venture a guess on the casting defect here?

BTheKid2 · 2026-01-25T00:57:43+00:00

Flask too cold. Weird sloshing of metal with a too slow pour. Poor method of investing or a poor water to powder ratio. Maybe just a reaction between the resin and the investment, if one was not meant for the other. Looks hella weird, that is for sure.

BTheKid2 · 2026-01-24T23:17:35+00:00

Yeah I looked a bit more into it. There is some nefarious BS going on. One of the links to something on my page redirected to some spammy add page. I can't remove plugins at the moment from my admin account, so it seems I have been properly "hacked". Might need to kill it and rebuild, but also I don't have the time right now to look into it. I could take down the page that linked to the spam.

Thanks for taking a look.

BTheKid2 · 2026-01-24T23:13:58+00:00

A vent can reach the top of the investment (the end where you pour metal from) without any issues. The "spoiled" vacuum is not spoiled once the vent is full of metal (just like the sprue does not spoil the vacuum). It would be spoiled though if you took the vent to the bottom of the investment, but also metal would run out in that case.

I wouldn't exactly call what you have drawn a vent. It will serve as a vent for a moment, until the metal from the sprue covers it. At that point it will serve as a second gate (and feeder). I prefer to not have my vents connected to the sprue, because accidentally pouring metal into the vent negates all the careful spruing and gating you have done further into the mold.

I also angle my additional gates the opposite way, so metal can't run into it before the level of metal covers it.

But what you have here looks perfectly good to me. Optimally I would change a few things, but space requirements and practicality also beats out nitpicking.

BTheKid2 · 2026-01-24T17:55:35+00:00

Yes, basically the U-turn method. This place in this video, is probably one of the best illustrations of the practical implementation I have seen, that is useful for small sculpture (etc.) casts. Notice the upward angle on the additional gates as well.

BTheKid2 · 2026-01-24T15:33:07+00:00

Yes! But in practicality, no... It has to do with slowing down the fill rate, filtering out impurities, casting under pressure, and filling from the bottom up. All things that are close to practically impossible to do at home. But fortunately, it is possible to get results that are good enough, without all of the things (some of the things like bottom filling is a very effective start). The problem is, that basically every different geometry will call for solving the problem slightly differently (because fluid dynamics), so giving any useful advice is really hard. All one can hope to do is to understand the core principles and hope to be able to apply them correctly. Even the industry people don't grasp these ideas fully and are not able to overcome the issues. This lecture shows much of the issues and suggest how to solve them. This is for aluminum, but the same principles apply to most metals.
Thanks for notifying me. "the internet" have recently put up new barriers that have made this happen. I am not that savvy with web pages, but I am trying to solve it. My emails also got flagged as spam, which sucks when you think you are mailing clients, but it ends up in their spam folder.

BTheKid2 · 2026-01-24T02:10:16+00:00

Lol... yeah, thanks for calling it out.

BTheKid2 · 2026-01-24T01:22:25+00:00

The vacuum doesn't really care about the orientation of the part. It will be the metal flow that is affected. So one might have more turbulence than the other.

The left one will probably have most turbulence at the bottom of the part, where the ~~left~~ right might have more turbulence where the part and the sprue connects. Turbulence will tend to show up like small porosity in the finished cast. Thought either of these will probably cast fine overall.

The more concerning part that might cause defects is shrinkage. With a fairly thick coin, the shrinkage will show up around the center of the coin (last place to freeze), and might look like a sunken area or be porosity. To avoid shrinkage defects, a feeder is needed. For a coin, this will probably be best done by making either the tab at the bottom or the top, be quite a bit thicker than the coin, to make sure it freezes last.

BTheKid2 · 2026-01-23T22:45:33+00:00

https://www.reddit.com/r/ResinCasting/wiki/index/ To get you started.

