Whether you need a metal detector today or not, it might be a good idea to set your plastics recycling system up for one down the road. The easiest way to plan for the future is to purchase a conveyor built to accept a metal detector or eddy current separator. Metal detectors can not be attached to the conveyor on the metal base because it will interfere with the operation of the magnet. A metal detector conveyor will have a section or sections of the sidewalls made from plastic which will not affect the operation without a metal detector.
If you think that you will have any need in the future, no matter how remote, for a metal separator, it's a good idea to spend the little extra upfront, than to have to replace your conveyor or try to modify it. You will save money in the long run.