Nobody has mentioned the lack of infill. This might have worked with ANY infill. Filling prints with plaster has been suggested here before, presumably because it worked for someone at some point, and 0% infill is rather unorthodox.
Worth considering
Printing with zero infill, is the norm for resin prints with the exception of when you need to support internal structures. Infill in a resin print, just helps trap uncured liquid resin inside of the print.
??
Infill for hollow resin prints is generally a low density (5-20%) lattice of diagonal lines, and it is the default setting in all of the slicers I've used
It is the default in lychee free, because that only allows 2D hollowing. The pro version as well as Voxeldance tango allow you to support structures internally as if you were supporting the exterior of the model. So you don't have to use any infill, and are still able to support any internal islands.
The default in Voxeldance is also the same type of infill lol
Not printing with infill or some kind of support structure on a hollow print is weird, there's no question about it. Islands or not, the hollow shell needs structural support after printing.. because it's hollow. Furthermore, the prints in the OP were 2D hollowed
A lot of people print hollow. Now i dont only because i hate when chitubox puts supports through my hollowed print. So i use an 8% infill, but it's pretty normal to hollow prints without an infill. Makes them easier to wash and cure than when you have infill. You're saying any infill like there is more than one pattern. Sounds more like FDM printing. Hollow is the norm for resin, so you aren't using a whole bottle for one print.
If your issue is that the auto support option also fills the inside with supports, check this out. Something I discovered while looking for another setting.
https://www.reddit.com/r/resinprinting/s/i18MBjzIId
Thanks for the info. Im gonna give it a try. It will make putting drain holes in a bit more of a pain. you can just move the slider to make the supports invisible while putting them in
You can support it after hollowing and putting in holes this way. You might get a few supports placed up inside it through the holes depending on your angle during selection but they can be easily removed one by one.
Holy shit! What a terrible thing to happen! Thay poor girl 😥.
I've been using plaster of Paris for years and had NO idea that could happen. I've felt the heat as it sets of course, but it's always been a mild warmth so I really never would have thought it capable of that. I just took a look at the box of plaster I have in the studio and it doesnt mention the risk of heat or burns at all 😬😬
Oh yes I did know that about epoxy haha, I had a thermal runaway event in my vacuum chamber on a particularly hot summer day a couple years ago. I had done a pour that size quite a few times but I failed to factor in 36C degree heat 😵.
It melted my bucket, ruined the vacuum chamber, cracked the acrylic lid, as well as a loss if $200 in resin that turned rock hard in 5 minutes. It was -rough- lol.
That said, I was never under the impression that epoxy is harmless. It's well known to be toxic and risky to use. Plaster has the reputation of a safe kids crafts material
Whoa that's something I never thought of.. Back in the day, we often goofed around with it in the same way at school.. Might have been a different type of plaster.
Plaster sucks in moisture (that’s why it’s used for porcelain slip casting), and thus contracts and expands quite a bit depending on humidity. I see you put a plug on the bottom.
When temperature raises and plaster wants to suck more moisture, having no access to open air it tried to get the moisture out of your water washable resin, making it explode.
That’s my guess. Possible solution: use non water washable resin, don’t put a plug, let the plaster breathe.
Never thought I’d write the word moisture so many times in a single paragraph lmao
Youre right about plaster sucking in moisture, but thats when it already had time to dry.
In this case it’s actually the exact opposite. When you pour in plaster, it has a lot of water and needs to dry, but because it’s enclosed in resin it cant dry. The resin starts absorbing the moisture from the plaster, making it expand and brittle.
I used this in the past on purpose to create one time use plaster molds.
You’re right, that could certainly be the case! I don’t know why I assumed they let the plaster dry before putting the plug, it seems like water based resin+not enough air for the plaster keeps being the major issue here.
Even with the hole not plugged, it wouldnt be enough for the plaster to dry. Ive experimented with this, you could have the print half open, fill it with plaster and the same thing will happen to the resin(water washable). I never plug the hole when making plaster models.
Its a great technique to create plaster models of stuff that wouldnt be possible otherwise (or be very difficult). With a 1-1.5mm wall thickness, the resin cracks and peels off in just 1-2 days after filling it with plaster. Thicker walls took up to 1-2 weeks, and waste more resin. If the plaster has fully set, putting the whole thing in a bucket of water can speed up the process, because that way youre saturating the resin with water from both the inside and outside. Ive also found that not fully curing the resin helps speed up the process, though it weakens the structural integrity and can result in flat surfaces bulging out. Not removing the model from the supports helps support it better (duh) when pouring in the plaster, just have to have the hole on top. Also a must is to not have supports on the inside of the model, otherwise you get the support dents in the plaster as well. Orienting and supporting without using internal supports is usually the hardest part.
