Plaster mold casting of A356

[BabyStress]

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

 

[arunmrao]

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?

 

[BabyStress]

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??

 

[arunmrao]

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

 

[swall]

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.

 

[BabyStress]

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

 

[kenvlach]

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.