as far as i know, the bobbin case is always metal. you're right about that. the bobbin case fits into a .....hmmm .... round space with a spindle poking up or out in the center, depending on whether you have a front loader or top loader. that round, empty space is called a chase, (it has another name that escapes me right now). the bobbin case rolls back and forth in there, hooking the upper thread to make the stitches. the chase on many newer machines are made of metal/plastic combos. those are the ones that Ernie the Repair Guy (my hero!) says should be loaded with only plastic bobbins. if the chase is made of all metal, then the bobbin should be metal.