Re: Need help with Plaster investment shell


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Posted by Phil B on September 07, 1999 at 18:41:28:

In Reply to: Need help with Plaster investment shell posted by Tom Kay on August 21, 1999 at 12:45:12:

I'm surprised you have not received any replies from the experts yet. I'm also a novice beginning with lost wax casting of aluminium parts for model engines. I'm about six months ahead of you. The blind leading the blind errrr. See

and follow-ups. To answer you questions: 50/50 plaster with sand is fine. The sand is physically mixed with the plaster. I use regular modelling plaster, it seems to work OK. You can reuse old mould material by grinding it up and mixing it upto 50% with new plaster, the result is called stucco. You will find that the necessary experimenting produces a lot of used mould material so re-use reduces the cost. I perform the initial burn out in an ordinary domestic oven at 200 deg. C (I live alone) followed by burn out at 600 deg. C for several hours in a muffle oven. My parts are much smaller than a quarter size Merlin crank case. You can either cast the wax pattern in the plaster/sand mixture (the shaw process) or you can dip or brush on the mixture (the Italian process). I currently prefer brushing as it is easier to avoid air bubbles and subsequent "potatoes" on a complex casting. The first brushed coat has great difficulty adhering to the wax and looks initially like it's not going to work but with perseverance the additional coats build up fine. I'm surprised that you have already progressed with making a wooden pattern for the crank case as you need to determine what to allow for both wax and aluminium shrinkage. Also I find that fractures/weak points occur in my wax pattern as it cools in the mould if it's shape (undercuts, cores etc.) prevents it from freely contracting without setting up internal stresses. I intend to make wax patterns for crank cases in two halves, followed by wax "welding" and clean up prior to making the final mold. The fractures are probably due to the fact that I use ordinary candle wax. The experts probably will have something to say on this topic (please). Anyway I would have experimented with some simple castings before progressing to something as labour intensive and 3 dimensional as a quarter scale crank case. Progressive cooling, both for the wax pattern and the final aluminuim casting, will be important for a thin walled casting like a crank case if shrinkage problems are to be avoided. Solving progressive cooling problems seems to require a degee of understanding of the cooling process together with hands on experience (read failures). You should try a book called Metal Casting - Appropriate technology in the small foundry - Steve Hurst - Intermediate Technology Publications Ltd. It's about third world technology but then this is appropriate to the amateur. Good luck, let me know how you get on.

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