The Association of Post Consumer Plastic Recyclers (yes, there is such a thing!) has an interesting website with daily recycling tips for consumers. Yesterday their tip was to remove bottle caps from plastic bottles before you put the bottles in your recycle bin. I was told that you should do this, but I never knew why.
Here's why. Most plastic bottles are made from either PET or HDPE plastic. Both are forms of polyethylene. The caps are typically polypropylene. So, when a capped bottle is put through recycling equipment the plastic from the top contaminates the batch of recycled plastic. Also, caps and lids can jam the machinery. That's bad for the machine, but it also presents a potential danger to the people working those machines.
So, remember that "caps off" helps the environment by allowing more plastic bottles to be effectively recycled. It also supports worker safety.