There are 3 qualities of plastic side seals: 

1. Polycarb is very basic with minimal qualities;
2. 2.  Delrin has natural lubricity which reduces friction, so smoother movement and superior solvent resistance;
3. 3.  Peek is the harder one and handles extreme fluid temperature, high pressure, and aggressive solvent exposures. 

Additional savings realized in cost of included A-Quality o-rings. 

A+  or A-Quality o-rings have much higher chemical resistance than the Buna or FKM (compatible to Viton) offered by most Sellers. 

A-Quality does not swell in solvents and lasts longer! Delrin does not scratch NEW mixing chambers. 

Do not confuse Delrin with Polycarb, they are different qualities and perform differently.