Creating various casts/splints for a 3d scanned right arm

[Anonymous]

Hi all,
I am seeking some advice for a first time user of meshmixer for the modelling of a cast over a 3d scanned right arm ( i have attached the file ). My project at university is the design and manufacture of a 3d printed cast for a typical fracture of a radius/ulna in the arm. I began with the hopes of modelling the cast in inventor however many people on the autodesk forums suggested i used meshmixer.

I hope to begin with a continuous cast which is typically used in hospitals, with a continous thickness, throughout the cast ( see attached photograph ). I then hope to model further casts made of more complex shapes and varying thickness ( for example thicker at the point at the break and less thick nearer the hand to allow some mobilisation of the fingers ). The purpose of having a cast with the structure of a mesh ( see attached photo ) is to allow a reduce in material, and air flow to the arm. It also has the pros of looking aesthetically pleasing and allowing the user to shower with the cast on if it is made with 3d printed plastics. I hope to create casts with various shapes ( ie made from circular shapes, triangular shapes, square shapes ).

I have watched some tutorials online of how to use meshmixer however i have not yet come accross a way in which to create a "mould" so to speak around the 3d scanned right arm i have. If anyone knows of any tutorials or methods in which I could use for the modelling aspect of my project that would very helpful.

Thanks,

Rayyan

[Anonymous]

Attached 3d scan of right arm in stl file for mesh mixer and ipt for inventor.

[Anonymous · Accepted solution]

Interesting that they pointed you to MM...

OK the standard cast:

Go to SELECT and select the region of the cast orange.

To get a smooth edges run Modify/SmoothBoundary on the selection.

By default this creates a face group too. Select this group and run Edit/Extract.

(I assume that there might be a need for some gab (here: 1mm) between arm and cast. If you want a perfect fit do the same with Offset at zero) Make sure that Direction is set to Normal.

Hit Y to separate the new surface as a separate Object.

SelectAll (Ctrl+A on Win or Cmd+A on Mac) and run Edit/Offset. Make sure Connected is checked:

To smooth the boundaries of the cast select These groups by double click and grow the selection via Modify/GrowRing several times.   

Run Deform/Smooth on this selection

As an alternative you might use SCULPT and its Smooth brush to do it manually.

[Anonymous]

For a pattern cast:

Do an extraction as above but set its Offset to a "center" value. Here I want to get a cast 8mm thick finally plus a gab of 1mm towards the arm so I need to set an offset of 5mm.

Again separate this surface as a separate object.

Now one way to get a pattern is to reduce the mesh. Each triangle edge will be a tube later on.

So SelectAll and run Edit/Reduce:

Leave SELECT and run EDIT/MakePattern on this result.

In MakePattern set the PatternType to Edges (you might use different modes here. Check them out).

Here I use SphereGradient mode in Gradient to get a stronger pattern on the wrist. Start- and EndScale is a factor of the set ElementDiameter. So StartScale at 2 times the ElementDiameter of 4 gives 8mm at the wrist while the thickness drops to one time 4mm at the ends. Note: This way the gab between arm and cast will be 3mm at the ends while it is 1mm at the wrist. You can move the black start point and the white end point to the desired position in 3D.

Accepting the result:

Another maybe interesting way is to define face groups before running MakePattern.

Here I did several EDIT/PlanCuts in CutType = SliceGroups:

On this you can use PatternType FaceGroupBorders in MakePattern:

to get something like this:

[Anonymous]

Hi MagWeb,

Thank you very much for taking the time to help me and going beyond that providing me with an extremely detailed walkthrough. I will keep you informed of my progress!
thanks again,
Ray

[Anonymous]

Hi Magweb,

Every time i try to create a pattern, i get a strange accumilation of lines to one point near the edges of the cast (please see attached photo). Can you suggest a way to get a "clean" edge like you have in your examples?

Cheers,

Ray

[Anonymous]

This attached image may show my issue better.

[Anonymous · Accepted solution]

Seems you forgot allowing a reduce of the boundary faces.

Disable PreserveBoundaries and PreserveGroupBoundaries in the Reduce tool.

That should fix it.

