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JohnInOttawa

Torsion box implications for infill material and layout?

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Good morning!  It is a balmy -25C with the windchill outside, so what better time to concentrate on the indoor to do list?

 

My usual disclaimer:  I'm clearly not an engineer.  I work in the end of the field that tries really hard not to break the nice toys that the engineers design for us.

 

As I look at the growing variety of materials and prepare to set up a test environment, I was looking at the torsion boxes I've built to hold some of my heavier equipment and began to wonder - I know that infill works on a very similar principle, but if I am aiming for the strongest, truest surface to withstand loads at the lowest cost, can I expect (and predict) strength and performance improvements for given combinations of infill and skin materials?

 

For example. We know that PLA can be very stiff but isn't particularly strong on its own and subject to longer term 'embrittlement' (pardon my abuse of the term).  But what happens, say, if we embed a PLA grid inside a nylon/CF box skin?  The skin provides stiffness and protection from UV, while the PLA should, in theory (depending on print orientation), serve pretty well to support the central area of the box against compression loads.   I recall that some torson boxes use corrugated cardboard for their internal structures and achieve amazing load bearing capacities.

 

Where am I going with all of this?  Some of the newer materials provide us with new levels of strength and stiffness - but the cost makes using these for larger prints a serious challenge.  If I was able to use an exotic filament for the skin to protect a less expensive but adequately strong matrix (and my question is really about determining 'adequately'), then all of a sudden these new materials start to look cost effective for a broader range of applications.

 

Looking forward to your thoughts!

John

Edited by JohnInOttawa

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Good Afternoon from Europe 😉

 

This is a very interesting concept, which has been implemented in a great variety of products (ranging from simple wood shell/cardboard honeycomb filled doors to high-strength sandwich panels used in aerospace applications), but as far as I know, hasn't been put to use in 3D printing very much.

 

A challenge I can think of is the bonding between the different materials, and also the question of how cost effective this really is, vs. using a lower infill percentage of the more expensive material. What could work, however, is for example CF-Nylon for the shell, and "normal" Nylon for the infill.

 

Regarding the issue of PLA - if I recall this correctly, it is not UV degradation, but slow crystallization of the molecules that makes it more brittle over time.

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Yes, inter-material compatibility is something I wonder about.  If the PLA change will happen regardless of environment, then I guess my next question would be whether that more brittle state leads to increased risk of internal failure or whether the internal matrix is protected from loads relevant to that change.

 

Related, there is a wide price range now of CF reinforced filaments.  Perhaps the answer is to go with a more generic filament internally that lacks the surface toughness or print quality of the exotic stuff, but is still good enough to be used internally.

 

Cost effectiveness would certainly be a factor dependent on what was being built.  And of course, if the internal fill material was also abrasive, now (on the UM3), we would be talking about two CC cores or Hardcores, which would drive costs somewhat.

 

J

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I once hit hard on a PLA model with thin wall and 20% infill, and both the wall and infill got severely crushed and dented at the place of impact. The damage looked weird and very "un-plastic like", but rather like a crashed car. So I am not sure PLA infill is going to protect, unless the soft shell would be very thick and distribute the loads widely enough, and the infill would be nearly solid and very thick too.

 

So I would rather print the whole thing in nylon if it has to be tough, or in thick solid PLA if it has to be hard.

 

Another option could be to print the shell, and then fill the inside with polyurethane. PU exists in foam, in rubberlike solids (after curing), and in very hard solid versions. I think a very tough and hard PU like used in skater wheels might be a good choice.

 

Or print a mould in PLA, carefully seal it with a non-stick layer, and cast it in PU. Then you have a very tough model in a single material. This might go a lot faster and cost less, once you have the setup right, if you need multuple models. A lot of artists and sculpters use this method in various forms. If it has to be light-weight, they tend to fill it with PU-foam. Or sometimes they work the other way round: first model the thing in PU foam, and cut and remove foam as desired. And then they seal the whole model with a hard and solid PU layer.

 

So, a combination of 3D-printing for a detailed model, and casting for strength and speed, might also work well. Then you have the best of both.

 

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