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Porosity on small 3D printed pieces casted in silver

Hello everyone,

I have just started casting earlier this year, I understand it is about a try and fail game, but for this one I think I need your help.

I own a Formlabs2 3D printer, on which I printed several pieces in Castable Wax Resin which I later set on a wax casting sprue. I used Plasticast investment, in a 38/100 ratio for water vs investment, waited for 2 hours before setting it in the kiln. I cast with an italian Bailo&Aldo vacuum machine - EasyCast and I respected the 8 hour burning schedule the provider from whom I bought the equipments (and who installed them in my workshop) recommended. The provider recomended to cast at 650 Celsius flask temperature and 950 Silver temperature. My flask dimensions are rather small - 9 cm in diameter/12 cm tall.

However, at the end of it, the large pieces turned out great, where the smaller ones (1mm thickness) have huge porosity. My plan was to cast once in Castable wax, then make vulcanised molds and inject wax, but these are not usable this way.

I kept reading several threads on this forum and I think either my metal might have been to hot, or the investment deteriorated where the small pieces were (which probably burned out quicker).

What do you suggest?

I don’t really subscribe to the try and … approach. However experience will be your best friend.

If the two rings are what you refer to as the large pieces then I suspect that the mass and thickness enabled good flow during casting. I would have liked to see the sprues.

The thinner parts appear to have a flow problem most likely because of the need for additional sprues or different locations. Again seeing the sprue would help.

Think of your wax and sprue and try to visualize the flow of a liquid being poured in and the possible paths and turbulence the bends are creating. Also the path that a volume on liquid must go thru the further along it goes from a sprue.

Someday someone will develop some software to simulate the flow, but I don’t know of any yet.
Regards RLW

Dear Ron,

Thank you so much for taking the time to answer my message.

I attached some photos of the sprues, maybe this helps to get a better image of the issue.

Thank you again for your time and answer!

I think that the first question could be “what is the nature of the pitting?”
Porosity sucks but it can also give a ton of information about what generated the problem and when in the process it occurred.

Larger castings are often plagued with fine sprays of micro porosity which becomes apparent when polishing to a higher finish. This “shrink spot porosity” is associated with the cooling and solidification of the metal as it shrinks and pulls metal from the wrong places.

Round pits may be gas porosity that occurs from too hot a melt, too much oxygen or investment breakdown formed from too high a burnout temp.

Jagged pits could be crud and contaminates and if filled with white investment could mean that the inside of the mold chamber has broken down—especially when partnered with strange, rough positive formations on the casting.

Rond dish-shaped pits that are haloed in bright metal could be flux inclusions…

Andy

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