Lost foam aluminium casting issues

[Gennady]

Hello team.
I wanted to give it a try to lost foam casting. I've watched several youtube videos and read quite a lot of articles - seems quite simple? Not really

First - faces of the part are extremely rough
Second - bigger problem - sand often is collapsing making cast completely ruined.

So here are few questions - sand is really dry, should I wet it a bit to prevent collapsing? Should I use a refractory coating - I've. seen many guys are using drywall compound to preserve sand from collapsing,

 

[bhowden]

I have done a couple. Some came out trash and others very good. I used drywall compound which is a bit messy and took longer to dry than I expected. I often wondered about a hybrid where I would pack the foam casting in green sand but I have not tried it.

Brian

 

[Gennady]

pack the foam casting in green sand

That I already tried - quite messy as well (for me) . You need to provide channels for gases to escape otherwise it is all with bubbles. For drywall compound I found that dunking model in the compound (liquified) several times and letting it dry in between is usually produces better results, still lot of mess, you are right.

 

[Dan Dubeau]

I have only done 2. Dry sand, and I used a thinned down drywall coating. While I got really good surface finishes, I've been plagued by venting and fill issues. I have one more pattern made ready to cast, and I think I have learned enough from the various mistakes so far and "expect" a good pour this last time around. If not, I'll make a weldment lol.

I learned a lot from watching Kelly Coffield's videos on youtube, and he helped me with issues when I posted over at https://forums.thehomefoundry.org/index.php

If we weren't in the middle of a heat wave, I would have had another shot at it the last two days. Maybe tomorrow morning, it's supposed to be a bit cooler. I'm hoping this last one is a success, as I'm hoping to employ this process for a few other various projects in the queue.

 

[Tom O]

Go for it you’ll be surprised just how much cooler it is once you back away from the furnace.

 

[djberta]

What I have done for success. Do not follow regular green sand or ptrobond practices. First foam prep, use the blue or pink foam from HD or Rona. Use low Melt hot glue to attach pieces together. Lightly fillet with the glue. You don’t need to glue the pieces directly together just fillet or cover the outside gap. Make sprues from the bottom and up, high points and anywhere with a decent under cut. Be sparing but cover all joints with the glue. If compound gets in then gasses can’t get out and AL cont get in. Dip in thinned drywall compound. Dry. Then dip in thick compound. Dry completely. Next you have to use loose sand it has to breath. Also you need some depth, I like 6 inches at least. Your sprues and gate should be large, I use 1.5 x 1.5 gates to a large soup can. With the soup can down about an inch or more over the gate. With the molten Al, fluxed with no dross. Pour steadily in to the gate until Al shows on the sprues. Leave for some time. You can lightly pack the sand but don’t ram it. But make sure it is all around your shape. I use a 7 gallon drum for casting, 5 gal works as well. Depends on the size.

If you watch Kellie’s videos he pours in to fairly tall flasks which is one of the reasons for success. He is also using a commercial compound and very precise tempature that he pours at.

Even with more failures then success, can be a steep learning curve. Once you hit your sweet spot it is incredibly satisfying. Next on my list is the parts for a float lock vise and a spindle mount.

 

[Gennady]

Thanks. I will give a try (again) I will need a trip to THD for drywall compound to have it in two different thicknesses.

parts for a float lock vise

Oooo - you are doing float lock vise?! Prezo design, I guess ?

 

[djberta]

Thanks. I will give a try (again) I will need a trip to THD for drywall compound to have it in two different thicknesses.


Oooo - you are doing float lock vise?! Prezo design, I guess ?

I’m working on the Mr Pete / blondihacks kind with cast parts like prezo’s. I’m not sure I’m committed to trying to make the spring latch style that prezo made. Also thinking of casting the main parts for tool post grinder. Someone else is doing an end mill grinder with castings, but for some reason I can’t find the thread.

My usual idea is to make a bunch of molds and pour them all in one sitting. I also usually make a couple plates as well. Just 6 or 8 inch wide, stock foam width and about a foot long. Then use them with projects. Usually cut pieces from making gates and sprues.

If you have not done so yet look at making a hot wire foam cutter using guitar string and a power supply. Makes roughing shapes and cutting the foam a lot easier and quicker. Mines just cobbled together from scrap parts. I made it at least 30 years ago when I made my gingery lathe.

 

[Gennady]

Agreed, Latch style is kinda complicated, Lead screw style is more "doable".

look at making a hot wire foam cutter

Yep. On my immediate project's list. I like your idea of making model parts from primitive shapes and just glue them together, also - to use fillets as a gluing media.

 

[Dan Dubeau]

https://canadianhobbymetalworkers.c...he-building-in-there.11706/page-8#post-182594

Here's the hot wire I made a while back. Works great. I've since switched to nichrome wire instead of guitar or safety wire. Works even better. My first wheel pattern I CNC'd the foam. Made a giant mess, and while nice, not really needed for a wheel pattern. The next two I cut simple circles from carboard and spray glued them onto foam. The wire follows the cardboard perfectly, and then I just glued the shapes together. I used wax fillets in the corners. Use low temp wax, and it burns out with the foam.

