MakePrintable is an automated 3D printing service that claims to be able to fix all of your 3D printing problems. The service provides an easy to understand process that involves uploading design, designating your printer and filament, select your desired output (Gcode, STL or OBJ file format), and download your new file. The types of problems that can be fixed include: solidity, transparency, hollowing, thickening, filling, speed, intersecting objects, flipped faces, non-manifold issues. Powered by MXD3D, if the MakePrintable website's clear, slick, and user-friendly design is any indication, the 3D printing scene has just received an invaluable service that will fix problems and encourage experimentation -- since MakePrintable can support your efforts in an automated fashion. Check out more details: http://3dprint.com/30943/makeprintable-fix-problems/ ‎


Below is an image from MakePrintable; they'll fix non-manifold issues: