Some chef's knives have a plastic handle. but the care instructions caution against putting the knife in the dishwasher. Why?
Although most plastic handles are dishwasher-safe (wooden handles aren't), sharp knives won't stay sharp if they go through the dishwasher. Water in the wash cycle can jostle knives, which can cause the blade to bump against the rack or other objects, significantly dulling or chipping the blade. Also, dishwasher detergent can dull and discolor chef's knives.
But the biggest reason to avoid the dishwasher is safety: No matter how you load knives—flat on a rack or standing in the basket—they can shift around, become hidden under other objects, or end up at awkward angles that make them dangerous to unload.
The bottom line: Hand-wash your chef's knives so that you stay safe and your knives remain in tip-top shape.