Joris Peels from the 3D printing startup Shapeways has been experimenting with making stovetop bioplastics, with mixed (but promising) results:
Overall making bioplastic at home was a fun activity. It had a certain magical, "oh my, I made plastic" quality to it. Bioplastic in thin sheet form does seem to be a fun and practical application as a build material. However right now the results, as far as I have found with molds, are not promising enough for one to be able to use it to make your own things or to produce plastic for your DIY projects. I need to qualify this by saying that any of the problems with the bioplastic could of course be my fault for not cooking it long enough, cooking it too high/low, measuring out the proportions incorrectly, using the wrong starch,drying the bioplastic in too hot/too humid/too dry a room etc.
Taking that into account, at the moment shrinkage, warping and the long drying times, as I have found them, make it unpractical. As for using it as a 3D printing material, I am not comfortable enough with this material at the moment to even properly test it in a 3D printer. The "gunky" quality and long drying times seem to currently make it impractical for use as a 3D printing material. Someone could possibly tweak the recipe and make it more practical both for 3D printing and molding and I hope this happens. Perhaps you could with some careful experimentation be the one to make bioplastic a practical reality for molding and 3d printing?