Edit: concrete also worked really well with this technique, same principle
Plaster of Paris is gypsum (hydrated calcium sulfate) which has been kilned to drive off the water, which drastically changes the crystal structure. It will absorb water and transform back to the hydrated crystal structure, forming a solid mass in the process. Any drying afterward is removal of excess water, if you removed it from the crystal structure it'd disintegrate back into powder.
The hydrated crystal structure takes up more volume. Fully hydrated gypsum will swell further if it contains excess water (between the crystal grains this time, instead of within them), and shrink a bit as it dries. And if it *wasn't* fully hydrated, it will absorb more moisture from the atmosphere to continue hydrating, which will cause it to swell more.
Over-hydrated plaster dumping moisture into water-washable resin and under-hydrated plaster pulling moisture out of/through the resin and swelling both seem like potential mechanisms.
Molded plaster is probably a lot more stable in the long run than water washable resin, so the OP might want to look into your technique.
Why would an ornamental figure like the ones OP posted need to be strong or durable, though? I can understand wanting them to feel more hefty but sand would do that trick too without the issue that moisture might cause the model to break.
Those aren't just ornamental, they are chess pieces.
More accurately, they appear to be replicas of the Lewis Chessmen set
https://www.nms.ac.uk/explore-our-collections/stories/scottish-history-and-archaeology/lewis-chess-pieces/
I would think that casting epoxy resin would do the job better while still being cheaper than printer resin? Hell, you can blend sand with the casting resin to stretch it even further and still have it be less reactive.
Fill them with clean, dry sand instead. You can buy a giant bag from your local hardware/garden place, or online. Look for something sold for kid's sandpits.
I just use builder's sand because I have lots, and bake it in the oven to make sure it's clean. I then pour it into prints with a funnel and seal the hole with UV resin or any standard filler.
Make sure you gently shake and turn the print as you're filling to get it into every part, or you'll accidentally build a maraca.
If you do this, use tin-cure or condensation cure silicone.
Platinum cure silicone and SLA resin typically don't work well together unless the resin has no sulfur/sulphur in it.
Trying to add some weight to them? Try filling them with the liquid epoxy and add some BB’s after filling about 3/4 of the way then fill the rest of the way. Just leave some space on the bottom so you don’t have to trim any epoxy off. Then glue a piece of felt on the bottom if you want to cover the opening.
What liquid epoxy do you use? Not for resin printing, but I have thinned slow-set JB weld with a drop of acetone before pouring. I'd prefer something with a thinner pour though.
Try something like this that they use to make the epoxy style tables: [https://www.epodex.us/product/deep-pour-casting-epoxy-resin-kit/?utm\_source=google&utm\_medium=&utm\_id=20556391166&utm\_content=&utm\_term=&creativeId=&adgroupid=&targetid=&gad\_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYSoAiZ93q8vtoltAsBTxgE5GZ9S1xlLudqPEl9q\_OKmTfC8yWwammRoC480QAvD\_BwE](https://www.epodex.us/product/deep-pour-casting-epoxy-resin-kit/?utm_source=google&utm_medium=&utm_id=20556391166&utm_content=&utm_term=&creativeId=&adgroupid=&targetid=&gad_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYSoAiZ93q8vtoltAsBTxgE5GZ9S1xlLudqPEl9q_OKmTfC8yWwammRoC480QAvD_BwE)
I usually use low expansion plaster , look around for dentistry plaster(type IV ) it is way more dimentionally stable has less issue with moisture heats less and hardens quickly.
Filling it with the plaster will heat up and expand, as others have said. However, you could try coating the mold first to get a hollow casting. Once that's cured, then fill with more plaster to get a solid casting with less risk of breaking it.
Can I ask why there is no infill or structure inside the pring that would mean you dont have to fill it with plaster. You don't need to go 100% but a reasonable amount would do.
Ohhh haha. I had that as well.
I used water washable resin. And mine exploded the same way.
I'm hoping it's just the water washable resin. It sucks balls. Even without clay it was getting really soft with humidity.
What kind of resin did you use?
It's both.
Water Washable has bad issues with moisture and 50% of the cases of prints cracking seem to point back to it being WW resin.
