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Putty Working

I've been looking for a fast machining durable material for mold making. Sand/media lost wax isn't going to cut it, and plaster of paris will break to easily. I was thinking maybe make cakes of Bondo and then machine them into flat blocks. They should machine quickly and be fairly durable.

I have two concerns with the idea. Will it handle the heat of molten lead being poured into it: Melting Point: 327.5 °C (600.65 K, 621.5 °F)

Related to that since Bondo can not be preheated like aluminum will it cool the material to quickly creating voids if it can take the heat, and/or will the material remain molten substantially longer due to the greater insulating properties of "Bondo" over aluminum molds.

Will the machined surfaces release the finished blank? With a releasing agent?

I'm looking for a material that machines faster and easier than aluminum for doing fast prototyping. Something I can rough out in one or two passes and and then do a finish pass.

I've done some experimentation with wood in the past. I even made wood bullet molds as a kid for my muzzle loader. Wood has two problems. The heat of molten lead will eat out the cavity relatively quickly. Was not an issue for my black powder gun since I made the bullets with a fairly large gap and shot them with a linen patch. I got about 20 bullets per cavity before I felt uncomfortable shooting them. When the bullets got to big I would just make another mold. The other is that wood distorts. It warps as it dries, and even kiln dried wood will warp and distort over time.

Reply to
Bob La Londe
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There are rigid polyurethanes that are used to cast zinc, lasting for dozens of shots, and which even will withstand a few shots of aluminum. They're used in centrifugal casting machines.

Bondo is filled polyester. It won't take that kind of heat. Metal-filled epoxy *can*, if it's formulated for it, but it will degrade pretty quickly.

Plaster of Paris is indeed weak, but there are high-temp plasters with much higher strength (some of them fiber-filled) that are used for low-pressure aluminum casting. Ferrari engines are sometimes cast in such molds.

Rayite or ThermoMold are possibilities. Also check Freeman's:

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Good luck.

Reply to
Ed Huntress

Have you considered machining a replica of what you want and then use silicon rubber to make a mold.

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Dan

Reply to
dcaster

Oops! I meant rigid silicones, not polyurethanes.

Reply to
Ed Huntress

That is an option. I would have to reverse my machining process and pocket around the blank. I guess I could make my blank out of just about anything that won't distort in a day then. I already have positive and negative aligning pin code snippets saved, and I just figured out offsetting (roughly using G10 or G92) last night so I can plug them in anywhere. I would cut a pair of blanks out of a block of wood even for that short time period. Leave a lip, and then pour media in the mold to make a mold. LOL. Make one with positive pins and one with negative pins.

I may just go back to aluminum and cut my molds directly. I just takes a long time. Time wasted if there is a mistake in my code.

Reply to
Bob La Londe

There is some math and shape dynamics that are much easier to do if I cut the mold directly. Simple things like making a cutter sized ball milled groove. I guess I could cut a mold out of a fast machining material, make a blank from the mold, and then use the blank to make a mold out of a pourable material.

Yer making my head hurt.

Reply to
Bob La Londe

Use something cheap and fast machining, like wax, foam or Bondo to test your code, and when it works, run it in the Aluminum?

Reply to
Stuart Wheaton

Machinable wax , harder than normal wax , can be made by melting candle wax and adding bits of polythene plastic until no moere polythene will disolve in the wax .Pour it into a block mold and let cool . Machine away.

Reply to
Kevin(Bluey)

Really!? That's way cool ... never heard of that before. Just the idea that polythene (aka polyethylene) would dissolve in wax is new.

How hard is the result?

Thanks, Bob

Reply to
Bob Engelhardt

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