23 June 2013

Todays result: 3D cellular fill patterns

3D cellular structures should be the standard fill mechanism used by slicing software. 

My recent work has lead me to the conclusion that 3D cellular structures make better fill patterns for 3D printing and that it would fit neatly as the fill pattern used by slicing software. Using cellular fill will have benefits over using horizontal alternating diagonal fills, and over 2D pattern fills such as hexagonal tessellations. 

Using cellular fill increases the strength of the part relative to the amount of material. Conversely, less material is needed to achieve the same overall strength. The cellular fill is a deliberate structure with plastic used sparingly but in a focused way. Each cell is a pocket of empty space with only the outer perimeter traced in plastic. This approximates a sphere producing a strong shape relative to the amount of material used. Each cell shares material with, and benefits from the stability and strength imparted by other cells. Using less material leads to less printing time.

This work has been my attempt to contribute back to the Reprap community. I have produced proof of concept model using the Pink panther woman but I feel pressured to release this idea before I can get it fully operational because of the recent plague of 3D patents and trolls.

In the near future, the use of cellular fill structures will be a normal and automatic part of slicing software instead of the current zig-zag fill a cellular pattern will used. At each layer the slicing engine will also slice (or more likely compute) the pattern and insert it into the object.

The generation of the pathways by slicing engines will also need to change. When sliced horizontally an object filled with cellular structure looks like a collection of different sized circles compressed together.  For the best outcome, every cell needs to be constructed as a series of complete circuits around the perimeter of the cell. In my experience, current slicing software tends to identify some cells as complete circuits, while others are afterthoughts once the complete cells are in place. The result is poor with the regions of the afterthought cells not connecting correctly to one or both adjacent walls. To date experimentation has been using objects filled with cellular structures within the model itself. 

Similar to the selecting layer height and print speed, the user will select the pattern used, wall thickness and size of the pattern. Orientation and location of the pattern may be advanced options.  By varying the settings the desired strength, density, and timing trade-offs can be controlled.

The main pattern of use is the truncated octahedron. It is a beautiful shape, made of six squares and eight hexagons all with equal length sides. It is single shape that will completely occupy 3d space with no gaps. It tessellates through uniformly via translation alone ie no rotations or other transforms. I have generated other more complicated patterns that could be used but I don’t believe at this point that they offer any additional advantages.

 Truncated octahedron pattern
This print is 100 * 100 * 50 mm and weight ~50gm.
It will support over 80kg placed on the thinner edge
I estimate it uses approximately the same quantity of plastic as a 10% zig zag fill

The Pink panther woman, scaled larger and cut into six segments.
Each segment is filled with a different cellular pattern
Printed in transparent ABS to show off the internal structure

Individual segments can be seen in previous posts.
The truncated octahedron model will be made available soon.