They say it here better than I can:

http://en.wikipedia.org/wiki/Rheology

Hello,

There are a lot of great presentations from Malvern Instruments that discuss many applicaion areas of Rheology, including how it relates to plastic processing. Using both a rotational rheometer and a capillary rheometer, one can generate insight into a materials structure and processing performance directly.

Use the search box on any page or go to the support/education area in their site.

http://www.malverninstruments.com/malvern/ondemand.nsf/search

http://www.malverninstruments.com/home/index.htm

http://www.malverninstruments.com/LabEng/products/iwtm/rheological_polymers.htm

If required, you might need to register, but this is free and the information is great.