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Modelling thermoform 'drape'... How can I do this???

aarongough

Stainless
Joined
Oct 27, 2014
Location
Toronto, Canada
Hey all!
I have come across a real head-scratcher of a problem. What I'm trying to do is to make a CAD model that is the same as a part that I normally thermoform from Kydex. The hot kydex is laid across this mold:


And then pressed down onto the mold using high density foam in a pneumatic press. This produces parts that are not an exact match to the mold. Instead because of the relatively thick kydex (0.080" thickness) they have 'drape' where the hard inside corners of the mold are transformed into flowing curves on the part. You can see what the resulting parts look like here:


After forming the sheet is CNC trimmed as you can see above.

The two halves are then attached together with rivets, which produces the sheath you see on the left here:


The resulting sheath uses elastic deformation of the kydex material to hold the knife in place without the need for straps or clips. When you push the knife in the 'lips' of the sheath are pushed outward, then snap back inward, into the finger groove in the knife handle, to hold the knife in place.

I am trying to re-design this part so it could potentially be 3D printed, and it has turned out that re-creating that 'snap fit' (for lack of a better term) has been extremely challenging. I have made some models that work, but tuning the strength of the fit is very challenging and the shape that I was able to make in CAD does not operate as smoothly as the thermoformed version...

Here are some photos of the printed versions I have made so far:


In addition to 3D printing I am also considering making a 'hard tool' from steel which would trim/form a sheet of hot kydex in a single stroke.


So my question is: how can I replicate the 'drape' of the physical thermoforming process in CAD? Surely there must be a tool out there that allows you to plastically deform one solid over another?

I know blender has some tools that 'kind of' do this, but they don't look like they're configurable enough to produce the result I need... I'm curious if anyone knows of other tools that will do this?

I'm honestly at the point where 3D scanning is starting to look like the right solution, but that seems a bit nuts!

-Aaron
 
If you are doing this for $hits and giggles, then bust out the drop mic and scale. If you are doing it for Benjamin's, get it 3D scanned. Talk to folks at a company with a 3D scanner to see what it would take to have it 3D scanned. PM me if you want to discuss further.

Chazsani

Sent from my G-Code editor @ 300kbs
Edit for clarity.
 
This kind of thing is better suited to subdivision surface modelling.

That said, it's been close to 20 years since I was really into that and my knowledge is certainly outdated. Modo was the hot stuff newcomer at that time, and it was great, but I'm not sure how it compares nowadays and it's not cheap. Cinema 4D always had really good subd tools as well, but again not cheap. Most of the "content creation" 3D software support subd modelling, including Blender, although how good it is in Blender I really don't know. Silo is another that seemed good many years ago and appears to be reasonably priced nowadays.

Seems like the CAD world is only now starting to get interested in subd modelling. Looks like Rhino maybe have introduced it recently, or are going to soon. I heard utterances of subd tools in Solidworks but again haven't paid too much attention.
 
This kind of thing is better suited to subdivision surface modelling.

That said, it's been close to 20 years since I was really into that and my knowledge is certainly outdated. Modo was the hot stuff newcomer at that time, and it was great, but I'm not sure how it compares nowadays and it's not cheap. Cinema 4D always had really good subd tools as well, but again not cheap. Most of the "content creation" 3D software support subd modelling, including Blender, although how good it is in Blender I really don't know. Silo is another that seemed good many years ago and appears to be reasonably priced nowadays.

Seems like the CAD world is only now starting to get interested in subd modelling. Looks like Rhino maybe have introduced it recently, or are going to soon. I heard utterances of subd tools in Solidworks but again haven't paid too much attention.

I believe the CAD system I'm using (Fusion 360) has this kind of tool built in. They call it the 'sculpt' or 'mesh' workspace. You can create primitives that are made from splines and then the surfaces in between are interpolated from the intersecting splines. You can subdivide an area to put in more detail in a particular spot.

I was able to get that to kinda/sorta approximate what I needed, but I felt like I didn't have enough control to fine tune the shapes. Perhaps I just need to learn how to use that workspace better...
 
If you are doing this for $hits and giggles, then bust out the drop mic and scale. If you are doing it for Benjamin's, get it 3D scanned. Talk to folks at a company with a 3D scanner to see what it would take to have it 3D scanned. PM me if you want to discuss further.

Chazsani

Sent from my G-Code editor @ 300kbs
Edit for clarity.

