Unions disappearing / turning invisible after creation

This issue is causing me a lot of problems too I recently made a boat model for a game im working on I published the game to roblox as I had to get off my pc for the night and in the morning the unions where invisible. Also, messing with the collisionfedelity crashes my studio ( The union issue also happened to every union I had in my game )

I have also experienced this issue. Even when saved as a model it is not able to separate the union - it just vanishes, so you’re stuck with a union you have to redo:

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This happens to me all the time, and drives me insane. I like working with unions, but the frequency with which they get corrupted makes them basically unusable.

My current workflow is: create union in Roblox, export it as .obj immediately so I don’t lose it, use Blender to bring the mesh back into Roblox so that I can turn it into a meshpart. This is a pretty quick and easy as long as the union isn’t so complicated that it requires additional work in Blender.

That isn’t a solution - separating it just destroys the instance entirely.


In my case, as I reported below, I need to use SubtractAsync, I need to create walls via Server Script, since the windows must be positioned in a flexible way, depending on the wall size.

So what now?
Do I have to remove this feature from my game?
Or how long should I wait until this bug is fixed?

This is due to ChildData missing from the union. PhysicsData remains afaik, but, when ChildData goes missing the Union will be separated into nothing (there is no instances stored for it to separate into), and will appear “invisible” despite having collision. I think additionally MeshData is lost? I don’t remember if MeshData is or isn’t lost… (These are all internal properties of PartOperation & PartOperationAsset instances, the latter being the instances stored in the Roblox cache for downloaded union content)

The reason that copy and pasting sometimes works is because the union can still have an asset ID with it on the web which will have old content. If your restart studio and save the game though, I’m fairly sure this ID gets overwritten because it gets uploaded as a new asset.

If you guys can provide me rbxl and rbxlx copies of effected Unions, I can possibly look into a way to recover or repair them hopefully. The more the better, but, it may not be possible as the data may just be gone entirely, we’ll see I suppose.

Also, if you have unions go missing in your place and its published, go into Game Settings > Places > (Sub menu) > Version History, you should be able to find old versions of your Unions still in tact there.


I’ve already looked into XML model files and it always manifests as a missing solidmodel asset ID. Just a null field. Doing anything useful with the compressed file format seems unlikely.

The union seems to get uploaded and the child data seems to get replaced with the uploaded asset whenever the place or model is saved in studio, which happens also on autosave, so recovering unions in other ways seems extremely unlikely and unreliable. The act of trying to save out the union will actually destroy it.

You can try this file if you want to test your theories, but I don’t think you’ll get much further. I have a tutorial up on how to recover your old unions that seems to do the job for most people who have lost significant work.

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Can someone explain to me how Bloxburg manages to overcome this bug?
There, for example, you can create a window anywhere on the wall. This is similar to my case, using SubtractAsync. But there, the wall does not disappear, as it happens with me …


We can’t tell you much without having a look into the source code itself and I don’t think anyone can do that besides Coeptus.

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This has also has happened to me, It is very annoying.

Yeah that is very annoying and i just rejoin studio

I’d imagine from the response time of building in bloxburg that CSG is not being used to generate windows lol, live CSG on that scale would take 10+ seconds to render. This is most likely just clever math.


If Roblox doesn’t fix this bug, I’ll have to change the strategy, creating four independent parts surrounding the window, and adjusting its sizes to match the desired result.

Even if the bug is fixed I would reccomend this strategy, it’s a lot easier to modify and live CSG times are way to slow to be practical in production games.


Thinking of quitting building, it’s really pissing me off.

Then the unions corrupted.

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For now it might be a good idea to switch back to the Legacy CSG system which is more stable and less likely to produce this bug, until Roblox eventually removes it.


@wobleh @ittrgrey

Not a good idea:


It’s still there for now, so you may as well take advantage of it whilst it’s still around

Or… you can try and recreate it in a 3d modeling program?

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I think the use of 3D programs is always the first choice for everything related to complex meshes, EXCEPT when the spacing between subparts must be varied, as in my case, where the position of holes for windows must be calculated automatically during the execution of the program; in this case, a static mesh will not do.