Nazdar 9700 (as mentioned by another poster) is the ink to use on CorPlast, if you can afford rack/air drying.
The 2 reasons for spiderwebbing: static and ink "gumming up" with prolonged pulling on the screen. The latter can be cured by adding retarder along with the thinner (scraping up used ink after every 50 pulls or so, and remixing)...sacrificing drying time of course.