Mike
The part thats troubling,is that the fix is a few keystrokes on the press while its running.
Sink marks are either not enough plastic in the mold,or (more than likely) they didn't "compress" the plastic in the mold after filling it.Think of it like when you're blowing up a baloon,when it gets harder to force air into the baloon,thats the same as the final stage of mold filling.All they have to do is alter when that happens and the sink marks are gone.