All plastics are compressible… therefore there is a compression that takes place with the polymer in front of the screw. There needs to be a cushion to compensate for variability, but too much cushion results in too much pressure loss before the nozzle.

Although a consistent cushion will help ensure a more reliable process… cushion control is not a reliable method of achieving this. You are best to monitor the cushion and adjust the shot size and transfer yourself when necessary.

-Andy