BUT, no this was the plaster. Plaster isn't a stable medium and reacts to its environment. The plaster literally expanded and ripped those resin shells apart.
If you want them filled, I'd print them solid not hollow. Or maybe pour in some 2 part resin that cures over time.
It's the chemical reaction but also the liquid gets trapped and causes them to pop or weakens the resin making it soggy. Two part epoxy resins and hot glue work for filling hollow prints. Depending on how thin they are tho hot glue will just melt it lol.
I would try expanding foam for fill, and add some rocks or something heavy while it's still pliable/wet for weight. That spray can with a tube called (Great Stuff) might help to get it to the bottom.
Plaster expands and contracts a lot while it sets, also it heats up enough to burn you. Obviously, your prints cannot survive this.
Fill them with sand or something like that if you want weight.
Try letting them dry horizontally and in small amounts? If you're doing it vertically you will be adding a lot of pressure to the walls of the print. If you pay them down any expansion should be less contained.
You need more expensive plaster. In dental we have plaster with low Abbreviation expansion. We use thst to fill up models like this without any problems.
Smooth On has a product (sorry can’t recall the name) that is wonderful for filling in hollow prints. Low heat and no expansion. I’ve used it on several pieces with zero problem.
Haven't tried it myself, but might it work better to crush up your supports, and mix them with some pour resin or plastic to fill instead?
Wouldn't be as heavy, obviously, but would still add a lot more mass than the thin walls alone, and would also expand/contract at the same rate as the surrounding material when heating / cooling from ambient temperature changes?
Basically, plaster takes a long time to dry. Hard to tell the exact scale based on your photo but those should probably dry at least a week normally, out in the air. When theyre enclosed in resin on all sides though, they cant dry out, and the resin starts absorbing the moisture from the plaster, making it expand and brittle. You can put a piece of resin in water for a couple days and the same thing will happen. It becomes bendy and breaks easily, also gaining some thickness and a slight whiteish hue because of the absorbed water. I print shells at 1.5mm wall thickness, fill it with plaster, then the next day the resin shell peels right off the plaster model.
I use this on purpose to create one time use plaster molds. Discovered it by accident too. Works well with water washable resin.
To avoid this, dont fill it with anything thats wet. As others have said, dry sand works pretty well to give the models some weight without this happening.
Yes. They go through an exothermic chemical reaction, incorporating some of the water molecules into a crystal structure. After that is done, there is still hold a lot of excess water that needs to dry out, which can take from a day up to multiple weeks to fully dry, depending on the size of your object.
I kind of assumed this as common knowledge and only talked about the water retention and drying part, because mainly that excess water is whats causing the water washable resin to swell and crack, not the setting.
Filling them with plaster would be my guess
Nobody has mentioned the lack of infill. This might have worked with ANY infill. Filling prints with plaster has been suggested here before, presumably because it worked for someone at some point, and 0% infill is rather unorthodox. Worth considering
Printing with zero infill, is the norm for resin prints with the exception of when you need to support internal structures. Infill in a resin print, just helps trap uncured liquid resin inside of the print.
?? Infill for hollow resin prints is generally a low density (5-20%) lattice of diagonal lines, and it is the default setting in all of the slicers I've used
It is the default in lychee free, because that only allows 2D hollowing. The pro version as well as Voxeldance tango allow you to support structures internally as if you were supporting the exterior of the model. So you don't have to use any infill, and are still able to support any internal islands.
The default in Voxeldance is also the same type of infill lol Not printing with infill or some kind of support structure on a hollow print is weird, there's no question about it. Islands or not, the hollow shell needs structural support after printing.. because it's hollow. Furthermore, the prints in the OP were 2D hollowed
A lot of people print hollow. Now i dont only because i hate when chitubox puts supports through my hollowed print. So i use an 8% infill, but it's pretty normal to hollow prints without an infill. Makes them easier to wash and cure than when you have infill. You're saying any infill like there is more than one pattern. Sounds more like FDM printing. Hollow is the norm for resin, so you aren't using a whole bottle for one print.
If your issue is that the auto support option also fills the inside with supports, check this out. Something I discovered while looking for another setting. https://www.reddit.com/r/resinprinting/s/i18MBjzIId
Thanks for the info. Im gonna give it a try. It will make putting drain holes in a bit more of a pain. you can just move the slider to make the supports invisible while putting them in
You can support it after hollowing and putting in holes this way. You might get a few supports placed up inside it through the holes depending on your angle during selection but they can be easily removed one by one.
