Question about lost PLA casting

[lee webster]

I was invited to watch a pour of cast iron where glass scrap was used to fuse to the dross and make the pour cleaner. I dont know about brass.

 

[creast]

Hmm. All the contributors here who regularly cast iron can comment on this. I personally have never heard of glass being used on iron.
Correct me if I'm wrong.

 

[lee webster]

John Woodward of JW Engineering here in Cornwall invited me to watch a pour. I think it was about 25kg or more of iron with several kg of glass. He was advised to add the glass by an old foundry worker. This is his web site.
https://www.jw-eng.co.uk/Foundry-Work

 

[Nerd1000]

I also recall having gas issues (no pun intended) with brass.

Holding a metal at an elevated temperature often causes absorption of gasses, and I could clearly see small gas bubbles all through my brass casting.

I have seen brass degassing agent for sale.
Don't breath any fumes coming off of any degassing agents.

Edit:
One of the benefits of melting gray iron is that it does not seem to have gas absorption problems, for whatever reason.

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Pretty much any metal can soak up some hydrogen from the furnace environment, copper alloys will also pick up oxygen (adding phosphorus removes this).

In practice however most of the porosity in a typical hobbyist casting is caused by entraining air in the pour. You can avoid that with good gating system design. Since adopting a tapered sprue with pouring cup and gates sized for a controlled fill rate (to avoid turbulence) I've consistently had relatively little porosity in my aluminium castings.

A side note: the chemical bonding in the metal is so strong that a gas bubble can't really form from dissolved gases unless there is an existing weakspot in the material. Typically this is an 'oxide film' formed by the oxidised surface of the pour getting mixed into the bulk metal. Even the best castings are full of these, they also act as crack initiation points which is why castings are typically less ductile and have lower ultimate tensile strength compared to wrought materials, even when their alloy composition is similar.

 

[GreenTwin]

Hmm. All the contributors here who regularly cast iron can comment on this. I personally have never heard of glass being used on iron.
Correct me if I'm wrong.

Glass is not required as a cover on gray iron.
Iron does not pick up gasses from the air, at least not enough to cause any gas bubbles in the casting.

And the iron slag is easily skimmed, so glass would be just a waste of time.

The amount of slag generated with iron can be minimized by using a reducing flame, instead of an oxidizing flame.

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[GreenTwin]

Pretty much any metal can soak up some hydrogen from the furnace environment, copper alloys will also pick up oxygen (adding phosphorus removes this).

In practice however most of the porosity in a typical hobbyist casting is caused by entraining air in the pour. You can avoid that with good gating system design. Since adopting a tapered sprue with pouring cup and gates sized for a controlled fill rate (to avoid turbulence) I've consistently had relatively little porosity in my aluminium castings.

A side note: the chemical bonding in the metal is so strong that a gas bubble can't really form from dissolved gases unless there is an existing weakspot in the material. Typically this is an 'oxide film' formed by the oxidised surface of the pour getting mixed into the bulk metal. Even the best castings are full of these, they also act as crack initiation points which is why castings are typically less ductile and have lower ultimate tensile strength compared to wrought materials, even when their alloy composition is similar.

Aluminum will have tiny gas bubbles all through it if it is overheated and held at melt temperature for any length of time.

These small bubbles are not the same as larger bubbles caused by entrained air in the sprue/runner/gate system.

The same tiny gas bubbles can form easily in brass.

I have had gas issues with gas bubbles in both aluminum and brass.

The best of castings are not full of bifilms because the foundry controls the ingot supply quality, does not stir their aluminum, and they do not use excessive velocity or turbulence in the sprue/runner/gate system.
I have seen white papers about this.
The worst thing you can ever do is stir molten aluminum, or allow it to splash around in a pour basin, sprue, runner or gate system.

I will look for some pictures of castings with gas defects in them.

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[GreenTwin]

Looking through my photos, I can't really find a good example of gassing in aluminum castings, but I recall having tiny pinholes in my early aluminum castings.

If you don't measure the aluminum temperature carefully, and if you don't pour immediately when you reach pour temperature (about 1,350 F), then you can get the pinholes.

The pinholes may not be noticeable to the untrained eye, but I did notice them in my early castings, expecially when I let the metal temperature overshoot (due to not paying attention), and letting the aluminum stay at or above pour temperature for perhaps 10 minutes.

The pinholes don't seem to affect the structural integrity of the aluminum 356 casting, but I have heard that for air-tight applications, such as a pressure vessel, you want to avoid the pinholes.

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[Nerd1000]

Looking through my photos, I can't really find a good example of gassing in aluminum castings, but I recall having tiny pinholes in my early aluminum castings.

If you don't measure the aluminum temperature carefully, and if you don't pour immediately when you reach pour temperature (about 1,350 F), then you can get the pinholes.

The pinholes may not be noticeable to the untrained eye, but I did notice them in my early castings, expecially when I let the metal temperature overshoot (due to not paying attention), and letting the aluminum stay at or above pour temperature for perhaps 10 minutes.

The pinholes don't seem to affect the structural integrity of the aluminum 356 casting, but I have heard that for air-tight applications, such as a pressure vessel, you want to avoid the pinholes.

.

I've got a fairly spectacular case in an ingot I could show (just need to go the the shop and take a photo, but that will have to wait until tomorrow). I'm fairly sure a large portion of it is air entrainment however, as my casting practice at the time was total garbage. There are certainly pinholes as well, not a shock given how dirty the metal was.

My newer castings have some tiny pinholes in them, so small you can't see them unless the surface has a very smooth finish. They're just on the edge of being visible to my unaided eye, and my father needs a magnifying glass to see them at all. Presumably these are gas porosity. Obviously very hard to photograph, but I'll see if I can capture some of those too.