Yeah 3D scanning does seem like the easiest route... The issue is that in the future if I want to make a variation on the design then I'd have to go right back to the start and make a physical prototype to scan and so on. Ideally I'd like to just be able to model directly, but I can see how 3D scanning might just be the 'simple' option.

As an aside: what's the ballpark on getting something like this 3D scanned in high resolution? The part is only 5x2.5" and only one half would need to be scanned as the other side is just a mirror image...
 
I don't have any help on the modeling side of it, but my question is what did you 3d print that on?
 
I don't have any help on the modeling side of it, but my question is what did you 3d print that on?

Unfortunately not my own printer! That 3D printed version was SLS printed from Nylon 6 powder on what I believe was an EOS printer, then dyed to get the black color. Overall a very nice print, though the features on the bottom surface are softer than the features on the top side.

I have also printed quite a few versions on my home FDM printer, which is honestly not a great printer but I have still gotten some very decent results:



The part on the left was printed with a 0.2mm layer height, 0.6mm diameter nozzle. Print material is carbon-fiber reinforced nylon... On a slightly better printer I think that part would look very crisp indeed! I have been very surprised at how well the carbon-fiber nylon materials print and at how strong they are...

I broke that part in the weakest direction (along the layer lines) and to break it I had to get both hands and my knee involved and it was still a struggle.
 
Yeah 3D scanning does seem like the easiest route... The issue is that in the future if I want to make a variation on the design then I'd have to go right back to the start and make a physical prototype to scan and so on. Ideally I'd like to just be able to model directly, but I can see how 3D scanning might just be the 'simple' option....

After scanning the part to a point cloud, move it to Solidworks or other compatible CAD and save it with surfaces. Use Solidworks to manipulate to your requirement(s).

Chazsani

Sent from my G-Code editor @ 300kbs
 
Takes about 1-2 hours for me to drag out my gear and take a scan for something that size.

I'm curious what scanner you have and if you're happy with it?

I have wondered whether buying a scanner would be a good investment. There are some inexpensive 'structured light' scanners coming out now that look like they are actually pretty high resolution, and in this case I wouldn't need 0.001" level accuracy I don't think...
 
I believe the CAD system I'm using (Fusion 360) has this kind of tool built in. They call it the 'sculpt' or 'mesh' workspace. You can create primitives that are made from splines and then the surfaces in between are interpolated from the intersecting splines. You can subdivide an area to put in more detail in a particular spot.

I was able to get that to kinda/sorta approximate what I needed, but I felt like I didn't have enough control to fine tune the shapes. Perhaps I just need to learn how to use that workspace better...

Maybe you need to learn it better, but I suspect the toolset in F360 is a limiting factor.

Here's a good example in C4D:

Cinema 4D Tutorial - Create Wax Type Using Collision Deformer - YouTube
 
Maybe you need to learn it better, but I suspect the toolset in F360 is a limiting factor.

Here's a good example in C4D:

Cinema 4D Tutorial - Create Wax Type Using Collision Deformer - YouTube

Damn! Yeah that is exactly the type of tooling that I was thinking of! Is that kind of tooling ever present in manufacturing CAD or is it really just in stuff like Blender and Cinema4D?

There is a tool in Fusion's mesh workspace that will 'attract' a mesh surface to other geometry, but it seems hard to get a result anywhere near what I actually need... I will play with it a bit more now
 
I'm curious what scanner you have and if you're happy with it?

I have wondered whether buying a scanner would be a good investment. There are some inexpensive 'structured light' scanners coming out now that look like they are actually pretty high resolution, and in this case I wouldn't need 0.001" level accuracy I don't think...

I've got a David 2 scanner (it uses structured light), purchased 5 years ago. Excellent results and it would work well for your size of parts. I place all the parts I scan on a rotary table and take scans usually every 10°. Detail is excellent. Downside is it's not the most compact setup, requiring a tripod and mount to hold the camera and projector. Post processing is usually the kicker (Geomagic $$$) but SolidWorks 2020 (and Fusion) can handle some large data clouds. A lot of the parts I scan are around the size of your sheath.
 