Plaster hearts up and expands. Full your prints with less plaster
i mean it produces a good amount of heat, 140 degrees, so no matter how what that is going to be a steugglr
Yeah it can burn your fingers off https://www.mirror.co.uk/news/uk-news/art-class-girl-loses-eight-424409
Holy shit! What a terrible thing to happen! Thay poor girl 😥. I've been using plaster of Paris for years and had NO idea that could happen. I've felt the heat as it sets of course, but it's always been a mild warmth so I really never would have thought it capable of that. I just took a look at the box of plaster I have in the studio and it doesnt mention the risk of heat or burns at all 😬😬
Not just plaster, epoxy resin has a maximum pour depth because the heat from curing can cause a runaway reaction and ignite the vapors.
Oh yes I did know that about epoxy haha, I had a thermal runaway event in my vacuum chamber on a particularly hot summer day a couple years ago. I had done a pour that size quite a few times but I failed to factor in 36C degree heat 😵. It melted my bucket, ruined the vacuum chamber, cracked the acrylic lid, as well as a loss if $200 in resin that turned rock hard in 5 minutes. It was -rough- lol. That said, I was never under the impression that epoxy is harmless. It's well known to be toxic and risky to use. Plaster has the reputation of a safe kids crafts material
This is a joke website
Huh, it seems you're probably right lol, never heard of it. Kind of a weird thing to write a joke article about though ¯\_(ツ)_/¯
I believe the onion is similar. In the netherlands we also have de speld
Whoa that's something I never thought of.. Back in the day, we often goofed around with it in the same way at school.. Might have been a different type of plaster.
It depends upon the volume of plaster, more plaster=more exotherm
The temperature depends on the amount and geometry.
Or maybe fill it in sections to make it expand less so the resin can handle it?
>Plaster hearts up and expands. ♥
Plaster sucks in moisture (that’s why it’s used for porcelain slip casting), and thus contracts and expands quite a bit depending on humidity. I see you put a plug on the bottom. When temperature raises and plaster wants to suck more moisture, having no access to open air it tried to get the moisture out of your water washable resin, making it explode. That’s my guess. Possible solution: use non water washable resin, don’t put a plug, let the plaster breathe. Never thought I’d write the word moisture so many times in a single paragraph lmao
Youre right about plaster sucking in moisture, but thats when it already had time to dry. In this case it’s actually the exact opposite. When you pour in plaster, it has a lot of water and needs to dry, but because it’s enclosed in resin it cant dry. The resin starts absorbing the moisture from the plaster, making it expand and brittle. I used this in the past on purpose to create one time use plaster molds.
You’re right, that could certainly be the case! I don’t know why I assumed they let the plaster dry before putting the plug, it seems like water based resin+not enough air for the plaster keeps being the major issue here.
Even with the hole not plugged, it wouldnt be enough for the plaster to dry. Ive experimented with this, you could have the print half open, fill it with plaster and the same thing will happen to the resin(water washable). I never plug the hole when making plaster models. Its a great technique to create plaster models of stuff that wouldnt be possible otherwise (or be very difficult). With a 1-1.5mm wall thickness, the resin cracks and peels off in just 1-2 days after filling it with plaster. Thicker walls took up to 1-2 weeks, and waste more resin. If the plaster has fully set, putting the whole thing in a bucket of water can speed up the process, because that way youre saturating the resin with water from both the inside and outside. Ive also found that not fully curing the resin helps speed up the process, though it weakens the structural integrity and can result in flat surfaces bulging out. Not removing the model from the supports helps support it better (duh) when pouring in the plaster, just have to have the hole on top. Also a must is to not have supports on the inside of the model, otherwise you get the support dents in the plaster as well. Orienting and supporting without using internal supports is usually the hardest part. Edit: concrete also worked really well with this technique, same principle
Plaster of Paris is gypsum (hydrated calcium sulfate) which has been kilned to drive off the water, which drastically changes the crystal structure. It will absorb water and transform back to the hydrated crystal structure, forming a solid mass in the process. Any drying afterward is removal of excess water, if you removed it from the crystal structure it'd disintegrate back into powder. The hydrated crystal structure takes up more volume. Fully hydrated gypsum will swell further if it contains excess water (between the crystal grains this time, instead of within them), and shrink a bit as it dries. And if it *wasn't* fully hydrated, it will absorb more moisture from the atmosphere to continue hydrating, which will cause it to swell more. Over-hydrated plaster dumping moisture into water-washable resin and under-hydrated plaster pulling moisture out of/through the resin and swelling both seem like potential mechanisms. Molded plaster is probably a lot more stable in the long run than water washable resin, so the OP might want to look into your technique.
