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I'm planning on casting an object in clear resin using a spherical silicone mold and did a test to see what I'd have to expect. It's came out quite well - better than I thought it would! - but around the circumference where the seal of the mold is, there are a number of little gouges:

Resin Mold

Some of them are quite deep, 3-4mm. How can I fill them? I thought of using more resin but I'd have to apply it to a centimetre or two of the casting at a time which sounds incredibly tedious. Is there something else I can use in place of the resin, like a paste, which sets faster? I've read about using things like milliput but they aren't transparent.

Alternatively, is it a good idea to just try to sand the whole sphere down a few millimetres to remove the gouges?

Thanks for any help!

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    How would you go about sandpapering the entire sphere uniformly? – Joachim Apr 24 '19 at 13:12
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    are you using a mold release agent? Additionally, are you vacuum or pressure treating the mold? – fred_dot_u Apr 24 '19 at 14:00
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    Resin spheres (or wood/resin spheres) are often turned on a lathe, including for the polishing. Example: youtube.com/watch?v=ulGB2ixZu1Y (also @Joachim) – Chris H Apr 25 '19 at 8:13
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    Just came across this question. Are you still looking for a solution? If so, a few questions to better understand the problem. 1. Is isn't clear what surface the gouges are on (the picture may be a bit of an optical illusion). Does the silicone mold have gouges in the surface that leave raised areas on the casting, or are you getting gouges in the casting from raised areas on the mold or from something going on during casting? 2. Are you looking to get one perfect sphere and you're done, or cast a lot of spheres (if the objective is to fix the process, how robust must the fix be)? (cont'd) – fixer1234 Jul 29 '19 at 2:19
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    3. Resin tends to be a lot more expensive than the mold. If the problem is with the mold, would it be practical to just get another one? 4. If you're talking about sanding the sphere, does that mean that the actual size isn't critical, and it doesn't need to be a perfect sphere? 5. Are there visible abnormalities in that area of the mold? 6. Have you ruled out a "process" issue (interaction in that area between resin and mold producing the pattern, or removal before curing in that area was complete)? (cont'd) – fixer1234 Jul 29 '19 at 2:19
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Use a clear two part epoxy resin to fill the gouges.

You can get one with a short drying time (5 minute epoxy) at most hardware stores or even the dollar store.

Ensure if its a two part mold, not to put the parts together while its drying or they will stick together.

Apply the glue in a ventilated area.

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It looks to me as though the marks you are seeing are resulting from your resin shrinking from the inside out (hottest part first) as it cures, while the resin on the outside (the coolest part) is still somewhat liquid, thus pulling away from the mold and leaving marks due to places where it has still adhered to the mold versus where it has pulled away. So one option you might look at is to try to find a type of resin that shrinks less (which typically means it also takes longer to cure), or try to cure your resin more slowly at a lower temperature. Another approach is to try to heat the outside of your mold (blowtorch? oven? hot air gun?) so that the resin cures first there, thus pulling away from the mold more uniformly. Another (generally always advisable regardless of what else you do) is to use spray mold release into the mold before you use it so that the resin won't try to stick to the mold and will just pull away everywhere as it shrinks, instead of pulling away unevenly as it has apparently been doing.

I would be cautious about using a different kind of resin (e.g. 5-minute epoxy) to fill such gaps: the other kind of resin may cure a different color, or may eventually turn a different color due to UV exposure (browning) at a different rate than the resin you used for the sphere. This may be noticeable, either initially or over time. The other resin having a different refractive index may also be an issue, resulting in distorted light transmission instead of a nice clear "spherical lens" effect.

If I found myself with your problem, I would probably try to use the resin I already had to solve it, either by applying it with a paintbrush to fill the gaps and then sanding the sphere smooth again, or by putting some mold release in the mold, then pooling a little bit of resin in the bottom of the mold, and then painting the surface of the mold with resin, and then painting some resin on the sphere (filling the crevices), and then re-inserting the sphere into the mold (being careful to watch out for bubbles!) so that the little bit of resin in the bottom of the mold is displaced by the sphere and is forced to flow up around the sphere (hopefully taking bubbles with it) until some pours out the top of the mold. (Essentially, re-mold the sphere with some additional resin, so that gaps will be filled.) Hopefully the second time around, the amount of shrinkage would be minimal since such a thin layer of resin is being cured.

Another approach would be to just put the sphere into the mold and then pour resin over the top of the sphere, allowing it to flow down around the edges of the sphere into the mold until it fills the space between the sphere and the mold and covers the sphere completely. This will only work well if your resin is relatively thin, however.

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