As mentioned in MAKE: click here
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:
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.
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:
- Dimensional accuracy and model rigidity. PLA is remarkably strong and our prints are typically accurate to plus-or-minus 0.005”.
- 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.
Image showing the internal structure of the print, which is 90% air:
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.
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.
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.