Hello all,
I am looking to cast using a lost polystyrene method, if I wanted to not have the polystyrene cells showing up in the cast, could I use masking tape to cover it, would it vaporise or would it cause issues?
Many thanks
Posted Jan 13, 2019, 5:00 PM UTC
It probably won’t burn out via direct pour but if you pre fire the mold it may burn out better.
The mold medium is important to surface finish as well. Green sand is pretty bad for surface. Petrabond is better. Plaster is likely best.
My casual recommendation is make a lost wax mold, dip the wax in a few layers of plaster to make a shell then ram up the shell in greensand to support it. Bake out the wax and cast.
In my lost pla casting I use paraffin canning wax it goes on very thin and it leaves a smooth surface
It wont work well at all. Burn out problems as mentioned. Use dry wall mud to finish. Take a look at what is happening over on “Alloy Avenue”. They do a lot of Excellent lost foam castings.
Joe
If the surface from styrofoam isn't good enough, and you're not willing to clean up the surface after casting, I think you want to look into lost wax casting instead of styrofoam.
The tape won't burn out, it will char instead. This might go ok, or it might go poorly, but I doubt it will produce the results you're hoping for.
But hey, running a test part is pretty easy, right? Fire up the furnace and give it a try on a small demo part. See what happens!
I've tried it, it will work. But you'll end up with tape lines instead of that foamy texture...
Extruded polystyrene foam (pink/blue insulation panels) doesn't have the cell texture of expanded polystyrene foam (aka beady white packaging material) and is easier to work without tearing it all up. It'll sand smooth quite well with a little care, and you can even invest your foamies in a really thin layer of thinned down premixed drywall mud to get a very faithful reproduction of the actual foam pattern's sanded smooth texture in your casting (let it fully dry before casting).
Personally, I usually skip the coating as I use extremely fine sand for my lost foam castings and find that gives a smooth enough texture for my needs.
Another solution would be to wipe some wax (lip balm, toilet gasket wax, anything that's really soft will be easier to work) into the pits between the expanded polystyrene cells. That is one way to make that type of foam pattern smoother, but be careful not to slather the wax on too thick or it'll give you problems in your castings.
Good luck!
try using extruded polyurethane instead ? or you can use ceramic shell and just coat the surface of the polystyrene with disclosing wax, you didnt mention whether youre ramming it up in sand, or if youre dipping the foam in shell. but either way if youre concerned about the surface then use polyurethane extruded foam its got two sides that are dead smooth at least.
That's a great idea!
It was a very rough just rammed in sand job.