A “cad” file? What file format are you thinking of? AutoCAD DWG?
Often people print to 3d printers using STL - there is an STL extension in the warehouse: Extension | SketchUp Extension Warehouse
Thom Thom is being too gracious, he’s actually developed a few extensions for SketchUp that ensure the models are suitable for 3D printing. So with the STL Extension Thom recommended and something like Solid Inspector, you may find that you can rescue your part for 3D printing in SketchUp.
MARK
Additionla info. Here is another approach to convert file. http://www.shapeways.com/tutorials/how_to_use_meshlab_and_netfabb.In addiditon Netfabb has a free service where in they will check and fix the stl and send back to you. Log on to their site and check service link. Other printers probably have similar service?
You certainly don’t need to pay someone to “convert’ your SketchUp model for 3D printing.
The SketchUp STL extension was created so anyone can export 3D printing STL files from SketchUp.
All you need to do is learn how to create a SketchUp model that’s printable.
A Solid in SketchUp is a single Group or Component whose geometry meets the certain standards.
When those conditions are met, Entity Info indicates the model is a Solid Group or Solid Component.
Also notice Entity Info indicates the Volume of a selected Solid.
• The geometry must form a single* airtight vessel.
Like a soap bubble … •No gaps •No holes •No leaks
• No extraneous Faces** inside or outside the vessel.
All Faces must serve to enclose the singular volume of the vessel.
• No stray Edges.
All Edges must serve to support a Face that in turn serves to enclose the airtight vessel.
• No nested Groups or Components.
*A Solid Group or Solid Component may contain one or more separate airtight vessels.
**Best that all Faces are oriented Front Side (white) facing out.