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I am trying to simulate this hinge inside this model of bottle. But when I hit play button on animation it acts weirdly and is pushed out of the bottle. I want the separation to swing down inside the bottle. How can I fix this.

This is the hinge before simulation

enter image description here

PS: The mechanism and hinge is working fine when I drag the whole hinge system outside the bottle into open space but not working inside the bottle like showed in the wireframe view above.

Hinge settings. enter image description here

Bottle settings: enter image description here

2 parts liquid system inside the bottle: enter image description here

The separation bridge, and the two liquids together: enter image description here

My end goal is to make bridge opening in the bottle that is two separate half semicircles which will be opened when the fluid from the bottom chamber of the bottle will be pushed upward and will open the bridge. So that fluid will basically apply the force on the two bridge gates or flaps and it will open them in upward direction inside the bottle (/ \ like this in shape). And then then both fluids in the top and bottom will mix together (say lemon juice in bottom and water in top). And yes, I want the gates/flaps to collide with the bottle inside when the bottom chamber water will be pushed upwards when a force will be applied on the bottom base of bottle.

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    $\begingroup$ Welcome to Blender Stack Exchange! Your question is a little bit unclear in that we can't see any settings or how you have your simulation setup. If you click the edit button at the bottom of your post you can add more details about what is going on, maybe some screenshots of settings on the hinge and bottle, or a gif showing what happens when you run the simulation. That being said, it sounds like a case of the objects intersecting with each other somehow. If your hinge is inside the wall of the bottle at all or is somehow colliding with the bottle that could be the issue. $\endgroup$ – Brenticus Mar 3 '18 at 19:18
  • $\begingroup$ One other thing is to check the collision margins on the objects. If they are higher than the physical distance between the closest points on the objects it will treat them like they are inside one another and that can cause issues. It sounds like you could get away with no collisions on the bottle, but I don't know what your end goal is so that might not be possible. $\endgroup$ – Brenticus Mar 3 '18 at 19:22
  • $\begingroup$ Please use the edit link at the bottom of your question (i.stack.imgur.com/lXFuK.png) and add more information. While the issue might seem obvious to you, It is hard to understand how you want the hinge to work (side to side, top to bottom?), what are the different parts? What would "correctly" mean in this context? What mechanism are you referring to? Help us understand your scene with images and consider sharing your .blend file so that others can inspect it. You can upload it at blend-exchange.giantcowfilms.com and then paste the resulting link as part of your question. $\endgroup$ – user1853 Mar 3 '18 at 19:42
  • $\begingroup$ Hi @Brenticus Thanks for the information. Yeah actually, the system is comprised of bottle as passive object and separation as active and is working through rigid body constraints with a hinge state. $\endgroup$ – Jayant Sikka Mar 3 '18 at 19:42
  • $\begingroup$ Actually, my end goal is to make bridge opening in the bottle that is two separate half semicircles which will be opened when the fluid from the bottom chamber of the bottle will be pushed upward. So that fluid will basically apply the force on the two bridge gates or flaps and it will open them in upward direction inside the bottle (/ \ like this in shape). And yes, I want the gates/flaps to collide with the bottle inside Any help on this? Also yes I have also added the hinge settings in the post. $\endgroup$ – Jayant Sikka Mar 3 '18 at 19:43
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From what I can tell, it appears that your setup has a number of issues (and please correct me if I am mistaken on anything here). The biggest one being that the physical design of the flaps and how you want them to open is impossible. I could try to explain it, but the gif below will show it best (I realize it's upside-down from what you want but the concept still applies).

enter image description here

If you tried to do this setup in the real world, the flaps would be locked in place and unable to move, as in the gif you can see that as soon as the flaps move they intersect with the walls of the bottle.

The solution to this will depend on your end goals. If you need the flaps to be visible, the only solution that I can think of would be to move the hinge to the middle of the bottle so that the flaps would fold up and meet in the middle. If you just want to keep two fluids separate and have them mix when the bottom one is forced up, there is probably a better solution for that, but I would ask a new question if that is your goal.

A couple of other things that I see that would also cause problems, even with a proper setup, are that the flaps start out partially inside the walls of the bottle. This will always break a simulation as it's a physically impossible condition. The second is that the bottle collision type is set to Convex Hull, which isn't the best for shapes with large convex areas. A better collision type would be Mesh, as it is the most accurate.

However, these are in addition to the main problem, and likely won't help unless the entire system is redesigned.

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