3d Printing: Lost PLA Investment Casting

As mentioned in MAKE: click here

make_project

We are always looking for new and innovative ways to meet our customers casting needs. Recently we conducted a successful beta test of a lost PLA investment casting in stainless steel. Others have tried this approach with aluminum, but we haven’t seen another example in stainless, here’s how we did it:

Background

This particular impeller proved difficult to cast from conventional sand tooling. The objective was to prove that we could produce a high quality investment casting without investing in expensive tooling. Below is a picture of the troublesome sand pattern we were trying to replace.

conventional_segmental core_sand_pattern

3d Printer

Our 3d printer uses a renewable bio-plastic (a polyester derived from corn to be precise) called PLA. We hypothesized that a scale model impeller in PLA could be used like the wax conventionally used in the investment casting process. Our main considerations were:

  1. Dimensional accuracy and model rigidity. PLA is remarkably strong and our prints are typically accurate to plus-or-minus 0.005”.
  2. Chemical and mechanical properties of PLA. PLA burns quite well and does not contain any nasty chemical or compounds that might cause emissions concerns during burn-out. The hollow structure of the model means that heat expansion of the plastic does not risk cracking the investment mould. PLA is water resistant, so the application of water-based ceramic slurry is not a problem either.

The photo below shows the finished PLA model. Note it was printed in two parts and  merged with glue.

3d_printed_PLA_impeller_for_investment_casting.jpg

Image showing the internal structure of the print, which is 90% air:

pla_impeller_internal_structure_90percent _air.jpg

Investment Casting

You can read more about investment casting here.

Our friends are Precise Castings were kind enough to help us with this experiment. The photos below show our model encased in a ceramic shell, and the furnace used the burn the PLA out of the shell prior to casting.

lost-PLA_investment_mould.jpg

PLA_burn_out_of_investment_mould.jpg

Finished Product

The finished product in ANSI 316 (CF8M) stainless steel was an excellent casting which could easily be finished with minimal machining. The cost of this test proved to be significantly lower than the same part cast in sand.

finished_casting_from_3d_printed_impeller_model.jpg

Thoughts on the Process

We conclude that lost-PLA is a viable prototype or small scale production technique. We look forward to using it for our customers in the near future.

If this looks like something you could use, or you want to learn more about our experiences with the process, please contact us for more information.