The Home Machinist!

If nothing else I try to do my research.
And then I invariably pick a project that requires extraordinary effort.
(I wish I could figure out why I do that...)
Anyhow did my first foam casting of a sundial.
Dial is 10" diameter, a total of 1/4" thick, with a 4" high gnomen (the triangle part on top)
Part of the 1/4" thickness includes a pattern and hour layout lines 1/8" deep.
Coated the pink styrofoam master with several coats of plaster.
Created a special cope and drag that would all me to place the sundial base down and surround with a minimum of 2" of sand.
Inverted the box... attached a square funnel 2" high, and coated with several coats of plaster.
Why dry, filled with sand to the top.
(photos attached)
Used a propane foundry to melt my aluminum - broken chunks of bar-b-que grill.
Got melt to a very liquid state, cleared the sludge, and poured.
Got the usual pour, burp, and orange flames.
I sorta expected the 1st pour to be less than perfect, but I did not expect catastrophic!
As you can see, the molten aluminum did NOT make it all the way to the edges of the disk, nor to the tip of gnomen (which was at the bottom and had gravity to help...
Sooooooooooo... What did my research, or perhaps more accurately MY analysis of that research, fail to compensate for?
How do I improve this type of pour????????

You need to put vents around the periphery. The gases around the outside had no place to go, so they pushed back into the molten Al. Some may have made it out/bubbled through, but not all of it could get out before the Al began to set up.

Believe it or not, it was a conscious decision...
A fellow metal-melter told me venting often created more problems than it solved.
But like most things, that's probably true in some situations, but not MY situation...
Next pour will definitely have soda straw vents!
But how many? 4? 8? 12?
There's probably a formula for something like this, but I'm not sure I could do the math!

Maybe a straw is to big. My mentor showed me to vent using a small wire to poke holes in the mold. Might be hard with lost foam. Good luck
.
Tom C.

A big part of your problem is the plaster. Regardless of what you do, it holds moisture which blows back steam. Also, plaster gives off CO2 gas when heated. Finally, the plaster is not very permeable to gas, so when the foam cooks out, the gas from it has a hard time getting out.

You would be better off using facing sand around the foam and then stab trhe area full of about 1/16" holes with a piece of sharpened spring steel wire. Facing sand is often finer sand than you would normally use and it is run through a riddle which is a frame or ring with a fine screen mesh bottom. This gives more detail with fewer surface defects.

To review, I've understood foam casting and green sand casting to be two completely different methods.
The demonstrations I've seen using foam casting, use foam covered with plaster in a loose sand base without any type of venting.
The molten aluminum is poured in and displaces the foam... any gasses generated in the process exit through the pour... hence the 'burp' that is always seen.
The plaster maintains the shape of the molten aluminum against the sand.
I've seen several successful pours using this approach.
In green sand casting it is important to use fine mesh sand to capture the detail of the pattern which is then removed.
And, yes, thin wire venting around the packed green sand is standard procedure, along with a major vent.
I appreciate everyone who is sharing their ideas, but I'm also hopeful that those who are sharing have actually done this type of pouring...
Chet

I've done that type of pour a few times. One bit of information that I finally found out was that the plaster is simply to keep the sand from the mold. Use the thinnest layer of plaster that you can possibly get. Another thing that that is important is to allow the gasses from the foam to escape. One very simple way is to create a sprue. That is simply a vent that allows the metal to rise back to the surface of the sand. That also gives a reserve supply of metal for the mold. For, when it begins to harden it shrinks. If the mold doesn't have enough metal it will have empty cavities. Keep trying, you are learning, expect mistakes... Good Luck...and have fun.. Mocklane

I have been doing Lost Foam since the late 1970's. Here are some examples of what I have made:

1. A top gated tender truck in foam showing the sprue and gate system.

2. Delta trailing trucks in foam. These were bottom gated (Top gating works better)

3. Bottom gated Delta trailing truck in 3-5's bronze as cast. Notice the high risers.

4. Several cleaned up castings in bronze, aluminum, and iron all done with "Lost Foam".

These foam patterns were invested by using the Sodium Silicate/CO-2 process for sand. Though I have used Petrobond, loose sand, and ceramic slurry, the hardened sand worked best for me.

Note that the tender truck was poured with the truck upside down.

Wow! Truly impressive work!
For my next attempt I'm going to mix up a batch of green sand to try my sundial pour again.
I'm also going to skip the drywall mud coatings, and make sure the pour is at a hotter temp.
I really like the idea of the rigidity you get with the sodium silicate/co2 process, but right now I'd like to avoid that additional level of technique.
Since you've done some foam pours with petrobond (hopefully the equivalent of green sand), do you have any special tips???
Your suggestions and photos are terrific!
Chet

Ronald, I wrote my last comment too quickly...
A follow up question...
How are you making those foam castings???
Chet

Chet, here are some posts that you should look at:

http://www.chaski.org/homemachinist/vie ... 14&t=84856
http://www.chaski.org/homemachinist/vie ... 14&t=78325
http://www.chaski.org/homemachinist/vie ... 14&t=76146
http://www.chaski.org/homemachinist/vie ... 14&t=75939
http://www.chaski.org/homemachinist/vie ... 14&t=73658
http://www.chaski.org/homemachinist/vie ... 14&t=73350
http://www.chaski.org/homemachinist/vie ... 14&t=73270
http://www.chaski.org/homemachinist/vie ... 14&t=73164
http://www.chaski.org/homemachinist/vie ... 14&t=50751

Starting into Lost Foam, like any other project, has a learning curve that depends partially on your past knowledge. If, like I did as a little kid, you glued balsa wood airplanes together, then graduated to HO trains and structures, and finally into larger scale railroading, making a pattern out of styrofoam is no problem.

You can work styrofoam just like you would wood. You can drill, mill, turn, sand, and coat it. However, gluing has to be done with a non-organic glue. Some use a hot wire, like they sell for making HO gauge scenery, but it's not very accurate.

What you make depends on you. I'm for the most part, making RR parts, so they have to be precise and to scale. For Sun Dials, there is no limit on your creativity!

Failed castings are "the name of the game", even for professionals. I have had interesting failures, from which, of course, I learned something.

Ronald, thank you for the research and listed references!
And, yes, I too went on a similar journey from balsa models to CNC (homemade) cutting.
One of my demons is wanting as much correct info as possible to avoid re-inventing the wheel.
But you can only read books on blacksmithing for so long before you have to heat some iron...
I'm finally 'heating the iron' by making my first batch of green sand.
And already I'm finding that the recommended ratios do not match the reality of my sand and bentonite.
(I'm assuming that the quality of the water is a constant... ; - )
Chet