Requesting Suggestions to eliminate porosity

Foundry Rat · Jan 25, 2023

Al2O3 said: ↑

Pretty much agree that most problems are melt management and temp related. Curiosity question for you: What are you using to degass? Rotary? and what metrology equipment do you have to measure H2 content/porosity of your castings.

You're obviously are an experienced foundry professional and happy to have you here but I must ask, is lost foam casting something you routinely practice?

You lose me when you start talking about venting lost foam castings especially the use of clothes hangars like poke vents. You do realize these posts are using unbound sand that has been vibrated to compact the mold.....no binder. Removing a wire would just cause the mold media to fill in behind it and cause the mold to be poorly packed in the vicinity of the extraction. If you're using green or some other bound sand, it can certainly work, but it's not standard practice nor utilizing the process to it's advantage.

I fully concede that most people (especially here on a home/hobby casting forum) attempting lost foam casting are willing to accept the gas evolving from the decomposed foam buoyantly rising through the molten metal of their castings and exiting through "vents" or even their sprue and feed system (worse yet). That may be ok for ornamental stuff and trying to insure higher first past success, as long as your definition of success doesn't include high metal quality.

....But, I would say it is certainly not optimal nor any more acceptable practice than allowing the same to occur in a conventional sand casting, and it's inviting folds, flaws, and in general poor metal quality castings. If you want good metal quality lost foam castings that are functioning machine parts with good mechanical properties, you'll need to eliminate that and get the gas to exit through the coating on the pattern wall as it evolves, not through the molten metal.......and it's easier said than done, but can be and is done......I didnt get what I considered good LF castings untill I was able to manage this.

Best,
Kelly
Click to expand...

Thanks Kelly, I am thinking that the media may not hold well because of no binder.
But this part venting is not the issue. I believe he holding the metal for too long at a High temp.

Foundry Rat · Jan 25, 2023

Al2O3 said: ↑

Pretty much agree that most problems are melt management and temp related. Curiosity question for you: What are you using to degass? Rotary? and what metrology equipment do you have to measure H2 content/porosity of your castings.

You're obviously are an experienced foundry professional and happy to have you here but I must ask, is lost foam casting something you routinely practice?

You lose me when you start talking about venting lost foam castings especially the use of clothes hangars like poke vents. You do realize these posts are using unbound sand that has been vibrated to compact the mold.....no binder. Removing a wire would just cause the mold media to fill in behind it and cause the mold to be poorly packed in the vicinity of the extraction. If you're using green or some other bound sand, it can certainly work, but it's not standard practice nor utilizing the process to it's advantage.

I fully concede that most people (especially here on a home/hobby casting forum) attempting lost foam casting are willing to accept the gas evolving from the decomposed foam buoyantly rising through the molten metal of their castings and exiting through "vents" or even their sprue and feed system (worse yet). That may be ok for ornamental stuff and trying to insure higher first past success, as long as your definition of success doesn't include high metal quality.

....But, I would say it is certainly not optimal nor any more acceptable practice than allowing the same to occur in a conventional sand casting, and it's inviting folds, flaws, and in general poor metal quality castings. If you want good metal quality lost foam castings that are functioning machine parts with good mechanical properties, you'll need to eliminate that and get the gas to exit through the coating on the pattern wall as it evolves, not through the molten metal.......and it's easier said than done, but can be and is done......I didnt get what I considered good LF castings untill I was able to manage this.

Best,
Kelly
Click to expand...

Degassing I use nitrogen, we start degassing as soon as the metal goes to full melt and continue while the metal is heating to pouring temp.
Rarely have a hydrogen issue....actually the main purpose of the degassing isn't to remove hydrogen, but to bring non-metallic inclusions to the top of the bath. The added benefit of removing the gas is a welcome bonus. Any gas we may have a chance of getting comes from a cold/wet charge, So all the charge metal gets preheated. and then it would be cold/wet ladles or skimmers.
The crew is very familiar with keeping tools hot.
If there is a question of hydrogen I pour a little of the metal off and put the liquid sample under a vacuum. Saw the cooled piece in half. That'll show real quick...
Here is something to look for as you are degassing...A gentle bubble is used, But when the bubble breaks the surface and you see a puff of flame, that is hydrogen coming to the surface.
Bubble till all the flames are gone....Also bubble the sides of your crucible, this does 2 things, removes gas from the side of the pot, (as hydrogen will stick to the side of your pot)and helps to keep the crucible clean by bringing ashes and other junk to the top.
The charge through the day is about 95% new ingot, and 5% returns, gates and risers.
I have found only one instance where I just couldn't get rid of the hydrogen. The only thing I could do was pig out the furnace and start again..The gased metal I was able to remelt with no problem of gas...go figure that one.

Foundry Rat · Jan 26, 2023

Here is a question.
Can stored foam hold moisture? How can one be sure the foam being used is dry. Does it matter?

ddmckee54 · Jan 26, 2023

Foam that has been "IN/UNDER" water for a length of time will get water-logged, but it takes a long time - months, maybe years. In some boats the volume under the floor-boards is filled with foam for additional floatation. If the hull leaks then, over time, this foam because it's sitting in water will get water-logged and become useless. I wouldn't worry about store-bought foam being water-logged, but if the foam seems overly heavy don't use it. If the foam were saturated with water, that might be a bad thing.

Al2O3 · Jan 26, 2023

I’d say it just doesn’t matter unless the foam has been buried or submerged for a protracted period, I’d say it doesn’t matter. The colored extrude polystyrene (XPS) foam board is closed cell. The white expanded ps (EPS) is closed within the bead but not between the beads.

I spray the patterns with a water based surfactant (soapy water) and the apply a water based refractory coating. In fact the polystyrene is so water repellent/ hygroscopic it’s hard to get the surface to properly wet with water based coatings. In general, they just won’t absorb water.

Best,
Kelly

Foundry Rat · Jan 26, 2023

Al2O3 said: ↑

I’d say it just doesn’t matter unless the foam has been buried or submerged for a protracted period, I’d say it doesn’t matter. The colored extrude polystyrene (XPS) foam board is closed cell. The white expanded ps (EPS) is closed within the bead but not between the beads.

I spray the patterns with a water based surfactant (soapy water) and the apply a water based refractory coating. In fact the polystyrene is so water repellent/ hygroscopic it’s hard to get the surface to properly wet with water based coatings. In general, they just won’t absorb water.

Best,
Kelly
Click to expand...

Thank you, that is an answer that tells me what I needed to know. I thought that because it is poly it would resist water, unless maybe the surface could be wetted. Even then it isn't holding water.

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Requesting Suggestions to eliminate porosity

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