I'm all setup to pour again. Just taking a break for lunch, and will fire up the forge in a bit.

 

[Gennady]

I'm all setup to pour again. Just taking a break for lunch, and will fire up the forge in a bit.

Yeaaa... heat is getting down. It is much more comfortable outside. Please, send some pictures - your models and castings.

 

[Dan Dubeau]

Casting went great this time. Only some very minor surface imperfections that machined out.



First look through the smoke....





Only some slight rippling on this edge and again, and under cut sand fill issue, but all in all a pretty good and useable casting.


Heres some shots of the pattern pre dip

 

[djberta]

Cool. Looks great. This thread has me in mind to do some casting for sure. If you find that issue with the undercuts. You can move your gate to minimize it. Also can vibrate the sand to get in to those areas. I have read but have not tried after doing your regular plaster coats, you can mix the plaster and sand together and then reinforce the area with the thicker sand plaster mix.

 

[Gennady]

I have read but have not tried after doing your regular plaster coats, you can mix the plaster and sand together and then reinforce the area with the thicker sand plaster mix.

I've seen that ceramic coating made similar way - dipping into ceramic slurry and then sprinkling with sand. Probably. same method can be applied with plaster.

 

[Tobho Mott]

I have only done 2. Dry sand, and I used a thinned down drywall coating. While I got really good surface finishes, I've been plagued by venting and fill issues. I have one more pattern made ready to cast, and I think I have learned enough from the various mistakes so far and "expect" a good pour this last time around. If not, I'll make a weldment lol.

I learned a lot from watching Kelly Coffield's videos on youtube, and he helped me with issues when I posted over at https://forums.thehomefoundry.org/index.php

If we weren't in the middle of a heat wave, I would have had another shot at it the last two days. Maybe tomorrow morning, it's supposed to be a bit cooler. I'm hoping this last one is a success, as I'm hoping to employ this process for a few other various projects in the queue.

Kelly Coffield has lost foam all figured out, +1 for that, you can't go wrong by following the methods he's documented in his videos and many forum posts on THF. Ie..dry unbonded sand packed by vibration, top gating, sufficient head pressure, and a dip coat that's barely thick enough to be opaque - thin enough to let the foam vapour vent through it into the sand. The coating isn't structural, it is only for improved surface finish (as good as the foam, no sand cast texture) and a barrier to sand inclusions on surfaces to be machined.

The mold will immediately collapse if you don't keep the gating fully primed until the whole mold is full - if your sprue splits into a Y to gate into 2 places for example, the 2 arms of the Y can't have a greater total cross section than the main sprue, or one or both of them will at least partly collapse. Lost foam molds will try to trick you into thinking they're full before they really are, too. So you stop pouring, only to immediately see your pouring cup gulp down any metal that's in it and cave in before you can resume.

I don't do a lot of lost foam but when I do I rarely use the coatings, because I only use this method for making quick and dirty lumps of metal, not making nice parts. But if you take the time to do it right like Kelly does, you really can make some very nice parts that way. You can even cast hollow castings with nothing but the same dry vibrated sand used for the cores (sorcery!)

Good luck!

Jeff

 

[LenVW]

I did process engineering for an investment casting company back in the 2000s.
We used to buildup the casting shells using a initial fine slurry to produce part details. Then followed by progressively coarser slurries to form the structural backing for the shells.
Shells were produced with a ‘wax’ model and typically 6 to 10 ceramic layers of slurry.

The process would result in parts that could be cast with fine details and tight tolerances.
At the time we were producing High Alloy ‘Impellers’ and components for centrifugal pumps that would operate at high RPMs and Fluid pressures.

 

[Gennady]

The coating isn't structural, it is only for improved surface finish (as good as the foam, no sand cast texture) and a barrier to sand inclusions on surfaces to be machined.

Thanks Tobho. That part wasn't entirely clear to me. I will give lost foam casting a try during this long weekend.

 

[Gippeto]

I use a single coating of thinned drywall compound with a little Dawn dish soap to aid wetting. Low temp hot glue or 50/50 toilet ring/paraffin wax for filleting/bonding. The coating just gives you control of the surface finish...what you had on the pattern is what you get on the part. Unbound playsand and vibration. Parts sprued up so as to eliminate undercuts that could otherwise be unsupported by the sand and result in mini "blow outs" in the casting. The knob blanks were a test, the one done without coating to see the surface finish.

 

[Dan Dubeau]

Toilet ring/parrafin mix is what use for fillets too. Melt it, suck it up into a syringe the plunge it out into a line on the bench. Fillet tools to press it into place.

I made my own fillet tools by welding various size balls to wire/rod. Even picked up an alcohol lamp from a local antique store. Showed my FIL and he laughed and said I could have just borrowed/had all his old ones, as he's a retired patternmaker, but where's the fun in that.

does the Styrofoam cup pouring basin work good? I bought some fiber board to make some reusable cups, but haven't had a chance yet. Just using cans for now.

 

[Gippeto]

The cups do work well, and as they are coated, keep any sand from entering with the pour. Can break them off fairly easy while they are short hard. Wheels for my furnace.