http://blog.ultimaker.com/2013/01/28/melting-2-different-pla-together-during-a-print/ (404)

How To:

- Pull out a lighter and briefly heated up both filament ends (don’t fire them up!)
- Merge them and wait a couple of seconds
- Form them with your fingers to be as smooth as possible
- Quickly file off any excess material (which could block within the bowden tube)
- Happy printing!