The purpose of routinely measuring screw and barrel wear to see the trend over time.  This practice allows you to preemptively correct for wear by scheduling repairs and replacements. for example, let’s suppose a screw is still working, but having some mixing or melting issues, you can document approximately how much wear, and clearance, indicates such complications. If you see a similar situation developing on another machine, you can schedule a screw replacement or repair, rather than react after you produce a bunch of scrap. You can also avoid certain jobs with sensitive materials or strict colorant requirements if you know a machine is suspect.

For comparison… top drag racers completely dismantle and re-assemble the engine after each race, replacing everything from pistons to pushrods (takes about 90 minutes). The components they remove are later measured and a determination is made whether to re-use, refurbish, or discard each component. Each team has its own set of records derived from experience to identify when a component is useful or useless. If the team fields different classes of cars, then the acceptable tolerances would vary from machine to machine. 

There are always rules of thumb, but your mechanics need to think about their jobs more like crew chiefs knowing the driver (mold) is stepping into a machine that is ready to do the job.

-Andy