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lost pla/pattern castings ?

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picclock:
I'm thinking of building a minnie traction engine on a budget, like NormanV's thread. The price of castings is pretty high so I thought I might investigate 3d printing the parts then have a go at casting them. I have quite a lot of scrap aluminium, brass and copper which could be used. Possibly bronze using very old non magnetic 1 and 2p pieces (pre 1993 ?)

Most of the parts just really need an impression in the plaster(?) rather than the full double sided casting process. I figured I would subject the plaster to vacuum to get rid of the bubbles then remove the pla using some form of mould release.

For the rear wheels I thought I would try an alloy of aluminium with 5% copper as Wikipedia indicates its not a bad idea. 

Any thoughts or advice on this much appreciated as I have yet to cast anything in metal.

Best Regards

picclock

Joules:
Wet sand the PLA parts, use sanding sealer on them and wet sand again.  For release agent you can use Vaseline.

vtsteam:
I suggest trying a small simple part first with your chosen method, cast in straight aluminum, without 5% copper.

You can melt that in even an open wood fire pit, after it produces glowing coals at the bottom. Put your crucible in there with the coals, and no need for draught.

It will melt, it will only take a few minutes longer than a furnace lined with $200 worth of refractory, plus gas burner, plus draught blower, and all the techno-wonder gear, etc.

This experiment will tell you everything about what needs improving or revision in your chosen molding method, with little expense or waste. Gradually work those details out before investing in anything else, like a furnace.

Furnaces do nothing but heat metal. Molding process is everything -- and that has to be learned through experience.

vtsteam:
The title says "lost PLA" which implies melting out the pattern material, either in the heat of the pour metal, or by mold baking, rather than releasing a permanent pattern (eg. with Vaseline).

I would think that baking would be the preferred "loss" method for a 3D printed pattern, as there is a larger volume of flammable material to get rid of otherwise.

Generally foam patterns are used for the metal replacement method, because they have a much smaller amount of flammable material. Though even then, cooler metals like zinc alloys can have problems completely eliminating pattern material before solidification.

In any case, as I said, try something very simple to start with, and see what happens -- this will guide you as you perfect your preferred method.

picclock:
@Joules
Why? My 3d prints come out really smooth. Vaseline for release agent I can understand. I thought that I would make some impressions in plaster of paris then when its set off remove the pattern and fill the hole with Aluminium or whatever.

@vtsteam
I already have refractory bricks, the kiln type, which I have used to silver solder steel parts. Once tried using an aluminium support and it melted like a candle  :palm:. Since then have always used steel parts for clamps/support. No problem melting ally or I suspect copper/Brass. Real problem is not overheating things, because although I have an infrared gun type thermometer it only goes to 650C.

Will try with a test piece and see what transpires. Maybe mold the chimney support in Aluminium then copper just to see what gives.

Would really like to cast the rear wheels as if it can be done done with an aluminium/copper alloy I should be able to incorporate the treads and not suffer from excessive wear or oxidation.

Thanks for the helpful comments.

Best Regards

picclock 

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