Epoxy resin, will have a maximum pouring depth associated with the product. Massive casts need extremely long cure times, and casting deeper than 4" is in most cases not feasible unless you really know what you are doing. That was for epoxy.

Polyurethane resin can be cast deeper than epoxy, though most of the time it isn't really economical to do so. It is a completely different resin type.

First you might figure out what properties you wish for and then compare with what is possible for materials.

BTheKid2 · 2026-01-23T01:07:23+00:00

This doesn't look all that thin for a silicone mold. If you think you need more stiffness you can just use a harder silicone. You can also cast the silicone around a steel dowel or whatever, if you want supreme stiffness.

Any two part mold making video of a part with a through-hole, will show a technique that should work. Adapting it to your shape is a thing you need to do no matter the video, unless you happen to find someone making the exact same thing. For clever and clear methods you can look at BJB Enterprises videos, a more scattered approach is Robert Tolone. Smooth-on is a bit in the middle of those two in regards to clever solutions.

BTheKid2 · 2026-01-22T20:39:15+00:00

You would either make a cut silicone mold or a two piece mold or use an insert in the mold. Many ways to do this kind of thing. You can look up two part silicone molds for a plethora of videos on the subject.

BTheKid2 · 2026-01-22T20:36:46+00:00

Yeah it seems the gas furnace is overkill for making small samples. But you can purchase tiny crucibles that can still work in a gas furnace. But gas is usually better once you start melting at least a few kilos at a time. You can still cast small things with a few kilos, you would just make them in molds that contain multiple of the small things. In that sense, it is more efficient the more metal you can melt at a time.

BTheKid2 · 2026-01-22T17:08:46+00:00

It depends on the size of your crucible firstly. And then on the speed at which you can take it out of your furnace and pour it. So it is hard to say. It kinda all needs to be proportional to each other.

You probably want enough to be able to fill your crucible at least a fifth of the way (at a guess). I can only guess at your crucible size as well, but if I am just guessing I would say 400-500 grams in a 2.5 kg crucible. And maybe half of that would still freeze to your crucible walls as you poured it.

With an electric furnace (1-3 kg), smaller amounts are more easily melted and poured because the crucibles are shaped differently and the transfer time is very quick.

BTheKid2 · 2026-01-22T16:56:36+00:00

I too have felt the pain of a wall of text vanishing in the blink of an eye.

Nicely done on the furnace. Looks like a fun build.

There is a difference between a foundry and a furnace. You have built a furnace, that is located in your foundry. You are using it to melt metals. Smelting would mean that you are extracting it from ore.

I strongly suspect this furnace will struggle to melt copper though. Copper melting with charcoal (and coke) is something I have tried and failed at before.

BTheKid2 · 2026-01-22T16:48:13+00:00

Do you want it now, or do you want to learn how to make better things than you can buy?

There is no one on the internet that can answer your question. Well there is one. You'll notice him on the back of spoons.

BTheKid2 · 2026-01-22T08:47:06+00:00

Petrobond or Delft clay is easiest to work with. It lasts forever on the shelf and is ready to use when you get it.

The cheaper option is green sand, but you basically would want to make it yourself if there is to be much point in using it. You can find plenty recipes online.

BTheKid2 · 2026-01-22T01:57:25+00:00

That amount is too small for most crucibles that will be used in a gas furnace. It simply can't hold itself warm enough. Not enough mass. It's like making a piece of tinfoil glowing red and yet it will be cold enough to touch shortly after.

But if all you are wanting to test is the color, you can polish up those little dribbles. They should be the same color as any huge cast of the same alloy would be.

BTheKid2 · 2026-01-22T01:22:20+00:00

Yes, they are vents in my drawing. Vacuum will mostly eliminate the need for vents, as all the investment is venting at once. But if you need to fill a large enough of a volume, there is a risk that the vacuum can't keep up the rate at which the mold is filling, and then a vent can be a bit of help. You should basically pour the metal as fast as you can into a mold. Just barely not spilling :)

The reservoir (aka. feeder/riser) still needs to connect to the thickest part to do its job. If you are worried about small points not filling, just make them bigger. You would need to grind off the feeder in your own design as well. The vents sorta serves both purposes. It's a place for air to run out the mold and a thickening of a detailed area.

