This media is formed by bonding abrasives (generally 40-70% by
weight) into polyester, usually by casting. After casting, this media is
typically pre-tumbled to remove flash. The plastic media is sofer, has a
lower density than preformed ceramic media, providesa cusioning effect,
and sometimes has a shorter life. It is well suitable for finishing
softer metals and fragile parts, achieving pre-plating requirements, and
producing smooth finishes as low as 5 micrometers. 

During finishing the
matrix which is softer than the abrasive, is eroded, leaving the
abrasive to perform the desired work, While some users believe it
assures parts free of impingement, this is not ture, All the mass
finishing processes expert for fixtured or compartmentalized operations
allow part-on-part contact. Plastic media can also be produced in a wide
variety of desirable shapes. In practice over a decade some shapes go
out of favor and new ones emerge.