Interesting. Perhaps during the sorting stage, materials should be grouped according to their contents. ie Food containers are separated from cleaning product containers etc, then each batch can be thoroughly cleaned before going on to be grouped into plastic types and then shredded. That way, items than are too difficult to clean thoroughly are kept separate and don’t soil the rest of the batch.
Sorry to hear about your inconvenience. But what will happen to those plastic if even you don’t want that? Think twice before you dispose those plastic.