This may be caused by a number of factors. Most commonly, it is because the work bowl is being over-filled or too much pressure is being exerted on ingredients as they fall through the feed tube. Doing this may cause the disc to come off balance and start sheering plastic.

Call Kitchenaid Customer Service and provide them with this description. They will replace the unit free of charge, regardless of it's age or warranty. Their toll-free telephone number is:

1.800.541.6390.