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Overcome poor multi-material compatibility: Feature request "Interlock overlapping meshes"

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As we all know one major stumbling block for multi-material printing is material compatibility. If you look at the material combinations sheet provided by Ultimaker there are way too much "X". The reason for that? Due to their chemical composition those materials are not friends, they don't stick to each other, they don't print well on top of each other. For example, covering a simple tool handle, made of PLA with a skin of TPU won't work well as the skin might rotate freely on its own.

Old school craftsmen know how to design strong joints without relying on bonding, like carpenters with dovetail structures. More generally spoken, they use topologically interlocking surfaces. But this "macro" scale interlock is challenging to adapt for arbitrary shaped componentens using 3D design software. At least, it requires profund scripting skills and information about the subsequent slicing process (layer height, line width, wall thickness) to produce something that slices well and achieves the desired goal.

But 3D printing has the unique ability to do this on micro scale! I don't talk about blending / fusing two materials as this will likely suffer from the same limitations as mentioned above. No, it's about small scale topological interlock, typically within 4-8 times the line width or layer height.

Cura already features a setting "Alternate Mesh Removal" which will make the printer alternate between overlapping meshes every layer. The good: Cura recognizes overlapping meshes. The bad: printing one layer of material A on top of one layer of material B will not produce a strong interlocking geometry and the printing itself will again suffer from the already mentioned incompabilities. So why not work on a feature  "Interlock Overlapping Meshes" where Cura will connect/interlock both meshes using certain smart strategies. In other words: The designer overlaps component A and component B to some extent and the slicer will use this volume to interlock both parts.

Some ideas:

Simple interlocking in xy plane. The "micro dovetail". As a side effect, the contact surface between  both materials is significantly increased, this should also support chemical bond. topo-interlock1.thumb.png.83344c4687597a0a8953286b8aad1bd3.png

Possible interlocking in Z direction. An aligned 50% linear infill on areas where meshes overlap. Either material should print fine on top of and underneath this grid topo-interlock2.thumb.png.437f708047369d1901b960d8f2c4d235.png.

So, @nallath what do the Cura devs think about this? And if it will never make it to the top 50 :p , any Cura contributors interested in this idea?

topo-interlock1.thumb.png.83344c4687597a0a8953286b8aad1bd3.png

topo-interlock2.thumb.png.437f708047369d1901b960d8f2c4d235.png

Edited by Guest
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Well, if you want to attach a skin, say 0.8 mm thick, to a core you would have to include its two inner perimeter lines in the overlapping volume, which is completetly located inside the core. In a way that the outer two perimeter lines remain untouched and the inner two start "dovetailing" into the core part. Ofc interlocking two thin parts isn't possible.

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I actually wanted to make a suggestion how to solve the problem for thin vertical interlocking structures but then I realised that there is one issue with the vertical interlocking anyway: How do you get to stick the very first line of the second material to stay where it has to (assuming it's not printed on the bed but on top of the other material)?

edit: answer: you need e.g. a horizontal expansion as it exists for the support in Cura.

A similar issue rises if the horizontal interlocking takes place on top of an already printed surface and not the print bed. The structure there have to be small enough in order not to get warping/curling of the second material.

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For the horizonzal interlock one could start with unmodifed perimeters of both parts and then incrementally protrude the dovetail until it's fully developed.

For the vertical interlock. True, but at least not more of an issue as without using this technique I think.

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For the vertical interlock. True, but at least not more of an issue as without using this technique I think.

Aka "doesn't work either"? ;)

So how can we get the first layer of the new material to stay/stick where it should? The glass is one possibility (horizontal expansion). But in most case this is not practical.

What about this idea: Print two layer thick stripes with material 1 (same orientation!). Make the second layer overlap the first by half the nozzle size or a quarter of the nozzle size. So you get a kind of channel which should be wider on the first of the two layers. Then you squeeze the second material into that channel, making some kind of interlock similar to the horizontal case. From there on it's history... :)

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