Resin casting question

Sounds like you got air trapped in the mold.

There are a variety of reasons that can affect your casting.
1.) Parts layout in the mold and the casting channels and "breather" channels by which air escapes. You will need to take undercuts into account of course.
2.) Resin has too short a setting time, resin comes in different hardening times 2 min, 5 min, 10 min, etc. This can also be affected by wrong catalyst/resin ratio(try for 1:1 Resin).
3.) Air trapped in the resin itself due to too rapid mixing or wrong stirring method.
4.) Pouring the Resin into the mold too quickly thus causing air to be trapped inside.
Always pour the resin along a stick into the mold.
5.) Not enough shaking/knocking of the mold while the resin hardens to get the extra air out.

Most professional casters use a variety of techniques/methods that are not available to use home casters.
1.) Vacuum chambers that will suck the air out of the liquid resin. Remember those old glass domes from science glass to create a vacuum.
2.) Spin casters this will spin the mold and force the resin into the mold and the air out of it. Rotor casters are used to make hollow castings to save Resin.
3.) Shaker table, the filled mold is strapped onto this and shacken/knocked.

Either way most molds will need a few "repairs" in order to get a good cast and all the air-bubbles out of it.

Here is a small hint for making the molds itself.
If you got an old mold, cut it up into small pieces and use those to add to the volume of the old. i.e. add new silicone till the part is covered and than make the mold walls from the old mold pieces and new silicone. Quiet a saving, a shop close to me lets me have old molds off-cuts for FREE.

If you can send me a pic of the mold and a casted part I might be able to give you more concrete ideas.

HTH.

QUOTE: Originally posted by Aaronw Any advice for casting round things like wheels?

Casting the wheel standing up in the mold will help the air to escape through the pour hole/casting channel but it is not a requirement as long as the air has another route to escape by.

I would asume that the flaw is on the side of he tire close towards the rim as I think the air would be trapped there.
Drilling a venting channel on the upper mold should get rid of the problem, but most likely you will need to tidy up 2 places at the back of the part if you do so.

QUOTE: Originally posted by Model Museum Most professional casters use a variety of techniques/methods that are not available to use home casters. 2.) Spin casters this will spin the mold and force the resin into the mold and the air out of it.

I read an article a few years ago on resin casting and the author suggested after poring the resin, quickly put the mold in a pair of panty hose and swing it around your head; poor man's centrifuge, I rekon. Never tried it, but I would strongly suggest doing this outside - just in case.

QUOTE: Originally posted by Hoser I read an article a few years ago on resin casting and the author suggested after poring the resin, quickly put the mold in a pair of panty hose and swing it around your head; poor man's centrifuge, I rekon. Never tried it, but I would strongly suggest doing this outside - just in case.

Sounds interesting, guess it would work too.

Spin casters actually use a mold that looks like a big donut.

QUOTE: Originally posted by Hoser I read an article a few years ago on resin casting and the author suggested after poring the resin, quickly put the mold in a pair of panty hose and swing it around your head; poor man's centrifuge, I rekon. Never tried it, but I would strongly suggest doing this outside - just in case.

nah, do it inside, what would the neighbours think ? if you do it inside, tell your family beforehand. grown men swinging pantyhose around raises eyebrows.