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Plaster mold casting of A356

Plaster mold casting of A356

Plaster mold casting of A356

(OP)
Hi

I have made a casting of A356 in a Plaster mold.
We have a lot of Hydrogen voids, 2,5 % voids.
We have used degasing tablets.
the heat have been meltet more than 8 hours.
Do anu one know why we get these voids. is it beacus the mlt is more tahn 8 hours old or is the watre in the Plast mold which evaporates

RE: Plaster mold casting of A356

It is 8 hours of melting time which has provided enough time for the gas to be picked up.How did you have such a long melting campaign?

RE: Plaster mold casting of A356

(OP)
The melting ovnen is quit small i think. (it is my subcontractor)

Can there come so much hyrdogen in the heat so the degassing tablets can get the hydrogen out of the heat???

How long is a desired melting and pouring cycle??

RE: Plaster mold casting of A356

Contact your nearest foundry consumables supplier like Foseco,they can help you out with good products to degas the metal.

RE: Plaster mold casting of A356

Are you sure these are hydgrogen voids rather than shrinkage? A plaster mold will not offer good directional solidification characteristics, since you can't control the thermal gradients and shrinkage becomes a real possibility.

RE: Plaster mold casting of A356

(OP)
No i am not sure
I can send you a microsope pic of the the part

RE: Plaster mold casting of A356

Not enough information.

Re "watre in the Plast mold which evaporates."
It is absolutely imperative to bake the moisture out of plaster molds to avoid defects & possible explosions.  Plaster of Paris (gypsum) must be baked above 320 oF (160 oC) to remove all moisture, check the product bulletin if using a special casting plaster. Gypsum is ~impermeable to gases -- was a more permeable plasters used?  Venting?

8 hours holding a melt???? Why not 8 minutes?
A356 doesn't have a lot of Mg (0.3-0.4%) or Zn (0.05% max), so maybe only lose some Mg.

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