needs to be turned to minimize the material need. Turn the part by entering 112.5 degrees into the corresponding field and click on “Rotation:X”. The part now turns. Then click on “Add to Pack”. The software will now calculates all tool paths.

Click here for larger image

All tool paths are shown (red for material, purple for support tool path) after the completion of the calculations. Now, verification is needed. The mold has an inner sphere (representing the combustion chamber), which is connected to the three cylinders (representing the channels). There has to be a connection to the outer mold shell, otherwise the interior geometry is not set in place correctly (loose parts inside). To verify this, click on  “Top View” to change perspective and choose the displayed layer to be “Top”.

Click here for larger image

By pressing “Page Up” or “Page Down” on the keyboard, one can switch within the layers.  Go down. You will reach a layer where suddenly, the spherical combustion chamber has three extruded fingers (left picture). As you continue down the layers, these fingers move along the dashed lines until they reach the outer mold shell (right picture). Thus, this design has a connection between the inner geometry and the outer shell.

Next click on the tab labeled "Pack" (upper left). Here, the positioning of the parts can be performed. This is especially interesting, if you would like to print the actuator mold several times. On the right side, details on your model and support material consumption, as well as the estimated printing time are given. Click “Print” to start. Then follow the instructions given on your 3D printer.

Click here for larger image

The Printing Process

The printing process is very reliable and robust. Nevertheless, a few things you should remember when using these 3D printers:

  • Platforms can be used several times. After a while, adhesion of the first layer is reduced and a part can peel off (creates a huge mess inside your printer and may damage your fuse head). Therefore it is advisable to use new platforms for large molds.
  • Calibrate X,Y and Z-axis periodically. This helps to give you stable printing performance. Also check whether your fuse has any polymer deposits nearby. These might interfere with any freshly printed layers.
  • Humidity affects your printing. Therefore store everything in a dry environment. Unload all materials, especially if you do not print for several weeks.

Mold Post-Treatment

The mold has to be washed in an alkaline bath to remove the support structure (base catalyzed degradation of PLA). This process can be sped up by heating to 60°C and introducing convection by a circulation pump. Also, specific flushing of the mold helps to dissolve the PLA. Once the support material is dissolved and washed away, the CDA mold is then dried and can be prepared for the casting process.

In principle, any model/support system can be used to print molds. The important step is the mold dissolution. If a solvent does not dissolve the model material, the casting technique does not work. More details are given in the section: Mold Removal.