Tom, upon further reflection, I think you do make a good point re PVC vs. "the rest of the polymer industry". I'll try to cite a few specifics I'm aware of----
1)- Most other polymers can be extruded or molded "as is" from the producer, perhaps with added color concentrate &/or filler or glass. PVC resin cannot be processed without formulating and compounding in lubricants,stabilizers(specific to process and end use), plasticisers, process aids. Color, fillers, glass also can be added as well.
2)-PVC evolved out of the rubber industry (most early producers were rubber companys--Goodyear,B.F.Goodrich,Uniroyal-U.S.Rubber,Firestone,General Tire. Rubber formulators and compounders were notorious for developing complicated formulations with a host of additives--antioxidants, curatives,lubricants,carbon blacks, etc.
3)- Most other polymers-Engineering as well as Olefins- evolved out of the petrochemical industry and other chemical companys. Most of these resins contain perhaps an antioxidant package, UV absorber, FR system, a minimal amount of total additives--and can be processed directly from the producer.
4)-Depending on how it's formulated, a given K-Value PVC could have a widely divergent set of fusion(melting),flow, and lubricity properties (all well seen in a Brabender curve). A "store-bought" compound producer (a Poly One,Georgia Gulf,ec.) might be able to furnish a single viscosity value for his compound, but not sure what it would be---don't know if can K values be determined on a compound?
5)-Yes, I'll agree there is a difference in thinking between PVC and other resin processors.There is also a difference in plant personnel thinking. I can recall several situations of plant-wide power failure, and watching the Chinese Fire Drill as everyone scurried to strip down pipe dies before onset of catastrophic degradation! As to extrusion screw designs, since the vast majority of PVC extrusion is via counter rotating twins--either conical or parallel--the main single screw development I'm aware of has been longer and longer L/D's, from 24:1 out to 30,32,36:1 L/D's, with the longer residence times allowing more mild temp./RPM conditions so as to run almost like a low shear twin. Carlon's (Lamson & Session)PVC pipe/conduit was made this way and they ran very low cost formulations.
I guess it's a question of preferring Fords or Chevys!!