LFMC12 - More Plaster

LFMC 12 - More Plaster - But Less...

Same shit different castings

These pieces were cast earlier in December of 2003, all are showing varying degrees of of the same flaw.

I call it the "Damp Plaster Flaw". As theworkshop.ca is not heated through the week when I'm at work the plaster coating never got a chance to dry out completely.

The dry loose sand that I use generally can absorb the venting foam vapour and a small amount of steam.

Each of these pieces made semi-violent burps and farts when they were poured. The edges of the feeders show flared out vanes and the center piece actually pushed the sand up a full inch and a half.

Back to Basics - and Casting Threads

I still get a surprising amount of email asking for more detail on how a Lost-Foam piece is cast. I try and summarize the process and provide a link to the "BackYardMetalCasting" forum if they need more details.

I've recently reviewed the entire LFMC section, and actually find it a bit repetitive, regardless here is a complete session.

A series of parts are made in foam, either by hand or machined. I like to have at least 3 or 4 pieces at minimum ready to cast in a session just to capitalize on the economy of the preparation steps

I use scrap lengths of 1" square foam as runners to feed the forms. The edges of the runners are rounded off with a coarse file.

The runners are cut to fit together as an apex where the metal will enter them. the angles are just eye-balled and cut with an old hacksaw blade.

The runners are hot glued to the forms and also at the apex where they join. To clear-up any confusion from earlier pages, the hot glue works great provided you get your metal to a proper heat, as runny as water, and some perceptible glow off the entire pot.

The image to the right is a repeat from the "ThisOldeLathe" pages where I started to describe using an extremely light refractory coating on the pieces. I mean like just a slight skin of plaster. The coating was completely dry by morning.

I never set-up any molds into sand until the day that I'm going to cast them.

The process essentially is the same way I started out with a couple of years ago. Put a piece of foam into a container and add sand around it, leaving some exposed foam to pour the metal into.

The tin can is a fairly recent addition that is working out great. Since upgrading the furnace to an oil fired unit, the original Alum Pop cans were melting on contact with the liquid metal being poured.

The entire container is tapped and banged on the floor repetitively to get the sand to settle as snugly around the form as possible.

An interesting side note is the idea to cast a set of "Starter Threads" into a piece that will be used on the new lathe.

The threads are easily formed by turning the pieces onto the spindle.

Here the threads are preserved with about 1/8th" of plaster rubbed into the grooves by hand (actually a finger)...

The entire hole is not filled as it would take days for the plaster to dry enough to cast properly. The dreaded "Damp Plaster Flaw"...

Here is the finished unit, even though it won't thread onto the lathe spindle, they look far better than what I expected and will likely clean up just fine with a tap.

Getting back to the Lost Foam Casting...

Here is a pot of metal glowing a nice warm orange. I try not to get too hung-up on the shade or colour of the pot as it is so variable depending on the ambient lighting and time of day.

The scum or dross has just been skimmed off the top and the metal beneath is a shimmery silver that looks just like mercury.

The pot is removed from the furnace and poured steadily and quickly into the can.

I always wear a full welders face shield, though admit I'm a chicken-shit... through the shield I'm always watching for the sand to heave either up meaning there's steam or down meaning that the form didn't completely fill and the sand just filled a cavity.

The sheet metal base will catch any spilt metal rather than having it hit the snow and possibly splash with steam on contact.

The casting is left for about two or three minutes as all the styrene burns off and watched for shifting in the sand.

The metal is liquid for close to 5 minutes after being poured, even in the winter time at sub zero temperatures.

While the metal is still liquid (test by poking the top of the casting with a steel rod), the tin can feeder is pulled carefully out of the sand.

I have yet to ruin a piece by doing this, but the can is pulled straight upward, disrupting the sand as little as possible.

Generally I will wait at least 15 minutes before removing a casting from the sand just ensure that it has had lots of time to solidify.

While I'm waiting, the Furnace is re-fired and filled with more metal... Go for a smoke, have a beer... avoid hard liquor and drugs until you're completely done and all gear is properly put away.

Today was a good day, everyone came home alive...

I imagine professional foundry men, must just piss their pants laughing at us hobby casters when we seem so proud of our onesy-twosy pieces that take days to make and cast.

Regardless, again I hope that this clarifies any missing gaps in the previous pages about the "Lost Foam Metal Casting" process.

In closing, this is just one of hundreds if not thousands of variations on a common theme.

Support theworkshop.ca

Back to Metal Casting