Ramblin' Dan's Workshop

The Sanctum of Fine Art and Invention

Some LW Casting Comments

DSC04933I admit I am still learning my way with vacuum assisted lost wax casting. I have lost about half of the models I have attempted to cast. In any kind of learning this sort of thing goes with the gaining of experience. I realize not every attempt is perfect.

I trained with the centripetal casting method. I never had a failure with that process. Probably because my instructor made sure I/we did it correctly. The vacuum assist casting is something I have tackled on my own.

After many attempts I still find it amazing that the investment (plaster like casting material) is porous enough for the vacuum to have any real effect. I understand the cast-in-a-vacuum method, but that is totally different than vacuum assist. Not in the same league with personal casting.

The process does work as many large professional casting companies use vacuum assist all the time. The reason is a size and volume limit to centripetal casting. Twenty rings on a single sprue is quite a mass to spin and keep balanced as the metal volume shifts.

I have stated I will eventually purchase a small “personal size” centripetal machine as I foresee most of my casting will be of small volume and single quantities of custom and specialty items.

My recent casting success with my last two flasks has encourage me that vacuum assist is actually a viable process for me. I did things a little different with the burn out (longer) and the casting process. I believe the real game changer for me in this latest attempt was a very quick pour and I kept the melting torch on the sprue button after the pour for about 10 to 15 seconds. This kept the button molten and I noticed it “draw down” during that extended heating period.

What I am thinking is, with the failures, my pours are not optimum and the button was setting up too fast and not providing extra material to the model as it shrunk with hardening. The cast parts would run out of material before they were full and create the incomplete casts I experienced. The cold button could also be blocking any large volume of air that was trying to escape, but I am assuming the vacuum is supposed to be taking care of the air.

What I do know is keeping the button molten for a few extra seconds definitely produced some good castings. I can’t wait until the next session and confirm my hypothesis. Let’s say I am not going to stop the extra heating it if keeps working.

There is no loss of investment (dollars) if I switch to centripetal casting as the vacuum pump is still required for degassing (removing air bubbles) from the casting investment material. It serves the same service with degassing rubber molding products too. The vacuum table was a very necessary investment.

I didn’t invest in any of the larger vacuum assist machines as I considered the smaller table version would be the perfect intermediate step and had the smallest “footprint” on shop space. I think that has been a good decision. I have also made a mental note that the Rio “third hand” assistant melting and pouring crucible sold with some vacuum assist machines would be a very beneficial (but pricey) accessory. Speed of pour is everything.

Here are some conclusions on personal lost wax casting

  • My conclusion is vacuum degassing is a necessary step.
  • Vacuum assist casting is difficult to do quickly freehand but can be a viable small shop method.
  • Rio “Third hand” assistant accessory would be a definite (but pricey) aid to vacuum assist casting.
  • Centripetal casting avoids variables of pour casting (vacuum assist) but limited to ultimate volume and size for mass production work.
  • As a single person craft specialty designer, I do not presently need a high capacity LW casting system.

My instructor is correct. Centripetal casting is the best low volume small item lost wax casting system. He also told me it is also the best choice for thin section and “lacy” type models with lots of detail. The reason it is used in wax carving class is it has the highest potential for obtaining good casts. Students are not happy when loosing perhaps weeks of work with a failed cast. It doesn’t make me happy but I know the risks of “lost” wax. I get over my “loss” but I don’t want to make it a regular practice.

Maybe I should have listened… but then, how would I have gained all this additional experience. J

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