The drawings are scanned on a networked photocopier and emailed to the teachers. They are converted from jpgs to svg files in Inkscape. Finally, the svg file is imported into Tinkercad, extruded, and turned into an stl for printing.
The workflow was inspired by the MakerHome blog post about turning a Sharpie drawing into a 3D print. However, we started running into issues with some of the drawings we were trying to print. Some models were printing with extra strings, while others were air printing, not extruding filament as the extruder worked its way around the model.
I tried adjusting the Trace Bitmap thresholds thinking that might help, but the models still wouldn't print.
Here is the solution I serendipitously stumbled across.
From the Path menu, select Trace Bitmap. Leave the threshold levels alone.
Click the Update button then close the Trace Bitmap window.
Select the image by clicking on it. If you drag the image you will notice that there is a tracing over the bitmap image.
In the image above you can see the bitmap on the left and the tracing on the right. Take a close look at the bitmap and you will see it is not uniform. I believe that Inkscape was including this data in the svg and Tinkercad was trying to model it, hence the ugly models, stringiness, and general failure of many of the print jobs.
Delete the bitmap portion of the drawing in Inkscape by selecting it and pressing Delete on your keyboard. Only the solid dark tracing should remain.
Save the file as an svg.
Import the svg file into Tinkercad at 10% size.
Clean up any "dots" left from the scan of the original image by using Box Holes to cover them. Size the silhouette to about 270 mm deep, so it will fill most of the build platform on a Replicator 2.
You will notice that the walls of the model are pretty smooth and free of holes.
Group the objects and download an stl of the model.
When you import the stl into MakerWare do not scale it to fit.
Use the Turn and Move tools to get the model completely on the build platform.
Your model should print beautifully with these settings.