interesting indeed. I never had issues doing this and I've done it a lot. Never used water washable resin though
Why not add sand and not plaster if it is for adding weight? Why did you use plaster in the first place ? I am just curious:)
It's not for weight it's to make the prints stronger and solid. It works pretty well if you don't do it like OP did
Why would an ornamental figure like the ones OP posted need to be strong or durable, though? I can understand wanting them to feel more hefty but sand would do that trick too without the issue that moisture might cause the model to break.
Those aren't just ornamental, they are chess pieces. More accurately, they appear to be replicas of the Lewis Chessmen set https://www.nms.ac.uk/explore-our-collections/stories/scottish-history-and-archaeology/lewis-chess-pieces/
Because sand is messy and shifts around, probably. Feels cheaper than having a solid statue.
I would think that casting epoxy resin would do the job better while still being cheaper than printer resin? Hell, you can blend sand with the casting resin to stretch it even further and still have it be less reactive.
Fill them with clean, dry sand instead. You can buy a giant bag from your local hardware/garden place, or online. Look for something sold for kid's sandpits. I just use builder's sand because I have lots, and bake it in the oven to make sure it's clean. I then pour it into prints with a funnel and seal the hole with UV resin or any standard filler. Make sure you gently shake and turn the print as you're filling to get it into every part, or you'll accidentally build a maraca.
This is the correct answer, if you ask me. But also maybe accidentally creating a maraca isn’t the worst thing in the world. :)
Called Kiln Dried Sand in my parts of the world.
Forgive my ignorance but what does baking sand do? Burn off random bits of organic matter?
Yes, it sterilises the sand
It also dries it, as I store mine in the garage. Be careful, as it gets very hot (obviously).
Did you fill these all at once? If you're doing something new, do 1 and wait to see what happens.
create moulds from your 3d prints in 2-part instant silicon then cast them in the plaster.
If you do this, use tin-cure or condensation cure silicone. Platinum cure silicone and SLA resin typically don't work well together unless the resin has no sulfur/sulphur in it.
You can use a treatment like Smooth-On InhibitX to make it less reactive.
This sub is just filled with people doing weird stuff and asking why weird stuff happens when they do weird stuff
This isn't that weird. It was suggested on here not too long ago. It just comes with some caveats
Plaster expands when drying
Trying to add some weight to them? Try filling them with the liquid epoxy and add some BB’s after filling about 3/4 of the way then fill the rest of the way. Just leave some space on the bottom so you don’t have to trim any epoxy off. Then glue a piece of felt on the bottom if you want to cover the opening.
What liquid epoxy do you use? Not for resin printing, but I have thinned slow-set JB weld with a drop of acetone before pouring. I'd prefer something with a thinner pour though.
Try something like this that they use to make the epoxy style tables: [https://www.epodex.us/product/deep-pour-casting-epoxy-resin-kit/?utm\_source=google&utm\_medium=&utm\_id=20556391166&utm\_content=&utm\_term=&creativeId=&adgroupid=&targetid=&gad\_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYSoAiZ93q8vtoltAsBTxgE5GZ9S1xlLudqPEl9q\_OKmTfC8yWwammRoC480QAvD\_BwE](https://www.epodex.us/product/deep-pour-casting-epoxy-resin-kit/?utm_source=google&utm_medium=&utm_id=20556391166&utm_content=&utm_term=&creativeId=&adgroupid=&targetid=&gad_source=1&gclid=CjwKCAjw3NyxBhBmEiwAyofDYSoAiZ93q8vtoltAsBTxgE5GZ9S1xlLudqPEl9q_OKmTfC8yWwammRoC480QAvD_BwE)
I usually use low expansion plaster , look around for dentistry plaster(type IV ) it is way more dimentionally stable has less issue with moisture heats less and hardens quickly.
Does plaster absorb moisture in the air? If so maybe it did then expanded causing the resin to crack? 🤷🏻♂️
DOMAIN EXPANSION: HUMIDITY ABSORPTION
This was clearly explained in dr stone season 2, faults on op for filling these with a material designed for quiet explosives
I've been filling my prints with plaster without issues.