Hi Aaron, it looks like you may be able to do it with the deform tool in Solidworks. It's hard to see what's going on here as the deform tool doesn't give you typical lines where curvature changes, so I've got the colour curvature on. This was 1.5mm material deformed by a .25" thick, vaguely knife shaped body. If you can DM me we could try a solid model of yours just for fun. I'm not sure how physical this deformation is. You can set the "distance" of deformation making the deformed material stiffer or softer, but to what degree is this realistic is unknown to me. Having said that, tuning the parameters in this method to reflect your actual part is more appealing to me than 3D scanning as the latter will make it hard to edit.

Edit: Here are better pictures and with the second taken as a cross section midway across the shape. This is actually a surface deformation so you have to select both sides.

Gough Test 3.jpg

Gough Test 2.jpg
 
Messed around a bit more. This is about the minimum allowable drape and that seems to be the limitation of this feature.

Gough Test 4.jpg

Deform is intended really to perturb surface patterns into more complex shapes, not as a vacuum forming tool. There's a good video about this and freeform by Spanner Product Development here: SolidWorks Webcast | Episode 16: Advanced Modeling Part 12 -The Deform and Freeform Features - YouTube I should note you can also do this with the sheet metal forming tool. But the forming tool gives you exactly the shape of the tool so you have to build the drape into the tool shape.
 
I'm wondering if you couldn't do this in FEA ? Model the knife, model the layer of plastic, then push the knife into the plastic sheet ?

That's an interesting question. It's a bit exotic for traditional FEA as the displacements are very large, but there are some new platforms out there that are much much better at soft materials as well as thin sections. e.g Dassault has something in their 3D Cloud application, whatever it's called.
 
I'm wondering if you couldn't do this in FEA ? Model the knife, model the layer of plastic, then push the knife into the plastic sheet ?

That's an interesting question. It's a bit exotic for traditional FEA as the displacements are very large, but there are some new platforms out there that are much much better at soft materials as well as thin sections. e.g Dassault has something in their 3D Cloud application, whatever it's called.

Soft body dynamics using FEM is used fairly extensively in the SFX/Post Production field, and in some game engines. Nothing really exists in the cadcam world that I can think of. Just about all the current 3d cad systems cad do a linear simulation that would indeed produce something that sort of looked like this if you played with the deformation scale enough, but there's not much that you'd be able to do with the results beyond look at them.

Hopefully Aaron is trying his hand at Blender as we speak, because that is where he is going to get the best results. (without spending money).

Damn! Yeah that is exactly the type of tooling that I was thinking of! Is that kind of tooling ever present in manufacturing CAD or is it really just in stuff like Blender and Cinema4D?

Maybe Rhino, probably NX.

Certainly not in Solidworks or Fusion.
 
The thing is, without proper FEA, which I very much doubt Rhino can do, you aren't going to get a realistic drape. Otherwise what you have to do is play with the deformation parameters until you end up with a solid that matches the original as best as you can. Which is actually what you would do with FEA in terms of playing with the moduli of the material until it behaves right, but if you were able to nail that you'd have a much more general tool. And either way you'd have to take your deformed shape and compare it, either by bringing in a 3D scan or, for example, machining or printing cross sections of the part and comparing them to the real thing until you're close. We do this cross section approach from time to time and waterjet test profiles from scrap. At this point, if had to do this I would use the Solidworks form tool and iterate draft angles and fillet radii until I matched the original shape.

Aaron, does the Kydex retain it's thickness through your process or does it thin out at the corners?
 
Sorry for the slow response guys!

Rcoope: very cool that Solidworks has that feature, haven't seen that before. Thanks for putting up the demo images! It does indeed look pretty darn close to what I would want. I don't think the minimum drape limitation will be an issue as the parts I make currently have larger drape radiuses than what I see in your images.

The kydex does mostly retain its thickness, I haven't seen much thinning, though I generally don't cut cross-sections of my sheaths either!

Follow up:

I haven't dug into Blender yet. I have been going through the process of re-drawing some of my models as they were originally done when I had a lot less experience with CAD/Fusion. In re-drawing them I have made much better use of components and grouped parts families into a single 'design' with multiple components, rather than into separate files as I had done previously.

This has allowed me to make derived geometry more easily in the sheath, and I was able to actually finally model a shape that worked:





You can see the fitting of the knife in action here: Login • Instagram

The model has an integral break-away support designed into the base that allows it to be printed standing up in an FDM printer. The material in this case is carbon-fibre nylon again.

I think I still prefer the look of the thermoformed sheaths though, so I am definitely not giving up on the idea of modelling the drape...

-Aaron
 








 
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