Wanting to KeyFrame / ShapeKey a mesh seperation for animation of a broken Piece of String


I am wanting to animate a piece of string/twine using Cloth physics for realism.
where something heavy is making the string/twine become thin in the centre then break.
i have been able to do that using shape keys to thin the mesh.
I have already split the mesh into two halves.

I have been trying various methods to accomplish this my animation but fail everytime.
I have tried the following

  1. two independent objects where I remove the mesh of opposite sides of the string/twine from the split point. and try using a hook to hold them together and didn’t work. (LOOKING FOR SOMETHING THAT CAN HOLD THE TWO CENTRE POINTS OF THE STRING TOGETHER)
  2. I have tried using a single object with the mesh split in the middle into two meshes of a single object but the cloth instantly makes the ends drop.

See below for details (My post from StackExchange)
Hello fellow Blenderer’s

as seen in my simple animation here. I am wanting the two middle points to stay joined for longer. Glued if you will!

enter image description here

the Object is a tube which is separated in the middle. I use KeyShapes to make the tube go from thick to skinny in the middle during the animation.

enter image description here enter image description here Not sure on how to accomplish gluing them together for longer


You could try to use the sewing feature of the cloth sim. It likely will give you at least a small gap in the center, but in can hold the strand together and can be switched off by keyframing.

Thankyou for your reply. I appreciate it

Can you take a look at my example file?
I’ve tried the sewing thing. I don’t get it!

P.s. the file above when opened in 2.79 vs 2.82 have very big differences in simulation. I swear i thought I was onto something!!

Ah, sorry, stupid me confused something about sewing. Loose edge always pull, just cannot collide. Thus it doesn’t help.

But I found another solution. With a shapekey (and dynamic mesh enabled) edges can lengthened arbitrarily, so they will behave as if not there (except for a small mass assigned to the middle vertex).


clothsim.blend (2.7 MB)

About the difference between versions, cannot tell the exact reason, but it is not uncommon. There have been some improvements to collision. Also what does make a big difference is switching from linear to angular bending model (but that shouldn’t happen automatically when opening an old file).

I cannot believe you worked it out! your a legend!
Can I ask,

I notice that when the mesh is separated, I can see in the viewport lines which join the two halves of the Long Cube.

Tho… When rendered, those lines are not shown!
Did you do anything special to the mesh to make it so those lines do not show up in renders?
BTW… I am testing with Cycles!

Hey OMGold… thanks so much for this, really. After seeing an artist do something in 3DsMax I believe he said it was done with nurbs… I dont know what they are but was told cannot in blender.
The thing he animated was like the effect of eating honey and taking it away from the mouth and watching the stretching string of honey thin out and drop.

I think with some keyshapes and trial and error I might be able to do something similar.

I have been wanting to know how to accomplish this for so long. this gives blender so much potential for the projects I am making.
I honestly have been trying and thinking of things for almost a year now.!


Again (as I already PMed) for everyone else, the topology at the break point is this:

The lines do not show up in renders, because only faces are rendered, and these I deleted.

About NURBS, Blender does have them very well. It’s called a curve object. But I wouldn’t know how to achieve this kind of breaking when doing a sim on one of these.

I owe you a few beers! :slight_smile:

Okay, next project a beer :wink:

I like it…,
Might flip fluids that with some real bubbles would look so good!!!

Question about the mesh in the cube. When rendering the frames before the KeyShape is triggered to 1
I can see a break in the cube
using cycles

any ideas. I tried pulling the mesh closer but the gap always remains after rebaking cloth.

It still might not close it completely, but it would be worth a try to enabling sewing now (but only until the break, you have to keyframe disabling it them).

I could be doing it wrong?
But enable the sewing did not help.

I am trying to pull the meshes into each other past the vertices which drop. So they cross each over but they flex too much and doesn’t look right I think I need to assign a few faces to one of the cloth settings.

You are right, it doesn’t really help for me either.

Cannot offer you a complete solution, but what I would try is to add another mesh to follow the sim mesh (in some way) which doesn’t show the gap. It is typical for cloth sims to do that anyway (with a surface deform modifier).It also may be good enough to just have a small ‘gap filler’ mesh which you make follow one of the rope ends with vertex parenting. You have to try to find out.

Hey OMG,

I have improved it a fair bit… the seam is not such a big deal in my example…
but I cannot use the angular setting for type if simulation
It needs to be linear but the linear option squashes the cube too much. especially vs your demo.
It might just be settings.

but would you kindly take a look and give me your opinions about keeping the structure more whole

My Blend File
ClothSim2.blend (1.2 MB)

I think it won’t work. I have an idea, though (which I probably should have come up with in the first place):

Use a single chain of vertices for simulation and then add a skin modifier to it. It seems to work to set RadiusX/Y=0 for the gap, resulting in no geometry at all there. Unfortunately there seems to be no way to dynamically change the radius, which would even allow you to have no gap at all for the unbroken string.

I would still have the requirement to actively configure Shape Keys to thin the center of the string before it breaks…

I will keep trying if you come up with any alternative ideas let me know.


Hmm, what you could do then is:

  • use the single chain of vertices for sim (as before)
  • hook the vertices of a curve object (with a reasonable number of vertices) to the sim mesh.

Then you have 2 options:

  • extrude the curve, setting the radius of the curve appropriately (this can be shapekeyed)
  • have a separate mesh for rendering (and shapekeying) which you make track the curve with a curve deform modifier

Just a thought:

  • make string a contiguous mesh with no splits
  • make a copy of string (topology must be the same), remove all modifiers and stash it somewhere invisible
  • on that copy, mark the tear as sharp edges
  • on the string, before cloth, add Mesh Data Transfer and Edge Split modifiers (so order is Mesh Data Transfer, Edge Split, Cloth)
  • on Edge Split, leave only “Sharp Edges” ticked
  • for Data Transfer, set source object to be that copy, check Edge data, enable Sharp, set Method to Topology
  • animate Mix Factor on the Data Transfer

Hi Stan,

I have attempted what you recommended in this file:
ClothSim3.blend (1.5 MB)

I have followed the information, provided.
It appears the cloth cannot simulate this properly as it sort of resets the sim from the frame I animate the mix setting

I have actually tried keyframing the Edge Split modifier using modifier enable button and achieves the same effect. the cloth sim fails. be it a blender bug or unsuported feature of cloth sim

any other ideas?