I Dont Want The Cookies I Want The Tray!

What do I need to change in my STL file, to have Shapeways recognize, that I want the cup cake tray printed (shown in Red see pic)

Make sure that your tray has some wall thickness, and run it through some mesh fixing software like meshmixer or netfabb studio to make sure that faces are oriented and there are no holes in the mesh.

What software did you make that in?

It would appear through your picture, that your "tray" has no printable thickness. As such, Shapeways auto check is closing off the bottoms of your "cookies" to make solids and removing everything else, i.e. the tray.
If you extrude and extend all of the surfaces of your tray, this should solve your problem

You are absolutely right! the .STL when uploaded to Shapeways cannot distinguish between an open or closed cube.

This is interesting, because when they print houses in China the walls are printing layer by layer by the CNC machine. The width of the wall is NOT an issue because the material used already has thickness & width, hence the CNC code would require only a single line for the outside walls.... In the same way the material Shapeways uses has thickness & width. Why would a single line not suffice in printing of a cube with one side missing?

The house printing you mention, whether in china or elsewhere, can only work like that when it is pre-defined what the default thickness should be. With the wide variety of customer models that shapeways prints, and the fact that more often than not the material uses is a very fine powder, specifying the wall thickness is required.

Thanks!.... I'm trying to become more knowledgeable about 3D printing. If I were to consider buying a small 3D Printer, the thickness of the material would be defined by the roll of ABS, PLA or PVA I'm using. Hence would a single line cube .STL produce the correct CNC code to print a CUBE with one side missing?

Not sure about that, but probably the printer control software would still divide by zero when trying to calculate how many passes of the printhead are needed to form the wall. (That is unless it is very simple software like on a really old pen plotter.)

What probably matters is the software that converts the stl into physical movements of the printer head. If that software can deal with open structures that have zero thickness walls then something like this might be possible. But even with an FDM print head the width of the print line will be less than the width of the material coming off the spool. Maybe there are printers that have bigger print nozzles but generally if you're wanting resolution prints it's not going to throw out a huge thick line of print material. You would need a special purpose printer meant to throw out material quickly with lower "spot" resolution. "Normal" 3D printers are going to try to fill in the area between boundary walls with print material.

If you're printing something like concrete you could probably print a wall using one big fat nozzle, but even then you'll want more thin paths for strength and better quality prints. There's not really much difference between a giant concrete printer and 3D printers that make smaller items. The nozzle paths will be a function of material being used and nozzle sizes. Ultimately there's a tool that runs along a vector path and deposits or solidifies material in any 3D printer.

Since you seem to be using Meshmixer you could convert an open surface into an object having wall thickness. There might be other ways to thicken the walls but I've used the "make pattern" function to create walls with thickness. Basically this will connect tubes between vertices to create a thick walled object. You might need to remesh to a higher resolution before doing this so that the walls don't have holes in them.

Here's an open box with zero wall thickness. I deleted the mesh from the top to create it.


Next I've used the make pattern tool to thicken walls, but you'll notice there are holes. You could adjust the line parameters to close the gaps but the surface might not end up being smooth enough. Note that the Element Dimens parameter will essentially set the wall thickness (2mm in this case). Also note that not all the parameter inputs are going to do anything for each pattern option. You'll need to play around with the tool to figure it out.



Here's another try after increasing the original mesh density by 50% before thickening the walls.



You can do something similar with your cookie tray.

As I understand the process, its not really a function of the machine to be able to print the thickness (thinness) you want, but rather the slicing software used to produce what is called G-code, to drive the printer.
If you were to draw a plain cube or open topped cube in a cad program, Sketchup for example, that software does not define the open topped cube as a solid as it is not "water tight." The cube however is.
When the cube is brought over to the slicer software, (cura, Simplify 3d, etc...) you can tell it how thick you want the walls and how much infill you want. So if I wanted a really thin walled cube, I would set the wall thickness to a ratio of the nozzle diameter of my printer, and 0% infill. This could produce what you are after, but it all starts with the design software first. If you don't have a watertight model to begin with, the slicer software will have a fit because it is not programmed to deal with such things that essentially have no mass and take up no space.
I believe the house printing machines essentially are set up to follow a path much more like a laser cutter does rather than a printer. The thickness is more determined by the concrete output rather than the software driving the "print head."