I was asked this question via email the other day…
We process a large amount of amorphous polymers including ABS, PS, and PC. We are looking to purchase come new screws for these materials, the screw manufacturer suggests using nearly half the screw for the transition zone, does this make sense? If so, Why?
Your supplier is making the correct suggestion. To avoid over-shearing the material, it is best to use a long transition zone to slowly heat and soften the amorphous polymer chains. This will help avoid un-melted gels from reaching the metering zone and the molded part.
In a 20:1 L/D screw, amorphous polymers require approximately 8-10 diameters of transition to ensure a proper melt.