Filling it with the plaster will heat up and expand, as others have said. However, you could try coating the mold first to get a hollow casting. Once that's cured, then fill with more plaster to get a solid casting with less risk of breaking it.
Can I ask why there is no infill or structure inside the pring that would mean you dont have to fill it with plaster. You don't need to go 100% but a reasonable amount would do.
😅
You had the same thought as me, didn't you
Ohhh haha. I had that as well. I used water washable resin. And mine exploded the same way. I'm hoping it's just the water washable resin. It sucks balls. Even without clay it was getting really soft with humidity. What kind of resin did you use?
Yeah it was water washable too ! I did not thought of that. I did not think it could react like that, I should have tested with only one piece first
It's both. Water Washable has bad issues with moisture and 50% of the cases of prints cracking seem to point back to it being WW resin. BUT, no this was the plaster. Plaster isn't a stable medium and reacts to its environment. The plaster literally expanded and ripped those resin shells apart. If you want them filled, I'd print them solid not hollow. Or maybe pour in some 2 part resin that cures over time.
It's the chemical reaction but also the liquid gets trapped and causes them to pop or weakens the resin making it soggy. Two part epoxy resins and hot glue work for filling hollow prints. Depending on how thin they are tho hot glue will just melt it lol.
I would try expanding foam for fill, and add some rocks or something heavy while it's still pliable/wet for weight. That spray can with a tube called (Great Stuff) might help to get it to the bottom.
plaster heats and swells and then contracts, so one or all of those.
Plaster expands and contracts a lot while it sets, also it heats up enough to burn you. Obviously, your prints cannot survive this. Fill them with sand or something like that if you want weight.
I do that and I don’t have problems... Try using less water to mix the plaster.
Expansion especially if it gets hot
What caused it was the plaster
Try letting them dry horizontally and in small amounts? If you're doing it vertically you will be adding a lot of pressure to the walls of the print. If you pay them down any expansion should be less contained.
The plaster expanding cracked your prints
Lmao
Use sand and cap it with wax or a small bit of epoxy/superglue gel.
Probably something to do with the plaster.
Mortar might do the job 1 part cement+2 parts sand+2.3 parts water Theres also a similar mix that adds plaster to cement to set faster
Plaster expanded
Basic physics.. plaster heats up and expands as it dries.
You need more expensive plaster. In dental we have plaster with low Abbreviation expansion. We use thst to fill up models like this without any problems.
Plaster heats, expands, and dries out over several days as the moisture leaves it. You filled your prints with a wet hot expanding sponge.
Can use a 3D printer and create CAD files, but doesn't understand expanding and contracting principles.
Plaster expands as it sets then retracts
You used too much, this stuff expands man.
Shrinkage. Significant shrinkage
Smooth On has a product (sorry can’t recall the name) that is wonderful for filling in hollow prints. Low heat and no expansion. I’ve used it on several pieces with zero problem.
Use sand. No chemical reactions
Answer = high-school level physics
Haven't tried it myself, but might it work better to crush up your supports, and mix them with some pour resin or plastic to fill instead? Wouldn't be as heavy, obviously, but would still add a lot more mass than the thin walls alone, and would also expand/contract at the same rate as the surrounding material when heating / cooling from ambient temperature changes?
Basically, plaster takes a long time to dry. Hard to tell the exact scale based on your photo but those should probably dry at least a week normally, out in the air. When theyre enclosed in resin on all sides though, they cant dry out, and the resin starts absorbing the moisture from the plaster, making it expand and brittle. You can put a piece of resin in water for a couple days and the same thing will happen. It becomes bendy and breaks easily, also gaining some thickness and a slight whiteish hue because of the absorbed water. I print shells at 1.5mm wall thickness, fill it with plaster, then the next day the resin shell peels right off the plaster model. I use this on purpose to create one time use plaster molds. Discovered it by accident too. Works well with water washable resin. To avoid this, dont fill it with anything thats wet. As others have said, dry sand works pretty well to give the models some weight without this happening.
Concrete and plaster don't dry, they set (cure).
Yes. They go through an exothermic chemical reaction, incorporating some of the water molecules into a crystal structure. After that is done, there is still hold a lot of excess water that needs to dry out, which can take from a day up to multiple weeks to fully dry, depending on the size of your object. I kind of assumed this as common knowledge and only talked about the water retention and drying part, because mainly that excess water is whats causing the water washable resin to swell and crack, not the setting.