The issue isn't metal or plastic, but rated and not. There are and have been specific boxes made and rated for ceiling fans. No matter what it is attached to you don't want to hang your heavy and expensive ceiling fan over your head or your kids from just any old box. I've seen octagon boxes screwed through the back to a ceiling joist actually separate at their folded or spot welded corner seams. If Sweets box is protruding and he isn't positive of it's durability, then what he needs is called a "pancake" box designed for a ceiling fan-it'll cost six bucks instead of 1.78 but you can hand a Hunter or Casablanca on it and not worry about finding the fan on the dining room table or in the middle of the bed. The expanding brace models Ron described also work quite well when the center of the ceiling falls between joists in an existing or old work situation. All of this stuff is available for less than 20 bucks at HD or Lowes ceiling fan departments not in the junction box aisle. good luck
I purchased a box designed for ceiling fans. It supports up to 70lbs. I reinforced the joist in my attic just in case. I mounted the fan and got everything connected. It works perfectly. I just want to thank everyone for their advice.