 

[100model]

Hmm. All the contributors here who regularly cast iron can comment on this. I personally have never heard of glass being used on iron.
Correct me if I'm wrong.

I have never used glass as a flux with cast iron but I always use glass when melting brass. If you use clean cast iron there is not much slag formed and it is easy to remove.

 

[werowance]

so on the glass, what do they actually use? broken up glass jars and bottles that are clean and dry? kind of like crumbling up saltine crackers before putting them on top of a bowl of soup? - now i want a bowl of tomato soup.......

 

[timo_gross]

so on the glass, what do they actually use? broken up glass jars and bottles that are clean and dry? kind of like crumbling up saltine crackers before putting them on top of a bowl of soup? - now i want a bowl of tomato soup.....

More like crumbling up palm fat on top, before you put the hot soup into the fridge then below a certain temperature the next day. The complete fat can be skimmed of in one big lump, with no risk of getting anny fat into your diet fat chicken soup?

Real guess work (aka dangerous half knowledge)
The SiO2 (glas) can make the slag more viscous or more liquid, depends on what is already there and what temperature.
Slag chemistry a rabbitwormhole.

 

[lee webster]

timo_gross.
I think that's the way John described the process, he does some very nice iron work.

 

[100model]

so on the glass, what do they actually use?

I use any kind of glass color does not matter. It is added when the crucible is cold and full of scrap brass. It floats to the top and protects the molten brass from burning zinc.

 

[Steamchick]

Does swarf melt without excessive oxidation - Slag? Or should I sell to the scrap man?
K2

 

[Dubi]

Looking through my photos, I can't really find a good example of gassing in aluminum castings, but I recall having tiny pinholes in my early aluminum castings.

If you don't measure the aluminum temperature carefully, and if you don't pour immediately when you reach pour temperature (about 1,350 F), then you can get the pinholes.

The pinholes may not be noticeable to the untrained eye, but I did notice them in my early castings, expecially when I let the metal temperature overshoot (due to not paying attention), and letting the aluminum stay at or above pour temperature for perhaps 10 minutes.

The pinholes don't seem to affect the structural integrity of the aluminum 356 casting, but I have heard that for air-tight applications, such as a pressure vessel, you want to avoid the pinholes.

.

I was told that if you had pinholes in a casting you could Teflon impregnate to reduce the pin holes. Is this correct?
Dubi

 

[bengt-olausson]

question about lost pla casting - often i will use red filler primer or heavy scratch primer to get rid of the print lines and make a smooth looking 3d print.

will that stuff burn out with the pla in my mold or will it leave a shell like a sheetrock mud coat might do ? thought about just plain parifin wax and painting it on the outside however i have warped pla prints with hot wax before so wanted to use the primer if it will burn out.

want to use plaster as the mold material, burn out the pla and hopefully the primer as well.

I,m a bit late om this, but there is a filament for casting moulds.MoldLay sold by 3dprima.
Bengt

 

[GreenTwin]

Does swarf melt without excessive oxidation - Slag? Or should I sell to the scrap man?
K2

I have never tried melting swarf, but have universally heard that it is not a good source of scrap metal.

If you are trying to make decorative pieces, swarf may be an option, since what do you have to lose?

Perhaps a bigger problem could be contamination with oil and/or water, which could cause problems if you added swarf to a hot melt.

.

 

[GreenTwin]

I was told that if you had pinholes in a casting you could Teflon impregnate to reduce the pin holes. Is this correct?
Dubi

I have heard that the pinholes can be sealed with some material, I think it was epoxy.

A better idea is to avoid the pinholes altogether, ie: bring the aluminum up to pour temperature as quickly as possible, don't stir the melt, skim and pour immediately when you reach 1,350 F.

I stopped having the pinholes when I used this method, and without using a degassing agent.

.

 

[GreenTwin]

I,m a bit late om this, but there is a filament for casting moulds.MoldLay sold by 3dprima.
Bengt

I have not tried lost-PLA, but I have seen examples of those who have successfully done it, and using gray iron too.

While I have heard of special filaments that are suppose to burn out more cleanly, the results I have seen with lost PLA would seem to indicate that in some/many cases, a special filament is not necessary for a clean burnout.

I would caution anyone about burning out thermoplastic, since I have heard that the fumes from burning plastic are exceptionally toxic (one whiff and you are gone).

Also note that on the 3dprima website, they talk about pouring concrete around your 3D print.
You should never use concrete in foundry work, since apparently it will hold moisture, which will flash when heated, and the concrete will explode.
There are some videos on ytube of violent explosions of furnaces that were made from concrete.
Never use concrete for foundry work.

.

 

[GreenTwin]

Here is a video of an amatuer trying some lost PLA (I think).

He seems very confused about terms and things, and it looks like he is doing his burnout inside, which is not a safe idea due to toxic fumes.

He comes up with some aluminum castings that look pretty good from the outside, but to the trained eye, his castings are full of defects.

I am not pointing out his casting defects to criticize the fellow and his casting work, but rather to show you things to avoid if you try aluminum castings.

One thing he does is use a pouring basin that keeps the sprue half full, and so he is aspirating air down the spure, which causes all sorts of defects.

And an example of gas holes can be seen in his video at 4:41.
His casting is full of gas holes.

These problems are easily avoided.
The gassing is easily avoided by not overheating the aluminum, heating the aluminum as quickly as possible, and by pouring immediately when you reach 1,350 F.

If you want good aluminum castings, you have to pay attention to details.
Most of the backyard casters on ytube are just pouring things for fun, and if that is the case, then basically anything goes as long as it is safe.