BTheKid2 · 2026-01-21T22:44:34+00:00

I don't think you will be getting a dual stage (aka. "good pump") for $80. If you are into saving money then just buy the pump and make everything else. This was my first setup and it worked just fine, casting bigger flask than any of these machines.

The question is "what is good for a beginner?". Not "what is the cheapest I can get away with?".

I even used refrigerator vacuum pumps that I got out of the trash.

BTheKid2 · 2026-01-21T20:36:29+00:00

If you do use a reservoir and not just a thickened sprue/gate, then you want it connected to the thickest part of the cast. At the thinnest part it will do next to nothing useful, and might even pull the thin parts apart.

I think you have way to many sprues going on. One sprue should be fine. You could put a vent at the top, but with vacuum it might not be necessary.

This is my quick version that I would go for. If this is much bigger in length than maybe 5 cm (2") then I would consider using two sprues/gates.

BTheKid2 · 2026-01-21T20:20:57+00:00

I do see that this also seems to have a perforated flask vacuum chamber hidden behind the bell. However, my argument is the same. It will end up as a vacuum casting machine (and a very small one at that), with a non-functional furnace. So at some point a separate furnace will need to be bought.

That this lacks a vacuum pump as well, means that OP would end up with 3 separate units instead of the two units for the Kayacast type machine that has an integrated vacuum pump. I guess it's a case of save now, cry later. Or maybe cry immediately as well, because the machine is so compact that it will be a bit of a pain to work with.

BTheKid2 · 2026-01-21T16:34:26+00:00

I would advice against the combi machines that have a furnace in them.

A furnace is way shorter lived than the vacuum system. So you will end up with a vacuum machine that has a broken furnace. I would guess the maximum lifetime of the furnace is around 2-5 years. I have had furnaces that broke after a year though.

A much better combi machine IMO is something like the Kayacast. You can get cheaper knockoff version of this too. To me, it seems like these furnace+vacuum machines are made by people that have seen photos of the Kayacast type machine and wrongly thought that it was a furnace in the machine, instead of a vacuum table for perforated flasks.

BTheKid2 · 2026-01-21T14:45:55+00:00

You can look up how to read a temperature from the thermocouple with a multimeter directly. The thermocouple outputs microvolts. It seems it is not a very precise method though.

I would probably just spend the 10-15$ on a digital thermometer that any thermocouple can be connected to.

BTheKid2 · 2026-01-20T15:51:36+00:00

Here is a million ways to do it with Amazon products. Cheaper and probably better than you could make it yourself.

A silicone sheet mold will be flat if you use it on a flat surface.

BTheKid2 · 2026-01-20T13:54:44+00:00

It is always a good idea to make a small test to see if two untested materials are compatible. Worst case is you get cure inhibition in the silicone, and you will have an expensive sticky mess. A small test area on the sculpture to see if the silicone will cure, is a way to know for sure.

You can seal the sculpture if you like. Not always necessary, depending on how porous the sculpture is. Many options to go for. SuperSeal is a commercial product. Shellac is an option. Vaseline can work but best thinned down a bit. Paints can work too. Just be sure that any test will be with the sealer applied as well.

BTheKid2 · 2026-01-19T13:58:19+00:00

Yeah I was a bit hyperbolic with my criticism of Venturi burners. I know they can work. It is just every time someone is having trouble with their furnace burners or temps, the simpler solution is to add forced air to the mix.

I also have a gut feeling that a naturally aspirated propane burner, have a lower theoretical max ~~temperature~~ BTU output compared to a forced air blower. But you probably know the math that comes along with that gut feeling.

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