3D Print Patterns For Investment Casting

February 20, 2019
Submitted by ebeard on Wed, 02/20/2019 - 22:07

3D printing patterns for investment casting is increasingly becoming the go-to solution for artisans, engineers, and industrial professionals alike. With LulzBot 3D Printers and PolyCast, users are able to test designs, iterate faster, and quickly produce short runs while reducing expensive tooling and long lead times. At a fraction of the cost of most additive manufacturing investment casting solutions, LulzBot 3D Printers enable companies of all sizes to speed innovation while reducing costs.

Step 1: 3D Printing With PolyCast

Award-winning LulzBot 3D Printers now come with ready-to-print profiles for PolyCast, a filament designed specifically for investment casting applications, producing a near-flawless burnout with less than .003% remaining ash residue. Simply load the filament into your LulzBot Mini 2 or LulzBot TAZ 6, select the PolyCast print profile in Cura LulzBot Edition, and hit print. In addition, use the following printing tips to ensure professional and dimensionally accurate results:

  • When placing your model on the build plate, select an orientation to minimize the need for support structure as much as possible. This will help improve the surface finish and printing time.
  • Print your model with minimal infill (ie;10%) and number of shells (2-3) to help facilitate the burnout process.
  • Shrinkage compensation will need to be applied to the .STL file to compensate for the dimensional change in the metal between the molten and the solid states. Modify the model size by the metal/alloy-dependent compensation factor, which is usually between 1.007-1.030. As an example, the compensation factor for steel is 1.025-1.030. If the dimension of the metal part is to be 1 m, the dimension of the printed pattern should be 1.025-1.030 m.
  • A layer height of 0.1-0.2 mm is ideal for this application. This helps with print resolution and post-processing.

Note: PolyCast absorbs moisture. It is highly recommended to store this filament under dry conditions (relative humidity of no more than 20%). 

Step 2: Post-Processing

For the investment casting process, it is important to start off with a smooth print surface that is free from defects and debris. Dip and spray polishing are two recommended methods of post-processing. Before beginning, we recommended using an 800 grit sand paper to remove any surface defects or seams from the printed part. When finished, be sure to remove any dust or debris from the object.

Option 1: Dip Polishing

  1. Attach a wire or a similar material for suspending the prints after post-processing is complete.
  2. Dip the printed pattern in isopropyl alcohol for 5-10 seconds. This can be done multiple times for a stronger polishing effect.
  3. Suspend the print and allow it to dry for 20-30 minutes. Steps 1-4 may be repeated, if needed.
  4. Place the part in a vacuum, or in a convection oven set at 40°C for one hour. This will ensure complete solvent evaporation and surface hardening. (You can also skip this step and allow the print to dry overnight, if preferred.)

Option 2: Spray Polishing

  1. Attach a wire or a similar material for suspending the prints after post-processing is complete, or place it on a base with a small contact area.
  2. Spray the surface of the print with isopropyl alcohol .
  3. Allow the print to dry for 20-30 minutes. Steps 1-4 may be repeated, if needed.
  4. Place the part in a vacuum, or in a convection oven set at 40°C for one hour. This will ensure complete solvent evaporation and surface hardening. (You can also skip this step and allow the print to dry overnight, if preferred.) 

Tip: The Polysher, by Polymaker, provides additional smoothing of layer lines in 20-40 minutes, while preserving macroscopic dimensional accuracy. Find it here!

Step 3: Parts Tree

The parts tree secures the cup to the part for casting, and ensures the metal flows smoothly and evenly into the part. 

  1. Attach the part to the cup with wax bars, or sprues, which will serve as channels for metal flow during the casting process. (The part should sit level on the cup for even pouring) Keep bends and twists in the sprues to a minimum, as these have the potential to slow the flow to the part.
  2. You may wish to attach smaller wax pieces to thinner areas of the part for even flow distribution.
  3. A small torch may be used on the final assembly to eliminate any holes in the sprues or the parts tree itself.

Step 4: Ceramic Dipping

The ceramic shell will serve as the final mold for casting once the printed material has vaporized during the burnout process. 

  1. Create the mold by alternately immersing the parts tree in a ceramic slurry and an extremely fine silica compound. Care should be taken to make sure the part is neatly and evenly coated during both steps, and that no gaps are present. 
  2. The number of times this process will need to be repeated varies. Generally, at least 5 coats are recommended, and for parts with intricate structures, 7-9 coats are recommended to prevent damage to the shell during casting.
  3. It is important to let each coat completely dry before repeating this step. Once the shell has reached desired thickness (approximately .3.75 in, or 9.525 mm on average), it is ready for burnout. 

Step 5: Burnout

The burnout process serves to both sinter, or harden, the ceramic shell, and to burn out the printed material inside to create the mold.

  1. Place the coated parts tree assembly in a kiln or furnace, cup side down. Generally, the shell will be heated to around 1100-1200°C for approximately 40-60 minutes. 
    (Note: The exact time and temperature will be determined by the type of furnace or kiln used, and by the specific metal part that will be produced.) 
  2. After the printed material has burned off, allow the shell to cool completely.

Step 6: Casting

The shell is prepared and pre-heated to accept the molten metal for casting.

  1. If there is any ash or debris remaining in the shell after burnout, rinse the inside of the shell before proceeding to the next step. 
  2. Pre-heat the shell for pouring by placing it in the kiln. Temperatures may vary, although generally they will be around 550-1100 °C. 
  3. Pour the molten metal into the shell while the shell is still hot. The metal will flow through the sprues and into the mold cavity. The mold and casting will cool and contract at the same rate, creating a part with excellent dimensional accuracy.
  4. Allow the metal to cool and solidify completely. Some castings may take longer to solidify than others depending on the material used and the thickness of the part.

Step 7: Cleanup

The part is removed from their ceramic shells and the sprues are removed, preparing it for its final finishing. 

  1. After the part has completely cooled, it is ready to be removed from the ceramic mold. The mold can be broken down and removed from the part manually, or with other methods like pneumatic jacks or water jets. 
  2. Remove the sprues from the part. This can be achieved with sawing, cutting, sanding, or by using a plasma cutter.

Step 8: Patina

The desired finish will depend on the material used and the part’s intended purpose. For some materials, a patina works well. For others, a protective coating to guard against corrosion is all that will be necessary. For reference purposes, we will address the two materials used in the video included with this tutorial. 

  1. For bronze parts, apply a coating of sulfurated potash. After baking in another oven , add a coat of ferric iron, followed by a layer of sealant to preserve the finish.
  2. For steel parts, especially those that will be used for functional prototypes, a coat of protective sealant is all that will be needed.