[Anonymous]

Hi Magweb,

Thank you again for your help. The final part of my model will be the strapping criteria (ie how the model is held together). I would ideally like to have the cast printed in 2 parts (ie bottom and top), which will be fastened together at certain lengths depending on the total length of  the cast. On both the bottom and top parts, i would like them to join like a male and female electrical component ( like a plug with a socket ), or another example would be like 2 pieces of lego. I have attached an image of a previous design which utilizes this concept. If you look closely, one of the parts will have 3 or 4 "plugs" which go into the other parts holes or "socket". I like these designs because they allow the cast to remain smooth and prevent the catching of the cast with other things and are essentialy manipulatiing existing material to make a "mate" or "joint". https://www.xkelet.com/ also has a great design where the 2 pieces of the cast join like a plug and a plastic ring holds them together.

Would it be possible to model such a thing? thank you again for your help.

Ray

[Anonymous]

Hi ! 

I use meshmixer since almost 2 Years for realise cast, brace . See my fb profile 

https://www.facebook.com/profile.php?id=100008369024410.

Ask for questions. 

Cyril from France

[hfcandrew]

Plane cut, add a small cylinder as the male plug at 3 or 4 points, make a a duplicate copy of all of them. Combine them to your one half. Do a Boolean difference to delete the space from the other half as the female hole.

[Anonymous]

thanks, i will give that a go!

[TrippyLighting]

@Anonymous I had not come across this thread before. Outstanding instructions! Thank you!

[theebadge]

This is very helpful.
Side question- when is it necessary to “Flip Normals” on the interior (skin facing) surface?  I watched a dental mold video where a negative mold was made from a scan, and they had to flip normals of the interior after offsetting. Not sure if that makes sense. 

[hfcandrew]

If you do an 'offset' with 'connected' checked, MM automatically flips the normal of the source surface. However I believe this was not the case in some old versions of MM from ~5 years ago. How old was this video you saw, can you post it?

For example if you want to make a mouth guard (sports guard) for a patient using a scan of their teeth and you want it to be 3mm thick. This is basically a negative, like moulding 3mm of putty over their teeth.:

1. Select the teeth
2. Offset (maybe like 0.5mm to give it some wiggle room)
3. Offset (with connected checked, 3mm)

This will automatically flip the normals.

There is a funny thing you can do where you can make a boolean difference operation behave like a boolean intersection by flipping the normals of one of the object, but that has no practical purpose. There are very few common practical instances now where normals ever need to be flipped as part of an MM workflow for this kind of stuff. 

Read: https://help.autodesk.com/view/MSHMXR/2019/ENU/?guid=GUID-90CE1063-9807-4A11-9E15-61E3AFD32A5E

[theebadge]

Ah. That makes sense, thanks. 
My question came from the first two minutes of this video: https://youtu.be/MKX7jErN03E

Concept-wise, is your orthotic fabrication workflow similar to this thread? 

[hfcandrew]

K I watched the video. He's doing it that way because he forgot to check 'connected' going:

1. Offset
2. Flip normals of inner shell
3. Plane cut

When instead he should go

1. Offset (with connected checked)
2. Plane cut

Also he is going export, then import which is not needed (just go EDIT>SEPARATE SHELLS). Also good practice until you get good is to make a couple duplicates of whatever you are working on, incase you realize somewhere along the way you messed.

For my orthotics, while I do not need to make injection moulds, Yes I've developed a similar work flow of the various MM tools.

[Anonymous]

Hi MagWeb,
Can you tell me the working principle of wireframe function, is it durable enough, and how to test their durability?

[Anonymous]

Sorry, I have no experience about the durability of such a splint in practice. There are several guys out there which seem to use those attempts in practice.

In theory and in general: Such networks give stiff structures with little weight - imagine the Paris Eifel tower. There's no solid center to bend around. Similar to bend a solid iron rod vs. bending an iron pipe. 

Now if one 3D prints such a structure its durability depends on the technology, the used material and your print settings. In most cases the week point is  the "welding" of one layer to another so the structure might break much easier bending the Z direction of your print. So it's a good idea to orient the object in your printer so the direction where you expect biggest forces is parallel to the printers bed.

To test durability: There are different forces as traction, compression, bending or shearing as well as time related things (long lasting or sudden force) to consider. Building professional test stands is an own science. A DIY setup to get e.g. bending forces a structure can stand might be to fix on end of the structure in a vice horizontally and to fix a hanging canister on the other side and fill it with water until the structure breaks. Easy to calculate the force resulting from a certain volume of water + the weight of the canister...

[Kate.hutchinson23]

Hi there, I am new to mesh-mixer and using a very similar process to this to design custom made wrist splints, However every time I go to extract or offset my selection it incorporates these spiky bits that seem to stick to the hand (image attached) . Do you know how I could get rid of these to achieve a clean offset like yours?