Is there a way to specify detailed internal structure

[ChrisAlbertson 0]

I want to do something, perhaps it is impossible with Cura.   

 

I am printing a structural part and strength to weight is important.   Let's say it is a motor mount plate and have 7 bolt holes.   Currently I find I need to print it with 2mm walls and 2mm top and bottom thickness then 40% infill and the part is strong enough.  This works but I see a way it could be lighter and stronger.

 

Walls are printed with all the plastic put down in parallel to the outside edge but I can only print walls along the edges.  It would be very good if I could make "walls" in the interior of the part.  Here are two use cases:

1. I would like to have "walls" radiating from a bolt hole to spread the stress out in to the skin  Cura makes a wall around the hole but the surface area of the outside is not enough to transfer the stress to the infill.  Interior walls in a spoke pattern would make the hole stronger and
2. I could make a truss design of thick "walls" that interconnect the bolt holes.  This way the stress is carried by 100% solid "beams" that are surrounded by a light 25% infill.   Lacking the ability to make these I must resort to higher infill even in sections of the part that are lightly loaded.

 

Here is how I might try doing this now...    In my CAD system (I use Fusion 360) I might make the part as I do now as a solid body then I make a second version of the same part but this version lacks the skin has  ONLY the internal beams that interconnect the bolt holes with a truss frame made of triangles.  SO I make an normal part and the same part again but as a skeleton truss.

 

Then I print in Cura 3.2 I use the second version of the part to modify the infill percent.  I set the infill for this to 100%.  So now I have beams made of 100% solid infill inside a much lighter structure and my overall strength to weight is better

 

Problem:  The "beams" are made of 45% crossed infill lines.  it would be MUCH stronger if the beams where printed like walls with all the plastic printed parallel

 

If this is not 100% clear think of steel reenforced concrete.   I want to print the steel bars the same way as walls are printed and I want to use low-percentage infill where the concrete would be.

 

Perhaps this is something Cura will never be able to do, or perhaps I can use some tricks with my STL files to trick Cura into making "walls" inside the part I'm printing.    Or perhaps there is another slicer that is better for making structural parts?

 

 

[gr5 2,224]

There a few ways to do this.  

 

1) The way you mentioned works pretty well.  Even though the pattern is "cross hatched" if you print it properly there should be no "grain".  I know that higher temp materials like ABS often have a grain but this just means you need to learn to print ABS hotter.  But I think you will like method 2 better:

 

2) Sometimes I model "needle thin walls" in my print.  model an internal cuboid inside your part that is thinner than paper.  Vertical walls so the slicer can't miss them.  Walls that don't reach the outer walls.   Imagine vertical pieces of paper - only as tall as you want your "ribbing".  But hollow instead of solid.  Hollow walls inside your part.  Radiating out from your screw holes.  You can model groups of these walls.  Make the width tiny - say .01mm.  You can put bundles of these radiating out from your screw holes.  .01 is so small it will be filled in.  So the printer will do "shell" around these "hollow walls" and create the structures you are looking for.  Space the walls at least 2X the nozzle width apart.  This will control the direction of "shell" lines in your ribbing so you can make them radiate outward and avoid that cross hatch pattern.  This will work on any slicer - not just Cura.  The height of these walls will control the height (inside your part) of these ribbing structures.
 

 

 

[burtoogle 516]

To add strength around holes or other features you can use "infill meshes" - these allow you to specify regions of your print that have modified infill so in my example below I have upped the infill density in the regions around my bolt holes. Interestingly, the infill mesh can have walls so your idea of having internal walls is actually achievable.

 

 

Hope this helps!

 

[gr5 2,224]

@smartavionicsI think that's exactly what he did.

[eldrick 65]

I must be missing something: using Moment of Inspiration (moi3d.com) I created a cube and a smaller sphere. I centered the sphere inside the cube and used Boolean Diff to hollow out the cube with the sphere. 

Saved as STL file (attached), and opened it in Cura 15.04, where the Layers view showed clearly that the cube had a hollow spherical center hole.

So the answer is that Yes, you can indeed create custom supports inside an object by making inner holes and printing the object mostly solid otherwise.

You might also want to try the ability to use Concentric with a large value of Number of Walls, to make the "grain" of the printing follow the outline of your internal supports.

 

hollow.stl

Edited May 6, 2018 by eldrick

[ChrisAlbertson 0]

8 hours ago, smartavionics said:

To add strength around holes or other features you can use "infill meshes" - these allow you to specify regions of your print that have modified infill

 

 

So it looks like you are dropping a cylinder onto the build plate after you place the part on the plate them moving it around by eye to center it on the hole.   That is the method I was using too.   Now what I want are shapes that are much more complex than a cylinder, something that looks like a skeleton or truss.

[burtoogle 516]

4 hours ago, ChrisAlbertson said:

 

So it looks like you are dropping a cylinder onto the build plate after you place the part on the plate them moving it around by eye to center it on the hole.   That is the method I was using too.   Now what I want are shapes that are much more complex than a cylinder, something that looks like a skeleton or truss.

 

No, my infill mesh is actually all of the cylinders together in their correct relationships as is shown in the image below, I have moved the meshes apart so you can clearly see them. OK, so my infill mesh is only 5 simple cylinders but it could be much more complicated, the principle is the same.

 

 

Edited May 6, 2018 by burtoogle

[mike_brandt 0]

gr5has the way to Design in the structure for fins

 

could you also put in . 1mm tubes to force a rod to be made

with the rod you could make a tessellated structure or a triangulated beam internaly to the skin.

if the rods/tubes did not touch the outside i would expect that you would need to cap the open end of the tube.

by increasing the size of the tube center the size of the rods could be increased with a minimal internal space.

 

a pre made rod framework could be made in solid and subtracted from the model

 this would preclude the need to cap the ends of internal ending tubes and simplify the design

 

this is all speculation and should be taken with grain of salt.