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I have been printing a few props for tabletop gaming: a statue, some walls, wine barrels... and I've noticed that resin gets trapped inside. If it stayed firmly inside I wouldn't care too much, but a few times now a model has cracked a tiny amount and liquid resin has started flowing out.

I know the obvious solution is just to make the model solid, but I assumed that since all the slicer tools have an option for hollowing with infill there is a solution to this.

I haven't tried too many things yet, because I am a little anxious to fiddle too much with the printer setting. I went from having 0.5 mm walls to 0.8 mm but it still happened. I have tried hex-grid infill and pillars. I also increased the exposure time by a second (from 8 to 9), but this was only to see if the object would be stronger and thus prevent the cracking.

I have considered increasing the Z-lift distance just to have the object longer out of the resin bath to drip off better.

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Don't hollow most miniatures below the 40 mm-scale

Tabletop miniatures are quite small in scale. Often they have very thin details. As a result, hollowing them out is not advisable in the first place and you will have the best results by printing them solidly. Most wargames use something between the 16 mm to 34 mm scale, but the problem is still present at the 40 mm scale. So bite the bullet and print solid for small items - it also gives the model a little weight to stay where they shall be on the table.

Only if you start to print things like small busts or vehicles that have quite some hollow pace, you could conceivably manage to include the needed geometry, as elaborated below, and then hollow out the models accordingly.

Hollow prints properly

In case you do have a hollowed print, you need to include two vents to allow the exchange of air and prevent cupping:

First, you need a vent for air to enter the model. This is best placed at the very top of where the included volume will wit on the printer and needs to be accessible to air once the model is raised out of the vat. For safety two bores to let in the air should be present.

The second vent is somewhat optional unless the print takes really long. If you include it, it needs to be at the very lowest point of the included volume to prevent trapping resin inside and allow it to drain the resin out of the model hanging from the print bed. Should the volume have separate lowest points, you'd need to include multiple drainage vents.

All vents also need to be sizeable enough to allow the viscous resin to drain from them. About 3 mm² (~2 mm diameter) is the absolute minimum for low viscous resin. High viscous resins require larger holes of about 10 mm² (~3.5 mm).

Also, no point of the model's interior volume should neck down to below those dimensions or you face the risk of having resin clog those neckings - which is again why you don't hollow small miniatures.

Further listening (especially for larger prints) is here from Angus/Maker Muse, who shows a way to hollow out with a hollow base but skipped the drain vent, and Mark Rhodes, who prefers to use 5 mm holes where possible.

Avoiding holes?

There are ways to avoid holes altogether, if you can orient the item in such a way that at all times one side of the print is open. Let's take for example a crate or barrel:

We could hollow the item and remove the face we want to be on the table later. This way we shape the item into a cup. We could add air access or resin vent, but we could also just put the missing wall to be facing in X or Y and then angle the item ever so slightly to give the item good resin drainage and totally avoid the need for an extra hole in a surface we want to retain. This is how you'd print a cup without a hole, as I had explained in Why cupping is bad for SLA

Or, you could time your return to the printer to right after the print is finished, you turn the print around so the resin that was trapped inside starts to drain down to the print platform and over that into the vat.

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    $\begingroup$ Thanks for your thorough answer. Yeah I had already realized that miniatures should be solid by now. What I am printing at the moment as I mentioned in my post are voluminous things like barrels and walls. Since I am printing them upside down and the bottom of the barrel is touching the build platform I guess I would need to add a 3mm hole in the side of the barrel near the bottom and then one in the lid. I guess I can fill these holes with some glue and hide them with paint. $\endgroup$ Jan 22 at 14:20
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    $\begingroup$ Or you print the lid separately and print the barrels as hollow cylinders. Or you print the barrel hollowed out, lacking the lower part and oriented sideways so that there is a wall missing. $\endgroup$
    – Trish
    Jan 22 at 15:36
  • $\begingroup$ @hansnordblad added some way to try to avoid it $\endgroup$
    – Trish
    Jan 22 at 16:19
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    $\begingroup$ I tend to cut my resin prints in half so that the top and bottom can be printed at the same time on the build plate, which halves the printing time for larger models, and then glue them together. This elevates the need for drainage holes. $\endgroup$ Jan 23 at 19:22
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    $\begingroup$ thanks again for all the help. I found a really nice stack of 5 barrels on a rack. When printing solid it comes to around 50ml resin and hollowed it about halves it. I am experimenting with hollowing it, then slicing it in half. This should allow all resin to drain out. Hopefully I will be able to glue it together nicely after printing. $\endgroup$ Feb 2 at 8:11

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